Very Low Mass Stars and Brown Dwarfs in Taurus-Auriga

By Russel J. White, G. Basri. 2002

459 citations recorded by the source corpus.

We present high-resolution optical spectra obtained with the HIRES spectrograph on the W. M. Keck I Telescope of seven low-mass T Tauri stars (LMTTs) and brown dwarfs in Taurus-Auriga. The observed Li I 6708 Å absorption, low surface gravity signatures, and radial velocities confirm that all are members of the Taurus star-forming region; no new spectroscopic binaries are identified. Four of the seven targets observed appear to be T Tauri brown dwarfs. Of particular interest is the previously classified "continuum T Tauri star" GM Tau, which has a spectral type of M6.5 and possibly a substellar mass. These spectra, in combination with previous high-resolution spectra of LMTTs, are used to understand the formation and early evolution of objects in Taurus-Auriga with masses near and below the stellar/substellar boundary. None of the LMTTs in Taurus are rapidly rotating (v sin i < 30 km s-1), unlike low-mass objects in Orion. Many of the slowly rotating, nonaccreting stars and brown dwarfs exhibit prominent Hα emission (equivalent widths of 3-36 Å), indicative of active chromospheres. We demonstrate empirically that the full width at 10% of the Hα emission profile peak is a more practical and possibly more accurate indicator of accretion than either the equivalent width of Hα or optical veiling: 10% widths > 270 km s-1 are classical T Tauri stars (i.e., accreting), independent of stellar spectral type. Although LMTTs can have accretion rates comparable to that of more typical, higher mass T Tauri stars (e.g., K7-M0 spectral types), the average mass accretion rate appears to decrease with decreasing mass. A functional form of Ṁ ∝ M is consistent with the available data, but the dependence is difficult to establish because of both selection biases in observed samples and the decreasing frequency of active accretion disks at low masses (M < 0.2 M☉). The diminished frequency of accretion disks for LMTTs, in conjunction with their lower, on average, mass accretion rates, implies that they are formed with less massive disks than higher mass T Tauri stars. The radial velocities, circumstellar properties, and known binaries do not support the suggestion that many of the lowest mass members of Taurus have been ejected from higher stellar density regions within the cloud. Instead, LMTTs appear to have formed and are evolving in the same way as higher mass T Tauri stars, but with smaller disks and shorter disk lifetimes.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/461aa44db889f4b57fd27c04632af60ebd029c08

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In Astronomy and Astrophysics

