FIRE Spectroscopy of the ultra-cool brown dwarf, UGPS J072227.51054031.2: Kinematics, Rotation and Atmospheric Parameters111This paper includes data gathered with the 6.5 meter Magellan Telescopes located at Las Campanas Observatory, Chile.
We present 6000 near–infrared spectroscopy of the nearby T9 dwarf, UGPS J072227.51054031.2, obtained during the commissioning of the Folded–Port Infrared Echellette Spectrograph on the Baade Magellan telescope at Las Campanas Observatory. The spectrum is marked by significant absorption from HO, CH and H. We also identify NH absorption features by comparing the spectrum to recently published line lists. The spectrum is fit with BT-Settl models, indicating K and . This corresponds to a mass of and an age of Gyr, however there are large discrepancies between the model and observed spectrum. The radial and rotational velocities of the brown dwarf are measured as 46.9 2.5 and 40 10 km s, respectively, reflecting a thin disk Galactic orbit and fast rotation similar to other T dwarfs, suggesting a young, possibly planetary-mass brown dwarf.
Subject headings:stars: low-mass, brown dwarfs — stars: individual (UGPS J072227.51054031.2) — stars: kinematics — stars: fundamental parameters — infrared: stars
Late–type T dwarfs (T 600 K) are among the dimmest, coldest and least massive products of star formation. As cooling brown dwarfs (BDs), these objects sample a broad range of age and mass, from old, relatively massive relics of the earliest epochs of Galactic star formation to recently–formed planetary–mass objects incapable of deuterium fusion ( 13 ; Burrows et al., 2001). Their numbers in the vicinity of the Sun help constrain the substellar initial mass function (e.g., Metchev et al., 2008; Burningham et al., 2010) and the minimum formation mass (e.g., Burgasser, 2004). Late-type T dwarfs also occupy the same physical parameter space (mass, age, ) as exoplanets, with moderate separations (0.5–1 AU) from solar–type stars, making them important benchmarks of exoplanet models (i.e., Burrows et al., 1997; Baraffe et al., 2003) and direct detection experiments (i.e., Macintosh et al., 2006).
Despite its astronomical utility, high–resolution spectroscopy of late T dwarfs is exceedingly rare. The main culprit is faint T dwarf luminosity ( for T9; Golimowski et al., 2004). Recently, surveys such as the Wide-field Infrared Survey Explorer (WISE; Wright et al., 2010) and the UKIRT Infrared Deep Sky Survey (UKIDSS; Lawrence et al., 2007) have produced deep multi–band photometry over thousands of square degrees in the near-infrared (NIR; 1-2.5m) and mid-IR ( 3-5 m). Yet many of the cool T dwarfs discovered in these surveys have been studied at low spectral resolutions ( = ), blending rich absorption bands produced by CH, NH and HO and complicating their atmospheric analyses. These low–resolution observations also limit the precision of radial and rotational velocity measurements. Higher resolution observations are only feasible for bright objects, biasing their kinematic analysis to the nearby BD population. For example, the largest study of T dwarfs with measured rotation velocities contained only nine objects (Zapatero Osorio et al., 2006).
Currently, there are brown dwarfs classified as T9 or later. One of the brightest of these sources is UGPS J072227.51054031.2 (hereafter UGPS 072205), discovered in the UKIDSS data by Lucas et al. (2010), and tentatively assigned a spectral type of T10 based on its strong molecular absorption and faint absolute magnitude. The spectral type of UGPS 072205 was revised by Kirkpatrick et al. (2011) and Cushing et al. (2011) to T9 and it has been designated as the infrared spectral standard. UGPS 072205 is a tantalizing target for followup studies, as it is relatively nearby ( 4.1 pc; see Table 1), bright (). In this paper, we present a moderate resolution (R 6000) NIR spectrum of UGPS 072205, acquired using the newly installed Folded-Port Infrared Echellette Spectrograph (FIRE; Simcoe et al., 2010). The observations are detailed in Section 2. In Section 3, the NIR SED and corresponding model fits are shown along with the rotational and radial velocities and Galactic orbit of UGPS 072205. Finally, our conclusions and paths for future investigations are presented in Section 4.
