arXiv subjects

F. Selsis

Publications and source records attributed to F. Selsis.

42 records · Page 3Linked to original sources

arXiv2006

Roche lobe effects on the atmospheric loss of "Hot Jupiters"

Observational evidence of a hydrodynamically evaporating upper atmosphere of HD209458b (Vidal-Madjar et al. 2003; 2004) and recent theoretical studies on evaporation scenarios of ``Hot Jupiters'' in orbits around solar-like stars with the age of the Sun indicate that the upper atmospheres of short-periodic exoplanets experience hydrodynamic blow-off conditions resulting in loss rates of the order of about 10^10 - 10^12 g s^-1 (Lammer et al. 2003; Yelle 2004; Baraffe et al. 2004; Lecavlier des Etangs et al. 2004; Jaritz et al. 2005, Tian et al. 2005; Penz et al. 2007). By studying the effect of the Roche lobe on the atmospheric loss from short-periodic gas giants we found, that the effect of the Roche lobe can enhance the hydrodynamic evaporation from HD209458b by about 2 and from OGLE-TR-56b by about 2.5 times. For similar exoplanets which are closer to their host star than OGLE-TR-56b, the enhancement of the mass loss can be even larger. Moreover, we show that the effect of the Roche lobe raises the possibility that ``Hot Jupiters'' can reach blow-off conditions at temperatures which are less than expected (< 10000 K) due to the stellar X-ray and EUV (XUV) heating.

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arXiv2006

Gaseous Planets, Protostars And Young Brown Dwarfs : Birth And Fate

We review recent theoretical progress aimed at understanding the formation and the early stages of evolution of giant planets, low-mass stars and brown dwarfs. Calculations coupling giant planet formation, within a modern version of the core accretion model, and subsequent evolution yield consistent determinations of the planet structure and evolution. Because of the uncertainties in the initial conditions, however, it is not possible to say whether young planets are faint or bright compared with low-mass young brown dwarfs. We review the effects of irradiation and evaporation on the evolution of short period planets and argue that substantial mass loss may have occurred for these objects. Concerning star formation, geometrical effects in protostar core collapse are examined by comparing 1D and 3D calculations. Spherical collapse is shown to overestimate the core inner density and temperature and thus to yield incorrect initial conditions for PMS or young brown dwarf evolution. Accretion is also shown to occur over a very limited fraction of the protostar surface. Accretion affects the evolution of young brown dwarfs and yields more compact structures for a given mass and age, thus fainter luminosities. This can lead to severe misinterpretations of the mass and/or age of young accreting objects from their location in the HR diagram. We argue that newborn stars and brown dwarfs should appear rapidly over an extended area in the HR diagram, depending on their accretion history, rather than on a well defined birth line. Finally, we suggest that the distinction between planets and brown dwarfs be based on an observational diagnostic, reflecting the different formation mechanisms between these two distinct populations, rather than on an arbitrary, confusing definition.

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arXiv2005

Radiative Habitable Zones in Martian Polar Environments

The biologically damaging solar ultraviolet (UV) radiation (quantified by the DNA-weighted dose) reaches the Martian surface in extremely high levels. Searching for potentially habitable UV-protected environments on Mars, we considered the polar ice caps that consist of a seasonally varying CO2 ice cover and a permanent H2O ice layer. It was found that, though the CO2 ice is insufficient by itself to screen the UV radiation, at 1 m depth within the perennial H2O ice the DNA-weighted dose is reduced to terrestrial levels. This depth depends strongly on the optical properties ofthe H2O ice layers (for instance snow-lile layes). The Earth-like DNA-weighted dose and Photosynthetically Active Radiation (PAR) requirements were used to define the upper and lower limits of the nortern and southern polar radiative habitable zone (RHZ) for which a temporal and spatial mapping was performed. Based on these studies we conclude that phtosynthetic life might be possible within the ice layers of the polar regions. The thickness varies along each Martian polar spring and summer between 1.5 m and 2.4 m for H2= ice-like layers, and a few centimeters for snow-like covers. These Martian Earth-like radiative habitable environments may be primary targets for future Martian astrobiological missions. Special attention should be paid to planetary protection, since the polar RHZ may also be subject to terrestrial contamination by probes.

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arXiv2005

Hot-Jupiters and hot-Neptunes: a common origin?

We compare evolutionary models for close-in exoplanets coupling irradiation and evaporation due respectively to the thermal and high energy flux of the parent star with observations of recently discovered new transiting planets. The models provide an overall good agreement with observations, although at the very limit of the quoted error bars of OGLE-TR-10, depending on its age. Using the same general theory, we show that the three recently detected hot-Neptune planets (GJ436, $ρ$ Cancri, $μ$ Ara) may originate from more massive gas giants which have undergone significant evaporation. We thus suggest that hot-Neptunes and hot-Jupiters may share the same origin and evolution history. Our scenario provides testable predictions in terms of the mass-radius relationships of these hot-Neptunes.

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arXiv2004

The effect of evaporation on the evolution of close-in giant planets

We include the effect of evaporation in our evolutionary calculations of close-in giant planets, based on a recent model for thermal evaporation taking into account the XUV flux of the parent star (Lammer et al. 2003). Our analysis leads to the existence of a critical mass for a given orbital distance $m_{\rm crit}(a)$ below which the evaporation timescale becomes shorter than the thermal timescale of the planet. For planets with initial masses below $m_{\rm crit}$, evaporation leads to a rapid expansion of the outer layers and of the total planetary radius, speeding up the evaporation process. Consequently, the planet does not survive as long as estimated by a simple application of mass loss rates without following consistently its evolution. We find out that the transit planet HD 209458b might be in such a dramatic phase, although with an extremely small probability. As a consequence, we predict that, after a certain time, only planets above a value $m_{\rm crit}(a)$ should be present at an orbital distance $a$ of a star. For planets with initial masses above $m_{\rm crit}$, evaporation does not affect the evolution of the radius with time.

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arXiv2003

A New Family of Planets ? "Ocean Planets"

A new family of planets is considered which is between rochy terrestrial planets and gaseous giant ones: "Ocean-Planets". We present the possible formation, composition and internal models of these putative planets, including that of their ocean, as well as their possible Exobiology interest. These planets should be detectable by planet detection missions such as Eddington and Kepler, and possibly COROT (lauch scheduled in 2006). They would be ideal targets for spectroscopic missions such as Darwin/TPF.

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