arXiv subjects

Raphael Hirschi

Publications and source records attributed to Raphael Hirschi.

At least 91 records · Page 5Linked to original sources

arXiv2010

Origin of CEMP stars: What helium and lithium can tell us about CEMP stars?

We show that the peculiar surface abundance patterns of Carbon Enhanced Metal Poor (CEMP) stars has been inherited from material having been processed by H- and He-burning phases in a previous generation of stars (hereafter called the ``Source Stars''). In this previous generation, some mixing must have occurred between the He- and the H-burning regions in order to explain the high observed abundances of nitrogen. In addition, it is necessary to postulate that a very small fraction of the carbon-oxygen core has been expelled (either by winds or by the supernova explosion). Therefore only the most outer layers should have been released by the Source Stars. Some of the CEMP stars may be He-rich if the matter from the Source Star is not too much diluted with the InterStellar Medium (ISM). Those stars formed from nearly pure ejecta would also be Li-poor.

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arXiv2009

Nucleosynthesis in Rotating massive stars and Abundances in the Early Galaxy

We discuss three effects of axial rotation at low metallicity. The first one is the mixing of the chemical species which is predicted to be more efficient in low metallicity environments. A consequence is the production of important quantities of primary $^{14}$N, $^{13}$C, $^{22}$Ne and a strong impact on the nucleosynthesis of the {\it s}-process elements. The second effect is a consequence of the first. Strong mixing makes possible the apparition at the surface of important quantities of CNO elements. This increases the opacity of the outer layers and may trigger important mass loss by line driven winds. The third effect is the fact that, during the main-sequence phase, stars, at very low metallicity, reach more easily than their more metal rich counterparts, the critical velocity\footnote{The critical velocity is the surface equatorial velocity such that the centrifugal acceleration compensates for the local gravity.}. We discuss the respective importance of these three effects as a function of the metallicity. We show the consequences for the early chemical evolution of the galactic halo and for explaining the CEMP stars. We conclude that rotation is probably a key feature which contributes in an important way to shape the evolution of the first stellar generations in the Universe.

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arXiv2008

Difficulties in Probing Nuclear Physics: A Study of $^{44}$Ti and $^{56}$Ni

The nucleosynthetic yield from a supernova explosion depends upon a variety of effects: progenitor evolution, explosion process, details of the nuclear network, and nuclear rates. Especially in studies of integrated stellar yields, simplifications reduce these uncertainties. But nature is much more complex, and to actually study nuclear rates, we will have to understand the full, complex set of processes involved in nucleosynthesis. Here we discuss a few of these complexities and detail how the NuGrid collaboration will address them.

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arXiv2008

NuGrid: Toward High Precision Double-Degenerate Merger Simulations with SPH in 3D

We present preliminary results from recent high-resolution double-degenerate merger simulations with the Smooth Particle Hydrodynamics (SPH) technique. We put particular emphasis on verification and validation in our effort and show the importance of details in the initial condition setup for the final outcome of the simulation. We also stress the dynamical importance of including shocks in the simulations. These results represent a first step toward a suite of simulations that will shed light on the question whether double-degenerate mergers are a viable path toward type 1a supernovae. In future simulations, we will make use of the capabilities of the NuGrid collaboration in post-processing SPH particle trajectories with a complete nuclear network to follow the detailed nuclear reactions during the dynamic merger phase.

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arXiv2008

Nucleosynthesis Calculations from Core-Collapse Supernovae

We review some of the uncertainties in calculating nucleosynthetic yields, focusing on the explosion mechanism. Current yield calculations tend to either use a piston, energy injection, or enhancement of neutrino opacities to drive an explosion. We show that the energy injection, or more accurately, an entropy injection mechanism is best-suited to mimic our current understanding of the convection-enhanced supernova engine. The enhanced neutrino-opacity technique is in qualitative disagreement with simulations of core-collapse supernovae and will likely produce errors in the yields. But piston-driven explosions are the most discrepant. Piston-driven explosion severely underestimate the amount of fallback, leading to order-of-magnitude errors in the yields of heavy elements. To obtain yields accurate to the factor of a few level, we must use entropy or energy injection and this has become the NuGrid collaboration approach.

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arXiv2008

Nucleosynthetic Yields from "Collapsars"

The "collapsar" engine for gamma-ray bursts invokes as its energy source the failure of a normal supernova and the formation of a black hole. Here we present the results of the first three-dimensional simulation of the collapse of a massive star down to a black hole, including the subsequent accretion and explosion. The explosion differs significantly from the axisymmetric scenario obtained in two-dimensional simulations; this has important consequences for the nucleosynthetic yields. We compare the nucleosynthetic yields to those of hypernovae. Calculating yields from three-dimensional explosions requires new strategies in post-process nucleosynthesis; we discuss NuGrid's plan for three-dimensional yields.

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arXiv2008

44Ti and 56Ni in core-collapse supernovae

We investigate the physical conditions where 44Ti and 56Ni are created in core-collapse supernovae. In this preliminary work we use a series of post-processing network calculations with parametrized expansion profiles that are representative of the wide range of temperatures, densities and electron-to-baryon ratios found in 3D supernova simulations. Critical flows that affect the final yields of 44Ti and 56Ni are assessed.