2. FIRE Observations and Reductions
The FIRE spectrograph (Simcoe et al., 2008, 2010) was installed and commissioned on the Baade Magellan telescope at Las Campanas Observatory during March and April 2010. FIRE is a single-object spectrograph with two modes: a cross–dispersed echellette mode with moderate resolution (), and a longslit low resolution mode (). The spectrum is imaged on a HAWAII-2RG chip, with continuous coverage from 0.85 - 2.5 m. In the cross–dispersed mode, the spectrum is spread over 21 orders, with some overlap in wavelength coverage at the edges of each order. Target acquisition is achieved with a second NIR imager and Mauna Kea Observatory filter focused on the entrance slit. FIRE was designed to be sensitive, employing the latest generation of HgCdTe detectors while limiting the number of reflective and transmissive surfaces, resulting in a zero-point of mag (for 1 count pixel s across ) for the echelle mode.
On April 6, 2010 UT, we obtained four 900s exposures of UGPS 072205 in FIRE’s echelle mode. The sky was clear with no cloud cover, and seeing was in at the time of observation. The 0 slit was used and aligned with the parallactic angle and the airmass was . The exposures were dithered along the slit in a ABBA pattern and a Fowler sampling of 8 was employed. An A0V star, HIP 63714, was observed for telluric correction and flux calibration purposes. Quartz flat fields and thorium-argon (ThAr) arcs were obtained after the science and telluric calibrator exposures.
The images were reduced using the FIRE reduction software package, FIREHOSE, which is based on the MASE pipeline (Bochanski et al., 2009) for the MagE spectrograph (Marshall et al., 2008). FIREHOSE, like MASE, was designed to reduce cross–dispersed echelle spectra with curved orders. Quartz lamp images were used to identify the order boundaries and derive flat-field and illumination corrections. A combination of OH telluric lines and ThAr arc images were used to determine the wavelength solution along the center of each order and its tilt in the spatial direction, which was used to construct a two dimensional vacuum wavelength map. The typical uncertainty of the wavelength solution was 0.15 pixels, corresponding to 0.04-0.4 Angstroms depending on the order. A 2D sky model was constructed using basis splines (Kelson, 2003) and subtracted from each order. This step eliminates the need for ABBA-type dithers for sky subtraction222However, for faint sources, multiple exposures are still needed to avoid saturating sky lines.. An optimal extraction routine was then performed on each order (Horne, 1986), extracting the object flux onto a heliocentric rest frame wavelength grid. Telluric corrections were incorporated into the pipeline using a modified version of xtellcor from Spextool (Cushing et al., 2004; Vacca et al., 2003). Telluric absorption was quantified by comparing the telluric stellar spectrum to a model Vega spectrum reddened using the color of HIP 63714, velocity shifted, and broadened using the 1.005 H I Pa line as a line kernel. The science target was also flux calibrated during this step. Multiple spectra of the same target were combined after flux calibration. Finally, the extracted orders were combined into a 1d spectrum, with overlap regions averaged together. The final spectrum is shown in Figure 1. The peak signals to noise in the and bands are 250, 350, 200, and 60 respectively. As a test of the flux calibration and telluric correction, we computed synthetic and colors from the final spectrum, and compared them to the measured values reported in Table 1. The color agreed within 0.1 mag, while differed by 0.5 mag, indicating the overall flux calibration between orders is sufficient for spectral analysis.
