3,196 research outputs found
SDSS J125637-022452: a high proper motion L subdwarf
We report the discovery of a high proper motion L subdwarf (
=0.617arcsec/yr) in the Sloan Digital Sky Survey spectral database. The optical
spectrum from the star SDSS J125637-022452 has mixed spectral features of both
late-M spectral subtype (strong TiO and CaH at 7000A) and mid-L spectral
subtype (strong wings of KI at 7700A, CrH and FeH), which is interpreted as the
signature of a very low-mass, metal-poor star (ultra-cool subdwarf) of spectral
type sdL. The near infrared (NIR) (J-Ks) colors from 2MASS shows the object to
be significantly bluer compared to normal L dwarfs, which is probably due a
strong collision induced absorption (CIA) due to H2 molecule. This is
consistent with the idea that CIA from H2 is more pronounced at low
metallicities. Proper motion and radial velocity measurements also indicate
that the star is kinematically "hot" and probably associated with the Galactic
halo population.Comment: 13 pages, 2 figures. Accepted for ApJ
Spectroscopic confirmation of UV-bright white dwarfs from the Sandage Two-Color Survey of the Galactic Plane
We present spectroscopic observations confirming the identification of hot
white dwarfs among UV-bright sources from the Sandage Two-color Survey of the
Galactic plane and listed in the Lanning (Lan) catalog of such sources. A
subsample of 213 UV bright Lan sources have been identified as candidate white
dwarfs based on the detection of a significant proper motion. Spectroscopic
observations of 46 candidates with the KPNO 2.1m telescope confirm 30 sources
to be hydrogen white dwarfs with subtypes in the DA1-DA6 range, and with one of
the stars (Lan 161) having an unresolved M dwarf as a companion. Five more
sources are confirmed to be helium white dwarfs, with subtypes from DB3 to DB6.
One source (Lan 364) is identified as a DZ 3 white dwarf, with strong lines of
calcium. Three more stars are found to have featureless spectra (to within
detection limits), and are thus classified as DC white dwarfs. In addition,
three sources are found to be hot subdwarfs: Lan 20 and Lan 480 are classified
as sdOB, and Lan 432 is classified sdB. The remaining four objects are found to
be field F star interlopers. Physical parameters of the DA and DB white dwarfs
are derived from model fits.Comment: To appear in the Astronomical Journa
Trigonometric parallaxes of ten ultracool subdwarfs
We measured absolute trigonometric parallaxes and proper motions with respect
to many background galaxies for a sample of ten ultracool subdwarfs.
The observations were taken in the H-band with the OMEGA2000 camera at the
3.5m-telescope on Calar Alto, Spain during a time period of 3.5 years. For the
first time, the reduction of the astrometric measurements was carried out
directly with respect to background galaxies. We obtained absolute parallaxes
with mean errors ranging between 1 and 3 mas.
With six completely new parallaxes we more than doubled the number of
benchmark ultracool (>sdM7) subdwarfs. Six stars in the M_{K_s} vs. J-K_s
diagram fit perfectly to model subdwarf sequences from M7 to L4 with [M/H]
between -1.0 and -1.5, whereas 4 are consistent with a moderately low
metallicity ([M/H]=-0.5) from M7 to T6. All but one of our objects have large
tangential velocities between 200 and 320 km/s typical of the Galactic halo
population.
Our results are in good agreement with recent independent measurements for
three of our targets and confirm the previously measured parallax and absolute
magnitude M_{K_s} of the nearest and coolest (T-type) subdwarf 2MASS 0937+29
with higher accuracy.
