Age consistency between exoplanet hosts and field stars

Abstract

Context. Transiting planets around stars are discovered mostly through photometric surveys. Unlike radial velocity surveys, photometric surveys do not tend to target slow rotators, inactive or metal-rich stars. Nevertheless, we suspect that observational biases could also impact transiting-planet hosts. Aims. This paper aims to evaluate how selection effects reflect on the evolutionary stage of both a limited sample of transiting-planet host stars (TPH) and a wider sample of planet-hosting stars detected through radial velocity analysis. Then, thanks to uniform derivation of stellar ages, a homogeneous comparison between exoplanet hosts and field star age distributions is developed. Methods. Stellar parameters have been computed through our custom-developed isochrone placement algorithm, according to Padova evolutionary models. The notable aspects of our algorithm include the treatment of element diffusion, activity checks in terms of log R HK and v sin i, and the evaluation of the stellar evolutionary speed in the Hertzsprung-Russel diagram in order to better constrain age. Working with TPH, the observational stellar mean density ρ allows us to compute stellar luminosity even if the distance is not available, by combining ρ with the spectroscopic log g. Results. The median value of the TPH ages is ∼5 Gyr. Even if this sample is not very large, however the result is very similar to what we found for the sample of spectroscopic hosts, whose modal and median values are [3, 3.5) Gyr and ∼4.8 Gyr, respectively. Thus, these stellar samples suffer almost the same selection effects. An analysis of MS stars of the solar neighbourhood belonging to the same spectral types bring to an age distribution similar to the previous ones and centered around solar age value. Therefore, the age of our Sun is consistent with the age distribution of solar neighbourhood stars with spectral types from late F to early K, regardless of whether they harbour planets or not. We considered the possibility that our selected samples are older than the average disc population.

Strong H i Lyman-α variations from an 11 Gyr-old host star: a planetary origin?

“…A K dwarf like Kepler-444 A can nonetheless be more active than the Sun, and we used the study of the transiting super-Earth HD 97658 b made by Bourrier et al (2016a) to estimate the variability of its K1-type host star at Ly-α. HD 97658 could be as old as Kepler-444 (9.7±2.8 Gyr;Bonfanti et al 2016), and was observed at three epochs in the Ly-α line. The peaks of the line revealed systematic variations, attributed to an active upper chromosphere/corona, which reach a maximum of ∼15% from one HST orbit to another.…”

MCMCI: A code to fully characterise an exoplanetary system

“…In addition, Denissenkov (2010) shows that stellar rotational speeds converge towards similar values after a few billion years, regardless of their initial spin rate; consistently Meibom et al (2015) confirm a well-defined periodage relation up to ∼ 2.5 Gyr. Taking all these considerations into account, Bonfanti et al (2016) explain how we use the gyrochronological relation by Barnes (2010) and the log R HK -age relation by Mamajek & Hillenbrand (2008) to compute conservative age lower limits, which they denote by τ v and τ HK , respectively. Just in assessing age lower limits, the code already takes care of the above-mentioned caveats such that the user is not obliged to be aware of them, and we attain the goal of possible discarding of unlikely pre-MS isochrones before interpolation.…”

“…Just in assessing age lower limits, the code already takes care of the above-mentioned caveats such that the user is not obliged to be aware of them, and we attain the goal of possible discarding of unlikely pre-MS isochrones before interpolation. Bonfanti et al (2016) also note that the parameter degeneracies on the HRD under the turn-off (TO) can be broken by using ρ . The mean stellar density is higher in the MS phase than in the pre-MS, thus it enables us to distinguish MS stars from pre-MS stars and to set a conservative age lower limit called τ ρ .…”

Asteroseismic potential of CHEOPS

“…To achieve the second goal, we considered the CHEOPS sample specified in Table 6. The estimate of stellar output parameters has been done thanks to the Isochrone Placement algorithm described in Bonfanti et al (2015Bonfanti et al ( , 2016. Interpolation in theoretical grids of tracks and isochrones have been made considering PARSEC 2 evolutionary models, version 1.2S (see Bressan et al 2012;Chen et al 2014;and references therein).…”