|Property||Value||Sourceaa1 - Lucas et al. (2010), 2 - This Paper, 3 - Cushing et al. (2011), 4 - Wright et al. (2010)|
|(mas yr)||-910 8||1|
|(mas yr)||1020 3||1|
|RV (km s)||46.9 2.5||2|
|(km s)||19 4||1|
|(km s)||40 10||2|
|(km s)||-42 2||2 bbComputed using the solar velocity of Schönrich et al. (2010).|
|(km s)||221 1||2 bbComputed using the solar velocity of Schönrich et al. (2010).|
|(km s)||4 1||2 bbComputed using the solar velocity of Schönrich et al. (2010).|
|20.51 0.09||1ccAverage of two reported values.|
3.1. Spectral Properties
Since the FIRE spectrum of UGPS 072205 is the highest resolution observation of one of the coolest BDs, we compiled the most recent laboratory line lists for HO, CH and NH to identify absorption features (Barber et al., 2006; Nassar & Bernath, 2003; Yurchenko et al., 2011, respectively). The HITRAN 2008 database was also used to supplement the linelists (Rothman et al., 2009). Prior to comparing to the FIRE spectrum of UGPS 072205, each linelist was cropped to 0.8-2.5 m and smoothed with a 50 km s Gaussian kernel, which corresponds to one FIRE resolution element. The absorption intensities for each linelist (in units of cm molecule) were scaled by the relative molecular abundances shown in Figure 3 of Saumon et al. (2006) for = 500 K. The non–equilibrium abundances were assumed for NH. The scaled values were then plotted along with the UGPS 072205 spectrum, and molecular features were manually identified. An example of our linelist comparisons are shown in Figure 2.
In Figure 3, we plot expanded views of the UGPS 072205 spectrum in the and bands. Prominent absorption features are labeled in each panel. Of note in Figure 3 is the significant number of absorption features throughout the spectrum that are attributed to HO, CH and NH. There are a large number of blends between the molecular features, especially with HO, but some isolated absorption bands do exist. We confirm the tentative identification of NH by Lucas et al. (2010) near 1.514 m. Additional isolated NH absorption features can be found near 1.234, 1.244, 1.52, 1.526, 1.542, 1.56, 1.566, 1.568 and 1.574 m. This suggests that observations spanning 1.5 - 1.6 m present the best chance at directly detecting NH at these temperatures. While the detection of NH has been suggested as the hallmark of the Y spectral class (i.e., Leggett et al., 2007), these weak features are consistent with the end of the T dwarf sequence as advocated by Cushing et al. (2011).
CH exhibits prominent absorption bands near 1.6 and 2.15 m. The structure seen in the UGPS 072205 spectrum is usually not detected at lower resolutions. These features may be used to derive spectral indices and provide isolated regions of the spectrum to derive atmospheric parameters (see Section 3.4).
3.2. Radial Velocity
The resolution of FIRE’s echelle mode permits the measurement of UGPS 072205’s radial velocity (RV) with a precision of a few km s. We cross–correlated the spectrum of UGPS 072205 against other T dwarfs observed with FIRE and model atmosphere predictions. The T dwarf RV standards (Table 2) were taken from Zapatero Osorio et al. (2007) and were observed with a similar setup on FIRE as part of a larger effort to quantify BD kinematics (Burgasser et al., in preparation). We also employed the T= 400 K, 500 K, 600 K and 700 K models (with = 5.0, ) from the BT-Settl grid (Allard et al., 2010). The 1.27-1.31 m spectral region in the band contains many strong molecular features, making it ideal for cross–correlation. The models were smoothed to match the resolution of the FIRE observations. Cross–correlations were computed using the xcorl IDL routine (Basri & Martín, 1999; Mohanty & Basri, 2003; West & Basri, 2009). The reported RV and uncertainty were computed by taking the unweighted mean and the standard deviation of the individual RV measurements, respectively. The measured RV and uncertainty for UGPS 072205 is 46.9 2.5 km s.
