964 research outputs found
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
The investigation of absolute proper motions of the XPM Catalogue
The XPM-1.0 is the regular version of the XPM catalogue. In comparison with
XPM the astrometric catalogue of about 280 millions stars covering entire sky
from -90 to +90 degrees in declination and in the magnitude range 10^m<B<22^m
is something improved. The general procedure steps were followed as for XPM,
but some of them are now performed on a more sophisticated level. The XPM-1.0
catalogue contains star positions, proper motions, 2MASS and USNO photometry of
about 280 millions of the sources. We present some investigations of the
absolute proper motions of XPM-1.0 catalogue and also the important information
for the users of the catalogue. Unlike previous version, the XPM-1.0 contains
the proper motions over the whole sky without gaps. In the fields, which cover
the zone of avoidance or which contain less than of 25 galaxies a quasi
absolute calibration was performed. The proper motion errors are varying from 3
to 10 mas/yr, depending on a specific field. The zero-point of the absolute
proper motion frame (the absolute calibration) was specified with more than 1
million galaxies from 2MASS and USNO-A2.0. The mean formal error of absolute
calibration is less than 1 mas/yr.Comment: 11 pages, 9 figures, accepte
Finding cool subdwarfs using a V-J reduced proper-motion diagram: Stellar parameters for 91 candidates
We present the results of a search for cool subdwarfs for which our
candidates were drawn from a V-J reduced proper-motion diagram constructed by
Salim & Gould (2002). Kinematic (U, V, and W) and self-consistent stellar
parameters (Teff, log g, [Fe/H], and V_t) are derived for 91 candidate
subdwarfs based on high resolution spectra. The observed stars span 3900K <
Teff < 6200K and -2.63 < [Fe/H] < 0.25 including only 3 giants (log g < 4.0).
Of the sample, 77 stars have MgH lines present in their spectra. With more than
56% of our candidate subdwarfs having [Fe/H] < -1.5, we show that the V-J
reduced proper-motion diagram readily identifies metal-poor stars.Comment: PASP (in press
Feasibility of an Infrared Parallax Program Using the Fan Mountain Tinsley Reflector
Despite the continuing importance of ground-based parallax measurements, few
active programs remain. Because new members of the solar neighborhood tend
towards later spectral types, infrared parallax programs are particularly
desirable. Consequently, the astrometric quality of the new infrared camera,
FanCam, developed by the Virginia Astronomical Instrumentation Laboratory
(VAIL) for the 31-in (0.8-m) Tinsley reflector at Fan Mountain Observatory was
assessed using 68 J-band exposures of an open cluster, NGC 2420, over a range
of hour angles during 2005. Positions of 16 astrometric evaluation stars were
measured and the repeatability of those positions was evaluated using the mean
error in a single observation of unit weight. Overall, a precision of 1.3 +/-
0.7 microns in x (RA) and 1.3 +/- 0.8 microns in y (Dec) was attained, which
corresponds to 0.04" +/- 0.02" in each axis. Although greater precision is
expected from CCDs in the visual and near-infrared, this instrument can achieve
precision similar to that of the ESO NTT infrared parallax program. Therefore,
measuring parallaxes in the infrared would be feasible using this equipment. If
initiated, such a program could provide essential distances for brown dwarfs
and very low mass stars that would contribute significantly to the solar
neighborhood census.Comment: accepted by New Astronomy, minor revisions per refere
Disk and Halo Wide Binaries from the Revised Luyten Catalog: Probes of Star Formation and MACHO Dark Matter
We present a catalog of 1147 candidate common proper motion binaries selected
from the revised New Luyten Two-Tenths Catalog. Among these, we identify 999
genuine physical pairs using the measured proper-motion difference and the
relative positions of each binary's components on a reduced proper-motion (RPM)
diagram. The RPM positions also serve to classify them as either disk
main-sequence (801), halo subdwarf (116), or pairs containing at least one
white dwarf (82). The disk and halo samples are complete to separations of
\theta=500" and \theta=900", which correspond to ~0.1 pc and ~1 pc,
respectively. At wide separations, both distributions are well described by
single power laws, dN/d\theta ~ \theta^{-\alpha}: \alpha=1.67+-0.07 for the
disk and \alpha=1.55+-0.10 for the halo. The fact that these distributions have
similar slopes (and similar normalizations as well) argues for similarity of
the star-formation conditions of these two populations. The fact that the halo
binaries obey a single power law out to ~1 pc permits strong constraints on
halo dark-matter candidates. At somewhat closer separations (10"<\theta<25"),
the disk distribution shows a pronounced flattening, which is detected at very
high statistical significance and is not due to any obvious systematic effect.
