760 research outputs found
Improved Astrometry and Photometry for the Luyten Catalog. I. Bright Stars
We outline the construction of an updated version of the New Luyten
Two-Tenths (NLTT) catalog of high proper motion stars, which will contain
improved astrometry and photometry for the vast majority of the ~59,000 stars
in NLTT. The bright end is constructed by matching NLTT stars to Hipparcos,
Tycho-2, and Starnet; the faint end by matching to USNO-A and 2MASS. In this
first paper, we detail the bright-end matching procedure. We show that for the
majority of stars in his catalog, Luyten measured positions accurate to 1" even
though he recorded his results much more coarsely. However, there is a long
tail of position errors, with one error as large as 11 deg. Proper-motion
errors for the stars with small position errors are 24 mas/yr (1 sigma) but
deteriorate to 34 mas/yr for stars with inferior positions. NLTT is virtually
100% complete for V15 deg, but completeness in this magnitude
range falls to about 75% at the Galactic plane. Incompleteness near the plane
is not uniform, but is rather concentrated in the interval -80<l<20, where the
Milky Way is brightest.Comment: Submitted to ApJ, 28 pages including 7 figure
Improved Astrometry and Photometry for the Luyten Catalog. II. Faint Stars and the Revised Catalog
We complete construction of a catalog containing improved astrometry and new
optical/infrared photometry for the vast majority of NLTT stars lying in the
overlap of regions covered by POSS I and by the second incremental 2MASS
release, approximately 44% of the sky. The epoch 2000 positions are typically
accurate to 130 mas, the proper motions to 5.5 mas/yr, and the V-J colors to
0.25 mag. Relative proper motions of binary components are measured to 3
mas/yr. The false identification rate is ~1% for 11 < V < 18 and substantially
less at brighter magnitudes. These improvements permit the construction of a
reduced proper motion diagram that, for the first time, allows one to classify
NLTT stars into main-sequence (MS) stars, subdwarfs (SDs), and white dwarfs
(WDs). We in turn use this diagram to analyze the properties of both our
catalog and the NLTT catalog on which it is based. In sharp contrast to popular
belief, we find that NLTT incompleteness in the plane is almost completely
concentrated in MS stars, and that SDs and WDs are detected almost uniformly
over the sky DEC > -33 deg. Our catalog will therefore provide a powerful tool
to probe these populations statistically, as well as to reliably identify
individual SDs and WDs.Comment: 16 figures. We will make the revised NLTT publicly available on
acceptance of the paper, or no later than July 18, 200
Classifying Luyten Stars Using An Optical-Infrared Reduced Proper Motion Diagram
We present a V-J reduced proper motion (RPM) diagram for stars in the New
Luyten Two-Tenths (NLTT) catalog. In sharp contrast to the RPM diagram based on
the original NLTT data, this optical-infrared RPM diagram shows distinct tracks
for white dwarfs, subdwarfs, and main-sequence stars. It thereby permits the
identification of white-dwarf and subdwarf candidates that have a high
probability of being genuine.Comment: Accepted ApJL version. 3 figures (2 in color). Table of candidate new
WDs closer than 20 pc is now include
Photometric Selection of QSO Candidates From GALEX Sources
We present a catalog of 36,120 QSO candidates from the Galaxy Evolution
Explorer (GALEX) Release Two (GR2) UV catalog and the USNO-A2.0 optical
catalog. The selection criteria are established using known quasars from the
Sloan Digital Sky Survey (SDSS). The SDSS sample is then used to assign
individual probabilities to our GALEX-USNO candidates. The mean probability is
~50%, and would rise to ~65% if better morphological information than that from
USNO were available to eliminate galaxies. The sample is ~40% complete for
i<=19.1. Candidates are cross-identified in 2MASS, FIRST, SDSS, and XMM-Newton
Slewing Survey (XMMSL1), whenever such counterparts exist. The present catalog
covers the 8000 square degrees of GR2 lying above 25 degrees Galactic latitude,
but can be extended to all 24,000 square degress that satisfy this criterion as
new GALEX data become available.Comment: AASTeX v5.2, 31 pages, 9 figures. Accepted for publication in ApJ.