3.2.1 Galactic Orbit
To frame the kinematics of UGPS 072205 in a Galactic context, we computed its orbit using its measured velocity and position as initial conditions. Using the distance, position, proper motion and radial velocity reported in Table 1, we computed the cylindrical velocity vector [, , ] where the local standard of rest is [0, 220, 0] km s (Kerr & Lynden-Bell, 1986), the solar motion is [11.1, 12.24, 7.25] km s (Schönrich et al., 2010) and the radial velocity component increases in the direction of the Galactic center (Johnson & Soderblom, 1987). The Sun’s radial position () was taken to be 8.5 kpc away from the Galactic center (Kerr & Lynden-Bell, 1986) and 27 pc above the Plane (Z; Chen et al., 2001; Jurić et al., 2008). Note that we are reporting velocities in a Galactocentric frame, rather than the traditional heliocentric frame. While the effect is negligible for UGPS 072205, using rather than a Galactic frame can introduce errors of a few km s for distances 100 pc. Employing a Galactocentric velocity frame will be important as more distant dwarfs are discovered in the next generation of surveys (i.e., LSST).
The orbit was integrated assuming a set of static, spherically-symmetric oblate Plummer’s sphere potentials for the Galactic halo, bulge and disk, using the forms described in Kuzmin (1956) and Miyamoto & Nagai (1975) and with parameters from Dauphole & Colin (1995). A Runge-Kutta integrator was used to calculate the orbit over a period of 250 Myr with a 10 kyr timestep, and both energy and the -component of angular momentum were conserved to better than one part in 10. To sample measurement uncertainties in the distance and velocity of UGPS 072205 relative to the Sun, we computed 100 realizations of the orbit through the Monte Carlo method, varying the starting conditions assuming normal distributions with means and widths given by the values in Table 1.
The baseline calculation is shown in Figure 4, revealing a flat orbit with small eccentricity (). The maximum vertical displacement of the source from the Galactic plane never exceeds pc, with radial excursions between 7 kpc 9.5 kpc, and = 0.110.02. This orbit is consistent with membership in the Galactic thin disk population (Bochanski et al., 2007) suggesting that UGPS 072205 is a relatively young brown dwarf. However, we strongly caution the use of kinematic properties as an age discriminant, as they should only be considered in a statistical manner. Computing orbits for larger samples of MLTY dwarfs will help place the orbit of UGPS 072205 in a broader context.
3.3. Rotational Velocity
The rotational velocity of UGPS 072205 was also measured with a technique used by a number of previous studies (e.g., Reid & Mahoney, 2000; Mohanty & Basri, 2003; West & Basri, 2009). Briefly, the science object (UGPS 072205) is cross–correlated with a rotationally unbroadened template (the T = 500 K, = 5.0 model, from BT-Settl, Allard et al., 2010). The model is convolved with a Gaussian of 50 km s to approximate the effects of FIRE’s line spread function. This cross–correlation function (CCF) is compared to the CCFs derived from correlating the unbroadened template to rotationally broadened versions of itself. We constructed a series of rotating templates ranging in from 10 to 120 km s, in steps of 10 km s using the technique described in Gray (1992). In Figure 5, we compare the auto–correlation of the template against the CCFs of the = 30, 40 and 60 km s templates and the CCF of UGPS 072205 over the same wavelength range described in Section 3.2. While = 40 km s was the closest match to the CCF of UGPS 072205, the = 30 and 50 km s were good fits to most of the CCF trough (see Figure 5). Thus, we report a for UGPS 072205 is km s.
This rotation velocity is similar to those of late-L and T dwarfs (e.g., Zapatero Osorio et al., 2006; Reiners & Basri, 2008). Of the nine T dwarfs observed by Zapatero Osorio et al. (2006), only one brown dwarf (SDSSJ134646.45003150.4), had a under 20 km s and the authors speculated that this may be due to inclination333Using the binomial function formalism from Browning et al. (2010), there is a chance of observing one slow rotator ( km s) in a sample of nine stars, assuming they all rotate at 40 km s.. The remaining 8 T dwarfs demonstrated rotational velocities between 20 and 40 km s and this distribution does not vary significantly from the observed rotation velocities of L dwarfs (Reiners & Basri, 2008). The of UGPS 072205 reinforces the findings of previous studies suggesting that T dwarfs are inefficient at rotational braking. In solar–type stars, magnetic fields power two sources of angular momentum loss: disk braking and flaring events (Skumanich, 1972; Bouvier et al., 1997). Large–scale magnetic fields can form in convective, rotating low–mass objects (i.e., Browning, 2008) and have been observed in some late M and L dwarfs (Reiners & Basri, 2008; Hallinan et al., 2008), but remain undetected in T dwarfs (Berger, 2006). This may indicate the lack of magnetic fields in T dwarfs, or alternatively, a weak coupling between the fields and the predominately neutral atmosphere. This de-coupling would reduce disk braking and flare frequency (due to reconnection events), which would decrease angular momentum loss in T dwarfs compared to solar–type stars.
