Prof Suzanne Aigrain

Abstract:

Space-based photometric transit searches, such as the ESA mission Eddington (planned for launch in 2007), are expected to detect large numbers of terrestrial planets outside the solar system, including some Earth-like planets. Combining simplicity of concept with efficiency, the transit method consists in detecting the periodic luminosity drop in stellar light curves caused by the transit of a planet in front of its parent star. In a previous paper (Aigrain & Favata 2002, hereafter Paper I), we developed a Bayesian transit detection algorithm and evaluated its performance on simulated light curves dominated by photon noise. In this paper, we examine the impact of intrinsic stellar variability. Running the algorithm on light curves with added stellar variability (constructed using data from the VIRGO/PMO6 instrument on board SoHO) demonstrated the need for pre-processing to remove the stellar noise. We have developed an effective variability filter, based on an ad-hoc optimal approach, and run extensive simulations to test the filter and detection algorithm combination for a range of stellar magnitudes and activity levels. These show that activity levels up to solar maximum are not an obstacle to habitable planet detection. We also evaluated the benefits of using colour information in the detection process, and concluded that in the case of Eddington they are outweighed by the implied loss of photometric accuracy.

Abstract:

The high-resolution optical spectra of the two X-ray active binaries RS CVn stars HD 113816 (IS Vir) and HD 119285 (V851 Cen) are analysed and their Na, Mg, Al, Si, Ca, Sc, Ti, Co and Ni contents determined, in the framework of a larger program of chemical analysis of RS CVn stellar atmosphere. The analysis of IS Vir and V851 Cen is performed with three different LTE methods. In the first one, abundances are derived for a large set of transitions (among which 28 Fe I lines, spanning a broad interval in excitation potential and equivalent width, and 6 Fe II transitions) using measured equivalent widths and Kurucz LTE model atmospheres as input for the MOOG software package. The input atmospheric parameters and abundances are iteratively modified until (i) the Fe I abundances exhibit no trend with excitation potential or equivalent width, (ii) Fe I and Fe II average abundances are the same and (iii) Fe and Alpha elements average abundances are consistent with the input values. The second method follows a similar approach, but uses a restricted line list (without the Fe I "low excitation potential" transitions) and relies on the B - V and V - I colour indices to determine the temperature. The third method uses the same restricted line list as the second method and relies on fitting the 6162 Å Ca I line wing profiles to derive the surface gravity. The reliability of these methods is investigated in the context of single line RS CVn stars. It is shown that the V - I photometric index gives, on a broader sample of stars, significantly cooler estimates of the effective temperature than the B - v index. All approaches give results in good agreement with each other, except the V - 1 based method. The analysis of IS Vir and V851 Cen results in both cases in their primaries being giant stars of near-solar metallicity. Their parameters as derived with the first method are respectively Teff = 4720 K, log g = 2.65, [Fe/H] = +0.04 and Teff = 4700 K, log g = 3.0 and [Fe/H] = -0.13. In the case of V851 Cen the derived iron content is significantly higher than a previous determination in the literature. Both stars exhibit relative overabundances of several elements (e.g. Ca) with respect to the solar mix.

Abstract:

A number of high-accuracy photometric missions will be launched in the next 6 years, starting with small, missions which will observe only a handful of objects and progressing to large size (1 m class), large field of view telescopes allowing the observation of up to some 100 000 stars simultaneously. While the prime science goals of these missions will be asteroseismology and terrestrial planet finding, their long-term, accurate photometric data will encode the surface activity pattern of each target star, and thus allow activity science to be performed on unbiased samples of stars of unprecedented size. The measurements which can be carried out in this way include the integrated activity level, the spot distribution and the rotation period. We discuss summarily the upcoming missions, and in some detail the activity measurements which can be performed with them. Eddington, an ESA mission whose primary science goals are asteroseismology and extra-solar terrestrial planet finding, and scheduled for launch in 2007, is also discussed in some detail.