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R. Hirschi

Publications and source records attributed to R. Hirschi.

At least 37 records · Page 2Linked to original sources
arXiv2021

Convective core entrainment in 1D main sequence stellar models

3D hydrodynamics models of deep stellar convection exhibit turbulent entrainment at the convective-radiative boundary which follows the entrainment law, varying with boundary penetrability. We implement the entrainment law in the 1D Geneva stellar evolution code. We then calculate models between 1.5 and 60 M$_{\odot}$ at solar metallicity ($Z=0.014$) and compare them to previous generations of models and observations on the main sequence. The boundary penetrability, quantified by the bulk Richardson number, $Ri_{\mathrm{B}}$, varies with mass and to a smaller extent with time. The variation of $Ri_{\mathrm{B}}$ with mass is due to the mass dependence of typical convective velocities in the core and hence the luminosity of the star. The chemical gradient above the convective core dominates the variation of $Ri_{\mathrm{B}}$ with time. An entrainment law method can therefore explain the apparent mass dependence of convective boundary mixing through $Ri_{\mathrm{B}}$. New models including entrainment can better reproduce the mass dependence of the main sequence width using entrainment law parameters $A \sim 2 \times 10^{-4}$ and $n=1$. We compare these empirically constrained values to the results of 3D hydrodynamics simulations and discuss implications.

astro-ph.SR↗
arXiv2021

Convective core sizes in rotating massive stars: I. Constraints from solar metallicity OB field stars

Spectroscopic studies of Galactic O and B stars show that many stars with masses above 8 M$_{\odot}$ are observed in the HR diagram just beyond the Main-Sequence (MS) band predicted by stellar models computed with a moderate overshooting. This may be an indication that the convective core sizes in stars in the upper part of the HR diagram are larger than predicted by these models. Combining stellar evolution models and spectroscopic parameters derived for a large sample of Galactic O and B stars, including brand new information about their projected rotational velocities, we reexamine the question of the convective core size in MS massive stars. We confirm that for stars more massive than about 8 M$_{\odot}$, the convective core size at the end of the MS phase increases more rapidly with the mass than in models computed with a constant step overshoot chosen to reproduce the main sequence width in the low mass range (around 2 M$_{\odot}$). This conclusion is valid for both the cases of non-rotating models and rotating models either with a moderate or a strong angular momentum transport. The increase of the convective core mass with the mass obtained from the TAMS position is, however, larger than the one deduced from the surface velocity drop for masses above about 15 M$_{\odot}$. Although observations available at the moment cannot decide what is the best choice between the core sizes given by the TAMS and the velocity drop, we discuss different methods to get out of this dilemma. At the moment, comparisons with eclipsing binaries seem to favor the solution given by the velocity drop. While we confirm the need for larger convective cores at higher masses, we find tensions in-between different methods for stars more massive than 15 M$_{\odot}$. The use of single-aged stellar populations (non-interacting binaries or stellar clusters) would be a great asset to resolve this tension.

astro-ph.SR↗
arXiv2021

Constraints on stellar rotation from the evolution of Sr and Ba in the Galactic halo

Recent studies show that the chemical evolution of Sr and Ba in the Galaxy can be explained if different production sites, hosting r- and s-processes, are taken into account. However, the question of unambiguously identifying these sites is still unsolved. Massive stars are shown to play an important role in the production of s-material if rotation is considered. In this work, we study in detail the contribution of rotating massive stars to the production of Sr and Ba, in order to explain their chemical evolution, but also to constrain the rotational behaviour of massive stars. A stochastic chemical evolution model was employed to reproduce the enrichment of the Galactic halo. We developed new methods for model-data comparison which help to objectively compare the stochastic results to the observations. We employed these methods to estimate the value of free parameters which describe the rotation of massive stars, assumed to be dependent on the stellar metallicity. We constrain the parameters using the observations for Sr and Ba. Employing these parameters for rotating massive stars in our stochastic model, we are able to correctly reproduce the chemical evolution of Sr and Ba, but also Y, Zr and La. The data supports a decrease of both the mean rotational velocities and their dispersion with increasing metallicity. Our results show that a metallicity-dependent rotation is a necessary assumption to explain the s-process in massive stars. Our novel methods of model-data comparison represent a promising tool for future galactic chemical evolution studies.

astro-ph.GA↗
arXiv2020

The evolutionary roads leading to low effective spins, high black hole masses, and O1/O2 rates of LIGO/Virgo binary black holes

