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Recent observations of volume-limited samples of magnetic white dwarfs (WD) have revealed a higher incidence of magnetism in older WDs. Specifically, these studies indicate that magnetism is more prevalent in WDs with fully or partially crystallized cores compared to those with entirely liquid cores. This has led to the recognition of a crystallization-driven dynamo as an important mechanism for explaining magnetism in isolated WDs. However, recent simulations challenged the capability of this mechanism to match both the incidence of magnetism and the field strengths detected in WDs. In this letter, we explore an alternative hypothesis for the surface emergence of magnetic fields in isolated WDs. WDs with masses $\gtrsim 0.55 M_\odot$ are the descendants of main-sequence stars with convective cores capable of generating strong dynamo magnetic fields. This idea is supported by asteroseismic evidence of strong magnetic fields buried within the interiors of red giant branch stars. Assuming that these fields are disrupted by subsequent convective zones, we have estimated magnetic breakout times for WDs. Due to the significant uncertainties in breakout times stemming from the treatment of convective boundaries and mass loss rates, we cannot provide a precise prediction for the emergence time of the main-sequence dynamo field. However, we can predict that this emergence should occur during the WD phase for WDs with masses $\gtrsim 0.65 M_\odot$. We also find that the magnetic breakout is expected to occur earlier in more massive WDs, consistently with observations from volume-limited samples and the well-established fact that magnetic WDs tend to be more massive than non-magnetic ones. Moreover, within the uncertainties of stellar evolutionary models, we find that the emergence of main-sequence dynamo magnetic fields can account for a significant portion of the magnetic WDs., Comment: accepted for publication in A&A letters

In this work, we study whether the engulfment of a brown dwarf (BD) by a solar-like main-sequence (MS) star can significantly alter the structure of the star and the Li content on its surface. We perform 3D Smoothed Particle Hydrodynamics simulations of the engulfment of a BD with masses 0.01 and 0.019 Msun, onto an MS star of 1 Msun and solar composition, in three different scenarios: a head-on collision, a grazing collision, and a merger. We study the dynamics of the interaction in detail, and the relevance of the type of interaction and the mass of the BD on the final fate of the sub-stellar object and the host star in terms of mass loss of the system, angular momentum transfer, and changes in the Li abundance in the surface of the host star. We found that most of the BD mass is diluted in the denser region of the MS star. Only in the merger scenario a significant fraction (40%) of the BD material would remain in the outer layers. We find a clear increase in the surface rotational velocity of the host star after the interaction, ranging between 25 km/s (grazing collision) to 50 km/s (merger). We also find a significant mass loss from the system (1e-4 - 1e-3 Msun) due to the engulfment, which in the case of the merger, may form a circumstellar disk-like structure. Assuming that neither the depth of the convective envelope of the host star nor its mass content are modified during the interaction, a small change in the surface Li abundance in the head-on and grazing collisions is found. However, in the merger we find large Li enhancements, by factors 20-30, depending on the BD mass. Some of these features could be detected observationally in the host star provide they remain long enough time., Comment: 18 pages, 16 figures, accepted for publication in Astronomy & Astrophysics

The precise progenitor system of type Ia supernovae (SNe Ia), whether it is a white dwarf (WD) close to the Chandrasekhar limit or substantially less massive, has been a matter of debate for decades. Recent research by our group on the accretion and simmering phases preceding the explosion of a massive WD has shown that the central density at thermal runaway lies in the range $(3.6-6.3)\times10^9$ g cm$^{-3}$ for reasonable choices of accretion rate onto the WD and progenitor metallicity. In this work, we have computed one-dimensional simulations of the explosion of such WDs, with special emphasis on the chemical composition of the ejecta, which in all cases is extremely rich in neutronized isotopes of chromium ($^{54}$Cr) and titanium ($^{50}$Ti). We show that, in order to reconcile such a nucleosynthesis with the isotopic abundances of the Solar System, Chandrasekhar-mass white dwarfs can account for at most 26 per cent of normal-luminosity SNe Ia, or at most 20 per cent of all SNe Ia., Comment: 5 pages, 1 table, 3 figures; Letter accepted by MNRAS

