The mass spectrum of stellar mass black holes (BHs) is highly uncertain. Dynamical mass measurements are available only for few (∼10) BHs in X-ray binaries, while theoretical models strongly depend ...on the hydrodynamics of supernova (SN) explosions and on the evolution of massive stars. In this paper, we present and discuss the mass spectrum of compact remnants that we obtained with sevn, a new public population-synthesis code, which couples the parsec stellar evolution tracks with up-to-date recipes for SN explosion (depending on the carbon–oxygen mass of the progenitor, on the compactness of the stellar core at pre-SN stage and on a recent two-parameter criterion based on the dimensionless entropy per nucleon at pre-SN stage). sevn can be used both as a stand-alone code and in combination with direct-summation N-body codes (starlab, higpus). The parsec stellar evolution tracks currently implemented in sevn predict significantly larger values of the carbon–oxygen core mass with respect to previous models. For most of the SN recipes we adopt, this implies substantially larger BH masses at low metallicity (≤2 × 10−3), than other population synthesis codes. The maximum BH mass found with sevn is ∼25, 60 and 130 M⊙ at metallicity Z = 2 × 10−2, 2 × 10−3 and 2 × 10−4, respectively. Mass loss by stellar winds plays a major role in determining the mass of BHs for very massive stars (≥90 M⊙), while the remnant mass spectrum depends mostly on the adopted SN recipe for lower progenitor masses. We discuss the implications of our results for the transition between neutron star and BH mass, and for the expected number of massive BHs (with mass >25 M⊙) as a function of metallicity.
ABSTRACT
The detection of GW190521 by the LIGO–Virgo collaboration has revealed the existence of black holes (BHs) in the pair-instability (PI) mass gap. Here, we investigate the formation of BHs in ...the PI mass gap via star–star collisions in young stellar clusters. To avoid PI, the stellar-collision product must have a relatively small core and a massive envelope. We generate our initial conditions from the outputs of a hydrodynamical simulation of the collision between a core helium burning star (∼58 M⊙) and a main-sequence star (∼42 M⊙). The hydrodynamical simulation allows us to take into account the mass lost during the collision (∼12 M⊙) and to build the chemical composition profile of the post-collision star. We then evolve the collision product with the stellar evolution codes parsec and mesa. We find that the post-collision star evolves through all the stellar burning phases until core collapse, avoiding PI. At the onset of core collapse, the post-collision product is a blue supergiant star. We estimate a total mass-loss of about 1 M⊙ during the post-collision evolution, due to stellar winds and shocks induced by neutrino emission in a failed supernova. The final BH mass is ≈87 M⊙. Therefore, we confirm that the collision scenario is a suitable formation channel to populate the PI mass gap.
We complement the parsec data base of stellar evolutionary tracks with new models of massive stars, from the pre-main-sequence phase to the central carbon ignition. We consider a broad range of ...metallicities, 0.0001 ≤ Z ≤ 0.04 and initial masses up to M
ini = 350 M⊙. The main difference with respect to our previous models of massive stars is the adoption of a recent formalizm accounting for the mass-loss enhancement when the ratio of the stellar to the Eddington luminosity, Γ
e
, approaches unity. With this new formalizm, the models are able to reproduce the Humphreys–Davidson limit observed in the Galactic and Large Magellanic Cloud colour–magnitude diagrams, without an ad hoc mass-loss enhancement. We also follow the predictions of recent wind models indicating that the metallicity dependence of the mass-loss rates becomes shallower when Γ
e
approaches unity. We thus find that the more massive stars may suffer from substantial mass-loss even at low metallicity. We also predict that the Humphreys–Davidson limit should become brighter at decreasing metallicity. We supplement the evolutionary tracks with new tables of theoretical bolometric corrections, useful to compare tracks and isochrones with the observations. For this purpose, we homogenize existing stellar atmosphere libraries of hot and cool stars (Potsdam Wolf–Rayet, atlas9 and phoenix) and we add, where needed, new atmosphere models computed with wm-basic. The mass, age and metallicity grids are fully adequate to perform detailed investigations of the properties of very young stellar systems, both in local and distant galaxies. The new tracks supersede the previous old padova models of massive stars.