|Standard||RV (km s)||NotesaaAll RV measurements were computed in the band from 1.27 to 1.31 m.|
|T = 400 K||47.9||BT-Settl model from Allard et al. (2010)|
|T = 500 K||49.2||BT-Settl model from Allard et al. (2010)|
|T = 600 K||48.5||BT-Settl model from Allard et al. (2010)|
|T = 700 K||48.5||BT-Settl model from Allard et al. (2010)|
|2M0415-0935||45.6||Assuming 49.6 km s for template (Zapatero Osorio et al., 2007)|
|2M0559-1404||46.8||Assuming -13.8 km s for template (Zapatero Osorio et al., 2007)|
|2M1553+1532||41.8||Assuming -32.9 km s for template (Zapatero Osorio et al., 2007)|
|Mean RV||46.9||2.5 km s uncertainty|
3.4. Atmospheric Model Fits
To examine the physical properties of UGPS 072205, we compared its FIRE spectrum to the BT-Settl models of Allard et al. (2010). These models are based on the PHOENIX code (Hauschildt et al., 1999), and reflect an update to the original Settl models of Allard et al. (2003) with a microturbulence velocity field determined from 2D hydrodynamic models (Freytag et al., 2010) and updated solar abundances from Asplund et al. (2009). We adopted a set of solar-metallicity ( = 0.0) models sampling = 400–900 K in 100 K steps, and = 3.5–5.5 cm s in 0.5 cm s steps, with the exceptions that violate evolutionary parameters (e.g., 700 K and = 5.5).
Our fitting procedure was based on the formalism developed by Cushing et al. (2008), Bowler et al. (2009) and Burgasser et al. (2010). Model surface fluxes (in units) were smoothed to a common resolution of = 6000 using a Gaussian kernel, and both models and FIRE data were interpolated onto a common wavelength grid spanning 0.9 to 2.4 . The FIRE data were also scaled to the observed magnitude of UGPS 072205. We then performed eight separate fits to the data, encompassing the full spectral range (excluding regions of strong telluric absorption), the spectral peaks, and three “narrow” regions (0.04–0.12 in width) sampling strong molecular absorption (Table 3). Data and models were compared using a statistic, with the degrees of freedom equal to the number of resolution elements sampled. The optimal scaling factor minimizing was computed following Cushing et al. (2008), and is equivalent to where is the radius of the brown dwarf and its distance from the Sun (Bowler et al., 2009). Two sets of fits were done, one in which the distance was treated as a free parameter and one in which the model-derived distance must agree with the parallax measurement of Lucas et al. (2010) to within 5. We also allowed for variations in the radial (50 km s in steps of 1.25 km s about the reported value) and rotational velocities (0–100 km s in steps of 3 km s) of the model templates to find a minimum. Means and uncertainties in the atmospheric parameters (, ) and associated physical parameters (mass, age, and radius based on the evolutionary models of Baraffe et al. 2003) were determined using the F-test probability distribution function (F-PDF) as a weighting factor, as described in Burgasser et al. (2010). We also propagated sampling uncertainties of 50 K and 0.25 dex for and , respectively.
