71 research outputs found
GAIA: Composition, Formation and Evolution of the Galaxy
The GAIA astrometric mission has recently been approved as one of the next
two `cornerstones' of ESA's science programme, with a launch date target of not
later than mid-2012. GAIA will provide positional and radial velocity
measurements with the accuracies needed to produce a stereoscopic and kinematic
census of about one billion stars throughout our Galaxy (and into the Local
Group), amounting to about 1 per cent of the Galactic stellar population.
GAIA's main scientific goal is to clarify the origin and history of our Galaxy,
from a quantitative census of the stellar populations. It will advance
questions such as when the stars in our Galaxy formed, when and how it was
assembled, and its distribution of dark matter. The survey aims for
completeness to V=20 mag, with accuracies of about 10 microarcsec at 15 mag.
Combined with astrophysical information for each star, provided by on-board
multi-colour photometry and (limited) spectroscopy, these data will have the
precision necessary to quantify the early formation, and subsequent dynamical,
chemical and star formation evolution of our Galaxy. Additional products
include detection and orbital classification of tens of thousands of
extra-Solar planetary systems, and a comprehensive survey of some 10^5-10^6
minor bodies in our Solar System, through galaxies in the nearby Universe, to
some 500,000 distant quasars. It will provide a number of stringent new tests
of general relativity and cosmology. The complete satellite system was
evaluated as part of a detailed technology study, including a detailed payload
design, corresponding accuracy assesments, and results from a prototype data
reduction development.Comment: Accepted by A&A: 25 pages, 8 figure
Building the cosmic distance scale: from Hipparcos to Gaia
Hipparcos, the first ever experiment of global astrometry, was launched by
ESA in 1989 and its results published in 1997 (Perryman et al., Astron.
Astrophys. 323, L49, 1997; Perryman & ESA (eds), The Hipparcos and Tycho
catalogues, ESA SP-1200, 1997). A new reduction was later performed using an
improved satellite attitude reconstruction leading to an improved accuracy for
stars brighter than 9th magnitude (van Leeuwen & Fantino, Astron. Astrophys.
439, 791, 2005; van Leeuwen, Astron. Astrophys. 474, 653, 2007).
The Hipparcos Catalogue provided an extended dataset of very accurate
astrometric data (positions, trigonometric parallaxes and proper motions),
enlarging by two orders of magnitude the quantity and quality of distance
determinations and luminosity calibrations. The availability of more than 20000
stars with a trigonometric parallax known to better than 10% opened the way to
a drastic revision of our 3-D knowledge of the solar neighbourhood and to a
renewal of the calibration of many distance indicators and age estimations. The
prospects opened by Gaia, the next ESA cornerstone, planned for launch in June
2013 (Perryman et al., Astron. Astrophys. 369, 339, 2001), are still much more
dramatic: a billion objects with systematic and quasi simultaneous astrometric,
spectrophotometric and spectroscopic observations, about 150 million stars with
expected distances to better than 10%, all over the Galaxy. All stellar
distance indicators, in very large numbers, will be directly measured,
providing a direct calibration of their luminosity and making possible detailed
studies of the impacts of various effects linked to chemical element
abundances, age or cluster membership. With the help of simulations of the data
expected from Gaia, obtained from the mission simulator developed by DPAC, we
will illustrate what Gaia can provide with some selected examples.Comment: 16 pages, 16 figures, Conference "The Fundamental Cosmic Distance
scale: State of the Art and the Gaia perspective, 3-6 May 2011, INAF,
Osservatorio Astronomico di Capodimonte, Naples. Accepted for publication in
Astrophysics & Space Scienc
Astrometric Methods and Instrumentation to Identify and Characterize Extrasolar Planets: A Review
I present a review of astrometric techniques and instrumentation utilized to
search for, detect, and characterize extra-solar planets. First, I briefly
summarize the properties of the present-day sample of extrasolar planets, in
connection with predictions from theoretical models of planet formation and
evolution. Next, the generic approach to planet detection with astrometry is
described, with significant discussion of a variety of technical, statistical,
and astrophysical issues to be faced by future ground-based as well as
space-borne efforts in order to achieve the required degree of measurement
precision. After a brief summary of past and present efforts to detect planets
via milli-arcsecond astrometry, I then discuss the planet-finding capabilities
of future astrometric observatories aiming at micro-arcsecond precision.
