Bibcode
Sanz-Forcada, J.; Micela, G.; Ribas, I.; Pollock, A. M. T.; Eiroa, C.; Velasco, A.; Solano, E.; García-Álvarez, D.
Bibliographical reference
Astronomy and Astrophysics, Volume 532, id.A6
Advertised on:
8
2011
Journal
Citations
344
Refereed citations
311
Description
Context. The current distribution of planet mass vs. incident stellar
X-ray flux supports the idea that photoevaporation of the atmosphere may
take place in close-in planets. Integrated effects have to be accounted
for. A proper calculation of the mass loss rate through photoevaporation
requires the estimation of the total irradiation from the whole XUV
(X-rays and extreme ultraviolet, EUV) range. Aims: The purpose of
this paper is to extend the analysis of the photoevaporation in
planetary atmospheres from the accessible X-rays to the mostly
unobserved EUV range by using the coronal models of stars to calculate
the EUV contribution to the stellar spectra. The mass evolution of
planets can be traced assuming that thermal losses dominate the mass
loss of their atmospheres. Methods: We determine coronal models
for 82 stars with exoplanets that have X-ray observations available.
Then a synthetic spectrum is produced for the whole XUV range (~1-912
Å). The determination of the EUV stellar flux, calibrated with
real EUV data, allows us to calculate the accumulated effects of the XUV
irradiation on the planet atmosphere with time, as well as the mass
evolution for planets with known density. Results: We calibrate
for the first time a relation of the EUV luminosity with stellar age
valid for late-type stars. In a sample of 109 exoplanets, few planets
with masses larger than ~1.5 MJ receive high XUV flux,
suggesting that intense photoevaporation takes place in a short period
of time, as previously found in X-rays. The scenario is also consistent
with the observed distribution of planet masses with density. The
accumulated effects of photoevaporation over time indicate that HD
209458b may have lost 0.2 MJ since an age of 20 Myr.
Conclusions: Coronal radiation produces rapid photoevaporation of the
atmospheres of planets close to young late-type stars. More complex
models are needed to explain the observations fully. Spectral energy
distributions in the XUV range are made available for stars in the
sample through the Virtual Observatory for the use in future planet
atmospheric models.
Appendices and Tables 3 and 4 are available in electronic form at http://www.aanda.org