Young Stellar Objects in the Gould Belt

Young Stellar Objects in the Gould Belt

Abstract

We present the full catalog of Young Stellar Objects (YSOs) identified in the 18 molecular clouds surveyed by the Spitzer Space Telescope “cores to disks” (c2d) and “Gould Belt” (GB) Legacy surveys. Using standard techniques developed by the c2d project, we identify 3239 candidate YSOs in the 18 clouds, 2966 of which survive visual inspection and form our final catalog of YSOs in the Gould Belt. We compile extinction corrected SEDs for all 2966 YSOs and calculate and tabulate the infrared spectral index, bolometric luminosity, and bolometric temperature for each object. We find that 326 (11%), 210 (7%), 1248 (42%), and 1182 (40%) are classified as Class 0+I, Flat-spectrum, Class II, and Class III, respectively, and show that the Class III sample suffers from an overall contamination rate by background AGB stars between 25% and 90%. Adopting standard assumptions, we derive durations of Myr for Class 0+I YSOs and Myr for Flat-spectrum YSOs, where the ranges encompass uncertainties in the adopted assumptions. Including information from (sub)millimeter wavelengths, one-third of the Class 0+I sample is classified as Class 0, leading to durations of Myr (Class 0) and Myr (Class I). We revisit infrared color-color diagrams used in the literature to classify YSOs and propose minor revisions to classification boundaries in these diagrams. Finally, we show that the bolometric temperature is a poor discriminator between Class II and Class III YSOs.

Subject headings:
infrared: stars – ISM: clouds – stars: formation - stars: low-mass
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1. Introduction

Most of the current star formation within 500 pc of the Sun occurs in the Gould Belt, a ring of nearby O-type stars inclined approximately 20 with respect to the Galactic Plane (Herschel, 1847; Gould, 1879). The Sun is located interior to the Gould Belt, which has a radius of about 350 pc (e.g., Clube, 1967; Stothers & Frogel, 1974; Comeron et al., 1992; de Zeeuw et al., 1999; Pöppel, 2001). All of the nearby, well-studied giant molecular clouds (e.g., Ophiuchus, Taurus, Perseus, Serpens, Orion, etc.) are located within this ring (see Figure 1 of Ward-Thompson et al., 2007, for a schematic diagram of the Gould Belt in galactic coordinates).

Due to the high visual extinction toward giant molecular clouds, a complete census of star formation in these clouds requires infrared observations. The Infrared Astronomical Satellite (IRAS) provided the first such unbiased survey of the Gould Belt (and the rest of the sky), but was limited in both sensitivity and spatial resolution. For young stellar objects (YSOs) still in the embedded phases of evolution, IRAS was only sensitive to objects with luminosities above 0.1 L (Myers et al., 1987). For more evolved YSOs, IRAS was even less sensitive. The spatial resolution provided by IRAS ranged from 0.5′ at 12 m ( pc at distances of AU) to 2′ at 100 m ( pc at distances of AU), insufficient to resolve individual stars forming in clustered regions of molecular clouds. Ground-based near-infrared surveys (e.g., the Two Micron All Sky Survey (2MASS); Skrutskie et al., 2006) provide much higher spatial resolution but are unable to detect the most deeply embedded, heavily extincted, and/or lowest luminosity YSOs. Ground-based mid-infrared studies are generally limited by the atmosphere to only the brightest and most luminous objects. Finally, selected regions of the Gould Belt were mapped in the mid-infrared by the Infrared Space Observatory (ISO). While ISO surveys identified many new deeply embedded and low luminosity YSOs, they only focused on small portions of molecular clouds (e.g., Bontemps et al., 2001; Persi et al., 2003; Kaas et al., 2004).

The launch of the Spitzer Space Telescope (Werner et al., 2004) in 2003 opened new avenues for infrared surveys of giant molecular clouds. With its combination of high sensitivity and large format detector arrays operating at m, Spitzer was the first mid-infrared facility capable of mapping entire molecular clouds within reasonable amounts of time. The Spitzer Legacy project ““From Molecular Cores to Planet-forming Disks” (Cores to Disks, or c2d) took advantage of these features by mapping seven large, nearby Gould Belt molecular clouds and nearly 100 isolated dense cores. An overview of the survey is given by Evans et al. (2003) and a summary of results for the large clouds is given by Evans et al. (2009). The c2d survey identified over 1000 YSOs, many of which were newly identified, and demonstrated Spitzer’s ability to detect YSOs that are too deeply embedded, too low luminosity, and/or too low mass to be detected by previous facilities.

The Spitzer Gould Belt (GB) Legacy survey followed up the c2d survey by mapping an additional 11 Gould Belt clouds using the same observation and data reduction strategies as c2d. Combined, the two surveys targeted nearly every major site of star formation within 500 pc of the Sun. The only two exceptions are the Taurus and Orion Molecular Clouds, both of which are so large they were the subject of their own, dedicated surveys by different teams (Rebull et al., 2010; Megeath et al., 2012). Adding the results of the GB survey to those obtained by c2d nearly triples the number of YSOs identified in the Gould Belt. A preliminary attempt to combine the results from the c2d, GB, Taurus, and Orion surveys was presented by Dunham et al. (2014), but a full combination, including reconciliation of different methods for observing, data reduction, source extraction and classification, is left for future work that combines the expertise of the respective surveys. Here we only consider the c2d and GB surveys.

While the full list of YSOs identified by Spitzer in the c2d and GB surveys has been used in several studies, including those on clustering of young stars (Bressert et al., 2010), star formation rates and thresholds (Heiderman et al., 2010), the luminosities of protostars (Dunham et al., 2013), relationships between gas and star formation (Evans et al., 2014), and the physical nature of empirically defined classes of young stars (Heiderman & Evans, 2015), this list itself has not yet appeared in publication. To rectify this, we present in this paper the full list of YSOs in the combined c2d and GB clouds. For the c2d clouds, this list updates and replaces the previous version given by Evans et al. (2009), including an analysis of contamination by background AGB stars, updated extinction corrections, and revised spectral energy distributions (SEDs). Although we use this YSO list to investigate classification of young stars and the durations of these classes, the primary purpose of this work is to present a full catalog of Spitzer c2d+GB YSOs. While useful on its own, this YSO list will reach its full potential in combination with results from the far-infrared and submillimeter surveys of the Gould Belt by the Herschel Space Observatory and SCUBA-2 on the James Clerk Maxwell Telescope, once they become fully available (Ward-Thompson et al., 2007; André et al., 2010; Harvey et al., 2013). By providing the full Spitzer YSO list, we enable such comparisons.

The organization of this paper is as follows. We describe our method in §2, including an overview of the c2d and GB surveys (§2.1), the procedure for identifying YSOs (§2.2), residual contamination in our YSO sample by background AGB stars (§2.3), and the construction of full SEDs and corrections for extinction (§2.4). We present our results in §3, including the full list of YSOs (§3.1) and a discussion of classification of young stars (§3.2 3.3). We discuss timescales for the classes of young stars in §4. Finally, we summarize our results in §5.

2. Method

A concise summary of these surveys, the adopted procedure for identifying young stellar objects, and the methods used to construct SEDs and to correct the photometry for foreground extinction are given in Section 2 of Dunham et al. (2013). Here we repeat the key information and refer the reader to Dunham et al. for more details.

2.1. Overview of the Surveys

The Spitzer c2d survey (PI: N. J. Evans) imaged seven large, nearby molecular clouds in the Gould Belt and approximately 100 isolated dense molecular cores. Evans et al. (2003) review both the primary science objectives of c2d and its basic observation plan, and Evans et al. (2009) summarize the results in the large clouds. The Spitzer GB survey (PI: L. E. Allen) is a follow-up program that imaged an additional 11 molecular clouds in the Gould Belt, completing most of the remaining clouds in the Gould Belt except for the Taurus and Orion molecular clouds (see §1). Both surveys imaged their respective targets at 3.6–8.0 m with the Spitzer Infrared Array Camera (IRAC; Fazio et al., 2004), and at 24–160 m images with the Multiband Imaging Photometer (MIPS; Rieke et al., 2004). A data pipeline developed by c2d was applied to the observations obtained by both programs. This pipeline includes data reduction, source extraction, and band-merging the results from each wavelength to produce final images and source catalogs, and has been described in detail elsewhere (Harvey et al., 2006; Evans et al., 2007).

For each cloud targeted by c2d or GB, Table 1 lists the parent survey, the assumed distance and reference for the distance, the total area mapped, the total cloud gas mass, and references to in-depth studies of each cloud based on the c2d or GB data, when available. The Spitzer Astronomical Observation Requests (AORs) for each cloud can be found in these references to individual cloud studies; Appendix A lists the AORs for the clouds without published data references. The isolated cores surveyed by c2d are not considered here except for a few cases where the cores are actually parts of larger cloud complexes and included in the results for those clouds (namely Cepheus and Ophiuchus North; Kirk et al., 2009; Hatchell et al., 2012). Note that the distances to these clouds are not all well determined, and some cloud distances are still under debate. One such example is the uncertainty over the distance to Aquila, with two distances (260 pc and 429 pc) commonly adopted in the literature (see, e.g., Harvey et al., 2006; Gutermuth et al., 2008; Dzib et al., 2011; Maury et al., 2011, for details). For this reason we do not list formal distance uncertainties and instead refer the reader to the references given in Table 1 for full discussions of the distances to each cloud.

The total cloud areas mapped and total cloud gas masses contained within these areas are taken from Heiderman et al. (2010), with the masses determined from extinction maps (see Heiderman et al. for details). For most clouds, the mapped areas were chosen to map the entire cloud above a minimum visual extinction in the range 2 A 3, with the exact value for each cloud depending on the total area that could be mapped within the allocated time. For Serpens and Aquila, only the cloud areas above A were mapped due to time constraints and difficulty in separating the cloud from the general extinction close to the Galactic midplane.

