arXiv subjects

S. K. Leggett

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

At least 109 records · Page 6Linked to original sources

arXiv2004

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

We present JHK photometry and spectroscopy for a sample of newly identified L and T dwarfs from the Sloan Digital Sky Survey (SDSS) and known dwarfs from the SDSS and the Two Micron All Sky Survey (2MASS). Spectral classification has been carried out using four infrared indices; we identify 9 new L8-9.5 dwarfs and 14 new T dwarfs from SDSS, including the latest SDSS dwarf yet found, the T7 SDSS J1758+4633. We classify 2MASS J0415-0935 as T9, the latest and coolest dwarf found to date. We combine these results with previously published data to produce a sample of 59 L dwarfs and 42 T dwarfs. We compare the near-IR colors and absolute magnitudes of dwarfs near the L-T transition with predictions made by models of the distribution and evolution of photospheric condensates. There is scatter in the spectral indices for L dwarfs, suggesting that these indices are probing different levels of the atmosphere. The near-IR colors of the L dwarfs also scatter, likely due to variations in the altitudes, spatial distributions and thicknesses of the condensate clouds. We identify a group of late L dwarfs that are blue for their spectral type and that have enhanced FeH, H2O and KI absorption, possibly due to an unusually small amount of condensates. The scatter seen in the H-K color for late T dwarfs can be reproduced by models with a range in surface gravity. The variation is probably due to the effect on the K-band flux of pressure-induced H2 opacity. The correlation of H-K color with gravity is supported by the strengths of the J-band KI doublet. Gravity is closely related to mass for field T dwarfs and the gravities implied by the H-K colors indicate that the T dwarfs in our sample have masses in the range 15-75 M_Jupiter, except for SDSS J1110+0116 which possibly has a mass of 10-15 M_Jupiter.

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arXiv2004

Preliminary Parallaxes of 40 L and T Dwarfs from the U.S. Naval Observatory Infrared Astrometry Program

We present preliminary trigonometric parallaxes and proper motions for 22 L dwarfs and 18 T dwarfs measured using the ASTROCAM infrared imager. Relative to absolute parallax corrections are made by employing 2MASS and/or SDSS photometry for reference frame stars. We combine USNO infrared and optical parallaxes with the best available CIT system photometry to determine M_J, M_H, and M_K values for 37 L dwarfs between spectral types L0 to L8 and 19 T dwarfs between spectral types T0.5 and T8 and present selected absolute magnitude versus spectral type and color diagrams, based on these results. Luminosities and temperatures are estimated for these objects. Of special interest are the distances of several objects which are at or near the L-T dwarf boundary so that this important transition can be better understood. The previously reported early-mid T dwarf luminosity excess is clearly confirmed and found to be present at J, H, and K. The large number of objects that populate this luminosity excess region indicates that it cannot be due entirely to selection effects. The T dwarf sequence is extended to M_J~16.9 by 2MASS J041519-0935 which, at d = 5.74 pc, is found to be the least luminous [log(L/L_sun)=-5.58] and coldest (T_eff~760 K) brown dwarf known. Combining results from this paper with earlier USNO CCD results we find that, in contrast to the L dwarfs, there are no examples of low velocity (V_tan < 20 km/s) T dwarfs. We briefly discuss future directions for the USNO infrared astrometry program.

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arXiv2003

Analysis of a Very Massive DA White Dwarf via the Trigonometric Parallax and Spectroscopic Methods

By two different methods, we show that LHS 4033 is an extremely massive white dwarf near its likely upper mass limit for destruction by unstable electron captures. From the accurate trigonometric parallax reported herein, the effective temperature (T=10,900 K) and the stellar radius (R=0.00368 R_sun) are directly determined from the broad-band spectral energy distribution -- the parallax method. The effective temperature and surface gravity are also estimated independently from the simultaneous fitting of the observed Balmer line profiles with those predicted from pure-hydrogen model atmospheres -- the spectroscopic method (T=10,760 K, log g=9.46). The mass of LHS 4033 is then inferred from theoretical mass-radius relations appropriate for white dwarfs. The parallax method yields a mass estimate of 1.310--1.330 M_sun, for interior compositions ranging from pure magnesium to pure carbon, respectively, while the spectroscopic method yields an estimate of 1.318--1.335 M_sun for the same core compositions. This star is the most massive white dwarf for which a robust comparison of the two techniques has been made.

