509 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 colorâmagnitude relations of low-mass subdwarfs and quantified their dependence on the newly refined metallicity parameter, ζ. We also developed a photometric metallicity parameter, ÎŽ(g â r), based on the gâââr and râââz 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 colorâmagnitude 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
A New Sample of Cool Subdwarfs from SDSS: Properties and Kinematics
We present a new sample of M subdwarfs compiled from the 7th data release of
the Sloan Digital Sky Survey. With 3517 new subdwarfs, this new sample
significantly increases the number of spectroscopically confirmed low-mass
subdwarfs. This catalog also includes 905 extreme and 534 ultra sudwarfs. We
present the entire catalog including observed and derived quantities, and
template spectra created from co-added subdwarf spectra. We show color-color
and reduced proper motion diagrams of the three metallicity classes, which are
shown to separate from the disk dwarf population. The extreme and ultra
subdwarfs are seen at larger values of reduced proper motion as expected for
more dynamically heated populations. We determine 3D kinematics for all of the
stars with proper motions. The color-magnitude diagrams show a clear separation
of the three metallicity classes with the ultra and extreme subdwarfs being
significantly closer to the main sequence than the ordinary subdwarfs. All
subdwarfs lie below (fainter) and to the left (bluer) of the main sequence.
Based on the average velocities and their dispersions, the extreme
and ultra subdwarfs likely belong to the Galactic halo, while the ordinary
subdwarfs are likely part of the old Galactic (or thick) disk. An extensive
activity analysis of subdwarfs is performed using H emission and 208
active subdwarfs are found. We show that while the activity fraction of
subdwarfs rises with spectral class and levels off at the latest spectral
classes, consistent with the behavior of M dwarfs, the extreme and ultra
subdwarfs are basically flat.Comment: 66 pages, 23 figures, accepted in Ap
The Very Short Period M Dwarf Binary SDSS J001641-000925
We present follow-up observations and analysis of the recently discovered
short period low-mass eclipsing binary, SDSS J001641-000925. With an orbital
period of 0.19856 days, this system has one of the shortest known periods for
an M dwarf binary system. Medium-resolution spectroscopy and multi-band
photometry for the system are presented. Markov chain Monte Carlo modeling of
the light curves and radial velocities yields estimated masses for the stars of
M1 = 0.54 +/- 0.07 Msun and M2 = 0.34 +/- 0.04 Msun, and radii of R1 = 0.68 +/-
0.03 Rsun and R2 = 0.58 +/- 0.03 Rsun respectively. This solution places both
components above the critical Roche overfill limit, providing strong evidence
that SDSS J001641-000925 is the first verified M-dwarf contact binary system.
Within the follow-up spectroscopy we find signatures of non-solid body rotation
velocities, which we interpret as evidence for mass transfer or loss within the
system. In addition, our photometry samples the system over 9 years, and we
find strong evidence for period decay at the rate of dP/dt ~8 s/yr. Both of
these signatures raise the intriguing possibility that the system is in
over-contact, and actively losing angular momentum, likely through mass loss.
This places SDSS J001641-000925 as not just the first M-dwarf over-contact
binary, but one of the few systems of any spectral type known to be actively
undergoing coalescence. Further study SDSS J001641-000925 is on-going to verify
the nature of the system, which may prove to be a unique astrophysical
laboratory.Comment: 11 figures, ApJ Accepte
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
M Dwarfs in SDSS Stripe 82: Photometric Light Curves and Flare Rate Analysis
We present a flare rate analysis of 50,130 M dwarf light curves in SDSS
Stripe 82. We identified 271 flares using a customized variability index to
search ~2.5 million photometric observations for flux increases in the u- and
g-bands. Every image of a flaring observation was examined by eye and with a
PSF-matching and image subtraction tool to guard against false positives.
Flaring is found to be strongly correlated with the appearance of H-alpha in
emission in the quiet spectrum. Of the 99 flare stars that have spectra, we
classify 8 as relatively inactive. The flaring fraction is found to increase
strongly in stars with redder colors during quiescence, which can be attributed
to the increasing flare visibility and increasing active fraction for redder
stars. The flaring fraction is strongly correlated with |Z| distance such that
most stars that flare are within 300 pc of the Galactic plane. We derive flare
u-band luminosities and find that the most luminous flares occur on the
earlier-type M dwarfs. Our best estimate of the lower limit on the flaring rate
(averaged over Stripe 82) for flares with \Delta u \ge 0.7 magnitudes on stars
with u < 22 is 1.3 flares hour^-1 square degree^-1 but can vary significantly
with the line-of-sight.Comment: 44 pages, 13 figure
The M Dwarf Problem in the Galaxy
We present evidence that there is an M dwarf problem similar to the
previously identified G dwarf and K dwarf problems: the number of
low-metallicity M dwarfs is not sufficient to match simple closed-box models of
local Galactic chemical evolution. We estimated the metallicity of 4141 M dwarf
stars with spectra from the Sloan Digital Sky Survey (SDSS) using a molecular
band strength versus metallicity calibration developed using high resolution
spectra of nearby M dwarfs. Using a sample of M dwarfs with measured
magnitudes, parallaxes, and metallicities, we derived a relation that describes
the absolute magnitude variation as a function of metallicity. When we examined
the metallicity distribution of SDSS stars, after correcting for the different
volumes sampled by the magnitude-limited survey, we found that there is an M
dwarf problem, with the number of M dwarfs at [Fe/H] ~ -0.5 less than 1% the
number at [Fe/H] = 0, where a simple model of Galactic chemical evolution
predicts a more gradual drop in star numbers with decreasing metallicity.Comment: To be published in Monthly Notices of the RAS by the Royal
Astronomical Society and Blackwell Publishing. 7 pages, 3 figure
The SLoWPoKES Catalog of Low-mass Ultra-wide Binaries: A Cool Stars Resource for Testing Fundamental Properties and for Constraining Binary Formation Theory
We present results from the Sloan Low-mass Wide Pairs of Kinematically
Equivalent Stars (SLoWPoKES) catalog of ultra-wide (10^3-10^5.5 AU), low-mass
(K5-M7) common proper motion binaries. We constructed a Galactic model, based
on empirical stellar number density and 3D velocity distributions, to select
bona fide pairs with probability of chance alignment <5%, making SLoWPoKES an
efficient sample for followup observations. Our initial catalog contains 1342
disk dwarf, subdwarf, and white dwarf-red dwarf systems and is the largest
collection of low-mass, wide binaries ever assembled. The diversity---in mass,
metallicity, age, and evolutionary states---of SLoWPoKES pairs makes it a
valuable resource of coeval laboratories to examine and constrain the physical
properties of low-mass stars. SLoWPoKES pairs show signatures of two (or more)
formation modes in the distribution of the physical separation and higher-order
multiplicity. Neither dynamical dissipation of primordial triples/quadruples or
dynamical capture of ejected stars can explain the observed populations by
itself. We use followup spectroscopic observations to recalibrate the
metallicity-sensitive {\zeta}_(TiO/CaH) index by assuming that both members of
the binary system have the same composition. Our new formulation is a
significantly better tracer of absolute metallicity, particularly for the
early-type M dwarfs. The SLoWPoKES catalog is publicly available on a custom
data visualization portal.Comment: To appear in Cool Star XVII proceeding
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