80 research outputs found
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
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