On the formation of oxygen-neon white dwarfs in close binary systems

The evolution of a star of initial mass 10 $M_{\odot}$, and metallicity $Z = 0.02$ in a Close Binary System (CBS) is followed from its main sequence until an ONe degenerate remnant forms. Restrictions have been made on the characteristics of the companion as well as on the initial orbital parameters in order to avoid the occurrence of reversal mass transfer before carbon is ignited in the core. The system undergoes three mass loss episodes. The first and second ones are a consequence of a case B Roche lobe overflow. During the third mass loss episode stellar winds may play a role comparable to, or even more important than Roche lobe overflow. In this paper, we extend the previously existing calculations of stars of intermediate mass belonging to close binary systems by following carefully the carbon burning phase of the primary component. We also propose different possible outcomes for our scenario and discuss the relevance of our findings. In particular, our main result is that the resulting white dwarf component of mass $1.1 M_\odot$ more likely has a core composed of oxygen and neon, surrounded by a mantle of carbon-oxygen rich material. The average abundances of the oxygen-neon rich core are $X({\rm O}^{16})=0.55$, $X({\rm Ne}^{20})=0.28$, $X({\rm Na}^{23})=0.06$ and $X({\rm Mg}^{24})=0.05$. This result has important consequences for the Accretion Induced Collapse scenario. The average abundances of the carbon-oxygen rich mantle are $X({\rm O}^{16})=0.55$, and $X({\rm C}^{12})=0.43$. The existence of this mantle could also play a significant role in our understanding of cataclysmic variables.Comment: 13 pages, 12 figures, accepted for publication in A&

Ferruccio Ritossa’s scientific legacy 50 years after his discovery of the heat shock response: a new view of biology, a new society, and a new journal

The pioneering discovery of the heat shock response by the Italian scientist Ferruccio Ritossa reached maturity this year, 2012. It was 50 years ago that Professor Ritossa, through an extraordinary combination of serendipity, curiosity, knowledge and inspiration, published the first observation that cells could mount very strong transcriptional activity when exposed to elevated temperatures, which was coined the heat shock response. This discovery led to the identification of heat shock proteins, which impact many areas of current biology and medicine, and has created a new avenue for more exciting discoveries. In recognition of the discovery of the heat shock response, Cell Stress Society International (CSSI) awarded Professor Ritossa with the CSSI medallion in October 2010 in Dozza, Italy. This article is based on a session of the Fifth CSSI Congress held in Québec commemorating Professor Ritossa and his discovery

On the origin of white dwarfs with carbon-dominated atmospheres: the case of H1504+65

We explore different evolutionary scenarios to explain the helium deficiency observed in H1504+65, the most massive known PG1159 star. We concentrate mainly on the possibility that this star could be the result of mass loss shortly after the born-again and during the subsequent evolution through the [WCL] stage. This possibility is sustained by recent observational evidence of extensive mass-loss events in Sakurai's object and is in line with the recent finding that such mass losses give rise to PG1159 models with thin helium-rich envelopes and large rates of period change, as demanded by the pulsating star PG1159-035. We compute the post born again evolution of massive sequences by taking into account different mass-loss rate histories. Our results show that stationary winds during the post-born-again evolution fail to remove completely the helium-rich envelope so as to explain the helium deficiency observed in H1504+65. Stationary winds during the Sakurai and [WCL] stages only remove at most half of the envelope surviving the violent hydrogen burning during the born-again phase. In view of our results, the recently suggested evolutionary connection born-again stars --> H1504+65 --> white dwarfs with carbon-rich atmospheres is difficult to sustain unless the whole helium-rich envelope could be ejected by non-stationary mass-loss episodes during the Sakurai stage.Comment: 5 pages, 2 figures. To be published in Astronomy & Astrophysic

On the formation of Super-AGB stars in intemediate mass close binary systems

The evolution of a star of initial mass 9 M_s, and Z = 0.02 in a Close Binary System is followed in the presence of different mass companions in order to study their influence on the final evolutionary stages and, in particular, on the structure and composition of the remnant components. We study two extreme cases. In the first one the mass of the secondary is 8 M_s, whereas in the second one the mass was assumed to be 1 M_s. For the first of those cases we have also explored the possible outcomes of both conservative and non-conservative mass-loss episodes. During the first mass transfer episode, several differences arise between the models. The system with the more extreme mass ratio is not able to survive the 1st. Roche lobe overflow, and spiral-in of the secondary onto the envelope of the primary is most likely. The system formed by two stars of comparable mass undergoes two mass transfer episodes in which the primary is the donor. We have performed two sets of calculations corresponding to this case in order to account for conservative and non-conservative mass transfer during the first mass loss episode. One of our main results is that for the non-conservative case the secondary becomes a Super-AGB. Such a star undergoes a final dredge-up episode, similar to that of a single star of comparable mass. The primary components do not undergo a Super-AGB phase, but instead a carbon-oxygen white dwarf is formed in both cases, before reversal mass transfer occurs. However, given the extreme mass ratios at this stage between the components of the binary system, the possibility of merger episodes remains likely. We also discuss the presumable final outcomes of the system and possible observational counterparts.Comment: 10 pages, 12figures, accepted for publication in A&

The gravitational collapse of ONe electron-degenerate cores and white dwarfs: the role of 24^{24}Mg and 12^{12}C revisited

