arXiv subjects

S. K. Leggett

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

112 records · Page 7Linked to original sources

arXiv2000

Molecular Abundances in the Atmosphere of the T Dwarf Gl 229B

We present new, high resolution, infrared spectra of the T dwarf Gliese 229B in the J, H, and K bandpasses. We analyze each of these as well as previously published spectra to determine its metallicity and the abundances of NH3 and CO in terms of the surface gravity of Gl 229B, which remains poorly constrained. The metallicity increases with increasing gravity and is below the solar value unless Gl 229B is a high-gravity brown dwarf with log g(cgs) ~ 5.5. The NH3 abundance is determined from both the H and the K band spectra which probe two different levels in the atmosphere. We find that the abundance from the K band data is well below that expected from chemical equilibrium, which we interpret as strong evidence for dynamical transport of NH3 in the atmosphere. This is consistent with the previous detection of CO and provides additional constraints on the dynamics of the atmosphere of this T dwarf.

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arXiv2000

Spectral Energy Distributions for Disk and Halo M--Dwarfs

We have obtained infrared (1 to 2.5 micron) spectroscopy for 42 halo and disk dwarfs with spectral type M1 to M6.5. These data are compared to synthetic spectra generated by the latest model atmospheres of Allard & Hauschildt. Photospheric parameters metallicity, effective temperature and radius are determined for the sample. We find good agreement between observation and theory except for known problems due to incomplete molecular data for metal hydrides and water. The metal-poor M subdwarfs are well matched by the models as oxide opacity sources are less important in this case. The derived effective temperatures for the sample range from 3600K to 2600K; at these temperatures grain formation and extinction are not significant in the photosphere. The derived metallicities range from solar to one-tenth solar. The radii and effective temperatures derived agree well with recent models of low mass stars.

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arXiv1999

Revised Fluxes for Gliese 229B

We have used the coronographic instrument CoCo with the IRTF's facility camera NSFCAM to obtain improved photometry at JHKL' of the giant planet/brown dwarf Gliese 229B. We have recalibrated the published spectra for this object, and re-calculated its luminosity. Our L' value, and our flux calibration of the spectra at JHK, are significantly different from those previously published. Our results show good agreement at all bands except H with evolutionary models by Burrows et al. which include grain condensation. The model comparison implies that Gliese 229B is likely to be a 0.5 Gyr-old 25 Jupiter-mass object with effective temperature around 900 K.

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arXiv1998

Infrared Colors at the Stellar/Substellar Boundary

We present new infrared photometry for 61 halo and disk stars around the stellar/substellar boundary. These data are combined with available optical photometry and astrometric data to produce color--color and absolute magnitude--color diagrams. The disk and halo sequences are compared to the predictions of the latest model atmospheres and structural models. We find good agreement between observation and theory except for known problems in the V and H passbands probably due to incomplete molecular data for TiO, metal hydrides and H$_2$O. The metal--poor M subdwarfs are well matched by the models as oxide opacity sources are less important in this case. The known extreme M subdwarfs have metallicities about one--hundredth solar, and the coolest subdwarfs have T$_{eff}\sim 3000$ K with masses $\sim $0.09M/M$_{\odot}$. The grainless models are not able to reproduce the flux distributions of disk objects with T$_{eff} <$ 2500 K, however a preliminary version of the NextGen--Dusty models which includes homogeneous formation and extinction by dust grains {\it is} able to match the colors of these very cool objects. The least luminous objects in this sample are GD165B, three DENIS objects --- DBD0205, DBD1058 and DBD1228 --- and Kelu-1. These have T$_{eff}\sim$ 2000 K and are at or below the stellar limit with masses $\leq$0.075M/M$_{\odot}$. Photometry alone cannot constrain these parameters further as the age is unknown, but published lithium detections for two of these objects (Kelu-1 and DBD1228) imply that they are young (aged about 1 Gyr) and substellar (mass $\leq$0.06M/M$_{\odot}$).

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