Aims: We investigate the stellar pancake mechanism during which a solar-type star is tidally flattened within its orbital plane passing close to a 10^6 solar masses black hole. We simulate the relativistic orthogonal compression process and follow the associated shock waves formation. Methods: We consider a one-dimensional hydrodynamical stellar model moving in the relativistic gravitational field of a non-rotating black hole. The model is numerically solved using a Godunov-type shock-capturing source-splitting method in order to correctly reproduce the shock waves profiles. Results: Simulations confirm that the space-time curvature can induce several successive orthogonal compressions of the star which give rise to several strong shock waves. The shock waves finally escape from the star and repeatedly heat up the stellar surface to high energy values. Such a shock-heating could interestingly provide a direct observational signature of strongly disruptive star - black hole encounters through the emission of hard X or soft gamma-ray bursts. Timescales and energies of such a process are consistent with some observed events such as GRB 970815.

We interest in the case of a main-sequence star deeply penetrating within the tidal radius of a massive black hole. We focus on the compression phase leading to a so-called pancake configuration of the star at the instant of maximal compression. The aim is to study the tidal compression process paying particular attention to the development of shock waves;to deduce reliable estimates of the thermodynamical quantities involved in the pancake star; and to solve a controversy about whether or not thermonuclear reactions can be triggered in the core of a tidally compressed star. We have set up a one-dimensional hydrodynamical model well-adapted to the geometry of the problem. Based on the high-resolution shock-capturing Godunov-type approach, it allows to study the compression phase undergone by the star in the direction orthogonal to its orbital plane. We show the existence of two regimes depending on whether shock waves develop before or after the instant of maximal core compression. In both cases we confirm high compression and heating factors in the stellar core able to trigger a thermonuclear explosion. Moreover, we show that the shock waves carry outwards a brief but very high peak of temperature from the centre to the surface of the star. We tentatively conclude that the phenomenon could give rise to hard electromagnetic radiation, to be compared to some X-ray flares already observed in some galactic nuclei harbouring massive black holes. Finally, we estimate that the rate of pancake stars should be about $10^{-5}$ per galaxy per year. If generated in hard X- or $γ$-ray band, several events of this kind per year should be detectable within the full observable universe.

The dependence of meiosis reinitiation upon pH and calcium has been investigated in oocytes of a prosobranch mollusk, the limpet Patella vulgata. In this species, 10 to 80% of the population of prophase-blocked oocytes with an intact germinal vesicle (GV) eventually reach the metaphase 1 stage (spontaneous maturation) and they do so even in the absence of external calcium and sodium. Release of the metaphase block was obtained by fertilization or treatment with ionophore A 23187 or excess KCl. Ionophore application is effective in the absence of calcium. These treatments are ineffective when applied to GV prophase-blocked oocytes. In contrast, such oocytes resumed maturation up to metaphase 1 when incubated in alkaline complete or calcium and sodium-free seawater in the presence of millimolar concentrations of weak bases (ammonia, procaine, or nicotine) or after microinjection with alkaline Hepes-KOH buffers. Furthermore, when insemination, ionophore or KCl treatments preceded the application of weak bases, full activation was obtained. These results suggest that activation of P. vulgata oocytes normally proceeds in two steps involving, respectively, a change in intracellular pH followed by a rise in intracellular free calcium.