All ten LIGO/Virgo binary black hole (BH-BH) coalescences reported from the O1/O2 runs have near zero effective spins. There are only three potential explanations of this fact. If the BH spin magnitudes are large then (i) either both BH spin vectors must be nearly in the orbital plane or (ii) the spin angular momenta of the BHs must be oppositely directed and similar in magnitude. Or, (iii) the BH spin magnitudes are small. We test the third hypothesis within the framework of the classical isolated binary evolution scenario of the BH-BH merger formation. We test three models of angular momentum transport in massive stars: a mildly efficient transport by meridional currents (as employed in the Geneva code), an efficient transport by the Tayler-Spruit magnetic dynamo (as implemented in the MESA code), and a very-efficient transport (as proposed by Fuller et al.) to calculate natal BH spins. We allow for binary evolution to increase the BH spins through accretion and account for the potential spin-up of stars through tidal interactions. Additionally, we update the calculations of the stellar-origin BH masses, include revisions to the history of star formation and to the chemical evolution across cosmic time. We find that we can match simultaneously the observed BH-BH merger rate density, BH masses, and effective spins. Models with efficient angular momentum transport are favored. The updated stellar-mass weighted gas-phase metallicity evolution now used in our models appears to be a key in better reproducing the LIGO/Virgo merger rate estimate. Mass losses during the pair-instability pulsation supernova phase are likely overestimated if the merger GW170729 hosts a BH more massive than 50 Msun. We also estimate rate of BH-NS mergers from recent LIGO/Virgo observations. Our updated models of BH-BH, BH-NS and NS-NS mergers are now publicly available at www.syntheticuniverse.org.

astro-ph.HE↗
arXiv2020

The Formation of a 70 Msun Black Hole at High Metallicity

A 70Msun BH was discovered in Milky Way disk in a long period and almost circular detached binary system (LB-1) with a high metallicity 8Msun B star companion. Current consensus on the formation of BHs from high metallicity stars limits the black hole mass to be below 20Msun. Using simple evolutionary model, we show that the formation of a 70Msun BH in high metallicity environment is possible if stellar wind mass loss rates are reduced by factor of 5. As observations indicate, a fraction of massive stars have surface magnetic fields which may quench the wind mass-loss, independently of stellar mass and metallicity. We also computed detailed stellar evolution models and we confirm such a scenario. A non-rotating 85Msun model at Z=0.014 with decreased winds ends up as a 71Msun star prior core-collapse with a 32Msun helium core and a 28Msun CO core. Such star avoids pair-instability pulsation supernova mass loss and may form a 70Msun BH in the direct collapse. Stars that can form such BHs expand to significant size with radius of R>600Rsun, exceeding the size of LB-1 orbit. Therefore, we can explain the formation of BHs upto 70Msun at high metallicity and this result is independent from LB-1. However, if LB-1 hosts a massive BH we are unable to explain how such a binary star system could have formed without invoking some exotic scenarios.

astro-ph.HE↗
arXiv2019

A direct measurement of the 17O(a,g)21Ne reaction in inverse kinematics and its impact on heavy element production

During the slow neutron capture process in massive stars, reactions on light elements can both produce and absorb neutrons thereby influencing the final heavy element abundances. At low metallicities, the high neutron capture rate of 16-O can inhibit s-process nucleosynthesis unless the neutrons are recycled via the 17O(a,n)20Ne reaction. The efficiency of this neutron recycling is determined by competition between the 17O(a,n)20Ne and 17O(a,g)21Ne reactions. While some experimental data are available on the former reaction, no data exist for the radiative capture channel at the relevant astrophysical energies. The 17O(a,g)21Ne reaction has been studied directly using the DRAGON recoil separator at the TRIUMF Laboratory. The reaction cross section has been determined at energies between 0.6 and 1.6 MeV Ecm, reaching into the Gamow window for core helium burning for the first time. Resonance strengths for resonances at 0.63, 0.721, 0.81 and 1.122 MeV Ecm have been extracted. The experimentally based reaction rate calculated represents a lower limit, but suggests that significant s-process nucleosynthesis occurs in low metallicity massive stars.

nucl-ex↗
arXiv2019

Impact of Uncertainties in Astrophysical Reaction Rates on Nucleosynthesis in the $νp$ Process

