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The state of the theory of long term behavior of cataclysmic variables is reviewed and the basic assumptions are specified. Next, despite the fact that the nova phenomenon is treated as a periodic one, secular changes caused by the changes in the masses of the binary system must be taken into account as well as other secular changes in the parameters of the system. In most reported calculation in the literature this extremely important changes are ignored. Preliminary results showing the effects of these changes on the evolution are presented.

We discuss several aspects of the phenomenon called Global Warming and refrain from any con- clusions. We leave the reader to draw his conclusions about the series of observations presented here. The phenomenon called Global Warming or in the new name Climate Change, refers to the be- havior of the ’Temperature of the Earth’ as a function of time. The ’temperature of the Earth’ is a complicated term as it is measured in different locations and one has to define at what location we discuss the temperature and how we average the temperature over the complicated surface of the Earth so as to obtain a single number. The atmosphere, or better said the troposphere, can be considered as a boundary layer. However, this layer is not isothermal and shows large temporal and location temperature variations. There are three basic questions the data should point to: • How the "Global Temperature" of the Earth is evaluated and how the statistical mean be- haves as a function of time, • How the temperature varies at a given fixed location, T(t), • What is the role of CO2 in driving the "Global Earth temperature". We examine in the paper the following topics: • The Global Earth Temperature and Definition of the Temperature Anomaly, • The rise of the oceans, • The behavior of the CO2, • The acidity of the seas, • Underwater volcanos, • The dependence of T(t) for a single place, • Theoretical predictions of the Global Temperature, • The Interaction of the Sun with Earth’s climate, • The phenomenon called ’Global Warming Pause. in an attempt to provide a comprehensive answer to the above questions.

This article reviews the scientific physical and mathematical evolution of models to calculate the Greenhouse effect (GHE) and radiative transfer in planetary atmospheres. It examines the set of assumptions made in the models and concludes with a short discussion about the next steps in GHE modeling.

Commission 35 (C35), “Stellar Constitution”, consists of members of the International Astronomical Union whose research spans many aspects of theoretical and observational stellar physics and it is mainly focused on the comprehension of the properties of stars, stellar populations and galaxies. The number of members of C35 increased progressively over the last ten years and currently C35 comprises about 400 members. C35 was part of Division IV (Stars) until 2014 and then became part of Division G (Stars and Stellar Physics), after the main IAU reorganisation in 2015. Four Working Groups have been created over the years under Division IV, initially, and Division G later: WG on Active B Stars, WG on Massive Stars, WG on Abundances in Red Giant and WG on Chemically Peculiar and Related Stars. In the last decade the Commission had 4 presidents, Wojciech Dziembowski (2003-2006), Francesca D'Antona (2006-2009), Corinne Charbonnel (2009-2012) and Marco Limongi (2012-2015), who were assisted by an Organizing Committee (OC), usually composed of about 10 members, all of them elected by the C35 members and holding their positions for three years. The C35 webpage (http://iau-c35.stsci.edu) has been designed and continuously maintained by Claus Leitherer from the Space Telescope Institute, who deserves our special thanks. In addition to the various general information on the Commission structure and activities, it contains links to various resources, of interest for the members, such as stellar models, evolutionary tracks and isochrones, synthetic stellar populations, stellar yields and input physics (equation of state, nuclear cross sections, opacity tables), provided by various groups. The main activity of the C35 OC is that of evaluating, ranking and eventually supporting the proposals for IAU sponsored meetings. In the last decade the Commission has supported several meetings focused on topics more or less relevant to C35. Since the primary aim of this document is to present the main activity of C35 over the last ten years, in the following we present some scientific highlights that emerged from the most relevant IAU Symposia and meetings supported and organized by C35 in the last decade.

One of the most interesting problems in Cataclysmic Variables is the long time scale evolution. This problem appears in long time evolution, which is also very important in the search for the progenitor of SN Ia. The classical approach to overcome this problem in the simulation of novae evolution is to assume: (1) A constant in time, rate of mass transfer. (2) The mass transfer rate that does not vary throughout the life time of the nova, even when many eruptions are considered. Here we show that these assumptions are valid only for a single thermonuclear flash and such a calculation cannot be the basis for extrapolation of the behavior over many flashes. In particular, such calculation cannot be used to predict under what conditions an accreting WD may reach the Chandrasekhar mass and collapse. We report on a new code to attack this problem. The basic idea is to create two parallel processes, one calculating the mass losing star and the other the accreting white dwarf. The two processes communicate continuously with each other and follow the time depended mass loss.

