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We estimate the effect of the shear flow in accretion disks on the radiation field. We apply the recent results of Wehrse et al. (2003, A&A, 401, 43) for the expansion line opacity in shear flows. We use an approximation and derive an approximate equation for the radiation field in an accretion disk in the diffusion approximation. We find that in the appropriate limit for the flow in disks: – The effect depends on a product of three factors: the first depends solely atomic physics and hence on the composition of the accreted matter, the second depends on the density while the third depends solely on the shear flow. – The radiation field in accretion disks around neutron stars, black holes and white dwarfs is strongly affected by the expansion line opacity. – The radiation field is very non isotropic leading to significantly higher specific intensities in the vertical direction, a higher temperature gradient and increased susceptibility to convection and other instabilities.

We use the Shaviv and Wehrse (1991) code to model the vertical structure and the emission properties of quiescent dwarf nova discs. We find that in the case of HT Cas the quiescent disc must be optically thin, in contradiction with the requirements of the standard disc instability model. We find a viscosity parameter alpha > 1. Although this is much less than values (~ 100) obtained in isothermal slab models it is not consistent with the accretion disc model assumptions., To be published in Physics Reports, proceedings of the conference Astrophysical Fluids: From Atomic Nuclei to Stars and Galaxies; 11 pages, 7 figures

The predictions of an improved standard solar model are compared with the observations of the four solar neutrino experiments. The improved model includes premain sequence evolution, element diffusion, partial ionization effects, and all the possible nuclear reactions between the main elements. It uses updated values for the initial solar element abundances, the solar age, the solar luminosity, the nuclear reaction rates and the radiative opacities. Neither nuclear equilibrium, nor complete ionization are assumed. The calculated $^8$B solar neutrino flux is consistent, within the theoretical and experimental uncertainties, with the solar neutrino flux measured by Kamiokande. The results from the $^{37}$Cl and $^{71}$Ga radiochemical experiments seem to suggest that the terrestrial $^7$Be solar neutrino flux is much smaller than that predicted. However, the present terrestrial ``defecit'' of $^7$Be solar neutrinos may be due to the use of inaccurate theoretical neutrino absorption cross sections near threshold for extracting solar neutrino fluxes from production rates. Conclusive evidence for a real deficit of $^7$Be solar neutrinos will require experiments such as BOREXINO or HELLAZ. A real defecit of $^7$Be solar neutrinos can be due to either astrophysical reasons or neutrino properties beyond the standard electroweak model. Only future neutrino experiments, such as SNO, Superkamiokande, BOREXINO and HELLAZ, will be able to provide conclusive evidence that the solar neutrino problem is a consequence of neutrino properties beyond the standard electroweak model. Earlier indications may be provided by long baseline neutrino oscillation experiments., Comment: Invited talk to be published in the Proceedings of ``Theoretical Aspects of Underground Physics'', Toledo, Spain, September 17-21, 1995 (Nucl. Phys. B, ed. A. Morales, in press)

High level of stellar UV radiation on a planet’s surface may prevent planet habitation by Earth-like life. Hence, we wish to estimate the UV Index as a function of the stellar parameters for Earth-like planets within the HZ of main-sequence stars. The stellar radiation that impinges the surface is calculated by a one-dimensional radiative transfer model which couples the photochemical reactions in the atmosphere. The calculations show that the oxygen and ozone molecules provide a sufficient protection from the stellar radiation, even for hot host stars, as the UV Index can reach a maximal value of 21 units (about twice higher than on Earth). Furthermore, the UV Index is highest on planets which orbit stars with an effective temperature of about 9,000 ;K and is lower for hotter and colder stars.

A thorough analysis of the low energy nuclear data pertinent to the nuclear reactions in the Sun and the solar neutrino problem leads to significant modifications in the most. important cross sections and to a new estimate for the solar neutrino flux. A review of the numerical method used to calculate the theoretical result indicates the degree of accuracy needed in such calculations.

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., 8 pages, 7 figures, submitted to MNRAS

The nuclear reactions responsible for the synthesis of the elements are sensitive to the properties of the nuclei. The resulting abundances of the elements are a direct consequence of nuclear properties. The question as to whether a certain element can be synthesized in a star depends on the temperature of the star, but the relative abundance of its isotopes depends on the nuclear properties. So let us investigate those nuclear properties which control the synthesis.