Extended tables available in the online edition of the journa
Completeness of USNO-B for High Proper-Motion Stars
I test the completeness of USNO-B detections of high proper-motion
(mu>0.18"/yr) stars and the accuracy of its measurements by comparing them to
the revised NLTT (rNLTT) catalog of Salim & Gould. For 14.5<V<18.5, only 6% of
such stars are missing from USNO-B while another 4% have large errors, mostly
too large to be useful. Including both classes, incompleteness is 10%. These
fractions rise toward both brighter and fainter magnitudes. Incompleteness
rises with proper motion to about 30% at mu=1"/yr. It also rises to about 35%
at the Galactic plane, although this is only determined for relatively bright
stars V<~14. For binaries, incompleteness rises from 10% at separations of 30"
to 47% at 10". The proper-motion errors reported internally by USNO-B are
generally correct. However, there is floor of sigma_mu~4mas/yr below which the
reported errors should not be taken at face value. The small number of stars
with relatively large reported errors (sigma_mu>~20mas/yr) may actually have
still larger errors than tabulated.Comment: Submitted to AJ, 26 pages including 10 figure
New Hipparcos-based Parallaxes for 424 Dim Stars
We present a catalog of 424 common proper motion companions to Hipparcos
stars with good (>3 sigma) parallaxes, thereby effectively providing new
parallaxes for these companions. Compared to stars in the Hipparcos catalog,
these stars are substantially dimmer. The catalog includes 20 WDs and an
additional 29 stars with M_V>14, the great majority of the latter being M
dwarfs.Comment: Submitted to ApJS, 20 page
A New Kinematic Distance Estimator to the LMC
The distance to the Large Magellanic Cloud (LMC) can be directly determined
by measuring three of its properties, its radial-velocity field, its mean
proper motion, and the position angle \phi_ph of its photometric line of nodes.
Statistical errors of 2% are feasible based on proper motions obtained with any
of several proposed astrometry satellites, the first possibility being the
Full-Sky Astrometric Mapping Explorer (FAME). The largest source of systematic
error is likely to be in the determination of \phi_ph. I suggest two
independent methods to measure \phi_ph, one based on counts of clump giants and
the other on photometry of clump giants. I briefly discuss a variety of methods
to test for other sources of systematic errors.Comment: submitted to ApJ, 13 page
Nearby Microlensing Events - Identification of the Candidates for the SIM
The Space Interferometry Mission (SIM) is the instrument of choice when it
comes to observing astrometric microlensing events where nearby, usually
high-proper-motion stars (``lenses''), pass in front of more distant stars
(``sources''). Each such encounter produces a deflection in the source's
apparent position that when observed by SIM can lead to a precise mass
determination of the nearby lens star. We search for lens-source encounters
during the 2005-2015 period using Hipparcos, ACT and NLTT to select lenses, and
USNO-A2.0 to search for the corresponding sources, and rank these by the SIM
time required for a 1% mass measurement.
For Hipparcos and ACT lenses, the lens distance and lens-source impact
parameter are precisely determined so the events are well characterized. We
present 32 candidates beginning with a 61 Cyg A event in 2012 that requires
only a few minutes of SIM time. Proxima Centauri and Barnard's star each
generate several events. For NLTT lenses, the distance is known only to a
factor of 3, and the impact parameter only to 1''. Together, these produce
uncertainties of a factor ~10 in the amount of SIM time required. We present a
list of 146 NLTT candidates and show how single-epoch CCD photometry of the
candidates could reduce the uncertainty in SIM time to a factor of ~1.5.Comment: ApJ accepted, 31 pages (inc. 5 tables), 5 figures. t SIM refine
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