2.2. Identification of Young Stellar Objects

The data reduction pipeline used by both the c2d and GB surveys creates band-merged source catalogs where the sources extracted at each Spitzer wavelength are matched with each other and with the 2MASS catalog, creating a final catalog of m photometry for each cloud. Candidate YSOs are identified using a method developed by Harvey et al. (2007a) and summarized in all of the publications focused on individual clouds that are listed in Table 1. In this method, the Spitzer SWIRE Legacy survey of the ELAIS N1 extragalactic field (Lonsdale et al., 2003) is processed to simulate the same sensitivity and extinction distribution of the c2d and GB clouds, and then used to locate background galaxies observed through the target clouds in various infrared color-magnitude diagrams. Using these results to define the expected colors and magnitudes of background galaxies, each source extracted in the cloud maps with infrared colors that can not be explained as an extincted background star is assigned an unnormalized “probability” of being a galaxy or YSO based on its position in each color-magnitude diagram, its morphology in the two shortest Spitzer IRAC bands (3.6 and 4.5 m), and its 24 and 70 m flux densities. Since it is unnormalized, this quantity is not a true probability but a relative parameter expressing the likelihood of being a background galaxy (higher values indicate higher likelihood of being galaxies; see Harvey et al., 2007a, for details). The final boundary between candidate YSO and candidate galaxy in this quantity is set to provide a nearly complete elimination of SWIRE sources, likely at the cost of excluding YSOs with overlapping colors and magnitudes (e.g., Hsieh & Lai, 2013).

One potential drawback to the c2d+GB method of identifying YSOs is that it requires detections in all four IRAC bands ( m) as well as the first MIPS band (24 m). As the 24 m MIPS band has lower resolution and sensitivity, some of the faintest and/or most clustered YSOs may be missed (e.g., Gutermuth et al., 2009, Gutermuth et al., in preparation.). We note that several alternative classification methods for separating background galaxies and YSOs have been presented in the literature (e.g., Gutermuth et al., 2009; Rebull et al., 2010; Kryukova et al., 2012; Hsieh & Lai, 2013). While they are all based on a similar philosophy of separating sources in various colors and magnitudes, they differ in their detailed implementations of this philosophy. A full comparison of these methods and ultimate consensus on best practices for extracting YSOs from infrared surveys of star-forming clouds is left for future work.

Applying these selection criteria results in the identification of 3239 candidate YSOs in the 18 c2d and GB clouds. We note here that we only consider sources that are located in the overlap regions imaged by both the IRAC and MIPS instruments, as these are the sources that can be most reliably classified using the method summarized above. For technical reasons, the MIPS observations often covered much larger areas (see the references listed in Table 1 for more details on the observations of each individual cloud). Several of the individual cloud studies listed in Table 1 also present candidate YSO lists in the MIPS-only regions using alternative identification and classification techniques, but such sources are not considered here. We visually inspected each of the 3239 candidate YSOs to remove resolved galaxies and image artifacts that are sometimes misidentified (see Evans et al., 2009, for details). Additionally, a few known YSOs that were saturated or undetected at one or more wavelengths were missing from the initial list and were added by hand. In the end, we were left with a final list of 2966 YSOs, and since all have passed visual inspection, we follow Evans et al. (2009) and drop the prefix “candidate” at this point.

Finally, we note that initial results from Herschel surveys of the c2d and GB clouds suggest that a modest number of the most deeply embedded protostars are undetected by Spitzer, increasing the number of Class 0 protostars24 by 15% – 30% (e.g., Harvey et al., 2013; Stutz et al., 2013; Sadavoy et al., 2014). Revisions to the YSO lists presented here will thus be necessary once complete Herschel results are available for each cloud.

2.3. Background Giant Star Contamination

Figure 1.— Left: Histograms of the infrared spectral index (, uncorrected for extinction) for the YSOs observed by Oliveira et al. (2009). The shaded histogram shows the full sample of 78 objects whereas the dashed histogram shows the 20 objects identified as background AGB stars by Oliveira et al. Right: The black symbols and line plot the contamination fraction in each bin, where this fraction is defined as the number of background AGB stars in this bin divided by the total number of objects in this bin, using the results from Oliveira et al. (2009). The colored symbols plot the contamination fraction for each Class (red for Class 0+I, green for flat-spectrum, blue for Class II, and purple for Class III). The Class II and Flat-spectrum results are plotted at the mean value of for each class, whereas the Class III and Class 0+I results are plotted at the mean values of the boundaries for each Class and the display ranges of the x axis. The dotted line in each panel marks the Class III boundary ().

Our final sample of YSOs is likely contaminated by background giant stars with infrared excesses. Oliveira et al. (2009) performed optical spectroscopy toward 78 YSOs in Serpens and identified 20 (26%) as background Asymptotic Giant Branch (AGB) stars. While the overall contamination rate is relatively low, Figure 1 shows that the contamination fraction strongly depends on the infrared spectral index 25. Starting from positive values, as decreases, the contamination fraction remains close to zero until the Class II/III boundary is reached at (see §3.2 for a full definition and discussion of YSO classes), and then steeply rises to 50% for more negative values of . On average, 62% of the Class III sources examined by Oliveira et al. are actually background AGB stars, whereas only 5% of the Class II sources they examined were found to be contaminants.

Very high rates of AGB star contamination among Class III sources are also found by Romero et al. (2012), also with optical spectra. They found Class III contamination rates of 100% based on observations of 30 objects in Lupus V and VI. While the fact that they only observed objects that met initial selection criteria for transition disks may bias their results, they observed 45% and 39% of the total Lupus V and VI Class III populations, respectively; thus the true contamination fractions are at least 40% in each cloud.

The true contamination fraction is likely a strong function of galactic latitude, and the specific clouds considered above (Serpens, Lupus V, and Lupus VI) all lie within  of the galactic plane. Indeed, Romero et al. (2012) found a lower contamination rate of 25% for Ophiuchus (), consistent with the Ophiuchus contamination rate of 20% measured by Cieza et al. (2010) using similar methods. To further investigate the contamination rate in our full sample of YSOs, Figure 2 plots a Spitzer color-color diagram consisting of [3.6]–[24] versus [3.6]–[4.5], using the observed photometry. Inspection of Figure 2 shows that the Class III YSO population can be divided into a population of objects clustered at lower (bluer) values of both colors, and a second, more distributed population extending to higher (redder) colors. Using optical spectra of candidate transition disks, Cieza et al. (2010), Cieza et al. (2012), and Romero et al. (2012) found that background AGB stars are predominately found at low values of [3.6]–[24], suggesting that the clustered population in Figure 2 may be dominated by AGB stars whereas the more distributed population may be dominated by YSOs. Indeed, the above studies found that all of their objects with [3.6]–[24]  1.5 were in fact AGB stars, with decreasing contamination rates at higher colors. We find that 298 of the objects in our final YSO sample have [3.6]–[24]  1.5, all but three of which are classified as Class III objects. This represents 25% of the Class III YSO sample and 10% of the total YSO sample. We consider these objects as likely AGB stars and mark them as such throughout the remainder of this paper. However, we do not remove them from the sample since the studies cited above only targeted a subset of objects that were identified as transition disk candidates and thus there is no guarantee that their finding of 100% contamination at the lowest values of [3.6]–[24] applies to our full sample. Confirmation of the status of these objects requires either spectroscopic or proper motion studies. Such work should be considered as a high priority for future investigations seeking to refine the YSO sample presented here.

Figure 2.— [3.6]–[24] versus [3.6]–[4.5] for all 2966 objects in our final YSO sample, using the observed photometry. The color of each symbol shows its spectral class determined using the extinction corrected infrared spectral index (see §3.1 and §3.2 for details on classification), with red indicating Class 0+I, green indicating Flat-spectrum, blue indicating Class II, and purple indicating Class III. The horizontal dashed line indicates the boundary of [3.6]–[24]  1.5 used to identify highly likely AGB stars contaminating our YSO sample. The vertical line of sources at [3.6]–[4.5]  0.5 is a rounding artifact that is discussed in detail in Appendix B.

The 25% Class III contamination by AGB stars found above is almost certainly a lower limit to the full number, since some contaminating AGB stars are found to have [3.6]–[24]  1.5 (Cieza et al., 2010, 2012; Romero et al., 2012). However, these studies target samples that are too small and too biased to determine contamination rates as a function of [3.6]–[24] color. To obtain an upper limit to the possible contamination fraction, Figure 3 compares the distributions of for our YSOs with those identified in young stellar clusters by Gutermuth et al. (2009), using a selection method that is also based on infrared colors and magnitudes but different in its details (see Gutermuth et al. for details). The c2d+GB sample contains a much higher fraction of Class III objects. While a full comparison of different methods for identifying YSOs is beyond the scope of this work, under the assumptions that the Gutermuth et al. method perfectly separates out background AGB stars and our regions contain the same fractions of sources in each Class, 87% of the Class III objects in the c2d+GB sample would need to be removed to match the Gutermuth et al. (2009) Class III fraction. We note here that the Gutermuth et al. survey targeted smaller regions centered on dense clusters whereas the c2d and GB surveys mapped much larger areas, including those more distant from cluster centers. As the Class II and III YSO populations are generally more distributed than earlier Classes, the intrinsic fraction of Class III sources is likely higher in the c2d and GB surveys than in Gutermuth et al. (2009). For this reason, our above assumption that the surveys contain the same fractions of sources in each Class results in an upper limit to the Class III contamination fraction.

Figure 3.— Histogram showing the distribution of infrared spectral index for the YSOs identified in the c2d+GB clouds (this work, shaded histogram) and by Gutermuth et al. (2009) in a survey of young stellar clusters (dashed histogram). The dotted line marks the Class III boundary ().

Synthesizing all of the above information, our sample of Class III YSOs suffers from between 25% and 90% contamination by AGB stars. These values are lower and upper limits, respectively, for the reasons listed above, and the most likely value is somewhere in between. Tighter constraints on the true contamination rate will require large optical spectroscopic campaigns and/or proper motion studies coupled with revisions to the selection criteria used to identify YSOs in infrared surveys. These contamination rates must be kept in mind when considering the results presented here and in other studies based on the c2d+GB YSO list (such as those discussed in §1).

