187 research outputs found
The Nearest Isolated Member of the TW Hydrae Association is a Giant Planet Analog
In a recent search for unusually red L and T dwarfs, we identified 2MASS
J11193254-1137466 as a likely young L7 dwarf and potential member of the TW
Hydrae association. We present spectra that confirm the youth of this object.
We also measure a radial velocity of 8.5 +/- 3.3 km/s that, together with the
sky position, proper motion and photometric distance, results in a 92%
probability of membership in the TW Hydrae association, with a calibrated field
contamination probability of 0.0005% using the BANYAN II tool. Using the age of
TW Hydrae and the luminosity of 2MASS J11193254-1137466, we estimate its mass
to be 4.3--7.6 MJup. It is the lowest-mass and nearest isolated member of TW
Hydrae at a kinematic distance of 28.9 +/- 3.6 pc, and the second-brightest
isolated <10 MJup object discovered to date.Comment: 6 pages, 3 figures, Accepted for publication in ApJ Letter
Ultracool Dwarfs Observed with the Spitzer Infrared Spectrograph -- III. Dust Grains in Young L Dwarf Atmospheres Are Heavier
Analysis of all archival 5--14 micron spectra of field ultracool dwarfs from
the Infrared Spectrograph on the Spitzer Space Telescope has shown that
absorption by silicates in the 8--11 micron region is seen in most L-type (1300
K to 2200 K) dwarfs. The absorption is caused by silicate-rich clouds in the
atmospheres of L dwarfs and is strongest at L4--L6 spectral types. Herein we
compare averages of the mid-infrared silicate absorption signatures of L3--L7
dwarfs that have low (10 cm s) vs.\ high
(10 cm s) surface gravity. We find that the silicate
absorption feature is sensitive to surface gravity and indicates a difference
in grain size and composition between dust condensates in young and old mid-L
dwarfs. The mean silicate absorption profile of low-gravity mid-L dwarfs
matches expectations for 1 micron-sized amorphous iron- and
magnesium-bearing pyroxene (MgFeSiO) grains. High-gravity mid-L
dwarfs have silicate absorption better represented by smaller (0.1
m) and more volatile amorphous enstatite (MgSiO) or SiO grains. This
is the first direct spectroscopic evidence for gravity-dependent sedimentation
of dust condensates in ultracool atmospheres. It confirms theoretical
expectations for lower sedimentation efficiencies in low-gravity atmospheres
and independently confirms their increased dustiness.Comment: Accepted for publication in MNRA
Adaptive Optics Observations of Vega: Eight Detected Sources and Upper Limits to Planetary-Mass Companions
From adaptive optics observations with the Palomar 5-meter telescope we place
upper limits on the masses of any planetary companions located between ~30-230
AU away from Vega, where our data are sensitive to depths ranging from H=12.5
mag to H=19.0 mag fainter than Vega itself. Our observations cover a
plus-shaped area with two 25"x57" elements, excluding 7"x7" centered on the
star. We have identified 2 double and 4 single point sources. These projected
companions are 14.9-18.9 mag fainter than Vega, and if physically associated
would have masses ranging from 4 to 35 Mjup and orbital radii 170-260 AU.
Recent simulations of dusty rings around Vega predict the presence of a
perturbing body with mass <2-3 Mjup and orbital radius ~40-100 AU, though more
massive (~10 Mjup) planets cannot be excluded. None of the detected objects are
this predicted planet. Based on a color-magnitude, spectroscopic, and proper
motion analysis, all objects are consistent with being background sources.
Given the glare of Vega, a 2 Mjup object near the expected orbital radii would
not have been visible at the 5-sigma level in our data, though any >10 Mjup
brown dwarf could have been seen at separation >80 AU.Comment: 16 pages, 1 table, 4 figures. Accepted for publication in Ap
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