Massive stars in the giant molecular cloud G23.30.3 and W41 ††thanks: Based on observations collected at the European Southern Observatory (ESO Programmes 084.D-0769, 085.D-019, 087.D-09609). ††thanks: MM is currently employed by the MPIfR. This works was partially carried out at RIT (2009), at ESA (2010), and at the MPIfR.
Key Words.:supergiants – Stars: supernovae – Galaxy: open clusters and associations
Context: Young massive stars and stellar clusters continuously form in the Galactic disk, generating new Hii regions within their natal giant molecular clouds and subsequently enriching the interstellar medium via their winds and supernovae.
Aims: Massive stars are among the brightest infrared stars in such regions; their identification permits the characterisation of the star formation history of the associated cloud as well as constraining the location of stellar aggregates and hence their occurrence as a function of global environment.
Methods:We present a stellar spectroscopic survey in the direction of the giant molecular cloud G23.30.3. This complex is located at a distance of kpc, and consists of several Hii regions and supernova remnants.
Results: We discovered 11 Of stars, one candidate Luminous Blue Variable, several OB stars, and candidate red supergiants. Stars with -band extinction from mag appear to be associated with the GMC G23.30.3; O and B-types satisfying this criterion have spectrophotometric distances consistent with that of the giant molecular cloud. Combining near-IR spectroscopic and photometric data allowed us to characterize the multiple sites of star formation within it. The O-type stars have masses from M, and ages of 5-8 Myr. Two new red supergiants were detected with interstellar extinction typical of the cloud; along with the two RSGs within the cluster GLIMPSE9, they trace an older burst with an age of 20–30 Myr. Massive stars were also detected in the core of three supernova remnants - W41, G22.70.2, and G22.75830.4917.
Conclusions:A large population of massive stars appears associated with the GMC G23.30.3, with the properties inferred for them indicative of an extended history of stars formation.
An understanding of the evolution, and fate of massive stars ( M) is of broad astronomical interest, and it is fundamental for studies of galaxies at all redshifts. Historically, the majority (70-90%) of massive stars were thought to be born in dense clusters, although recent observations also support formation in low-density environments (Lada & Lada 2003; de Wit et al. 2005; Wright et al. 2014). In turn, such star clusters appear to form in large molecular complexes (Clark & Porter 2004; Clark et al. 2009; Davies et al. 2012), and a direct proportionality is often assumed between the cluster masses and the masses of the collapsing clouds (e.g. Krumholz & Bonnell 2007; Alves et al. 2007). However, observational constraints on the distribution (clusters versus stars in isolation) and evolution of massive stars are difficult to obtain, because of their rarity, and heavy dust obscuration of the richest star-forming regions of the Galaxy.
The recent completion of multiple radio and infrared surveys of the Galactic plane111The Multi-Array Galactic Plane Imaging Survey (MAGPIS)(White et al. 2005; Helfand et al. 2006), the Two Micron All Sky Survey (2MASS) (Cutri et al. 2003), the Deep Near Infrared Survey of the Southern Sky (DENIS) (Epchtein et al. 1994), the UKIRT Infrared Deep Sky Survey (UKIDSS) (Lucas et al. 2008), the VISTA Variables in the Via Lactea survey (VVV) (Soto et al. 2013), the Midcourse Space Experiment (MSX) (Egan et al. 2003; Price et al. 2001), the Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE) (Churchwell et al. 2009) and WISE the Wide-field Infrared Survey Explore (WISE) (Cutri & et al. 2012). has opened a golden epoch for studying the formation, evolution, and environment of massive stars. Over the past decade, multi-wavelength analyses of the Galactic plane have revealed several hundred new Hii regions, and candidate supernova remnants (SNRs, e.g. Green 2009; Brogan et al. 2006; Helfand et al. 2006). Moreover, an impressive large number of new candidate stellar clusters and ionizing stars have been reported; more than 1800 candidate clusters were detected with 2MASS data (e.g. Bica et al. 2003), more than 90 candidates were found with GLIMPSE data (Mercer et al. 2005), and candidates with the VVV survey (Borissova et al. 2011).