We revise the theoretical initial mass-final luminosity relation for progenitors of type IIP and IIL supernovae. The effects of the major uncertainties, as those due to the treatment of convection, semiconvection, rotation, mass loss, nuclear reaction rates and neutrinos production rates are discussed in some details. The effects of mass transfer between components of close-binary systems are also considered. By comparing the theoretical predictions to a sample of type II supernovae for which the initial mass of the progenitors and the pre-explosive luminosity are available, we conclude that stellar rotation may explain a few progenitors which appear brighter than expected in case of non-rotating models. In the most extreme case, SN2012ec, an initial rotational velocity up to 300 km s$^{-1}$ is required. Alternatively, these objects could be mass-loosing components of close binaries. However, most of the observed progenitors appear fainter than expected. This occurrence seems to indicate that the Compton and pair neutrino energy-loss rates, as predicted by the standard electro-weak theory, are not efficient enough and that an additional negative contribution to the stellar energy balance is required. We show that axions coupled with parameters accessible to currently planned experiments, such as IAXO and, possibly, Baby-IAXO and ALPS II, may account for the missing contribution to the stellar energy-loss., Comment: paper accepted to be publiished on ApJ

Gamma-ray astronomy explores the most energetic photons in nature to address some of the most pressing puzzles in contemporary astrophysics. It encompasses a wide range of objects and phenomena: stars, supernovae, novae, neutron stars, stellar-mass black holes, nucleosynthesis, the interstellar medium, cosmic rays and relativistic-particle acceleration, and the evolution of galaxies. MeV gamma-rays provide a unique probe of nuclear processes in astronomy, directly measuring radioactive decay, nuclear de-excitation, and positron annihilation. The substantial information carried by gamma-ray photons allows us to see deeper into these objects, the bulk of the power is often emitted at gamma-ray energies, and radioactivity provides a natural physical clock that adds unique information. New science will be driven by time-domain population studies at gamma-ray energies. This science is enabled by next-generation gamma-ray instruments with one to two orders of magnitude better sensitivity, larger sky coverage, and faster cadence than all previous gamma-ray instruments. This transformative capability permits: (a) the accurate identification of the gamma-ray emitting objects and correlations with observations taken at other wavelengths and with other messengers; (b) construction of new gamma-ray maps of the Milky Way and other nearby galaxies where extended regions are distinguished from point sources; and (c) considerable serendipitous science of scarce events -- nearby neutron star mergers, for example. Advances in technology push the performance of new gamma-ray instruments to address a wide set of astrophysical questions., Comment: 14 pages including 3 figures

In this study we include axions in stellar evolution models adopting the current stringest constraints for their coupling to photons and electrons. We obtain that the minimum stellar mass of Core Collapse Supernova (CCSN) progenitors is shifted up by nearly 2 Mo. This result seems to be in tension with the observationaly derived minimum mass of CCSN progenitors., Comment: Contributed to the 13th Patras Workshop on Axions, WIMPs and WISPs, Thessaloniki, May 15 to 19, 2017

Previous determinations of the oxygen isotopic ratios in AGB carbon stars were at odds with the existing theoretical predictions. We aim to redetermine the oxygen ratios in these stars using new spectral analysis tools and further develop discussions on the carbon and nitrogen isotopic ratios in order to elucidate this problem. Oxygen isotopic ratios were derived from spectra in the K-band in a sample of galactic AGB carbon stars of different spectral types and near solar metallicity. Synthetic spectra calculated in LTE with spherical carbon-rich atmosphere models and updated molecular line lists were used. The CNO isotope ratios derived in a homogeneous way, were compared with theoretical predictions for low-mass (1.5-3 M_o) AGB stars computed with the FUNS code assuming extra mixing both during the RGB and AGB phases. For most of the stars the 16O/17O/18O ratios derived are in good agreement with theoretical predictions confirming that, for AGB stars, are established using the values reached after the FDU according to the initial stellar mass. This fact, as far as the oxygen isotopic ratios are concerned, leaves little space for the operation of any extra mixing mechanism during the AGB phase. Nevertheless, for a few stars with large 16O/17O/18O, the operation of such a mechanism might be required, although their observed 12C/13C and 14N/15N ratios would be difficult to reconcile within this scenario. Furthermore, J-type stars tend to have lower 16O/17O ratios than the normal carbon stars, as already indicated in previous studies. Excluding these peculiar stars, AGB carbon stars occupy the same region as pre-solar type I oxide grains in a 17O/16O vs. 18O/16O diagram, showing little spread. This reinforces the idea that these grains were probably formed in low-mass stars during the previous O-rich phases., Comment: Accepted fo publication in A&A

