Amedeo Tornambe

Helium-burning evolutionary phases in population II stars. I Breathing pulses in horizontal branch stars

The result of an investigation into the evolutionary characteristics of a typical horizontal-branch (HB) model are presented. A new treatment of semiconvection has been used which overlaps Robertson and Faulkners prescription in the major phase of central He burning and which allows a meaningful treatment of the last phases of He exhaustion at the center. The occurrence of convective instabilities near the He exhaustion in the central core is confirmed, finding that three major convection pulses occur before the exhaustion of He. Consequences regarding HB lifetimes and post-HB evolution are briefly discussed. 19 references.

Carbon-Oxygen White Dwarfs Accreting CO-rich Matter. I. A Comparison between Rotating and Nonrotating Models

We investigate the lifting effect of rotation on the thermal evolution of CO white dwarfs accreting CO-rich matter. We find that rotation induces the cooling of the accreting structure so that the delivered gravitational energy causes a greater expansion with respect to the standard nonrotating case. The increase in the surface radius produces a decrease in the surface value of the critical angular velocity and, therefore, the accreting white dwarf becomes gravitationally unbound (Roche instability). This occurrence is due to an increase in the total angular momentum of the accreting white dwarf and depends critically on the amount of specific angular momentum deposited by the accreted matter. If the specific angular momentum of the accreted matter is equal to that of the outer layers of the accreting structure, the Roche instability occurs well before the accreting white dwarf can attain the physical conditions for carbon burning. If the values of both initial angular velocity and accretion rate are small, we find that the accreting white dwarf undergoes a secular instability when its total mass approaches 1.4 M☉. At this stage, the ratio between the rotational energy and the gravitational binding energy of the white dwarf becomes of the order of 0.1, so that the star must deform by adopting an elliptical shape. In this case, since the angular velocity of the white dwarf is as large as ~1 rad s-1, the anisotropic mass distribution induces the loss of rotational energy and angular momentum via gravitational wave radiation. We find that, independent of the braking efficiency, the white dwarf contracts and achieves the physical conditions suitable for explosive carbon burning at the center so that a Type Ia supernova event is produced.

Hydrogen-accreting Carbon-Oxygen White Dwarfs

Matter of solar system composition has been added to the surfaces of two initially cool carbon-oxygen (CO) white dwarfs of masses 0.5 M☉ and 0.8 M☉ at rates in the range 10-8 to 10-6 M☉ yr-1. Four different regimes are encountered. (1) At the highest accretion rates, models become red giants after the accretion of only a very small amount of mass. As the accretion rate is decreased, models are encountered that (2) burn hydrogen at the same rate at which it is accreted, (3) experience a series of nondynamical hydrogen shell flashes followed eventually by a powerful helium shell flash, and, finally, (4) experience nova-like hydrogen shell flashes. Although all of the regimes have been explored, special attention has been given to models that experience recurrent mild hydrogen-burning pulses or burn hydrogen at a stationary rate. For lower accretion rates, the helium flash is so powerful that the convective layer forced by helium burning penetrates deeply into the hydrogen-rich envelope; this penetration may lead to the ejection of external layers even if the helium flash would not of itself have become dynamical. For higher accretion rates, even when convection does not penetrate into hydrogen-rich layers, the helium layer expands, and much, if not most, of the accreted matter is lost during the event because of the interaction of the expanded envelope with the companion star. Analysis of the results suggests that it is unlikely that, in the real world, a hydrogen-accreting CO white dwarf with a typical initial mass will attain the Chandrasekhar mass. Dynamical helium-burning flashes are probable.

Dust formation around AGB and SAGB stars: a trend with metallicity?

