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Astron. Astrophys. 361, 265-272 (2000)

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1. Introduction

High precision (some m s-1) radial-velocity measurements give astronomers the possibility of detecting giant planets around other stars. To date, more than 30 extra-solar giant planets were discovered, and many others are expected to be found in the next few years (see e.g. Marcy et al. 2000 or Marcy & Butler 1998 for a review on the subject, and Queloz et al. 2000a, Udry et al. 2000, Santos et al. 2000a, Mazeh et al. 2000, Vogt et al. 2000, Naef et al. 2000, and Marcy et al. 2000 for the most recent announcements).

These techniques however, are not sensible only to the motion of a star around the center of mass of a star/planet system. Intrinsic variations, such as non-radial pulsation (Brown et al. 1998), inhomogeneous convection or spots (Saar & Donahue 1997) are expected to induce radial velocity variations, which can prevent us from finding planets (if the perturbation is larger than the orbital radial-velocity variation) or give us false candidates (if they produce a periodic signal over a few rotational periods).

The physics of the photospheric perturbations that produce the intrinsic radial-velocity variations is not easy to model (at least in detail). In one hand, the amplitude of the perturbations should depend, for example, on the size of the spots and on the velocities of the convection inhomogeneities (Saar & Donahue 1997), or on the rotational velocity of the star (e.g. Mayor et al. 1998). These are functions of the spectral type and age: a younger star is expected to have a higher rotation rate; the magnetic fields produced will then be more or less strong, depending on the depth of the convective zone, i.e. on the spectral type (e.g. Noyes et al. 1984). On the other hand, the presence of active regions is deeply associated with photospheric features (like spots or convective inhomogeneities, e.g. Schrijver et al. 1989) that can be responsible for intrinsic radial-velocity "jitter". This fact may permit us to study in a simple but indirect way the photospheric phenomena responsible for intrinsic radial-velocity "jitter", by measuring a chromospheric-activity index, such as the Mount-Wilson "S" index (Vaughan et al. 19781.

In June 1998 we started a long term extra-solar planet search programme at ESO (La Silla) using the CORALIE echelle high-resolution ([FORMULA] 50.000) spectrograph installed at the 1.2-m Euler Swiss telescope. This programme makes use of a large (about 1650) volume-limited sample of stars in the southern sky (Udry et al. 2000).

The precision obtained with CORALIE is of the order of 7  m s-1 (e.g. Queloz et al. 2000b). At this level, and given the nature of the sample (we have all kinds of dwarfs with spectral types from F8 to M0) it is very important to know the limits imposed by chromospheric activity, so that we can disentangle activity-related phenomena from real planetary candidates, and possibly exclude bad candidates for this survey and for future even more sensible programmes (Pepe et al. 2000).

In order to monitor the chromospheric activity of the stars in our survey, we use CORALIE spectra to compute a new chromospheric activity index, [FORMULA], based on the flux in the center of the CaII H line. In Sect. 2 we present the [FORMULA] index, describing the technique and some tests. We then calibrate our activity index values to the Mount-Wilson "S" system (hereafter [FORMULA]). In Sect. 3 we present our sample of stars with [FORMULA] values, and in Sect. 4 we discuss our results for F, G and K dwarfs. In particular we focus on the study of the relation between the radial-velocity "jitter" and the chromospheric activity as expressed by the fractional CaII H and K flux corrected for photospheric flux ([FORMULA], as defined by Noyes et al. 1984).

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© European Southern Observatory (ESO) 2000

Online publication: September 5, 2000