2.4. Constructing SEDs and Correcting for Extinction

Following the method first summarized by Evans et al. (2009) and updated by Dunham et al. (2013), we compile as complete an SED as possible for each of the 2966 YSOs. We refer to Dunham et al. for a full description of this process, but give a summary here. To construct the SEDs, we included:

  1. Selected ground-based optical and infrared photometry as compiled by the authors of the individual cloud papers listed in Table 1.

  2. Wide-field Infrared Survey Explorer (WISE; Wright et al., 2010) 12 and 22 m photometry from the all-sky catalog26.

  3. Spitzer 160 m photometry for sources that are detected but not located in saturated or confused regions. Since the c2d pipeline does not extract 160 m sources, we measured flux densities using aperture photometry (using the IDL procedure aper) and standard aperture corrections as given by the MIPS Instrument Handbook27.

  4. Selected 350 m photometry from a targeted survey of protostars with SHARC-II28 (Wu et al., 2007, A. Suresh et al. 2015, submitted).

  5. Selected 450 and 850 m photometry from the SCUBA29 Legacy Catalog (Di Francesco et al., 2008).

  6. Other submillimeter and millimeter photometry from the literature, where available from large surveys of nearby molecular clouds (Young et al., 2005, 2006; Enoch et al., 2006, 2007; Belloche et al., 2011b, a; Maury et al., 2011).

Regarding the last three items, we have access to complete submillimeter or millimeter surveys for only 6 out of the 18 clouds (Chamaeleon I, Chamaeleon II, Chamaeleon III, Ophiuchus, Perseus, and Serpens), plus a partial survey of Aquila and piecemeal coverage of other clouds from the SCUBA Legacy Catalog (Di Francesco et al., 2008) and our own 350 m observations (Wu et al., 2007, A. Suresh et al. 2015, submitted). We consider a Spitzer source to match a submillimeter or millimeter detection if it is located within one beam of the detection, using the appropriate beam size for each survey. If multiple Spitzer sources match a single submillimeter or millimeter detection, we do not attempt to split the flux density and simply match it to the closest Spitzer source unless the SED clearly indicates a better match is obtained with a different Spitzer source.

Finally, we correct all photometry for foreground extinction following the procedure outlined by Evans et al. (2009). We only wish to correct for the foregound extinction from both the clouds and the material between us and the clouds, and not the local extinction from dense cores surrounding protostars (this extincted emission is reprocessed to longer wavelengths and thus included in our compiled SEDs). Thus, we adopt extinction values as follows:

  1. Whenever possible, we adopt extinction values from the literature for Class II and III YSOs (classified via infrared spectral index; see §3.2), as determined from optical or near-infrared spectroscopic studies of each cloud that provide reliable spectral type (and thus extinction) measurements. We refer to the data references listed in Table 1 for more details on these studies.

  2. For all Class II and III YSOs with no published extinction values, we de-redden to the intrinsic near-infrared colors of an assumed spectral type of K7. This particular spectral type is chosen because it is found to be representative of the YSOs in the c2d clouds (Oliveira et al., 2009, 2010, see also Evans et al. 2009 for details).

  3. We calculate the mean extinction toward all Class II YSOs in each cloud, and then adopt that mean value for each of the Class 0+I and Flat-spectrum YSOs in that cloud.

With extinctions assigned as above, we de-redden the SED of each YSO using the Weingartner & Draine (2001) extinction law for . Whereas Evans et al. (2009) adopted a version of the Weingartner & Draine (2001) extinction law that only included dust absorption, here we update the procedure to correct for both dust absorption and scattering. The choice of this particular extinction law is motivated by results showing that it is appropriate for the dense molecular clouds in which stars form (e.g., Chapman et al., 2009). While we do caution that our approach only provides approximate extinction corrections, especially for the Class 0+I and Flat-spectrum sources where the corrections only account for the mean cloud extinction, it is nevertheless the best that can currently be done and is significantly more reliable than ignoring the effects of extinction altogether.

3. Results

3.1. Full List of Young Stellar Objects

For each of the 2966 YSOs, Table 2 lists a running index, the cloud in which the YSO is found, the Spitzer source name (which also gives the source coordinates), the visual extinction used for extinction corrections, and the infrared spectral index (), the bolometric temperature (), and the bolometric luminosity (). These last three quantities are calculated using both the observed and extinction corrected photometry, and are denoted with primes in the latter case (, , and ). The second-to-last column of Table 2 indicates whether or not each object has [3.6]–[24]  1.5 and is thus likely a background AGB star, as described above in §2.3.

The final column of Table 2 indicates whether or not each object is associated with a dense core as traced by submillimeter or millimeter continuum emission at m. As a lack of such an association does not necessarily imply that no core is present (see below), to avoid any confusion in the interpretation of this column we list either y or no entry rather than y or n. Dunham et al. (2013) used such associations to define the sample of protostars in the c2d+GB clouds, with the rationale behind this definition being that detections in existing submillimeter and millimeter surveys of star-forming regions trace dense cores but not circumstellar disks due to the relatively low spatial resolutions and mass sensitivities of these surveys (see Dunham et al., 2013, for details). By requiring a dense core but making no assumptions about the infrared colors of protostars, this definition ensures a reliable sample of protostars that includes those viewed edge-on down outflow cavities, but comes at the cost of incompleteness to those protostars that are embedded in low-mass cores or located in clouds with incomplete or no available submillimeter or millimeter continuum surveys (see Dunham et al., 2014, for details). Associations with dense cores and the resulting sample of protostars must be revisited once the JCMT SCUBA-2 survey of the Gould Belt is complete, the first results of which are just starting to become available (e.g., Pattle et al., 2015).

Figure 4.— The extinction corrected bolometric luminosity () plotted versus the extinction corrected bolometric temperature () for all 2966 YSOs. The color of each symbol shows its spectral class determined from  (the extinction corrected infrared spectral index), with red indicating Class 0+I, green indicating Flat-spectrum, blue indicating Class II, and purple indicating Class III. Black symbols indicate sources with [3.6]–[24]  1.5 that are considered likely to be background AGB stars (see §2.3). Filled circles mark sources associated with submillimeter or millimeter continuum emission tracing dense cores (see text for details), while the plus signs mark sources with no such associations. The three thick lines plot model tracks from Young & Evans (2005) for singular isothermal spheres of different initial masses undergoing inside-out collapse with 100% efficiency (all mass initially in the cores end up in the stars). These models end when infall stops and the cores have fully accreted onto the stars and are thus only showing evolution in the protostellar stage. The three thin lines plot model tracks from Myers et al. (1998) for cores of different initial masses collapsing with accretion rates that decrease exponentially with time and efficiencies less than one (i.e., only a fraction of the mass initially in the cores ends up in the stars, see Myers et al., 1998, for details). As above, these models are only relevant for the protostellar stage of evolution. The heavy dashed line on the left is the ZAMS from 0.1 to 2 M taken from D’Antona & Mazzitelli (1994). The  class boundaries as defined by Chen et al. (1995) are plotted as vertical dashed lines.

The infrared spectral index (, ) is defined as the slope of the infrared SED in log() vs. log(), where is the wavelength and is the flux density at that wavelength:

(1)

In practice, we calculate with a linear least squares fit to all available 2MASS and Spitzer photometry between 2 and 24 m. The bolometric temperature of a source is defined to be the temperature of a blackbody with the same flux-weighted mean frequency (Myers & Ladd, 1993). It is in essence a protostellar equivalent of stellar effective temperature; it starts at very low values ( K) for cold, starless cores and eventually increases to the effective temperature of the central (proto)star once the core and disk have fully dissipated. , , , and are calculated by integrating over the full SEDs:

(2)
(3)

In practice, we calculate these integrals using the trapezoid rule to integrate over the finitely sampled SEDs.

Figure 4 plots  versus  (a “BLT” diagram; Myers & Ladd, 1993) for all 2966 YSOs identified in the c2d and Gould Belt clouds. The luminosities of Class 0 and Class I sources are distributed over greater than 4 orders of magnitude and extend to much lower luminosites than predicted by simple evolutionary models. A full discussion of this “protostellar luminosity problem” can be found in Offner & McKee (2011), Dunham et al. (2013), and Dunham et al. (2014), but to summarize, mass-dependent and/or time variable mass accretion is required to explain the observed luminosities of protostars. Class II and Class III objects exhibit both a very narrow range of bolometric temperatures and a bifurcation in  into two groups. Both effects are artifacts introduced by our extinction corrections and are discussed in Appendix C. To summarize, the first effect is because the bolometric temperature of a Class II or III source depends more on the spectral type of the object than the amount and characteristics of circumstellar material, and for most objects a constant spectral type of K7 is assumed (see above). The second effect (bifurcation) is a consequence of optical data only being available for a subset of our YSOs.

Tables 3 and 4 list, for each YSO, the same running index as in Table 2, followed by flux density and flux density uncertainty pairs for the 2MASS (1.25, 1.65, and 2.17 m) and Spitzer (3.6, 4.5, 5.8, 8.0, 24, and 70 m) wavelengths. All flux densities and flux density uncertainties are listed in mJy and rounded to two significant digits. Table 3 lists the observed values while Table 4 lists the extinction corrected values. The observed and extinction corrected flux densities that we have compiled at all other wavelengths in order to construct SEDS, as described above, are tabulated in Appendix D.

3.2. Classification of Young Stellar Objects

Over the past few decades a standard model for low-mass star formation has emerged. This model, developed by Adams et al. (1987) and summarized by Shu et al. (1987), resulted from the merger of an empirical classification system based on the infrared spectral indices of YSOs (Lada & Wilking, 1984) with theoretical studies of the stages of the collapse of a dense, rotating core (Shu, 1977; Terebey et al., 1984). Recent reviews on classification of young stars and the associations between observed SED classes and physical evolutionary stages are given by Evans et al. (2009) and Dunham et al. (2014).

Including the revisions by Greene et al. (1994), these empirical Classes are as follows:

  • Class 0+I:  ;

  • Flat-spectrum:  ;

  • Class II:  ;

  • Class III:  .

