472 research outputs found
Gaia astrometric science performance - post-launch predictions
The standard errors of the end-of-mission Gaia astrometry have been
re-assessed after conclusion of the in-orbit commissioning phase of the
mission. An analytical relation is provided for the parallax standard error as
function of Gaia G magnitude (and V-I colour) which supersedes the pre-launch
relation provided in de Bruijne (2012).Comment: To be published in the proceedings of the GREAT-ITN conference "The
Milky Way Unravelled by Gaia: GREAT Science from the Gaia Data Releases", 1-5
December 2014, University of Barcelona, Spain, EAS Publications Series, eds
Nicholas Walton, Francesca Figueras, and Caroline Soubira
A Hipparcos census of the nearby OB associations
A comprehensive census of the stellar content of the nearby OB associations
is presented, based on Hipparcos positions, proper motions, and parallaxes.
Moving groups are identified by combining de Bruijne's refurbished convergent
point method with the `Spaghetti method' of Hoogerwerf & Aguilar. Monte Carlo
simulations are used to estimate the expected number of interloper field stars.
Astrometric members are listed for 12 young stellar groups, out to a distance
of ~650 pc. These are the 3 subgroups Upper Scorpius, Upper Centaurus Lupus and
Lower Centaurus Crux of Sco OB2, as well as Vel OB2, Tr 10, Col 121, Per OB2,
alpha Persei (Per OB3), Cas-Tau, Lac OB1, Cep OB2, and a new group designated
as Cep OB6. The selection procedure corrects the list of previously known
astrometric and photometric B- and A-type members, and identifies many new
members, including a significant number of F stars, as well as evolved stars,
e.g., the Wolf-Rayet stars gamma^2 Vel (Vel OB2) and EZ CMa (Col 121), and the
classical Cepheid delta Cep in Cep OB6. In the nearest associations the
later-type members include T Tauri objects and other pre-main sequence stars.
Astrometric evidence for moving groups in the fields of R CrA, CMa OB1, Mon
OB1, Ori OB1, Cam OB1, Cep OB3, Cep OB4, Cyg OB4, Cyg OB7, and Sct OB2, is
inconclusive, due to their large distance or unfavorable kinematics.
The mean distances of the well-established groups are systematically smaller
than previous estimates. The mean motions display a systematic pattern, which
is discussed in relation to the Gould Belt. Six of the 12 detected moving
groups do not appear in the classical list of nearby OB associations. The
number of unbound young stellar groups in the Solar neighbourhood may be
significantly larger than thought previously.Comment: 51 pages, 30 PostScript figures, 6 tables in PostScript format,
default LaTeX using psfig.sty; accepted for publication in the Astronomical
Journal, scheduled for January 1999 issue. Abbreviated abstrac
OB Associations
Since the previous (1990) edition of this meeting enormous progress in the
field of OB associations has been made. Data from X-ray satellites have greatly
advanced the study of the low-mass stellar content of associations, while
astrometric data from the Hipparcos satellite allow for a characterization of
the higher-mass content of associations with unprecedented accuracy. We review
recent work on the OB associations located within 1.5 kpc from the Sun, discuss
the Hipparcos results at length, and point out directions for future research.Comment: To appear in The Physics of Star Formation and Early Stellar
Evolution II, eds C.J. Lada & N. Kylafis (Kluwer Academic), 30 pages, 9
EPS-figures, LaTeX using crckapb.sty, epsfig.sty, amssymb.st
Detecting stars, galaxies, and asteroids with Gaia
(Abridged) Gaia aims to make a 3-dimensional map of 1,000 million stars in
our Milky Way to unravel its kinematical, dynamical, and chemical structure and
evolution. Gaia's on-board detection software discriminates stars from spurious
objects like cosmic rays and Solar protons. For this, parametrised
point-spread-function-shape criteria are used. This study aims to provide an
optimum set of parameters for these filters. We developed an emulation of the
on-board detection software, which has 20 free, so-called rejection parameters
which govern the boundaries between stars on the one hand and sharp or extended
events on the other hand. We evaluate the detection and rejection performance
of the algorithm using catalogues of simulated single stars, double stars,