The Galactic giant molecular cloud (GMC) GMC G23.30.3 (object ”[23,78]” in Dame et al. (1986)) is found at a distance of 4–5 kpc (Albert et al. 2006). A remarkable number of candidate stellar clusters appear associated with this region (e.g. Messineo et al. 2010), and four SNRs (G, G022.700.2, W41, and G22.75830.4917, Green 2009; Helfand et al. 2006; Leahy & Tian 2008) are projected against it (as shown by Messineo et al. 2010). The presence of SNRs suggests that massive star formation has been active in multiple sites of this GMC, as do the stellar cluster number 9 in Mercer et al. (2005) (hereafter GLIMPSE9, Messineo et al. 2010), cluster number 10 in Mercer et al. (2005) (hereafter GLIMPSE10), 117, and 118 (Bica et al. 2003). Additional regions with massive stars were identified by Messineo et al. (2010).
Given this, G23.30.3 appears to be an ideal laboratory for the investigation of massive stars and multi-seeded star formation. The rich star clusters associated with the complex allow us to study the mode and progression of star formation in this region and to sample rare evolutionary phases of massive stars, such as Wolf-Rayets (WRs), red supergiants (RSGs), and luminous blue variables (LBVs). The presence of supernova remnants (SNRs) indicates that star formation has been progressing for some time, with the current stellar population providing information on the initial masses of the supernova progenitors, and on the fate of massive stars.
In this paper, we present the result of a spectroscopic survey of selected bright stars in the direction of GMC G23.30.3. In Sect. 2, the spectroscopic observations and data reduction are presented, along with available photometric data. In Sect. 3, we describe the spectral types, the reddening properties, and the selection of massive stars likely associated with the GMC. Luminosities of the massive stars are derived. Eventually, in Sect. 4, we summarize the results, and briefly discuss the spatial distribution of the detected massive stars, their ages, and their connection with the supernova remnants.
2 Observations and data reduction
|REG1/ [BDS2003]118||18 34 15.1||08 20 42||1.2||G23.56670.0333 (SNR5)||1|
|GLIMPSE9Large||18 34 09.6||09 13 53||3.0||border of G22.70.2 (SNR2)||new|
|near G22.75830.4917 (SNR3)|
|GLIMPSE9 (cluster)||18 34 09.6||09 13 53||0.3||2, 3|
|REG2||18 34 41.1||08 34 22||4.0||border of W41||2|
|REG4/GLIMPSE10||18 34 31.6||08 46 47||5.0||core of W41||2, 3|
|REG5||18 34 20.0||08 59 48||5.0||G22.99170.3583 (SNR4)||2|
|REG7/[BDS2003]117||18 34 27.7||09 15 52||2.0||core of G22.75830.4917 (SNR3)||2, 1|
|RSGCX1||18 33 08.9||09 09 14||4.5||core of G22.70.2 (SNR2)||new|
Notes. A larger region enclosing the GLIMPSE9 cluster was surveyed. The quoted radius encloses only the bulk of the nebulosity seen at 3.6 m.
2.1 SINFONI data
The observations were made with the Spectrograph for INtegral Field Observations in the Near Infrared (SINFONI) (Eisenhauer et al. 2003) on the Yepun Very Large Telescope, under the ESO programs 084.D-0769 and 085.D-0192 (P.I. Messineo). We observed stars with K mag and K mag from selected fields (see Table 1); their color-color distribution is shown in Sect. 3.2. A total number of 89 data-cubes were obtained, and a total number of 104 stellar spectra were extracted from these cubes.
We used SINFONI in non-AO mode, with a pixel scale of pix, the -grating (1.95-2.45 m), and a resolving power R .
Exposures were taken in a target-sky-sky-target sequence, using a fixed sky position. Integration times () ranged from 1 s to 53 s in period 84, and from 1 s to 93 s in period 85. Two exposures were taken for each position. Telluric standard stars of B-type were observed at an airmass within 0.2 dex from the airmass of the science observations, and immediately before or after the science observation.
