395 research outputs found
2MASS J06164006-6407194: The First Outer Halo L Subdwarf
We present the serendipitous discovery of an L subdwarf, 2MASS
J06164006-6407194, in a search of the Two Micron All Sky Survey for T dwarfs.
Its spectrum exhibits features indicative of both a cool and metal poor
atmosphere including a heavily pressured-broadened K I resonant doublet, Cs I
and Rb I lines, molecular bands of CaH, TiO, CrH, FeH, and H2O, and enhanced
collision induced absorption of H2. We assign 2MASS 0616-6407 a spectral type
of sdL5 based on a comparison of its red optical spectrum to that of near
solar-metallicity L dwarfs. Its high proper motion (mu =1.405+-0.008 arcsec
yr-1), large radial velocity (Vrad = 454+-15 km s-1), estimated uvw velocities
(94, -573, 125) km s-1 and Galactic orbit with an apogalacticon at ~29 kpc are
indicative of membership in the outer halo making 2MASS 0616-6407 the first
ultracool member of this population.Comment: Accepted for publication in Ap
2MASSJ035523.51+113337.4: A Young, Dusty, Nearby, Isolated Brown Dwarf Resembling A Giant Exoplanet
We present parallax and proper motion measurements, near-infrared spectra,
and WISE photometry for the low surface gravity L5gamma dwarf
2MASSJ035523.37+113343.7 (2M0355). We use these data to evaluate photometric,
spectral, and kinematic signatures of youth as 2M0355 is the reddest isolated L
dwarf yet classified. We confirm its low-gravity spectral morphology and find a
strong resemblance to the sharp triangular shaped -band spectrum of the 10
Myr planetary-mass object 2M1207b. We find that 2M0355 is underluminous
compared to a normal field L5 dwarf in the optical and MKO J,H, and K bands and
transitions to being overluminous from 3-12 microns, indicating that enhanced
photospheric dust shifts flux to longer wavelengths for young, low-gravity
objects, creating a red spectral energy distribution. Investigating the
near-infrared color magnitude diagram for brown dwarfs confirms that 2M0355 is
redder and underluminous compared to the known brown dwarf population, similar
to the peculiarities of directly imaged exoplanets 2M1207b and HR8799bcd. We
calculate UVW space velocities and find that the motion of 2M0355 is consistent
with young disk objects (< 2-3 Gyr) and it shows a high likelihood of
membership in the AB Doradus association.Comment: 23 pages, 10 figures, 5 Tables, Submitted to AJ 13 May 201
A Monitoring Campaign for Luhman 16AB. I. Detection of Resolved Near-Infrared Spectroscopic Variability
[abbreviated] We report resolved near-infrared spectroscopic monitoring of
the nearby L dwarf/T dwarf binary WISE J104915.57-531906.1AB (Luhman 16AB), as
part of a broader campaign to characterize the spectral energy distribution and
temporal variability of this system. A continuous 45-minute sequence of
low-resolution IRTF/SpeX data spanning 0.8-2.4 micron were obtained, concurrent
with combined-light optical photometry with ESO/TRAPPIST. Our spectral
observations confirm the flux reversal of this binary, and we detect a
wavelength-dependent decline in the relative spectral fluxes of the two
components coincident with a decline in the combined-light optical brightness
of the system over the course of the observation. These data are successfully
modeled as a combination of brightness and color variability in the T0.5 Luhman
16B, consistent cloud variations; and no significant variability in L7.5 Luhman
16A. We estimate a peak-to-peak amplitude of 13.5% at 1.25 micron over the full
lightcurve. Using a two-spot brightness temperature model, we infer an average
cloud covering fraction of ~30-55% for Luhman 16B, varying by 15-30% over a
rotation period. A Rhines scale interpretation for the size of the variable
features explains an apparent correlation between period and amplitude for
three highly variable T dwarfs, and predicts relatively fast winds (1-3 km/s)
for Luhman 16B consistent with lightcurve evolution on an advective time scale
(1-3 rotation periods). Our observations support the model of a patchy
disruption of the mineral cloud layer as a universal feature of the L dwarf/T
dwarf transition.Comment: 11 pages, 7 figures; accepted for publication in Astrophysical
Journa
Brown Dwarf Retrievals on FIRE!: Atmospheric Constraints and Lessons Learned from High Signal-to-Noise Medium Resolution Spectroscopy of a T9 Dwarf
Brown dwarf spectra offer vital testbeds for our understanding of the
chemical and physical processes that sculpt substellar atmospheres. Recently,
atmospheric retrieval approaches have been applied to a number of
low-resolution (R~100) spectra of brown dwarfs, yielding constraints on the
abundances of chemical species and temperature structures of these atmospheres.