The $νp$ process appears in proton-rich, hot matter which is expanding in a neutrino wind and may be realised in explosive environments such as core-collapse supernovae or in outflows from accretion disks. The impact of uncertainties in nuclear reaction cross sections on the finally produced abundances has been studied by applying Monte Carlo variation of all astrophysical reaction rates in a large reaction network. As the detailed astrophysical conditions of the $νp$ process still are unknown, a parameter study was performed, with 23 trajectories covering a large range of entropies and $Y_\mathrm{e}$. The resulting abundance uncertainties are given for each trajectory. The $νp$ process has been speculated to contribute to the light $p$ nuclides but it was not possible so far to reproduce the solar isotope ratios. It is found that it is possible to reproduce the solar $^{92}$Mo/$^{94}$Mo abundance ratio within nuclear uncertainties, even within a single trajectory. The solar values of the abundances in the Kr-Sr region relative to the Mo region, however, cannot be achieved within a single trajectory. They may still be obtained from a weighted superposition of different trajectories, though, depending on the actual conditions in the production site. For a stronger constraint of the required conditions, it would be necessary to reduce the uncertainties in the 3$α$ and $^{56}$Ni(n,p)$^{56}$Co rates at temperatures $T>3$ GK.

astro-ph.HE↗
arXiv2019

The s process in rotating low-mass AGB stars. Nucleosynthesis calculations in models matching asteroseismic constraints

Aims. We investigate the s-process during the AGB phase of stellar models whose cores are enforced to rotate at rates consistent with asteroseismology observations of their progenitors and successors. Methods. We calculated new 2M$_{\odot}$, Z=0.01 models, rotating at 0, 125, and 250 km/s at the start of main sequence. An artificial, additional viscosity was added to enhance the transport of angular momentum in order to reduce the core rotation rates to be in agreement with asteroseismology observations. We compared rotation rates of our models with observed rotation rates during the MS up to the end of core He burning, and the white dwarf phase. Results. We present nucleosynthesis calculations for these rotating AGB models that were enforced to match the asteroseismic constraints on rotation rates of MS, RGB, He-burning, and WD stars. In particular, we calculated one model that matches the upper limit of observed rotation rates of core He-burning stars and we also included a model that rotates one order of magnitude faster than the upper limit of the observations. The s-process production in both of these models is comparable to that of non-rotating models. Conclusions. Slowing down the core rotation rate in stars to match the above mentioned asteroseismic constraints reduces the rotationally induced mixing processes to the point that they have no effect on the s-process nucleosynthesis. This result is independent of the initial rotation rate of the stellar evolution model. However, there are uncertainties remaining in the treatment of rotation in stellar evolution, which need to be reduced in order to confirm our conclusions, including the physical nature of our approach to reduce the core rotation rates of our models, and magnetic processes.

astro-ph.SR↗
arXiv2019

Impact of Uncertainties in Nuclear Reaction Cross Sections on p-Nucleosynthesis in Thermonuclear Supernovae

The propagation of uncertainties in reaction cross sections and rates of neutron-, proton-, and alpha-induced reactions into the final isotopic abundances obtained in nucleosynthesis models is an important issue in studies of nucleosynthesis and Galactic Chemical Evolution. We developed a Monte Carlo method to allow large-scale postprocessing studies of the impact of nuclear uncertainties on nucleosynthesis. Temperature-dependent rate uncertainties combining realistic experimental and theoretical uncertainties are used. From detailed statistical analyses uncertainties in the final abundances are derived as probability density distributions. Furthermore, based on rate and abundance correlations an automated procedure identifies the most important reactions in complex flow patterns from superposition of many zones or tracers. The method so far was already applied to a number of nucleosynthesis processes. Here we focus on the production of p-nuclei in white dwarfs exploding as thermonuclear (type Ia) supernovae. We find generally small uncertainties in the final abundances despite of the dominance of theoretical nuclear uncertainties. A separate analysis of low- and high-density regions indicates that the total uncertainties are dominated by the high-density regions.

astro-ph.HE↗
arXiv2019

Uncertainties in $ν$p-process nucleosynthesis from Monte Carlo variation of reaction rates

