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The Astrophysical Journal | 2003

The NIRSPEC Brown Dwarf Spectroscopic Survey. I. Low-Resolution Near-Infrared Spectra

Ian S. McLean; Mark R. McGovern; Adam J. Burgasser; J. Davy Kirkpatrick; L. Prato; Sungsoo S. Kim

We present the first results of a near-infrared (0.96-2.31 μm) spectroscopic survey of M, L, and T dwarfs obtained with NIRSPEC on the Keck II telescope. Our new survey has a resolving power of R = λ/Δλ ~ 2000 and is comprised of two major data sets: 53 J-band (1.14-1.36 μm) spectra covering all spectral types from M6 to T8 with at least two members in each spectral subclass (wherever possible), and 25 flux-calibrated spectra from 1.14 to 2.31 μm for most spectral classes between M6 and T8. Sixteen of these 25 objects have additional spectral coverage from 0.96 to 1.14 μm to provide overlap with optical spectra. Spectral flux ratio indexes for prominent molecular bands are derived, and equivalent widths (EWs) for several atomic lines are measured. We find that a combination of four H2O and two CH4 band strengths can be used for spectral classification of all these sources in the near-infrared and that the H2O indexes are almost linear with spectral type from M6 to T8. The H2O indexes near 1.79 and 1.96 μm should remain useful beyond T8. In the near-infrared a notable feature at the boundary between the M and L types is the disappearance of relatively weak (EW ~ 1-2 A) atomic lines of Al I and Ca I, followed by Fe I around L2. At the boundary between L and T dwarfs it is the appearance of CH4 in all near-infrared bands (J, H, and K) that provides a significant spectral change, although we find evidence of CH4 as early as L7 in the K band. The FeH strength and the equivalent width of the K I lines are not monotonic, but in combination with other factors provide useful constraints on spectral type. The K I lines are sensitive to surface gravity. The CO band strength near 2.30 μm is relatively insensitive to spectral class. The peak calibrated flux (Fλ) in the 0.96-2.31 μm region occurs near 1.10 μm at M6 but shifts to about 1.27 μm at T8. In addition, the relative peak flux in the J, H, and K bands is always in the sense J > H > K except around L6, where the differences are small. One object, 2MASS 2244+20 (L6.5), shows normal spectral behavior in the optical but has an infrared spectrum in which the peak flux in J band is less than at H and K.


The Astrophysical Journal | 2004

IDENTIFYING YOUNG BROWN DWARFS USING GRAVITY-SENSITIVE SPECTRAL FEATURES

Mark R. McGovern; J. Davy Kirkpatrick; Ian S. McLean; Adam J. Burgasser; L. Prato; Patrick J. Lowrance

We report the initial results of the Brown Dwarf Spectroscopic Survey Gravity Project to study gravity sensitive features as indicators of youth in brown dwarfs. Low-resolution (R ~ 2000) J-band and optical (R ~ 1000) observations using NIRSPEC and LRIS at the W. M. Keck Observatory reveal transitions of TiO, VO, K I, Na I, Cs I, Rb I, CaH, and FeH. By comparing these features in late-type giants and in old field dwarfs, we show that they are sensitive to the gravity (g = GM/R2) of the object. Using low-gravity spectral signatures as age indicators, we observed and analyzed J-band and optical spectra of two young brown dwarfs, G196-3B (20-300 Myr) and KPNO Tau 4 (1-2 Myr) and two possible low-mass brown dwarfs in the σ Orionis cluster (3-7 Myr). We report the identification of the bands of TiO near 1.24 μm and the A-X band of VO near 1.18 μm together with extremely weak J-band lines of K I in KPNO Tau 4. This is the first detection of TiO and VO in the J band in a substellar-mass object. The optical spectrum of KPNO Tau 4 exhibits weak K I and Na I lines, weak absorption by CaH, and strong VO bands, also signatures of a lower gravity atmosphere. G196-3B shows absorption features in both wavelength regions, like those of KPNO Tau 4, suggesting that its age and mass are at the lower end of published estimates. Whereas σ Ori 51 appears to be consistent with a young substellar object, σ Ori 47 shows signatures of high gravity most closely resembling an old L1.5/L0 and cannot be a member of the σ Orionis cluster.


