19 research outputs found
The Binary White Dwarf LHS 3236
The white dwarf LHS 3236 (WD1639+153) is shown to be a double-degenerate
binary, with each component having a high mass. Astrometry at the U.S. Naval
Observatory gives a parallax and distance of 30.86 +/- 0.25 pc and a tangential
velocity of 98 km/s, and reveals binary orbital motion. The orbital parameters
are determined from astrometry of the photocenter over more than three orbits
of the 4.0-year period. High-resolution imaging at the Keck Observatory
resolves the pair with a separation of 31 and 124 mas at two epochs. Optical
and near-IR photometry give a set of possible binary components. Consistency of
all data indicates that the binary is a pair of DA stars with temperatures near
8000 and 7400 K and with masses of 0.93 and 0.91 M_solar; also possible, is a
DA primary and a helium DC secondary with temperatures near 8800 and 6000 K and
with masses of 0.98 and 0.69 M_solar. In either case, the cooling ages of the
stars are ~3 Gyr and the total ages are <4 Gyr. The combined mass of the binary
(1.66--1.84 M_solar) is well above the Chandrasekhar limit; however, the
timescale for coalescence is long.Comment: Accepted for the Astrophysical Journa
Trigonometric Parallaxes of Central Stars of Planetary Nebulae
Trigonometric parallaxes of 16 nearby planetary nebulae are presented,
including reduced errors for seven objects with previous initial results and
results for six new objects. The median error in the parallax is 0.42 mas, and
twelve nebulae have parallax errors less than 20 percent. The parallax for
PHL932 is found here to be smaller than was measured by Hipparcos, and this
peculiar object is discussed. Comparisons are made with other distance
estimates. The distances determined from these parallaxes tend to be
intermediate between some short distance estimates and other long estimates;
they are somewhat smaller than estimated from spectra of the central stars.
Proper motions and tangential velocities are presented. No astrometric
perturbations from unresolved close companions are detected.Comment: 24 pages, includes 4 figures. Accepted for A
The USNO-B Catalog
USNO-B is an all-sky catalog that presents positions, proper motions,
magnitudes in various optical passbands, and star/galaxy estimators for
1,042,618,261 objects derived from 3,643,201,733 separate observations. The
data were obtained from scans of 7,435 Schmidt plates taken for the various sky
surveys during the last 50 years. USNO-B1.0 is believed to provide all-sky
coverage, completeness down to V = 21, 0.2 arcsecond astrometric accuracy at
J2000, 0.3 magnitude photometric accuracy in up to five colors, and 85%
accuracy for distinguishing stars from non-stellar objects. A brief discussion
of various issues is given here, but the actual data are available from
http://www.nofs.navy.mil and other sites.Comment: Accepted by Astronomical Journa
The Identification of the X-ray Counterpart to PSR J2021+4026
We report the probable identification of the X-ray counterpart to the
gamma-ray pulsar PSR J2021+4026 using imaging with the Chandra X-ray
Observatory ACIS and timing analysis with the Fermi satellite. Given the
statistical and systematic errors, the positions determined by both satellites
are coincident. The X-ray source position is R.A. 20h21m30.733s, Decl. +40 deg
26 min 46.04sec (J2000) with an estimated uncertainty of 1.3 arsec combined
statistical and systematic error. Moreover, both the X-ray to gamma-ray and the
X-ray to optical flux ratios are sensible assuming a neutron star origin for
the X-ray flux. The X-ray source has no cataloged infrared-to-visible
counterpart and, through new observations, we set upper limits to its optical
emission of i' >23.0 mag and r' > 25.2mag. The source exhibits an X-ray
spectrum with most likely both a powerlaw and a thermal component. We also
report on the X-ray and visible light properties of the 43 other sources
detected in our Chandra observation.Comment: Accepted for publication in the Astrophysical Journa
Trigonometric Parallaxes for Two Late-Type Subdwarfs: LSR1425+71 (sdM8.0) and the Binary LSR1610-00 (sd?M6pec)
Trigonometric parallax astrometry and BVI photometry are presented for two
late-type subdwarf candidates, LSR1425+71 (sdM8.0) and LSR1610-00 (sd?M6pec).
