Can Nonlinear Structure Form at the Era of Decoupling?

The effects that large scale fluctuations had on small scale isothermal modes at the epoch of recombination are analysed. We find that: (a) Albeit the fact that primordial fluctuations were at this epoch still well in the linear regime, a significant nonlinear radiation hydrodynamic interaction could have taken place. (b) Short wavelength isothermal fluctuations are unstable. Their growth rate is exponential in the amplitude of the large scale fluctuations and is therefore very sensitive to the initial conditions. (c) The observed CMBR fluctuations are of order the limit above which the effect should be significant. Thus, according to their exact value, the effect may be negligible or lead to structure formation out of isothermal fluctuations within the period of recombination. (d) If the cosmological parameters are within the prescribed regime, the effect should be detectable through induced deviations in the Planck spectrum. (e) The sensitivity of the effect to the initial conditions provides a tool to set limits on various cosmological parameters with emphasis on the type and amplitude of the primordial fluctuation spectrum. (f) Under proper conditions, the effect may be responsible for the formation of sub-globular cluster sized objects at particularly high red shifts. (g) Under certain circumstances, it can also affect horizon sized large scale structure.Comment: To appear in MNRAS, 17 pages, 8 figure

The Fate of a WD Accreting H-Rich Material at High Rates

We study C/O white dwarfs with masses of 1.0 to 1.4 Msun accreting solar-composition material at very high accretion rates. We address the secular changes in the WDs, and in particular, the question whether accretion and the thermonuclear runaways result is net accretion or erosion. The present calculation is unique in that it follows a large number of cycles, thus revealing the secular evolution of the WD system. We find that counter to previous studies, accretion does not give rise to steady state burning. Instead, it produces cyclic thermonuclear runaways of two types. During most of the evolution, many small cycles of hydrogen ignition and burning build a helium layer over the surface of the white dwarf. This He layer gradually thickens and progressively becomes more degenerate. Once a sufficient amount of He has accumulated, several very large helium burning flashes take place and expel the accreted envelope, leaving no net mass accumulation. The results imply that such a system will not undergo an accretion induced collapse, nor will it lead to a SN Type Ia, unless a major new physical process is found.Comment: 8 pages, 7 figures, submitted to MNRA

The Super-Eddington Nature of Super Massive Stars

Supermassive stars (SMS) are massive hydrogen objects, slowly radiating their gravitational binding energy. Such hypothetical primordial objects may have been the seed of the massive black holes (BHs) observed at the centre of galaxies. Under the standard picture, these objects can be approximately described as n=3 polytropes, and they are expected to shine extremely close to their Eddington luminosity. Once however, one considers the porosity induced by instabilities near the Eddington limit, which give rise to super-Eddington states, the standard picture should be modified. We study the structure, evolution and mass loss of these objects. We find the following. First, the evolution of SMSs is hastened due to their increased energy release. They accelerate continuum driven winds. If there is no rotational stabilization, these winds are insufficient to "evaporate" the objects, such that they can collapse to form a supermassive BHs, however, they do prevent SMSs from emitting a copious amount of ionizing radiation. If the SMSs are rotationally stabilized, the winds "evaporate" the objects until a normal sub-Eddington star remains, having a mass of a few 100Msun.Comment: 10 pages, 7 figure