A Test of Pre-Main-Sequence Evolutionary Models across the Stellar/Substellar Boundary Based on Spectra of the Young Quadruple GG Tauri

We present spatially separated optical spectra of the components of the young hierarchical quadruple GG Tau. Spectra of GG Tau Aa and Ab (separation 025 ~ 35 AU) were obtained with the Faint Object Spectrograph on board the Hubble Space Telescope. Spectra of GG Tau Ba and Bb (separation 148 ~ 207 AU) were obtained with both the HIRES and the LRIS spectrographs on the W. M. Keck telescopes. The components of this minicluster, which span a wide range in spectral type (K7-M7), are used to test both evolutionary models and the temperature scale for very young, low-mass stars under the assumption of coeval formation. Of the evolutionary models tested, those of Baraffe et al. yield the most consistent ages when combined with a temperature scale intermediate between that of dwarfs and giants. The version of the Baraffe et al. models computed with a mixing length nearly twice the pressure scale height is of particular interest, as it predicts masses for GG Tau Aa and Ab that are in agreement with their dynamical mass estimate. Using this evolutionary model and a coeval (at 1.5 Myr) temperature scale, we find that the coldest component of the GG Tau system, GG Tau Bb, is substellar with a mass of 0.044 ± 0.006 M☉. This brown dwarf companion is especially intriguing as it shows signatures of accretion, although this accretion is not likely to alter its mass significantly. GG Tau Bb is currently the lowest mass, spectroscopically confirmed companion to a T Tauri star, and is one of the coldest, lowest mass T Tauri objects in the Taurus-Auriga star-forming region.

[1]  F. Vrba,et al.  X-ray sources in regions of star formation. III - Naked T Tauri stars associated with the Taurus-Auriga complex , 1988 .

[2]  G. Rieke,et al.  The Low-Mass Initial Mass Function in Young Clusters: L1495E , 1998 .

[3]  L. Hartmann,et al.  How to unveil a t Tauri star , 1989 .

[4]  Harland W. Epps,et al.  THE KECK LOW-RESOLUTION IMAGING SPECTROMETER , 1995 .

[5]  S. Beckwith,et al.  Gas and Dust in the Pre--Main-Sequence Multiple System GG Tauri , 1993 .

[6]  F. Rogers,et al.  Radiative atomic Rosseland mean opacity tables , 1992 .

[7]  M. Giampapa,et al.  Rotational Velocities and Chromospheric/Coronal Activity of Low-Mass Stars in the Young Open Clusters IC 2391 and IC 2602 , 1997 .

[8]  Spectroscopy of Brown Dwarf Candidates in the ρ Ophiuchi Molecular Core , 1998, astro-ph/9810508.

[9]  A. Boss.  Collapse and Fragmentation of Molecular Cloud Cores. IV. Oblate Clouds and Small Cluster Formation , 1996 .

[10]  G. Marcy,et al.  A surprise at the bottom of the main sequence: Rapid rotation and NO H-alpha emission , 1995 .

[11]  Graham Berriman,et al.  Infrared Spectra of Low-Mass Stars: Toward a Temperature Scale for Red Dwarfs , 1996 .

[12]  M. Schwartz,et al.  Discovery of Seven T Tauri Stars and a Brown Dwarf Candidatein the Nearby TW Hydrae Association , 1998, astro-ph/9812189.

[13]  Nicholas B. Suntzeff,et al.  The Pre-Main-Sequence Eclipsing Binary TY Coronae Australis: Precise Stellar Dimensions and Tests of Evolutionary Models , 1998 .

[14]  L. Hartmann,et al.  A Young Star near the Hydrogen-burning Limit , 1998 .

[15]  M. Irwin,et al.  Ultra-Cool M Dwarfs Discovered by QSO Surveys.I: The APM Objects , 1997 .

[16]  G. Rieke,et al.  The interstellar extinction law from 1 to 13 microns. , 1985 .

[17]  L. Hartmann,et al.  A Search for Very Low Mass Pre-Main-Sequence Stars in Taurus , 1998 .

[18]  J. Elias.  A study of the Taurus dark cloud complex , 1978 .

[19]  I. Mazzitelli,et al.  Further improvements of a new model for turbulent convection in stars , 1992 .

[20]  Todd J. Henry,et al.  A Standard Stellar Spectral Sequence in the Red/Near-Infrared: Classes K5 to M9 , 1991 .

[21]  C. Bertout,et al.  Accretion disks around T Tauri stars. II: Balmer emission , 1989 .

[22]  M. Bessell,et al.  Subdwarf studies. IV - Abundance ratios in extremely metal-deficient stars , 1991 .

[23]  Neill Reid,et al.  Low-mass stars in the Hyades , 1993 .

[24]  M. Osorio,et al.  Brown dwarfs and extrasolar planets , 1998 .

[25]  I. Bonnell,et al.  Fragmentation of elongated cylindrical clouds. VI: Comparison with observations , 1993 .

[26]  G. Basri,et al.  Hamilton echelle spectra of young stars. I: Optical veiling , 1990 .

[27]  David R. Alexander,et al.  Low-Temperature Rosseland Opacities , 1975 .

[28]  M. Couture,et al.  HIRES: the high-resolution echelle spectrometer on the Keck 10-m Telescope , 1994, Astronomical Telescopes and Instrumentation.

