Modelling solar-like variability for the detection of Earth-like planetary transits

I. Performance of the three-spot modelling and harmonic function fitting

¹ Dipartimento di Fisica e Astronomia, Sezione Astrofisica, Università degli Studi di Catania, Italy
² INAF – Osservatorio Astrofisico di Catania, via S. Sofia 78, 95123 Catania, Italy e-mail: [aldo.bonomo;nuccio.lanza]@oact.inaf.it

Received: 11 October 2007
Accepted: 20 February 2008

Abstract

Aims. We present a comparison of two methods of fitting solar-like variability to increase the efficiency of the detection of Earth-like planetary transits across the disc of a Sun-like star. One of them is the harmonic fitting method that coupled with the Box fitting Least-Square (BLS) detection algorithm demonstrated the best performance during the first CoRoT blind test.

Methods. 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.

Results. We find that a model based on three point-like active regions reduces the impact of stellar microvariability more effectively than a best fit with a linear combination of 200 harmonic functions provided that the standard deviation of the noise is  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.

Conclusions. Our results show the advantage of a model that includes 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.