225 research outputs found
Time domain period determination techniques
Two simple period determination schemes are discussed. They are well suited to problems involving non-sinusoidal periodic phenomena sampled at a few irregularly spaced points. Statistical properties are discussed. The techniques are applied to the double mode Cepheids BK Cen and TU Cas as test cases
Search for a circum-planetary material and orbital period variations of short-period Kepler exoplanet candidates
A unique short-period Mercury-size Kepler exoplanet candidate KIC012557548b
has been discovered recently by Rappaport et al. (2012). This object is a
transiting disintegrating exoplanet with a circum-planetary material -
comet-like tail. Close-in exoplanets, like KIC012557548b, are subjected to the
greatest planet-star interactions. This interaction may have various forms. In
certain cases it may cause formation of the comet-like tail. Strong interaction
with the host star, and/or presence of an additional planet may lead to
variations in the orbital period of the planet. Our main aim is to search for
comet-like tails similar to KIC012557548b and for long-term orbital period
variations. We are curious about frequency of comet-like tail formation among
short-period Kepler exoplanet candidates. We concentrate on a sample of 20
close-in candidates with a period similar to KIC012557548b from the Kepler
mission.Comment: 19 pages, 75 figures, AN accepte
On a new theoretical framework for RR Lyrae stars I: the metallicity dependence
We present new nonlinear, time-dependent convective hydrodynamical models of
RR Lyrae stars computed assuming a constant helium-to-metal enrichment ratio
and a broad range in metal abundances (Z=0.0001--0.02). The stellar masses and
luminosities adopted to construct the pulsation models were fixed according to
detailed central He burning Horizontal Branch evolutionary models. The
pulsation models cover a broad range in stellar luminosity and effective
temperatures and the modal stability is investigated for both fundamental and
first overtones. We predict the topology of the instability strip as a function
of the metal content and new analytical relations for the edges of the
instability strip in the observational plane. Moreover, a new analytical
relation to constrain the pulsation mass of double pulsators as a function of
the period ratio and the metal content is provided. We derive new
Period-Radius-Metallicity relations for fundamental and first-overtone
pulsators. They agree quite well with similar empirical and theoretical
relations in the literature. From the predicted bolometric light curves,
transformed into optical (UBVRI) and near-infrared (JHK) bands, we compute the
intensity-averaged mean magnitudes along the entire pulsation cycle and, in
turn, new and homogenous metal-dependent (RIJHK) Period-Luminosity relations.
Moreover, we compute new dual and triple band optical, optical--NIR and NIR
Period-Wesenheit-Metallicity relations. Interestingly, we find that the optical
Period-W(V,B-V) is independent of the metal content and that the accuracy of
individual distances is a balance between the adopted diagnostics and the
precision of photometric and spectroscopic datasets.Comment: 51 pages, 20 figures, 9 tables, accepted for publication on Ap
Second Overtone Pulsators Among Delta Scuti Stars
We investigate the modal stability of stellar models at masses and luminosity
levels corresponding to post main sequence luminous delta scuti pulsators. The
envelope models have been computed at fixed mass value, luminosity level and
chemical composition (Y=0.28, Z=0.02). According to a nonlinear approach to
radial oscillations the present investigation predicts the occurrence of stable
second overtone pulsators for the first time. The shape of both light and
velocity curves are presented and discussed, providing a useful tool for the
identification of second overtone pulsators among the known groups of radially
pulsating stars. The period ratios of mixed mode pulsators obtained by
perturbing the first and the second overtone radial eigenfunctions are in
agreement with observative values. Finally, the physical structure and the
dynamical properties of second overtone pulsators are discussed in detail. The
role played by the nodal lines in the destabilization of second overtone
pulsators is also pointed out.Comment: 20 pages, 11 Postscript figures, uses aaspp4.sty and tighten.st
Nonlinear Beat Cepheid and RR Lyrae Models
The numerical hydrodynamic modelling of beat Cepheid behavior has been a long
standing quest in which purely radiative models had failed consistently. We
find that beat pulsations occur naturally when {\it turbulent convection} is
accounted for in our hydrodynamics code. The development of a relaxation code
and of a Floquet stability analysis greatly facilitates the search for and the
analysis of beat Cepheid models. The conditions for the occurrence of beat
behavior can be understood easily and at a fundamental level with the help of
amplitude equations.Comment: 8 pages, 5 figures, Astronomy and Astrophysics, submitte
Limit-cycle behavior in one-zone convective models
We present the results of a detailed set of one-zone models that account for
the coupling between pulsation and convection following the original
prescriptions of Stellingwerf (1986). Motivated by the arbitrary nature of the
input parameters adopted in this theoretical framework, we computed several
sequences of models that cover a substantial fraction of the parameter space
and a longer integration time. We also included a turbulent pressure term and
found that this physical mechanism plays a crucial role in the pulsation
characteristics of the models by removing the sharp discontinuities along the
light and the velocity curves showed by models that do not account for
turbulent pressure. Finally, we investigated the vibrational and the
pulsational stability of completely convective models. We consider as the most
important finding of the present work the identification of a well-defined
region in the parameter space where they approach limit-cycle stability.
Several numerical experiments performed by adopting different values of the
adiabatic exponent and of the shell thickness indicate that the coupling
between pulsation and convection is the key driving mechanism for LPVs, a
finding supported by recent theoretical predictions.Comment: 11 pages, 10 figures; accepted for publication in Astrophysical
Journa
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