Kepler-91b: a planet at the end of its life

Abstract

Context. The evolution of planetary systems is intimately linked to the evolution of their host stars. Our understanding of the whole planetary evolution process is based on the wide planet diversity observed so far. Only a few tens of planets have been discovered orbiting stars ascending the red giant branch. Although several theories have been proposed, the question of how planets die remains open owing to the small number statistics, making it clear that the sample of planets around post-main sequence stars needs to be enlarged. Aims. In this work we study the giant star Kepler-91 (KOI-2133) in order to determine the nature of a transiting companion. This system was detected by the Kepler Space Telescope, which identified small dims in its light curve with a period of 6.246580 ± 0.000082 days. However, its planetary confirmation is needed due to the large pixel size of the Kepler camera, which can hide other stellar configurations able to mimic planet-like transit events. Methods. We analysed Kepler photometry to 1) re-calculate transit parameters; 2) study the light-curve modulations; and 3) to perform an asteroseismic analysis (accurate stellar parameter determination) by identifying solar-like oscillations on the periodogram. We also used a high-resolution and high signal-to-noise ratio spectrum obtained with the Calar Alto Fiber-fed Échelle spectrograph (CAFE) to measure stellar properties. Additionally, false-positive scenarios were rejected by obtaining high-resolution images with the AstraLux lucky imaging camera on the 2.2 m telescope at the Calar Alto Observatory. −0.22 R away from the stellar atmosphere at the pericentre. We also detected three small dims in the phase-folded light curve. The combination of two of them agrees with the theoretical characteristics expected for secondary eclipse. Conclusions. Kepler-91b could be the previous stage of the planet engulfment, which has recently been detected for BD+48 740. Our estimations show that Kepler-91b will be swallowed by its host star in less than 55 Myr. Among the confirmed planets around giant stars, this is the closest to its host star. At pericentre, the star subtends an angle of 48• , covering around 10% of the sky as seen from the planet. The planetary atmosphere seems to be inflated probably due to the high stellar irradiation.

Radial velocity confirmation of Kepler-91 b

“…In contrast, Angerhausen et al (2014) provided a high-level analysis of the entire light-curve (encompassing all available Kepler observations). They detected several dimmings in the phase-folded light-curve, achieved similar conclusions as Lillo-Box et al (2014b), and suggested that the detected dimmings are poorly explained with a secondary eclipse alone. Sliski & Kipping (2014) estimated (also with transit-fitting) the mean stellar density of the star and compared it with the asteroseismic determination.…”

“…The modelling of these modulations can provide the mass of the bound object, which makes it an alternative method with which to confirm its planetary nature. Lillo-Box et al (2014b) used this photometric technique (together with a careful characterisation of the host star) to establish the planetary nature of the object transiting the star Kepler-91 (KOI-2133 or KIC 8219268). It was detected by the Kepler mission (Borucki et al 2011) and announced as a planet candidate in the second release of the mission on February 2012 (Batalha et al 2013).…”

“…It was detected by the Kepler mission (Borucki et al 2011) and announced as a planet candidate in the second release of the mission on February 2012 (Batalha et al 2013). The exhaustive analysis of the light-curve modulations, transit dim, and asteroseismic signals performed by Lillo-Box et al (2014b) led us to classify this planet as an inflated hot Jupiter (M p = 0.88 +0.17 −0.33 and R p = 1.384 +0.011 −0.054 ) that orbits very close (r = 2.32 +0.07 −0.22 R at periastron passage) to a K3 III giant star (M = 1.31 ± 0.10 M , R = 6.30 ± 0.16 R ).…”

Determining stellar parameters of asteroseismic targets: going beyond the use of scaling relations

“…In this respect a way forward would be to determine the star's mean density by using the full set of observed acoustic modes, not just their average frequency spacing. This approach was carried out in at least two RGB stars (Huber et al 2013;Lillo-Box et al 2014), and led to determination of the stellar mean density which is ∼ 5 − 6 per cent higher than derived from assuming scaling relations, and with a much improved precision of ∼ 1.4 per cent. Furthermore, the impact of surface effects on the inferred mean density is mitigated when determining the mean density using individual mode frequencies rather than using the average large separation (e.g., see Chaplin & Miglio 2013).…”

Asteroseismology of red giants: From analysing light curves to estimating ages

“…This approach was carried out in at least two RGB stars (Huber et al 2013;Lillo-Box et al 2014), and led to determination of the stellar mean density which is ∼ 5-6 % higher than derived from assuming scaling relations, and with a much improved precision of ∼ 1.4 %.…”