Data reduction was performed as described in Messineo et al. (2007). The construction of a wavelength-calibrated data-cube, along with the removal of the instrumental signatures, was performed with version 3.9.0 of the ESO SINFONI pipeline (Schreiber et al. 2004; Modigliani et al. 2007). Each science frame was sky-subtracted, and flat-fielded. Dead/hot pixels were removed by interpolation; geometric distortions were corrected. A wavelength-calibration map was obtained using daytime arc-lamp lines. Possible shifts in wavelengths (up to 0.4 pixels) were checked, and corrected with observed OH sky lines (Oliva & Origlia 1992; Rousselot et al. 2000) by cross-correlating the OH line positions with a template spectrum with OH lines at zero velocity.
Stellar traces were extracted from the cubes, and corrected for atmospheric and instrumental responses by dividing the spectra of the targets by the spectra of B-type stars. The Br and He I lines were removed from the spectra of the standards with a linear interpolation, and the resulting spectra were multiplied by a black body curve, F, with the effective temperature of the star. Some spectra with low signal-to-noise displayed residuals of OH sky lines; in these seven stars, we removed the residuals of the OH sky lines at 2.0008 m, 2.0276 m, 2.0413 m, 2.0563 m, 2.0729 m, 2.1506 m, 2.1802 m, 2.1955 m, 2.2126 m, and 2.2312 m with a linear interpolation. The absolute coordinates of the SINFONI fields generally agree with the 2MASS coordinates within 1 or 2. The astrometry of each field was aligned with a 2MASS image or UKIDSS image.
We examined stellar traces with a signal-to-noise ratio above 20-40.
|[hh mm ss]||[deg mm ss]||[K]|
|1||18 33 18.14||24 09.9||SofI||OBe||24300 8800||-0.12||-0.06|
|2||18 33 52.19||10 38.2||SofI||O9-9.5e||29300 1800||-0.16||-0.07||BD4766|
|3||18 34 00.86||15 41.5||SINFONI||O6-7f+||35700 1000||-0.21||-0.10|
|4||18 34 05.74||16 00.6||SINFONI||O7-8.5f+||31800 1500||-0.21||-0.10|
|5||18 34 06.25||15 17.9||SINFONI||O6-7f+||35700 1000||-0.21||-0.10|
|6||18 34 08.75||13 59.9||SINFONI||B0-3||23800 6700||-0.16||-0.08|
|7||18 34 09.25||03 06.0||SINFONI||B4-A2||12700 3600||-0.02||0.00|
|8||18 34 10.50||14 04.4||SINFONI||B0-3||23800 6700||-0.16||-0.08|
|9||18 34 10.59||13 43.9||SINFONI||O7-8.5f+||33100 1500||-0.21||-0.10|
|10||18 34 10.70||13 58.7||SINFONI||OF||-0.06||-0.01|
|11||18 34 11.30||13 56.4||SINFONI||OF||-0.06||-0.01|
|12||18 34 11.81||55 44.9||SINFONI||B4-A2||12700 3600||-0.02||0.00|
|13||18 34 12.14||00 23.6||SINFONI||B4-A2||12700 3600||-0.02||0.00|
|14||18 34 12.17||12 29.9||SINFONI||O6-7f+||34500 1200||-0.21||-0.10|
|15||18 34 13.47||14 31.9||SINFONI||O6-7f+||34500 1200||-0.21||-0.10|
|16||18 34 14.47||44 22.9||SINFONI||O9-9.5e||29300 1800||-0.16||-0.07|
|17||18 34 15.88||45 45.2||SINFONI||O9-9.5f+||31400 1100||-0.19||-0.09|
|18||18 34 17.26||46 50.0||SINFONI||O6-7f+||34500 1200||-0.21||-0.10|
|19||18 34 18.14||57 18.4||SINFONI||B4-A2||12700 3600||-0.02||0.00|
|20||18 34 18.85||45 32.9||SINFONI||B4-A2||12700 3600||-0.02||0.00|
|21||18 34 19.19||46 17.6||SINFONI||B7.5-A2||12900 3900||-0.02||0.00|
|22||18 34 21.70||28 20.9||SofI||cLBV||13200 2300||0.01||-0.01|