The Astrophysical Journal | 2003

The First Substellar Subdwarf? Discovery of a Metal-poor L Dwarf with Halo Kinematics

Adam J. Burgasser; J. Davy Kirkpatrick; Adam Burrows; James Liebert; I. Neill Reid; John E. Gizis; Mark R. McGovern; L. Prato; Ian S. McLean

We present the discovery of the first L-type subdwarf, 2MASS J05325346+8246465. This object exhibits enhanced collision-induced H2 absorption, resulting in blue near-infrared (NIR) colors (J-Ks = 0.26 ± 0.16). In addition, strong hydride bands in the red optical and NIR, weak TiO absorption, and an optical/J-band spectral morphology similar to the L7 DENIS 0205-1159AB imply a cool, metal-deficient atmosphere. We find that 2MASS 0532+8246 has both a high proper motion, μ = 260 ± 015 yr-1, and a substantial radial velocity, vrad = -195 ± 11 km s-1, and its probable proximity to the Sun (d = 10-30 pc) is consistent with halo membership. Comparison to subsolar-metallicity evolutionary models strongly suggests that 2MASS 0532+8246 is substellar, with a mass of 0.077 M 0.085 M☉ for ages 10-15 Gyr and metallicities Z = 0.1-0.01 Z☉. The discovery of this object clearly indicates that star formation occurred below the hydrogen burning mass limit at early times, consistent with prior results indicating a flat or slightly rising mass function for the lowest mass stellar subdwarfs. Furthermore, 2MASS 0532+8246 serves as a prototype for a new spectral class of subdwarfs, additional examples of which could be found in NIR proper-motion surveys.


The Astrophysical Journal | 2003

Astrophysics of Young Star Binaries

L. Prato; Thomas P. Greene; M. Simon

This paper describes our study of the astrophysics of individual components in close pre-main-sequence binaries. We observed both stars in 17 systems, located in four nearby star-forming regions, using low-resolution (R = 760) infrared spectroscopy and photometry. For 29 components we detected photospheric absorption lines and were able to determine spectral type, extinction, K-band excess, and luminosity. The other five objects displayed featureless or pure emission line spectra. In ~50% of the systems, the extinction and K-band excess of the primary stars dominate those of the secondaries. Masses and ages were determined for these 29 objects by placing them on the H-R diagram, overlaid with theoretical pre-main-sequence tracks. Most of the binaries appear to be coeval. The ages span 5 ? 105 to 1 ? 107 yr. The derived masses range from the substellar, 0.06 M?, to 2.5 M?, and the mass ratios from M2/M1 = 0.04 to 1.0. Fourteen stars show evidence of circumstellar disks. The K-band excess is well correlated with the K-L color for stars with circumstellar material.


The Astrophysical Journal | 2002

Stellar Companions to Stars with Planets

J. Patience; R. White; Andrea M. Ghez; Caer McCabe; Ian S. McLean; James E. Larkin; L. Prato; Sungsoo S. Kim; James P. Lloyd; Michael C. Liu; James R. Graham; Bruce A. Macintosh; Donald T. Gavel; Claire E. Max; Brian J. Bauman; Scot S. Olivier; Peter L. Wizinowich; D. S. Acton

A combination of high-resolution and wide-field imaging reveals two binary stars and one triple star system among the sample of the first 11 stars with planets detected by radial velocity variations. High-resolution speckle or adaptive optics (AO) data probe subarcsecond scales down to the diffraction limit of the Keck 10 m or the Lick 3 m, and direct images or AO images are sensitive to a wider field, extending to 10 or 38, depending on the camera. One of the binary system—HD 114762—was not previously known to be a spatially resolved multiple system; additional data taken with the combination of Keck adaptive optics and NIRSPEC are used to characterize the new companion. The second binary system—τ Boo—was a known multiple with two conflicting orbital solutions; the current data will help constrain the discrepant estimates of periastron time and separation. Another target—16 Cyg B—was a known common proper motion binary, but the current data resolve a new third component, close to the wide companion 16 Cyg A. Both the HD 114762 and 16 Cyg B systems harbor planets in eccentric orbits, while the τ Boo binary contains an extremely close planet in a tidally circularized orbit. Although the sample is currently small, the proportion of binary systems is comparable to that measured in the field over a similar separation range. Incorporating the null result from another companion search project lowers the overall fraction of planets in binary systems, but the detections in our survey reveal that planets can form in binaries separated by less than 50 AU.


