arXiv subjects

S. K. Leggett

Publications and source records attributed to S. K. Leggett.

At least 91 records · Page 5Linked to original sources

arXiv2007

The Late-T Dwarf Companion to the Exoplanet Host Star HD 3651: A New Benchmark for Gravity and Metallicity Effects in Ultracool Spectra

(Abridged) We present near-IR photometry and spectroscopy of HD 3651B, the low-luminosity, wide-separation (480 AU) companion to the K0V exoplanet host star HD 3651A. We find a spectral type of T7.5+/-0.5, a temperature of 780-840 K, and a mass of 40-72 Mjup; its bolometric luminosity and effective temperature are among the lowest measured for any brown dwarf. We estimate an age of 3-12 Gyr for the primary star HD 3651A and find that it is ~3x older than the K4V star Gl 570A (1-5 Gyr), the host star of the T7.5 dwarf Gl 570D. HD 3651B belongs to the rare class of substellar objects that are companions to main-sequence stars and thus provides a new benchmark for studying very low-temperature objects. Given their similar temperatures (within ~30 K) and metallicities (within ~0.1 dex) but different ages, a comparison of HD 3651B and GL 570D allows us to examine gravity-sensitive diagnostics in ultracool spectra. We find that the expected signature of HD 3651B's higher surface gravity due to its older age, namely a suppressed K-band flux relative to GL 570D, is not seen. Instead, the K-band flux of HD 3651B is enhanced compared to GL 570D, indicative of a younger age. Thus, the relative ages derived from interpretation of T dwarf spectra and from stellar activity indicators appear to be in discord. One likely explanation is that the K-band fluxes are also very sensitive to metallicity differences. Metallicity variations may be as important as surface gravity variations in causing spectral differences among field late-T dwarfs.

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arXiv2006

Trigonometric Parallaxes of Central Stars of Planetary Nebulae

Trigonometric parallaxes of 16 nearby planetary nebulae are presented, including reduced errors for seven objects with previous initial results and results for six new objects. The median error in the parallax is 0.42 mas, and twelve nebulae have parallax errors less than 20 percent. The parallax for PHL932 is found here to be smaller than was measured by Hipparcos, and this peculiar object is discussed. Comparisons are made with other distance estimates. The distances determined from these parallaxes tend to be intermediate between some short distance estimates and other long estimates; they are somewhat smaller than estimated from spectra of the central stars. Proper motions and tangential velocities are presented. No astrometric perturbations from unresolved close companions are detected.

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arXiv2006

Physical parameters of two very cool T dwarfs

We present new infrared spectra of the T8 brown dwarf 2MASS J04151954-0935066: 2.9-4.1 micron spectra obtained with the Infrared Camera and Spectrograph on the Subaru Telescope, and 5.2-14.5 micron spectra obtained with the Infrared Spectrograph on the Spitzer Space Telescope. We use these data and models to determine an accurate bolometric luminosity of log L_bol/L_sun=-5.67 and to constrain the effective temperature, gravity, mass and age to 725-775K, log g=5.00-5.37, M=33-58 M_Jupiter and age=3-10Gyr. We perform the same analysis using published 0.6-15 micron spectra for the T7.5 dwarf 2MASS J12171110-0311131, for which we find a metal-rich composition ([Fe/H]~0.3) and log L_bol/L_sun=-5.31, T_eff=850-950K, log g=4.80-5.42, M=25-66M_Jupiter and age=1-10Gyr. These luminosities and effective temperatures straddle those determined with the same method and models for Gl 570D by Saumon et al. (2006) and make 2MASS J04151954-0935066 the coolest and least luminous T dwarf with well-determined properties. We find that synthetic spectra generated by the models reproduce the observed red through mid-infrared spectra of 2MASS J04151954-0935066 and 2MASS J12171110-0311131 very well, except for known discrepancies which are most likely due to the incomplete CH4 opacities. Both objects show evidence of departures from strict chemical equilibrium and we discuss this result in the context of other late T dwarfs where disequilibrium phenomena have been observed.