We denote the infrared spectral index as  to emphasize that we only consider extinction corrected values in this study, and we refer to the first SED class as Class 0+I instead of the more commonly used Class I to emphasize that Class 0 and I objects are both included in this definition (see below).

Figure 5.— A four-color bar is plotted for each cloud, where each color corresponds to an evolutionary Class (similar to Figure 4, red corresponds to Class 0+I, green corresponds to Flat-spectrum, blue corresponds to Class II, and purple corresponds to Class III). The fractional length of each color represents the fraction of YSOs in each cloud with the corresponding Class. The clouds are sorted by Class 0+I fraction, with the largest fractions at the top and the smallest fractions at the bottom, and all 2966 YSOs are included.
Figure 6.— Same as Figure 5, except after removing the 298 objects identified as likely background AGB stars (see §2.3).

Table 5 lists, for each cloud, the numbers of YSOs in each Class using the extinction corrected infrared spectral indices, both with and without the likely background AGB stars removed. Summed over all clouds, out of the 2966 total YSOs, 326 (11%) are classified as Class 0+I, 210 (7%) are classified as Flat-spectrum, 1248 (42%) are classified as Class II, and 1182 (40%) are classified as Class III. If we first remove the 298 sources identified as likely background AGB stars, out of the 2668 remaining YSOs, 326 (12%) are classified as Class 0+I, 210 (8%) are classified as Flat-spectrum, 1245 (47%) are classified as Class II, and 887 (33%) are classified as Class III. We emphasize that there are large variations between clouds in the relative numbers in each Class. To reinforce this point visually, Figures 5 and 6 plot, for each cloud, four-color bars where each color corresponds to a Class and the fractional length of each color represents the fraction of YSOs with the corresponding Class. Figure 5 displays the results including all 2966 YSOs whereas Figure 6 displays the results after removing the 298 likely background AGB stars. Finally, we also emphasize that our method of YSO identification is only capable of identifying Class III sources with detectable infrared excesses and is thus incomplete to the full populations of pre-main sequence stars in these clouds.

As noted above, we refer to the first Class as Class 0+I, but the original definition of this Class was simply Class I (Lada & Wilking, 1984; Greene et al., 1994). Class 0 objects were later added as an earlier Class for protostars too deeply embedded to detect in the infrared but inferred through other means (i.e., outflow presence; Andre et al., 1993). They are defined as objects with a ratio of submillimeter to bolometric luminosity () greater than 0.5% (Andre et al., 1993), where  is calculated for m. While they are defined observationally, the intended corresponding physical definition of Class 0 objects are protostars with at least 50% of their total mass still residing in the surrounding, infalling core (Andre et al., 1993). An alternative observational definition is given in terms of the bolometric temperature as objects with   K (Myers & Ladd, 1993; Chen et al., 1995). While  is generally a better discriminator between Class 0 and Class I objects than , and is less sensitive to source geometry (e.g., Young & Evans, 2005; Dunham et al., 2010, 2014), it cannot be calculated without 350 m photometry. As we do not have 350 m photometry available for many of our YSOs, we only consider  in this study.

Figure 7.— Extinction corrected infrared spectral index () plotted versus extinction corrected bolometric temperature () for all 2966 YSOs, with the colors and symbols the same as in Figure 4. The horizontal lines show the class boundaries in  as revised by Greene et al. (1994) and adopted in this work, and the vertical dashed lines show the  class boundaries as defined by Chen et al. (1995).

With the sensitivity of Spitzer, Class 0 sources are easily detected in the infrared (e.g., Noriega-Crespo et al., 2004; Young et al., 2004; Jørgensen et al., 2006; Dunham et al., 2006, 2008; Tobin et al., 2010). Figure 7 plots  versus  for all 2966 YSOs. In general, the two quantities are anti-correlated, although there are some notable exceptions. There are a small number of SED Class 0+I sources (plotted in red in Figure 7) with sufficiently high  values to be classified as Class II or III by . None of these objects are associated with dense cores and are likely simply more evolved YSOs viewed through large extinctions that are not fully extinction-corrected (recall that, for SED Class 0+I sources, only the average cloud extinction is accounted for). There are also a few SED Class II YSOs (plotted in blue in Figure 7) with sufficiently low  values to be classified as Class 0 by . These are likely true protostars viewed at nearly pole-on inclinations through outflow cavities (e.g., Whitney et al., 2003; Robitaille et al., 2006; Crapsi et al., 2008), although detailed follow-up study is required to rule out chance alignments between more evolved YSOs and dense cores.

While there is a general anti-correlation between  and , this trend weakens for objects classified as Class 0+I by  (objects with  ). With substantial overlap in  for objects classified as Class 0 or Class I by , it is clear that these two Classes overlap in their infrared spectral indices. These results confirm those presented previously by Enoch et al. (2009) and Evans et al. (2009) and indicate that mid-infrared data alone are insufficient to separate Class 0 and Class I objects; instead, full SEDs are required.

Using our compiled SEDs, and restricting ourselves only to those YSOs associated with a dense core (both to restrict ourselves to confirmed protostars and to ensure available submillimeter or millimeter detections for reliable measurements of ), our  measurements result in 65 Class 0 protostars and 129 Class I protostars. Thus, we find that approximately one third () of protostars are classified as Class 0, in agreement with previous results based on the smaller c2d-only sample (Enoch et al., 2009; Evans et al., 2009). While future work must revisit this ratio once complete, sensitive submillimeter coverage is available for all clouds through the Herschel and JCMT Gould Belt surveys, preliminary Herschel results suggest only modest increases to the number of Class 0 objects (generally 15% – 30%, Harvey et al., 2013; Stutz et al., 2013; Sadavoy et al., 2014), leading to only small upward revisions in the ratio of Class 0 to Class I protostars.

3.3. Infrared Color-Color Diagrams

Figure 8.— Extinction corrected color-color diagrams for the four IRAC bands, with the colors and symbols the same as in Figure 4. The box in (a) indicates the region identified with Class II sources by Allen et al. (2004), whereas in (b) it indicates our proposed modifications to this region (see text for details).

Figure 8 presents an extinction corrected infrared color-color diagram of all YSOs identified in this work constructed from the four IRAC wavelengths: [3.6]  [4.5] plotted versus [5.8]  [8.0]. The different Classes of YSOs (classified via ) are generally well-separated in this diagram, as first noted by Allen et al. (2004). In cases where photometry is only available at m and thus the infrared spectral index cannot be reliably calculated, this diagram can be used to approximately classify YSOs. Indeed, Allen et al. (2004) used the following two conditions to define a box where Class II sources are located:

  1. 0.4 [5.8]  [8.0] 1.1, and

  2. 0.0 [3.6]  [4.5] 0.8.

The original definition of this “Class II box” by Allen et al. (2004) used observed photometry. Inspection of Figure 8 reveals that minor revisions are required when using extinction corrected photometry. While we acknowledge that polygons with greater than four sides could possibly give better results, for simplicity we only consider rectangular regions. To define a new Class II box, we allow all four boundaries to vary and select those which simultaneously maximize the probability that a Class II YSO is located within the box (defined as the number of Class II YSOs located within the box divided by the total number of Class II YSOs) and the probability that a YSO located within the box is classified as Class II (defined as the number of Class II YSOs located within the box divided by the total number of YSOs located within the box). In practice, we implement these definitions by selecting the boundaries which maximize the sum of these two probabilities. Our proposed new Class II box, plotted in panel (b) of Figure 8, has the following boundaries:

  1. 0.45 [5.8]  [8.0] 1.55, and

  2. 0.00 [3.6]  [4.5] 0.70.

We note that the shape of this revised box is qualitatively similar to that proposed by Gutermuth (2005), who followed a similar procedure. With these revised boundaries, the fraction of Class II YSOs located within the box increases from 84.5% to 89.9%, and the fraction of YSOs in the box that are Class II increases from 78.8% to 82.4%.

Figure 9 presents a second extinction corrected infrared color-color diagram of all YSOs which uses both the IRAC and MIPS observations by plotting [3.6]  [5.8] versus [8.0]  [24]. Again the different Classes of YSOs (classified via ) are generally well-separated, as first noted by Muzerolle et al. (2004). Indeed, Muzerolle et al. defined three regions in this color-color diagram to separate Class III/stellar, Class II, and Class 0+I YSOs; these regions are marked in panel (a) of Figure 9 and are defined in Table 6. As above, Muzerolle et al. used observed photometry, and we find that minor revisions are required when using extinction corrected photometry. Furthermore, the original boundaries defined by Muzerolle et al. did not include a region for Flat-spectrum objects. Thus, we follow the same procedure as above, again only considering rectangular regions, and maximize the sum of the eight probabilities (two for each Class), subject to the following constraints: (1) the minimum x and y Class III boundaries be held fixed at the Muzerolle et al. values, (2) there are no maximum x and y Class 0+I boundaries, and (3) the regions can not overlap. Our revised boundaries are plotted in panel (b) of Figure 9 and listed in Table 6. Table 6 also lists each of the two probabilities considered for both the original and revised boundaries.

Ultimately, no region in either color-color diagram will classify YSOs with a perfect one-to-one correspondence to the infrared spectral index, but in cases where insufficient photometry at 2 and/or 24 m is available, approximate YSO classification is possible via infrared color-color diagrams.

Figure 9.— Extinction corrected color-color diagram using three of the four IRAC bands and the first MIPS band, with the colors and symbols the same as in Figure 4. The boxes in (a) indicate the regions identified with Class III/stellar, Class II, and Class 0+I sources by Muzerolle et al. (2004), going from lower left to upper right. The boxes in (b) indicate our proposed modifications to these regions, now with four regions (Class III/stellar, Class II, Flat-spectrum, and Class 0+I, again from lower left to upper right).

