20 research outputs found
A Survey for Faint Stars of Large Proper Motion Using Extra Poss II Plates
We have conducted a search for new stars of high proper motion (0.4
arcsec yr) using POSS II fields for which an extra IIIaF (red) plate of
good quality exists, along with useable IIIaJ (blue) and IV-N (infrared) plates
taken at epochs differing by a minimum of 1.5 years. Thirty-five fields at
Galactic latitudes 20 were measured, covering some 1378 deg,
or 3.3% of the sky. Searches with three plate combinations as well as all four
plates were also made. Seven new stars were found with 0.5 arcsec
yr, which were therefore missed in the Luyten Half Second (LHS) Catalog.
One of these is a common proper motion binary consisting of two subdwarf M
stars; another is a cool white dwarf with probable halo kinematics. As a test
of our completeness -- and of our ability to test that of Luyten -- 216 of 230
catalogued high proper motion stars were recovered by the software, or 94%.
Reasons for incompleteness of the LHS are discussed, such as the simple fact
that POSS II plates have deeper limiting magnitudes and greater overlap than
did POSS I. Nonetheless, our results suggest that the LHS is closer to 90%
complete than recent estimates in the literature (e.g 60%), and we propose a
reason to account for one such lower estimate. The conclusion that the LHS
Catalog is more complete has implications for the nature of the halo dark
matter. In particular it strengthens the constraint on the local density of
halo stars, especially white dwarfs at M17-18.Comment: Astronomical Journal, in press, 16 pages, 3 figure
Parallax and Luminosity Measurements of an L Subdwarf
We present the first parallax and luminosity measurements for an L subdwarf,
the sdL7 2MASS J05325346+8246465. Observations conducted over three years by
the USNO infrared astrometry program yield an astrometric distance of
26.7+/-1.2 pc and a proper motion of 2.6241+/-0.0018"/yr. Combined with
broadband spectral and photometric measurements, we determine a luminosity of
log(Lbol/Lsun) = -4.24+/-0.06 and Teff = 1730+/-90 K (the latter assuming an
age of 5-10 Gyr), comparable to mid-type L field dwarfs. Comparison of the
luminosity of 2MASS J05325346+8246465 to theoretical evolutionary models
indicates that its mass is just below the sustained hydrogen burning limit, and
is therefore a brown dwarf. Its kinematics indicate a ~110 Myr, retrograde
Galactic orbit which is both eccentric (3 <~ R <~ 8.5 kpc) and extends well
away from the plane (Delta_Z = +/-2 kpc), consistent with membership in the
inner halo population. The relatively bright J-band magnitude of 2MASS
J05325346+8246465 implies significantly reduced opacity in the 1.2 micron
region, consistent with inhibited condensate formation as previously proposed.
Its as yet unknown subsolar metallicity remains the primary limitation in
constraining its mass; determination of both parameters would provide a
powerful test of interior and evolutionary models for low-mass stars and brown
dwarfs.Comment: Accepted to ApJ 10 September 2007; 13 pages, 5 figures, 3 tables,
formatted in emulateapj styl
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
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
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
Extensive spiral structure and corotation resonance
Spiral density wave theories demand that grand design spiral structure be
bounded, at most, between the inner and outer Lindblad resonances of the spiral
pattern. The corotation resonance lies between the outer and inner Lindblad
resonances. The locations of the resonances are at radii whose ratios to each
other are rather independent of the shape of the rotation curve. The measured
ratio of outer to inner extent of spiral structure for a given spiral galaxy
can be compared to the standard ratio of corotation to inner Lindblad resonance
radius. In the case that the measured ratio far exceeds the standard ratio, it
is likely that the corotation resonance is within the bright optical disk.
Studying such galaxies can teach us how the action of resonances sculpts the
appearance of spiral disks. This paper reports observations of 140 disk
galaxies, leading to resonance ratio tests for 109 qualified spirals. It lists
candidates that have a good chance of having the corotation resonance radius
within the bright optical disk.Comment: 29 pages, 7 figure files, AAS late
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