New models for the evolution of Post-Asymptotic Giant Branch stars and Central Stars of Planetary Nebulae

The Post Asymptotic Giant Branch (AGB) phase is arguably one of the least understood phases of the evolution of low- and intermediate- mass stars. The two grids of models presently available are based on outdated micro- and macro-physics and do not agree with each other. We study the timescales of post-AGB and CSPNe in the context of our present understanding of the micro- and macro-physics of stars. We want to assess whether new post-AGB models, based on the latter improvements in TP-AGB modeling, can help to understand the discrepancies between observation and theory and within theory itself. We compute a grid of post-AGB full evolutionary sequences that include all previous evolutionary stages from the Zero Age Main Sequence to the White Dwarf phase. Models are computed for initial masses between 0.8 and 4 $M_\odot$ and for a wide range of initial metallicities ($Z_0=$0.02, 0.01, 0.001, 0.0001), this allow us to provide post-AGB timescales and properties for H-burning post-AGB objects with masses in the relevant range for the formation of planetary nebulae ($\sim$ 0.5 - 0.8, $M_\odot$). We find post-AGB timescales that are at least $\sim 3$ to $\sim 10$ times shorter than those of old post-AGB stellar evolution models. This is true for the whole mass and metallicity range. The new models are also $\sim$ 0.1 - 0.3 dex brighter than the previous models with similar remnant masses. Post-AGB timescales show only a mild dependence on metallicity. The shorter post-AGB timescales derived in the present work are in agreement with recent semiempirical determinations of the post-AGB timescales from the CSPNe in the Galactic Bulge. Due to the very different post-AGB crossing times, initial-final mass relation and luminosities of the present models, they will have a significant impact in the predictions for the formation of planetary nebulae and the planetary nebulae luminosity function.Comment: Main Article: 16 pages, 12 figures and 3 tables. Main Article + Appendices: 22 Pages, 16 figures and 6 tables. Accepted for publication in A&A. (Revised to match the final version accepted for publication in A&A

A Red Giants' Toy Story

In spite of the spectacular progress accomplished by stellar evolution theory some simple questions remain unanswered. One of these questions is ``Why do stars become Red Giants?''. Here we present a relatively simple analytical answer to this question. We validate our analysis by constructing a quantitative toy-model of a red giant and comparing its predictions to full stellar evolutionar models. We find that the envelope forces the value of $\nabla=d \ln T/d \ln P$ at, and above, the burning shell into a very narrow range of possible values. Together with the fact that the stellar material at the burning shell both provides and transports most of the stellar luminosity, this leads to tight relations between the thermodynamic variables at the burning shell and the mass and radius of the core -- $T_s(M_c,R_s)$, $P_s(M_c,R_s)$ and $\rho_s(M_c,R_s)$. When complemented by typical mass-radius relations of the helium cores, this implies that for all stellar masses the evolution of the core dictates the values of $T_s$, $P_s$ and $\rho_s$. We show that for all stellar masses evolution leads to an increase in the pressure and density contrasts between the shell and the core, forcing a huge expansion of the layers on top of the burning shell. Besides explaining why stars become red giants our analysis also offers a mathematical demonstration of the so-called shell homology relations, and provides simple quantitative answers to some properties of low-mass red giants.Comment: Accepted for Publication in The Astrophysical Journal. Updated with minor corrections to match the accepted version after proofs. 27 Pages. 15 Figures. 5 appendixe

New evolutionary sequences for extremely low mass white dwarfs: Homogeneous mass and age determinations, and asteroseismic prospects

We provide a fine and homogeneous grid of evolutionary sequences for He-core white dwarfs with masses 0.15-0.45 Msun, including the mass range for ELM white dwarfs (<0.20Msun). The grid is appropriate for mass and age determination, and to study their pulsational properties. White dwarf sequences have been computed by performing full evolutionary calculations that consider the main energy sources and processes of chemical abundance changes during white dwarf evolution. Initial models for the evolving white dwarfs have been obtained by computing the non-conservative evolution of a binary system consisting of a Msun ZAMS star and a 1.4 Msun neutron star for various initial orbital periods. To derive cooling ages and masses for He-core white dwarf we perform a least square fitting of the M(Teff, g) and Age(Teff, g) relations provided by our sequences by using a scheme that takes into account the time spent by models in different regions of the Teff-g plane. This is useful when multiple solutions for cooling age and mass determinations are possible in the case of CNO-flashing sequences. We also explore the adiabatic pulsational properties of models near the critical mass for the development of CNO flashes (~0.2 Msun). This is motivated by the discovery of pulsating white dwarfs with stellar masses near this threshold value. We obtain reliable and homogeneous mass and cooling age determinations for 58 very low-mass white dwarfs, including 3 pulsating stars. Also, we find substantial differences in the period spacing distributions of g-modes for models with stellar masses ~ 0.2 Msun, which could be used as a seismic tool to distinguish stars that have undergone CNO flashes in their early cooling phase from those that have not. Finally, for an easy application of our results, we provide a reduced grid of values useful to obtain masses and ages of He-core white dwarf.Comment: 12 pages, 9 figures, to be published in Astronomy and Astrophysic