4. Timescales for Young Stellar Objects

4.1. Class 0+I and Flat-spectrum YSOs

The standard method for determining the time spent in each YSO Class is to calculate the ratio of the number of sources in that Class to the number in a reference Class, and then multiply by the duration of the reference Class (e.g., Wilking et al., 1989; Evans et al., 2009; Dunham et al., 2014). Implicit in this method are the assumptions that star formation is continuous over at least the duration of the reference class and that time is the only variable. Regarding the first assumption, we strive to average out cloud-to-cloud variations in the recent star formation history by averaging over many clouds. Regarding the second, our results are best thought of as an average over all other relevant variables (e.g., environment, stellar mass; see Evans et al., 2009). In this section, we investigate the effects of different assumptions for the reference class and its duration on the timescales of Class 0+I and Flat-spectrum YSOs.

We first follow the method of Evans et al. (2009) and Dunham et al. (2014), who adopt Class II as the reference Class and assume a Class II duration of 2 Myr. This duration is taken from analyses of the fraction of cluster members with disks versus cluster age for various young clusters, which generally show a disk disperal time of a few Myr (Wyatt, 2008; Ribas et al., 2014, 2015). As discussed extensively by Evans et al. (2009), this time is best considered as a disk half-life rather than an absolute duration. The first row of Table 7 presents durations for the Class 0+I and Flat-spectrum Classes calculated following this method, using the extinction corrected numbers: 0.52 and 0.34 Myr for Class 0+I and Flat-spectrum objects, respectively.

Although the ages of young stars are highly uncertain, recent studies suggest a revised half-life for disk survival of Myr on both observational (e.g., Kraus et al., 2012; Bell et al., 2013; Soderblom et al., 2014) and theoretical (e.g., Alexander & Armitage, 2009) grounds. Very recent results by Ribas et al. (2014) and Ribas et al. (2015), using spectroscopically confirmed members of various young clusters, show that this duration depends on the wavelength where the infrared excess indicative of the presence of a disk first appears, ranging from 1.9 Myr for m to 3.0 Myr for m (note that these times have been converted from the times presented by Ribas et al. to half-life times). As our Spitzer data are sensitive to infrared excesses out to 24 m, and our Spitzer c2d+GB data have comparable mid-infrared sensitivities to the Ribas et al. Spitzer and WISE data used to determine disk lifetimes, their derived lifetimes suggest that 3 Myr is a more appropriate choice for the duration of the reference Class in this work. The second row of Table 7 lists the revised durations: 0.78 and 0.50 Myr for Class 0+I and Flat-spectrum objects, respectively.

While it has become somewhat standard to adopt Class II as the reference Class, this choice may result in a logical inconsistency. The duration of either 2 or 3 Myr based on disk fraction vs. cluster age is derived by determining the ratio of cluster members with infrared excesses to all cluster members (in other words, the ratio of disk sources to disk no disk sources). By adopting Class II as the reference Class, we omit all of the Class III YSOs that still have disks, even though these objects are included when other studies (such as those by Ribas et al. ) derive disk lifetimes. Since infrared colors indicative of dust are the fundamental ingredient in our identifcation of YSOs (see §2.2), by definition the Class III YSOs identified in this work have disks (if they are all in fact YSOs; the issue of Class III contamination will be addressed in the following paragraph). Since the recent work by Ribas et al. (2014) and Ribas et al. (2015) is based on mid-infrared data from Spitzer and WISE with comparable sensitivity to mid-infrared excesses as our c2d+GB data, and since we use disk lifetimes derived by Ribas et al. to set the duration of the reference Class, and further given that all Class II and Class III sources identified here have disks, we thus argue that it is more appropriate to adopt the sum of Class II and Class III as the reference Class than Class II alone. Thus, the third row of Table 7 lists the revised durations assuming a duration of 3 Myr for the sum of Class II and III sources: 0.40 and 0.26 Myr for Class 0+I and Flat-spectrum objects, respectively.

The one major complication with the above analysis is the fact that our Class III sample suffers from 25% – 90% contamination, as discussed in §2.3. On the other hand, contamination is not a problem in the Ribas et al. (2014, 2015) samples since they only consider spectroscopically confirmed cluster members. As a result, the above calculations of Class durations likely overcount the number of objects in the reference Class and thus underestimate the Class 0+I and Flat-spectrum durations. To account for this range of contamination rates, the last two columns of Table 7 list the calculated durations of the Class 0+I and Flat-spectrum Classes with the Class II + Class III reference Class corrected for contamination and a reference Class duration of 3 Myr. We consider these values to be our most accurate and logically consistent lifetimes: Myr for Class 0+I sources, and Myr for Flat-spectrum sources.

4.2. Class 0 YSOs

In §3.2, we reported that 33.5% of the objects associated with dense cores (and thus considered protostars according to the definition adopted in this paper) are classified as Class 0 by , with the remainder classified as Class I. Under the same assumptions as above (continuous star formation and time is the only variable) and our definition of a protostar as a YSO associated with a dense core, our results imply that 33.5% of the total protostellar duration is spent in the Class 0 phase, with the remaining 66.5% of the time spent in the Class I phase. If we assume that the lifetime of the SED Class 0+I objects is a good proxy for the lifetime of the protostellar stage of evolution (see §4.3 below for justification of this assumption), the range of SED Class 0+I durations listed in Table 7 ( Myr) imply resulting durations of Myr for Class 0 protostars and Myr for Class I protostars (see the last two columns of Table 7). If we only consider the last two rows of Table 7 (including Class III YSOs in the reference Class, correcting for estimated contamination rates in the Class III sample, and adopting a reference Class duration of 3 Myr), the resulting durations are Myr for Class 0 and Myr for Class I.

As noted in §3.2, we generally find the same ratios of SED Class 0 to SED Class 0+I YSOs as in previous studies (Evans et al., 2009; Enoch et al., 2009; Dunham et al., 2014). While those studies derived a Class 0 duration of Myr, our expanded range of possible durations is due to our expanded analysis of the contents and duration of the reference Class.

4.3. Lifetimes of Protostars

As noted in §3.2, the standard picture of low-mass star formation resulted from the merger of an empirical classification system based on the infrared spectral indices of YSOs (Lada & Wilking, 1984) with theoretical studies of the stages of the collapse of a dense, rotating core (Shu, 1977; Terebey et al., 1984). In this picture, SED Class 0+I YSOs are protostars still embedded in and accreting from dense cores, SED Class II YSOs are T Tauri stars with optically thick, accreting disks, and SED Class III YSOs are pre-main sequence stars with little or no disks left (all Spitzer-identified Class III YSOs, by definition, still have infrared excesses, thus the population of Class III YSOs without disks is not identified in the work presented here). Flat-spectrum YSOs have no clear link to a distinct physical stage, and in reality the correlation between observational SED Class and physical Stage is weakened by the effects of geometry, extinction, and accretion history (e.g., Whitney et al., 2003; Young & Evans, 2005; Robitaille et al., 2006; Crapsi et al., 2008; Dunham et al., 2010).

While our study provides the statistics necessary to measure durations of the observed SED Classes, it does not provide the necessary information to measure durations of physical Stages. In a separate, complementary study, Heiderman & Evans (2015) surveyed nearly all of the SED Class 0+I and Flat-spectrum YSOs in the c2d and GB surveys for emission from the dense gas tracer HCO (3–2). Using criteria based on the detection and strength of this line, they sorted objects into Stage 0+I (protostar) and Stage II (disk) sources. They found that the Flat-spectrum YSOs are approximately evenly divided between protostar and disk sources, with no evidence that they occupy a physically distinct evolutionary stage. They also found that the number of Flat-spectrum YSOs classified as protostars is balanced by the number of SED Class 0+I YSOs classified as disks, coincidentally resulting in a nearly identical number of protostars (defined in their work based on the presence of dense gas traced by HCO line emission) and SED Class 0+I YSOs (326 Class 0+I YSOs identified here, 335 protostars identified by Heiderman & Evans). Thus, based on their results, the range of possible protostellar stage durations is the same as the range of possible SED Class 0+I durations listed in Table 7.

5. Summary

In this paper we have presented the full catalog of young stellar objects identified in the molecular clouds surveyed by the Spitzer Space Telescope “cores to disks” (c2d) and “Gould Belt” (GB) Legacy surveys. In the case of the c2d clouds, our catalog updates a previous one published by Evans et al. (2009), and in the case of the GB clouds, our catalog represents the first catalog of Spitzer-identified YSOs in the full ensemble of these clouds. We summarize our main results as follows:

  • We have used standard techniques developed by the c2d project to identify 3239 candidate YSOs in the 18 c2d and GB clouds, 2966 of which survive visual inspection and form our final catalog of young stellar objects in the Gould Belt.

  • We compile SEDs for all 2966 YSOs and correct these SEDs for foreground extinction.

  • We calculate and tabulate the infrared spectral index, bolometric luminosity, and bolometric temperature for each object, both with and without extinction corrections. We also tabulate information on whether or not each YSO is considered highly likely to be a background AGB star, and whether or not each YSO is associated with a dense core as traced by submillimeter or millimeter continuum emission.

  • After correcting for extinction, we find that 326 out of 2966 (11%) are classified as Class 0+I, 210 out of 2966 (7%) are classified as Flat-spectrum, 1248 out of 2966 (42%) are classified as Class II, and 1182 out of 2966 (40%) are classified as Class III. The Class III sample suffers from an overall contamination rate by background AGB stars between 25% and 90%.

  • Adopting standard assumptions, we derive durations of Myr for Class 0+I YSOs and Myr for Flat-spectrum YSOs, with most likely durations of Myr (Class 0+I) and Myr (Flat-spectrum). The ranges encompass uncertainties in the adopted assumptions.

  • Including information from submillimeter and millimeter wavelengths, one-third of the Class 0+I sample is classified as Class 0. With this result, we calculate durations of Myr (Class 0) and Myr (Class I), with most likely durations of Myr (Class 0) and Myr (Class I). As above, the ranges encompass uncertainties in the adopted assumptions.

  • We revisit infrared color-color diagrams used in the literature to classify YSOs and use our large, extinction corrected sample to propose revisions to classification boundaries in these diagrams.

