Adding RV and Photometry data to reduce jitter

Deriving the radial-velocity variations induced by stellar activity from high-precision photometry - Test on HD189733 with simultaneous MOST/SOPHIE data
http://arxiv.org/list/astro-ph.EP/new

Quote :

Stellar activity induces apparent radial velocity (RV) variations in late-type main-sequence stars that may hamper the detection of low-mass planets and the measurement of their mass. We use simultaneous measurements of the active planet host star HD189733 with high-precision optical photometry by the MOST satellite and high-resolution spectra by SOPHIE. We apply on this unique dataset a spot model to predict the activity-induced RV variations and compare them with the observed ones. The model is based on the rotational modulation of the stellar flux. A maximum entropy regularization is applied to find a unique and stable solution for the distribution of the active regions versus stellar longitude. The RV variations are synthesized considering the effects on the line profiles of the brightness perturbations due to dark spots and bright faculae and the reduction of the convective blueshifts in the active regions. The synthesized RV time series shows a remarkably good agreement with the observed one although variations on timescales shorter than 4-5 days cannot be reproduced by our model. Persistent active longitudes are revealed by the spot modelling. They rotate with slightly different periods yielding a minimum relative amplitude of the differential rotation of \Delta \Omega / \Omega = 0.23 \pm 0.10. The method proves capable of reducing the power of the activity-induced RV variations by a factor from 2 to 10 at the rotation frequency and its harmonics up to the third. Thanks to the high-precision space-borne photometry delivered by CoRoT, Kepler, or later PLATO, it is possible to map the longitudinal distribution of active regions in late-type stars and apply the method presented in this paper to reduce remarkably the impact of stellar activity on their RV jitter allowing us to confirm the detection of low-mass planets or refine the measurement of their mass

Generalizing the approach to Kepler transit candidates host stars

http://arxiv.org/abs/1110.1034

Quote :

We present a new, simple method to predict activity-induced radial velocity variations using high-precision time-series photometry. It is based on insights from a simple spot model, has only two free parameters (one of which can be estimated from the light curve) and does not require knowledge of the stellar rotation period. We test the method on simulated data and illustrate its performance by applying it to MOST/SOPHIE observations of the planet host-star HD189733, where it gives almost identical results to much more sophisticated, but highly degenerate models, and synthetic data for the Sun, where we demonstrate that it can reproduce variations well below the m/s level. We also apply it to Quarter 1 data for Kepler transit candidate host stars, where it can be used to estimate RV variations down to the 2-3m/s level, and show that RV amplitudes above that level may be expected for approximately two thirds of the candidates we examined.

From the conclusions:

Quote :

we have shown that the FF0 method can be used to estimate RV amplitudes down to the 2-3 ms1 level
using Kepler data. Approximately two thirds of the 600 planet candidate host stars to which we applied it are expected to display peak-to-peak RV amplitudes over 30-day timescales that are above that level. This confirms that obtaining mass measurements for many of the Kepler terrestrial and/or longer-period planet candidates will be challenging. However, it also opens up a range of possibilities for identifying the most promising candidates to follow up, and perhaps disentangling activity- and planet-induced RV signals for those objects. [...]
The method can thus be applied to large samples of light curves such as those produced by the CoRoT and Kepler missions, to assess the RV jitter properties of a large sample of stars, and their implications for the detection of habitable planets by RV.