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arXiv2006

3.6-7.9 um Photometry of L and T Dwarfs and the Prevalence of Vertical Mixing in their Atmospheres

We present new L' (3.75 um) photometry of six L and T dwarfs, and M' (4.70 um) photometry of ten L and T dwarfs, observed at Gemini Observatory, and new 3.55, 4.49, 5.73 and 7.87 um photometry of nine L and T dwarfs, obtained with the Spitzer Space Telescope. The sample includes unusually blue and red dwarfs from our near-infrared studies. The data are combined with published L', M' and Spitzer photometry of L and T dwarfs, and trends of colors with spectral type and other colors are examined. Model atmospheres by Marley and Saumon are used to generate synthetic colors for ranges of effective temperature, gravity, grain sedimentation efficiency, metallicity and vertical mixing efficiency. We explore how these parameters affect the mid-infrared colors of L and T dwarfs and find that the data are modelled satisfactorily only if substantial vertical mixing occurs in both L- and T-dwarf atmospheres. The location and range of the L and T dwarf sequences in IRAC color-color and color - magnitude diagrams is also only reproduced if this mixing occurs, with a range of efficiency described by K_zz ~ 10^2-10^6 cm^2/s. The colors of the unusually red dwarfs are best reproduced by non-equilibrium models with low sedimentation efficiency, i.e. thick cloud decks, and those of the unusually blue dwarfs by non-equilibrium models with high sedimentation efficiency, i.e. thin cloud decks. The K-L' and Spitzer [3.55]-[4.49] colors can be used as indicators of effective temperature for L and T dwarfs, but care must be taken to include gravity and metallicity effects for late-T dwarfs and vertical mixing for both late-L and T dwarfs.

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arXiv2006

JHK Observations of Faint Standard Stars in the Mauna Kea Near-Infrared Photometric System

JHK photometry in the Mauna Kea Observatory (MKO) near-IR system is presented for 115 stars. Of these, 79 are UKIRT standards and 42 are LCO standards. The average brightness is 11.5 mag, with a range of 10 to 15. The average number of nights each star was observed is 4, and the average of the internal error of the final results is 0.011 mag. These JHK data agree with those reported by other groups to 0.02 mag. The measurements are used to derive transformations between the MKO JHK photometric system and the UKIRT, LCO and 2MASS systems. The 2MASS-MKO data scatter by 0.05 mag for redder stars: 2MASS-J includes H2O features in dwarfs and MKO-K includes CO features in giants. Transformations derived for stars whose spectra contain only weak features cannot give accurate transformations for objects with strong absorption features within a filter bandpasses. We find evidence of systematic effects at the 0.02 mag level in the photometry of stars with J<11 and H,K<10.5. This is due to an underestimate of the linearity correction for stars observed with the shortest exposure times; very accurate photometry of stars approaching the saturation limits of infrared detectors which are operated in double-read mode is difficult to obtain. Four stars in the sample, GSPC S705-D, FS 116 (B216-b7), FS 144 (Ser-EC84) and FS 32 (Feige 108), may be variable. 84 stars in the sample have 11< J< 15 and 10.5<H,K<15, are not suspected to be variable, and have magnitudes with an estimated error <0.027 mag; 79 of these have an error of <0.020 mag. These represent the first published high-accuracy JHK stellar photometry in the MKO photometric system; we recommend these objects be employed as primary standards for that system [abridged].

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arXiv2006

A Spitzer Infrared Spectrograph (IRS) Spectral Sequence of M, L, and T Dwarfs

We present a low-resolution (R = 90), 5.5-38 micron spectral sequence of a sample of M, L, and T dwarfs obtained with the Infrared Spectrograph (IRS) onboard the Spitzer Space Telescope. The spectra exhibit prominent absorption bands of H_2O at 6.27 microns, CH_4 at 7.65 microns, and NH_3 at 10.5 microns and are relatively featureless at lambda > 15 microns. Three spectral indices that measure the strengths of these bands are presented; H_2O absorption features are present throughout the MLT sequence while the CH_4 and NH_3 bands first appear at roughly the L/T transition. Although the spectra are, in general, qualitatively well matched by synthetic spectra that include the formation of spatially homogeneous silicate and iron condensate clouds, the spectra of the mid-type L dwarfs show an unexpected flattening from roughly 9 to 11 microns. We hypothesize that this may be a result of a population of small silicate grains that are not predicted in the cloud models. The spectrum of the peculiar T6 dwarf 2MASS J0937+2931 is suppressed from 5.5-7.5 microns relative to typical T6 dwarfs and may be a consequence of its mildly metal-poor/high surface gravity atmosphere. Finally, we compute bolometric luminosities of a subsample of the M, L, and T dwarfs by combining the IRS spectra with previously published 0.6-4.1 micron spectra and find good agreement with the values of Golimowski et al. who use L'- and M'-band photometry and to account for the flux emitted at lambda > 2.5 microns.