  • The bolometric temperature of a YSO surrounded by a circumstellar disk but no longer embedded in a dense core is primarily determined by the spectral type of the YSO and the availability of optical photometry, and not by the properties of the circumstellar material. As such, the bolometric temperature should not be used to distinguish between Class II and Class III YSOs.

The YSO catalog presented here represents the most complete catalog of star formation within 500 pc of the Sun yet assembled. It includes nearly every major site of star formation in the solar neighborhood except for the Taurus and Orion molecular clouds, each of which was the subject of its own, dedicated Spitzer Legacy survey. While this work represents a major step forward in assembling a complete and reliable catalog of Young Stellar Objects in the Gould Belt, future work to further refine the YSO catalog presented here is needed on the following three fronts: (1) combining the results of the c2d+GB, Taurus, and Orion surveys using uniform data reduction and analysis techniques, (2) identifying and removing background AGB stars currently contaminating the sample, and (3) revisiting the associations between Spitzer sources and dense cores traced by submillimeter and millimeter detections once the full results from the Herschel and SCUBA-2 Gould Belt surveys are available.

We thank the referee for thoughtful comments that have improved the quality of this paper, and we acknowledge helpful discussions with H. Arce and L. Kristensen. This work is based primarily on observations obtained with the Spitzer Space Telescope, operated by the Jet Propulsion Laboratory, California Institute of Technology. It also uses data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation. These data were provided by the NASA/IPAC Infrared Science Archive, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA. This research has made use of NASA’s Astrophysics Data System (ADS) Abstract Service, the IDL Astronomy Library hosted by the NASA Goddard Space Flight Center, and the SIMBAD database operated at CDS, Strasbourg, France. MMD acknowledges support from NASA ADAP grant NNX13AE54G and from the Submillimeter Array (SMA) through an SMA postdoctoral fellowship and from H. Arce at Yale University, where a portion of this work was performed. NJE is supported by NSF Grant AST-1109116 to the University of Texas at Austin. LAC was supported by ALMA-CONICYT grant 31120009, CONICYT-FONDECYT grant 1140109, and the Millennium Science Initiative (Chilean Ministry of Economy) grant RC130007. ALH is supported by an NSF AAPF under award AST1302978. KEY acknowledges support from the Louisiana Space Consortium Research Enhancement Award through NASA EPSCoR grant number NNX10AI40H.

Appendix A A.  Spitzer Astronomical Observation Requests (AORs) for Unpublished Clouds

Three clouds listed in Table 1 do not have published data references (Chamaeleon I, Chamaeleon III, and Musca). Additionally, only a small piece of Aquila was published in the reference listed for that cloud (Gutermuth et al., 2008). Consequently, for these four clouds there are no published lists of the Spitzer Astronomical Observation Requests (AORs) that targeted them. We thus list their AORs in Table 8.

Appendix B B.  Rounding Artifacts in Color-Color Diagrams

Inspection of Figure 2 reveals a vertical line of sources at [3.6]–[4.5] 0.5. If an object has identical flux densities at two Spitzer wavelengths, the magnitude difference between these two wavelengths is then simply equal to:

(B1)

where and are the fluxes for zero magnitude at each wavelength. For 3.6 and 4.5 m, this gives [3.6]–[4.5] = 0.473.

Figure 10.— Same as Figure 2 ([3.6]–[24] versus [3.6]–[4.5] for all 2966 objects in our final YSO sample, using the observed photometry), except plotted using the original data before rounding. The vertical line of sources at [3.6]–[4.5] 0.5 has now disappeared as YSOs with similar flux densities have not now all been rounded to identical flux densities.

There are 147 objects plotted in Figure 2 with identical flux densities at 3.6 and 4.5 m, resulting in 147 objects plotted at the same value on the x-axis and thus giving rise to the vertical line of sources. We note that this is an artifact introducing by rounding all flux densities to two significant digits. To demonstrate this point, Figure 10 plots the same color-color diagram as Figure 2, except now using the original data before rounding. As evident from inspection of this figure, the vertical line of sources at [3.6]–[4.5] 0.5 has now disappeared.

The same artifact is also present in Figure 2 in the [3.6]–[24] color. For 3.6 and 24 m, identical flux densities result in a magnitude difference of [3.6]–[24] = 3.959. There are only 23 objects plotted in Figure 2 with identical flux densities at 3.6 and 24 m, so while the vertical line of sources at [3.6]–[24] 4 is present, it is not as apparent. Similar artifacts also affect the colors plotted in Figures 8 and 9 using the extinction corrected photometry. For the four colors plotted in these two figures, between 38 and 95 objects with identical flux densities are plotted. While corresponding vertical and horizontal lines of sources are present, none are apparent because they all occur at colors where the source densities are too high for these small subsets of sources to be visible. These rounding artifacts have no effect on the results presented in this paper.

Appendix C C.  Bolometric Temperatures of Class II and III Sources

As noted in §3.1, the bolometric temperatures of our Class II and III YSOs are generally tightly clustered into two narrow  ranges (see Figures 4 and 7). To understand the origins of these effects, we construct simple continuum radiative transfer models of circumstellar disks using the three-dimensional Monte Carlo radiative transfer package RADMC-3D30. We assume an axisymmetric physical structure and use RADMC-3D to first calculate the two-dimensional dust temperature profile through the disks and then generate full model SEDs from optical to millimeter wavelengths.

We adopt a simple, parameterized disk density structure following a power-law in the radial coordinate and a Gaussian in the vertical coordinate. The exact density profile is given as follows:

(C1)

where is the distance above the midplane (, with and the usual radial and zenith angle spherical coordinates), is the distance in the midplane from the origin (), and is the density at the reference midplane distance , normalized such that the total disk mass is correct (see below). The quantity is the disk scale height and is given by , where is the scale height at . The parameter , which describes how the scale height changes with and sets the flaring of the disk, is treated as a free parameter (see below). We set AU at AU. The disk surface density profile, calculated by integrating Equation C1 over the vertical coordinate , has a radial power-law index of , where . For these models, we hold fixed at 2.25 and the disk outer radius fixed at 100 AU.

To consider a diverse set of disks, we treat the disk inner radius, flaring parameter , and total disk mass as free parameters. Allowed values for each parameter are as follows: 0.1, 1, 10, 20, and 50 AU for the disk inner radius; 1.0, 1.25, 1.5, 1.75 for , and 0.0005, 0.001, 0.005, 0.01, and 0.05 M for the disk mass. We thus consider 100 total models and generate SEDs at 9 inclinations31, starting at  and increasing in steps of 10, resulting in 900 total model SEDs. We adopt the dust opacities of Ossenkopf & Henning (1994) appropriate for thin ice mantles after yr of coagulation at a gas density of cm (OH5 dust). The radiative transfer is performed with no external heating by the interstellar radiation field, but isotropic scattering off dust grains is included. The input stellar spectrum is assumed to be a simple blackbody with L and of either 4050 K or 3050 K (corresponding approximately to K7 and M5 spectral type pre-main sequence stars, respectively; Pecaut & Mamajek, 2013). Finally, for each of the 1800 total model SEDs (two stellar models each surrounded by 100 different circumstellar disk models, each viewed at 9 inclinations), we calculate two values of the bolometric temperature: one with the full SED from optical to millimeter wavelengths, and another excluding all optical wavelengths (all wavelengths shortward of 1.25 m).

Figure 11.— Histograms showing the distribution of  for the models described in the text. Left: The distributions of  for the K7 disk models, calculated both with (shaded) and without (dashed) optical photometry included in the SEDs. Right: The distributions of  for the K7 (shaded) and M5 (dashed) disk models, in both cases calculated with optical photometry included in the SEDs.

The left panel of Figure 11 shows the distributions of  for the K7 disk models, calculated both with and without optical photometry included in the SEDs. Since the radiative transfer model includes no foreground extinction, these values are equivalent to our observed values after correcting for extinction. When optical photometry is included in the SEDs, the distribution of  exhibits a narrow peak around 3500 K with a width of only a few hundred K, plus a long tail to lower values corresponding to SEDs viewed at edge-on inclinations. A similar behavior is exhibited when optical photometry is left out of the SEDs, except both the peak and long tail are shifted to lower values of . This behavior is expected since the inclusion of shorter wavelength data shifts the flux-weighted mean frequency to higher values.

The right panel of Figure 11 shows the distributions of  for the K7 and M5 disk models, in both cases calculated with optical photometry included in the SEDs. Each set of models exhibits narrow peaks, with the K7 model peak located at higher values than the M5 model. Even for the large range of disk properties considered here, it is evident from this Figure that the measured  for YSOs with disks is mostly determined by the spectral type of the central source. The disk properties introduce only a very narrow spread except for models viewed at nearly edge-on inclinations.

The tight clustering into two narrow ranges of  for most of our Class II and III YSOs, as evident from Figures 4 and 7, is naturally explained by these results. A K7 spectral type is assumed for most sources when deriving extinction corrections, explaining the tight clustering, and optical photometry is only available for a subset of our YSOs (see §2.4), explaining the bifurcation into two separate ranges of . One consequence of this situation is that, even though a boundary in bolometric temperature between Class II and Class III is defined by Chen et al. (1995),  is not a good discriminator between these two Classes.

Appendix D D.  Tables of Ancillary Photometry Collected from the Literature

Here we tabulate all ancillary photometry used to construct YSO SEDs, where we define ancillary to mean all photometry beyond the 2MASS and Spitzer photometry measured, band-merged, and cataloged by the c2d pipeline. This definition of ancillary includes Spitzer 160 m photometry. All flux densities and flux density uncertainties are listed in mJy and rounded to two significant digits.

Tables 9 and 10 list, for each YSO, the same running index as in Table 2, followed by flux density and flux density uncertainty pairs for all photometry compiled between m. Table 9 lists observed values whereas Table 10 lists extinction corrected values.

Tables 11 and 12 list, for each YSO, the same running index as in Table 2, followed by flux density and flux density uncertainty pairs for all ancillary photometry compiled between m. Table 11 lists observed values whereas Table 12 lists extinction corrected values.