Lastly, I outline a number experiments that can be conducted by means of
high-precision astrometry during the next decade, to illustrate its potential
for important contributions to planetary science, in comparison with other
indirect and direct methods for the detection and characterization of planetary
systems.Comment: 61 pages, 8 figures, PASP, accepted (October 2005 issue
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
The HgMn Binary Star Phi Herculis: Detection and Properties of the Secondary and Revision of the Elemental Abundances of the Primary
Observations of the Mercury-Manganese star Phi Herculis with the Navy
Prototype Optical Interferometer (NPOI) conclusively reveal the previously
unseen companion in this single-lined binary system. The NPOI data were used to
predict a spectral type of A8V for the secondary star Phi Her B. This
prediction was subsequently confirmed by spectroscopic observations obtained at
the Dominion Astrophysical Observatory. Phi Her B is rotating at 50 +/-3
km/sec, in contrast to the 8 km/sec lines of Phi Her A. Recognizing the lines
from the secondary permits one to separate them from those of the primary. The
abundance analysis of Phi Her A shows an abundance pattern similar to those of
other HgMn stars with Al being very underabundant and Sc, Cr, Mn, Zn, Ga, Sr,
Y, Zr, Ba, Ce, and Hg being very overabundant.Comment: Accepted to ApJ, 45 pages, 11 figure
Two Boehm-Vitense gaps in the main sequence of the Hyades
Hipparcos proper motions and trigonometric parallaxes allow the derivation of
secular parallaxes which fix the distances to individual stars in the Hyades
cluster to an accuracy of \sim 2 percent. The resulting color-absolute
magnitude diagram for 92 high-fidelity single members of the cluster displays a
very narrow main sequence, with two turn-offs and associated gaps. These occur
at the locations where the onset of surface convection affects the B-V colors,
as predicted by Boehm-Vitense thirty years ago. The new distances provide
stringent constraints on the transformations of colors and absolute magnitudes
to effective temperatures and luminosities, and on models of stellar interiors.Comment: 4 pages, 2 PostScript figures, LaTeX using aastex and
emulateapj5.sty; accepted for publication in Astrophysical Journal Letter
The Laser Astrometric Test of Relativity Mission
This paper discusses new fundamental physics experiment to test relativistic
gravity at the accuracy better than the effects of the 2nd order in the
gravitational field strength. The Laser Astrometric Test Of Relativity (LATOR)
mission uses laser interferometry between two micro-spacecraft whose lines of
sight pass close by the Sun to accurately measure deflection of light in the
solar gravity. The key element of the experimental design is a redundant
geometry optical truss provided by a long-baseline (100 m) multi-channel
stellar optical interferometer placed on the International Space Station. The
geometric redundancy enables LATOR to measure the departure from Euclidean
geometry caused by the solar gravity field to a very high accuracy. LATOR will
not only improve the value of the parameterized post-Newtonian (PPN) parameter
gamma to unprecedented levels of accuracy of 1 part in 1e8, it will also reach
ability to measure effects of the next post-Newtonian order (1/c^4) of light
deflection resulting from gravity's intrinsic non-linearity. The solar
quadrupole moment parameter, J2, will be measured with high precision, as well
as a variety of other relativistic. LATOR will lead to very robust advances in
the tests of fundamental physics: this mission could discover a violation or
extension of general relativity, or reveal the presence of an additional long
range interaction in the physical law. There are no analogs to the LATOR
experiment; it is unique and is a natural culmination of solar system gravity
experiments.Comment: 8 pages, 2 figures, invited talk given at the Second International
Conference on Particle and Fundamental Physics in Space (SpacePart'03), 10-12
December 2003, Washington, D
The design and performance of the Gaia photometric system
The European Gaia astrometry mission is due for launch in 2011. Gaia will rely on the proven principles of the ESA Hipparcos mission to create an all-sky survey of about one billion stars throughout our Galaxy and beyond, by observing all objects down to 20 mag. Through its massive measurement of stellar distances, motions and multicolour photometry, it will provide fundamental data necessary for unravelling the structure, formation and evolution of the Galaxy. This paper presents the design and performance of the broad- and medium-band set of photometric filters adopted as the baseline for Gaia. The 19 selected passbands (extending from the UV to the far-red), the criteria and the methodology on which this choice has been based are discussed in detail. We analyse the photometric capabilities for characterizing the luminosity, temperature, gravity and chemical composition of stars. We also discuss the automatic determination of these physical parameters for the large number of observations involved, for objects located throughout the entire Hertzsprung-Russell diagram. Finally, the capability of the photometric system (PS) to deal with the main Gaia science case is outline