We investigate the impact of stellar rotation on the formation of black holes (BHs) by means of our population synthesis code sevn. Rotation affects the mass function of BHs in several ways. In ...massive metal-poor stars, fast rotation reduces the minimum zero-age main sequence (ZAMS) mass for a star to undergo pair instability and pulsational pair instability. Moreover, stellar winds are enhanced by rotation, peeling off the entire hydrogen envelope. As a consequence of these two effects, the maximum BH mass we expect from the collapse of a rotating metal-poor star is only ∼45 M , while the maximum mass of a BH born from a nonrotating star is ∼60 M . Furthermore, stellar rotation reduces the minimum ZAMS mass for a star to collapse into a BH from ∼18-25 M to ∼13-18 M . Finally, we have investigated the impact of different core-collapse supernova (CCSN) prescriptions on our results. While the threshold value of compactness for direct collapse and the fallback efficiency strongly affect the minimum ZAMS mass for a star to collapse into a BH, the fraction of the hydrogen envelope that can be accreted onto the final BH is the most important ingredient in determining the maximum BH mass. Our results confirm that the interplay between stellar rotation, CCSNe and pair instability plays a major role in shaping the BH mass spectrum.
ABSTRACT
Pair-instability (PI) is expected to open a gap in the mass spectrum of black holes (BHs) between ≈40–65 and ≈120 M⊙. The existence of the mass gap is currently being challenged by the ...detection of GW190521, with a primary component mass of $85^{+21}_{-14}$ M⊙. Here, we investigate the main uncertainties on the PI mass gap: the 12C(α, γ)16O reaction rate and the H-rich envelope collapse. With the standard 12C(α, γ)16O rate, the lower edge of the mass gap can be 70 M⊙ if we allow for the collapse of the residual H-rich envelope at metallicity Z ≤ 0.0003. Adopting the uncertainties given by the starlib database, for models computed with the 12C(α, γ)16O rate $-1\, \sigma$, we find that the PI mass gap ranges between ≈80 and ≈150 M⊙. Stars with MZAMS > 110 M⊙ may experience a deep dredge-up episode during the core helium-burning phase, that extracts matter from the core enriching the envelope. As a consequence of the He-core mass reduction, a star with MZAMS = 160 M⊙ may avoid the PI and produce a BH of 150 M⊙. In the $-2\, {}\sigma {}$ case, the PI mass gap ranges from 92 to 110 M⊙. Finally, in models computed with 12C(α, γ)16O $-3\, {}\sigma {}$, the mass gap is completely removed by the dredge-up effect. The onset of this dredge-up is particularly sensitive to the assumed model for convection and mixing. The combined effect of H-rich envelope collapse and low 12C(α, γ)16O rate can lead to the formation of BHs with masses consistent with the primary component of GW190521.
Binary black holes in the pair instability mass gap Di Carlo, Ugo N; Mapelli, Michela; Bouffanais, Yann ...
Monthly notices of the Royal Astronomical Society,
09/2020, Letnik:
497, Številka:
1
Journal Article
Recenzirano
Odprti dostop
ABSTRACT
Pair instability (PI) and pulsational PI prevent the formation of black holes (BHs) with mass ≳60 M⊙ from single star evolution. Here, we investigate the possibility that BHs with mass in ...the PI gap form via stellar mergers and multiple stellar mergers, facilitated by dynamical encounters in young star clusters. We analyse 104 simulations, run with the direct N-body code nbody6++gpu coupled with the population synthesis code mobse. We find that up to ∼6 per cent of all simulated BHs have mass in the PI gap, depending on progenitor’s metallicity. This formation channel is strongly suppressed in metal-rich (Z = 0.02) star clusters because of stellar winds. BHs with mass in the PI gap are initially single BHs but can efficiently acquire companions through dynamical exchanges. We find that ∼21 per cent, 10 per cent, and 0.5 per cent of all binary BHs have at least one component in the PI mass gap at metallicity Z = 0.0002, 0.002, and 0.02, respectively. Based on the evolution of the cosmic star formation rate and metallicity, and under the assumption that all stars form in young star clusters, we predict that ∼5 per cent of all binary BH mergers detectable by advanced LIGO and Virgo at their design sensitivity have at least one component in the PI mass gap.