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arXiv2008

Nucleosynthesis simulations for a wide range of nuclear production sites from NuGrid

Simulations of nucleosynthesis in astrophysical environments are at the intersection of nuclear physics reaction rate research and astrophysical applications, for example in the area of galactic chemical evolution or near-field cosmology. Unfortunately, at present the available yields for such applications are based on heterogeneous assumptions between the various contributing nuclear production sites, both in terms of modeling the thermodynamic environment itself as well as the choice of specifc nuclear reaction rates and compilations. On the other side, new nuclear reaction rate determinations are often taking a long time to be included in astrophysical applications. The NuGrid project addresses these issues by providing a set of codes and a framework in which these codes interact. In this contribution we describe the motivation, goals and first results of the NuGrid project. At the core is a new and evolving post-processing nuclesoynthesis code (PPN) that can follow quiescent and explosive nucleosynthesis following multi-zone 1D-stellar evolution as well as multi-zone hydrodynamic input, including explosions. First results are available in the areas of AGB and massive stars.

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arXiv2008

NuGrid: s process in massive stars

The s-process production in massive stars at very low metallicities is expected to be negligible due to the low abundance of the neutron source 22Ne, to primary neutron poisons and decreasing iron seed abundances. However, recent models of massive stars including the effects of rotation show that a strong production of 22Ne is possible in the helium core, as a consequence of the primary nitrogen production (observed in halo metal poor stars). Using the PPN post-processing code, we studied the impact of this primary 22Ne on the s process. We find a large production of s elements between strontium and barium, starting with the amount of primary 22Ne predicted by stellar models. There are several key reaction rate uncertainties influencing the s-process efficiency. Among them, 17O(alpha,gamma) may play a crucial role strongly influencing the s process efficiency, or it may play a negligible role, according to the rate used in the calculations. We also report on the development of a new parallel (MPI) post-processing code (MPPNP) designed to follow the complete nucleosynthesis in stars on highly resolved grids. We present here the first post-processing run from the ZAMS up to the end of helium burning for a 15 solar mass model.

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arXiv2008

Spatial Distribution of Nucleosynthesis Products in Cassiopeia A: Comparison Between Observations and 3D Explosion Models

We examine observed heavy element abundances in the Cassiopeia A supernova remnant as a constraint on the nature of the Cas A supernova. We compare bulk abundances from 1D and 3D explosion models and spatial distribution of elements in 3D models with those derived from X-ray observations. We also examine the cospatial production of 26Al with other species. We find that the most reliable indicator of the presence of 26Al in unmixed ejecta is a very low S/Si ratio (~0.05). Production of N in O/S/Si-rich regions is also indicative. The biologically important element P is produced at its highest abundance in the same regions. Proxies should be detectable in supernova ejecta with high spatial resolution multiwavelength observations.

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arXiv2008

Complete nucleosynthesis calculations for low-mass stars from NuGrid

Many nucleosynthesis and mixing processes of low-mass stars as they evolve from the Main Sequence to the thermal-pulse Asymptotic Giant Branch phase (TP-AGB) are well understood (although of course important physics components, e.g. rotation, magnetic fields, gravity wave mixing, remain poorly known). Nevertheless, in the last years presolar grain measurements with high resolution have presented new puzzling problems and strong constraints on nucleosynthesis processes in stars. The goal of the NuGrid collaboration is to present uniform yields for a large range of masses and metallicities, including low$-$mass stars and massive stars and their explosions. Here we present the first calculations of stellar evolution and high-resolution, post-processing simulations of an AGB star with an initial mass of 2 M_sun and solar-like metallicity (Z=0.01), based on the post-processing code PPN. In particular, we analyze the formation and evolution of the radiative 13C-pocket between the 17th TP and the 18th TP. The s-process nucleosynthesis profile of a sample of heavy isotopes is also discussed, before the next convective TP occurrence.

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arXiv2008

Evolution and chemical and dynamical effects of high-mass stars

We review general characteristics of massive stars, present the main observable constraints that stellar models should reproduce. We discuss the impact of massive star nucleosynthesis on the early phases of the chemical evolution of the Milky Way (MW). We show that rotating models can account for the important primary nitrogen production needed at low metallicity. Interestingly such rotating models can also better account for other features as the variation with the metallicity of the C/O ratio. Damped Lyman Alpha (DLA) systems present similar characteristics as the halo of the MW for what concern the N/O and C/O ratios. Although in DLAs, the star formation history might be quite different from that of the halo, in these systems also, rotating stars (both massive and intermediate) probably play an important role for explaining these features. The production of primary nitrogen is accompanied by an overproduction of other elements as $^{13}$C, $^{22}$Ne and s-process elements. We show also how the observed variation with the metallicity of the number ratio of type Ibc to type II supernovae may be a consequence of the metallicity dependence of the line-driven stellar winds.