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arXiv2003

L' and M' Standard Stars for the Mauna Kea Observatories Near-Infrared System

We present L'and M' photometry, obtained at UKIRT using the Mauna Kea Observatories Near-IR filter set, for 46 and 31 standard stars, respectively. The L' standards include 25 from the UKIRT in-house "Bright Standards" with magnitudes deriving from Elias et al. (1982) and observations at the IRTF in the early 1980s, and 21 fainter stars. The M' magnitudes derive from the results of Sinton & Tittemore (1984). We estimate the average external error to be 0.015 mag for the bright L' standards and 0.025 mag for the fainter L' standards, and 0.026 mag for the M' standards. The new results provide a network of homogeneously observed standards, and establish reference stars for the MKO system, in these bands. They also extend the available standards to magnitudes which should be faint enough to be accessible for observations with modern detectors on large and very large telescopes.

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arXiv2002

Model Atmosphere Analysis of Two Very Cool White Dwarfs

A detailed analysis of the very cool white dwarfs SDSS 1337+00 and LHS 3250 is presented. Model atmosphere calculations with improved collision-induced absorptions by molecular hydrogen indicate that a pure hydrogen composition can be ruled out, and that the strong infrared absorption observed in these cool stars is better explained in terms of collisions of H2 with neutral helium. It is shown that even though the overall shape of the observed energy distributions can be reproduced reasonably well with helium-rich models, the peak of the energy distribution near 6000 A is always predicted too narrow. The extreme helium-rich composition inferred for both objects is discussed in the broader context of the extremely cool white dwarfs reported in various surveys.

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arXiv2001

Atmospheric Analysis of the M/L- and M/T-Dwarf Binary Systems LHS 102 and Gliese 229

We present 0.9-2.5um spectroscopy with R~800 and 1.12-1.22um spectroscopy with R~5800 for the M dwarfs Gl 229A and LHS 102A, and for the L dwarf LHS 102B. We also report IZJHKL' photometry for both components of the LHS 102 system, and L' photometry for Gl 229A. The data are combined with previously published spectroscopy and photometry to produce flux distributions for each component of the kinematically old disk M/L-dwarf binary system LHS 102 and the kinematically young disk M/T-dwarf binary system Gliese 229. The data are analyzed using synthetic spectra generated by the latest "AMES-dusty" and "AMES-cond" models by Allard & Hauschildt. Although the models are not able to reproduce the overall slope of the infrared flux distribution of the L dwarf, most likely due to the treatment of dust in the photosphere, the data for the M dwarfs and the T dwarf are well matched. We find that the Gl 229 system is metal-poor despite having kinematics of the young disk, and that the LHS 102 system has solar metallicity. The observed luminosities and derived temperatures and gravities are consistent with evolutionary model predictions if the Gl 229 system is very young (age ~30 Myr) with masses (A,B) of (0.38,>0.007)M(sun), and the LHS 102 system is older, aged 1-10 Gyr with masses (A,B) of (0.19,0.07)M(sun).

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arXiv2001

Towards Spectral Classification of L and T Dwarfs: Infrared and Optical Spectroscopy and Analysis

We present 0.6-2.5um, R~400 spectra of twenty-seven cool, low luminosity stars and substellar objects. Based on these and previously published spectra we develop a preliminary spectral classification system for L and T dwarfs. For late L and T types the classification system is based entirely on four spectral indices in the 1-2.5um interval. Two of these indices are derived from water absorption bands at 1.15um and 1.4um, the latter of which shows a smooth increase in depth through the L and T sequences and can be used to classify both spectral types. The other two indices make use of methane absorption features in the H and K bands, with the K band index also applicable to mid to late L dwarfs. Continuum indices shortward of 1um used by previous authors to classify L dwarfs are found to be useful only through mid L subclasses. We employ the 1.5um water index and the 2.2um methane index to complete the L classification through L9.5 and to link the new system with a modified version of the 2MASS ``Color-d'' index. By correlating the depths of the methane and water absorption features, we establish a T spectral sequence from types T0 to T8, based on all four indices, which is a smooth continuation of the L sequence. We reclassify two 2MASS L8 dwarfs as L9 and L9.5 and identify one SDSS object as L9. In the proposed system methane absorption appears in the K band approximately at L8, two subclasses earlier than its appearance in the H band. The L and T spectral classes are distinguished by the absence and presence, respectively, of H band methane absorption.

