arXiv subjects

Raphael Hirschi

Publications and source records attributed to Raphael Hirschi.

101 records · Page 6Linked to original sources

arXiv2006

Very low metallicity massive star models: Pre-SN evolution and primary nitrogen production

Two series of models were computed. The first series consists of 20 solar mass models with varying initial metallicity (Z=0.02 down to Z=10^{-8}) and rotation (V_{ini}=0-600 km/s). The second one consists of models with an initial metallicity of Z=10^{-8}, masses between 9 and 85 solar masses and fast initial rotation velocities (V_{ini}=600-800 km/s). The most interesting models are the models with Z=10^{-8} ([Fe/H]~-6.6). In the course of helium burning, carbon and oxygen are mixed into the hydrogen burning shell. This boosts the importance of the shell and causes a reduction of the CO core mass. Later in the evolution, the hydrogen shell deepens and produces large amount of primary nitrogen. For the most massive models (M>~60 solar masses), significant mass loss occurs during the red supergiant stage. This mass loss is due to the surface enrichment in CNO elements via rotational and convective mixing. The 85 solar mass model ends up as a WO type Wolf-Rayet star. Therefore the models predict SNe of type Ic and possibly long and soft GRBs at very low metallicities. The rotating 20 solar mass models can best reproduce the observed CNO abundances at the surface of extremely metal poor (EMP) stars and the metallicity trends when their angular momentum content is the same as at solar metallicity (and therefore have an increasing surface velocity with decreasing metallicity). The wind of the massive star models can also reproduce the CNO abundances of the most metal-poor carbon-rich star known to date, HE1327-2326.

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arXiv2006

The evolution of massive stars in the context of V838 Monocerotis

The aim of this paper is to look at the evolution of massive stars in order to determine whether or not the progenitor of V838 Mon may be a massive star. In the first part of this paper, the evolution of massive stars around solar metallicity is described, especially the evolution in the Hertzsprung-Russell (HR) diagram. Then, using the observational constraints, the probable progenitors (and their evolution) are described. Using models of single stars, no progenitor can be found amongst massive stars that can satisfy all the observational constraints. Wolf-Rayet stars (stars with initial masses above about 30 solar masses, which have lost their hydrogen rich envelopes) could explain 10 to 100 solar masses of circumstellar material but they are very luminous (L > 100,000 solar luminosities). Main sequence stars crossing the HR diagram and becoming red supergiants (RSG) can have very low effective temperatures but take thousands of years to cross over. Be stars (fast rotating stars with a mass around 10 solar masses), which form disk or B stars accreting matter from a binary companion of a similar mass would need to be compared in detail with the observational constraints. In the future, there will hopefully be further observational constraints on the models coming from the mass and nature (interstellar or circumstellar) of the material producing the light echo and from a frequency estimate of spectacular objects such as V838 Mon.

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arXiv2006

PopII 1/2 stars: very high N14 and low O16 yields

Nine 20 Mo models were computed with metallicities ranging from solar, through $Z=10^{-5}$ ([Fe/H]=~-3.1) down to $Z=10^{-8}$ ([Fe/H]=~-6.1) and with initial rotational velocities between 0 and 600 km/s to study the impact of initial metallicity and rotational velocity. The very large amounts of N14 observed (~0.03 Mo) are only produced at $Z=10^{-8}$ (PopII 1/2). The strong dependence of the N14 yields on rotation and other parameters like the initial mass and metallicity may explain the large scatter in the observations of N14 abundance. The metallicity trends are best reproduced by the models with Omega_ini/Omega_c=~0.75, which is slightly above the mean observed value for OB solar metallicity stars. Indeed, in the model with Vini=600 km/s at $Z=10^{-8}$, the O16 yield is reduced due to strong mixing. This allows in particular to reproduce the upturn for C/O and a slightly decreasing [C/Fe], which are observed below [Fe/H]=~-3.

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arXiv2006

Rotating massive stars @ very low Z: high C and N production

Two series of models and their yields are presented in this paper. The first series consists of 20 Mo models with varying initial metallicity (solar down to $Z=10^{-8}$) and rotation (Vini=0-600 km/s). The second one consists of models with an initial metallicity of $Z=10^{-8}$, masses between 20 and 85 Mo and average rotation velocities at these metallicities (Vini=600-800 km/s). The most interesting models are the models with $Z=10^{-8}$ ([Fe/H]=~-6.6). In the course of helium burning, carbon and oxygen are mixed into the hydrogen burning shell. This boosts the importance of the shell and causes a reduction of the size of the CO core. Later in the evolution, the hydrogen shell deepens and produces large amount of primary nitrogen. For the most massive models (M>~60 Mo), significant mass loss occurs during the red supergiant stage. This mass loss is due to the surface enrichment in CNO elements via rotational and convective mixing. The yields of the fast rotating 20 Mo models can best reproduce (within our study) the observed abundances at the surface of extremely metal poor (EMP) stars. The wind of the massive models can reproduce the CNO abundances of the carbon--rich UMPs, in particular for the most metal poor star known to date, HE1327-2326.

