Maximal red shifts of neutron stars

It has been recently established that there exists a maximal red shiftz _max for a homogeneous star of given massM. The relationshipz _max(M) is obtained for neutron stars in the mass range 0.71M/M 12.06.     

Critical parameters for supermassive objects in general relativity

Evolutionary tracks up to the point of dynamical instability are obtained for isentropic objects with rest masses ranging from 10² M to 10⁷ M . Accurate values for the red shift, specific entropy, luminosity and effective temperature at the onset of collapse are given.     

Dynamic phases in the evolution of a 1.25M ⊙ C−O star

The evolution of a 1.25M _⊙ carbon and oxygen (equal fractions by mass) homogeneous star is followed by means of a computer code capable of dealing with dynamic evolutionary phases. After carbon ignition at the center, followed by successive shell flashes and the formation of aT-inversion, convection begins at the surface and the model evolves through a very short but strong dynamic phase (viz. a pulsation) after which it settles down to a white dwarf configuration.     

Isentropic stars in general relativity

In an investigation of the evolution of homogeneous, isentropic, stars through stages of diminishing entropy, Rakavy and Shaviv (1968) have recently found that stars of mass less thanM _c (Chandrasekhar's limiting mass for white dwarfs) evolve into white dwarfs, while stars of mass greater thanM _c approach a (singular) state of minimum entropy. An elementary explanation of these results is given and qualitative effects of general relativity are discussed. It is found that stars which are lighter than the Oppenheimer and Volkoff (1939) limit become white dwarfs, while heavier stars must become dynamically unstable at a finite stage in their evolution.     

Maximal masses of Fe56 and Ni56 stars

Upper bounds are derived for the masses of pure Ni⁵⁶ and Fe⁵⁶ stars, which are dynamically stable with respect to photodisintegration. Possible effects on stellar evolution are discussed.     

Relativistic isentropic stellar models

Relativistic, isentropic, homogeneous models are constructed by a method that automatically detects instabilities, and evolutionary tracks of central conditions are shown on a (T, ) diagram. Models heavier than 20M become unstable because of pair creation. Iron photodisintegration causes instability in the mass range between 1.5M and 20M . General relativistic effects bring about the onset of instability in models of 1.2–1.5M when the central density is about 10¹⁰ g/cm³. Lighter models become white dwarfs. It is pointed out that general relativistic instability will prevent the formation of neutron stars through hydrostatic evolution and may be relevant in setting off low-mass supernovae.     

Nuclei of planetary nebulae and white dwarf envelopes

Upper bounds to the masses of He envelopes of white dwarfs are derived by examining the maximum He envelope that a nucleus of a planetary nebulae can have consistent with observation.     

The variable\dot M scenario for nova outbursts

An evolutionary scenario for classical novae is proposed, which is intended to solve the discrepancies that exist between theory and observations:the space densities of classical novae deduced from surveys in the solar neighbourhood are lower by about two orders of magnitude than those predicted theoretically, and the mass transfer rates in nova binaries, as estimated from observed luminosities in quiescence, are higher than those allowed by the thermonuclear runaway model for nova outbursts. These discrepancies disappear if mass transfer (at a high rate) takes place for only a few hundred years before and a few hundred years after an eruption, but declines afterwards and remains off for most of the time between outbursts. We show that such a behavior is to be expected if one takes into account the variation of binary separation, due to mass ejection on the one hand and angular momentum losses on the other hand.One of the aspects of this scenario, on which we report in more detail, is the possibility of enhanced Roche-lobe overflow of the secondary, due to its expansion that results from irradiation by the high nova luminosity. We followed the evolution of a 0.5 M main sequence star illuminated by a changing flux, typical of a classical nova. The numerical results indicate that, in spite of the slight binary separation that may occur after eruption, mass loss from the irradiated and thus bloated secondary should continue for a few hundred years. Other aspects of the variable scenario are briefly summarized.Paper presented at the IAU Colloquium No. 93 on Cataclysmic Variables. Recent Multi-Frequency Observations and Theoretical Developments, held at Dr. Remeis-Sternwarte Bamberg, F.R.G., 16–19 June, 1986.     

A new, efficient stellar evolution code for calculating complete evolutionary tracks

We present a new stellar evolution code and a set of results, demonstrating its capability at calculating full evolutionary tracks for a wide range of masses and metallicities. The code is fast and efficient, and is capable of following through all evolutionary phases, without interruption or human intervention. It is meant to be used also in the context of modelling the evolution of dense stellar systems, for performing live calculations for both normal star models and merger products.
The code is based on a fully implicit, adaptive-grid numerical scheme that solves simultaneously for structure, mesh and chemical composition. Full details are given for the treatment of convection, equation of state, opacity, nuclear reactions and mass loss.
Results of evolutionary calculations are shown for a solar model that matches the characteristics of the present sun to an accuracy of better than 1 per cent; a  1 M_⊙  model for a wide range of metallicities; a series of models of stellar Populations I and II, for the mass range 0.25 to  64 M_⊙  , followed from pre-main-sequence to a cool white dwarf or core collapse. An initial–final mass relationship is derived and compared with previous studies. Finally, we briefly address the evolution of non-canonical configurations, merger products of low-mass main-sequence parents.