It has been suggested that a $ν$p process can occur when hot, dense, and proton-rich matter is expanding within a strong flux of anti-neutrinos. In such an environment, proton-rich nuclides can be produced in sequences of proton captures and (n,p) reactions, where the free neutrons are created in situ by $\overlineν_\mathrm{e}+\mathrm{p} \rightarrow \mathrm{n}+\mathrm{e}^+$ reactions. The detailed hydrodynamic evolution determines where the nucleosynthesis path turns off from N = Z line and how far up the nuclear chart it runs. In this work, the uncertainties on the final isotopic abundances stemming from uncertainties in the nuclear reaction rates were investigated in a large-scale Monte Carlo approach, simultaneously varying ten thousand reactions. A large range of model conditions was investigated because a definitive astrophysical site for the $ν$p process has not yet been identified. The present parameter study provides, for each model, identification of the key nuclear reactions dominating the uncertainty for a given nuclide abundance. As all rates appearing in the $ν$p process involve unstable nuclei, and thus only theoretical rates are available, the final abundance uncertainties are larger than those for nucleosynthesis processes closer to stability. Nevertheless, most uncertainties remain below a factor of three in trajectories with robust nucleosynthesis. More extreme conditions allow production of heavier nuclides but show larger uncertainties because of the accumulation of the uncertainties in many rates and because the termination of nucleosynthesis is not at equilibrium conditions. It is also found that the solar ratio of the abundances of ${}^{92}$Mo and ${}^{94}$Mo could be reproduced within uncertainties.

astro-ph.SR↗
arXiv2019

Constraining transport of angular momentum in stars: Combining asteroseismic observations of core helium burning stars and white dwarfs

Transport of angular momentum has been a challenging topic within the stellar evolution community, even more since the recent asteroseismic surveys. All published studies on rotation using asteroseismic observations show a discrepancy between the observed and calculated rotation rates, indicating there is an undetermined process of angular momentum transport active in these stars. We aim to constrain the efficiency of this process by investigating rotation rates of 2.5 M$_{\odot}$ stars. First, we investigated whether the Tayler-Spruit dynamo could be responsible for the extra transport of angular momentum for stars with an initial mass of 2.5 M$_{\odot}$. Then, by computing rotating models including a constant additional artificial viscosity, we determined the efficiency of the missing process of angular momentum transport by comparing the models to the asteroseismic observations of core helium burning stars. Parameter studies were performed to investigate the effect of the stellar evolution code used, initial mass, and evolutionary stage. We evolved our models into the white dwarf phase, and provide a comparison to white dwarf rotation rates. The Tayler-Spruit dynamo is unable to provide enough transport of angular momentum to reach the observed values of the core helium burning stars investigated in this paper. We find that a value for the additional artificial viscosity $ν_{\rm{add}}$ around 10$^7$ cm$^2$ s$^{-1}$ provides enough transport of angular momentum. However, the rotational period of these models is too high in the white dwarf phase to match the white dwarf observations. From this comparison we infer that the efficiency of the missing process must decrease during the core helium burning phase. When excluding the $ν_{\rm{add}}$ during core helium burning phase, we can match the rotational periods of both the core helium burning stars and white dwarfs.

astro-ph.SR↗
arXiv2019

Dependence of Convective Boundary Mixing on Boundary Properties and Turbulence Strength

Convective boundary mixing is one of the major uncertainties in stellar evolution. In order to study its dependence on boundary properties and turbulence strength in a controlled way, we computed a series of 3D hydrodynamical simulations of stellar convection during carbon burning with a varying boosting factor of the driving luminosity. Our 3D implicit large eddy simulations were computed with the PROMPI code. We performed a mean field analysis of the simulations within the Reynolds-averaged Navier-Stokes framework. Both the vertical RMS velocity within the convective region and the bulk Richardson number of the boundaries are found to scale with the driving luminosity as expected from theory. The positions of the convective boundaries were estimated through the composition profiles across them, and the strength of convective boundary mixing was determined by analysing the boundaries within the framework of the entrainment law. We find that the entrainment is approximately inversely proportional to the bulk Richardson number. Although the entrainment law does not encompass all the processes occurring at boundaries, our results support the use of the entrainment law to describe convective boundary mixing in 1D models, at least for the advanced phases. The next steps and challenges ahead are also discussed.

astro-ph.SR↗
arXiv2018

The s-process nucleosynthesis in low mass stars: impact of the uncertainties in the nuclear physics determined by Monte Carlo variations

We investigated the impact of uncertainties in neutron-capture and weak reactions (on heavy elements) on the s-process nucleosynthesis in low-mass stars using a Monte-Carlo based approach. We performed extensive nuclear reaction network calculations that include newly evaluated temperature-dependent upper and lower limits for the individual reaction rates. Our sophisticated approach is able to evaluate the reactions that impact more significantly the final abundances. We found that beta-decay rate uncertainties affect typically nuclides near s-process branchings, whereas most of the uncertainty in the final abundances is caused by uncertainties in neutron capture rates, either directly producing or destroying the nuclide of interest. Combined total nuclear uncertainties due to reactions on heavy elements are approximately 50%.

