Analysis of astrometric catalogues with vector spherical harmonics

Comparison of stellar catalogues with position and proper motion components using a decomposition on a set of orthogonal vector spherical harmonics. We show the theoretical and practical advantages of this technique as a result of invariance properties and the independence of the decomposition from a prior model. We describe the mathematical principles used to perform the spectral decomposition, evaluate the level of significance of the multipolar components and examine the transformation properties under space rotation. The principles are illustrated with a characterisation of the systematic effects in the FK5 catalogue compared to Hipparcos and with an application to the extraction of the rotation and dipole acceleration in the astrometric solution of QSOs expected from Gaia.Comment: accepted for publication in Astronomy & Astrophysic

Evidence for Prolonged Main Sequence Stellar Evolution of F Stars in close binaries

Binary F stars exhibit large brightness anomaly, which is defined here as the difference between the absolute magnitude from the uvby photometry and the actual absolute magnitude of the star. We have found that the anomaly inversely correlates with the binary components separation. There is evidence that the correlation reflects actual population differences between close and wide binary pairs, in which case it indicates that the anomaly is somehow associated with the interaction of binary's components. The anomaly has also been found to correlate with both kinematics and metallicity. The sense of the correlations implies that the anomaly increases as the star evolves, suggesting a peculiar evolution of a primary F star in a tight binary pair. This conclusion has further been supported by the study of the age-velocity relation (AVR) of F stars that are cataloged in the HIPPARCOS as single. Among these stars, those with brightness anomaly were previously shown to be most likely unidentified close binaries. We have found that the AVR of these binary candidates is different from that of the ``truly single'' F stars. The discrepancy between the two AVRs indicates that the putative binaries are, on average, older than similar normal single F stars at the same effective temperature and luminosity, which is consistent with the inferred peculiar evolution in close binaries. It appears that this peculiarity is caused by the impact of the components interaction in a tight pair on stellar evolution, which results in the prolonged main sequence lifetime of the primary F star.Comment: 8 pages, 7 figures, accepted by Astronomy and Astrophysic

The Physics of Bodily Tides in Terrestrial Planets, and the Appropriate Scales of Dynamical Evolution

Any model of tides is based on a specific hypothesis of how lagging depends on the tidal-flexure frequency. For example, Gerstenkorn (1955), MacDonald (1964), and Kaula (1964) assumed constancy of the geometric lag angle, while Singer (1968) and Mignard (1979, 1980) asserted constancy of the time lag. Thus, each of these two models was based on a certain law of scaling of the geometric lag. The actual dependence of the geometric lag on the frequency is more complicated and is determined by the rheology of the planet. Besides, each particular functional form of this dependence will unambiguously fix the appropriate form of the frequency dependence of the tidal quality factor, Q. Since at present we know the shape of the dependence of Q upon the frequency, we can reverse our line of reasoning and single out the appropriate actual frequency-dependence of the angular lag. This dependence turns out to be different from those employed hitherto, and it entails considerable alterations in the time scales of the tide-generated dynamical evolution. Phobos' fall on Mars is an example we consider.Comment: arXiv admin note: substantial text overlap with arXiv:astro-ph/060552

Science Alerts with Gaia

5 p.International audienceGaia is before all a survey mission designed to observe the sky in a continuous manner. The sky coverage results from the spin of the satellite over a period of 6h, combined with a much slower motion of the spin axis, allowing after six months complete sky coverage. The CCD counts are stored on-board and sent to the ground station every day during the visibility period of the spacecraft by the ground antenna. The satellite design has been optimized for a survey mission, with ground treatment and not for an immediate access to the data, let alone to some scientifically immediately usable information. The processing comprises several more or less independent pipelines, each involving some sort of global processing requiring the accumulation of a substantial amount of data over several weeks or months. However, it remains possible to carry out a quick, but crude in regard of the accuracy achievable on a longer term, analysis of the data stream arriving on the ground to detect transient phenomena, like photometric burst or fast motion of solar system objects. This dedicated processing, largely distinct from the general processing, and the associated validation systems put in place, is referred to as Science Alerts within the DPAC community

Gaia astrometric accuracy in the past

Program available at: http://www.imcce.fr/hosted_sites/naroo/program.htmlInternational audienceGaia is due for launch in the third quarter of 2013 and will open a new page in fundamental astronomy. The astrometric accuracy achievable of about 20 muas/yr at G=15 to below 10 muas/yr at the bright end will permit to know the positions of millions stars with mas accuracy almost 100 years back in time. I will present the current expectations regarding the accuracy of Gaia astrometry at the end of the mission and explain how to propagate the stellar positions and their covariance matrix several decades away from the Gaia epoch without introducing modeling error. I will show also diagrams illustrating the positional accuracy down to 1900 as a function of magnitude and sky position

Rate of correct detection of periodic signal with the Gaia satellite

The Gaia satellite was selected as a cornerstone mission of the European Space Agency (ESA) in 2000 October and confirmed in 2002 with a current target launch date of 2011. The Gaia mission will gather on the same observational principles as Hipparcos detailed astrometric, photometric and spectroscopic properties of about one billion sources brighter than V= 20 mag. The nature of the measured objects ranges from near-Earth objects (NEOs) to gamma-ray burst afterglows, and encompasses virtually any kind of stars in our Galaxy. Gaia will provide multicolour (in about 20 passbands extending over the visible range) photometry with typically 250 observations distributed over 40 well-separated epochs during the 5-yr mission. The multi-epoch nature of the project will permit the detection and analysis of variable sources whose number is currently estimated in the range of several tens of millions, among the detectable sources. In this paper, we assess the performances of Gaia in analysing photometric periodic phenomena. We first quickly present the overall observational principle before discussing the implication of the scanning law in the time-sampling. Then from extensive simulations one assesses the performances in the recovery of periodic signals as a function of the period, signal-to-noise ratio and position on the sky for simple sinusoidal variabilit

Micro-arcsecond light bending by Jupiter

The detectors designed for Gaia, the next ESA space astrometry mission to be launched in 2011, will allow to observe repeatedly stars very close to Jupiter's limb. This will open a unique opportunity to test General Relativity by performing many Eddington-like experiments through the comparison between the pattern of a starfield observed with or without Jupiter. We have derived the main formulas relevant for the monopole and quadrupole light deflection by an oblate planet and developed a simulator to investigate the processing of the Gaia astrometric observation in the vicinity of the planet. The results show that such an experiment carried out with the Gaia data will provide a new fully independent determination of the PPN parameter gamma by means of differential astrometric measurements and, more importantly, for the first time will evidence the bending effect due to the quadrupole moment with a 3-sigma confidence level. Given the accuracy of the experiment for the monopole deflection, this will permit to test alternative modelling of the light bending by moving masses.Comment: 22 pages, 16 figures. submitted to Class. Quantum Gra