The metallicity of the progenitor system producing a type Ia supernova (SN Ia) could play a role in its maximum luminosity, as suggested by theoretical predictions. We present an observational study to investigate if such a relationship there exists. Using the 4.2m WHT we have obtained intermediate-resolution spectroscopy data of a sample of 28 local galaxies hosting SNe Ia, for which distances have been derived using methods independent to those based on the own SN Ia parameters. From the emission lines observed in their optical spectrum, we derived the gas-phase oxygen abundance in the region where each SN Ia exploded. Our data show a trend, with a 80% of chance not to be due to random fluctuation, between SNe Ia absolute magnitudes and the oxygen abundances of the host galaxies, in the sense that luminosities tend to be higher for galaxies with lower metallicities. This result seems like to be in agreement with both the theoretically expected behavior, and with other observational results. This dependence $M_{B}$-Z might induce to systematic errors when is not considered in deriving SNe Ia luminosities and then using them to derive cosmological distances., Comment: 9 pages, 4 figures, accepted for publication in ApJ Letters

We derive a strong bound on the axion-photon coupling $g_{a\gamma}$ from the analysis of a sample of 39 Galactic Globular Clusters. As recognized long ago, the R parameter, i.e. the number ratio of stars in horizontal over red giant branch of old stellar clusters, would be reduced by the axion production from photon conversions occurring in stellar cores. In this regard we have compared the measured R with state-of-the-art stellar models obtained under different assumptions for $g_{a\gamma}$. We show that the estimated value of $g_{a\gamma}$ substantially depends on the adopted He mass fraction Y, an effect often neglected in previous investigations. Taking as benchmark for our study the most recent determinations of the He abundance in H II regions with O/H in the same range of the Galactic Globular Clusters, we obtain an upper bound $g_{a\gamma}<0.66\times 10^{-10}$ GeV$^{-1}$ at 95$\%$ confidence level. This result significantly improves the constraints from previous analyses and is currently the strongest limit on the axion-photon coupling in a wide mass range., Comment: (v2: 6 pages, 1 eps figure. Revised version. Discussion enlarged and references added. To appear on Physical Review Letters.)

By using updated stellar low mass stars models, we can systematically investigate the nucleosynthesis processes occurring in AGB stars, when these objects experience recurrent thermal pulses and third dredge-up episodes. In this paper we present the database dedicated to the nucleosynthesis of AGB stars: the FRUITY (FRANEC Repository of Updated Isotopic Tables & Yields) database. An interactive web-based interface allows users to freely download the full (from H to Bi) isotopic composition, as it changes after each third dredge-up episode and the stellar yields the models produce. A first set of AGB models, having masses in the range 1.5 < M/Msun < 3.0 and metallicities 1e-3 < Z < 2e-2, is discussed here. For each model, a detailed description of the physical and the chemical evolution is provided. In particular, we illustrate the details of the s-process and we evaluate the theoretical uncertainties due to the parametrization adopted to model convection and mass loss. The resulting nucleosynthesis scenario is checked by comparing the theoretical [hs/ls] and [Pb/hs] ratios to those obtained from the available abundance analysis of s-enhanced stars. On the average, the variation with the metallicity of these spectroscopic indexes is well reproduced by theoretical models, although the predicted spread at a given metallicity is substantially smaller than the observed one. Possible explanations for such a difference are briefly discussed. An independent check of the third dredge-up efficiency is provided by the C-stars luminosity function. Consequently, theoretical C-stars luminosity functions for the Galactic disk and the Magellanic Clouds have been derived. We generally find a good agreement with observations., Comment: Accepted for Publication on The Astrophysical Journal Supplements

