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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.

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.

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

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)

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.

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

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.

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 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.

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.

We provide a formal analytic solution of the radiative transfer equation for a gray moving atmosphere in a plane parallel geometry. A formal solution in the diffusion and the free-streaming limit is also provided in the case of a spherically extended atmosphere. The formal solutions are written explicitly for scattering atmospheres in which the density and the velocity fields are given by a power law. A self-consistent temperature profile accurate to O(Beta = v/c) is provided for the case in which the absorption or the scattering are temperature independent. The gray extinction temperature profile is considerably simplified in the case of a scattering atmosphere. Steady state flow and homologous expansion are special cases that are considered in detail.

We discuss the question of the form of the radiative transfer equation in an expanding envelope when a quasi-static approximation is applied. We find that the radiative transfer equation written in the fluid's rest frame must include the `advection' term.

We present, for the first time, a closed integral-form solution to the accretion shock structures for the case where the cooling is due to optically thin bremsstrahlung emission and a series of power-law cooling functions of density and temperature. Our results can provide useful checks on numerical calculations and simple accurate estimates for valuable parameters such as the shock height. For the case where the cooling rate j = (2/3)Arho(exp 2)(P/rho)(exp 1/2)(1 + epsilon (sub s)(P/P(sub s)(exp alpha)(rho(sub s)/rho)(exp beta)), we find that a substantial amount of the accretion energy is released at the base of the accretion shock in the form of bremsstrahlung radiation. This implies that for a cyclotron-dominated shock (qualitatively given by alpha = 2.0, beta = 3.85, and epsilon(sub s) is much greater than 1), bremsstrahlung cooling still plays a crucial role in determining the shock structure. Our results are shown to be consistent with detailed numerical calculations.

We consider a statistical model in which gas clouds (the particles of the generalized gas) interact with one another and merge to form larger objects. We derive the particle mass distribution for such a gas. Our assumptions are equivalent to the basic assumptions of the statistical mechanics of sticky gas. We show, numerically, that the upper part of the mass distributions has an asymptotic shape of the following form: N(m)∼(m/ ) −λ exp (−βm/ ), where λ=λ(α) and β=β(α) depend on the parameter α of the cross section for merger and is the mean mass. The only time dependence is via the dependence of on time, which is found to be a power law. The distribution has a maximum, namely there is a mass that is the most frequent one

Previous studies of time-dependent accretion onto magnetic white dwarfs, in which the cooling was assumed to be due to bremsstrahlung emission, have shown that the accretion shock undergoes oscillations. However, when cyclotron cooling is also included, the oscillations are damped for sufficiently strong magnetic fields. Here we demonstrate that the oscillations can be sustained by accretion-fluctuation-induced excitations. The frequency of the QPOs are shown to increase quadratically with the magnetic field strength. We interpret the oscillations as a two-phase process in which bremsstrahlung cooling dominates in one half-cycle and cyclotron cooling in the other. Such a process may have very different consequences compared to a single-phase process where the functional form of the cooling is essentially the same throughout the cycle. If in the two-phase process damping occurs mainly in the cyclotron cooling half-cycle, there will be a universal effective damping factor which tends to suppress all oscillation modes indiscriminately. The oscillations of the accretion shock also could be a limit cycle process in which the system vacillates between two branches.

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.

The amount of 26 Al in the interstellar medium, which has been detected both by the HEAO 3 γ-ray spectrometer and the SMM experiment, in combination with the measured overabundance of 26 Mg in meteorites, is predicted to have been produced by nucleosynthesis in nova explosions. To test this prediction, the nucleosynthesis of 26 Al is investigated during hot hydrogen burning along temperature-density-time profiles obtained from hydrodynamic simulations of nova outbursts.

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 interaction of the boundary layer with the stellar interior and possible mixing between the hydrogen-rich accreted matter and the core material is discussed in view of recent numerical simulations of the boundary layer.

The paper presents simulations of the outburst of U Sco by accreting material, with a solar abundance of the CNO nuclei, at a rate of 1.1 x 10 to the -6th solar mass/yr onto a 1.35-solar mass white dwarf. This evolutionary sequence takes about 2.6 yr to reach runaway conditions and ultimately eject 4 x 10 to the -7th solar mass moving with speeds exceeding about 400 km/s. It is found that ejection occurred by radiation pressure rather than by explosive CNO burning. 22 references.

Etude de l'hypothese communement faite de l'accretion dans les objets compacts, ou la matiere s'etend lentement a la surface. On trouve que cette hypothese n'est pas justifiee pour les parametres courants des taux d'accretion et les effets de deviations dus a cette hypothese sur le runaway thermonucleaire sont importantes

In the present analytical model for a power law X-ray continuum production in active galactic nuclei, the dissipation of turbulent energy flux above the accretion disk forms an optically thin transition layer with an inverted temperature gradient. The emitted thermal radiation has a power law spectrum in the 0.1-100 keV range, with a photon energy spectral index gamma of about 0.4-1.0. Thermal X-ray contribution from the layer is 5-10 percent of the total disk luminosity. The gamma value of 0.75 is suggested as a 'natural' power law index for Seyfert galaxies and QSOs.

Un ensemble de conditions aux limites sont etablies pour l'accretion sur des etoiles de la sequence principale. Une condition aux limites thermiques permettant d'inclure l'energie d'accretion et la luminosite d'accretion dans l'evolution est egalement fournie. Ces conditions sont utilisees avec un code d'evolution stellaire hydrodynamique pour etudier l'accretion de masse a taux eleve sur une etoile de la sequence principale avec une masse de 1.31 M ○. afin de simuler l'un des scenarios proposes pour le sursaut de la nova recurrente RS Oph

We show that cool UV accretion disks, if present in active galactic nuclei,can be sources of line-driven winds. Subcritical disks are shown to be capable of producing winds optically thin in the continuum, which are accelerated by radiation pressure in lines (i.e., by absorption and scattering of disk-continuum photons in the Doppler-shifted optically thick UV resonance lines). Results of a numerical simulation to the outflow from a disk around a supermassive black hole are presented and discussed. The necessary condition for initiating the wind is that the local radiative flux must be peaked in the energy range of existing lines. The base of the wind must therefore be shielded from the central region, which is assumed to emit power-law-distributed radiation. The disk atmosphere can provide such shielding. The winds are characterized by steep velocity gradients and high asymptotic velocities, approx.10/sup 9/ cm s/sup -1/, as observed in broad emission-line regions of QSOs and Seyfert galaxies, and are thermally unstable, as they heat up. This results in a two-phase equilibrium: cold clouds embedded in a hot, Compton-heated background. The hot wind provides the pressure confinement for the dense and massive clouds. The relevant time scales for the cloud formation and theirmore » future evolution are estimated, and the implications for the dynamics of the broad emission-line regions are discussed.« less

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.