cosmic rays, Solar protons, unresolved galaxies, and asteroids. We optimised
the rejection parameters, improving - with respect to the functional baseline -
the detection performance of single and double stars, while, at the same time,
improving the rejection performance of cosmic rays and of Solar protons. We
find that the minimum separation to resolve a close, equal-brightness double
star is 0.23 arcsec in the along-scan and 0.70 arcsec in the across-scan
direction, independent of the brightness of the primary. We find that, whereas
the optimised rejection parameters have no significant impact on the
detectability of de Vaucouleurs profiles, they do significantly improve the
detection of exponential-disk profiles. We also find that the optimised
rejection parameters provide detection gains for asteroids fainter than 20 mag
and for fast-moving near-Earth objects fainter than 18 mag, albeit this gain
comes at the expense of a modest detection-probability loss for bright,
fast-moving near-Earth objects. The major side effect of the optimised
parameters is that spurious ghosts in the wings of bright stars essentially
pass unfiltered.Comment: Accepted for publication in A&
Radial velocities for the Hipparcos-Gaia Hundred-Thousand-Proper-Motion project
(abridged) The Hundred-Thousand-Proper-Motion (HTPM) project will determine
the proper motions of ~113500 stars using a 23-year baseline. The proper
motions will use the Hipparcos data, with epoch 1991.25, as first epoch and the
first intermediate-release Gaia astrometry, with epoch ~2014.5, as second
epoch. The expected HTPM proper-motion standard errors are 30-190 muas/yr,
depending on stellar magnitude. Depending on the characteristics of an object,
in particular its distance and velocity, its radial velocity can have a
significant impact on the determination of its proper motion. The impact of
this perspective acceleration is largest for fast-moving, nearby stars. Our
goal is to determine, for each star in the Hipparcos catalogue, the
radial-velocity standard error that is required to guarantee a negligible
contribution of perspective acceleration to the HTPM proper-motion precision.
We employ two evaluation criteria, both based on Monte-Carlo simulations, with
which we determine which stars need to be spectroscopically (re-)measured. Both
criteria take the Hipparcos measurement errors into account. For each star in
the Hipparcos catalogue, we determine the confidence level with which the
available radial velocity and its standard error, taken from the XHIP
compilation catalogue, are acceptable. We find that for 97 stars, the radial
velocities available in the literature are insufficiently precise for a 68.27%
confidence level. We also identify 109 stars for which radial velocities are
currently unknown yet need to be acquired to meet the 68.27% confidence level.
To satisfy the radial-velocity requirements coming from our study will be a
daunting task consuming a significant amount of spectroscopic telescope time.
Fortunately, the follow-up spectroscopy is not time-critical since the HTPM
proper motions can be corrected a posteriori once (improved) radial velocities
become available.Comment: Accepted in A&
Temperature determination via STJ optical spectroscopy
ESA's Superconducting Tunnel Junction (STJ) optical photon-counting camera
(S-Cam2) incorporates an array of pixels with intrinsic energy sensitivity.
Using the spectral fitting technique common in X-ray astronomy, we fit black
bodies to nine stellar spectra, ranging from cool flare stars to hot white
dwarfs. The measured temperatures are consistent with literature values at the
expected level of accuracy based on the predicted gain stability of the
instrument. Having also demonstrated that systematic effects due to count rate
are likely to be small, we then proceed to apply the temperature determination
method to four cataclysmic variable (CV) binary systems. In three cases we
measure the temperature of the accretion stream, while in the fourth we measure
the temperature of the white dwarf. The results are discussed in the context of
existing CV results. We conclude by outlining the prospects for future versions
of S-Cam.Comment: 9 pages, 9 figures (11 files); uses aa.cls; accepted for publication
in A&
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