405 research outputs found
Mapping the Local Halo: Statistical Parallax Analysis of SDSS Low-Mass Subdwarfs
We present a statistical parallax study of nearly 2000 M subdwarfs with photometry and spectroscopy from the Sloan Digital Sky Survey (SDSS). Statistical parallax analysis yields the mean absolute magnitudes, mean velocities, and velocity ellipsoids for homogenous samples of stars. We selected homogeneous groups of subdwarfs based on their photometric colors and spectral appearance. We examined the color–magnitude relations of low-mass subdwarfs and quantified their dependence on the newly refined metallicity parameter, ζ. We also developed a photometric metallicity parameter, δ(g − r), based on the g − r and r − z colors of low-mass stars and used it to select stars with similar metallicities. The kinematics of low-mass subdwarfs as a function of color and metallicity were also examined and compared to main-sequence M dwarfs. We find that the SDSS subdwarfs share similar kinematics to the inner halo and thick disk. The color–magnitude relations derived in this analysis will be a powerful tool for identifying and characterizing low-mass metal-poor subdwarfs in future surveys such as Gaia and LSST, making them important and plentiful tracers of the stellar halo
Near-infrared Detection of WD 0806-661 B with the Hubble Space Telescope
WD 0806-661 B is one of the coldest known brown dwarfs (T=300-345 K) based on
previous mid-infrared photometry from the Spitzer Space Telescope. In addition,
it is a benchmark for testing theoretical models of brown dwarfs because its
age and distance are well-constrained via its primary star (2+/-0.5 Gyr,
19.2+/-0.6 pc). We present the first near-infrared detection of this object,
which has been achieved through F110W imaging (~Y+J) with the Wide Field Camera
3 on board the Hubble Space Telescope. We measure a Vega magnitude of
m110=25.70+/-0.08, which implies J~25.0. When combined with the Spitzer
photometry, our estimate of J helps to better define the empirical sequence of
the coldest brown dwarfs in M4.5 versus J-[4.5]. The positions of WD 0806-661 B
and other Y dwarfs in that diagram are best matched by the cloudy models of
Burrows et al. and the cloudless models of Saumon et al., both of which employ
chemical equilibrium. The calculations by Morley et al. for 50% cloud coverage
differ only modestly from the data. Spectroscopy would enable a more stringent
test of the models, but based on our F110W measurement, such observations are
currently possible only with Hubble, and would require at least ~10 orbits to
reach a signal-to-noise ratio of ~5
Hunting The Most Distant Stars in the Milky Way: Methods and Initial Results
We present a new catalog of 404 M giant candidates found in the UKIRT
Infrared Deep Sky Survey (UKIDSS). The 2,400 deg available in the UKIDSS
Large Area Survey Data Release 8 resolve M giants through a volume four times
larger than that of the entire Two Micron All Sky Survey. Combining
near-infrared photometry with optical photometry and proper motions from the
Sloan Digital Sky Survey yields an M giant candidate catalog with less M dwarf
and quasar contamination than previous searches for similarly distant M giants.
Extensive follow-up spectroscopy of this sample will yield the first map of our
Galaxy's outermost reaches over a large area of sky. Our initial spectroscopic
follow-up of 30 bright candidates yielded the positive identification of
five M giants at distances kpc. Each of these confirmed M giants
have positions and velocities consistent with the Sagittarius stream. The
fainter M giant candidates in our sample have estimated photometric distances
kpc (assuming = 0.0), but require further spectroscopic
verification. The photometric distance estimates extend beyond the Milky Way's
virial radius, and increase by for each 0.5 dex decrease in assumed
. Given the number of M giant candidates, initial selection efficiency,
and volume surveyed, we loosely estimate that at least one additional
Sagittarius-like accretion event could have contributed to the hierarchical
build-up of the Milky Way's outer halo.Comment: 16 pages, 11 figures, emulateapj format. Accepted by A
The Sloan Digital Sky Survey Data Release 7 Spectroscopic M Dwarf Catalog III: The Spatial Dependence of Magnetic Activity in the Galaxy
We analyze the magnetic activity of 59,318 M dwarfs from the Sloan Digital Sky Survey (SDSS) Data Release 7. This analysis explores the spatial distribution of M dwarf activity as a function of both vertical distance from the Galactic plane (Z) and planar distance from the Galactic center (R). We confirm the established trends of decreasing magnetic activity (as measured by Hα emission) with increasing distance from the mid-plane of the disk and find evidence for a trend in Galactocentric radius. We measure a non-zero radial gradient in the activity fraction in our analysis of stars with spectral types dM3 and dM4. The activity fraction increases with R and can be explained by a decreasing mean stellar age with increasing distance from the Galactic center
The Most Distant Stars in the Milky Way
We report on the discovery of the most distant Milky Way (MW) stars known to
date: ULAS J001535.72015549.6 and ULAS J074417.48253233.0. These stars
were selected as M giant candidates based on their infrared and optical colors
and lack of proper motions. We spectroscopically confirmed them as outer halo
giants using the MMT/Red Channel spectrograph. Both stars have large estimated
distances, with ULAS J001535.72015549.6 at kpc and ULAS
J074417.48253233.0 at 238 64 kpc, making them the first MW stars
discovered beyond 200 kpc. ULAS J001535.72015549.6 and ULAS
J074417.48253233.0 are both moving away from the Galactic center at km s and km s, respectively. Using their
distances and kinematics, we considered possible origins such as: tidal
stripping from a dwarf galaxy, ejection from the MW's disk, or membership in an
undetected dwarf galaxy. These M giants, along with two inner halo giants that
were also confirmed during this campaign, are the first to map largely
unexplored regions of our Galaxy's outer halo.Comment: Accepted and in print by ApJL. Seven pages, 2 figure
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