Asteroseismology of red giants & galactic archaeology
Red-giant stars are low- to intermediate-mass (~M)
stars that have exhausted hydrogen in the core. These extended, cool and hence
red stars are key targets for stellar evolution studies as well as galactic
studies for several reasons: a) many stars go through a red-giant phase; b) red
giants are intrinsically bright; c) large stellar internal structure changes as
well as changes in surface chemical abundances take place over relatively short
time; d) red-giant stars exhibit global intrinsic oscillations. Due to their
large number and intrinsic brightness it is possible to observe many of these
stars up to large distances. Furthermore, the global intrinsic oscillations
provide a means to discern red-giant stars in the pre-helium core burning from
the ones in the helium core burning phase and provide an estimate of stellar
ages, a key ingredient for galactic studies. In this lecture I will first
discuss some physical phenomena that play a role in red-giant stars and several
phases of red-giant evolution. Then, I will provide some details about
asteroseismology -- the study of the internal structure of stars through their
intrinsic oscillations -- of red-giant stars. I will conclude by discussing
galactic archaeology -- the study of the formation and evolution of the Milky
Way by reconstructing its past from its current constituents -- and the role
red-giant stars can play in that.Comment: Lecture presented at the IVth Azores International Advanced School in
Space Sciences on "Asteroseismology and Exoplanets: Listening to the Stars
and Searching for New Worlds" (arXiv:1709.00645), which took place in Horta,
Azores Islands, Portugal in July 201
<i>Gaia</i> Data Release 1. Summary of the astrometric, photometric, and survey properties
Context. At about 1000 days after the launch of Gaia we present the first Gaia data release, Gaia DR1, consisting of astrometry and photometry for over 1 billion sources brighter than magnitude 20.7.
Aims. A summary of Gaia DR1 is presented along with illustrations of the scientific quality of the data, followed by a discussion of the limitations due to the preliminary nature of this release.
Methods. The raw data collected by Gaia during the first 14 months of the mission have been processed by the Gaia Data Processing and Analysis Consortium (DPAC) and turned into an astrometric and photometric catalogue.
Results. Gaia DR1 consists of three components: a primary astrometric data set which contains the positions, parallaxes, and mean proper motions for about 2 million of the brightest stars in common with the HIPPARCOS and Tycho-2 catalogues – a realisation of the Tycho-Gaia Astrometric Solution (TGAS) – and a secondary astrometric data set containing the positions for an additional 1.1 billion sources. The second component is the photometric data set, consisting of mean G-band magnitudes for all sources. The G-band light curves and the characteristics of ∼3000 Cepheid and RR-Lyrae stars, observed at high cadence around the south ecliptic pole, form the third component. For the primary astrometric data set the typical uncertainty is about 0.3 mas for the positions and parallaxes, and about 1 mas yr−1 for the proper motions. A systematic component of ∼0.3 mas should be added to the parallax uncertainties. For the subset of ∼94 000 HIPPARCOS stars in the primary data set, the proper motions are much more precise at about 0.06 mas yr−1. For the secondary astrometric data set, the typical uncertainty of the positions is ∼10 mas. The median uncertainties on the mean G-band magnitudes range from the mmag level to ∼0.03 mag over the magnitude range 5 to 20.7.
Conclusions. Gaia DR1 is an important milestone ahead of the next Gaia data release, which will feature five-parameter astrometry for all sources. Extensive validation shows that Gaia DR1 represents a major advance in the mapping of the heavens and the availability of basic stellar data that underpin observational astrophysics. Nevertheless, the very preliminary nature of this first Gaia data release does lead to a number of important limitations to the data quality which should be carefully considered before drawing conclusions from the data
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