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Published March 20, 2017 | Published + Submitted
Journal Article Open

Surface Gravities for 228 M, L, and T Dwarfs in the NIRSPEC Brown Dwarf Spectroscopic Survey

Abstract

We combine 131 new medium-resolution (R ~ 2000) J-band spectra of M, L, and T dwarfs from the Keck NIRSPEC Brown Dwarf Spectroscopic Survey (BDSS) with 97 previously published BDSS spectra to study surface-gravity-sensitive indices for 228 low-mass stars and brown dwarfs spanning spectral types M5–T9. Specifically, we use an established set of spectral indices to determine surface gravity classifications for all of the M6–L7 objects in our sample by measuring the equivalent widths (EW) of the K I lines at 1.1692, 1.1778, and 1.2529 μm, and the 1.2 μm FeH_J absorption index. Our results are consistent with previous surface gravity measurements, showing a distinct double peak—at ~L5 and T5—in K I EW as a function of spectral type. We analyze the K I EWs of 73 objects of known ages and find a linear trend between log(Age) and EW. From this relationship, we assign age ranges to the very low gravity, intermediate gravity, and field gravity designations for spectral types M6–L0. Interestingly, the ages probed by these designations remain broad, change with spectral type, and depend on the gravity-sensitive index used. Gravity designations are useful indicators of the possibility of youth, but current data sets cannot be used to provide a precise age estimate.

Additional Information

© 2017 The American Astronomical Society. Received 2016 August 12; revised 2017 February 22; accepted 2017 February 23; published 2017 March 27. The data presented herein were obtained at the W.M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. This research has benefitted from the M, L, T, and Y dwarf compendium housed at DwarfArchives.org. This research has made use of the SIMBAD database, operated at CDS, Strasbourg, France. We thank the anonymous referee for their insightful comments which improved the paper. Facility: Keck:II(NIRSPEC) - KECK II Telescope.

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Published - Martin_2017_ApJ_838_73.pdf

Submitted - 1703.03811.pdf

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August 19, 2023
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October 25, 2023