The Astrophysical Journal | 2006

Investigating Disk Evolution: A High Spatial Resolution Mid-Infrared Survey of T Tauri Stars

Caer McCabe; Andrea M. Ghez; L. Prato; Gaspard Duchene; Robert Scott Fisher; Charles M. Telesco

We present a high spatial resolution, 10-20 μm survey of 65 T Tauri binary stars in Taurus, Ophiuchus, and Corona Australis using the Keck 10 m telescopes. Designed to probe the inner ~1 AU region of the circumstellar disks around the individual stellar components in these binary systems, this study increases the number of binaries with spatially resolved measurements at 10 μm by a factor of ~5. Combined with resolved near-infrared photometry and spectroscopic accretion diagnostics, we find that ~10% of stars with a mid-infrared excess do not appear to be accreting. In contrast to an actively accreting disk system, these passive disks have significantly lower near-infrared colors that are, in most cases, consistent with photospheric emission, suggesting the presence of an inner disk hole. In addition, there appears to be a spectral type/mass dependence associated with the presence of a passive disk, with all passive disks occurring around M-type stars. The presence of a passive disk does not appear to be related to the fact that these objects are in visual binary systems; the passive disk systems span the entire range of binary separations present in the sample, and a similar fraction of passive disks is observed in a sample of single stars. The possibility that the passive disks are caused by the presence of an as yet undetected companion at a small separation (0.3-3 AU) is possible for any individual system; however, it cannot account for the spectral type dependence of the passive disk sample as a whole. We propose that these passive disks represent a subset of T Tauri stars that are undergoing significant disk evolution. The fraction of observed passive disks and the observed spectral type dependence can both be explained by models of disk evolution that include disk photoevaporation from the central star.


The Astrophysical Journal | 2000

Deep 10 and 18 Micron Imaging of the HR 4796A Circumstellar Disk: Transient Dust Particles and Tentative Evidence for a Brightness Asymmetry

Charles M. Telesco; R. S. Fisher; Robert K. Pina; R. F. Knacke; Stanley F. Dermott; Mark C. Wyatt; K. Grogan; Elizabeth Katherine Holmes; Andrea M. Ghez; L. Prato; Lee Hartmann; Ray Jayawardhana

We present new 10.8 and 18.2 km images of HR 4796A, a young A0 V star that was recently dis- covered to have a spectacular, nearly edge-on, circumstellar disk prominent at D20 km (Jayawardhana and coworkers ; Koerner and coworkers). These new images, obtained with OSCIR (the University of Florida Observatory Spectrometer/Camera for the Infrared) at Keck II, show that the disks size at 10 km is comparable to its size at 18 km. Therefore, the 18 kmemitting dust may also emit some, or all, of the 10 km radiation. Using these multiwavelength images, we determine a ii characteristic ˇˇ diameter of 2¨3 km for the mid-infraredemitting dust particles if they are spherical and composed of astronomical silicates. Particles this small are expected to be blown out of the system by radiation pressure in a few hundred years, and therefore these particles are unlikely to be primordial. Rather, as inferred in a com- panion paper (Wyatt and coworkers), they are probably products of collisions that dominate both the creation and the destruction of dust in the HR 4796A disk. Dynamical modeling of the disk, the details of which are presented in the companion paper, indicates that the disk surface density is relatively sharply peaked near 70 AU, which agrees with the mean annular radius deduced by Schneider and coworkers from their NICMOS images. Interior to 70 AU, the model density drops steeply by a factor of 2 between 70 and 60 AU, falling to zero by 45 AU, which corresponds to the edge of the previously discovered central hole ; in the context of the dynamical models, this ii soft ˇˇ edge for the central hole occurs because the dust particle orbits are noncircular. The optical depth of mid-infraredemitting dust in the hole is D3% of the optical depth in the disk, and the hole is therefore relatively very empty. We present evidence (D1.8p signi—cance) for a brightness asymmetry that may result from the presence of the hole and the gravitational perturbation of the disk particle orbits by the low-mass stellar companion or a planet. This ii pericenter glow,ˇˇ which must still be con—rmed, results from a very small (a few AU) shift of the disks center of symmetry relative to the central star HR 7496A ; one side of the inner bound- ary of the annulus is shifted toward HR 4796A, thereby becoming warmer and more infrared-emitting. The possible detection of pericenter glow implies that the detection of even complex dynamical eUects of planets on disks is within reach. Subject headings : circumstellar matterinfrared : starsstars : individual (HR 4796A)