We calculate the dust formed around asymptotic giant branch (AGB) and super-AGB stars of metallicity Z = 0.008 by following the evolution of models with masses in the range 1 M⊙ ≤ M ≤ 8 M⊙ through the thermal pulses phase, assuming that dust forms via condensation of molecules within a wind expanding isotropically from the stellar surface. We find that, because of the strong hot bottom burning (HBB) experienced, high-mass models produce silicates, whereas lower mass objects are predicted to be surrounded by carbonaceous grains; the transition between the two regimes occurs at a threshold mass of 3.5 M⊙. These findings are consistent with the results presented in a previous investigation, for Z = 0.001. However, in the present higher metallicity case, the production of silicates in the more massive stars continues for the whole AGB phase, because the HBB experienced is softer at Z = 0.008 than at Z = 0.001; thus, the oxygen in the envelope, essential for the formation of water molecules, is never consumed completely. The total amount of dust formed for a given mass experiencing HBB increases with metallicity, because of the higher abundance of silicon, and the softer HBB, both factors favouring a higher rate of silicates production. This behaviour is not found in low-mass stars, because the carbon enrichment of the stellar surface layers, due to repeated third dredge-up episodes, is almost independent of the metallicity. Regarding cosmic dust enrichment by intermediate-mass stars, we find that the cosmic yield at Z = 0.008 is a factor of ∼5 larger than at Z = 0.001. In the lower metallicity case carbon dust dominates after ∼300 Myr, but at Z = 0.008 the dust mass is dominated by silicates at all times, with a prompt enrichment occurring after ∼40 Myr, associated with the evolution of stars with masses M ∼ 7.5–8 M⊙. These conclusions are partly dependent on the assumptions concerning the two important macrophysics inputs needed to describe the AGB phase, and still unknown from first principles: the treatment of convection, which determines the extent of the HBB experienced and of the third dredge-up following each thermal pulse, and mass-loss, essential in fixing the time-scale on which the stellar envelope is lost from the star.

Optical and Infrared Observations of the Supernova SN 1999el

Optical and near-infrared light curves of the Type IIn supernova SN 1999el in NGC 6951 are presented. A period of 220 days (416 days in the near-infrared) is covered from the first observation obtained a few days before maximum light. Spectroscopic observations are also discussed. Using as a distance calibrator the Type Ia SN 2000E, which occurred some months later in the same galaxy, and fitting a blackbody law to the photometric data, we obtain a maximum bolometric luminosity for SN 1999el of ~1044 ergs s-1. In general, the photometric properties of SN 1999el are very similar to those of SN 1998S, a bright and well-studied Type IIn supernova, showing a fast decline in all observed bands similar to those of Type II-L supernovae. The differences with SN 1998S are analyzed and ascribed to the differences in a preexisting circumstellar envelope in which dust was already present at the moment of the SN outburst. We infer that light echoes may play a possibly significant role in affecting the observed properties of the light curves, although improved theoretical models are needed to account for the data. We conclude that mass loss in the progenitor RG stars is episodic and occurs in an asymmetric way. This implies that collapsing massive stars appear as normal Type II supernovae if this occurs far from major mass-loss episodes, whereas they appear as Type IIn supernovae if a large mass-loss episode is in progress.

The 'Red Giant Clock' as an indicator for the efficiency of central mixing in horizontal-branch stars

New evolutionary evaluations have been compared with observational data for the globular cluster M5 in order to assess the kind of mixing (semiconvection with and without the inclusion of the breathing pulses or overshooting) which takes place in real stars during the central He-burning phase. It is suggested that the evolutionary time scale on the red giant branch should be used to assess the absolute time scale of evolution on the asymptotic giant branch. On this basis, it is found that ordinary semiconvection without breathing pulses leads to the best match to the observed data for M5. 18 refs.

