Report - School of Physics

HARPS: The Observatoire de Genève together with the Physikalisches Institut der
Universität Bern, the Observatoire de Haute-Provence, and the Service d’Aéronomie
du CNRS and in collaboration with ESO, developed the HARPS spectrograph installed
on ESO’s 3.6-m Telescope at La Silla. The instrument is a high-resolution,
high-efficiency fibre-fed echelle spectrograph designed to efficiently search for extrasolar
planets reaching a precision of 1 m s −1 on radial-velocity measurements. Typically,
this precision is reached in 1 minute for a V = 7.5 G-dwarf star. The long-term
precision is ensured by the spectrograph’s stability: the spectrograph resides in a
pressure- and temperature-controlled vacuum tank, with a drift usually well below
1 m s −1 during one night, which can be further corrected using the simultaneous
thorium technique. HARPS has been available to the community since October
2003.
For the development of this instrument, the HARPS consortium has been granted
500 guaranteed nights over 5 years (100 nights per year). The HARPS survey is
designed to address several specific questions:
(a) only a few of the hundred detected planets have masses less than the mass of
Saturn, and due to the present precision of radial velocity surveys the distribution of
planetary masses is heavily biased (or completely unknown) for masses less than half
the mass of Jupiter. The high precision of HARPS will allow searches for low-mass
planets: for a sample of preselected non-active solar-type stars (from the Coralie
planet-search sample), the aim is to explore the domain of the mass-function for
short-period planets below the mass of Saturn down to a few Earth masses;
(b) in a continuation of the planet-search programmes conducted over ∼10 years, a
quick screening of a large volume-limited sample of ∼1000 still unobserved stars will
be performed in order to identify new ‘hot Jupiters’ and other Jovian-type planets.
Increasing the list of ‘hot Jupiters’ will improve the prospects of finding further stars
with a planetary transit among relatively bright stars. Better statistics are needed
to identify new properties of the distribution of exo-planet parameters. This part
of the programme has already revealed two new short-period planets (Pepe et al.,
2004);
(c) a systematic search for planets will be made for a volume-limited sample of
slowly-rotating non-binary M-dwarfs closer than 11 pc. Such a survey of very low
mass stars should constrain the frequency of planets as a function of stellar mass.
Up to now only one planetary system orbiting an M-dwarf is known. For the less
massive stars short-period planets of only a few times the mass of the Earth could
be detected. Since most of these objects are faint, high efficiency is required. These
objects are of prime importance for future astrometric studies to be carried out with
the VLTI or SIM;
(d) stars with detected giant planets exhibit an impressive excess of metallicity in
contrast to stellar samples without giant planets. The excess of metallicity does
not seem related to the mass of the convective zone and probably originates in the
11