11,266 research outputs found
Comparing the performance of stellar variability filters for the detection of planetary transits
We have developed a new method to improve the transit detection of
Earth-sized planets in front of solar-like stars by fitting stellar
microvariability by means of a spot model. A large Monte Carlo numerical
experiment has been designed to test the performance of our approach in
comparison with other variability filters and fitting techniques for stars of
different magnitudes and planets of different radius and orbital period, as
observed by the space missions CoRoT and Kepler. Here we report on the results
of this experiment.Comment: 4 pages, 3 postscript figures, Transiting Planets Proceeding IAU
Symposium No.253, 200
Star-planet interactions
Stars interact with their planets through gravitation, radiation, and
magnetic fields. I shall focus on the interactions between late-type stars with
an outer convection zone and close-in planets, i.e., with an orbital semimajor
axis smaller than approximately 0.15 AU. I shall review the roles of tides and
magnetic fields considering some key observations and discussing theoretical
scenarios for their interpretation with an emphasis on open questions.Comment: 20 pages, 5 figures, invited talk at the 18th Cambridge Workshop on
Cool Stars, Stellar Systems, and the Sun, Proceedings of Lowell Observatory,
edited by G. van Belle & H. Harri
Star-planet magnetic interaction and evaporation of planetary atmospheres
Stars interact with their close-in planets through radiation, gravitation,
and magnetic fields. We investigate the energy input to a planetary atmosphere
by reconnection between stellar and planetary magnetic fields and compare it to
the energy input of the extreme ultraviolet (EUV) radiation field of the star.
We quantify the power released by magnetic reconnection at the boundary of the
planetary magnetosphere that is conveyed to the atmosphere by accelerated
electrons. We introduce simple models to evaluate the energy spectrum of the
accelerated electrons and the energy dissipated in the atmospheric layers in
the polar regions of the planet upon which they impinge. A simple transonic
isothermal wind flow along field lines is considered to estimate the increase
in mass loss rate in comparison with a planet irradiated only by the EUV flux
of its host star. We find that energetic electrons can reach levels down to
column densities of 10^{23}-10^{25} m^{-2}, comparable with or deeper than EUV
photons, and increase the mass loss rate up to a factor of 30-50 in close-in (<
0.10 AU), massive (> 1.5 Jupiter masses) planets. Mass loss rates up to
(0.5-1.0)x10^{9} kg/s are found for atmospheres heated by electrons accelerated
by magnetic reconnection at the boundary of planetary magnetospheres. On the
other hand, average mass loss rates up to (0.3-1.0)x10^{10} kg/s are found in
the case of magnetic loops interconnecting the planet with the star. The
star-planet magnetic interaction provides a remarkable source of energy for
planetary atmospheres, generally comparable with or exceeding that of stellar
EUV radiation for close-in planets. Therefore, it must be included in models of
chemical evolution or evaporation of planetary atmospheres as well as in
modelling of light curves of transiting planets at UV wavelengths.Comment: 13 pages, 8 figures, accepted by Astronomy and Astrophysic
On the correlation between stellar chromospheric flux and the surface gravity of close-in planets
The chromospheric emission of stars with close-in transiting planets has been
found to correlate with the surface gravity of their planets. Stars with
low-gravity planets have on average a lower chromospheric flux. We propose that
this correlation is due to the absorption by circumstellar matter that comes
from the evaporation of the planets. Planets with a lower gravity have a
greater mass-loss rate which leads to a higher column density of circumstellar
absorption and this in turn explains the lower level of chromospheric emission
observed in their host stars. We estimated the required column density and
found that planetary evaporation can account for it. We derived a theoretical
relationship between the chromospheric emission as measured in the core of the
Ca II H&K lines and the planet gravity. We applied this relationship to a
sample of transiting systems for which both the stellar Ca II H&K emission and
the planetary surface gravity are known and found a good agreement, given the
various sources of uncertainties and the intrinsic variability of the stellar
emissions and planetary evaporation rates. We consider implications for the
radial velocity jitter applied to fit the spectroscopic orbits and for the age
estimates of planetary systems based on the chromospheric activity level of
their host stars.Comment: 5 pages, 2 figures, accepted as a Letter to the Editor of Astronomy
and Astrophysic
Close-by planets and flares in their host stars
The interaction between the magnetic fields of late-type stars and their
close-by planets may produce stellar flares as observed in active binary
systems. However, in spite of several claims, conclusive evidence is still
lacking. We estimate the magnetic energy available in the interaction using
analytical models to provide an upper bound to the expected flare energy. We
investigate three different mechanisms leading to magnetic energy release. The
first two can release an energy up to , where
is the surface field of the star, its radius, and the
magnetic permeability of the plasma. They operate in young active stars whose
