H. Voss

Gaia broad band photometry

The scientific community needs to be prepared to analyse the data from Gaia, one of the most ambitious ESA space missions, to be launched in 2012. The purpose of this paper is to provide data and tools in order to predict in advance how Gaia photometry is expected to be. To do so, we provide relationships among colours involving Gaia magnitudes and colours from other commonly used photometric systems (Johnson-Cousins, SDSS, Hipparcos and Tycho). The most up-to-date information from industrial partners has been used to define the nominal passbands and based on the BaSeL3.1 stellar spectral energy distribution library, relationships were obtained for stars with different reddening values, ranges of temperatures, surface gravities and metallicities. The transformations involving Gaia and Johnson-Cousins V-I_C and Sloan DSS g-z colours have the lowest residuals. A polynomial expression for the relation between the effective temperature and the colour G_BP-G_RP was derived for stars with T > 4500 K. Transformations involving two Johnson or two Sloan DSS colours yield lower residuals than using only one colour. We also computed several ratios of total-to-selective absorption including absorption A_G in the G band and colour excess E(G_BP-G_RP) for our sample stars. A relationship, involving A_G/A_V and the intrinsic (V-I_C) colour, is provided. The derived Gaia passbands have been used to compute tracks and isochrones using the Padova and BASTI models. Finally, the performances of the predicted Gaia magnitudes have been estimated according to the magnitude and the celestial coordinates of the star. The provided dependencies among colours can be used for planning scientific exploitation of Gaia data, performing simulations of the Gaia-like sky, planning ground-based complementary observations and for building catalogues with auxiliary data for the Gaia data processing and validation.

Comparative blind test of five planetary transit detection algorithms on realistic synthetic light curves

Because photometric surveys of exoplanet transits are very promising sources of future discoveries, many algorithms are being developed to detect transit signals in stellar light curves. This paper compares such algorithms for the next generation of space-based transit detection surveys like CoRoT, Kepler, and Eddington. Five independent analyses of a thousand synthetic light curves are presented. The light curves were produced with an end-to-end instrument simulator and include stellar micro-variability and a varied sample of stellar and planetary transits diluted within a much larger set of light curves. The results show that different algorithms perform quite differently, with varying degrees of success in detecting real transits and avoiding false positives. We also find that the detection algorithm alone does not make all the difference, as the way the light curves are filtered and detrended beforehand also has a strong impact on the detection limit and on the false alarm rate. The microvariability of sun-like stars is a limiting factor only in extreme cases, when the fluctuation amplitudes are large and the star is faint. In the majority of cases it does not prevent detection of planetary transits. The most sensitive analysis is performed with periodic box-shaped detection filters. False positives are method-dependent, which should allow reduction of their detection rate in real surveys. Background eclipsing binaries are wrongly identified as planetary transits in most cases, a result which confirms that contamination by background stars is the main limiting factor. With parameters simulating the CoRoT mission, our detection test indicates that the smallest detectable planet radius is on the order of 2 Earth radii for a 10-day orbital period planet around a K0 dwarf.

Gaia Data Release 1 - The photometric data

Context. This paper presents an overview of the photometric data that are part of the first Gaia data release. Aims. The principles of the processing and the main characteristics of the Gaia photometric data are presented. Methods. The calibration strategy is outlined briefly and the main properties of the resulting photometry are presented. Results. Relations with other broadband photometric systems are provided. The overall precision for the Gaia photometry is shown to be at the milli-magnitude level and has a clear potential to improve further in future releases.

Gaia Data Release 1 - Principles of the photometric calibration of the G band

Context. Gaia is an ESA cornerstone mission launched on 19 December 2013 aiming to obtain the most complete and precise 3D map of our Galaxy by observing more than one billion sources. This paper is part of a series of documents explaining the data processing and its results for Gaia Data Release 1, focussing on the G band photometry. Aims. This paper describes the calibration model of the Gaia photometric passband for Gaia Data Release 1. Methods. The overall principle of splitting the process into internal and external calibrations is outlined. In the internal calibration, a self-consistent photometric system is generated. Then, the external calibration provides the link to the absolute photometric flux scales. Results. The Gaia photometric calibration pipeline explained here was applied to the first data release with good results. Details are given of the various calibration elements including the mathematical formulation of the models used and of the extraction and preparation of the required input parameters (e.g. colour terms). The external calibration in this first release provides the absolute zero point and photometric transformations from the Gaia G passband to other common photometric systems. Conclusions. This paper describes the photometric calibration implemented for the first Gaia data release and the instrumental effects taken into account. For this first release no aperture losses, radiation damage, and other second-order effects have not yet been implemented in the calibration.

Spectrophotometry with Gaia

Gaia is an all-sky survey satellite, to be launched by ESA in late 2011, to obtain parallaxes and proper motions to microarcsecond precision, radial velocities and astrophysical parameters for about 109 objects down to a limiting magnitude of 20mag, which means about 104 times more stars than observed with Hipparcos mission. The astrophysical information for all sources will be derived from broad-band photometry and low-resolution spectrophotometry complemented with astrometric and high-resolution spectroscopy measurements around CaII triplet. This paper describes the instrument and its capabilities in terms of stellar parameters determination, as well as the current status of the mission.

Calibration Model for Gaia Photometry and Spectrophotometry

From the astrometric measurements of unfiltered (white) light, Gaia will produce broad band G magnitudes, while the spectral energy distribution of each source will be sampled by a dedicated spectrophotometric instrument providing low resolution spectra in the blue (BP, 330–680 nm) and the red (RP, 650–1,050 nm). We present the data reduction scheme for this data. It is foreseen as an iterative process updating the mean spectra and the calibration parameters. Uncertainties from a prototype of the data reduction chain are also evaluated.

Stellar parameters through high precision parallaxes

Before Hipparcos, the determination of absolute luminosity was usually done through calibrations based on a few stellar parallaxes measured at the highest precision. The Hipparcos mission meant a giant step on the knowledge of luminosities and fine structure of the HR diagram. The Gaia mission will go an enormous step further. Besides luminosities, Gaia will allow us to derive other stellar parameters like temperature and extinction, gravity, chemical composition, age and mass by the combination of astrometric and spectrometric data. Through simulations during the mission preparation, it has been shown that the astrometric parallax information is essential to deal with the degeneracy between gravity and chemical composition ([Fe/H] and [α/Fe]), that cannot be treated using only spectrophotometry. We show the expected HR diagram for the Gaia domain and the accuracies of stellar parameters.