Incomplete carbon-oxygen detonation with reactions terminating after burning of 12C in the leading 12C + 12C reaction (C-detonation) may occur in the low density outer layers of white dwarfs exploding as Type Ia supernovae (SNIa). Previous studies of carbon-oxygen detonation structure and stability at low densities were performed under the assumption that the "velocity" of a detonation wave derives from complete burning of carbon and oxygen to iron. In fact, at densities <10^6 g/cm3 the detonation in SNIa may release less than a half of the available nuclear energy. In this paper we study basic properties of such detonations. We find that the length of an unsupported steady-state C-detonation is 30-100 times greater than previously estimated, and that the decreased energy has a drastic effect on the detonation stability. In contrast to complete detonations which are one-dimensionally stable, C-detonations may be one-dimensionally unstable and propagate by periodically re-igniting themselves via spontaneous burning. The re-ignition period at rho < 10^6 g/cm3 is estimated to be greater than the time-scale of a SNIa explosion. This suggests that propagation and quenching of C-detonations at these densities could be affected by the instability. Potential observational implications of this effect are discussed., Comment: 36 pages, 29 figures, accepted for publication in ApJ

We have analyzed the influence of the stellar populations, from which SN progenitors come from, on the observational outcome, including the metal free Pop. III. We use our models to study the evolution of the progenitor, the subsequent explosion and the light curves. For Type Ia, the variation of the main sequence mass of the progenitor of the exploding WD, produces an offset in the maximum-decline relation of 0.2 mag. This effect is critical for the use of high redshift Type Ia SNe as cosmological standard candles. In contrast, the metallicity does not change the above relation (at maximum, Delta M_V < 0.06 mag). For Type II, we find a dependence of the light curve properties with both, main sequence mass and metallicity of the progenitor, and we identify a rather homogeneous subclass, {\it Extreme} II-P, that may be used as a quasi-standard candle. Note that, although not as good as Type Ia for distance determinations, Type II are expected to occur since the first stars were formed., Comment: 5 pages, to appear in proceedings of IAU Colloquium 192, "Supernovae (10 years of 1993J)", Valencia, Spain 22-26 April 2003, eds. J.M. Marcaide, K.W. Weiler, Springer Verlag

We have extended our published set of low mass AGB stellar models to lower metallicity. Different mass loss rates have been explored. Interpolation formulae for luminosity, effective temperature, core mass, mass of dredge up material and maximum temperature in the convective zone generated by thermal pulses are provided. Finally, we discuss the modifications of these quantities as obtained when an appropriate treatment of the inward propagation of the convective instability, caused by the steep rise of the radiative opacity occurring when the convective envelope penetrates the H-depleted region, is taken into account., Comment: 11 pages, 4 figures, accepted for publication in Publications of the Astronomical Society of Australia (PASA)

Pulsating white dwarfs provide constraints to the evolution of progenitor stars. We revise He-burning stellar models, with particular attention to core convection and to its connection with the nuclear reactions powering energy generation and chemical evolution Theoretical results are compared to the available measurements for the variable white dwarf GD 358, which indicate a rather large abundance of central oxygen. We show that the attempt to constrain the relevant nuclear reaction rate by means of the white dwarf composition is faced with a large degree of uncertainty related to evaluating the efficiency of convection-induced mixing.By combining the uncertainty of the convection theory with the error on the relevant reaction rate we derive that the present theoretical prediction for the central oxygen mass fraction in white dwarfs varies between 0.3 and 0.9. Unlike previous claims, we find that models taking into account semiconvection and a moderate C12(alpha,gamma)O16 reaction rate are able to account for a high central oxygen abundance. The rate of the C12(alpha,gamma)O16 used in these models agrees with the one recently obtained in laboratory experiments (Kunz et al. 2002). On the other hand, when semiconvection is inhibited, as in the case of classical models (bare Schwarzschild criterion) or in models with mechanical overshoot, an extremely high rate of the C12(\alpha,\gamma)O16 reaction is needed to account for a large oxygen production. Finally, we show that the apparent discrepancy between our result and those reported in previous studies depends on the method used to avoid the convective runaways (the so called breathing pulses), which are usually encountered in modeling late stage of core He-burning phase., Comment: 19 pages, 4 figures, accepted for publication in The Astrophysical Journal