We review the works of Helmholtz and Kelvin and their timescale and show that it is not what is called in classical text books the Kelvin–Helmholtz time scale. Next we review the work of Ritter and show that he was the first to calculate what is erroneously attributed today to Kelvin and Helmholtz. We conclude that the correct name is the Ritter–Kelvin–Helmholtz timescale.

The purpose of this paper is to explore the steady state and dynamical behavior of thin, axisymmetric, viscous accretion disks. To facilitate an analytical treatment we replace the energy equation with a general polytropic assumption. The asymptotic expansion of Kluzniak & Kita (2000, Three-dimensional structure of an alpha accretion disk (arXiv:astro-ph/0006266)), which extended the method of Regev (1983, A&A, 126, 146) to a full steady polytropic disk (with n = 3/2), is further developed and implemented for both the steady (for any polytropic index) and time-dependent problems. The spatial form and temporal behavior of selected dynamical disturbances are studied in detail. It is shown that the transient dynamics resulting from initial perturbations on the linearly stable steady state gives rise to substantial growth of perturbations. We identify the initial perturbation space which leads to such transient growth and provide analytical solutions which manifest this behavior three terms (physical causes) responsible for the appearance of transient dynamics are identified. Two depend explicitly on the viscosity while the third one is relevant also for inviscid disks. The main conclusion we draw is that transient dynamics and, in particular, significant perturbation energy amplification occurs in disks on a global scale. We speculate on the possible implications of these findings to accretion disk theory.

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.

The linear stability of accretion disks is revisited. The governing equations are expanded asymptotically and solved to first order in the expansion parameter $\epsilon$ defined by the ratio of the disk's vertical thickness to its radial extent. Algebraically growing solutions are found for global perturbations on the radial accretion flow of thin inviscid compressible Keplerian disks. The algebraic temporal behavior is exhibited in the vertical velocities and the thermodynamic variables and has the form $t\sin\Omega_0 t$ locally in the disk where $\Omega_0$ is the Keplerian rotation rate. The physical implications and relations to the Solberg-Hoiland stability criteria are discussed., Comment: Submitted to A+A Letters on November 15, 2004. 4 pages, no figures. Accepted January 20, 2005. Section 3 rewritten and references updated

In an attempt to clarify the behavior of the screening phenomenon in N-body systems at thermodynamics equi- librium, under the particular conditions at the core of the Sun, we define the screening energy and summarize the two main types of methods to calculate it: (a) by using classical mean field in Statistical Mechanics and (b) ab initio, by using molecular dynamics and first principles only. We invent a pair interaction with a finite parametrized range and strength, and carry out a numerical experiment which demonstrates the difference between the two aforementioned methods and underlying assumptions as a function of the range of the interaction (or equivalently the number of particles in the interaction zone). We study the effect that fluctuations in the number of particles in the screening/neutralizing cloud have on the screening, and the deviations from mean field results which ensue. We also demonstrate that the classical mean field theory is inaccurate under the conditions prevailing in stellar cores and essentially over estimates the screening. We show how the molecular dynamics results tend to the mean field Statistical Mechanics limits when the number of particles in the neutralizing domain increases.

The screening of nuclear reactions in stars takes place under the special conditions that the total number of particles in the screening cloud is of the order of unity. Hence, standard mean field theory is not expected tobe valid. We demonstrate by the use of Molecular dynamics that fluctuations dominate the screening and lead to a smaller screening compared with the predictions of the classical mean field theory.

We apply the method of Shaviv & Shaviv to evaluate from first principles the plasma screening correction to the rate of nuclear reactions under conditions similar to those prevailing in the present-day Sun. We calculate the screening factor for the p-p chain and the CN cycle nuclear reactions. We find the following

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 stand-off shock formed in the accretion flow on to a stationary wall, such as the surface of a white dwarf, may be thermally unstable, depending on the cooling processes which dominate the post-shock flow. Some processes lead to instability, while others tend to stabilize the shock. We consider competition between the destabilizing influence of thermal bremsstrahlung cooling, and a stabilizing process which is a power law in density and temperature. Cyclotron cooling and processes which are of order 1, 3/2 and 2 in density are considered. The relative efficiency and power-law indices of the second mechanism are varied, and particular effects on the stability properties and frequencies of oscillation modes are examined.

