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Title: Uniform Atmospheric Retrieval Analysis of Ultracool Dwarfs. II. Properties of 11 T dwarfs

Abstract

Brown dwarf spectra are rich in information revealing of the chemical and physical processes operating in their atmospheres. We apply a recently developed atmospheric retrieval tool to an ensemble of late-T dwarf (600–800 K) near-infrared (1–2.5 μ m) spectra. With these spectra we are able to directly constrain the molecular abundances for the first time of H{sub 2}O, CH{sub 4}, CO, CO{sub 2}, NH{sub 3}, H{sub 2}S, and Na+K, surface gravity, effective temperature, thermal structure, photometric radius, and cloud optical depths. We find that ammonia, water, methane, and the alkali metals are present and that their abundances are well constrained in all 11 objects. We find no significant trend in the water, methane, or ammonia abundances with temperature, but find a very strong (>25 σ ) decreasing trend in the alkali metal abundances with decreasing effective temperature, indicative of alkali rainout. As expected from previous work, we also find little evidence for optically thick clouds. With the methane and water abundances, we derive the intrinsic atmospheric metallicity and carbon-to-oxygen ratios. We find in our sample that metallicities are typically subsolar (−0.4 < [ M /H] < 0.1 dex) and carbon-to-oxygen ratios are somewhat supersolar (0.4 < C/O < 1.2), differentmore » than expectations from the local stellar population. We also find that the retrieved vertical thermal profiles are consistent with radiative equilibrium over the photospheric regions. Finally, we find that our retrieved effective temperatures are lower than previous inferences for some objects and that some of our radii are larger than expectations from evolutionary models, possibly indicative of unresolved binaries. This investigation and method represent a new and powerful paradigm for using spectra to determine the fundamental chemical and physical processes governing cool brown dwarf atmospheres.« less

Authors:
 [1]; ;  [2];  [3];  [4];  [5];  [6];  [7];  [8];  [9]
  1. School of Earth and Space Exploration, Arizona State University, Tempe AZ 85287 (United States)
  2. NASA Ames Research Center, Mail Stop 245-3, Moffett Field, CA 94035 (United States)
  3. Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822 (United States)
  4. Centre for Astrophysics Research, School of Physics, Astronomy and Mathematics, University of Hertfordshire, Hatfield AL10 9AB (United Kingdom)
  5. Department of Astronomy, Harvard University, Cambridge, MA 02138 (United States)
  6. Department of Physics and Astronomy, Vanderbilt University, Nashville, TN 37235 (United States)
  7. Observatories of the Carnegie Institution for Science, 813 Santa Barbara Street, Pasadena, CA 91101 (United States)
  8. Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA 95064 (United States)
  9. BAER Institute/NASA Ames Research Center, Mail Stop 245-3, Moffett Field, CA 94035 (United States)
Publication Date:
OSTI Identifier:
22679760
Resource Type:
Journal Article
Journal Name:
Astrophysical Journal
Additional Journal Information:
Journal Volume: 848; Journal Issue: 2; Other Information: Country of input: International Atomic Energy Agency (IAEA); Journal ID: ISSN 0004-637X
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ABUNDANCE; ALKALI METALS; AMMONIA; ATMOSPHERES; CARBON; CARBON DIOXIDE; CARBON MONOXIDE; DWARF STARS; EQUILIBRIUM; GRAVITATION; HYDROGEN SULFIDES; METHANE; OXYGEN; RADIANT HEAT TRANSFER; SPECTRA; STARS; SURFACES; WATER

Citation Formats

Line, Michael R., Marley, Mark S., Freedman, Richard, Liu, Michael C., Burningham, Ben, Morley, Caroline V., Hinkel, Natalie R., Teske, Johanna, Fortney, Jonathan J., and Lupu, Roxana, E-mail: mrline@asu.edu. Uniform Atmospheric Retrieval Analysis of Ultracool Dwarfs. II. Properties of 11 T dwarfs. United States: N. p., 2017. Web. doi:10.3847/1538-4357/AA7FF0.
Line, Michael R., Marley, Mark S., Freedman, Richard, Liu, Michael C., Burningham, Ben, Morley, Caroline V., Hinkel, Natalie R., Teske, Johanna, Fortney, Jonathan J., & Lupu, Roxana, E-mail: mrline@asu.edu. Uniform Atmospheric Retrieval Analysis of Ultracool Dwarfs. II. Properties of 11 T dwarfs. United States. doi:10.3847/1538-4357/AA7FF0.
Line, Michael R., Marley, Mark S., Freedman, Richard, Liu, Michael C., Burningham, Ben, Morley, Caroline V., Hinkel, Natalie R., Teske, Johanna, Fortney, Jonathan J., and Lupu, Roxana, E-mail: mrline@asu.edu. Fri . "Uniform Atmospheric Retrieval Analysis of Ultracool Dwarfs. II. Properties of 11 T dwarfs". United States. doi:10.3847/1538-4357/AA7FF0.
@article{osti_22679760,
title = {Uniform Atmospheric Retrieval Analysis of Ultracool Dwarfs. II. Properties of 11 T dwarfs},
author = {Line, Michael R. and Marley, Mark S. and Freedman, Richard and Liu, Michael C. and Burningham, Ben and Morley, Caroline V. and Hinkel, Natalie R. and Teske, Johanna and Fortney, Jonathan J. and Lupu, Roxana, E-mail: mrline@asu.edu},
abstractNote = {Brown dwarf spectra are rich in information revealing of the chemical and physical processes operating in their atmospheres. We apply a recently developed atmospheric retrieval tool to an ensemble of late-T dwarf (600–800 K) near-infrared (1–2.5 μ m) spectra. With these spectra we are able to directly constrain the molecular abundances for the first time of H{sub 2}O, CH{sub 4}, CO, CO{sub 2}, NH{sub 3}, H{sub 2}S, and Na+K, surface gravity, effective temperature, thermal structure, photometric radius, and cloud optical depths. We find that ammonia, water, methane, and the alkali metals are present and that their abundances are well constrained in all 11 objects. We find no significant trend in the water, methane, or ammonia abundances with temperature, but find a very strong (>25 σ ) decreasing trend in the alkali metal abundances with decreasing effective temperature, indicative of alkali rainout. As expected from previous work, we also find little evidence for optically thick clouds. With the methane and water abundances, we derive the intrinsic atmospheric metallicity and carbon-to-oxygen ratios. We find in our sample that metallicities are typically subsolar (−0.4 < [ M /H] < 0.1 dex) and carbon-to-oxygen ratios are somewhat supersolar (0.4 < C/O < 1.2), different than expectations from the local stellar population. We also find that the retrieved vertical thermal profiles are consistent with radiative equilibrium over the photospheric regions. Finally, we find that our retrieved effective temperatures are lower than previous inferences for some objects and that some of our radii are larger than expectations from evolutionary models, possibly indicative of unresolved binaries. This investigation and method represent a new and powerful paradigm for using spectra to determine the fundamental chemical and physical processes governing cool brown dwarf atmospheres.},
doi = {10.3847/1538-4357/AA7FF0},
journal = {Astrophysical Journal},
issn = {0004-637X},
number = 2,
volume = 848,
place = {United States},
year = {2017},
month = {10}
}