A set of hydrodynamical models based on stellar evolutionary progenitors is used to study the nature of SN 2011dh. Our modeling suggests that a large progenitor star -with R ∼ 200 R ⊙ -, is needed to reproduce the early light curve of SN 2011dh. This is consistent with the suggestion that the yellow super-giant star detected at the location of the SN in deep pre-explosion images is the progenitor star. From the main peak of the bolometric light curve and expansion velocities we constrain the mass of the ejecta to be ≈ 2 M ⊙ , the explosion energy to be E = 6 − 10 × 10 50 erg, and the 56 Ni mass to be approximately 0.06 M ⊙ . The progenitor star was composed of a helium core of 3 to 4 M ⊙ and a thin hydrogen-rich envelope of ≈ 0.1 M ⊙ with a main sequence mass estimated to be in the range of 12-15 M ⊙ . Our models rule out progenitors with helium-core masses larger than 8 M ⊙ , which correspond to M ZAMS 25 M ⊙ . This suggests that a single star evolutionary scenario for SN 2011dh is unlikely.

We present bolometric light curves of Type II-plateau supernovae (SNe II-P) obtained using a newly developed, one-dimensional Lagrangian hydrodynamic code with flux-limited radiation diffusion. Using our code we calculate the bolometric light curve and photospheric velocities of SN 1999em obtaining a remarkably good agreement with observations despite the simplifications used in our calculation. The physical parameters used in our calculation are E = 1.25 foe, M = 19 M ⊙ , R = 800 R ⊙ and M Ni = 0.056 M ⊙ . We find that an extensive mixing of 56 Ni is needed in order to reproduce a plateau as flat as that shown by the observations. We also study the possibility to fit the observations with lower values of the initial mass consistently with upper limits that have been inferred from pre-supernova imaging of SN 1999em in connection with stellar evolution models. We cannot find a set of physical parameters that reproduce well the observations for models with pre-supernova mass of ≤ 12 M ⊙ , although models with 14 M ⊙ cannot be fully discarded.

In this paper we investigate the effects of element diffusion on the structure and evolution of low‐mass helium white dwarfs. Attention is focused mainly on the occurrence of hydrogen‐shell flashes induced by diffusion processes during cooling phases. Physically sound initial models with stellar masses of 0.406, 0.360, 0.327, 0.292, 0.242, 0.196, 0.169 and 0.161 M⊙ are constructed by applying mass‐loss rates at different stages of the red giant branch evolution of a solar model up to the moment the model begins to evolve to the blue part of the HR diagram. The multicomponent flow equations describing gravitational settling, and chemical and thermal diffusion are solved and the diffusion calculations are coupled to an evolutionary code. In addition, the same sequences are computed but neglecting diffusion. Results without diffusion are similar to recent results of Driebe, Schönberner, Blöcker and Herwig. We find that element diffusion strongly affects the structure and cooling history of helium white dwarfs. In particular, diffusion induces the occurrence of hydrogen‐shell flashes in models with masses ranging from 0.18 to 0.41 M⊙, which is in sharp contrast with the situation when diffusion is neglected. In connection with further evolution, these diffusion‐induced flashes lead to much thinner hydrogen envelopes, preventing stable nuclear burning from being a sizeable energy source at advanced stages of evolution. This implies much shorter cooling ages than in the case when diffusion is neglected. These new evolutionary models are discussed in light of recent observational data on some millisecond pulsar systems with white dwarf companions. In this context, we find that discrepancies between spin‐down ages and the predictions of standard evolutionary models appear to be the result of ignoring element diffusion in such evolutionary models. Indeed, such discrepancies vanish when account is taken of diffusion.