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.

It is generally tacitly assumed that the atmosphere of a habitable planet absorbs in the infrared and is transparent in the visible. We consider the cases in which the optical depth in the visible is not negligible. We show that such planets can harbor life under conditions similar to those found on Earth. Moreover, it is conceivable that planetary evolution starts with phases in which the optical depth in the visible is not negligible and evolves out of it or passes through such a phase during the evolution of the planet.

The stability of accretion shocks in magnetic white dwarfs is investigated. Shock oscillations are simulated numerically, and in the simulations bremsstrahlung and cyclotron cooling are considered in an explicit form. We have found that for shocks with magnetic fields lower than 10 MG, shock oscillations can be sustained; while for magnetic fields greater than 20 MG, the oscillations are suppressed.

Novae in outburst can be used as distance indicators. The distance measurement process is considerably simplified if novae could be regarded as standard candles. The term standard candles in the context of this paper means that the absolute magnitude at maximum for different novae is, to a good approximation, constant. The t3 – Mb diagram (Schmidet 1957) implies that this is the case for M31 and LMC novae (Della Valle & Livio 1995). However, this conclusion does not necessarily apply to non Local Group novae (Livio 1992; Della Valle & Livio 1995); for example the Virgo novae are not similar to Local Group novae (Della Valle & Livio 1995). Here we focus on a question of principle, namely: Does the fact that novae have similar spectra imply that they have similar luminosities?

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 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 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

We generalize the problem of the semi-gray model to cases in which a non-negligible fraction of the stellar radiation falls on the long-wavelength range, and/or that the planetary long-wavelength emission penetrates into the transparent short wavelength domain of the absorption. Second, applying the most general assumptions and independently of any particular properties of an absorber, we show that the greenhouse effect saturates and any Earth-like planet has a maximal temperature which depends on the type of and distance to its main-sequence star, its albedo and the primary atmospheric components which determine the cutoff frequency below which the atmosphere is optically thick. For example, a hypothetical convection-less planet similar to Venus, that is optically thin in the visible, could have at most a surface temperature of 1200-1300K irrespective of the nature of the greenhouse gas. We show that two primary mechanisms are responsible for the saturation of the runaway greenhouse effect, depending on the value of the wavelength above which the atmosphere becomes optically thick. Unless this wavelength is small and resides in the optical region, saturation is achieved by radiating the thermal flux of the planet through the short wavelength tail of the thermal distribution. This has the observational implication, the radiation from such a planet should be skewed towards the NIR. Otherwise, saturation takes place by radiating through windows in the FIR., 13 pages 14 figures

We stressed that the observed composition of stars is uniform. All stars have, to a very good approximation, the same composition. The justification for this fundamental result was the success of the Harvard spectral type classification, based on spectral line intensity ratios. The observations demonstrated time and again that, to a very good approximation, the overwhelming majority of stars (at least those nearby) exhibit similar line ratios for the same effective surface temperatures, thus indicating similar compositions. This general tenet survived until the beginning of the 1950s, when it became clear that this was only approximately correct. The problem was then completely turned round, to ask what was the source of the small difference in the abundances of the heavy elements.

Figure 8.1 shows the light nuclei arranged in the \((Z,N)\) plane, where \(Z\) is the number of protons and \(N\) the number of neutrons. We see that there are no stable nuclei for \(A=Z+N=5\) and \(A=Z+N=8\). This means that hydrogen cannot fuse with helium to form any \(A=5\) nucleus and helium cannot fuse with another helium to form an \(A=8\) nucleus. These are the \(A=5\) and 8 nuclear barriers. But we know that the nuclei heavier than helium are synthesized. So how does this come about?

The idea of atoms as forming the ultimate constituents of matter already arose in the great speculative or philosophical minds of the ancient world, although no exact chemical or physical atomic theory actually evolved from it at the time, and although, most importantly, there was no direct evidence for it. The fundamental questions were: What are the chemical elements? What are the basic units which carry the identity of a chemical element? Can matter be divided indefinitely while still preserving its identity?