Lithium abundance derived in metal-poor main-sequence (MS) stars is about three times lower than the value of primordial Li predicted by the standard big bang nucleosynthesis when the baryon density ...is taken from the cosmic microwave background or the deuterium measurements. This disagreement is generally referred as the lithium problem. We here reconsider the stellar Li evolution from the pre-main sequence (PMS) to the end of the MS phase by introducing the effects of convective overshooting (OV) and residual mass accretion. We show that 7Li could be significantly depleted by convective OV in the PMS phase and then partially restored in the stellar atmosphere by a tail of matter accretion which follows the Li-depletion phase and that could be regulated by EUV photoevaporation. By considering the conventional nuclear burning and microscopic diffusion along the MS, we can reproduce the Spite plateau for stars with initial mass m
0 = 0.62–0.80 M⊙, and the Li declining branch for lower mass dwarfs, e.g. m
0 = 0.57–0.60 M⊙, for a wide range of metallicities (Z = 0.00001 to Z = 0.0005), starting from an initial Li abundance A(Li) = 2.72. This environmental Li evolution model also offers the possibility to interpret the decrease of Li abundance in extremely metal-poor stars, the Li disparities in spectroscopic binaries and the low Li abundance in planet hosting stars.
We present the colibri code for computing the evolution of stars along the thermally pulsing asymptotic giant branch (TP-AGB) phase. Compared to purely synthetic TP-AGB codes, colibri relaxes a ...significant part of their analytic formalism in favour of a detailed physics applied to a complete envelope model, in which the stellar structure equations are integrated from the atmosphere down to the bottom of the hydrogen-burning shell. This allows us to predict self-consistently: (i) the effective temperature, and more generally the convective envelope and atmosphere structures, correctly coupled to the changes in the surface chemical abundances and gas opacities; (ii) the conditions under which sphericity effects may significantly affect the atmospheres of giant stars; (iii) the core mass-luminosity relation and its possible break-down due to the occurrence of hot-bottom burning (HBB) in the most massive AGB stars, by taking properly into account the nuclear energy generation in the H-burning shell and in the deepest layers of the convective envelope; (iv) the HBB nucleosynthesis via the solution of a complete nuclear network (including the pp chains, and the CNO, NeNa and MgAl cycles) coupled to a diffusive description of mixing, suitable to follow also the synthesis of 7Li via the Cameron-Fowler beryllium transport mechanism; (v) the intershell abundances left by each thermal pulse via the solution of a complete nuclear network applied to a simple model of the pulse-driven convective zone (PDCZ); (vi) the onset and quenching of the third dredge-up, with a temperature criterion that is applied, at each thermal pulse, to the result of envelope integrations at the stage of the post-flash luminosity peak.
At the same time, colibri pioneers new techniques in the treatment of the physics of stellar interiors, not yet adopted in full TP-AGB models. It is the first evolutionary code ever to use accurate on-the-fly computation of the equation of state (EoS) for roughly 800 atoms, ions, molecules and of the Rosseland mean opacities throughout the atmosphere and the deep envelope. This ensures a complete consistency, step by step, of both EoS and opacity with the evolution of the chemical abundances caused by the third dredge-up and HBB. Another distinguishing aspect of colibri is its high computational speed, which allows to generate complete grids of TP-AGB models in just a few hours. This feature is absolutely necessary for calibrating the many uncertain parameters and processes that characterize the TP-AGB phase.