|23||18 34 23.79||49 18.1||SINFONI||O6-7f+||35700 1000||-0.21||-0.10|
|24||18 34 26.38||00 49.1||SINFONI||OF||-0.06||-0.01|
|25||18 34 27.67||15 51.1||SINFONI||O4f+||38200 2500||-0.21||-0.10|
|26||18 34 28.48||59 31.1||SINFONI||B4-A2||12700 3600||-0.02||0.00|
|27||18 34 30.15||44 40.6||SINFONI||OF||-0.06||-0.01|
|28||18 34 30.84||58 40.1||SINFONI||B4-A2||12700 3600||-0.02||0.00|
|29||18 34 30.95||58 37.8||SINFONI||OF||-0.06||-0.01|
|30||18 34 33.83||32 57.9||SINFONI||OF||-0.06||-0.01|
|31||18 34 33.92||32 59.6||SINFONI||B0-3||23800 6700||-0.16||-0.08|
|32||18 34 35.17||00 39.9||SINFONI||B4-A2||12700 3600||-0.02||0.00|
|33||18 34 35.74||01 27.6||SINFONI||OF||-0.06||-0.01|
|34||18 34 36.94||47 54.7||SINFONI||OF||-0.06||-0.01|
|35||18 34 38.36||50 49.7||SINFONI||OF||-0.06||-0.01|
|36||18 34 42.63||45 01.9||SINFONI||O6-7f+||34500 1200||-0.21||-0.10|
|37||18 34 42.86||45 02.9||SINFONI||OF||-0.06||-0.01|
|38||18 34 50.71||46 16.0||SINFONI||B0-3||23800 6700||-0.16||-0.08|
|MFD2010 3||18 34 08.68||14 11.1||B0-3||21500 6000||-0.08||-0.04||MFD2010 3|
|MFD2010 4||18 34 08.54||14 11.8||B0-3||21500 6000||-0.08||-0.04||MFD2010 4|
|MVM2011 39||18 33 47.64||23 07.7||WC8||65000 5000||0.43||0.38||MVM2011 39|
Notes. Identification numbers are followed by celestial coordinates, instrument, spectral types, estimated effective temperatures, T, intrinsic near-infrared colors, and comments. Two B supergiants detected by Messineo et al. (2010), and a WR discovered by Mauerhan et al. (2011) are appended to the table. We used the collection of infrared colors and temperatures per spectral types as listed in the Appendix of Messineo et al. (2011). For every star (for example a O6-7 star), we assumed the mean temperature of the range considered, and as error half range. () Messineo et al. (2010). () Mauerhan et al. (2011).
2.2 SofI data
An additional 47 objects were detected with the Son of Isaac (SofI) spectrograph on the ESO New Technology Telescope (NTT) on La Silla during the ESO program 087.D-09609 (P.I, Messineo), on the nights of June 10, 11, and 12, 2011.
Observations with SofI on the NTT were performed with the medium resolution grism, a slit-width of 1, and the K filter. A coverage from 2.0 m to 2.3 m at a resolving power of was obtained. Medium resolution spectra in band were taken only for one target, a candidate LBV; a slit with a width of 1 was used, which provided a coverage from 1.5 m to 1.8 m at a resolving power of . The objects were nodded along the slit to obtain pairs of frames, which were subtracted and flat-fielded. In a few observations, the stellar traces did not move (no nodding, no jitter), and we subtracted each frame with darks. The two-dimensional frames were rectified with a bilinear interpolation of stellar traces and arc lines. Stellar traces were extracted from individual frames, aligned in wavelength, and co-added. Correction for atmospheric and instrumental responses were performed with spectra of B-type standards (taken in the same manner as for the targets, and with linearly interpolated Br and He I lines). We multiplied the results by a black body curve, F.