New stellar models with mass ranging between 4 and 8 Mo, Z=0 and Y=0.23 are presented. The models have been evolved from the pre Main Sequence up to the Asymptotic Giant Branch (AGB). At variance with previous claims, we find that these updated stellar models do experience thermal pulses in the AGB phase. In particular we show that: a) in models with mass larger than 6 Mo, the second dredge up is able to raise the CNO abundance in the envelope enough to allow a "normal" AGB evolution, in the sense that the thermal pulses and the third dredge up settle on; b) in models of lower mass, the efficiency of the CNO cycle in the H-burning shell is controlled by the carbon produced locally via the 3alpha reactions. Nevertheless the He-burning shell becomes thermally unstable after the early AGB. The expansion of the overlying layers induced by these weak He-shell flashes is not sufficient by itself to allow a deep penetration of the convective envelope. However, immediately after that, the maximum luminosity of the He flash is attained and a convective shell systematically forms at the base of the H-rich envelope. The innermost part of this convective shell probably overlaps the underlying C-rich region left by the inter-shell convection during the thermal pulse, so that fresh carbon is dredged up in a "hot" H-rich environment and a H flash occurs. This flash favours the expansion of the outermost layers already started by the weak thermal pulse and a deeper penetration of the convective envelope takes place. Then, the carbon abundance in the envelope rises to a level high enough that the further evolution of these models closely resembles that of more metal rich AGB stars. These stars provide an important source of primary carbon and nitrogen., Comment: 28 pages, 5 tables and 17 figures. Accepted for publication in ApJ

At this moment, the use of SNIa for cosmology lies on the assumption that the SNe at high redshifts are equal to the local ones. However, some observations indicate a correlation between the light curve (LC) properties and the morphological type of the host galaxy. This could indicate a dependence with the age (mass/composition) of the underlying population. In this work we have chosen the delayed detonation explosion model in CO Chandrasekhar mass WDs to explore the dependence of the SNIa LC and nucleosynthesis with the initial mass and composition of the WD progenitor. The progenitor influences the final SNIa via the mass of the CO core formed and the C/O ratio within it (1D explosion models). We have followed the evolution of stars with masses between 1.5 and 8 Mo and metallicity, Z=0, 1.E-05, 0.001 and 0.02, from the pre-main sequence to the TP-AGB phase. The differences obtained in the final C/O ratio within the explosive WD are smaller than 22%. This results in a difference at maximum of 0.03 mag and of 0.1 mag when the brightness-decline relation is applied., Comment: 4 pages, 1 figure, needs espcrc1.sty; conference "Nuclei in the Cosmos 2000", held in Arhus, Denmark, June 27-July 1, 2000; submitted to Nucl. Phys. A

We present theoretical evolutionary models for Z=0 stars in the mass range 4< M/Mo <8 from the pre-main sequence up to the AGB phase. Contrary to previous calculations we found that these stars develop normal thermal pulses and third dredge-up episodes. Special attention is devoted to analyze the chemical enrichment in the envelope due to the above mechanisms. As a consequence, we show that these stars become carbon and nitrogen rich. Using different IMFs proposed in the literature for the Population III stars, we study their contribution to the pre-galactic chemical enrichment. It is found that Z=0 AGB stars could significantly contribute to 7Li, 12C and 14N and produce extreme non-solar 24Mg/25Mg/26Mg ratios. However, the net contribution is very sensitive to the IMF adopted and to the fraction of primordial matter which goes into stars., Comment: 3 pages, needs espcrc1.sty; conference "Nuclei in the Cosmos 2000", held in Arhus, Denmark, June 27-July 1, 2000; submitted to Nucl. Phys. A

