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Register a new userImpact of the Rotation and Compactness of Progenitors on the Mass of Black Holes
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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 a rotating metal-poor star is only $sim{}45$ M$_odot$, while the maximum mass of a BH born from a non-rotating star is $sim{}60$ M$_odot$. Furthermore, stellar rotation reduces the minimum ZAMS mass for a star to collapse into a BH from $sim{}18-25$ M$_odot$ to $sim{}13-18$ M$_odot$. 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 hydrogen envelope that can be accreted onto the final BH is the most important ingredient to determine 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.
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If a black hole has a low spin value, it must double its mass to reach a high spin parameter. Although this is easily accomplished through mergers or accretion in the case of supermassive black holes in galactic centers, it is impossible for stellar-mass black holes in X-ray binaries. Thus, the spin distribution of stellar-mass black holes is almost pristine, largely reflective of the angular momentum imparted at the time of their creation. This fact can help provide insights on two fundamental questions: What is the nature of the central engine in supernovae and gamma-ray bursts? and What was the spin distribution of the first black holes in the universe?
Models of pair-instability supernovae (PISNe) predict a gap in black hole (BH) masses between $sim 45M_odot-120M_odot$, which is referred to as the upper BH mass-gap. With the advent of gravitational-wave astrophysics it has become possible to test this prediction, and there is an important associated effort to understand what theoretical uncertainties modify the boundaries of this gap. In this work we study the impact of rotation on the hydrodynamics of PISNe, which leave no compact remnant, as well as the evolution of pulsational-PISNe (PPISNe), which undergo thermonuclear eruptions before forming a compact object. We perform simulations of non-rotating and rapidly-rotating stripped helium stars in a metal poor environment $(Z_odot/50)$ in order to resolve the lower edge of the upper mass-gap. We find that the outcome of our simulations is dependent on the efficiency of angular momentum transport, with models that include efficient coupling through the Spruit-Tayler dynamo shifting the lower edge of the mass-gap upwards by $sim 4%$, while simulations that do not include this effect shift it upwards by $sim 15%$. From this, we expect the lower edge of the upper mass-gap to be dependent on BH spin, which can be tested as the number of observed BH mergers increases. Moreover, we show that stars undergoing PPISNe have extended envelopes ($Rsim 10-1000~R_odot$) at iron-core collapse, making them promising progenitors for ultra-long gamma-ray bursts.
We build an evolution model of the central black hole that depends on the processes of gas accretion, the capture of stars, mergers as well as electromagnetic torque. In case of gas accretion in the presence of cooling sources, the flow is momentum-driven, after which the black hole reaches a saturated mass; subsequently, it grows only by stellar capture and mergers. We model the evolution of the mass and spin with the initial seed mass and spin in $Lambda$CDM cosmology. For stellar capture, we have assumed a power-law density profile for the stellar cusp in a framework of relativistic loss cone theory that include the effects of black hole spin, Carters constant, loss cone angular momentum, and capture radius. Based on this, the predicted capture rates of $10^{-5}$--$10^{-6}$ yr$^{-1}$ are closer to the observed range. We have considered the merger activity to be effective for $z lesssim 4$, and we self-consistently include the Blandford-Znajek torque. We calculate these effects on the black hole growth individually and in combination, for deriving the evolution. Before saturation, accretion dominates the black hole growth ($sim 95%$ of the final mass), and subsequently, stellar capture and mergers take over with roughly equal contribution. The simulations of the evolution of the $M_{bullet} - sigma$ relation using these effects are consistent with available observations. We run our model backward in time and retrodict the parameters at formation. Our model will provide useful inputs for building demographics of the black holes and in formation scenarios involving stellar capture.
We suggest in this note that spider systems are the naturally expected progenitors of the highest neutron star masses, and possibly low-mass black holes, based on their long-term evolutionary features and actual mass measurements.
We aim to study the progenitor properties and expected rates of the two lowest-mass binary black hole (BH) mergers, GW 151226 and GW 170608, detected within the first two Advanced LIGO-Virgo runs, in the context of the isolated binary-evolution scenario. We use the public MESA code, which we adapted to include BH formation and unstable mass transfer developed during a common-envelope (CE) phase. Using more than 60000 binary simulations, we explore a wide parameter space for initial stellar masses, separations, metallicities, and mass-transfer efficiencies. We obtain the expected distributions for the chirp mass, mass ratio, and merger time delay by accounting for the initial stellar binary distributions. Our simulations show that, while the progenitors we obtain are compatible over the entire range of explored metallicities, they show a strong dependence on the initial masses of the stars, according to stellar winds. All the progenitors follow a similar evolutionary path, starting from binaries with initial separations in the $30-200~R_odot$ range, experiencing a stable mass transfer interaction before the formation of the first BH, and a second unstable mass-transfer episode leading to a CE ejection that occurs either when the secondary star crosses the Hertzsprung gap or when it is burning He in its core. The CE phase plays a fundamental role in the considered low-mass range: only progenitors experiencing such an unstable mass-transfer phase are able to merge in less than a Hubble time. We find integrated merger-rate densities in the range $0.2-5.0~{rm yr}^{-1}~{rm Gpc}^{-3}$ in the local Universe for the highest mass-transfer efficiencies explored. The highest rate densities lead to detection rates of $1.2-3.3~{rm yr}^{-1}$, being compatible with the observed rates. A high CE-efficiency scenario with $alpha_{rm CE}=2.0$ is favored when comparing with observations. ABRIDGED.
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