Report - School of Physics

The idea of ‘stacking up’ many radial velocity observations to average the effects of
stellar oscillations is appealing, but faces several complications: (i) even if p-mode
oscillation effects can be minimised, beating amongst these modes may induce large
radial-velocity variations (up to 10 m s −1 peak-to-peak) over timescales of a few
hours, specifically some 5–6 hours for µ Arae (Bouchy, private communication).
The star will therefore need to be observed over several hours for each epoch (radial
velocity point); (ii) simulations by Bouchy (private communication) show that a
precision of ∼ 1 m s −1 is reached in about 15–20 min, while the gain is much less
rapid with increasing observation time. A precision of 0.1 m s −1 (still insufficient
for the detection of the Earth around the Sun) will therefore be very expensive in
terms of telescope time; (iii) a wavelength calibration precision from night-to-night is
then needed at the level of the long-term precision targeted. Reference calibration to
0.1 m s −1 will require further improvements in calibration techniques. With HARPS,
a precision of about 0.5 m s −1 is reached, as illustrated by asteroseismology results
on µ Arae with 250 observations each. Investigations are ongoing (Udry, private
communication) into the possibility of having a reference at the 0.02 m s −1 level for
an instrument on OWL, while the HARPS GTO programme anyway pushing in this
direction will soon help to better characterise the question. In conclusion, a very
high radial velocity precision seems possible, but at a very high cost.
There is a significant difference in the case of transiting candidates: now the period
and phase are known, and with e ∼ 0 for short period planets, a series of accumulated
measurements can be used to constrain the radial velocity semi-amplitude. With
HARPS at a precision of 1 m s −1 , for short-period planets, it is expected that limits
of a few Earth-masses, for P < 10 days, can be reached. If the transiting object
is larger, then the radial velocity effect will be larger and easier to detect. False
positive detections will be the main problem.
(b) Photometric (transit) limits below the Earth’s atmosphere are typically a little
below the 1% photometric precision, limited by variations in extinction, scintillation
and background noise (depending on telescope aperture size), corresponding to
masses of about 1 M J for solar-type stars. One main challenge is to reach differential
photometric accuracies of around 1 mmag over a wide field of view, in which
airmass, transparency, differential refraction and seeing all vary significantly. The
situation improves above the atmosphere, and a number of space experiments are
planned to reach the 0.01% limits required for the detection of Earth-mass planets.
HST can place much better limits on transit photometry than is possible from the
ground, as exemplified by HD 209458 (see Sections 2.1.2 and 2.2.1). Simulations
have been made by the COROT teams in order to estimate the transit detection
threshold due to stellar activity. In the case of a very active star, the detection of
an Earth (80 ppm) is not possible. In the case of a quiet star (like the Sun), it is
possible if several transits are summed. In the case of COROT, 1.6 M ⊕ is detected
after 10–30 transits. Another complication is again false-positives, where statistical
effects, stellar activity, and background binaries can all mimic transit events, and
which call for independent confirmation of detections in general.
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