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.

While the \(s\)-process can be described as a low neutron flux occurring over a long period of time, the \(r\)-process can be described as the opposite, namely, irradiation by a high neutron flux for a very short time. Hence, while the \(s\)-process could be described as a quiet, slow, hydrostatic evolution, the \(r\)-process is usually associated with the most violent phases in stellar life—the explosion that puts an end to the ‘normal’ life of the star. It seems that no in-between process exists, only the two extremes. Notwithstanding, problems with the explosion scenario have led astrophysicists to ponder on less violent circumstances where a milder variant of the \(r\)-process might take place. But whatever the situation, we are now looking for an extreme source of neutrons in an expanding environment. The adjective ‘extreme’ refers to the nuclear properties while the requirement of expansion refers to the astrophysical site, so that the synthesized isotopes can be ejected into space. Alpher, Bethe, and Gamow considered the Big Bang to be just such an explosive situation, but in this specific case we are supposedly immersed in the products.

The famous curve of the binding energy per nucleon tells us that the most stable nuclei are in the iron group. Stellar evolution is therefore the formation of more and more stable nuclei, accompanied by the extraction of the nuclear binding energy which is then radiated away. The nuclei beyond the iron group are less and less stable, and their formation requires energy.

As early as 1916, Eddington proclaimed that the stars obey the same laws of physics as we discover in the laboratory. The known universe was small in those days, but today we can observe much further away and look back many more millions of years in time. The obvious question to ask is: were the laws of physics in general, and spectroscopy in particular, valid, say, 5 billion years ago? How confident can we be in the derived abundances of the elements, in objects that are ten billion light years away, using the physical laws we find today?

Once it became known that hydrogen was the most abundant element in the Universe (in 1931), and once the binding energy per nucleon could be obtained as a function of the atomic weight (in 1935), it was quite natural to assume that the fusion of light elements into heavier ones was the source of stellar energy. But the mechanism remained mysterious. There is a basic nuclear difference between hydrogen and heavier elements in their neutron to proton ratio. While in hydrogen there are no neutrons, helium has two neutrons and two protons, and if four protons fuse to form a helium nucleus, then two protons must \(\upbeta \)-decay. We say that \(Y_\mathrm{ e}=n(p)/[n(p)+n(n)]\) changes from \(Y_\mathrm{ e}=1\) for hydrogen to \(Y_\mathrm{ e}=1/2\) for He and heavier nuclei. How does this change take place? Inside nuclei or during a collision? This is the basic difference between the CN cycle and the proton–proton chain.

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.

All our knowledge about the composition of cosmic objects is obtained via spectroscopy. Two key disciplines are required to extract this information from observations: the theory of radiative transfer through stellar material and the theory of atomic structure. Spectroscopy is as old as modern science. It began with Johannes Kepler (1571–1630m) and later Isaac Newton (1643–1727m), who knew about the effect of the prism on sunlight. When they cast the outgoing light of the prism on a screen, they discovered all the colors of the rainbow. Naturally, Newton used a circular aperture, and consequently his spectrum was not pure. Despite this early start, progress was slow at the beginning, and even after major breakthroughs, about 400 years were needed before reliable information about stellar composition could be obtained.

As soon as stellar spectra were analyzed spectroscopically in the second half of the nineteenth century, it became clear that these spectra exhibit a bewildering variety. The need for some order, or rather some ‘classification’, became urgent. Of the many schemes suggested for stellar classification, the Harvard scheme, proposed by Pickering, was accepted in 1910 as the standard.

Bethe’s discoveries explained the source of energy of the main sequence stars but did not elucidate how the elements heavier than helium were formed. Three years later Bethe posed the problem of the synthesis of the elements and suggested, somewhat in despair and for want of a better explanation, that the synthesis of the elements might be independent of the energy source of stars.

We present a two-dimensional time dependent collocation Chebyshev spectral method as a tool for the solution of certain astrophysical flows. In this paper, we solve the Navier-Stokes equations in spherical coordinates. The problem of the propagation of an initial perturbation in an atmosphere of a compact star is used to demonstrate the method and its power. In particular, viscosity and rotation are included.