Searching transients in large-scale surveys. A method based on the Abbe value

(Abridged) A new method is presented to identify transient candidates in large-scale surveys based on the variability pattern in their light curves. The method is based on the Abbe value, that estimates the smoothness of a light curve, and on a newly introduced value called the excess Abbe that estimates the regularity of the light curve variability pattern over the duration of the observations. Based on simulated light curves, transients are shown to occupy a specific region in the Abbe versus Excess Abbe diagram, distinct from sources presenting pulsating-like features in their light curves or having featureless light curves. The method is tested on real light curves taken from EROS-2 and OGLE-II surveys in a 0.50deg x 0.17deg field of the sky in the LMC. The method identifies 43 EROS-2 transient candidates out of a total of ~1300 variable stars, and 19 more OGLE-II candidates. The efficiency of the method is further tested by comparing the list of transient candidates with known Be stars in the literature. It is shown that all Be stars known in the studied field of view with detectable bursts or outbursts are successfully extracted by the method. In addition, four new transient candidates displaying bursts and/or outbursts are found in the field, of which at least two are good new Be candidates. The new method proves to be a potentially powerful tool to extract transient candidates from large-scale multi-epoch surveys. The better the photometric measurement uncertainties are, the cleaner the list of detected transient candidates is. In addition, the diagram is shown to be a good diagnostic tool to check the data quality of multi-epoch photometric surveys. A trend of instrumental and/or data reduction origin, for example, will manifest itself by an unexpected distribution of points in the diagram.Comment: Accepted for publication in A&

Diffusion in stellar interiors: critical tests of three numerical methods

We describe and discuss the properties of three numerical methods for solving the diffusion equation for the transport of the chemical species and of the angular momentum in stellar interiors. We study through numerical experiments both their accuracy and their ability to provide physical solutions. On the basis of new tests and analyses applied to the stellar astrophysical context, we show that the most robust method to follow the secular evolution is the implicit finite differences method. The importance of correctly estimating the diffusion coefficient between mesh points is emphasized and a procedure for estimating the average diffusion coefficient between a convective and a radiative zone is described.Comment: 15 pages, 8 figures. accepted for publication in Astronomy and Astrophysic

The survival of 205Pb in intermediate-mass AGB stars

The now extinct 205Pb is a pure s-process radionuclide (t_{1/2} = 1.5x10^7 y) of possible substantial cosmochemical interest. As a necessary complement to the detailed theoretical study of the nuclear physics and astrophysics aspects of the 205Pb - 205Tl pair carried out by Yokoi et al. (1985), and to the recent calculation of the 205Pb production in Wolf-Rayet stars by Arnould et al. (1997), this paper addresses for the first time in some detail the question of the survival of this radionuclide in thermally pulsing AGB stars. This problem is made difficult by the high sensitivity to temperature and density of the rates of the weak interaction processes that are able to produce or destroy 205Pb. In view of this sensitivity, a recourse to detailed stellar models is mandatory. With the help of some simplifying assumptions concerning in particular the third dredge-up characteristics, some of which (like its depth) being considered as free parameters, predictions are made for the 205Pb contamination of the stellar surface at the end of a pulse-interpulse cycle following a series of a dozen of pulses in three different intermediate-mass stars (M=3M_sun,Z=0.02; M=6M_sun,Z=0.02; M=3M_sun,Z=0.001). It is concluded that the chances for a significant 205Pb surface enrichment are likely to increase with M for a given Z, or to increase with decreasing Z for a given M. More specifically, following the considered pulses at least, the enrichment appears to be rather unlikely in the 3M_sun star with Z=0.02, while it seems to be much more probable in the other two considered stars. It is also speculated that the (3M_sun,Z=0.02) star could possibly experience some 205Pb enrichment following later pulses than the ones considered in this paper.Comment: 10 pages, 10 figures, Latex A&A, ps file available at ftp://obsftp.unige.ch/pub/mowlavi/pbtl.ps; accepted for publication in A&

Gaia and variable stars

The study of variable phenomena (periodic, irregular or transient) provides a unique way to acquire knowledge about objects in our Universe. Currently, we are going through a rapid expansion of time-domain astrophysics. One reason for this expansion is the technological developments materialised in small to medium size observational projects such as HAT, OGLE, Catalina, PTF and upcoming very large projects such as Gaia or LSST. In this article, we are focusing on the ESA cornerstone mission Gaia. This spacecraft will provide astrometric, photometric and spectroscopic measurements for one billion stars. Among the existing and planned multi-epoch projects Gaia is unique because it will provide exquisite astrometric measurements for all objects it observes. We provide a brief overview of the literature concerning this mission and its expected contribution to variability studie

The Galactic Cepheid period-luminosity relation revisited using bona fide cluster Cepheids

Classical Cepheids in Galactic open clusters (cluster Cepheids: CCs) have been studied extensively for multiple decades, thanks to their importance as calibrators of the Galactic Cepheid period-luminosity relation (PLR). Here we revisit the calibration of the Galactic PLR using a new sample of CCs, since even recent calibrations show significant discrepancies. The CC sample employed for the calibration is based on the preliminary results of a self-consistent, eight-dimensional all-sky census. This census is based mostly on literature data, supplemented with high-precision radial-velocity observations from both hemispheres. New CCs are identified from our census and the degree of confidence in membership is quantified for known candidates. Using only bona fide CCs, we obtain MV = (−3.08 ± 0.50) log P + (−0.94 ± 0.42) mag, which is in perfect agreement with the results by Sandage, Tammann, and Reindl, albeit with larger error bars and an rms of 0.21 mag. The key to obtaining a meaningful calibration is to employ accurate cluster distance moduli and space reddening values. A homogeneous study of all bona fide host clusters would be desirable to increase precision and confidence in the calibratio

Models for Pop I stars: implications for age determinations

Starting from a few topical astrophysical questions which require the knowledge of the age of Pop I stars, we discuss the needed precision on the age in order to make progresses in these areas of research. Then we review the effects of various inputs of the stellar models on the age determination and try to identify those affecting the most the lifetimes of star