Exoplanets and Stellar Physics

Abstract:

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 microvariability 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 KO dwarf. © ESO 2005.

Abstract:

Accurately determining the star formation history in NGC 4038/4039 -- ``The Antennae'' is hampered by extinction. We therefore used near infrared images obtained with ISAAC at the VLT and with SOFI at the NTT to determine the recent star formation history in this merger. In combination with archival HST data, we determined ages, extinction and other parameters for single star clusters, and properties of the cluster population as a whole. About 70% of the K_s-band detected star clusters with masses >= 10^5 M_sun are younger than 10 Myrs (approximately an e-folding time for cluster ages), which we interpret as evidence for rapid dissolution but not free expansion. The total mass of K-band selected clusters is about 5-10x10^8 M_sun and represents about 3-6% of the total molecular gas. This takes into account only the detected clusters and in view of the rapid dissolution means that this is only a lower limit to the total mass of stars produced in clusters during the burst. Studies of cluster formation in other galaxies recently suggested short cluster dissolution timescales, too, which means that star formation rates may have been severely underestimated in the past. Extinction is strongly variable and very high in some regions, but around A_V=1.3 mag on average. Even though most clusters are detected at least in I-band, only the information about individual cluster ages and extinction allows to avoid uncertainties of orders of magnitude in star formation rate estimates determined from optical fluxes. From the distribution of individual cluster extinction vs. age, which is significantly higher for clusters below 8-9 Myr than for older clusters, we infer that this is the time by which a typical cluster blows free of its native dust cocoon.