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arXiv2001

Infrared Classification of L and T Dwarfs

We use new and published medium resolution 0.6--2.5 $μ$m spectra of L and T dwarfs to develop a unified classification system for both of these new spectral classes. Two indices of the system at 1.2 $μ$m and 1.5 $μ$m are based on nearby absorption bands of water vapor and two are associated with methane bands near 1.6 $μ$m and 2.2 $μ$m. The 1.5 $μ$m index is monotonic through the L and T sequences, and forms the backbone of the system; the indices for the other bands provide extensive, but only partial, coverage. We correlate the 1.5 $μ$m index with continuum indices shortward of 1 $μ$m devised by others for classifying L dwarfs, in order to obtain a tight link between optical and infrared classifications. Our proposed system defines ten spectral subclasses for L (L0--L9) and nine for T (T0--T8). The boundary between L and T is defined to be the onset of absorption by methane in the H band. Methane absorption in the K band near 2.2 $μ$m is found to begin approximately at L8.

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arXiv2001

Infrared Colors of L and T Dwarfs

We discuss the behaviour of the JHKL'M' colors of L and T dwarfs, based on photometric and spectroscopic data obtained at the United Kingdom Infrared Telescope in Hawaii. We have measured the first accurate M' photometry for L and T dwarfs. The K-M' colors of T dwarfs are much bluer than predicted by published models, suggesting that CO may be more abundant than expected, as has been found spectroscopically for the T6 dwarf Gl 229B. We also find that K-L' increases monotonically through most of the M, L, and T subclasses, but it is approximately constant between types L6 and T5, due to the onset of methane absorption at the blue edge of the L' bandpass. The JHK colors of L dwarfs show significant scatter, suggesting variations in the amount and properties of photospheric dust, and indicating that it may not be possible to associate a unique effective temperature with a given L spectral type. The H-K colors of the later T dwarfs also show some scatter which we suggest is due to variations in pressure-induced molecular hydrogen opacity, which is sensitive to gravity (or age for a brown dwarf) and metallicity.

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arXiv2001

A Survey of z>5.8 Quasars in the Sloan Digital Sky Survey I: Discovery of Three New Quasars and the Spatial Density of Luminous Quasars at z~6

We present the results from a survey of i-dropout objects selected from ~1550 deg^2 of multicolor imaging data from the Sloan Digital Sky Survey, to search for luminous quasars at z>5.8. Objects with i*-z*>2.2 and z*<20.2 are selected, and follow-up J band photometry is used to separate L and T type cool dwarfs from high-redshift quasars. We describe the discovery of three new quasars, at z=5.82, 5.99 and 6.28, respectively. Their spectra show strong and broad Ly alpha+NV emission lines, and very strong Ly alpha absorption, with a mean continuum decrement D_A > 0.90. The ARC 3.5m spectrum of the z=6.28 quasar shows that over a range of 300 A immediately blueward of the Ly alpha emission, the average transmitted flux is only 0.003 +/-0.020 times that of the continuum level, consistent with zero flux, and suggesting a tentative detection of the complete Gunn-Peterson trough. The existence of strong metal lines suggests early chemical enrichment in the quasar enviornment. The three new objects, together with the previously published z=5.8 quasar form a complete color-selected flux-limited sample at z>5.8. We estimate that at $z=6$, the comoving density of luminous quasars at M_1450 < -26.89 (h=0.5, Omega=1)is 1.1x10^-9 Mpc^-3. This is a factor of ~2 lower than that at z~5, and is consistent with an extrapolation of the observed quasar evolution at low-z. We discuss the contribution of quasars to the ionizing background at z~6. The luminous quasars discussed in the paper have central black hole masses of several times 10^9 M_sun by the Eddington argument. Their observed space density provides a sensitive test of models of quasar and galaxy formation at high redshift. (Abridged)