The Astrophysical Journal | 2003

The Mass Ratio Distribution in Main-Sequence Spectroscopic Binaries Measured by Infrared Spectroscopy

Tsevi Mazeh; Michal Simon; L. Prato; B. Markus; Shay Zucker

We report infrared spectroscopic observations of a large well-defined sample of main-sequence, single-lined spectroscopic binaries to detect the secondaries and derive the mass ratio distribution of short-period binaries. The sample consists of 51 Galactic disk spectroscopic binaries found in the Carney and Latham high proper motion survey, with primary masses in the range 0.6-0.85 M☉. Our infrared observations detect the secondaries in 32 systems, two of which have mass ratios, q = M2/M1, as low as ~0.20. Together with 11 systems previously identified as double-lined binaries by visible light spectroscopy, we have a complete sample of 62 binaries, of which 43 are double lined. The mass ratio distribution is approximately constant over the range q = 1.0-0.3. The distribution appears to rise at lower q values, but the uncertainties are sufficiently large that we cannot rule out a distribution that remains constant. The mass distribution derived for the secondaries in our sample and that of the extrasolar planets apparently represent two distinct populations.


The Astrophysical Journal | 2002

The Smallest Mass Ratio Young Star Spectroscopic Binaries

L. Prato; Michal Simon; Tsevi Mazeh; Ian S. McLean; D. Norman; Shay Zucker

Using high-resolution near-infrared spectroscopy with the Keck Telescope, we have detected the radial velocity signatures of the cool secondary components in four optically identified pre-main-sequence, single-lined spectroscopic binaries. All are weak-lined T Tauri stars with well-defined center-of-mass velocities. The mass ratio for one young binary, NTTS 160905-1859, is M2/M1 = 0.18 ± 0.01, the smallest yet measured dynamically for a pre-main-sequence spectroscopic binary. These new results demonstrate the power of infrared spectroscopy for the dynamical identification of cool secondaries. Visible-light spectroscopy, to date, has not revealed any pre-main-sequence secondary stars with masses <0.5 M☉, while two of the young systems reported here are in that range. We compare our targets with a compilation of the published young, double-lined spectroscopic binaries and discuss our unique contribution to this sample.


The Astrophysical Journal | 2004

S ORIONIS 70: JUST A FOREGROUND FIELD BROWN DWARF?

Adam J. Burgasser; J. Davy Kirkpatrick; Mark R. McGovern; Ian S. McLean; L. Prato; I. Neill Reid

We examine recent claims that the T-type brown dwarf S Ori 053810.1� 203626 (S Ori 70) is a spectro- scopically verified low-mass (3 þ5 � 1 MJup )m ember of the 1-8 MyrOrionis cluster. Comparative arguments by Martin & Zapatero Osorio asserting that S Ori 70 exhibits low surface gravity spectral features indicative of youth and low mass are invalidated by the fact that their comparison object was not the field T dwarf 2MASS 0559� 1404, but rather a nearby background star. Instead, we find that the 1-2.5 � m spectra of S Ori 70 are well matched to older (agefew Gyr) field T6-T7 dwarfs. Moreover, we find that spectral model fits to late- type field T dwarf spectra tend to yield low surface gravities (log g ¼ 3:0 3:5), and thus young ages (P 5M yr) and low masses (P3 MJup), inconsistent with expected and/or empirical values. Finally, we show that the identification of one T dwarf in the field imaged by Zapatero Osorio et al. is statistically consistent with the expected foreground contamination. Based on the reexamined evidence, we conclude that S Ori 70 may simply be an old, massive (30-60 MJup) field brown dwarf lying in the foreground of theOrionis cluster. This interpretation should be considered before presuming the existence of so-called cluster planets. Subject headings: open clusters and associations: individual (� Orionis) — planetary systems: formation — stars: formation — stars: individual (S Orionis 70) — stars: low-mass, brown dwarfs — techniques: spectroscopic

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Ian S. McLean

University of California

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J. Davy Kirkpatrick

California Institute of Technology

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Miriani Griselda Pastoriza

Universidade Federal do Rio Grande do Sul

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Andrea M. Ghez

University of California

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Alberto Rodriguez-Ardila

National Council for Scientific and Technological Development

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