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تسجيل مستخدم جديدWolf-Rayet stars and radioisotope production
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Radioisotopes are natural clocks which can be used to estimate the age of the solar system. They also influence the shape of supernova light curves. In addition, the diffuse emission at 1.8 MeV from the decay of 26Al may provide a measure of the present day nucleosynthetic activity in the Galaxy. Therefore, even if radionuclides represent only a tiny fraction of the cosmic matter, they carry a unique piece of information. A large number of radioisotopes are produced by massive stars at the time of their supernova explosion. A more or less substantial fraction of them are also synthesized during the previous hydrostatic burning phases. These nuclides are then ejected either at the time of the supernova event, or through stellar winds during their hydrostatic burning phases. This paper focusses of the non explosive ejection of radionuclides by non-rotating or rotating Wolf-Rayet stars.
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New models of rotating and non-rotating stars are computed for initial masses between 25 and 120 Msun and for metallicities Z = 0.004, 0.008, 0.020 and 0.040 with the aim of reexamining the wind contribution of Wolf-Rayet (WR) stars to the F19 enrich
ment of the interstellar medium. Models with an initial rotation velocity vini = 300 km/s are found to globally eject less F19 than the non-rotating models. We compare our new predictions with those of Meynet & Arnould (2000), and demonstrate that the F19 yields are very sensitive to the still uncertain F19(alpha,p)Ne22 rate and to the adopted mass loss rates. Using the recommended mass loss rate values that take into account the clumping of the WR wind and the NACRE reaction rates when available, we obtain WR F19 yields that are significantly lower than predicted by Meynet & Arnould (2000), and that would make WR stars non-important contributors to the galactic F19 budget. In view, however, of the large nuclear and mass loss rate uncertainties, we consider that the question of the WR contribution to the galactic F19 remains quite largely open.
Massive stars deeply influence their surroundings by their luminosity and the injection of kinetic energy. So far, they have mostly been studied with spatially unresolved observations, although evidence of geometrical complexity of their wind are num
erous. Interferometry can provide spatially resolved observations of massive stars and their immediate vicinity. Specific geometries (disks, jets, latitude-dependent winds) can be probed by this technique. The first observation of a Wolf-Rayet (WR) star (gamma^2 Vel) with the AMBER/VLTI instrument yielded to a re-evaluation of its distance and an improved characterization of the stellar components, from a very limited data-set. This motivated our team to increase the number of WR targets observed with AMBER. We present here new preliminary results that encompass several spectral types, ranging from early WN to evolved dusty WC. We present unpublished data on WR79a, a massive star probably at the boundary between the O and Wolf- Rayet type, evidencing some Wolf-Rayet broad emission lines from an optically thin wind. We also present new data obtained on gamma^2 Vel that can be compared to the up-to-date interferometry-based orbital parameters from North et al. (2007). We discuss the presence of a wind-wind collision zone in the system and provide preliminary analysis suggesting the presence of such a structure in the data. Then, we present data obtained on 2 dusty Wolf-Rayet stars: WR48a-b and WR118, the latter exhibiting some clues of a pinwheel-like structure from the visibility variations.
The Wolf-Rayet (WR) phenomenon is widespread in astronomy. It involves classical WRs, very massive stars (VMS), WR central stars of planetary nebula CSPN [WRs], and supernovae (SNe). But what is the root cause for a certain type of object to turn int
o an emission-line star? In this contribution, I discuss the basic aspects of radiation-driven winds that might reveal the ultimate difference between WR stars and canonical O-type stars. I discuss the aspects of (i) self-enrichment via CNO elements, (ii) high effective temperatures Teff, (iii) an increase in the helium abundance Y, and finally (iv) the Eddington factor Gamma. Over the last couple of years, we have made a breakthrough in our understanding of Gamma-dependent mass loss, which will have far-reaching consequences for the evolution and fate of the most massive stars in the Universe. Finally, I discuss the prospects for studies of the WR phenomenon in the highest redshift Ly-alpha and He II emitting galaxies.
Meynet and Arnould (1993) have suggested that Wolf-Rayet (WR) stars could significantly contaminate the Galaxy with 19F. In their scenario, 19F is synthesized at the beginning of the He-burning phase from the 14N left over by the previous CNO-burning
core, and is ejected in the interstellar medium when the star enters its WC phase. Recourse to CNO seeds makes the 19F yields metallicity-dependent. These yields are calculated on grounds of detailed stellar evolutionary sequences for an extended range of initial masses (from 25 to 120 Msol) and metallicities (Z = 0.008, 0.02 and 0.04). The adopted mass loss rate prescription enables to account for the observed variations of WR populations in different environments. The 19F abundance in the WR winds of 60 Msol model stars is found to be about 10 to 70 times higher than its initial value, depending on the metallicity. This prediction is used in conjunction with a very simple model for the chemical evolution of the Galaxy to predict that WR stars could be significant (dominant?) contributors to the solar system fluorine content. We also briefly discuss the implications of our model on the possible detection of fluorine at high redshift.
We present the results of an ongoing investigation to provide a detailed view of the processes by which massive stars shape the surrounding interstellar medium (ISM), from pc to kpc scales. In this paper we have focused on studying the environments o
f Wolf-Rayet (WR) stars in M31 to find evidence for WR wind-ISM interactions, through imaging ionized hydrogen nebulae surrounding these stars. We have conducted a systematic survey for HII shells surrounding 48 of the 49 known WR stars in M31. There are 17 WR stars surrounded by single shells, or shell fragments, 7 stars surrounded by concentric limb brightened shells, 20 stars where there is no clear physical association of the star with nearby H-alpha emission, and 4 stars which lack nearby H-alpha emission. For the 17+7 shells above, there are 12 which contain one or two massive stars (including a WR star) and that are <=40 pc in radius. These 12 shells may be classical WR ejecta or wind-blown shells. Further, there may be excess H-alpha point source emission associated with one of the 12 WR stars surrounded by putative ejecta or wind-blown shells. There is also evidence for excess point source emission associated with 11 other WR stars. The excess emission may arise from unresolved circumstellar shells, or within the extended outer envelopes of the stars themselves. In a few cases we find clear morphological evidence for WR shells interacting with each other. In several H-alpha images we see WR winds disrupting, or punching through, the walls of limb-brightened HII shells.
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