The best–fit models and data are plotted in Figure 6. The upper panel displays the best fit to the entire spectrum, with the distance limit enforced. There are significant deviations between the model and data in the bands, suggesting that there remain missing or incorrect molecular opacity in the BT-Settl model calculations. However, the agreement between the best–fit distance–restricted model and data improves in the “narrow” regions, as shown in Figure 6. Given the deviations between the models and data, caution is warranted in using the physical parameters listed in Table 3, however some general trends do emerge. First, we examined the relative effect of the distance restriction. The distance restricted fits prefer a cooler , 500 K instead of 700 K. The distance restricted fits also suggest a higher surface gravity and older age, but there is no clear behavior in the mass determination. We also note that most of the fits with unrestricted distances prefer distances much larger than the measured parallax.
We adopt the restricted distance and wavelength sets ( and ) for further discussion. These clipped wavelength sets were chosen to sample strong molecular absorption, mostly due to CH. Agreement between the data and model within these windows indicates that the BT-Settl models may have the proper opacity included in these windows, but due to the dearth of benchmark brown dwarfs (Konopacky et al., 2010, and references therein), the physical parameters derived from these fits may have large systematic uncertainties. These fits suggest an object with K, , mass and an age of Gyr. These parameters agree with the results of Lucas et al. (2010), who reported K, , mass and an age of Gyr. To examine the accuracy in other bandpasses, synthetic photometry was computed in the IRAC and WISE bands reported in Table 1 using the K, = 4.0 model and reported parallax. In general, the synthetic photometry matched the reported values within 1 mag, and agreed within mag for the and bandpasses, suggesting the calculated opacities in these filters may be correct. We note that the region produced identical results with and without the distance restriction, suggesting that the BT-Settl models may perform well in this wavelength range. However, adopting the uncertainties from the physical parameters from only this range probably underestimates the systematic errors, especially since only one set of models was considered.
|Fit Ranges ()||0.9–2.35||0.9–1.15||1.15–1.35||1.27–1.31||1.45–1.8||1.50–1.57,||1.9–2.35||2.1–2.18|
|No Distance Restriction|
We have presented an analysis of FIRE observations of one of the coldest brown dwarfs known, the T9 infrared spectral standard UGPS 072205444The FIRE spectrum is available online at http://personal.psu.edu/jjb29/0722.html.. Using current line lists and atmospheric models, we characterized the NIR SED and constrained the physical parameters of UGPS 072205. At the resolutions achieved with FIRE, we are able to identify individual molecular features throughout the spectrum. As more cold brown dwarfs are discovered through new surveys such as WISE (Wright et al., 2010), the Canada–France Brown Dwarf Survey (CFBDS; Albert et al., 2011) and VISTA (Irwin et al., 2004), these features may be useful in determining fundamental parameters and discriminating between spectral types. Unfortunately, the BT-Settl model atmospheres do not adequately reproduce the spectral features observed at these low temperatures across the NIR regime, although fits over restricted regions are more robust. By limiting our analysis to the wavelength and distance restricted fits, we derive a of K, of , mass of and age of Gyr. These values agree well with the results from Lucas et al. (2010), however the data should be re–examined as models are further refined.
The radial velocity of UGPS 072205 was measured as 46.9 km s to a precision of a few km s. Combined with parallax and proper motion measurements from Lucas et al. (2010), the Galactic orbit of UGPS 072205 was computed to investigate its parent population. Its orbit is similar to many thin disk objects, exhibiting low eccentricity and vertical excursions taking it only 60 pc away from the plane. This orbit also agrees well with the age of Gyr derived from the atmospheric fits. As larger catalogs of cold brown dwarfs with well measured kinematic properties are assembled, their ensemble properties will be important for testing the predictions of Milky Way kinematic structure models (i.e. Roškar et al., 2008; Loebman et al., 2011).
The rotational velocity of UGPS 072205 was also measured, employing the BT-Settl atmospheric model as a template. The object is rotating at 40 10 km s, similar to other late–type L and T dwarfs and further evidence that rotational braking is not efficient in brown dwarfs (Zapatero Osorio et al., 2006; Reiners & Basri, 2008). In future investigations, we will secure observations of brown dwarfs with small projected rotational velocities, such as SDSSJ134646.45003150.4 (Zapatero Osorio et al., 2006) to serve as empirical templates.
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