The need to go to
The science goals of PLATO require the detection of a large number of
exoplanets and the detailed analysis of their central stars, including
seismic analysis. They are achievable by a very high precision, very
long duration and high duty cycle photometric monitoring from space of
a very large sample of stars. The Earth's atmosphere provides strong
disturbances which limit the achievable performance to millimag
accuracies, mostly through scintillation noise. The small amplitude of
the photometric dips caused by terrestrial planets are therefore beyond
the range of ground-based observations. Scintillation noise in
ground-based photometric observations also prevents the detection of
low amplitude oscillations in cool stars. The noise in ground-based
photometric observations is such that only giant planets with sizes
larger than that of Saturn can be detected via their transits, while
oscillations of cool stars are totally beyond reach of this technique.
In addition, long, uninterrupted observations, that only space-based
instruments can provide, are necessary to optimize the probability of
transit detection, and to see several successive transits in order to
measure the orbital period, as well as to avoid sidelobes in stellar
oscillation power spectra.
Space is therefore necessary on one hand because of its tranquility and
the absence of photometric disturbances, and on the other hand because
of the possibility it offers to perform the long, uninterrupted
observations that are needed to detect exoplanets and to perform
seismic analysis of stars.
Alternative techniques can be used from the ground to achieve exoplanet
detection as well as oscillation mode detection, and has seen tremendous
progress in recent years. The most efficient of these relies on radial
velocity measurements, performed in high resolution spectroscopy, with
either echelle cross-dispersed spectrographs (Bouchy et
al. 2001) or Fourier transform spectrographs (Mosser et al.
2003). In particular, the HARPS spectrograph installed on the ESO
3.6m telescope at La Silla showed recently that it can reach radial
velocity precision as low as about 1 ms-1. Such a low level
of radial velocity noise recently led to the discovery of a planet with
a mass of 5sini Earth masses
on a close-in orbit around the 0.31 solar mass M dwarf Gl 581 (Udry et al.
2007). Although quite impressive, this performance cannot compare
with those of a space photometric mission, and would not allow us to
reach the science goals of PLATO.
One drawback of the radial velocity technique is that the mass
determination suffers from the sini
ambiguity, except in the rare cases where the inclination angle i is known. For instance, the
recently discovered planet around Gl 581 may well be a much more
massive one whose orbit is seen at high inclination.
Second, there is a hard limit for the radial velocity precision
achievable with this technique. A Doppler shift of 1 ms-1
corresponds to a spectrum
displacement of about 10 nm on the detector of a HARPS-type
spectrograph at 100,000 spectral resolution. The data on which the
recent exoplanet discovery around Gl 581 is based are not photon-noise
limited, indicating that this hard limit has probably been met.
Therefore, there is little hope for improving significantly the
performance of the Doppler technique in the future. This means that
smaller planets, with sizes and masses comparable to those of the
Earth, will never be detected by this technique, and that the
exploration of the exoplanet distribution down to at least Earth size,
one of the most important science goals of PLATO, is not achievable
from the ground.
Observations with HARPS have also revealed solar-like oscillations in
nearby bright stars (e.g. Mosser et
al. 2005). However these observations have not allowed us to
perform full asteroseismic analysis so far, for three reasons:
- the noise level is still too high to detect a large number of
- the day-night alternance creates strong day-aliases which pollute the
power spectra and make their exploitation impossible;
- the total duration of these observations is too short to allow us to
measure the oscillation frequencies with a sufficient accuracy.
Besides, asteroseismology with the radial velocity technique is limited
to stars with projected rotation velocities smaller than about 10 kms-1
which makes the list of accessible targets very limited.
There is little hope to improve significantly these three major
drawbacks of ground-based asteroseismology. The noise level will not be
easily decreased further, as discussed earlier. Moreover, these
observations will remain severely limited in terms of target magnitude
and vsini. Seismology
programmes with HARPS are limited to stars with mV about 6,
above which photon noise is simply too high to allow us to detect any
oscillation mode. In order to reach stars with mV = 11,
which is needed to study open cluster members for instance, much larger
telescopes with diameters of the order of 40m should be used. High
efficiency, high stability, high resolution spectrographs for these
extremely large telescopes will be difficult to build, and obtaining
high duty cycles for long periods of time on these telescopes is out of
question, as detailed below.
The duty cycle of ground-based observations can be improved by
multi-site networks of telescopes equipped with appropriate
spectrographs; however, even if such an ambitious ground-based network
can be setup in the future, the drift of sidereal time limits to only a
couple of months the total time during which a high duty cycle can be
obtained. An attractive alternative is to perform these observations
from Antarctica. However, even in the overly optimistic scenario where
an extremely large telescope would be installed in a high quality site
in Antarctica, such as Dome C, and made available exclusively to
asteroseismology programmes, the total duration of high duty cycle
observations would not exceed three months.
We therefore conclude that asteroseismology from the ground with the
Doppler technique, which has already been achieved on a few targets
with modest duty cycle and total monitoring time, will remain limited
to bright stars with low projected rotation velocities, and high duty
cycle observations will be very rarely obtained and for limited total
amounts of time.