1. K. Luhman
2. L. Hillenbrand
3. Eric Mamajek
4. G. Herczeg
5. N. Evans II
6. A. Kraus
7. L. Hartmann
8. Michael C. Liu

In Instrumentation

1. K. Luhman
2. L. Hillenbrand
3. Michael C. Liu
4. Eric Mamajek
5. A. Kraus
6. G. Herczeg
7. N. Evans II
8. A. Reiners

Citations in the corpus

• Evidence for a T Tauri Phase in Young Brown Dwarfs (2003, methodology): “Following White & Basri (2003), we used a combination of giant and dwarf spectra for the template (see detailed discussion in Mohanty & Basri 2003 and Mohanty et al. 2003a).”
• Spectroscopic Properties of Cool Stars in the Sloan Digital Sky Survey: An Analysis of Magnetic Activity and a Search for Subdwarfs (2004, background): “…integration to sum the flux under the emission line, taking care to avoid and/or account for regions that are affected by local TiO features including the continuum bump just to the blue of H (Pettersen & Coleman 1981) and the absorption trough to the red (Tinney & Reid 1998; White & Basri 2003).”
• A REVISED AGE FOR UPPER SCORPIUS AND THE STAR FORMATION HISTORY AMONG THE F-TYPE MEMBERS OF THE SCORPIUS–CENTAURUS OB ASSOCIATION (2011, background): “White & Basri (2003) find that independent of spectral type, a full width at 10% peak > 270 km s−1 is a good indicator of accretion.”
• ACCRETION RATES FOR T TAURI STARS USING NEARLY SIMULTANEOUS ULTRAVIOLET AND OPTICAL SPECTRA (2013, methodology): “Both the equivalent width of Hα, EW(Hα), and the width of Hα at 10% of the maximum flux are often used as accretion proxies (White & Basri 2003; Natta et al. 2004).”
• CARMENES input catalogue of M dwarfs. I. Low-resolution spectroscopy with CAFOS (2015, background): “Four of them lie at the boundary between chromospheric and accretion emission, as defined by Barrado y Navascués & Martı́n (2003), White & Basri (2003), and Fang (2010).”
• Accretion in the ρ-Ophiuchi pre-main sequence stars (2006, methodology): “The results are from Gullbring et al. (1998), Muzerolle et al. (1998b, 2003, 2005), White & Ghez (2001), White & Basri (2003), and Calvet et al. (2004); note that, for homogeneity, we have redetermined M using DM98 tracks for all objects.”
• The Spitzer c2d Survey of Weak-Line T Tauri Stars. II. New Constraints on the Timescale for Planet Building (2007, background): “…likely to be driven by the presence of weakly accreting stars in our sample of wTTs. Muli-epoch high-resolution spectroscopy of the sample of wTTs with disks would be highly desirable to establish, from their Hα velocity profiles, which objects are in fact actively accreting (White & Basri 2003).”
• Measuring Accretion in Young Substellar Objects: Approaching the Planetary Mass Regime (2005, background): “…brown dwarfs has indeed provided considerable evidence for magnetically mediated disk accretion, including broad permitted emission-line profiles (especially H ) and optical continuum veiling (White & Basri 2003, hereafter WB03; Muzerolle et al. 2003b, hereafter, M03; Jayawardhana et al. 2003b).”
• Accretion in Young Stellar/Substellar Objects (2003, background): “Radial and Rotational Velocities We determined v sin i values for the HIRES sample only—the resolution of ESI is too low for any meaningful limits, given typical values for low-mass young stars (see, e.g., Hartmann et al. 1986; White & Basri 2003, hereafter WB03).”
• IDENTIFYING THE YOUNG LOW-MASS STARS WITHIN 25 PC. I. SPECTROSCOPIC OBSERVATIONS (2009, methodology): “…2MASS J2234, 2MASS J1553, and 2MASS J0335 are accreting T Tauri stars while 2MASS J0557 is on the borderline, with upper limits on the ages of 5, 3, 10, and 10 Myr, respectively, based on the criteria devised by White & Basri (2003), Mohanty et al. (2005), and Barrado y Navascués & Martı́n (2003).”
• Galactic Stellar and Substellar Initial Mass Function (2003, background): “Such a scenario raises also other problems including the BD radial velocity dispersion, binary frequency and circumstellar properties (see e.g. Joergens & Guenther 2001, Bate et al. 2003, Close et al. 2003, White & Basri 2003).”
• Theory of Star Formation (2007, background): “A recent compilation of observations (White & Basri, 2003; Muzerolle et al., 2003; Calvet et al., 2004) shows an approximate dependence of the accretion rate on stellar mass Ṁdisk ∝ m2∗, although with considerable scatter (Muzerolle et al., 2005).”
• Star and Protoplanetary Disk Properties in Orion's Suburbs (2009, methodology): “Power law fits of the formṀacc∝Mα∗ have yielded values in the range 1.0−2.1 for the exponentα (White & Basri 2003; Muzerolle et al. 2003; Calvet et al. 2004; Muzerolle et al. 2005; Mohanty et al. 2005; Natta et al. 2006; Garcia Lopez et al. 2006; Herczeg & Hillenbrand 2008; Gatti et al. 2008).”
• On the formation of Hα line emission around classical T Tauri stars (2006, methodology): “Using the criteria of White & Basri (2003), half of the models shown in the figure can be classified as CTTS, and the other half as WTTS.”