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arXiv2001

Infrared Photometry of Late-M, L, and T Dwarfs

We present ZJHKL'M' photometry of a sample of 58 late-M, L, and T dwarfs, most of which are identified from the Sloan Digital Sky Survey and the Two Micron All-Sky Survey. Near-infrared spectra and spectral classifications for most of this sample are presented in a companion paper by Geballe et al. We derive the luminosities of 18 dwarfs in the sample and the results imply that the effective temperature range for the L dwarfs in our sample is approximately 2200-1300 K and for the T dwarfs 1300-800 K. We obtained new photometric data at the United Kingdom Infrared Telescope for: 42 dwarfs at Z, 34 dwarfs at JHK, 21 dwarfs at L', as well as M' data for two L dwarfs and two T dwarfs. The M' data provide the first accurate photometry for L and T dwarfs in this bandpass - for a T2 and a T5 dwarf, we find K-M'=1.2 and 1.6, respectively. These colors are much bluer than predicted by models suggesting that CO may be more abundant in these objects than expected, as has been found for the T6 dwarf Gl 229B. We also find that K-L' increases monotonically through most of the M, L, and T subclasses, but it is almost constant between types L6 and T5. The degeneracy is probably due to the onset of methane absorption at the blue edge of the L' bandpass. The JHK colors of L dwarfs show significant scatter, suggesting that the fluxes in these bandpasses are sensitive to variations in photospheric dust properties. The H-K colors of the later T dwarfs also show some scatter which we suggest is due to variations in pressure-induced molecular hydrogen opacity, which is sensitive to gravity and metallicity.

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arXiv2001

Infrared Observations and Modeling of one of the Coolest T Dwarfs, Gl 570D

We have obtained a good quality R$\sim$400 0.8-2.5 $μ$m spectrum and accurate photometry of Gl 570D, one of the coolest and least luminous brown dwarfs currently known. The spectrum shows that Gl 570D has deeper absorptions in the strong water and methane bands at 1.12-1.17 $μ$m, 1.33-1.45 $μ$m, 1.62-1.88 $μ$m, and 2.20-2.45 $μ$m and is both bluer at J-K and redder at $K-L^{\prime}$ than previously observed T dwarfs. Data analysis using model spectra coupled with knowledge of the well-understood primary implies that for the same surface gravity, Gl 570D is about 160 K cooler than Gl 229B. For an age range of 2-5 Gyr Gl 570D has an effective temperature in the range 784-824 K, a log gravity in the range 5.00-5.27 cm s$^{-2}$, and a luminosity in the range 2.88-2.98 $\times$ 10$^{-6}$ L$_\odot$.

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arXiv2001

JHK Standard Stars for Large Telescopes: the UKIRT Fundamental and Extended Lists

We present high-precision JHK photometry with the 3.8m UK Infrared Telescope (UKIRT) of 83 standard stars, 28 from the widely used preliminary list known as the "UKIRT Faint Standards" (Casali & Hawarden, 1992), referred to here as the Fundamental List, and 55 additional stars referred to as the Extended List. The stars have 9.4<K<15.0 and most should be readily observable with imaging array detectors in normal operating modes on telescopes of up to 10m aperture. Many are accessible from the southern hemisphere. Arcsec-accuracy positions (J2000, Epoch ~1998) are given, together with optical photometry and spectral types from the literature, where available, or inferred from the J-K colour. Finding charts are provided for stars with proper motions exceeding 0.3"/yr. On 30 nights between late 1994 and early 1998 the stars from the Fundamental List, which were used as standards for the whole programme, were observed on an average of 10 nights each, and those from the Extended List 6 nights. The average internal standard error of the mean results for K is 0.005 mag; for J-H it is 0.003 mag for the Fundamental List stars and 0.006 mag for the Extended List; for H-K the average is 0.004 mag. The results are on the natural system of the IRCAM3 imager, which used a 256x256 InSb detector array with "standard" JHK filters, behind gold-coated fore-optics and a gold- or silver-dielectric coated dichroic. We give colour transformations onto the CIT, Arcetri and LCO/Palomar NICMOS systems, and preliminary transformations onto the system defined by the new Mauna Kea Observatory filter set.

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arXiv2001

Infrared Spectroscopy of Brown Dwarfs: the onset of methane absorption in L dwarfs and the L/T transition

We present infrared spectra of brown dwarfs with spectral types from mid-L to T. The 0.9-2.5 $μ$m spectra of three dwarfs found by the Sloan Digital Sky Survey contain absorption bands of both methane and carbon monoxide and bridge the gap between late L and previously observed T dwarfs. These dwarfs form a clear spectral sequence, with CH$_{4}$ absorption increasing as the CO absorption decreases. Water vapor band strengths increase in parallel with the methane bands and thus also link the L and T types. We suggest that objects with detectable CO and CH$_{4}$ in the H and K bands should define the earliest T subclasses. From observations of bright (K~$\leq$~13 mag) L dwarfs found by 2MASS, we find that the onset of detectable amounts of CH$_{4}$ occurs near spectral type L5. For this spectral type methane is observable in the 3.3~$μ$m $ν$$_{3}$ band only, and not in the overtone and combination bands at H and K.

