488 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
Masses of the components of SB2 binaries observed with Gaia. II. Masses derived from PIONIER interferometric observations for Gaia validation
In anticipation of the Gaia astrometric mission, a 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 combined with Gaia astrometric measurements. In order to validate the
masses derived from Gaia, interferometric observations are obtained for three
SB2s in our sample with F-K components: HIP 14157, HIP 20601 and HIP 117186.
The masses of the six stellar components are derived. Due to its edge-on
orientation, HIP 14157 is probably an eclipsing binary. We note that almost all
the derived masses are a few percent larger than the expectations from the
standard spectral-type-mass calibration and mass-luminosity relation. Our
calculation also leads to accurate parallaxes for the three binaries, and the
Hipparcos parallaxes are confirmed.Comment: 10 pages, 3 figures, accepted by 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
Correlated errors in Hipparcos parallaxes towards the Pleiades and the Hyades
We show that the errors in the Hipparcos parallaxes towards the Pleiades and
the Hyades open clusters are spatially correlated over angular scales of 2 to 3
deg, with an amplitude of up to 2 mas. This correlation is stronger than
expected based on the analysis of the Hipparcos catalog. We predict the
parallaxes of individual cluster members, pi_pm, from their Hipparcos proper
motions, assuming that all cluster members have the same space velocity. We
compare pi_pm with their Hipparcos parallaxes, pi_Hip, and find that there are
significant spatial correlations in pi_Hip. We derive a distance modulus to the
Pleiades of 5.58 +- 0.18 mag using the radial-velocity gradient method. This
value, agrees very well with the distance modulus of 5.60 +- 0.04 mag
determined using the main-sequence fitting technique, compared with the value
of 5.33 +- 0.06 inferred from the average of the Hipparcos parallaxes of the
Pleiades members. We show that the difference between the main-sequence fitting
distance and the Hipparcos parallax distance can arise from spatially
correlated errors in the Hipparcos parallaxes of individual Pleiades members.
Although the Hipparcos parallax errors towards the Hyades are spatially
correlated in a manner similar to those of the Pleiades, the center of the
Hyades is located on a node of this spatial structure. Therefore, the parallax
errors cancel out when the average distance is estimated, leading to a mean
Hyades distance modulus that agrees with the pre-Hipparcos value. We speculate
that these spatial correlations are also responsible for the discrepant
distances that are inferred using the mean Hipparcos parallaxes to some open
clusters. Finally, we note that our conclusions are based on a purely geometric
method and do not rely on any models of stellar isochrones.Comment: 33 pages including 10 Figures, revised version accepted for
publication in Ap
Screening the Hipparcos-based astrometric orbits of sub-stellar objects
The combination of Hipparcos astrometric data with the spectroscopic data of
putative extrasolar planets seems to indicate that a significant fraction of
these low-mass companions could be brown or M dwarfs Han et al. (2001). We show
that this is due to the adopted reduction procedure, and consequently that the
Hipparcos data do not reject the planetary mass hypothesis in all but one
cases. Additional companions, undetected so far, might also explain the large
astrometric residuals of some of these stars.Comment: 9 pages, 7 figures, accepted for publication in A&
Spectroscopic survey of the Galaxy with Gaia I. Design and performance of the Radial Velocity Spectrometer
The definition and optimisation studies for the Gaia satellite spectrograph,
the Radial Velocity Spectrometer (RVS), converged in late 2002 with the
adoption of the instrument baseline. This paper reviews the characteristics of
the selected configuration and presents its expected performance. The RVS is a
2.0 by 1.6 degree integral field spectrograph, dispersing the light of all
sources entering its field of view with a resolving power R=11 500 over the
wavelength range [848, 874] nm. The RVS will continuously and repeatedly scan
the sky during the 5 years of the Gaia mission. On average, each source will be
observed 102 times over this period. The RVS will collect the spectra of about
100-150 million stars up to magnitude V~17-18. At the end of the mission, the
RVS will provide radial velocities with precisions of ~2 km/s at V=15 and
\~15-20 km/s at V=17, for a solar metallicity G5 dwarf. The RVS will also
provide rotational velocities, with precisions (at the end of the mission) for
late type stars of sigma_vsini ~5 km/s at V~15 as well as atmospheric
parameters up to V~14-15. The individual abundances of elements such as Silicon
and Magnesium, vital for the understanding of Galactic evolution, will be
obtained up to V~12-13. Finally, the presence of the 862.0 nm Diffuse
Interstellar Band (DIB) in the RVS wavelength range will make it possible to
derive the three dimensional structure of the interstellar reddening.Comment: 17 pages, 9 figures, accepted for publication in MNRAS. Fig. 1,2,4,5,
6 in degraded resolution; available in full resolution at
http://blackwell-synergy.com/links/doi/10.1111/j.1365-2966.2004.08282.x/pd
Fundamental stellar parameters of zeta Pup and gamma^2 Vel from HIPPARCOS data
We report parallax measurements by the HIPPARCOS satellite of zeta Puppis and
gamma^2 Velorum. The distance of zeta Pup is d=429 (+120/ -77) pc, in agreement
with the commonly adopted value to Vela OB2. However, a significantly smaller
distance is found for the gamma^2 Vel system: d=258 (+41/-31) pc. The total
mass of gamma^2 Vel derived from its parallax, the angular size of the
semi-major axis as measured with intensity interferometry, and the period is
M(WR+O)=29.5 (+/-15.9) Msun. This result favors the orbital solution of Pike et
al. (1983) over that of Moffat et al. (1986). The stellar parameters for the O
star companion derived from line blanketed non-LTE atmosphere models are:
Teff=34000 (+/-1500) K, log L/Lsun=5.3 (+/-0.15) from which an evolutionary
mass of M=29 (+/-4) Msun and an age of 4.0 (+0.8/-0.5) Myr is obtained from
single star evolutionary models. With non-LTE model calculations including He
and C we derive a luminosity log L/Lsun~4.7 (+/-0.2) for the WR star. The
mass-luminosity relation of hydrogen-free WR stars implies a mass of M(WR)~5
(+/-1.5) Msun. From our data we favor an age of ~10 Myr for the bulk of the
Vela OB2 stars. Evolutionary scenarios for zeta Pup and gamma^2 Vel are
discussed in the light of our results.Comment: Submitted to ApJ Letters (misprints corrected
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