An investigation into the radial velocity variations of CoRoT-7

Abstract

Context. CoRoT-7b, the first transiting "superearth" exoplanet, has a radius of 1.7 R ⊕ and a mass of 4.8 M ⊕ . The HARPS radial velocity (RV) measurements used for deriving this mass also detected an additional companion with a period of 3.7 days and a mass of 8.4 M ⊕ . The mass of CoRoT-7b is a crucial parameter for planet structure models, but is difficult to determine because CoRoT-7 is a modestly active star and there is at least one additional companion. Aims. The aims of this paper are to assess the statistical significance of the RV variations of CoRoT-7b and CoRoT-7c, to obtain a better measurement of the planet mass for CoRoT-7b, and to search for additional companions in the RV data. Methods. A Fourier analysis is performed on the HARPS spectral data of CoRoT-7. These data include RV measurements, spectral line bisectors, the full width at half maximum of the cross-correlation function, and Ca II emission. The latter 3 quantities vary due to stellar activity and were used to assess the nature of the observed RV variations. An analysis of a sub-set of the RV measurements where multiple observations were made per night was also used to estimate the RV amplitude from CoRoT-7b that was less sensitive to activity variations. Results. Our analysis indicates that the 0.85-d and 3.7-d RV signals of CoRoT-7b and CoRoT-7c are present in the spectral data with a high degree of statistical significance. We also find evidence for another significant RV signal at 9 days. An analysis of the activity indicator data reveals that this 9-d signal most likely does not arise from activity, but possibly from an additional companion. If due to a planetary companion the mass is m = 19.5 M ⊕ , assuming co-planarity with CoRoT-7b. A dynamical study of the three planet system shows that it is stable over several hundred millions of years. Our analysis yields a RV amplitude of 5.04 ± 1.09 m s −1 for CoRoT-7b which corresponds to a planet mass of m = 6.9 ± 1.4 M ⊕ . This increased mass would make the planet CoRoT-7b more Earth-like in its internal structure. Conclusions. CoRoT-7 is confirmed to be a planet system with at least 2 and possibly 3 exoplanets having masses in the range 7−20 M ⊕ . If the third companion can be confirmed then CoRoT-7 may represent a case of an ultra-compact planetary system.

On the mass determination of super-Earths orbiting active stars: the CoRoT-7 system

“…When the increase due to the activity (about 3 m s −1 ) is added to it, one may verify that the part of the unexplained RV increase is still large, but just one third of the increase seen in the RV measurements. An alternative approach suggested by Hatzes et al (2010), which has the advantage of offering no explicit hypotheses on the activity (except its slow variation), has also been improved and combined with a biased Monte Carlo approach. The resulting mass of CoRoT-7b lies close to results obtained using the 3-harmonic filter.…”

“…The poor constraining of the solution for CoRoT-7c deserves some discussion. The alternative technique proposed by Hatzes et al (2010) is good for separating low-and high-frequency contributions. The period of CoRoT-7c is not short enough to be considered as low.…”

“…However, like CoRoT-7b, they may be found around non-quiet stars, so improvement of the techniques of mass and orbit determination used to study such cases is important. This need is behind many investigations and the number of papers dealing with the mass of CoRoT-7b (QBM; Boisse et al 2011;Hatzes et al 2010;Pont et al 2011) is increasing. In addition to this interest, we have to consider that HARPS, which is currently the only instrument able to make measurements of CoRoT-7 with the required precision, is in demand for a great deal of other targets and we cannot foresee when a new series of measurements with the same quality of the existing one will become possible.…”

“…8 and 9. The approach discussed in section 8 follows a sugestion by Hatzes et al (2010) and only uses the observations from nights where multiple observations were done. This is of particular importance because it allows an analysis that is independent of any explicit hypotheses on the behavior of the star activity.…”

Physical state of the deep interior of the CoRoT-7b exoplanet

“…Although the planet's exact mass is still subject to debate (e.g, Pont et al 2011;Hatzes et al 2010), a total mass of about five times that of the Earth seems to be most favored (Bruntt et al 2010;Queloz et al 2009). Combined with the measured radius of (1.58 ± 0.10) M ⊕ (Bruntt et al 2010), this suggests an average compressed density of (7.2 ± 1.8) Mg m −3 for CoRoT-7b, being consistent with a terrestrial bulk composition (Fig.…”