coronae have closed magnetic field lines up to the distance of their close-by
planets that can trigger the energy release. The third mechanism operates in
weakly or moderately active stars having a coronal field with predominantly
open field lines at the distance of their planets. The released energy is of
the order of and depends on the ratio of the
planetary to the stellar fields, thus allowing an indirect measurement of the
former when the latter is known. We compute the released energy for different
separations of the planet and different stellar parameters finding the
conditions for the operation of the proposed mechanisms. An application to
eight selected systems is presented. The computed energies and dissipation
timescales are in agreement with flare observations in the eccentric system HD
17156 and in the circular systems HD 189733 and HD 179949. This kind of
star-planet interaction can be unambiguously identified by the higher flaring
frequency expected close to periastron in eccentric systems.Comment: 17 pages, 9 figures, 5 tables; accepted to Astronomy and Astrophysic
Modelling solar-like variability for the detection of Earth-like planetary transits. I. Performance of the three-spot modelling and harmonic function fitting
We present a comparison of two methods of fitting solar-like variability to
increase the efficiency of detection of Earth-like planetary transits across
the disk of a Sun-like star. One of them is the harmonic fitting method that
coupled with the BLS detection algorithm demonstrated the best performance
during the first CoRoT blind test. We apply a Monte Carlo approach by
simulating a large number of light curves of duration 150 days for different
values of planetary radius, orbital period, epoch of the first transit, and
standard deviation of the photon shot noise. Stellar variability is assumed in
all the cases to be given by the Total Solar Irradiance variations as observed
close to the maximum of solar cycle 23. After fitting solar variability,
transits are searched for by means of the BLS algorithm. We find that a model
based on three point-like active regions is better suited than a best fit with
a linear combination of 200 harmonic functions to reduce the impact of stellar
microvariability provided that the standard deviation of the noise is 2-4 times
larger than the central depth of the transits. On the other hand, the
200-harmonic fit is better when the standard deviation of the noise is
comparable to the transit depth. Our results show the advantage of a model
including a simple but physically motivated treatment of stellar
microvariability for the detection of planetary transits when the standard
deviation of the photon shot noise is greater than the transit depth and
stellar variability is analogous to solar irradiance variations.Comment: 8 pages, 6 figures, accepted by Astronomy & Astrophysic
Photospheric activity, rotation, and star-planet interaction of the planet-hosting star CoRoT-6
The CoRoT satellite has recently discovered a hot Jupiter that transits
across the disc of a F9V star called CoRoT-6 with a period of 8.886 days. We
model the photospheric activity of the star and use the maps of the active
regions to study stellar differential rotation and the star-planet interaction.
We apply a maximum entropy spot model to fit the optical modulation as observed
by CoRoT during a uninterrupted interval of about 140 days. Photospheric active
regions are assumed to consist of spots and faculae in a fixed proportion with
solar-like contrasts. Individual active regions have lifetimes up to 30-40
days. Most of them form and decay within five active longitudes whose different
migration rates are attributed to the stellar differential rotation for which a
lower limit of \Delta \Omega / \Omega = 0.12 \pm 0.02 is obtained. Several
active regions show a maximum of activity at a longitude lagging the
subplanetary point by about 200 degrees with the probability of a chance
occurrence being smaller than 1 percent. Our spot modelling indicates that the
photospheric activity of CoRoT-6 could be partially modulated by some kind of
star-planet magnetic interaction, while an interaction related to tides is
highly unlikely because of the weakness of the tidal force.Comment: 9 pages, 7 figures, accepted to Astronomy & Astrophysic
Tides and angular momentum redistribution inside low-mass stars hosting planets: a first dynamical model
We introduce a general mathematical framework to model the internal transport
of angular momentum in a star hosting a close-in planetary/stellar companion.
By assuming that the tidal and rotational distortions are small and that the
deposit/extraction of angular momentum induced by stellar winds and tidal
torques are redistributed solely by an effective eddy-viscosity that depends on
the radial coordinate, we can formulate the model in a completely analytic way.
It allows us to compute simultaneously the evolution of the orbit of the
companion and of the spin and the radial differential rotation of the star. An
illustrative application to the case of an F-type main-sequence star hosting a
hot Jupiter is presented. The general relevance of our model to test more
sophisticated numerical dynamical models and to study the internal rotation
profile of exoplanet hosts, submitted to the combined effects of tides and
stellar winds, by means of asteroseismology are discussed.Comment: 32 pages, 10 figures, one table; accepted to Celestial Mechanics and
Dynamical Astronomy, special issue on tide
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