We also present a list of 11 previously unknown halo stars with parallaxes that
are recognized here as companions of Hipparcos stars.Comment: 56 pages, 16 figures; replaced with version accepted for publication
in Ap
On high proper motion white dwarfs from photographic surveys
The interpretation of high proper motion white dwarfs detected by Oppenheimer
et al (2001) was the start of a lively controversy. While the discoverers
identify a large fraction of their findings as dark halo members, others
interpret the same sample as essentially made of disc and/or thick disc stars.
We use the comprehensive description of Galactic stellar populations provided
by the "Besancon" model to produce a realistic simulation of Oppenheimer et al.
data, including all observational selections and calibration biases. The
conclusion is unambiguous: Thick disc white dwarfs resulting from ordinary
hypotheses on the local density and kinematics are sufficient to explain the
observed objects, there is no need for halo white dwarfs. This conclusion is
robust to reasonable changes in model ingredients. The main cause of the
misinterpretation seems to be that the velocity distribution of a proper motion
selected star sample is severely biased in favour of high velocities. This has
been neglected in previous analyses. Obviously this does not prove that no such
objects like halo white dwarfs can exist, but Oppenheimer et al. observations
drive their possible contribution in the dark matter halo down to an extremely
low fraction.Comment: 4 pages, 1 figure, A&A Letters, accepte
Assessment of left atrial volume before and after pulmonary thromboendarterectomy in chronic thromboembolic pulmonary hypertension.
BackgroundImpaired left ventricular diastolic filling is common in chronic thromboembolic pulmonary hypertension (CTEPH), and recent studies support left ventricular underfilling as a cause. To investigate this further, we assessed left atrial volume index (LAVI) in patients with CTEPH before and after pulmonary thromboendarterectomy (PTE).MethodsForty-eight consecutive CTEPH patients had pre- & post-PTE echocardiograms and right heart catheterizations. Parameters included mean pulmonary artery pressure (mPAP), pulmonary vascular resistance (PVR), cardiac index, LAVI, & mitral E/A ratio. Echocardiograms were performed 6 ± 3 days pre-PTE and 10 ± 4 days post-PTE. Regression analyses compared pre- and post-PTE LAVI with other parameters.ResultsPre-op LAVI (mean 19.0 ± 7 mL/m2) correlated significantly with pre-op PVR (R = -0.45, p = 0.001), mPAP (R = -0.28, p = 0.05) and cardiac index (R = 0.38, p = 0.006). Post-PTE, LAVI increased by 18% to 22.4 ± 7 mL/m2 (p = 0.003). This change correlated with change in PVR (765 to 311 dyne-s/cm5, p = 0.01), cardiac index (2.6 to 3.2 L/min/m2, p = 0.02), and E/A (.95 to 1.44, p = 0.002).ConclusionIn CTEPH, smaller LAVI is associated with lower cardiac output, higher mPAP, and higher PVR. LAVI increases by ~20% after PTE, and this change correlates with changes in PVR and mitral E/A. The rapid increase in LAVI supports the concept that left ventricular diastolic impairment and low E/A pre-PTE are due to left heart underfilling rather than inherent left ventricular diastolic dysfunction
Unveiling the nature of three INTEGRAL sources through optical spectroscopy
The results of an optical spectroscopy campaign performed at the Astronomical
Observatory of Bologna in Loiano (Italy) on three hard X-ray sources detected
by INTEGRAL (IGR J17303-0601, IGR J18027-1455 and IGR J21247+5058) are
presented. These data have allowed a determination of the nature for two of
them, with IGR J17303-0601 being a low mass X-ray binary in the Galaxy and IGR
J18027-1455 a background Type 1 Seyfert galaxy at redshift z = 0.035. IGR
J21247+5058, instead, has a quite puzzling spectroscopic appearance, with a
broad, redshifted H_alpha complex superimposed onto a `normal' F/G-type
Galactic star continuum: these features, together with the spatially coincident
extended radio emission, might suggest a chance alignment between a relatively
nearby star and a background radio galaxy. These results underline the still
non-negligible importance of smaller telescopes in modern astrophysics.Comment: 4 pages, 2 figures. Accepted for publication in A&A Letter
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