astro-ph.SR↗
arXiv2018

NuGrid Stellar Data Set. II. Stellar Yields from H to Bi for Stellar Models with Mzams = 1 to 25Msun and Z = 0.0001 to 0.02

We provide here a significant extension of the NuGrid Set 1 models in mass coverage and toward lower metallicity, adopting the same physics assumptions. The combined data set now includes the initial masses M/Msun = 1, 1.65, 2, 3, 4, 5, 6, 7, 12, 15, 20, 25 for Z = 0.02, 0.01, 0.006, 0.001, 0.0001 with alpha-enhanced composition for the lowest three metallicities. These models are computed with the MESA stellar evolution code and are evolved up to the AGB, the white dwarf stage, or until core collapse. The nucleosynthesis was calculated for all isotopes in post-processing with the NuGrid mppnp code. Explosive nucleosynthesis is based on semi-analytic 1D shock models. Metallicity-dependent mass loss, convective boundary mixing in low- and intermediate mass models and H and He core burning massive star models is included. Convective O-C shell mergers in some stellar models lead to the strong production of odd-Z elements P, Cl, K and Sc. In AGB models with hot dredge-up the convective boundary mixing efficiency is reduced to accommodate for its energetic feedback. In both low-mass and massive star models at the lowest metallicity H-ingestion events are observed and lead to i-process nucleosynthesis and substantial N-15 production. Complete yield data tables, derived data products and online analytic data access are provided.

astro-ph.SR↗
arXiv2018

Uncertainties in s-process nucleosynthesis in low mass stars determined from Monte Carlo variations

The main s-process taking place in low mass stars produces about half of the elements heavier than iron. It is therefore very important to determine the importance and impact of nuclear physics uncertainties on this process. We have performed extensive nuclear reaction network calculations using individual and temperature-dependent uncertainties for reactions involving elements heavier than iron, within a Monte Carlo framework. Using this technique, we determined the uncertainty in the main s-process abundance predictions due to nuclear uncertainties link to weak interactions and neutron captures on elements heavier than iron. We also identified the key nuclear reactions dominating these uncertainties. We found that $β$-decay rate uncertainties affect only a few nuclides near s-process branchings, whereas most of the uncertainty in the final abundances is caused by uncertainties in neutron capture rates, either directly producing or destroying the nuclide of interest. Combined total nuclear uncertainties due to reactions on heavy elements are in general small (less than 50%). Three key reactions, nevertheless, stand out because they significantly affect the uncertainties of a large number of nuclides. These are $^{56}$Fe(n,$γ$), $^{64}$Ni(n,$γ$), and $^{138}$Ba(n,$γ$). We discuss the prospect of reducing uncertainties in the key reactions identified in this study with future experiments.

astro-ph.SR↗
arXiv2018

Sensitivity to neutron captures and beta-decays of the enhanced s-process in rotating massive stars at low metallicities

The s-process in massive stars, producing nuclei up to $A\approx 90$, has a different behaviour at low metallicity if stellar rotation is significant. This enhanced s-process is distinct from the s-process in massive stars around solar metallicity, and details of the nucleosynthesis are poorly known. We investigated nuclear physics uncertainties in the enhanced s-process in metal-poor stars within a Monte-Carlo framework. We applied temperature-dependent uncertainties of reaction rates, distinguishing contributions from the ground state and from excited states. We found that the final abundance of several isotopes shows uncertainties larger than a factor of 2, mostly due to the neutron capture uncertainties. A few nuclei around branching points are affected by uncertainties in the $β$-decay.

astro-ph.SR↗
arXiv2018

Impacts of nuclear-physics uncertainties in the s-process determined by Monte-Carlo variations

The s-process, a production mechanism based on slow-neutron capture during stellar evolution, is the origin of about half the elements heavier than iron. Abundance predictions for s-process nucleosynthesis depend strongly on the relevant neutron-capture and $β$-decay rates, as well as on the details of the stellar model being considered. Here, we have used a Monte-Carlo approach to evaluate the nuclear uncertainty in s-process nucleosynthesis. We considered the helium burning of massive stars for the weak s-process and low-mass asymptotic-giant-branch stars for the main s-process. Our calculations include a realistic and general prescription for the temperature dependent uncertainty for the reaction cross sections. We find that the adopted uncertainty for (${\rm n},γ$) rates, tens of per cent on average, effects the production of s-process nuclei along the line of $β$-stability, and that the uncertainties in $β$-decay from excited state contributions, has the strongest impact on branching points.

astro-ph.SR↗