Roche limit

Abstract: In a recent paper, Abramowicz, Czerny, Lasota and Szuszkiewicz (1988) have found a new branch of equilibrium solutions for stationary accretion discs around black holes. These solutions correspond to moderately super-Eddington accretion rates. The existence of the new branch is a consequence of cooling due to general relativistic Roche lobe overflow and horizontal advection of heat. On an accretion rate versus surface density plane the new branch forms, together with the two “standard” branches (corresponding to to the Shakura-Sunyaev accretion discs models) a characteristically S-shaped curve. The relation between this “S-curve” and the variability of accretion discs is under study and seems to allow, at least in principle, for a limit cycle behaviour.

Abstract: The planet in the system HD209458 is the first one for which repeated transits across the stellar disk have been observed1,2. Together with radial velocity measurements3, this has led to a determination of the planet's radius and mass, confirming it to be a gas giant. But despite numerous searches for an atmospheric signature4,5,6, only the dense lower atmosphere of HD209458b has been observed, through the detection of neutral sodium absorption7. Here we report the detection of atomic hydrogen absorption in the stellar Lyman α line during three transits of HD209458b. An absorption of 15 ± 4% (1σ) is observed. Comparison with models shows that this absorption should take place beyond the Roche limit and therefore can be understood in terms of escaping hydrogen atoms.

Abstract: Context. Heavy metals in the atmospheres of white dwarfs are thought in many cases to be accreted from a circumstellar debris disk, which was formed by the tidal disruption of a rocky planetary body within the Roche radius of the star. The abundance analysis of photospheric elements and conclusions about the chemical composition of the accreted matter are a new and promising method of studying the composition of extrasolar planetary systems. However, ground-based searches for metal-polluted white dwarfs that rely primarily on the detection of the Ca ii K line become insensitive at Teff > 15 000 K because this ionization state depopulates. Aims. We present the results of the first unbiased survey for metal pollution among hydrogen-atmosphere (DA type) white dwarfs with cooling ages in the range 20–200 Myr and 17 000 K 23 000 K, in excellent agreement with the absence of infrared excess from dust around these warmer stars. The median, main sequence progenitor of our sample corresponds to an A-type star of ≈2 M� , and we find 13 of 23 white dwarfs descending from