Supernova research is presently in the limelight, and new types of supernova are being discovered, leading to a zoo of types, each with their own particular behavior. As discussed by Shaviv, the theory of supernovas (SN) has not yet produced any ‘successful explosion’, so we describe here only what the extensive work by various modelers seems to suggest. Masses given relate to the moment just prior to the SN explosion. The initial star on the main sequence would certainly have been more massive and would have lost significant amounts of matter during its evolution toward the critical moment at which it exploded.

All stars observed today show various amounts of heavy elements. But we know that heavy elements could not have been formed in the Big Bang. So what about the heavy elements seen in the most heavy-element-poor stars? Where were they formed? While the amount of heavy elements in the most metal-poor stars exceeds by many orders of magnitude what can be synthesized in the standard Big Bang, no star completely lacking in heavy elements has ever been observed.

A classical nova may accrete a non-homogeneousenvelope, which can result in the ignition of a local thermonuclear runaway on the surface of the white dwarf. We studied the propagation of thermal flows along the meridian in the hydrogen rich envelope, to find the conditions under which a thermonuclear runaway is not spherically symmetric. We included mass accretion and tested the effect of temperature inhomogeneities in the secular evolution of the envelope, supposing perturbations with different wavelengths, ranging from very small to comparable with the radius of the white dwarf. The calculations were stopped at the onset of a thermonuclear runaway, when the hydrodynamic expansion starts. We found the ranges of accretion rates and masses of white dwarfs for which the runaway ignites locally. The propagation time of the runaway along the meridian may be as long as days and weeks. “Local” thermonuclear runaways can explain the asymmetries and inhomogeneities of the nova shells and account for the slow rise time to maximum (about one week) of many novae.

The evolutionary track of low-mass red giant stars (0.7–0.9M⊙) is computed with the aim to demonstrate the conditions under which low-mass white dwarfs (WDs) can form through the evolution of single stars. Also, the influence of the mixing length to the scale height ratio on the radius of the star is calculated and the coupling between the mixing-length and the mass-loss rate parameters is investigated. Our conclusions are that the uncertainties in mass-loss and mixing-length to scale-height ratio leave enough parameter space to allow the formation of low-mass WD via single star evolution. We also conclude that the gap between proto-WD stars without any nebula and stars with well-defined nebulae is bridged by stars which have a dilute gas cloud around them which cannot be observed as a nebula.

When Bunsen and Kirchoff observed the stars spectroscopically, they opened up a new field of observation, and stellar spectroscopy soon became a routine. What the astronomers discovered was a bewildering variety of stellar spectra, to the point of confusion and disarray. There was an urgent need for some order, for once order is established, theories about the evolution of stars, as well as their energy source, can be conceived and checked against observation. It was stellar classification that revealed to astrophysicists where the elements are synthesized and how stars evolve. However, it was not an easy ride.

The photospheres of type II supernovae during the coasting phase ( ~ 1 week to 1 month after outburst) are characterized by moderate temperatures (Teff ~ 7...12 × 10^3K) , large radii (R ~ 10^13...10^15cm) , very low densities (p ~ 10^-13... 10^-16 gcm-3) and high expansions velocities (up to ~ 30000 km-1 in the outer layers). These properties lead to large geometrical extensions (i.e. the photons can travel a large fraction of the radius without being absorbed) and to electron scattering as the main source of opacity. Therefore these photospheres are very different from those of stars in hydrostatic equilibrium. In this review we first describe the input physics for models and essential numerical aspects of the model construction. In particular we stress the central role of an adequate treatment of the radiative transfer equation which should simultaneously take into account the sphericity, a high ratio of scattering to absorption, and relativistic effects. Subsequently we discuss details of the emergent spectra and their relevance for parameter determinations. Finally, model spectra are compared with observations of the bright supernovae SN 1979C and SN 1980 K, which seem to be more average examples of type II supernovae than SN 1987A, and possibilities for using these objects for extragalactic distance determinations are investigated.

Continuum and line blanketing models for the photospheres of novae in the early stages of their outbursts are presented. The expanding envelopes are characterized by a very slow increase of density with decreasing radius which leads to very large geometrical extensions and large temperature differences between the inner and outer parts. The spectra show a large IR excess and a small Balmer jump which may be either in absorption or in emission. For the parameters considered (T eff = 104,1.5 x 104, 2 x 104K, R = 1011 cm, solar composition), most lines are in absorption. The effects of both modifications in the temperature structure (e.g. by heating from shock fronts) and changes in the abundances of the heavy elements on the emergent spectra are briefly discussed.