For all targets, we also obtained infrared J,H,K_s photometry at a level of a
few milli-magnitudes relative to 2MASS standards.Comment: Letter 4 pages 1 figure. accepted by Astronomy and Astrophysic
Wide companions to Hipparcos stars within 67 pc of the Sun
A catalog of common-proper-motion (CPM) companions to stars within 67 pc of
the Sun is constructed based on the SUPERBLINK proper-motion survey. It
contains 1392 CPM pairs with angular separations 30" < \rho < 1800", relative
proper motion between the two components less than 25 mas/yr, magnitudes and
colors of the secondaries consistent with those of dwarfs in the (M_V,V-J)
diagram. In addition, we list 21 candidate white-dwarf CPM companions with
separations under 300", about half of which should be physical. We estimate a
0.31 fraction of pairs with red-dwarf companions to be physical systems (about
425 objects), while the rest (mostly wide pairs) are chance alignments. For
each candidate companion, the probability of a physical association is
evaluated. The distribution of projected separations s of the physical pairs
between 2 kAU and 64 kAU follows f(s) ~ s^{-1.5}, which decreases faster than
\"Opik's law. We find that Solar-mass dwarfs have no less than 4.4% +/- 0.3%
companions with separations larger than 2 kAU, or 3.8% +/- 0.3% per decade of
orbital separation in the 2 to 16 kAU range. The distribution of mass ratio of
those wide companions is approximately uniform in the 0.1<q<1.0 range, although
we observe a dip at q=0.5 which, if confirmed, could be evidence of bimodal
distribution of companion masses. New physical CPM companions to two exoplanet
host stars are discovered.Comment: To appear in the Astronomical Journal. Electronic data table
available on request from the author
The Effect of Women’s Bargaining Power on Child Nutrition in Rural Senegal
We examine how women’s bargaining power affects child nutritional status using data from rural Senegal. In order to correct for the potential endogeneity of women’s bargaining power we use information on a mother’s ethnicity relative to that of the community she resides in order to construct an arguably exogenous exclusion restriction. While standard OLS estimates suggest that if a mother has more bargaining power, her children will have a better nutritional status, our IV estimates indicate that the true impact is underestimated if the endogeneity of bargaining power is not taken into account
The Na 8200 Angstrom Doublet as an Age Indicator in Low-Mass Stars
We investigate the use of the gravity sensitive neutral sodium (NaI) doublet
at 8183 Angstroms 8195 Angstroms (Na 8200 Angstrom doublet) as an age indicator
for M dwarfs. We measured the Na doublet equivalent width (EW) in giants, old
dwarfs, young dwarfs, and candidate members of the Beta Pic moving group using
medium resolution spectra. Our Na 8200 Angstrom doublet EW analysis shows that
the feature is useful as an approximate age indicator in M-type dwarfs with
(V-K_s) >= 5.0, reliably distinguishing stars older and younger than 100 Myr. A
simple derivation of the dependence of the Na EW on temperature and gravity
supports the observational results. An analysis of the effects of metallicity
show that this youth indicator is best used on samples with similar
metallicity. The age estimation technique presented here becomes useful in a
mass regime where traditional youth indicators are increasingly less reliable,
is applicable to other alkali lines, and will help identify new-low mass
members in other young clusters and associations.Comment: 26 pages, 8 figures, accepted to the Astronomical Journa
Twenty-three new ultra-cool subdwarfs from the Sloan Digital Sky Survey
A search of the Sloan Digital Sky Survey spectroscopic database has turned up
23 new ultra-cool subdwarfs, low-mass metal-poor stars of spectral subtype M
7.0 or later. Spectra from these red objects all show very strong molecular
bands of CaH but relatively weak bands of TiO, indicative of a cool, metal-poor
atmosphere. Five of the stars are formally classified as M subdwarfs
(sdM7.0-sdM8.5), 13 as more metal-poor extreme subdwarfs (esdM7.0-esdM8.0), and
five as extremely metal-poor ultra subdwarfs (usdM7.0-usdM7.5). In the
[H_r,r-z] reduced proper motion diagram, these subdwarfs clearly populate the
locus of low-luminosity stars with halo kinematics. It is argued that the
objects are all very low-mass, metal-poor stars from the Galactic halo
(Population II). These new discoveries more than double the census of
spectroscopically confirmed ultra-cool subdwarfs. We show that the stars stand
out remarkably in the [g-r,r-i] color-color diagram; a proposed color and
proper motion selection scheme is expected to be extremely efficient in
identifying more of these old, very low mass stars in the vicinity of the Sun.Comment: 4 pages, submitted to the Astrophysical Journal Letters, version 2
includes minor correction
A Very High Proper Motion Star and the First L dwarf in the Kepler Field
We report two nearby high proper motion dwarfs of special interest identified
using the Preliminary Data Release of the Wide-field Infrared Survey Explorer
(WISE) and the Two Micron All-Sky Survey (2MASS). WISEP J191239.91-361516.4 has
a motion of 2.1 arcseconds per year. Photometry identifies it as a mid-M dwarf.
WISEP J190648.47+401106.8 is a spectroscopically confirmed L1 dwarf in the
Kepler Mission field with a motion of 0.48 arcseconds per year. The estimated
distance is 17 parsecs. Both lie at relatively low galactic latitudes and
demonstrate the possibility of discovering proper motion stars independently of
the historic photographic sky surveys.Comment: 11 pages, accepted by Astrophysical Journal Letter
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