For the former we measure an absolute parallax of 13.37+/-0.51 mas yielding
Mv=15.25+/-0.09. The astrometry for LSR1610-00 shows that this object is an
astrometric binary with a period of 1.66+/-0.01 yr. The photocentric orbit is
derived from the data; it has a moderate eccentricity (e ~ 0.44+/-0.02) and a
semi-major axis of 0.28+/-0.01 AU based on our measured absolute parallax of
31.02+/-0.26 mas. Our radial velocity measure of -108.1+/-1.6 km/s for
LSR1610-00 at epoch 2006.179, when coupled with the observation of -95+/-1 km/s
at epoch 2005.167 by Reiners & Basri, indicates a systemic radial velocity of
-101+/-1 km/s for the LSR1610-00AB pair. The galactic velocity components for
LSR1425+71 and LSR1610-00AB -- (U,V,W)=(84+/-6, -202+/-13, 66+/-14) km/s and
(U,V,W)=(36+/-2, -232+/-2, -61+/-2) km/s, respectively. For both stars, the
velocities are characteristic of halo population kinematics. However, modeling
shows that both stars have orbits around the galaxy with high eccentricity that
pass remarkably close to the galactic center. LSR1425+71 has a luminosity and
colors consistent with its metal-poor subdwarf spectral classification, while
LSR1610-00 has a luminosity and most colors indicative of being only mildly
metal-poor, plus a uniquely red B-V color. The companion to LSR1610-00 must be
a low-mass, substellar brown dwarf. We speculate on the paradoxical nature of
LSR1610-00 and possible sources of its peculiarities.Comment: Accepted for ApJ. 37 pages, including 8 figure
Astrometry and Photometry for Cool Dwarfs and Brown Dwarfs
Trigonometric parallax determinations are presented for 28 late type dwarfs
and brown dwarfs, including eight M dwarfs with spectral types between M7 and
M9.5, 17 L dwarfs with spectral types between L0 and L8, and three T dwarfs.
Broadband photometry at CCD wavelengths (VRIz) and/or near-IR wavelengths (JHK)
are presented for these objects and for 24 additional late-type dwarfs.
Supplemented with astrometry and photometry from the literature, including ten
L and two T dwarfs with parallaxes established by association with bright,
usually HIPPARCOS primaries, this material forms the basis for studying various
color-color and color-absolute magnitude relations. The I-J color is a good
predictor of absolute magnitude for late-M and L dwarfs. M_J becomes
monotonically fainter with I-J color and with spectral type through late-L
dwarfs, then brightens for early-T dwarfs. The combination of zJK colors alone
can be used to classify late-M, early-L, and T dwarfs accurately, and to
predict their absolute magnitudes, but is less effective at untangling the
scatter among mid- and late-L dwarfs. The mean tangential velocity of these
objects is found to be slightly less than that for dM stars in the solar
neighborhood, consistent with a sample with a mean age of several Gyr. Using
colors to estimate bolometric corrections, and models to estimate stellar
radii, effective temperatures are derived. The latest L dwarfs are found to
have T_eff ~ 1360 K.Comment: 48 pages, including 7 figures and 6 tables. Accepted for A
LSST: from Science Drivers to Reference Design and Anticipated Data Products
(Abridged) We describe here the most ambitious survey currently planned in
the optical, the Large Synoptic Survey Telescope (LSST). A vast array of
science will be enabled by a single wide-deep-fast sky survey, and LSST will
have unique survey capability in the faint time domain. The LSST design is
driven by four main science themes: probing dark energy and dark matter, taking
an inventory of the Solar System, exploring the transient optical sky, and
mapping the Milky Way. LSST will be a wide-field ground-based system sited at
Cerro Pach\'{o}n in northern Chile. The telescope will have an 8.4 m (6.5 m
effective) primary mirror, a 9.6 deg field of view, and a 3.2 Gigapixel
camera. The standard observing sequence will consist of pairs of 15-second
exposures in a given field, with two such visits in each pointing in a given
night. With these repeats, the LSST system is capable of imaging about 10,000
square degrees of sky in a single filter in three nights. The typical 5
point-source depth in a single visit in will be (AB). The
project is in the construction phase and will begin regular survey operations
by 2022. The survey area will be contained within 30,000 deg with
, and will be imaged multiple times in six bands, ,
covering the wavelength range 320--1050 nm. About 90\% of the observing time
will be devoted to a deep-wide-fast survey mode which will uniformly observe a
18,000 deg region about 800 times (summed over all six bands) during the
anticipated 10 years of operations, and yield a coadded map to . The
remaining 10\% of the observing time will be allocated to projects such as a
Very Deep and Fast time domain survey. The goal is to make LSST data products,
including a relational database of about 32 trillion observations of 40 billion
objects, available to the public and scientists around the world.Comment: 57 pages, 32 color figures, version with high-resolution figures
available from https://www.lsst.org/overvie