Planets and X-rays: a radiation diet

Source: arXiv

ABSTRACT According to theory, high energy emission from the coronae of cool stars can
severely erode the atmosphere of orbiting planets. To test the long term
effects of the erosion we study a large sample of planet-hosting stars observed
in X-rays. The results reveal that massive planets (Mp sin i > 1.5 Mj) may
survive only if exposed to low accumulated coronal radiation. The planet HD
209458 b might have lost more than 1 Mj already, and other cases, like tau Boo
b, could be losing mass at a rate of 3.4 Earth masses per Gyr. The strongest
erosive effects would take place during the first stages of the stellar life,
when the faster rotation generates more energetic coronal radiation. The
planets with higher density seem to resist better the radiation effects, as
foreseen by models. Current models need to be improved to explain the observed
distribution of planetary masses with the coronal radiation received.

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    ABSTRACT: Aims. We study the relative role of EUV and X-ray radiation in the heating of hydrogen-rich planet atmospheres with different composition and electron content.Methods. An accurate photo-ionization model has been used to follow the primary photo-electron energy deposition throughout the atmosphere.Results. Heating rates and efficiencies have been computed, together with column density cut-offs at which photons of given energies stop their heating production inside the atmosphere. Assuming 100 eV as the energy borderline between the extreme ultraviolet spectral range and X-rays we find that when the absorbing hydrogen column density is higher than $10^{20}$ cm$^{-2}$ only X-rays can heat the gas. Extreme ultraviolet photons heat the upper atmospheric layers.Conclusions. Using emission spectra from a sample of solar-type stars of different ages representative of the Sun's main sequence lifetime, we have derived the corresponding heating rates. We find that the existence of an energetic cross-over in atmospheric heating is present for all stars in the sample.
    Astronomy and Astrophysics 03/2009; · 5.08 Impact Factor
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    ABSTRACT: Aims.We investigate the influence of high-energy stellar radiation at close-in orbits on atmospheric mass loss during the stellar evolution of a G-type star.Methods.High-energy stellar luminosity varies over a wide range for G field stars. The X-ray luminosity distributions from the Pleiades, the Hyades, and the field are used to derive a scaling law for the evolution of the stellar X-ray luminosity distribution. A modified energy-limited escape approach is taken for calculating atmospheric mass loss for a broad range of planetary parameters.Results.We show that the evolution of close-in exoplanets strongly depends on the detailed X-ray luminosity history of their host stars, which varies over several orders-of-magnitude for G stars. Stars located in the high-energy tail of the luminosity distribution can evaporate most of its planets within 0.5 AU, while a significant fraction of planets can survive if exposed to a moderate X-ray luminosity. We show the change on an initial planetary mass distribution caused by atmospheric escape.
    Astronomy and Astrophysics 01/2008; · 5.08 Impact Factor
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    ABSTRACT: We report on the results of the Sun in Time multi-wavelength program (X-rays to the UV) of solar analogs with ages covering ~0.1-7 Gyr. The chief science goals are to study the solar magnetic dynamo and to determine the radiative and magnetic properties of the Sun during its evolution across the main sequence. The present paper focuses on the latter goal, which has the ultimate purpose of providing the spectral irradiance evolution of solar-type stars to be used in the study and modeling of planetary atmospheres. The results from the Sun in Time program suggest that the coronal X-ray-EUV emissions of the young main-sequence Sun were ~100-1000 times stronger than those of the present Sun. Similarly, the transition region and chromospheric FUV-UV emissions of the young Sun are expected to be 20-60 and 10-20 times stronger, respectively, than at present. When considering the integrated high-energy emission from 1 to 1200 A, the resulting relationship indicates that the solar high-energy flux was about 2.5 times the present value 2.5 Gyr ago and about 6 times the present value about 3.5 Gyr ago (when life supposedly arose on Earth). The strong radiation emissions inferred should have had major influences on the thermal structure, photochemistry, and photoionization of planetary atmospheres and also played an important role in the development of primitive life in the Solar System. Some examples of the application of the Sun in Time results on exoplanets and on early Solar System planets are discussed. Comment: 20 pages, 8 figures, accepted for publication in ApJ
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