Radiation from accretion discs in cataclysmic variable stars (CVs) provides fundamental information about the properties of these close binary systems and about the physics of accretion in general. The detailed diagnostics of accretion disc structure can be achieved by including in its description all the relevant heating and cooling physical mechanism, in particular the convective energy transport that, although dominant at temperatures less than about 10 000 K, is usually not taken into account when calculating spectra of accretion discs. We constructed a radiative transfer code coupled with a code determining the disc's hydrostatic vertical structure. We have obtained for the first time model spectra of cold, convective accretion discs. As expected, these spectra are mostly flat in the optical wavelengths with no contribution from the UV, which in quiescence must be emitted by the white dwarf. The disc structures obtained with our radiative-transfer code compare well with the solutions of equations used to describe the dwarf-nova outburst cycle according to the thermal-viscous disc instability model thus allowing the two to be combined. Our code allows calculating the spectral evolution of dwarf nova stars through their whole outburst cycle, providing a new tool for testing models of accretion discs in cataclysmic variables. We show that convection plays an important role in determining the vertical disc structure and substantially affects emitted spectra when, as often the case, it is effective at optical depths tau ~ 1. The emergent spectrum is independent of the parameters of the convection model.(Abstract shortened), Comment: Astronomy & Astrophysics in press. Deeply revised version. Corrected OP data taken into account. New figures (except for Figs. 1 & 2)

We present spectra of accretion discs around white-dwarfs calculated with an improved and updated version of the Shaviv & Wehrse (1991) model. The new version includes line opacities and convective energy transport and can be used to calculate spectra of hot discs in bright systems (nova--like variables or dwarf novae in outburst) as well as spectra of cold accretion discs in quiescent dwarf novae., 10 pages. Talk presented at "Jean-Pierre Lasota, X-ray binaries, accretion disks and compact stars" (October 2007); Abramowicz, M. Ed., New Astron. Rev., in press

We have calculated a large grid of hydrogen-rich supernova photospheres, in which radii, effective temperatures, density profiles, and expansion velocities have been varied. Spherical geometry, radiative equilibrium and LTE level populations are assumed. In the quasi-exact radiative transfer, the dilution of the radiation field, and scattering as well as absorption (by all relevant continuous processes and up to 150 000 lines in some models) are accurately considered. Good agreement can be obtained with the UV and IR spectra of supernovae 1979C, 1980K, and 1987A as observed during the coasting phase. Potential methods of parameter determinations for SN II are briefly discussed.

Model atmospheres for supernovae of type II have been calculated taking into account the effects of sphericity and velocity fields. We obtain a good fit for the energy distribution of the recent supernovae SN 1980 K and SN 1987 A.

The $^8$B solar neutrino flux as measured by Super-Kamiokande is consistent with the $^{37}$Ar production rate in $^{37}$Cl at Homestake. GALLEX and SAGE, continue to observe $^{71}$Ge production rates in $^{71}$Ga that are consistent with the minimal signal expected from the solar luminosity. The observed $^8$B solar neutrino flux is in good agreement with that predicted by the standard solar model of Dar and Shaviv with nuclear reaction rates that are supported by recent measurements of nuclear fusion cross sections at low energies. The measurements of Super-Kamiokande, SAGE and GALLEX suggest that the expected the pep, $^7$Be and NO solar neutrino fluxes are strongly suppressed. This can be explained by neutrino oscillations and the Mikheyev-Smirnov-Wolfenstein effect. Since neither a flavor change, nor a terrestrial variation, nor a spectral distortion of the $^8$B solar neutrino flux has been observed yet, the solar neutrino problem does not provide conclusive evidence for neutrino properties beyond the standard electroweak model. The deviations of the experimental results from those predicted by the standard solar models may reflect the approximate nature of of solar models and of our knowledge of nuclear reaction rates, radiation transport and particle diffusion in dense stellar plasmas. Only future observations of spectral distortions, or terrestrial modulation or flavor change of solar neutrinos in solar neutrino experiments, such as Super-Kamiokande, SNO, Borexino and HELLAZ will be able to establish that neutrino properties beyond the minimal standard electroweak model are responsible for the solar neutrino problem., To be published in Physics Reports, Proceedings of ``From Atomic Nuclei to Stars and Galaxies'' Haifa, January 12-16, 1998. No enclosed figures

We discuss recent numerical and analytical results that show that different sets of input parameters for supernova/nova model atmospheres yield synthetic spectra that are indistinguishable from one another. The underlying physical reason for this phenomenon is that the atmospheres are Thomson scattering-dominated, and have a gray scattering coefficient. Implications for distance measurements by means of the expanding photosphere method are briefly discussed.

We discuss the theory of optically thick winds from Novae, how they are treated numerically, and the effect of the mass loss on the long term evolution of the WD.