Medium-resolution (R~1e3) spectra of brown dwarfs offer significant additional
insight, as molecular features are more easily disentangled from one another
and the thermal structure of the upper atmosphere is more readily probed. We
present results from a GPU-based retrieval analysis of a high signal-to-noise,
medium-resolution (R~6000) FIRE spectrum from 0.85-2.5 microns of a T9 dwarf.
At 60x higher spectral resolution than previous brown dwarf retrievals, a
number of novel challenges arise. We examine the strong effect of different
opacity sources on our retrieved constraints, in particular for CH4.
Furthermore, we find that flaws in the data such as errors from order stitching
can greatly bias our results. We compare these results to those obtained for a
R~100 spectrum of the same object, revealing how constraints on atmospheric
abundances and temperatures improve by an order of magnitude or more (depending
on the species) with increased spectral resolution. In particular, we precisely
constrain the abundance of H2S, which is undetectable at lower spectral
resolution. While these medium-resolution retrievals offer the potential of
precise, stellar-like constraints on atmospheric abundances (~0.02 dex), our
retrieved radius is unphysically small (R~0.50 R), indicating lingering
shortcomings with our modeling framework. This work is an initial investigation
into brown dwarf retrievals at medium spectral resolution, offering guidance
for future ground-based studies and JWST observations of substellar objects.Comment: 28 pages, 28 figures, 4 tables. Accepted to Ap
A Widely-Separated, Highly-Occluded Companion to the Nearby Low-Mass T Tauri Star TWA 30
We report the discovery of TWA 30B, a wide (~3400 AU), co-moving M dwarf
companion to the nearby (~42 pc) young star TWA 30. Companionship is confirmed
from their statistically consistent proper motions and radial velocities, as
well as a chance alignment probability of only 0.08%. Like TWA 30A, the
spectrum of TWA 30B shows signatures of an actively accreting disk (H I and
alkali line emission) and forbidden emission lines tracing outflowing material
([O I], [O II], [O III], [S II], and [N II]). We have also detected [C I]
emission in the optical data, marking the first such detection of this line in
a pre-main sequence star. Negligible radial velocity shifts in the emission
lines relative to the stellar frame of rest (Delta V < 30 km/s) indicate that
the outflows are viewed in the plane of the sky and that the corresponding
circumstellar disk is viewed edge-on. Indeed, TWA 30B appears to be heavily
obscured by its disk, given that it is 5 magnitudes fainter than TWA 30A at
K-band despite having a slightly earlier spectral type (M4 versus M5). The
near-infrared spectrum of TWA 30B also evinces an excess that varies on day
timescales, with colors that follow classical T Tauri tracks as opposed to
variable reddening (as is the case for TWA 30A). Multi-epoch data show this
excess to be well-modeled by a blackbody component with temperatures ranging
from 630 to 880 K and emitting areas that scale inversely with the temperature.
The variable excess may arise from disk structure such as a rim or a warp at
the inner disk edge located at a radial distance of ~3-5 R_sun. As the second
and third closest actively accreting and outflowing stars to the Sun (after TWA
3), TWA 30AB presents an ideal system for detailed study of star and planetary
formation processes at the low-mass end of the hydrogen-burning spectrum.Comment: 34 pages, 6 figures, AJ in press; Replaced Figure 4 with a better
color version, added 3 references and slightly amended Section 3.2.
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