Untitled - Laboratoire d'Astrophysique de l'Observatoire de Grenoble

process by comparing its outcome for solar-type stars and for stars of much lower (or larger) mass than the
Sun. Are planetary systems as frequent or not? Do their orbital and mass distributions differ? The answer to
these questions is at present totally unknown.
Surveys for planets around very low mass stars and massive stars are difficult because very low mass stars
are faint, making the measurement of accurate radial velocities expensive in telescope. On the other hand,
massive stars combine high rotational velocities and a small number of photospheric lines, making accurate
spectroscopy a challenge.
In our group, because of the long time collaborations we have been maintaining with the Geneva observatory,
in particular with its exoplanet group, we have been involved early on in radial velocity surveys for planets
around solar-type stars. This was a natural follow-up to the astrometry programmes that had been going-on
for solar-type and low mass stars (led in particular by C.Perrier). This section of our activities is not described
in details here. Instead, we focus on the more challenging planet hunts around low-mass and high mass stars.
6.5.1 Search for planets around very low mass stars
In 2003 members of FOST integrated the HARPS consortium with the duty to pilot the search for planets
around M-dwarfs (Delfosse is PI). 10% of the Harps Guaranteed Time goes to this program (10 nights/yr
during 5 years). The accuracy of the radial velocities reached by HARPS on very low mass stars is between 1 to
3 m.s −1 and semi-amplitudes of 10 m.s −1 are detectable easily. They correspond to planets of 6 Earth masses
in 3-day orbits around a 0.15M⊙ star (13 Earth masses for a 0.6 M⊙ star), or 40 Earth mass planets for a 1000d
period around a 0.15M⊙ stars (100 Earth mass around a 0.6 M⊙ star).
Bonfils et al. (2005b) reported the discovery of a Neptune-mass planet around Gl 581 (M3V). The planet
has a circular orbit of P = 5.366 days (see Fig. 6.11). The minimum mass of the planet (m2 sin i) is only
0.052 MJup = 0.97 MNep = 16.6 MEarth making Gl 581b one of the lightest extra-solar planet known to date.
The Gl 581 planetary system is only the third centered on an M dwarf, joining the Gl 876 three-planet system
(Delfosse et al. 1998, A&A, 338, L67) and the lone planet around Gl 436 (Butler et al. 2004, ApJ, 617, 580).
Its discovery reinforces the emerging tendency for such planets to be of low mass, and found at short orbital
periods.
Figure 6.11: Upper panel: Phased radial velocities for Gl 581. Lower panel: Residuals around the fitted solution
versus time. The weighted RMS of the residuals around the fit is only 2.5 m s −1 . ¿From Bonfils et al. (2005).
In 2006 we plan to start a large program of radial velocity measurements with SOPHIE, the new spectrograph
of the OHP-1.93m telescope. An accuracy of ∼ 3m.s −1 is expected. With both programs, our sample will reach
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