We present non-LTE model atmosphere calculations for the LMC nova 1988 #1 during the optically thick phase shortly after the outburst. The models are able to reproduce the characteristic features of the observed spectra and will be used to determine the atmospheric parameters and chemical abundances of LMC novae.

Recent nucleosynthesis and hydrodynamic calculations of the consequences of accretion onto massive ONeMg white dwarf stars show that under certain circumstances significant amounts of the beta-unstable nuclei can be produced and ejected by the resulting explosion. We use these calculations as a guide in order to obtain the conditions under which the heating of the ejected material by the nonthermal electrons and positrons produced by the decays of the beta-unstable nuclei is sufficient to overcome the cooling from adiabatic expansion and lead to the production of X-ray-emitting coronal gas. These conditions are as follows: (1) a mass fraction for Na-22 of the order of 10(exp -3) or greater, (2) an expansion velocity in the range approximately 10(exp 2) - 10(exp 3) km/s, (3) a photospheric radius of approximately 10(exp 14) cm, (4) if the density distribution in the atmosphere satisfies a power law, then the exponent must be less than 3 for heating to overcome adiabatic cooling. Both the simulations of the outburst and the model atmosphere fits to the observed energy distributions, however, imply that the exponent is greater than or = 3 during the early phases of the outburst. Nevertheless, for a value of the exponent of 2, we predict the time when hot coronal gas can form during the expansion phases of the envelope.

In the continuum and line-blanketed models presented here, nova atmospheres are characterized by a very slow decrease of density with increasing radius. This feature leads to very large geometrical extensions so that there are large temperature differences between the inner and outer parts of the line-forming regions. The theoretical spectra show a large IR excess and a small Balmer jump which may be either in absorption or in emission. For the parameters considered (effective temperature of about 10 exp 4 K, L = 2 x 10 exp 4 solar luminosities, outer boundary density of about 3 x 10 exp -15 g cm exp -3, mass-loss rate of 10 exp -5 solar masses/yr), most lines are in absorption. The effects of changes in the abundances of the heavy elements on the emergent spectra are discussed. The strong unidentified features observed in ultraviolet spectra of novae are found in actuality to be regions of transparency within the Fe 'forest'. Ultraviolet spectra obtained from the IUE archives are displayed, and spectral synthesis of these spectra is done using the theoretical atmospheres.

Accretion discs have come to be known as a frequent phenomenon in astrophysics (Frank, King and Raine 1985). While accretion discs are of various dimensions, scales, temperatures and so on, we limit the discussion here to accretion discs around white dwarfs. Extension of the present discussion to other conditions, in particular accretion discs in premain sequence stars and self gravitating accretion discs, will be made in subsequent communications.

We show that cool UV accretion disks, if present in active galactic nuclei, can be sources of line driven winds. Continuum subcritical disks are shown to possess optically thin continuum winds which are accelerated by radiation pressure in lines.

Recent development in the theory of the boundary layer is considered. The model for the formation of a corona above the WD is discussed in some detail.

The question of local thermonuclear runaways (LTNR) on the surface of white dwarfs is investigated. 1-D equations of energy and of motion are applied, including the accretion process in the continuity equation. The accretion is assumed to be spherically symmetric and to create non spherically symmetric finite gradients in temperature density or composition. We found that the TNR tends to occur locally when the mass and the luminosity of the white dwarf and the accretion rate are high. For certain sets of parameters the size of the LTNR is a significant part of the WD total surface area. The propagation of the TNR may take from few hours to months, producing a slow rise to maximum in novae (unexplained by models so far). Hydrodynamical flows caused by temperature in homogeneities or by rotation can explain the asymmetries observed in nova shells.

The evolution of a 0.7-solar mass red giant is followed along the red giant branch, with mass loss included according to Reimers's formula. The red giant star completes its evolution by totally exhausting its envelope, without igniting the helium in the core. At this point, the core mass, which turns into a white dwarf, is 0.43 solar mass. The consequences of such an evolutionary path are discussed, together with the constraints that these results impose on the mass-loss rate during this evolutionary phase. 30 references.

ABS>The question of overshooting from interior convective cores in stellar models is examined in the framework of a simplified nonlocal mixing- length model for convection. The most important results are (a) a finite nonnegligible overshoot is obtained for parameter combinations typical of stellar convective cores; (b) a finite nonzero overshoot is obtained even in the limit of adiabatic convection (achieved when convective speeds tend to zero). A general thermedynamic argument is presented to show that the limiting results are not model dependent.