A new set of stellar models in the mass range 1.2 to 9 $M_{\odot}$ is presented. The adopted chemical compositions cover the typical galactic values, namely $0.0001 \le Z \le 0.02$ and $0.23 \le Y \le 0.28$. A comparison among the most recent compilations of similar stellar models is also discussed. The main conclusion is that the differencies among the various evolutionary results are still rather large. For example, we found that the H-burning evolutionary time may differ up to 20 %. An even larger disagreement is found for the He-burning phase (up to 40-50 %). Since the connection between the various input physics and the numerical algorithms could amplify or counterbalance the effect of a single ingredient on the resulting stellar model, the origin of this discrepancies is not evident. However most of these discrepancies, which are clearly found in the evolutionary tracks, are reduced on the isochrones. By means of our updated models we show that the ages inferred by the theory of stellar evolution is in excellent agreement with those obtained by using other independent methods applied to the nearby Open Clusters. Finally, the theoretical initial/final mass relation is revised., Comment: 35 pages, 24 figures, 4 tables, accepted for publication in the Astrophisycal Journal

Due to their high luminosity at maximum and degree of homogeneity, Type Ia supernovae have been extensively used for cosmological purpouses, in particular to estimate extragalactic distances and the Hubble constant. Recently the number of Type Ia supernovae detected at high redshift has increased, opening the possibility of determining the mass density parameter, the cosmological constant and the deceleration parameter. The observed supernovae appear to be further than expected -even for an empty Universe-, implying a low density Universe and moreover an accelerating Universe. Among the various uncertainties, we address the possibility that old supernovae are not equal to current supernovae. From first principles, an evolution of progenitors with time is expected. Additionally, some observations show a dependence of the observed properties on galaxy type and colour. Our aim in this work is to study the outcome of exploding CO white dwarfs following the evolution of the progenitor intermediate mass stars with different masses and metallicities. Once this influence of the progentitor has been determined, the observations may be corrected. At the present stage of this project we are not able to quantify this effect properly. One result is clear, that the differences at maximum are expected to be small ($\sim$0.2 mag) but this is of the same order as all the evidence for a positive cosmological constant ($\sim$0.25 mag)., Comment: 12 pages, 1 figure, 2 tables (memsait.sty, epsf.sty) in Future Directions of Supernovae Research: Progenitors to Remnants. Assergi (Italy) Sep-Oct 1998

We show that the inclusion of axion emission during stellar evolution introduces important changes into the evolutionary behaviour of AGB stars. The mass of the resulting C/O white dwarf is much lower than the equivalent obtained from standard evolution. This implies a deficit in luminous AGB stars and in massive WDs. Moreover the total mass processed in the nuclear burning shells that is dredged-up to the surface ($3^{rd} D_{up}$) increases when axion emission is included, modifying the chemical composition of the photosphere. We conclude that the AGB is a promising phase to put constraints on particle physics, Comment: 8 pages, 3 tables, 8 figures, accepted for publication in MNRAS

The influence of the initial composition and structure of the exploding white dwarf on the nucleosynthesis and structure of Type Ia Supernovae has been studied. The progenitor structures are based on detailed stellar evolutionary tracks for stars in the mass range between 1 to 9 $M_\odot$ using the state of the art code FRANEC. The calculations of the thermonuclear explosions are based on a set of delayed detonation models which give a good account of the optical and infrared light curves and of the spectral evolution. Our code solves the hydrodynamical equations explicitly by the piecewise parabolic method. Nuclear burning is taken into account using an extended network of 218 nuclei. In principle, our calculations allow the observed spectra and light curve to be linked to the progenitor. Moreover, our study is relevant to estimate potential evolution in the progenitor population at cosmological time scales., Comment: 4 pages, 2 figures. Contribution in the Proceedings of "Nuclei in the Cosmos V" Volos (Greece)