Quiescent nuclear burning in low-metallicity white dwarfs

We discuss the impact of residual nuclear burning in the cooling sequences of hydrogen-rich DA white dwarfs with very low metallicity progenitors ($Z=0.0001$). These cooling sequences are appropriate for the study of very old stellar populations. The results presented here are the product of self-consistent, fully evolutionary calculations. Specifically, we follow the evolution of white dwarf progenitors from the zero-age main sequence through all the evolutionary phases, namely the core hydrogen-burning phase, the helium-burning phase, and the thermally pulsing asymptotic giant branch phase to the white dwarf stage. This is done for the most relevant range of main sequence masses, covering the most usual interval of white dwarf masses --- from 0.53\, M_{\sun} to 0.83\, M_{\sun}. Due to the low metallicity of the progenitor stars, white dwarfs are born with thicker hydrogen envelopes, leading to more intense hydrogen burning shells as compared with their solar metallicity counterparts. We study the phase in which nuclear reactions are still important and find that nuclear energy sources play a key role during long periods of time, considerably increasing the cooling times from those predicted by standard white dwarf models. In particular, we find that for this metallicity and for white dwarf masses smaller than about 0.6\, M_{\sun}, nuclear reactions are the main contributor to the stellar luminosity for luminosities as low as \log(L/L_{\sun})\simeq -3.2. This, in turn, should have a noticeable impact in the white dwarf luminosity function of low-metallicity stellar populations.Comment: 4 pages, 3 figures. Accepted for publication in ApJ Letter

The white dwarf cooling sequence of 47 Tucanae

47 Tucanae is one of the most interesting and well observed and theoretically studied globular clusters. This allows us to study the reliability of our understanding of white dwarf cooling sequences, to confront different methods to determine its age, and to assess other important characteristics, like its star formation history. Here we present a population synthesis study of the cooling sequence of the globular cluster 47 Tucanae. In particular, we study the distribution of effective temperatures, the shape of the color-magnitude diagram, and the corresponding magnitude and color distributions. We do so using an up-to-date population synthesis code based on Monte Carlo techniques, that incorporates the most recent and reliable cooling sequences and an accurate modeling of the observational biases. We find a good agreement between our theoretical models and the observed data. Thus, our study, rules out previous claims that there are still missing physics in the white dwarf cooling models at moderately high effective temperatures. We also derive the age of the cluster using the termination of the cooling sequence, obtaining a good agreement with the age determinations using the main-sequence turn-off. Finally, we find that the star formation history of the cluster is compatible with that btained using main sequence stars, which predict the existence of two distinct populations. We conclude that a correct modeling of the white dwarf population of globular clusters, used in combination with the number counts of main sequence stars provides an unique tool to model the properties of globular clusters.Comment: 6 pages and 4 figures accepted for publication in A &

On the relevance of bubbles and potential flows for stellar convection

Recently Pasetto et al. have proposed a new method to derive a convection theory appropriate for the implementation in stellar evolution codes. Their approach is based on the simple physical picture of spherical bubbles moving within a potential flow in dynamically unstable regions, and a detailed computation of the bubble dynamics. Based on this approach the authors derive a new theory of convection which is claimed to be parameter free, non-local and time-dependent. This is a very strong claim, as such a theory is the holy grail of stellar physics. Unfortunately we have identified several distinct problems in the derivation which ultimately render their theory inapplicable to any physical regime. In addition we show that the framework of spherical bubbles in potential flows is unable to capture the essence of stellar convection, even when equations are derived correctly.Comment: 14 pages, 3 figures. Accepted for publication in Monthly Notices of the Royal Astronomical Society. (Comments and criticism are welcomed