The SOPHIE search for northern extrasolar planets

Abstract

Context. The mass domain where massive extrasolar planets and brown dwarfs lie is still poorly understood. Indeed, not even a clear dividing line between massive planets and brown dwarfs has been established yet. This is partly because these objects are very scarce in close orbits around solar-type stars, the so-called brown dwarf desert. Owing to this, it has proven difficult to set up a strong observational base with which to compare models and theories of formation and evolution. Aims. We search to increase the current sample of massive sub-stellar objects with precise orbital parameters, and to constrain the true mass of detected sub-stellar candidates. Methods. The initial identification of sub-stellar candidates was made using precise radial velocity measurements obtained with the SOPHIE spectrograph at the 1.93-m telescope of the Haute-Provence Observatory. Subsequent characterisation of these candidates, with the principal aim of identifying stellar companions in low-inclination orbits, was made by means of different spectroscopic diagnostics such as the measurement of the bisector velocity span and the study of the correlation mask effect. With this objective, we also employed astrometric data from the Hipparcos mission, and a novel method of simulating stellar cross-correlation functions. Results. Seven new objects with minimum masses between ∼10 M Jup and ∼90 M Jup are detected. Out of these, two are identified as low-mass stars in low-inclination orbits, and two others have masses below the theoretical deuterium-burning limit, and are therefore planetary candidates. The remaining three are brown dwarf candidates; the current upper limits for their the masses do not allow us to conclude on their nature. Additionally, we have improved the parameters of an already-known brown dwarf (HD 137510b), confirmed by astrometry.

The SOPHIE search for northern extrasolar planets

“…Seeing change at the fiber input introduced radial velocity changes. This so-called seeing effect on SOPHIE has been described by Boisse et al (2010Boisse et al ( , 2011, Díaz et al (2012), and Bouchy et al (2013). Considering that variations in the pupil illumination produce a differential RV effect along spectral orders, the difference between RVs measured in each half of the spectral orders can be used to partially correct the main RV.…”

“…Following the approach described by Díaz et al (2012), we examined the bisector span of the SOPHIE CCF of our BD candidates to check whether significant variation and a correlation with RV measurements can reveal a relatively luminous companion polluting the peak of the primary star. The detection of a correlation indicates that the velocity signal is the resultat least in part -of a deformation of the stellar CCF.…”

“…For each value of q we computed the dispersion of the average of CCF residuals and searched for a significant peak centered at the systemic velocity. We tested our approach on HD 16702, which was first identified by Díaz et al (2012) to have a companion with a minimum mass of 48.7 M Jup . These authors showed that H astrometric analysis and CCF simulations lead to a companion mass of 0.55 ± 0.14 M and 0.40 ± 0.05 M , respectively.…”

“…Most of the brown dwarf companions have been discovered in radial velocity (RV) surveys (e.g., Nidever et al 2002;Sahlmann et al 2011;Díaz et al 2012). In these programs, which were designed to find exoplanets, brown dwarfs are easily detected by their strong RV signatures.…”

SOPHIE velocimetry ofKeplertransit candidates

“…The small number of BD companions to solar-type stars prevents reliable statistical studies from being made, and although radial-velocity surveys have detected a few tens of objects with minimum masses in the BD regime (e.g. Sahlmann et al 2011;Díaz et al 2012), the inclination degeneracy does not allow us to be certain that these objects are actually sub-stellar. Since the mass function is expected to rise in the stellar regime, the inclination effect is more severe for BD companions (Ho & Turner 2011).…”