815 research outputs found
Distances and absolute magnitudes from trigonometric parallaxes
We first review the current knowledge of Hipparcos systematic and random
errors, in particular small-scale correlations. Then, assuming Gaussian
parallax errors and using examples from the recent Hipparcos literature, we
show how random errors may be misinterpreted as systematic errors, or
transformed into systematic errors.
Finally we summarise how to get unbiased estimates of absolute magnitudes and
distances, using either Bayesian or non-parametrical methods. These methods may
be applied to get either mean quantities or individual estimates. In
particular, we underline the notion of astrometry-based luminosity, which
avoids the truncation biases and allows a full use of Hipparcos samples.Comment: 20 pages, 8 figures, Invited paper in Haguenau Colloquium
"Harmonizing Cosmic Distance Scales in a Post-Hipparcos Era", 14-16/09/98, to
appear in ASP Conf. Series, D. Egret and A. Heck ed
An updated maximum likelihood approach to open cluster distance determination
An improved method for estimating distances to open clusters is presented and
applied to Hipparcos data for the Pleiades and the Hyades. The method is
applied in the context of the historic Pleiades distance problem, with a
discussion of previous criticisms of Hipparcos parallaxes. This is followed by
an outlook for Gaia, where the improved method could be especially useful.
Based on maximum likelihood estimation, the method combines parallax, position,
apparent magnitude, colour, proper motion, and radial velocity information to
estimate the parameters describing an open cluster precisely and without bias.
We find the distance to the Pleiades to be pc, in accordance
with previously published work using the same dataset. We find that error
correlations cannot be responsible for the still present discrepancy between
Hipparcos and photometric methods. Additionally, the three-dimensional space
velocity and physical structure of Pleiades is parametrised, where we find
strong evidence of mass segregation. The distance to the Hyades is found to be
pc, also in accordance with previous results. Through the use
of simulations, we confirm that the method is unbiased, so will be useful for
accurate open cluster parameter estimation with Gaia at distances up to several
thousand parsec.Comment: 14 pages, 8 figures, 6 tables, 3 appendices. Accepted in A&
Open clusters with Hipparcos I. Mean astrometric parameters
New memberships, mean parallaxes and proper motions of all 9 open clusters
closer than 300 pc (except the Hyades) and 9 rich clusters between 300 and 500
pc have been computed using Hipparcos data. Precisions, ranging from 0.2 to 0.5
mas for parallaxes and 0.1 to 0.5 mas/yr for proper motions, are of great
interest for calibrating photometric parallaxes as well as for kinematical
studies. Careful investigations of possible biases have been performed and no
evidence of significant systematic errors on the mean cluster parallaxes has
been found. The distances and proper motions of 32 more distant clusters, which
may be used statistically, are also indicated.Comment: 15 pages, A&A in pres
On the derivation of radial velocities of SB2 components: a "CCF vs TODCOR" comparison
The radial velocity (RV) of a single star is easily obtained from
cross-correlation of the spectrum with a template, but the treatment of
double-lined spectroscopic binaries (SB2s) is more difficult. Two different
approaches were applied to a set of SB2s: the fit of the cross-correlation
function with two normal distributions, and the cross-correlation with two
templates, derived with the TODCOR code. It appears that the minimum masses
obtained through the two methods are sometimes rather different, although their
estimated uncertainties are roughly equal. Moreover, both methods induce a
shift in the zero point of the secondary RVs, but it is less pronounced for
TODCOR. All-in-all the comparison between the two methods is in favour of
TODCOR.Comment: 5 pages, 4 figures, SF2A Conference 201
Masses of the components of SB2 binaries observed with Gaia. I. Selection of the sample and mass ratios of 20 new SB2s discovered with Sophie
In anticipation of the Gaia astrometric mission, a large sample of
spectroscopic binaries is being observed since 2010 with the Sophie
spectrograph at the Haute--Provence Observatory. Our aim is to derive the
orbital elements of double-lined spectroscopic binaries (SB2s) with an accuracy
sufficient to finally obtain the masses of the components with relative errors
as small as 1% when the astrometric measurements of Gaia are taken into
account. Simultaneously, the luminosities of the components in the Gaia
photometric band G will also be obtained. Our observation program started with
200 SBs, including 152 systems that were only known as single-lined. Thanks to
the high efficiency of the Sophie spectrograph, an additional component was
found for 25 SBs. After rejection of 5 multiple systems, 20 new SB2s were
retained, including 8 binaries with evolved primary, and their mass ratios were
derived. Our final sample contains 68 SB2s, including 2 late-type giants and 10
other evolved stars.Comment: 8 pages, 3 figures, accepted for publication in MNRA
Validation of the new Hipparcos reduction
Context.A new reduction of the astrometric data as produced by the Hipparcos
mission has been published, claiming accuracies for nearly all stars brighter
than magnitude Hp = 8 to be better, by up to a factor 4, than in the original
