Planetary detection limits taking into account stellar noise
I. Observational strategies to reduce stellar oscillation and granulation effects

X. Dumusque, S. Udry, C. Lovis, N. C. Santos, M. J. P. F. G. Monteiro

Abstract
Context. Stellar noise produced by oscillations, granulation phenomena (granulation, mesogranulation and supergranulation) and activity affects radial velocity measurements. The signature of this noise in radial velocity is small, around the meter-per-second, but already too much for the detection of Earth mass planets in habitable zones.
Aims. In this paper, we address the important role played by observational strategies in averaging out the radial velocity signature of stellar noise. We also derive the planetary mass detection limits expected in presence of stellar noise.
Methods. We start with HARPS asteroseismology measurements for 4 stars (β Hyi, α Cen A, μ Ara and τ Ceti) available in the ESO archive plus very precise measurements of α Cen B. This sample covers different spectral types, from G2 to K1 and different evolutionary stage, from subgiant to dwarf stars. Since the span of our data ranges between 5 to 8 days, only stellar noise sources with a time scale smaller than this time span will be extracted from these observations. Therefore, we will have access to oscillation modes and granulation phenomena, without important contribution of activity noise which is present at larger time scales. For those 5 stars, we generate synthetic radial velocity measurements after fitting corresponding models of stellar noise in Fourier space. These measurements allows us to study the radial velocity variation due to stellar noise for different observational strategies as well as the corresponding planetary mass detection limits.
Results. Applying 3 measurements per night of 10 minutes exposure each, 2 hours apart, seems to average out most efficiently the stellar noise considered. For quiet K1V stars as α Cen B, such a strategy allows us to detect planets of ~ 3 times the mass of Earth with an orbital period of 200 days, corresponding to the habitable zone of the star. Our simulations moreover suggest that planets smaller than typically 5M_⊕ can be detected with HARPS over a wide range of separations around most non-active solar type dwarfs. Since activity is not yet included in our simulation, these detection limits correspond to a case, which exist, where the host star has few magnetic features. In this case stellar noise is dominated by oscillation modes and granulation phenomena. For our star sample, a trend between spectral type and surface gravity and the level of radial velocity variation is also emphasized by our simulations.

Keywords
planetary systems - stars: oscilation - techniques: radial velocities

Notes
Based on observations collected at the La Silla Parana Observatory, ESO (Chile), with the HARPS spectrograph at the 3.6-m telescope.
See The Extrasolar Planets Encyclopaedia, http://exoplanet.eu

Astronomy and Astrophysics
Volume 525, Page A140_1
January 2011

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