Evidence for Type Ia Supernova Diversity from Ultraviolet Observations with the Hubble Space Telescope

We present ultraviolet (UV) spectroscopy and photometry of four Type Ia supernovae (SNe 2004dt, 2004ef, 2005M, and 2005cf) obtained with the UV prism of the Advanced Camera for Surveys on the Hubble Space Telescope. This data set provides unique spectral time series down to 2000 A. Significant diversity is seen in the near-maximum-light spectra (~2000-3500 A) for this small sample. The corresponding photometric data, together with archival data from Swift Ultraviolet/Optical Telescope observations, provide further evidence of increased dispersion in the UV emission with respect to the optical. The peak luminosities measured in the uvw1/F250W filter are found to correlate with the B-band light-curve shape parameter Δm 15(B), but with much larger scatter relative to the correlation in the broadband B band (e.g., ~0.4 mag versus ~0.2 mag for those with 0.8 mag 3σ), being brighter than normal SNe Ia such as SN 2005cf by ~0.9 mag and ~2.0 mag in the uvw1/F250W and uvm2/F220W filters, respectively. We show that different progenitor metallicity or line-expansion velocities alone cannot explain such a large discrepancy. Viewing-angle effects, such as due to an asymmetric explosion, may have a significant influence on the flux emitted in the UV region. Detailed modeling is needed to disentangle and quantify the above effects.

He-accreting white dwarfs: accretion regimes and final outcomes

The behaviour of carbon-oxygen white dwarfs (WDs) subject to direct helium accretion is extensively studied. We aim to analyze the thermal response of the accreting WD to mass deposition at different time scales. The analysis has been performed for initial WDs masses and accretion rates in the range (0.60 - 1.02) Msun and 1.e-9 - 1.e-5 Msun/yr, respectively. Thermal regimes in the parameters space M_{WD} - dot{M}_{He}, leading to formation of red-giant-like structure, steady burning of He, mild, strong and dynamical flashes have been identified and the transition between those regimes has been studied in detail. In particular, the physical properties of WDs experiencing the He-flash accretion regime have been investigated in order to determine the mass retention efficiency as a function of the accretor total mass and accretion rate. We also discuss to what extent the building-up of a He-rich layer via H-burning could be described according to the behaviour of models accreting He-rich matter directly. Polynomial fits to the obtained results are provided for use in binary population synthesis computations. Several applications for close binary systems with He-rich donors and CO WD accretors are considered and the relevance of the results for the interpretation of He-novae is discussed.

On the Formation of Massive C-O White Dwarfs: the Lifting Effect of Rotation

The effect of stellar rotation on the late evolution of intermediate-mass stars has been explored, employing very simple numerical methods. At the epoch of central He exhaustion and C-O core formation, even an initially small rotation may induce, for the first time, a nonnegligible effect on the evolutionary outcome, as a consequence of the huge contraction of the core radius that occurs at this stage. The role of various hydrodynamic instabilities is discussed, verifying the consistency of our approach. The most important characteristic of the rotating models is the slow increase in temperature in the region where He burning occurs. As a consequence, the elapsed time before the onset of the second dredge-up is greater than that found for nonrotating models, and the C-O core mass is markedly increased during the early asymptotic giant branch (AGB) phase. In such a way, massive C-O white dwarfs, near the Chandrasekhar limiting mass for nonrotating stars, would be formed after envelope ejection during the thermal pulse phase. The properties of early AGB and thermally pulsing models obtained by including rotation are compared to those of nonrotating models.

On the Formation of O-Ne White Dwarfs in Metal-rich Close Binary Systems

The evolution of primary components of initially close binary systems has been followed assuming specific initial conditions such as to have the formation of an O-Ne white dwarf at the end of a nonconservative evolution of the system parameters. More specifically, the evolution of a 10.5 M ○. , Z=Z ○. , and Y=0.27 star model has been traced through the two major episodes of mass transfer in order to make a direct comparison with the previous computations of Iben & Tutukov, which we reproduce quite well. In this case, mass transfer finishes at the beginning of the C-burning phase. Analogous experiments have been performed with both a 10 M ○. and a 10.5 M ○. (Z=2×Z ○. and Y=0.32) star models. In these cases, the second mass-transfer episode continues also during the carbon-burning phase