The final stages of the evolution of electron--degenerate ONe cores, resulting from carbon burning in ``heavy weight'' intermediate--mass stars (8 M_{\sun}\la M \la 11 M_{\sun}) and growing in mass, either from carbon burning in a shell or from accretion of matter in a close binary system, are examined in the light of their detailed chemical composition. In particular, we have modelled the evolution taking into account the abundances of the following minor nuclear species, which result from the previous evolutionary history: $^{12}$C, $^{23}$Na, $^{24}$Mg, and $^{25}$Mg. Both $^{23}$Na and $^{25}$Mg give rise to Urca processes, which are found to be unimportant for the final outcome of the evolution. $^{24}$Mg was formerly considered a major component of ONe cores (hence called ONeMg cores), but updated evolutionary calculations in this mass range have severely reduced its abundance. Nevertheless, we have parameterized it and we have found that the minimum amount of $^{24}$Mg required to produce NeO burning at moderate densities is $\sim 23$, a value exceedingly high in the light of recent evolutionary models. Finally, we have determined that models with relatively small abundances of unburnt carbon ($X(^{12}$C)$\sim 0.015$) could be a channel to explosion at low to moderate density ($\sim 1\times 10^9$ g cm$^{-3}$). This is clearly below the current estimate for the explosion/collapse threshold and would have interesting consequences.Comment: 8 pages, 6 figures, to appear in Astronomy and Astrophysic

EC-SNe from super-AGB progenitors: theoretical models vs. observations

Using a parametric approach, we determine the configuration of super-AGB stars at the explosion as a function of the initial mass and metallicity, in order to verify if the EC-SN scenario involving a super-AGB star is compatible with the observations regarding SN2008ha and SN2008S. The results show that both the SNe can be explained in terms of EC-SNe from super-AGB progenitors having a different configuration at the collapse. The impact of these results on the interpretation of other sub-luminous SNe is also discussed.Comment: Accepted for publication in ApJ

Pulsations of massive ZZ Ceti stars with carbon/oxygen and oxygen/neon cores

We explore the adiabatic pulsational properties of massive white dwarf stars with hydrogen-rich envelopes and oxygen/neon and carbon/oxygen cores. To this end, we compute the cooling of massive white dwarf models for both core compositions taking into account the evolutionary history of the progenitor stars and the chemical evolution caused by time-dependent element diffusion. In particular, for the oxygen/neon models, we adopt the chemical profile resulting from repeated carbon-burning shell flashes expected in very massive white dwarf progenitors. For carbon/oxygen white dwarfs we consider the chemical profiles resulting from phase separation upon crystallization. For both compositions we also take into account the effects of crystallization on the oscillation eigenmodes. We find that the pulsational properties of oxygen/neon white dwarfs are notably different from those made of carbon/oxygen, thus making asteroseismological techniques a promising way to distinguish between both types of stars and, hence, to obtain valuable information about their progenitors.Comment: 11 pages, including 11 postscript figures. Accepted for publication in Astronomy and Astrophysic

Constraints on the origin of the massive, hot, and rapidly rotating magnetic white dwarf RE J 0317-853 from an HST parallax measurement

We use the parallax measurements of RE J 0317-853 to determine its mass, radius, and cooling age and thereby constrain its evolutionary origins. We observed RE J 0317-853 with the the Hubble Space Telescope's Fine Guidance System to measure the parallax of RE J 0317-853 and its binary companion, the non-magnetic white dwarf LB 9802. In addition, we acquired spectra of comparison stars with the Boller & Chivens spectrograph of the SMARTS telescope to correct the parallax zero point. For the corrected parallax, we determine the radius, mass, and the cooling age with the help of evolutionary models from the literature. The properties of RE J 0317-853 are constrained using the parallax information. We discuss the different cases of the core composition and the uncertain effective temperature. We confirm that RE J 0317-853 is close to the Chandrasekhar's mass limit in all cases and almost as old as its companion LB 9802. The precise evolutionary history of RE J 0317-853 depends on our knowledge of its effective temperature. It is possible that it had a single star progenitor possible if we assume that the effective temperature is at the cooler end of the possible range from 30000 to 50000; if T_eff is instead at the hotter end, a binary-merger scenario for RE J 0317-853 becomes more plausible.Comment: 11 pages, 6 figures; language revised edition with added references. Accepted for publication in Astronomy and Astrophysic

Mass-radius relations for massive white dwarf stars

We present detailed theoretical mass-radius relations for massive white dwarf stars with oxygen-neon cores. This work is motivated by recent observational evidence about the existence of white dwarf stars with very high surface gravities. Our results are based on evolutionary calculations that take into account the chemical composition expected from the evolutionary history of massive white dwarf progenitors. We present theoretical mass-radius relations for stellar mass values ranging from 1.06 to 1.30 Mo with a step of 0.02 Mo and effective temperatures from 150000 K to approx. 5,000 K. A novel aspect predicted by our calculations is that the mass-radius relation for the most massive white dwarfs exhibits a marked dependence on the neutrino luminosity. Extensive tabulations for massive white dwarfs, accessible from our web site, are presented as well.Comment: 7 pages, including 8 postscript figures. Accepted for publication in Astronomy and Astrophysic