26,092 research outputs found
Stellar magnetic cycles
The solar activity cycle is a manifestation of the hydromagnetic dynamo
working inside our star. The detection of activity cycles in solar-like stars
and the study of their properties allow us to put the solar dynamo in
perspective, investigating how dynamo action depends on stellar parameters and
stellar structure. Nevertheless, the lack of spatial resolution and the limited
time extension of stellar data pose limitations to our understanding of stellar
cycles and the possibility to constrain dynamo models. I briefly review some
results obtained from disc-integrated proxies of stellar magnetic fields and
discuss the new opportunities opened by space-borne photometry, made available
by MOST, CoRoT, Kepler, and GAIA, and by new ground-based spectroscopic or
spectropolarimetric observations. Stellar cycles have a significant impact on
the energetic output and circumstellar magnetic fields of late-type active
stars which affects the interaction between stars and their planets. On the
other hand, a close-in massive planet could affect the activity of its host
star. Recent observations provide circumstantial evidence of such an
interaction with possible consequences for stellar activity cycles.Comment: 10 pages, Invited paper at the IAU Symposium 264, held during the
2009 IAU General Assembly in Rio de Janeiro, Brasil, from 3 to 7 August 2009;
Editors: A. H. Andrei, A. G. Kosovichev and J.-P. Rozelo
Modelling the time variation of the surface differential rotation in AB Doradus and LQ Hydrae
Sequences of Doppler images of the young, rapidly rotating late-type stars AB
Dor and LQ Hya show that their equatorial angular velocity and the amplitude of
their surface differential rotation vary versus time. Such variations can be
modelled to obtain information on the intensity of the azimuthal magnetic
stresses within stellar convection zones. We introduce a simple model in the
framework of the mean-field theory and discuss briefly the results of its
application to those solar-like stars.Comment: 4 pages, 1 figures, accepted by Astronomical Notes (Astronomische
Nachrichten
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
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
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