999 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
Model order reduction for stochastic dynamical systems with continuous symmetries
Stochastic dynamical systems with continuous symmetries arise commonly in
nature and often give rise to coherent spatio-temporal patterns. However,
because of their random locations, these patterns are not well captured by
current order reduction techniques and a large number of modes is typically
necessary for an accurate solution. In this work, we introduce a new
methodology for efficient order reduction of such systems by combining (i) the
method of slices, a symmetry reduction tool, with (ii) any standard order
reduction technique, resulting in efficient mixed symmetry-dimensionality
reduction schemes. In particular, using the Dynamically Orthogonal (DO)
equations in the second step, we obtain a novel nonlinear Symmetry-reduced
Dynamically Orthogonal (SDO) scheme. We demonstrate the performance of the SDO
scheme on stochastic solutions of the 1D Korteweg-de Vries and 2D Navier-Stokes
equations.Comment: Minor revision
Fluorine production in intermediate-mass stars
The 19F production during the first dozen thermal pulses of AGB stars with
(M=3,Z=0.02), (M=6,Z=0.02) and (M=3,Z=0.001) is investigated on grounds of
detailed stellar models and of revised rates for 15N(a,g)19F and 18O(a,g)22Ne.
These calculations confirm an early expectation that 19F {\it is} produced in
AGB thermal pulses. They also enlarge substantially these previous results by
showing that the variations of the level of 19F production during the evolution
is very sensitive to the maximum temperature reached at the base of the pulse.
These variations are analyzed in detail, and are shown to result from a subtle
balance between different nuclear effects (mainly 19F production or destruction
in a pulse, and 15N synthesis during the interpulse), possibly super-imposed on
dilution effects in more or less extended pulse convective tongues. Our
calculations, as most others, do not predict the third dredge-up self-
consistently. When parametrized, it appears that our models of
intermediate-mass AGB stars are able to account only for the lowest 19F
overabundances observed in solar-metallicity MS, S and C stars. That conclusion
is expected to hold true for low-mass stars when F production results from
secondary 13C only. Massive AGB stars, on the other hand, are not expected to
build up large surface F abundanc- es. Therefore, the large F overabundance
reported for the super Li-rich star WZ Cas (where HBB is supposed to be
operating) remains unexplained so far. Our results for the (M=3,Z=0.001) star
indicate that F surface overabundances can also be expected in low-metallicity
stars provided that third dredge-ups occur after the early cool pulses. The
relative increase in the surface 19F/12C ratio is, however, lower in the
low-metallicity than in the solar-metallicity star. No observations areComment: 27 pages, includes figures, postcript file (A&A format, 15 pages
including figures) can be found via anonymous ftp at
ftp://obsftp.unige.ch/pub/mowlavi/fluor.ps.gz ; accepted by A&
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&
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