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arXiv2005

Evolution of rotating stars at very low metallicity

At very low metallicity, the effects of differential rotation have a more important impact on the evolution of stars than at high metallicity. Rotational mixing leads to the production of great quantities of helium and of primary $^{14}$N by massive stars. Rotation induces important mass loss and allows stars to locally strongly enrich the interstellar medium in CNO elements. Stars formed from interstellar clouds enriched by the winds of fast rotating massive stars would present surface abundances similar to those of C-rich extremely metal-poor stars. C-rich stars can also be formed by mass accretion in a binary system where the primary would be a fast rotating intermediate mass star in the early-AGB phase. Fast rotation may also lead to the formation of collapsars even at very low metallicity and make the most massive stars avoid the pair instability.

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arXiv2004

Chemical Abundances and Yields from Massive Stars

Stellar rotation produces an internal mixing of the elements due to shear instability and meridional circulation. This leads to observable $N/C$ enhancements in massive stars above about 7--9 $M_{\odot}$. Rotation also favours mass loss by stellar winds. Mass loss effects dominate for masses above 30 $M_{\odot}$, while mixing dominates below that limit. The effects of mixing are also much larger at lower metallicity $Z$, because the internal $Ω$--gradients are steeper. This appears to be in agreement with observations in the SMC. At very low $Z$ and Z=0, mixing between the He--burning core and the H--burning shell leads to the production of primary N in intermediate mass stars. Such enrichments increase the metallicity of the rotating star, also massive Z=0 stars with moderate initial velocities currently reach break--up velocity during a fraction of the MS phase. Both effects favour mass loss in Z=0 stars, which have ejecta with abundance anomalies very similar to those of C--rich very metal poor stars.

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arXiv2004

Effects of Rotation on Presupernovae Models

We show that rotation strongly affects the nature of the supernova progenitor (blue/red supergiant or Wolf--Rayet star), and thus the supernova types. In particular our models well reproduce the variations of the number ratio SNIb/Ic to SNII with metallicity. Rotation also produces envelope enrichments of the N/C ratio, and increases the size of the CO cores. We show the evolution of the specific angular momentum up to the preSN stage and make comparison with neutron stars. We suggest that the rare WO stars, preferentially formed at low metallicity, are the progenitors of GRB.

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arXiv2004

Evolution of the Most Massive Stars

We discuss the physics of the $ΩΓ$-- Limit, i.e. when the star is unbound as a result of {\emph{both}} rotation and radiation pressure. We suggest that the $ΩΓ$-- Limit is what makes the Humphreys--Davidson Limit. Stellar filiations are discussed, with an emphasis on the final stages, in particular on the final masses, their angular momentum and the chemical yields with account of rotation. A possible relation between WO stars and GRB is emphasized.

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arXiv2003

Pre-suprenova evolution of rotating massive stars

The Geneva evolutionary code has been modified to study the advanced stages (Ne, O, Si burnings) of rotating massive stars. Here we present the results of four 20 solar mass stars at solar metallicity with initial rotational velocities of 0, 100, 200 and 300 km/s in order to show the crucial role of rotation in stellar evolution. As already known, rotation increases mass loss and core masses (Meynet and Maeder 2000). A fast rotating 20 solar mass star has the same central evolution as a non-rotating 26 solar mass star. Rotation also increases strongly net total metal yields. Furthermore, rotation changes the SN type so that more SNIb are predicted (see Meynet and Maeder 2003 and N. Prantzos and S. Boissier 2003). Finally, SN1987A-like supernovae progenitor colour can be explained in a single rotating star scenario.

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arXiv2003

Dynamical shear instability

The dynamical shear instability is an important mixing process in the advanced stages of the evolution of massive stars. We calculated different models of 15 Mo with an initial rotational velocity, v(rot,ini)= 300 km/s to investigate its efficiency. We found that the dynamical shear instability has a timescale shorter than Oxygen burning timescale and that it slightly enlarges the convective zones and smoothens the omega gradients throughout the evolution. However, its effect is too localized to slow down the core of the star.

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arXiv2003

Stellar yields in CNO from rotating stellar models

For $^{12}$C and $^{16}$O, rotating models predict in general enhanced yields. At high metallicity, the carbon and oxygen yields from the very high mass stars, which go through a WR phase, are little affected by rotation. For $^{14}$N, rotation allows the production of important amounts of primary nitrogen in {\it intermediate mass stars} at very low metallicity. The process invoked for this production is different, from the classically accepted scenario {\it i.e.} the Hot Bottom Burning (HBB) in Asymptotic Giant Branch (AGB) stars. Rotating models also predict important productions of primary $^{13}$C, $^{17}$O and $^{22}$Ne at very low metallicity.

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