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arXiv2000

Photometric and Spectroscopic Analysis of Cool White Dwarfs with Trigonometric Parallax Measurements

A photometric and spectroscopic analysis of 152 cool white dwarf stars is presented. The discovery of 7 new DA white dwarfs, 2 new DQ white dwarfs, 1 new magnetic white dwarf, and 3 weak magnetic white dwarf candidates, is reported, as well as 19 known or suspected double degenerates. The photometric energy distributions, the Halpha line profiles, and the trigonometric parallax measurements are combined and compared to model atmosphere calculations to determine the effective temperature and the radius of each object, and also to constrain the atmospheric composition. New evolutionary sequences with C/O cores with thin and thick hydrogen layers are used to derive masses and ages. We confirm the existence of a range in Teff between 5000 and 6000K where almost all white dwarfs have H-rich atmospheres. There is little evidence for mixed H/He dwarfs, with the exception of 2 He-rich DA stars, and 5 C2H white dwarfs which possibly have mixed H/He/C atmospheres. The DQ sequence terminates near 6500K, below which they are believed to turn into C2H stars. True DC stars slightly above this temperature are found to exhibit H-like energy distributions despite the lack of Halpha absorption. Attempts to interpret the chemical evolution show the problem to be complex. Convective mixing is necessary to account for the non-DA to DA ratio as a function of temperature. The presence of helium in cool DA stars, the existence of the non-DA gap, and the peculiar DC stars are also explained in terms of convective mixing, although our understanding of how this mechanism works needs to be revised. The oldest object in our sample is about 7.9 Gyr or 9.7 Gyr old depending on whether thin or thick hydrogen layer models are used. The mean mass of our sample is 0.65 +/- 0.20 Msun.

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arXiv2000

Infrared Spectra and Spectral Energy Distributions of Late-M- and L-Dwarfs

We have obtained 1.0-2.5um spectra at R~600 of 14 disk dwarfs with spectral types M6 to L7. For four of the dwarfs we have also obtained infrared spectra at R~3000 in narrow intervals. In addition, we present new L' photometry for four of the dwarfs in the sample, which allows improved determinations of their bolometric luminosities. We resolve the L-dwarf Denis-P J 0205-1159 into an identical pair of objects separated by 0.35". The spectra, with the published energy distribution for one other dwarf, are compared to synthetic spectra generated by upgraded model atmospheres. Good matches are found for 2200> Teff K>1900 (spectral types around M9 to L3), but discrepancies exist at Teff> 2300 K (M8) and for Teff<1800 K (L4-L7). At the higher temperatures the mismatches are due to incompleteness in the water vapor linelist. At the lower temperatures the disagreement is probably due to our treatment of dust: we assume a photospheric distribution in equilibrium with the gas phase. We derive effective temperatures for the sample from the comparison with synthetic spectra and also by comparing our observed total intrinsic luminosities to structural model calculations (which are mostly independent of the atmosphere but are dependent on the unknown masses and ages of the targets). The two derivations agree to ~200 K except for the faintest object in the sample where the discrepancy is larger. Agreement with other temperature determinations is also ~200 K, except for the L7 dwarf.

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arXiv2000

The Onset of Methane in L Dwarfs

We have detected weak absorption features produced by the strong nu(3) methane band at 3.3 microns in two L dwarfs, 2MASSW J1507476-162738 and 2MASSI J0825196+211552, classified by Kirkpatrick et al. (2000) as spectral types L5 and L7.5 respectively. These absorptions occur in objects warmer than any in which methane previously has been detected, and mark the first appearance of methane in the ultracool star-to-brown dwarf spectral sequence.

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arXiv2000

The Missing Link: Early Methane ("T") Dwarfs in the Sloan Digital Sky Survey

We report the discovery of three cool brown dwarfs which fall in the effective temperature gap between the latest L dwarfs currently known, with no methane absorption bands in the 1-2.5um range, and the previously known methane (T) dwarfs, whose spectra are dominated by methane and water. The newly discovered objects were detected as very red objects in the Sloan Digital Sky Survey imaging data and have JHK colors between the red L dwarfs and the blue Gl229B-like T dwarfs. They show both CO and methane absorption in their near-infrared spectra in addition to water, with weaker methane absorption features in the H and K bands than those in all other methane dwarfs reported to date. Due to the presence of methane in these bands, we propose that these objects are early T dwarfs. The three form part of the brown dwarf spectral sequence and fill in the large gap in the overall spectral sequence from the hottest main sequence stars to the coolest methane dwarfs currently known.

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