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arXiv2008

Massive star evolution: from the early to the present day Universe

Mass loss and axial rotation are playing key roles in shaping the evolution of massive stars. They affect the tracks in the HR diagram, the lifetimes, the surface abundances, the hardness of the radiation field, the chemical yields, the presupernova status, the nature of the remnant, the mechanical energy released in the interstellar medium, etc... In this paper, after recalling a few characteristics of mass loss and rotation, we review the effects of these two processes at different metallicities. Rotation probably has its most important effects at low metallicities, while mass loss and rotation deeply affect the evolution of massive stars at solar and higher than solar metallicities.

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arXiv2008

Developments in physics of massive stars

New constraints on stellar models are provided by large surveys of massive stars, interferometric observations and asteroseismology. After a review of the main results so far obtained, we present new results from rotating models and discuss comparisons with observed features. We conclude that rotation is a key feature of massive star physics.

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arXiv2008

Massive Stars as Cosmic Engines through the Ages

Some useful developments in the model physics are briefly presented, followed by model results on chemical enrichments and WR stars. We discuss the expected rotation velocities of WR stars. We emphasize that the (C+O)/He ratio is a better chemical indicator of evolution for WC stars than the C/He ratios. With or without rotation, at a given luminosity the (C+O)/He ratios should be higher in regions of lower metallicity Z. Also, for a given (C+O)/He ratio the WC stars in lower Z regions have higher luminosities. The WO stars, which are likely the progenitors of supernovae SNIc and of some GRBs, should preferentially be found in regions of low Z and be the descendants of very high initial masses. Finally, we emphasize the physical reasons why massive rotating low Z stars may also experience heavy mass loss

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arXiv2007

Mass loss and very low-metallicity stars

Mass loss plays a dominant role in the evolution of massive stars at solar metallicity. After discussing different mass loss mechanisms and their metallicity dependence, we present the possibility of strong mass loss at very low metallicity. Our models at Z=1e-8 show that stars more massive than about 60 solar masses may lose a significant fraction of their initial mass in the red supergiant phase. This mass loss is due to the surface enrichment in CNO elements via rotational and convective mixing. Our 85 solar mass model ends its life as a fast rotating WO type Wolf-Rayet star. Therefore the models predict the existence of type Ic SNe and long and soft GRBs at very low metallicities. Such strong mass loss in the red supergiant phase or the Omega-Gamma limit could prevent the most massive stars from ending as pair-creation supernovae. The very low metallicity models calculated are also very interesting from the nucleosynthesis point of view. Indeed, the wind of the massive star models can reproduce the CNO abundances of the most metal-poor carbon-rich star known to date, HE1327-2326. Finally, using chemical evolution models, we are able to reproduce the evolution of CNO elements as observed in the normal extremely metal poor stars.

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arXiv2007

The impact of reduced mass loss rates on the evolution of massive stars

Mass loss is a very important aspect of the life of massive stars. After briefly reviewing its importance, we discuss the impact of the recently proposed downward revision of mass loss rates due to clumping (difficulty to form Wolf-Rayet stars and production of critically rotating stars). Although a small reduction might be allowed, large reduction factors around ten are disfavoured. We then discuss the possibility of significant mass loss at very low metallicity due to stars reaching break-up velocities and especially due to the metal enrichment of the surface of the star via rotational and convective mixing. This significant mass loss may help the first very massive stars avoid the fate of pair-creation supernova, the chemical signature of which is not observed in extremely metal poor stars. The chemical composition of the very low metallicity winds is very similar to that of the most metal poor star known to date, HE1327-2326 and offer an interesting explanation for the origin of the metals in this star. We also discuss the importance of mass loss in the context of long and soft gamma-ray bursts and pair-creation supernovae. Finally, we would like to stress that mass loss in cooler parts of the HR-diagram (luminous blue variable and yellow and red supergiant stages) are much more uncertain than in the hot part. More work needs to be done in these areas to better constrain the evolution of the most massive stars.

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arXiv2006

Mass loss of rotating stars at very low metallicity

Some indirect observations, as the high fraction of Be stars at low metallicity, or the necessity for massive stars to be important sources of primary nitrogen, seem to indicate that very metal poor stars were fast rotators. As a consequence of this fast rotation, these stars, contrarily to current wisdom, might lose large amounts of mass during their lifetime. In this paper, we review various mechanisms triggered by rotation which may induce strong mass loss at very low metallicity. The most efficient process comes from surface enrichments in CNO elements which then drive mass loss by stellar winds. Due to this process, a fast rotating 60 M$_\odot$ with metallicities in the range of $Z=10^{-8}$ and $10^{-5}$, can lose between 30 and 55% of its initial mass. This rotationally wind ejected material participates to the chemical evolution of the interstellar medium, enriching it exclusively in H- and He-burning products. In particular, metal poor fast rotating stars may play a key role for explaining the origin of the peculiar abundance pattern observed at the surface of the extremely metal-poor C-rich stars, for explaining the chemical inhomogeneities observed in globular clusters, and the presence of stars in $ω$ Cen with a very high helium content .

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