Characterisation of extrasolar planetary transit candidates

Abstract

Abstract. The detection of transits is an efficient technique to uncover faint companions around stars. The full characterisation of the companions (M-type stars, brown dwarfs or exoplanets) requires high-resolution spectroscopy to measure properly masses and radii. With the advent of massive variability surveys over wide fields, the large number of possible candidates makes such a full characterisation for all of them impractical. We explore here a fast technique to pre-select the most promising candidates using either near-IR photometry or low resolution spectroscopy. We develop a new method based on the well-calibrated surface brightness relation along with the correlation between mass and luminosity for main sequence stars, so that not only can giant stars be excluded but also accurate effective temperatures and radii measured. The main source of uncertainty arises from the unknown dispersion of extinction at a given distance. We apply this technique to our observations of a sample of 34 stars extracted from the 62 low-depth transits identified by OGLE during their survey of some 10 5 stars in the Carina fields of the Galactic disc. We infer that at least 78% of the companions of the stars which are well characterised in this sample are not exoplanets. Stars OGLE-TR-105, OGLE-TR-109 and OGLE-TR-111 are the likeliest to host exoplanets and deserve high-resolution follow-up studies. Most recently, OGLE-TR-111 was confirmed as an exoplanet with M planet 0.53 ± 0.11 M Jup , confirming the efficiency of our method in pre-selecting reliable planetary transit candidates.

Characterisation of extrasolar planetary transit candidates

“…In this work, which represents the continuation of the work published by Gallardo et al (2005), we present new optical and near-IR photometry and measure the properties of 27 stars with observed transits (among the 113 just mentioned), constraining astrophysical parameters of both stellar and companion objects. A&A 522, A4 (2010) We use the method developed by Gallardo et al (2005) and focus our analysis on the G, K, and M-type parent stars.…”

“…In this sub sample we included one target previously observed by Gallardo et al (2005) i.e., OGLE-TR-111 (a confirmed planet), and 5 eclipsing binary systems observed and analyzed by Pont et al (2005) i.e., OGLE-TR-72, OGLE-TR-78, OGLE-TR-105, OGLE-TR-106 and OGLE-TR-123, to compare their results with our conclusions (see discussion in Sect. 5).…”

“…Many of these planetary companions have been discovered by various photometric extra-solar planets surveys, such as OGLE (optical gravitational lensing experiment), STARE (STellar Astrophysics & Research on Exoplanets), WASP (wide angle search for planets), and by the COROT space telescope (COnvection, ROtation & planetary Transits, launched at the end of 2006), which claimed out the discovered of the first telluric planet (Léger et al 2009). Despite the success of these projects, a major problem in identifying transiting planets has been that photometry alone does not allow us to unambiguously distinguish between extrasolar planets and other astrophysical low-luminosity objects such as very late M-type stars, brown dwarf, or grazing binaries, because all these objects have similar characteristics (e.g., Dreizler et al 2002Dreizler et al , 2007Gallardo et al 2005;Silva & Cruz 2006). Transit surveys are able to monitor orders of magnitude more stars in a comparatively shorter observing time than radial velocity surveys, hence have the ability to discover many interesting exoplanet candidates, although non-planetary events must be filtered out.…”

“…Near-IR photometry may also be used to identify and remove giant stars and stars with an IR excess and larger companions identify by observing host-star ellipsoidal variability (e.g., Sirko & Paczynski 2003). Gallardo et al (2005) developed a method based on the well-calibrated surface brightness relation along with the correlation between mass and luminosity for main-sequence stars. This allows not only the classification of the most promising candidates, but also the measurement of the effective temperature and radius of the parent star.…”

“…A&A 522, A4 (2010) We use the method developed by Gallardo et al (2005) and focus our analysis on the G, K, and M-type parent stars. We select the most interesting stellar objects to host extrasolar planets from the depth in its transit light curve, and then derive stellar parameters such as effective temperature, stellar radius, and distance.…”