pastis: Bayesian extrasolar planet validation – II. Constraining exoplanet blend scenarios using spectroscopic diagnoses

Abstract

The statistical validation of transiting exoplanets proved to be an efficient technique to secure the nature of small exoplanet signals which cannot be established by purely spectroscopic means. However, the spectroscopic diagnoses are providing us with useful constraints on the presence of blended stellar contaminants. In this paper, we present how a contaminating star affects the measurements of the various spectroscopic diagnoses as function of the parameters of the target and contaminating stars using the model implemented into the PASTIS planet-validation software. We find particular cases for which a blend might produce a large radial velocity signal but no bisector variation. It might also produce a bisector variation anticorrelated with the radial velocity one, as in the case of stellar spots. In those cases, the full width half maximum variation provides complementary constraints. These results can be used to constrain blend scenarios for transiting planet candidates or radial velocity planets. We review all the spectroscopic diagnoses reported in the literature so far, especially the ones to monitor the line asymmetry. We estimate their uncertainty and compare their sensitivity to blends. Based on that, we recommend the use of BiGauss which is the most sensitive diagnosis to monitor line-profile asymmetry. In this paper, we also investigate the sensitivity of the radial velocities to constrain blend scenarios and develop a formalism to estimate the level of dilution of a blended signal. Finally, we apply our blend model to re-analyse the spectroscopic diagnoses of HD16702, an unresolved face-on binary which exhibits bisector variations.

SOPHIE velocimetry ofKeplertransit candidates

“…The errors on the RV are estimated using the method explained in Bouchy et al (2001) and in Appendix A of Boisse et al (2010). For the bisector and FWHM, we used the photon noise factors listed in Santerne et al (2015). These spectroscopic diagnostics are used to reveal the presence of contaminating stars, therefore likely false positives that might be the source of the transit event (Santos et al 2002;Torres et al 2005).…”

“…Moreover, if the eclipsing system is physically bound with the target star, they are most likely blended in both photometry and spectroscopy. Using the variation in the line profile (the bisector and the FWHM, see Santerne et al 2015), we identified some triple systems with relatively faint companions compared with the target star. However, if the eclipse host contributes to less than about 5% of the total flux of the system (magnitude difference more than 3, or mass ratio lower than ∼0.5), we would not be able to detect the second set of lines in the cross-correlation functions or its impact on the target line-profile shape.…”

An extreme planetary system around HD 219828

“…These values are useful to diagnose radial-velocity variations that are intrinsic to the star, for instance caused by stellar activity on its different timescales (e.g. Queloz et al 2000;Santos et al 2010;Figueira et al 2010;Dumusque et al 2012;Santerne et al 2015). Values for the chromospheric activity index log R HK were also derived (see Sect.…”

“…We therefore have no evidence that stellar activity may be contributing to produce the observed signals or any other additional signal in the data, at any timescale. The lack of any long-term significant variability on the BIS and FWHM also excludes the possibility that HD 219828 c is a solar-like stellar companion in a highly inclined orbit (Santerne et al 2015), since such a companion is expected to leave a trace on these parameters.…”

The SOPHIE search for northern extrasolar planets

“…This value also agrees with the identification of a second component in the spectra by Kolbl et al (2015) with an estimated T eff of 3500 ± 250 K and contributing to 1.06% of the total flux. More recently, Santerne et al (2015) performed blend models with PASTIS (Díaz et al 2014) and derived a companion mass of 0.35 ± 0.03 M .…”