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

300 objects, a size sufficient for statistical studies.
Notwithstanding the caveats raised in the beginning of this section regarding the cost of measuring accurate
radial velocity for faint M-dwarfs, this type of stars remains extremely interesting for future planet search
programmes, in particular when potent imagers will come on-line, like ESO’s Planet Finder. Indeed, M dwarfs
are by far the most abundant objects in the Solar neighbourhood, with 100 of the 120 nearest stars being
M dwarfs. Their proximity makes them very favourable targets for astrometric searches. Similarly, planetary
system around M dwarfs are ideal targets for astrometric measurement of the reflex motion: the very low mass
of the star results in a more favourable mass ratio, and, again, the (statistically) smaller distance produces a
larger angular motion for a given linear displacement. We very recently obtained the first astrometric mass
determination of a planet (Benedict, McArthur, Forveille, Delfosse et al. 2002) around Gl 876. Needless to say,
the planets discovered by the present program will be ideal targets for the PRIMA instrument of the VLTI, if
a suitable reference can be found, as well as for the space astrometry missions. Finally, the brightness contrast
between an M-star and a Jupiter-like planet is very favourable in the infrared for a planet imaging campaign
like the one envisaged with ESO’s PLANET FINDER instrument.
6.5.2 Search for planets around F- and A-stars
The massive Main Sequence stars (spectral type earlier than F8V) have not been investigated for planets until
recently because exhibit a low number of stellar lines (typically a few dozens to a few hundreds for A-F type
stars versus a few thousands for Solar-type stars), and these lines are generally broadened by high rotational
velocities (typically 20-200 km/s versus a few km/s). However, knowing about the presence of planets or brown
dwarfs around more massive objects is also of importance. We already know that the disks around these massive
stars and Solar-type stars are different in terms of properties at similar ages: indeed T Tauri disks aged a few
Myrs appear to be less evolved than those around massive stars aged also a few Myrs such as HD 141569 or
HR 4796, which tend to show that these disks, as the parent stars, evolve more rapidly (Lagrange and Augereau,
2004). The occurence and time scale of planet formation have to be investigated and compared similarly.
Figure 6.12: ELODIE radial velocity data and orbital solutions for a F6V star showing periodic radial velocity
variations corresponding to the presence of a 9.1 MJup planet orbiting at 1.1 AU (period of 388 days). The
eccentricity is 0.34. Top: Radial velocities. Bottom: Residuals to the fitted orbital solution.
The way to process the data and extract the information on radial velocity variations on lower mass stars is
not straightforwardly applicable to more massive stars (Griffin et al, 2000, A&A Suppl. Ser., 147, 299-321). We
introduced (Chelli, 2000, A&A, 358, L59) a new radial velocity measurement method, consisting in correlating,
for a given star and in the Fourier space, each spectrum and a reference spectrum built by summing up all the
spectra, which are specific of the considered star.
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