No abstract

Recent measurements by Hipparcos present observational evidence supporting the existence of some white dwarf (WD) stars with iron-rich core composition. In connection with this, the present paper is aimed at exploring the structure and evolution of iron-core WDs by means of a detailed and updated evolutionary code. In particular, we examined the evolution of the central conditions, neutrino luminosity, surface gravity, crystallization, internal luminosity profile and ages. We find that the evolution of iron-rich WDs is markedly different from that of their carbon±oxygen counterparts. In particular, cooling is strongly accelerated (up to a factor of 5 for models with pure iron composition) as compared with the standard case. Thus, if iron WDs were very numerous, some of them would have had time enough to evolve at lower luminosities than that corresponding to the fall-off in the observed WD luminosity function.

The recent detection in archival HST images of an object at the the location of supernova (SN) iPTF13bvn may represent the first direct evidence of the progenitor of a Type Ib SN. The object's photometry was found to be compatible with a Wolf-Rayet pre-SN star mass of ≈ 11 M ⊙ . However, based on hydrodynamical models we show that the progenitor had a pre-SN mass of ≈ 3.5 M ⊙ and that it could not be larger than ≈ 8 M ⊙ . We propose an interacting binary system as the SN progenitor and perform evolutionary calculations that are able to self-consistently explain the light-curve shape, the absence of hydrogen, and the pre-SN photometry. We further discuss the range of allowed binary systems and predict that the remaining companion is a luminous O-type star of significantly lower flux in the optical than the pre-SN object. A future detection of such star may be possible and would provide the first robust identification of a progenitor system for a Type Ib SN.

The purpose of this work is to explore the evolution of helium-core white dwarf stars in a self-consistent way with the predictions of detailed non-gray model atmospheres and element diffusion. To this end, we consider helium-core white dwarf models with stellar masses of 0.406, 0.360, 0.327, 0.292, 0.242, 0.196 and 0.169 solar masses and follow their evolution from the end of mass loss episodes during their pre-white dwarf evolution down to very low surface luminosities. We find that when the effective temperature decreases below 4000K, the emergent spectrum of these stars becomes bluer within time-scales of astrophysical interest. In particular, we analyse the evolution of our models in the colour-colour and colour-magnitude diagrams and we find that helium-core white dwarfs with masses ranging from approx. 0.18 to 0.3 solar masses can reach the turn-off in their colours and become blue again within cooling times much less than 15 Gyr and then remain brighter than M_V approx. 16.5. In view of these results, many low-mass helium white dwarfs could have had time enough to evolve to the domain of collision-induced absorption from molecular hydrogen, showing blue colours.Comment: 11 pages, 9 figures. Accepted for publication in MNRA

The present work is designed to explore the evolution of helium‐core white dwarf (He WD) stars for the case of metallicities much lower than the solar metallicity (Z= 0.001 and 0.0002). Evolution is followed in a self‐consistent way with the predictions of detailed and new non‐grey model atmospheres, time‐dependent element diffusion and the history of the white dwarf progenitor. Reliable initial models for low‐mass He WDs are obtained by applying mass‐loss rates to a 1‐M⊙ stellar model in such a way that the stellar radius remains close to the Roche lobe radius. The loss of angular momentum caused by gravitational wave emission and magnetic stellar wind braking are considered. Model atmospheres, based on a detailed treatment of the microphysics entering the WD atmosphere (such as the formalism of Hummer–Mihalas to deal with non‐ideal effects) and hydrogen line and pseudo‐continuum opacities, enable us to provide accurate colours and magnitudes at both early and advanced evolutionary stages. We find that most of our evolutionary sequences experience several episodes of hydrogen thermonuclear flashes. In particular, the lower the metallicity, the larger the minimum stellar mass for the occurrence of flashes induced by CNO cycle reactions. The existence of a mass threshold for the occurrence of diffusion‐induced CNO flashes leads to a marked dichotomy in the age of our models. Another finding of this study is that our He WD models experience unstable hydrogen burning via PP nuclear reactions at late cooling stages as a result of hydrogen chemically diffusing inwards. Such PP flashes take place in models with very low metal content. We also find that models experiencing CNO flashes exhibit a pronounced turn‐off in most of their colours at MV≈ 16. Finally, colour–magnitude diagrams for our models are presented and compared with recent observational data of He WD candidates in the globular clusters NGC 6397 and 47 Tucanae.