[29]  P. Hartigan,et al.  Disk Accretion and Mass Loss from Young Stars , 1995 .

[30]  Alycia J. Weinberger,et al.  Speckle Imaging Measurements of the Relative Tangential Velocities of the Components of T Tauri Binary Stars , 1995 .

[31]  Italo Mazzitelli,et al.  New pre-main-sequence tracks for M less than or equal to 2.5 solar mass as tests of opacities and convecti on model , 1994 .

[32]  Suzanne L. Hawley,et al.  Brown Dwarfs in the Hyades and Beyond? , 1998, astro-ph/9811347.

[33]  Oasa,et al.  Isolated and companion young brown dwarfs in the taurus and chamaeleon molecular clouds , 1998, Science.

[34]  S. Holfeltz,et al.  Measurement of T Tauri Binaries Using the Hubble Space Telescope Fine Guidance Sensors , 1996 .

[35]  L. Hartmann,et al.  Optical veiling, disk accretion, and the evolution of T Tauri stars , 1990 .

[36]  F. Comerón,et al.  ROSAT X-ray detection of a young brown dwarf in the chamaeleon I dark cloud , 1998, Science.

[37]  Andrea M. Ghez,et al.  High Spatial Resolution Imaging of Pre-Main-Sequence Binary Stars: Resolving the Relationship between Disks and Close Companions , 1997 .

[38]  Forrest J. Rogers,et al.  Updated Opal Opacities , 1996 .

[39]  I. Reid.  Glucocorticoid Osteoporosis , 1999, The Journal of asthma : official journal of the Association for the Care of Asthma.

[40]  C. Lada,et al.  Low-Mass Star Formation and the Initial Mass Function in IC 348 , 1998 .

[41]  L. Hartmann,et al.  An Age Estimate for the beta Pictoris Analog HR 4796A , 1995 .

[42]  D. Popper.  A Pre--Main-Sequence Star in the Detached Binary EK Cephei , 1987 .

[43]  L. Hartmann,et al.  Additional measurements of pre-main-sequence stellar rotation , 1989 .

[44]  J. Klapp,et al.  Collapse and Fragmentation Models of Prolate Molecular Cloud Cores. I. Initial Uniform Rotation , 1997 .

[45]  IPAC,et al.  A Candidate Substellar Companion to CD –33°7795 (TWA 5) , 1999 .

[46]  E. Martín,et al.  Weak and Post-T Tauri Stars around B-Type Members of the Scorpius-Centaurus OB Association , 1998 .

[47]  Kevin T. C. Jim,et al.  Adaptive Optics Imaging of GG Tauri: Optical Detection of the Circumbinary Ring , 1996 .

[48]  L. Hillenbrand.  On the Stellar Population and Star-Forming History of the Orion Nebula Cluster , 1997 .

[49]  L. Hartmann,et al.  Pre-Main-Sequence Evolution in the Taurus-Auriga Molecular Cloud , 1995 .

[50]  T. Guillot,et al.  A Nongray Theory of Extrasolar Giant Planets and Brown Dwarfs , 1997, astro-ph/9705201.

[51]  J. Liebert,et al.  Spectroscopy of a Young Brown Dwarf in the ρ Ophiuchi Cluster , 1997 .

[52]  L. Hartmann,et al.  A new optical extinction law and distance estimate for the Taurus-Auriga molecular cloud , 1994 .

[53]  F. Rogers,et al.  The Hyades lithium problem revisited , 1994 .

[54]  David Montes,et al.  Library of high and mid-resolution spectra in the Ca ii H K?? , 1996 .

[55]  James Liebert,et al.  M dwarf spectra from 0.6 to 1.5 micron - A spectral sequence, model atmosphere fitting, and the temperature scale , 1993 .

[56]  Italo Mazzitelli,et al.  Evolution of low mass stars , 1997 .

[57]  M. Bessell,et al.  JHKLM PHOTOMETRY: STANDARD SYSTEMS, PASSBANDS, AND INTRINSIC COLORS , 1988 .

[58]  David G. Monet,et al.  U.S. Naval Observatory CCD parallaxes of faint stars. I - Program description and first results , 1992 .

[59]  R. Rebolo,et al.  Spectroscopy of New Substellar Candidates in the Pleiades: Toward a Spectral Sequence for Young Brown Dwarfs , 1996 .

[60]  Christopher M. Johns-Krull,et al.  The Spectral Variability of the T Tauri Star DF Tauri , 1997 .

[61]  Beatriz Barbuy,et al.  The Near-infrared Nai Doublet Feature in M Stars * , 1996 .

[62]  L. Hartmann,et al.  Disk Accretion Rates for T Tauri Stars , 1998 .

[63]  Stefano Casertano,et al.  THE PHOTOMETRIC PERFORMANCE AND CALIBRATION OF WFPC2 , 1995 .

[64]  L. Hartmann,et al.  Optical excess emission in T Tauri stars , 1991 .

[65]  Forrest J. Rogers,et al.  Spin-Orbit Interaction Effects on the Rosseland Mean Opacity , 1992 .

[66]  M. Skrutskie,et al.  Correlations of Optical and Infrared Excesses in T Tauri Stars , 1990 .

[67]  J. Kirkpatrick,et al.  Low Mass Companions to Nearby Stars: Spectral Classification and its Relation to the Stellar/Substellar Break , 1994 .