We illustrate the many unique features of colibri by means of detailed evolutionary tracks computed for several choices of model parameters, including initial star masses, chemical abundances, nuclear reaction rates, efficiency of the third dredge-up, overshooting at the base of the PDCZ, etc. Future papers in this series will deal with the calibration of all these and other parameters using observational data of AGB stars in the Galaxy and in nearby systems, a step that is of paramount importance for producing reliable stellar population synthesis models of galaxies up to high redshift.
We extend the PARSEC library of stellar evolutionary tracks by computing new models of massive stars, from 14 to 350 M⊙. The input physics is the same used in the PARSEC V1.1 version, but for the ...mass-loss rate from considering the most recent updates in the literature. We focus on low metallicity, Z = 0.001 and Z = 0.004, for which the metal-poor dwarf irregular star-forming galaxies, Sextans A, the Wolf–Lundmark–Melotte galaxy and NGC 6822, provide simple but powerful workbenches. The models reproduce fairly well the observed colour-magnitude diagrams (CMDs) but the stellar colour distributions indicate that the predicted blue loop is not hot enough in models with a canonical extent of overshooting. In the framework of a mild extended mixing during central hydrogen burning, the only way to reconcile the discrepancy is to enhance the overshooting at the base of the convective envelope (EO) during the first dredge-up. The mixing scales required to reproduce the observed loops, EO = 2H
P or EO = 4H
P, are definitely larger than those derived from, e.g. the observed location of the red-giant-branch bump in low mass stars. This effect, if confirmed, would imply a strong dependence of the mixing scale below the formal Schwarzschild border, on the stellar mass or luminosity. Reproducing the features of the observed CMDs with standard values of envelope overshooting would require a metallicity significantly lower than the values measured in these galaxies. Other quantities, such as the star formation rate and the initial mass function, are only slightly sensitive to this effect. Future investigations will consider other metallicities and different mixing schemes.
We present the dust ejecta of the new stellar models for the thermally pulsing asymptotic giant branch (TP-AGB) phase computed with the colibri code. We use a formalism of dust growth coupled with a ...stationary wind for both M- and C-stars. In the original version of this formalism, the most efficient destruction process of silicate dust in M-giants is chemisputtering by H2 molecules. For these stars, we find that dust grains can only form at relatively large radial distances (r ∼ 5R
*), where they cannot be efficiently accelerated, in agreement with other investigations. In the light of recent laboratory results, we also consider the alternative case that the condensation temperature of silicates is determined only by the competition between growth and free evaporation processes (i.e. no chemisputtering). With this latter approach we obtain dust condensation temperatures that are significantly higher (up to T
cond ∼ 1400 K) than those found when chemisputtering is included (T
cond ∼ 900 K), and in better agreement with condensation experiments. As a consequence, silicate grains can remain stable in inner regions of the circumstellar envelopes (r ∼ 2 R
*), where they can rapidly grow and can be efficiently accelerated. With this modification, our models nicely reproduce the observed trend between terminal velocities and mass-loss rates of Galactic M-giants.
For C-stars the formalism is based on the homogeneous growth scheme where the key role is played by the carbon over oxygen excess. The models reproduce fairly well the terminal velocities of Galactic stars and there is no need to invoke changes in the standard assumptions. At decreasing metallicity the carbon excess becomes more pronounced and the efficiency of dust formation increases. This trend could be in tension with recent observational evidence in favour of a decreasing efficiency, at decreasing metallicity. If confirmed by more observational data, it would indicate that either the amount of the carbon excess, determined by the complex interplay between mass-loss, third dredge-up and hot-bottom burning, or the homogeneous growth scheme should be revised. Finally, we analyse the differences in the total dust production of M-stars that arise from the use of the two approaches (i.e. with or without chemisputtering). We find that, in spite of the differences in the expected dust stratification, for a given set of TP-AGB models, the ejecta are only weakly sensitive to the specific assumption. This work also shows that the properties of TP-AGB circumstellar envelopes are important diagnostic tools that may be profitably added to the traditional calibrators for setting further constraints on this complex phase of stellar evolution.
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