|[hh mm ss]||[deg mm ss]||[AA]||[K]||[K]||[%]|
|39||18 32 36.02||9 08 03.5||SofI||29||M5||3450 203||K5||3869 137||8|
|40||18 33 08.89||9 08 32.6||SofI||33||3223 226||M0||3790 124||11||IRAS18303-0910|
|41||18 33 13.90||9 06 23.2||SofI||23||M1||3745 130||K3||3985 121||0|
|42||18 33 15.02||9 08 32.2||SofI||23||M1||3745 130||K3||3985 121||10|
|43||18 33 35.24||8 47 57.7||SofI||32||3223 226||M0||3790 124||18||BG|
|44||18 33 37.80||9 21 38.1||SofI||21||M0||3790 124||K2||4049 131||8|
|45||18 33 40.98||9 03 25.2||SofI||26||M3||3605 120||K3||3985 121||16|
|46||18 34 10.36||9 13 52.9||SINFONI||64||3223 226||M3||3605 120||76||[MFD2010]8|
|47||18 34 23.17||8 48 38.6||SINFONI||61||3223 226||M2||3660 140||6|
|48||18 34 33.86||8 44 21.2||SINFONI||47||M6||3336 226||K5||3869 137||16|
|MFD2010 5||18 34 09.86||9 14 23.8||SINFONI||3223 226||M1.5||3710 152||MFD2010 5|
|BD08 4635||18 34 51.88||8 36 40.8||SINFONI||3223 226||M2||3660 140||BD08 4635|
|BD08 4639||18 35 31.06||8 41 23.4||SINFONI||3223 226||K2||4049 131||BD08 4639|
|BD08 4645||18 36 21.66||8 52 40.0||SINFONI||3223 226||M2||3660 140||BD08 4645|
Notes. Identification numbers are followed by celestial coordinates, instrument, EW(CO)s, spectral types, T, HO indexes, and comments. Two spectral types are reported; the first was obtained using the relation for red giants (Sp[RGB]), the latter using that for red supergiants (Sp[RSG]). We appended to the table RSG [MFD2010]5 (Messineo et al. 2010), RSG BD 4645, BD 4635, and BD 4639 (Skiff 2013). () Temperature errors account for accuracy in spectral types of . () The HO index depends on the correction for A; a variation of 10% in A typically affects the HO by 20%. () BG= object in the background of the cloud. () Messineo et al. (2010). () Skiff (2013).
2.3 Infrared photometry
We searched for counterparts of the observed stars in the 2MASS Catalog of Point Sources (Cutri et al. 2003), in the third release of DENIS data at CDS (catalog B/denis) (Epchtein et al. 1994), in the GLIMPSE catalog (Churchwell et al. 2009), and in the WISE catalog (Cutri & et al. 2012); we used the closest match within a search radius of 2. We searched in the UKIDSS catalog (Lucas et al. 2008) with a search radius of 1, and retained only counterparts in the linear regime ( mag). The II/293 (GLIMPSE) catalog from CDS is a combination of the original GLIMPSE-I (v2.0), GLIMPSE-II (v2.0), and GLIMPSE-3D catalogs. We also searched for counterparts in the Version 2.3 of the MSX Point Source Catalog (Egan et al. 2003; Price et al. 2001) with a search radius of 5. MSX upper limits were removed. WISE counterparts were retained only if their signal-to-noise ratio was larger than 2.0. Near-infrared and GLIMPSE counterparts were visually checked with 2MASS/UKIDSS and GLIMPSE charts. For most of sources, WISE band-3 and band-4 provided upper limit magnitudes, due to confusion.
In addition, we searched for possible , , -band matches in The Naval Observatory Merged Astrometric Dataset (NOMAD) by Zacharias et al. (2004). The photometric data are listed in Table 4. For a few targets (missing in both 2MASS and UKIDSS), K counterparts were estimated from the SINFONI cubes (with a typical uncertainty of mag). For stars [MFD2010]3, [MFD2010]4, and [MFD2010]5, and -band measurements were obtained with the Near Infrared Camera and Multi-Object Spectrometer (NICMOS, Skinner et al. 1998) (Messineo et al. 2010).
Notes. () 2MASS upper limits and confused stars were removed all, but star #4. () Small corrections ( mag, mag, mag) were applied to match the 2MASS photometric system. () UKIDSS values were used. () K was estimated from the SINFONI data-cube. () and K were taken from Messineo et al. (2010). () Identification numbers are taken from Table 2, 3, and 12.