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arXiv2006

Ammonia as a tracer of chemical equilibrium in the T7.5 dwarf Gliese 570D

We present the first analysis of an optical to mid-infrared spectrum of the T7.5 dwarf Gliese 570D with model atmospheres, synthetic spectra, and brown dwarf evolution sequences. We obtain precise values for the basic parameters of Gl 570D: Teff=800 - 820K, log g (cm/s^2)=5.09 - 5.23, and log L/Lsun= -5.525 to -5.551. The Spitzer IRS spectrum shows prominent features of ammonia (NH3) that can only be fitted by reducing the abundance of NH3 by about one order of magnitude from the value obtained with chemical equilibrium models. We model departures from chemical equilibrium in the atmosphere of Gl 570D by considering the kinetics of nitrogen and carbon chemistry in the presence of vertical mixing. The resulting model spectrum reproduces the data very well.

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arXiv2006

SDSS J1534+1615AB: A Novel T Dwarf Binary Found with Keck Laser Guide Star Adaptive Optics and the Potential Role of Binarity in the L/T Transition

We have resolved the newly discovered T dwarf SDSS J1534+1615 into a 0.11'' binary using the Keck sodium laser guide star adaptive optics system. With an integrated-light spectral type of T3.5, this binary provides a new benchmark for studying the distinctive J-band brightening previously noted among early and mid-T dwarfs, using two brown dwarfs with different spectral types but having a common metallicity and age and very similar surface gravities. We estimate spectral types of T1.5+/-0.5 and T5.5+/-0.5 for the two components based on their near-IR colors, consistent with modeling the integrated-light spectrum as the blend of two components. The observed near-IR flux ratios are unique compared to all previously known substellar binaries: the component that is fainter at H and K' is brighter at J. This inversion of the near-IR fluxes is a manifestation of the J-band brightening within this individual binary system. Therefore, SDSS 1534+1615 demonstrates that the brightening can be intrinsic to ultracool photospheres (e.g., arising from cloud disruption and/or rapid increase in cloud sedimentation) and does not necessarily result from physical variations among the observed ensemble of T dwarfs (e.g., a range in masses, ages and/or metallicities). We suggest that the apparently large amplitude of the J-band brightening may be due to a high incidence of unresolved binaries and that the true amplitude of the phenomenon could be more modest. This scenario would imply that truly single objects in these spectral subclasses are relatively rare, in agreement with the small effective temperature range inferred for the L/T transition.

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arXiv2006

Debris Disks Around White Dwarfs: The DAZ Connection

We present near-infrared spectroscopic observations of 20 previously known DAZ white dwarfs obtained at the NASA Infrared Telescope Facility. Two of these white dwarfs (G29-38 and GD362) are known to display significant K-band excesses due to circumstellar debris disks. Here we report the discovery of excess K-band radiation from another DAZ white dwarf, WD0408-041 (GD56). Using spectroscopic observations, we show that the excess radiation cannot be explained by a stellar or substellar companion, and is likely to be caused by a warm debris disk. Our observations strengthen the connection between the debris disk phenomena and the observed metal abundances in cool DAZ white dwarfs. However, we do not find any excess infrared emission from the most metal rich DAZs with $T_{\rm eff}=$ 16000 -- 20000 K. This suggests that the metal abundances in warmer DAZ white dwarfs may require another explanation.

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arXiv2006

Seventy-One New L and T Dwarfs from the Sloan Digital Sky Survey

We present near-infrared observations of 71 newly discovered L and T dwarfs, selected from imaging data of the Sloan Digital Sky Survey (SDSS) using the i-dropout technique. Sixty-five of these dwarfs have been classified spectroscopically according to the near-infrared L dwarf classification scheme of Geballe et al. and the unified T dwarf classification scheme of Burgasser et al. The spectral types of these dwarfs range from L3 to T7, and include the latest types yet found in the SDSS. Six of the newly identified dwarfs are classified as early- to mid-L dwarfs according to their photometric near-infrared colors, and two others are classified photometrically as M dwarfs. We also present new near-infrared spectra for five previously published SDSS L and T dwarfs, and one L dwarf and one T dwarf discovered by Burgasser et al. from the Two Micron All Sky Survey. The new SDSS sample includes 27 T dwarfs and 30 dwarfs with spectral types spanning the complex L-T transition (L7-T3). We continue to see a large (~0.5 mag) spread in J-H for L3 to T1 types, and a similar spread in H-K for all dwarfs later than L3. This color dispersion is probably due to a range of grain sedimentation properties, metallicity, and gravity. We also find L and T dwarfs with unusual colors and spectral properties that may eventually help to disentangle these effects.