Tables 13 and 14 list, for each YSO, the same running index as in Table 2, followed by flux density and flux density uncertainty pairs for all ancillary photometry compiled between m. Table 13 lists observed values whereas Table 14 lists extinction corrected values.

Distance Distance Area Mass Data
Cloud Survey (pc) Reference32 (pc) (M) Reference(s)33
Aquila GB 260 1 179.0 24400 3000 1
Auriga/CMC GB 450 2 52.4 4844 1152 2
Cepheus GB 200–32534 3 38.0 2610 170 3
Chamaeleon I GB 150 4 9.4 857 210
Chamaeleon II c2d 178 5 9.2 637 300 4, 5, 6
Chamaeleon III GB 150 4 28.0 1330 390
Corona Australis GB 130 6 3.0 279 110 7
IC5146 GB 950 7 149.0 8550 2170 8
Lupus I c2d 150 8 8.9 513 310 9, 10
Lupus III c2d 200 8 15.4 912 320 9, 10
Lupus IV c2d 150 8 2.5 189 95 9, 10
Lupus V GB 150 8 11.7 705 220 11
Lupus VI GB 150 8 6.7 455 140 11
Musca GB 160 9 6.8 335 110
Ophiuchus c2d 125 10 29.6 3120 1800 12
Ophiuchus North GB 130 11 7.3 621 17 13
Perseus c2d 250 12 73.2 6590 3600 14, 15
Serpens c2d 429 13 17.0 2340 640 16, 17, 18
Table 1Molecular Clouds Surveyed by the c2d and GB Surveys
Spitzer Observed Extinction Corrected
Source Name Likely
Index Cloud (SSTc2d or SSTgb +) (mag) (K) (L) (K) (L) AGB? Core?
1 Aquila J180144.8-044941 8.2 -2.16 1700 3.0 -2.52 2000 8.2 N
2 Aquila J180145.8-045902 8.0 -2.14 1700 1.4 -2.56 2000 3.7 N
3 Aquila J180154.2-043753 12.4 -2.57 2100 0.52 -3.22 2600 4.7 Y
4 Aquila J180229.2-051552 8.3 -2.03 1700 3.6 -2.38 2000 9.8 N
5 Aquila J180302.0-044233 12.4 -2.45 2100 0.11 -3.15 2500 0.92 N
6 Aquila J180308.0-043523 8.0 -2.18 1700 0.24 -2.52 2000 0.64 Y
7 Aquila J180313.8-042845 6.9 -2.22 1800 0.72 -2.53 2000 1.7 Y
8 Aquila J180423.9-042248 9.0 -2.15 1700 2.2 -2.44 2000 6.3 Y
9 Aquila J180435.7-044700 8.1 -2.26 1700 1.9 -2.65 2000 5.3 Y
10 Aquila J180444.5-043706 12.4 0.53 540 0.055 -0.10 1000 0.11 N
Table 2Properties of the c2d+GB YSOs
IRAC IRAC IRAC IRAC IRAC IRAC IRAC IRAC MIPS MIPS MIPS MIPS
Index 1.25 m 1.25 m 1.65 m 1.65 m 2.17 m 2.17 m 3.6 m 3.6 m 4.5 m 4.5 m 5.8 m 5.8 m 8.0 m 8.0 m 24 m 24 m 70 m 70 m
1 290 7.3 820 33 1100 21 620 36 420 24 350 17 220 11 67 6.2
2 140 3.3 370 14 480 9.3 300 15 160 7.7 150 6.9 92 4.3 36 3.3
3 220 5.2 180 3.4 120 2.3 53 2.8 34 1.7 23 1.1 13 0.65 3.2 0.38 36 9.0
4 340 9.9 890 36 1200 30 800 41 530 26 490 24 330 16 100 9.6
5 37 1.0 40 1.1 30 0.83 13 0.64 8.8 0.42 5.9 0.28 3.5 0.17 1.5 0.22
6 23 0.62 67 1.6 81 1.7 49 2.5 33 1.6 26 1.3 19 0.88 4.7 0.48
7 88 1.9 200 4.8 250 4.9 140 7.4 96 4.7 77 3.7 50 2.4 14 1.3
8 180 4.3 540 18 750 18 510 25 340 17 310 15 200 9.5 48 4.5
9 200 4.8 520 16 700 16 410 21 230 11 200 9.6 120 5.8 40 3.7
10 0.62 0.050 2.2 0.090 3.6 0.14 4.7 0.23 6.9 0.33 12 0.57 27 1.3 67 6.2 110 16
Table 3Observed Spitzer and 2MASS Flux Densities, in mJy, of the c2d+GB YSOs
IRAC IRAC IRAC IRAC IRAC IRAC IRAC IRAC MIPS MIPS MIPS MIPS
Index 1.25 m 1.25 m 1.25 m 1.65 m 2.17 m 2.17 m 3.6 m 3.6 m 4.5 m 4.5 m 5.8 m 5.8 m 8.0 m 8.0 m 24 m 24 m 70 m 70 m
1 2200 55 2800 120 2400 49 1000 59 630 35 490 23 320 16 82 7.6
2 1000 24 1300 46 1100 21 490 25 240 12 200 9.6 130 6.3 43 4.0
3 4700 110 1200 22 430 8.3 110 5.9 63 3.1 39 1.8 24 1.2 4.4 0.51 38 9.4
4 2600 78 3200 130 2900 70 1300 68 800 40 680 34 490 23 130 12
5 810 22 260 7.2 110 3.0 28 1.4 16 0.78 9.7 0.46 6.3 0.30 2.0 0.30
6 170 4.5 220 5.4 190 3.9 79 4.0 49 2.3 37 1.7 27 1.3 5.7 0.58
7 480 11 570 14 510 9.9 220 11 140 6.6 100 4.9 69 3.3 17 1.6
8 1700 40 2100 69 1900 45 880 44 530 27 450 21 300 14 60 5.6
9 1500 35 1800 53 1600 36 680 34 340 17 270 13 170 8.4 48 4.5
10 13 1.1 14 0.59 13 0.50 10 0.49 13 0.61 20 0.95 48 2.2 90 8.4 110 17
Table 4Extinction Corrected Spitzer and 2MASS Flux Densities, in mJy, of the c2d+GB YSOs
Extinction Corrected, All Sources Extinction Corrected, Excluding Likely AGB Stars
Cloud 0+I Flat II III Total 0+I Flat II III Total
Aquila 83 65 330 841 1319 83 65 327 634 1109
Auriga/CMC 35 8 73 17 133 35 8 73 15 131
Cepheus 18 11 61 13 103 18 11 61 12 102
Chamaeleon I 4 5 62 15 86 4 5 62 14 85
Chamaeleon II 2 0 19 5 26 2 0 19 4 25
Chamaeleon III 1 0 0 3 4 1 0 0 0 1
Corona Australis 9 6 22 17 54 9 6 22 16 53
IC5146 28 10 79 15 132 28 10 79 14 131
Lupus I 1 2 8 2 13 1 2 8 0 11
Lupus III 2 5 38 24 69 2 5 38 14 59
Lupus IV 1 1 3 7 12 1 1 3 5 10
Lupus V 0 0 7 36 43 0 0 7 25 32
Lupus VI 1 0 2 42 45 1 0 2 25 28
Musca 1 0 1 11 13 1 0 1 4 6
Ophiuchus 28 43 174 47 292 28 43 174 32 277
Ophiuchus North 2 1 3 4 10 2 1 3 3 9
Perseus 76 35 235 39 385 76 35 235 31 377
Serpens 34 18 131 44 227 34 18 131 39 222
Total 326 210 1248 1182 2966 326 210 1245 887 2668
Table 5Number of YSOs by Cloud and Class
Quantity Class III/stellar Class II Flat Class 0+I
Original x 0.5 2.0 3.50
Original x 1.5 5.0
Original y 0.3 1.50
Original y 0.5 1.5
Original P35 65.2% 77.9% 66.0%
Original P36 99.5% 84.2% 84.1%
Revised x 0.5 1.5 2.5 2.5
Revised x 4.0 5.0 4.5
Revised y 0.4 1.1 1.9
Revised y 0.4 1.1 1.8
Revised P37 83.2% 79.2% 63.2% 65.6%
Revised P38 92.2% 91.8% 59.1% 90.0%
Table 6Classification Via [3.6]  [5.8] versus [8.0]  [24]
39 40 41 42 43
Reference Class (Myr) (Myr) (Myr) (Myr) (Myr)
All Class II 2.0 0.52 0.34 0.17 0.35
All Class II 3.0 0.78 0.50 0.26 0.52
All Class II + All Class III 3.0 0.40 0.26 0.13 0.27
All Class II + 75% of Class III 3.0 0.46 0.30 0.15 0.31
All Class II + 10% of Class III 3.0 0.72 0.46 0.24 0.48
Table 7Timescales for Young Stellar Objects
Cloud MIPS Target Name MIPS AOR IRAC Target Name IRAC AOR
Aquila SERAQU_E_1 19968768 SERAQU_E_1 20008448
19968256 20008704
SERAQU_E_2 20001536 20008960
20001024 19970816
SERAQU_E_3 19970560 19971072
19970304 19971328
SERAQU_E_4 20002048 SERAQU_E_2 20005120
20001792 20005376
20005632
19964672
19965440
19965952
SERAQU_E_3 20002304
20002560
20003072
19957248
19958016
19958272
SERAQU_E_4 19997952
19998464
19998720
19999488
20018176
20018688
20018944
20019200
SERAQU_E_5 14511104
SERAQU_E_6 27042304
27042560
SERAQU_NE 14510336 SERAQU_NE 19991552
19966464 19988480
19966976 19991296
20009728
19987968
20009216
SERAQU_SW_1 24195328 SERAQU_SW 19972352
24195584 19971840
SERAQU_SW_2 24195840 19972608
24196096 19986944
19986688
19987200
SERAQU_W_1 19974656 SERAQU_W_1a 19970048
19974400 19985664
SERAQU_W_1b 19962368
19980800
SERAQU_W_2 20006912 SERAQU_W_2 19957760
20006656 19956992
19975424
19974912
Chamaeleon I CHA_I 03661312 CHA_I_1 19986432
03962112 20006400
19979264 CHA_I_2 03960320
20011008 CHA_I_3 20015104
19978496 20014592
20010240 CHA_I_4 03651328
CHA_I_5 19992832
20012800
Chamaeleon III CHA_III_1 19980032 CHA_III_1 19995392
19979776 20015872
CHA_III_2 20012544 CHA_III_2a 19989248
19977472 19989760
20012032 20007168
19976448 20007424
CHA_III_2b 19985152
20003328
CHA_III_2c 19961344
19977216
19984896
19961600
19998208
20002816
27040256
27040512
27041536
27042048
Musca MUSCA 20004096 MUSCA_1 27040768
19973120 27041024
20003840 MUSCA_2 27043072
19972864 27043328
MUSCA_3 20009472
19975680
19976704
20009984
19975936
19977728
Table 8Spitzer AORs for Unpublished Clouds
Index 0.36 m 0.36 m 0.44 m 0.44 m 0.55 m 0.55 m 0.64 m 0.64 m 0.665 m 0.665 m 0.79 m 0.79 m 0.915 m 0.915 m 0.96 m 0.96 m
1
2
3
4
5
6
7
8
9
10
44
Table 9Observed m Flux Densities, in mJy, of the c2d+GB YSOs
Index 0.36 m 0.36 m 0.44 m 0.44 m 0.55 m 0.55 m 0.64 m 0.64 m 0.665 m 0.665 m 0.79 m 0.79 m 0.915 m 0.915 m 0.96 m 0.96 m
1
2
3
4
5
6
7
8
9
10
45
Table 10Extinction Corrected m Flux Densities, in mJy, of the c2d+GB YSOs
Index 3.4 m 3.4 m 5.0 m 5.0 m 6.7 m 6.7 m 12 m 12 m 14 m 14 m 22 m 22 m
1 160 2.3 87 2.3
2 70 1.0 47 1.6
3 7.6 0.21 3.7 0.99
4 240 3.3 120 2.9
5 1.7 0.16
6 14 0.28 8.2 0.85
7 33 0.51 19 1.1
8 120 1.5 63 2.1
9 81 1.2 50 1.7
10 27 0.47 65 2.2
46
Table 11Observed m Flux Densities, in mJy, of the c2d+GB YSOs
Index 3.4 m 3.4 m 5.0 m 5.0 m 6.7 m 6.7 m 12 m 12 m 14 m 14 m 22 m 22 m
1 230 3.4 110 2.9
2 100 1.5 59 2.0
3 14 0.39 5.2 1.4
4 360 4.9 160 3.6
5 3.1 0.29
6 21 0.42 10 1.1
7 46 0.72 23 1.3
8 190 2.4 81 2.7
9 120 1.8 62 2.1
10 49 0.87 92 3.2
47
Table 12Extinction Corrected m Flux Densities, in mJy, of the c2d+GB YSOs
Index 160 m 160 m 350 m 350 m 450 m 450 m 850 m 850 m 1100 m 1100 m 1200 m 1200 m 1300 m 1300 m
1
2
3
4
5
6
7
8
9
10
48
Table 13Observed m Flux Densities, in mJy, of the c2d+GB YSOs
Index 160 m 160 m 350 m 350 m 450 m 450 m 850 m 850 m 1100 m 1100 m 1200 m 1200 m 1300 m 1300 m
1
2
3
4
5
6
7
8
9
10
49
Table 14Extinction Corrected m Flux Densities, in mJy, of the c2d+GB YSOs

