81 research outputs found
Surface Gravities for 228 M, L, and T Dwarfs in the NIRSPEC Brown Dwarf Spectroscopic Survey
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 M6-L7 objects in our sample by
measuring equivalent widths (EW) of the K I lines at 1.1692, 1.1778, 1.2529 um,
and the 1.2 um FeHJ 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 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
datasets cannot be used to provide a precise age estimate.Comment: 33 pages, 13 figures, ApJ in pres
The Exemplar T8 Subdwarf Companion of Wolf 1130
We have discovered a wide separation (188.5") T8 subdwarf companion to the
sdM1.5+WD binary Wolf 1130. Companionship of WISE J200520.38+542433.9 is
verified through common proper motion over a ~3 year baseline. Wolf 1130 is
located 15.83 +/- 0.96 parsecs from the Sun, placing the brown dwarf at a
projected separation of ~3000 AU. Near-infrared colors and medium resolution
(R~2000-4000) spectroscopy establish the uniqueness of this system as a
high-gravity, low-metallicity benchmark. Although there are a number of
low-metallicity T dwarfs in the literature, WISE J200520.38+542433.9 has the
most extreme inferred metallicity to date with [Fe/H] = -0.64 +/- 0.17 based on
Wolf 1130. Model comparisons to this exemplar late-type subdwarf support it
having an old age, a low metallicity, and a small radius. However, the
spectroscopic peculiarities of WISE J200520.38+542433.9 underscore the
importance of developing the low-metallicity parameter space of the most
current atmospheric models.Comment: Accepted to ApJ on 05 September 2013; 33 pages in preprint format, 8
figures, 3 table
Measuring and Replicating the 1-20 um Energy Distributions of the Coldest Brown Dwarfs: Rotating, Turbulent and Non-Adiabatic Atmospheres
Cold, low-mass, field brown dwarfs are important for constraining the
terminus of the stellar mass function, and also for optimizing atmospheric
studies of exoplanets. In 2020 new model grids for such objects were made
available: Sonora-Bobcat and ATMO 2020. Also, new candidate cold brown dwarfs
were announced, and new spectroscopic observations at lambda ~4.8 um were
published. In this paper we present new infrared photometry for some of the
coldest brown dwarfs, and put the new data and models together to explore the
properties of these objects. We reconfirm the importance of mixing in these
atmospheres, which leads to CO and NH_3 abundances that differ by orders of
magnitude from chemical equilibrium values. We also demonstrate that the new
models retain the known factor >~3 discrepancy with observations at 2 <~ lambda
um <~ 4, for brown dwarfs cooler than 600 K. We show that the entire 1 <~
lambda um <~ 20 energy distribution of six brown dwarfs with 260 <= T_eff K <=
475 can be well reproduced, for the first time, by model atmospheres which
include dis-equilibrium chemistry as well as a photospheric temperature
gradient which deviates from the standard radiative/convective equilibrium
value. This change to the pressure-temperature profile is not unexpected for
rotating and turbulent atmospheres which are subject to diabatic processes. A
limited grid of modified-adiabat model colors is generated, and used to
estimate temperatures and metallicities for the currently known Y dwarfs. A
compilation of the photometric data used here is given in the Appendix.Comment: 40 pages which includes 16 Figures and 10 Tables. The Journal
publication will include data behind the Figures for Figures 5, 8 and 9, and
a machine readable version of Table 1
Solar Contamination in Extreme-precision Radial-velocity Measurements: Deleterious Effects and Prospects for Mitigation
Solar contamination, due to moonlight and atmospheric scattering of sunlight, can cause systematic errors in stellar radial velocity (RV) measurements that significantly detract from the ~10 cm s−1 sensitivity required for the detection and characterization of terrestrial exoplanets in or near habitable zones of Sun-like stars. The addition of low-level spectral contamination at variable effective velocity offsets introduces systematic noise when measuring velocities using classical mask-based or template-based cross-correlation techniques. Here we present simulations estimating the range of RV measurement error induced by uncorrected scattered sunlight contamination. We explore potential correction techniques, using both simultaneous spectrometer sky fibers and broadband imaging via coherent fiber imaging bundles, that could reliably reduce this source of error to below the photon-noise limit of typical stellar observations. We discuss the limitations of these simulations, the underlying assumptions, and mitigation mechanisms. We also present and discuss the components designed and built into the NEID (NN-EXPLORE Exoplanet Investigations with Doppler spectroscopy) precision RV instrument for the WIYN 3.5 m telescope, to serve as an ongoing resource for the community to explore and evaluate correction techniques. We emphasize that while "bright time" has been traditionally adequate for RV science, the goal of 10 cm s−1 precision on the most interesting exoplanetary systems may necessitate access to darker skies for these next-generation instruments
The AllWISE Motion Survey, Part 2
We use the AllWISE Data Release to continue our search for WISE-detected
motions. In this paper, we publish another 27,846 motion objects, bringing the
total number to 48,000 when objects found during our original AllWISE motion
survey are included. We use this list, along with the lists of confirmed
WISE-based motion objects from the recent papers by Luhman and by Schneider et
al. and candidate motion objects from the recent paper by Gagne et al. to
search for widely separated, common-proper-motion systems. We identify 1,039
such candidate systems. All 48,000 objects are further analyzed using
color-color and color-mag plots to provide possible characterizations prior to
spectroscopic follow-up. We present spectra of 172 of these, supplemented with
new spectra of 23 comparison objects from the literature, and provide
classifications and physical interpretations of interesting sources. Highlights
include: (1) the identification of three G/K dwarfs that can be used as
standard candles to study clumpiness and grain size in nearby molecular clouds
because these objects are currently moving behind the clouds, (2) the
confirmation/discovery of several M, L, and T dwarfs and one white dwarf whose
spectrophotometric distance estimates place them 5-20 pc from the Sun, (3) the
suggestion that the Na 'D' line be used as a diagnostic tool for interpreting
and classifying metal-poor late-M and L dwarfs, (4) the recognition of a triple
system including a carbon dwarf and late-M subdwarf, for which model fits of
the late-M subdwarf (giving [Fe/H] ~ -1.0) provide a measured metallicity for
the carbon star, and (5) a possible 24-pc-distant K5 dwarf + peculiar red L5
system with an apparent physical separation of 0.1 pc.Comment: 62 pages with 80 figures, accepted for publication in The
Astrophysical Journal Supplement Series, 23 Mar 2016; second version fixes a
few small typos and corrects the footnotes for Table
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