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arXiv2005

SDSS J042348.57-041403.5AB: A Brown Dwarf Binary Straddling the L/T Transition

We present the discovery of SDSS J042348.57-041403.5 as a closely-separated (0"16) brown dwarf binary, resolved by the Hubble Space Telescope Near Infrared Camera and Multi-Object Spectrometer. Physical association is deduced from the angular proximity of the components and constraints on their common proper motion. SDSS0423-0414AB appears to be composed of two brown dwarfs with spectral types L6+/-1 and T2+/-1. Hence, this system straddles the transition between L dwarfs and T dwarfs, a unique evolutionary phase of brown dwarfs characterized by substantial shifts in spectral morphology over an apparently narrow effective temperature range. Binarity explains a number of unusual properties of SDSS 0423-0414, including its overluminosity and high effective temperature compared to other early-type T dwarfs, and possibly its conflicting spectral classifications (L7.5 in the optical, T0 in the near infrared). The relatively short estimated orbital period of this system (~15-20 yr) and the presence of Li I absorption in its combined light spectrum make it an ideal target for both resolved spectroscopy and dynamical mass measurements. SDSS 0423-0414AB joins a growing list of late-L/early-T dwarf binaries, the high percentage of which (~50%) may provide a natural explanation for observed peculiarities across the L/T transition.

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arXiv2005

A Unified Near Infrared Spectral Classification Scheme for T Dwarfs

A revised near infrared classification scheme for T dwarfs is presented, based on and superseding prior schemes developed by Burgasser et al. and Geballe et al., and defined following the precepts of the MK Process. Drawing from two large spectroscopic libraries of T dwarfs identified largely in the Sloan Digital Sky Survey and the Two Micron All Sky Survey, nine primary spectral standards and five alternate standards spanning spectral types T0 to T8 are identified that match criteria of spectral character, brightness, absence of a resolved companion and accessibility from both northern and southern hemispheres. The classification of T dwarfs is formally made by the direct comparison of near infrared spectral data of equivalent resolution to the spectra of these standards. Alternately, we have redefined five key spectral indices measuring the strengths of the major H$_2$O and CH$_4$ bands in the 1-2.5 micron region that may be used as a proxy to direct spectral comparison. Two methods of determining T spectral type using these indices are outlined and yield equivalent results. These classifications are also equivalent to those from prior schemes, implying that no revision of existing spectral type trends is required. The one-dimensional scheme presented here provides a first step toward the observational characterization of the lowest luminosity brown dwarfs currently known. Future extensions to incorporate spectral variations arising from differences in photospheric dust content, gravity and metallicity are briefly discussed. A compendium of all currently known T dwarfs with updated classifications is presented.

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arXiv2005

Excess Infrared Radiation from a Massive DAZ White Dwarf: GD362 - a Debris Disk?

We report the discovery of excess K-band radiation from a massive DAZ white dwarf star, GD362. Combining infrared photometric and spectroscopic observations, we show that the excess radiation cannot be explained by a stellar or substellar companion, and is likely to be caused by a debris disk. This would be only the second such system known, discovered 18 years after G29-38, the only single white dwarf currently known to be orbited by circumstellar dust. Both of these systems favor a model with accretion from a surrounding debris disk to explain the metal abundances observed in DAZ white dwarfs. Nevertheless, observations of more DAZs in the mid-infrared are required to test if this model can explain all DAZs.

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arXiv2005

On the Interpretation of High Velocity White Dwarfs as Members of the Galactic Halo

A detailed analysis of 32 of the 38 halo white dwarf candidates identified by Oppenheimer et al. is presented, based on model atmosphere fits to observed energy distributions built from optical BVRI and infrared JHK CCD photometry. Effective temperatures and atmospheric compositions are determined for all objects, as well as masses and cooling ages when trigonometric parallax measurements are available. This sample is combined with that of other halo white dwarf candidates and disk white dwarfs to study the nature of these objects in terms of reduced proper motion diagrams, tangential velocities, and stellar ages. We reaffirm the conclusions of an earlier analysis based on photographic magnitudes of the same sample that total stellar ages must be derived in order to associate a white dwarf with the old halo population, and that this can only be accomplished through precise mass and distance determinations.