Footnotes

  1. affiliation: Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, MS 78, Cambridge, MA 02138, USA
  2. affiliation: mdunham@cfa.harvard.edu
  3. affiliation: National Optical Astronomy Observatories, Tucson, AZ, USA
  4. affiliation: Department of Astronomy, The University of Texas at Austin, 2515 Speedway, Stop C1400, Austin, TX 78712-1205, USA
  5. affiliation: Department of Physics & Astronomy, University of Victoria, Victoria, BC, V8W 3P6, Canada
  6. affiliation: Núcleo de Astronomíía de la Facultad de Ingenieríía, Universidad Diego Portales, Av. Ejército 441, Santiago, Chile
  7. affiliation: Millenium Nucleus “Protoplanetary Disks in ALMA Early Science,” Chile
  8. affiliation: National Research Council of Canada, Herzberg Astronomy & Astrophysics Programs, 5071 West Saanich Road, Victoria, BC, Canada V9E 2E7
  9. affiliation: Department of Astronomy, University of Massachusetts, Amherst, MA 01003, USA
  10. affiliation: Department of Astronomy, The University of Texas at Austin, 2515 Speedway, Stop C1400, Austin, TX 78712-1205, USA
  11. affiliation: Physics and Astronomy, University of Exeter, Stocker Road, Exeter EX4 4QL, UK
  12. affiliation: Department of Astronomy, University of Virginia, P.O. Box 400325, Charlottesville, VA 22904, USA
  13. affiliation: National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA
  14. affiliation: Department of Astronomy, University of Maryland, College Park, MD 20742, USA
  15. affiliation: Columbia Astrophysics Laboratory, Columbia University, New York, NY 10027, USA
  16. affiliation: National Research Council of Canada, Herzberg Astronomy & Astrophysics Programs, 5071 West Saanich Road, Victoria, BC, Canada V9E 2E7
  17. affiliation: Department of Physics and Astronomy, University of Victoria, Victoria, BC V8P 1A1, Canada
  18. affiliation: Jeremiah Horrocks Institute, University of Central Lancashire, Preston, PR1 2HE, United Kingdom
  19. affiliation: National Research Council of Canada, Herzberg Astronomy & Astrophysics Programs, 5071 West Saanich Road, Victoria, BC, Canada V9E 2E7
  20. affiliation: Gemini Observatory, 670 N. A’ohoku Pl, Hilo, HI 96720, USA
  21. affiliation: Space Science Institute, 4750 Walnut Street, Suite 205, Boulder, CO 80301, USA
  22. affiliation: Department of Physical Sciences, Nicholls State University, PO Box 2022, Thibodaux, LA 70310
  23. slugcomment: Accepted for publication in ApJS
  24. Class 0 protostars are the youngest class of YSOs; see §3.1 for the formal definition of this class of objects.
  25. The quantity is defined as the slope of the infrared SED in log() vs. log() and is used to classify YSOs, as discussed in detail in §3.1 and §3.2. While calculated from extinction corrected photometry is used in later sections, here we use the values calculated from the observed photometry for consistency with the previous studies to which we compare.
  26. Available at http://irsa.ipac.caltech.edu/cgi-bin/Gator/nph-scan?mission=irsa&submit=Select&projshort=WISE
  27. Available at http://irsa.ipac.caltech.edu/data/SPITZER/docs/ mips/mipsinstrumenthandbook/
  28. The Submillimeter High Angular Resolution Camera II (SHARC-II) was a 350 m bolometer array operated at the Caltech Submillimeter Observatory (Dowell et al., 2003).
  29. The Submillimeter Common-User Bolometer Array (SCUBA) was a 450 and 850 m bolometer array operated at the James Clerk Maxwell Telescope.
  30. Available at: http://www.ita.uni-heidelberg.de/dullemond/software/radmc-3d/
  31. An inclination of  corresponds to a pole-on (face-on) system, while an inclination of  corresponds to an edge-on system.
  32. Reference for the distances quoted in this work: (1) Maury et al. (2011); (2) Lada et al. (2009); (3) Kirk et al. (2009); (4) Belloche et al. (2011b); (5) Whittet et al. (1997); (6) Neuhäuser & Forbrich (2008); (7) Harvey et al. (2008); (8) Comerón (2008); (9) Knude & Hog (1998); (10) Wilking et al. (2008); (11) de Geus et al. (1989); (12) Enoch et al. (2006); (13) Dzib et al. (2011).
  33. References presenting the Spitzer IRAC and MIPS observations: (1) Gutermuth et al. (2008); (2) Broekhoven-Fiene et al. (2014); (3) Kirk et al. (2009); (4) Young et al. (2005); (5) Porras et al. (2007); (6) Alcalá et al. (2008); (7) Peterson et al. (2011); (8) Harvey et al. (2008); (9) Chapman et al. (2007); (10) Merín et al. (2008); (11) Spezzi et al. (2011); (12) Padgett et al. (2008); (13) Hatchell et al. (2012); (14) Jørgensen et al. (2006); Rebull et al. (2007); (16) Harvey et al. (2006); (17); Harvey et al. (2007b); (18) Harvey et al. (2007a).
  34. Different regions within Cepheus are located at different distances; see Kirk et al. (2009) for details.
  35. The full table is published online in a machine-readable format. Only the first ten lines are shown here.The full table is published online in a machine-readable format. Only the first ten lines are shown here.The full table is published online in a machine-readable format. Only the first ten lines are shown here.
  36. The probability that a YSO located within the region has the desired Class.
  37. The probability that a YSO of the desired Class is located within the region.
  38. The probability that a YSO located within the region has the desired Class.
  39. Assumed duration of the reference Class.
  40. Calculated duration of Class 0+I sources ().
  41. Calculated duration of Flat-spectrum sources ().
  42. Calculated duration of Class 0 sources (); see §4.2 for details.
  43. Calculated duration of Class I sources (); see §4.2 for details.
  44. footnotetext: The full table is published online in a machine-readable format. Only the first ten lines are shown here.
  45. footnotetext: The full table is published online in a machine-readable format. Only the first ten lines are shown here.
  46. footnotetext: The full table is published online in a machine-readable format. Only the first ten lines are shown here.
  47. footnotetext: The full table is published online in a machine-readable format. Only the first ten lines are shown here.
  48. footnotetext: The full table is published online in a machine-readable format. Only the first ten lines are shown here.
  49. footnotetext: The full table is published online in a machine-readable format. Only the first ten lines are shown here.

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