catalogue. Aims.The new Hipparcos astrometric catalogue is checked for the
quality of the data and the consistency of the formal errors as well as the
possible presence of error correlations. The differences with the earlier
publication are explained. Methods. The internal errors are followed through
the reduction process, and the external errors are investigated on the basis of
a comparison with radio observations of a small selection of stars, and the
distribution of negative parallaxes. Error correlation levels are investigated
and the reduction by more than a factor 10 as obtained in the new catalogue is
explained. Results.The formal errors on the parallaxes for the new catalogue
are confirmed. The presence of a small amount of additional noise, though
unlikely, cannot be ruled out. Conclusions. The new reduction of the Hipparcos
astrometric data provides an improvement by a factor 2.2 in the total weight
compared to the catalogue published in 1997, and provides much improved data
for a wide range of studies on stellar luminosities and local galactic
kinematics.Comment: 12 pages, 19 figures, accepted for publication by Astronomy and
Astrophysic
A spectroscopic survey of faint, high-galactic latitude red clump stars. II. The medium resolution sample
Aims. The goal of our survey is to provide accurate and multi-epoch radial
velocities, atmospheric parameters (Teff, log g and [M/H]), distances and space
velocities of faint Red Clump stars. Methods. We recorded high signal-to-noise
(S/N >= 200) spectra of Red Clump stars, over the 4750-5950 Ang range, at a
resolving power 5500. The target stars are distributed over the great circle of
the celestial equator. Radial velocities were obtained via cross-correlation
against IAU radial velocity standards. Atmospheric parameters were derived via
chi^2 fit to a synthetic spectral library. A large number of RC stars from
other surveys were re-observed to check the consistency of our results.
Results. A total of 245 Red Clump stars were observed (60 of them with a second
epoch observation separated in time by about three months), and the results are
presented in an output catalog. None of them is already present in other
surveys of Red Clump stars. In addition to astrometric and photometric support
data from external sources, the catalog provides radial velocities (accuracy
sigma(RV)=1.3 km/s), atmospheric parameters (sigma(Teff)=88 K, sigma(log
g)=0.38 dex and sigma([M/H])=0.17 dex), spectro-photometric distances, (X,Y,Z)
galacto-centric positions and (U,V,W) space velocities.Comment: in press in A&
Triage of the Gaia astrometric orbits. I. A sample of binaries with probable compact companions
In preparation for the release of the astrometric orbits of Gaia, Shahaf et
al. (2019) proposed a triage technique to identify astrometric binaries with
compact companions based on their astrometric semi-major axis, parallax, and
primary mass. The technique requires the knowledge of the appropriate
mass-luminosity relation to rule out single or close-binary main-sequence
companions. The recent publication of the Gaia DR3 astrometric orbits used a
schematic version of this approach, identifying 735 astrometric binaries that
might have compact companions. In this communication, we return to the triage
of the DR3 astrometric binaries with more careful analysis, estimating the
probability for its astrometric secondary to be a compact object or a
main-sequence close binary. We compile a sample of 177 systems with
highly-probable non-luminous massive companions, which is smaller but cleaner
than the sample reported in Gaia DR3. The new sample includes 8 candidates to
be black-hole systems with compact-object masses larger than 2.4 . The
orbital-eccentricitysecondary-mass diagram of the other 169 systems suggests
a tentative separation between the white-dwarf and the neutron-star binaries.
Most white-dwarf binaries are characterized by small eccentricities of about
0.1 and masses of 0.6 , while the neutron star binaries display
typical eccentricities of 0.4 and masses of 1.3 .Comment: Submitted to MNRAS; 12 pages, 13 figure
Interstellar extinction towards open clusters and galactic structure
We study the distribution of interstellar matter near the galactic plane on
the basis of open star clusters and reviewed the correlation of extinction with
different physical parameters of the clusters to understand the galactic
structure. It is seen from the extinction towards open clusters that about 90%
of the absorbing material lie within -5<b<5 deg of the galactic plane. The mean
thickness of the absorbing material, which is determined in terms of half-width
value \beta, is estimated to be about 125+/-21 pc. We show that the
interstellar absorption follows a sinusoidal variation with galactic longitude
and maximum and minimum absorptions occur at l ~ 48+/-4 deg and l ~ 228+/-4 deg
respectively. It is found that the galactic plane defined by the reddening
material is inclined by an angle of 0.6+/-0.4 deg to the formal galactic plane
and inclination is maximum at l ~ 54+/-6 deg. The reddening analysis has been
used to constrain the Solar offset which is found to be about 22.8+/-3.3 pc
above the reddening plane. We obtained a scale height of 53+/-5 pc for the
distribution of open clusters while it is 186+/-25 pc for the distribution of
reddening material from the reddening plane.Comment: 9 pages, 12 figures, accepted for the publication in MNRA
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