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arXiv2004

Near-Infrared Photometry and Spectroscopy of L and T Dwarfs: the Effects of Clouds, Gravity, and Effective Temperature

We present new JHKL'M' photometry on the MKO system for a large sample of L and T dwarfs identified from SDSS and 2MASS and classified according to the scheme of Geballe et al. (2002). We have compiled a sample of 105 L and T dwarfs that are uniformly classified and measured on a single photometric system. The scattered JHK spectral indices and colors of L dwarfs are likely caused by variations in the altitudes, distributions, and thicknesses of condensate clouds. Scatter in the H-K colors of late T dwarfs probably reflects the sensitivity of the K-band flux to pressure induced H_2 opacity, which itself is sensitive to surface gravity. The M' luminosities of late-T dwarfs are 1.5-2.5 times fainter than predicted under conditions of chemical equilibrium. We have computed L_bol and T_eff for 42 L and T dwarfs whose trigonometric parallaxes have been measured. We find that T_eff ~ 1450 K for types L7-T4, which supports recent models that attribute the changing JHK-band luminosities and spectral features across the L-T transition to rapid changes in the condensate clouds over a narrow range of T_eff. We compute T_eff = 600-750 K for 2MASS 0415-0935 (T9), which supplants Gl 570D as the coolest known brown dwarf.

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arXiv2004

Spectral Classification Beyond M

Significant populations of field L and T dwarfs are now known, and we anticipate the discovery of even cooler dwarfs by Spitzer and ground-based infrared surveys. However, as the number of known L and T dwarfs increases so does the range in their observational properties, and difficulties have arisen in interpreting the observations. Although modellers have made significant advances, the complexity of the very low temperature, high pressure, photospheres means that problems remain such as the treatment of grain condensation as well as incomplete and non-equilibrium molecular chemistry. Also, there are several parameters which control the observed spectral energy distribution - effective temperature, grain sedimentation efficiency, metallicity and gravity - and their effects are not well understood. In this paper, based on a splinter session, we discuss classification schemes for L and T dwarfs, their dependency on wavelength, and the effects of the parameters T_eff, f_sed, [m/H] and log g on optical and infrared spectra. We will discuss the various hypotheses that have been presented for the transition from the dusty L types to the clear atmosphere T types. We conclude with a brief discussion of the spectral class beyond T. Authors of each Section are identified by their initials.

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arXiv2004

Spitzer Infrared Spectrograph Observations of M, L, and T Dwarfs

We present the first mid-infrared spectra of brown dwarfs, together with observations of a low-mass star. Our targets are the M3.5 dwarf GJ 1001A, the L8 dwarf DENIS-P J0255-4700, and the T1/T6 binary system epsilon Indi Ba/Bb. As expected, the mid-infrared spectral morphology of these objects changes rapidly with spectral class due to the changes in atmospheric chemistry resulting from their differing effective temperatures and atmospheric structures. By taking advantage of the unprecedented sensitivity of the Infrared Spectrograph on the Spitzer Space Telescope we have detected the 7.8 micron methane and 10 micron ammonia bands for the first time in brown dwarf spectra.

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arXiv2004

L' and M' Photometry of Ultracool Dwarfs

We have compiled L' and M' photometry of 63 single and binary M, L,and T dwarfs obtained at UKIRT using the MKO filter set. This compilation includes new L' data for 8 L dwarfs and 13 T dwarfs and new M' data for 7 L dwarfs, 5 T dwarfs, and Gl 229A. We compute L_bol, BC_K, and T_eff for 42 dwarfs whose flux-calibrated JHK spectra, L' photometry, and trigonometric parallaxes are available, and we estimate these quantities for 9 other dwarfs whose parallaxes and flux-calibrated spectra have been obtained. BC_K is a well-behaved function of near-IR spectral type with a dispersion of ~0.1 mag for types M6-T5; it is significantly more scattered for types T5-T9. T_eff declines steeply and monotonically for types M6-L7 and T4-T9, but is nearly constant at ~1450 K for types L7-T4 with assumed ages of ~3 Gyr. This constant T_eff is evidenced by nearly unchanging values of L'-M' between types L6 and T3. We compare the observed K, L', and M' luminosities of L and T dwarfs in our sample with those predicted by model atmospheres with varying surface gravities and sedimentation efficiencies. The models indicate that the L3-T4.5 dwarfs generally have higher gravities (log g = 5.0-5.5) than the T6-T9 dwarfs (log g = 4.5-5.0). The predicted M' luminosities of T dwarfs are 1.5-2.5 times larger than those derived empirically for the T dwarfs in our sample. This discrepancy is attributed to absorption at 4.5-4.9 um by CO, which is not expected under the condition of chemical equilibrium assumed in the models. We determine that the L3 dwarf Kelu-1 and the T0 dwarf SDSS J0423-0414 are probable binary systems. We compute log(L_bol/L_sun) = -5.73 +/- 0.05 and T_eff = 600-750 K for the T9 dwarf 2MASS J0415-0935, which supplants Gl 570D as the least luminous and coolest brown dwarf presently known.

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