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تسجيل مستخدم جديدBirth events, masses and the maximum mass of Compact Stars
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تأليف
Jorge E. Horvath
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We give in this Chapter an overview of the problem of neutron star mass distribution, the issue of the maximum mass as inferred from the existing sample and the new gravitational wave events, and the connection with the formation events. It is shown that at least two different mass-scales (and possibly three) are favored by Bayesian and frequentist analysis, and the resulting maximum mass on empirical grounds only is $sim 2.5 M_{odot}$, making room for an interpretation of the lighter component of the GW190408 merger as a neutron star (as suggested by the GW event population analysis). We discuss in some length the challenges for a theoretical construction of a stiff equation of state and the status of available supernova explosions (single and binary simulations) and AIC expected to provide the masses at birth.
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We conjecture and verify a set of universal relations between global parameters of hot and fast-rotating compact stars, including a relation connecting the masses of the mass-shedding (Kepler) and static configurations. We apply these relations to th
e GW170817 event by adopting the scenario in which a hypermassive compact star remnant formed in a merger evolves into a supramassive compact star that collapses into a black hole once the stability line for such stars is crossed. We deduce an upper limit on the maximum mass of static, cold neutron stars $ 2.15^{+0.10}_{-0.07}le M^star_{mathrm{TOV}} le 2.24^{+0.12}_{-0.10} $ for the typical range of entropy per baryon $2 le S/A le 3$ and electron fraction $Y_e = 0.1$ characterizing the hot hypermassive star. Our result implies that accounting for the finite temperature of the merger remnant relaxes previously derived constraints on the value of the maximum mass of a cold, static compact star.
We present in this article an overview of the problem of neutron star masses. After a brief appraisal of the methods employed to determine the masses of neutron stars in binary systems, the existing sample of measured masses is presented, with a high
light on some very well-determined cases. We discuss the analysis made to uncover the underlying distribution and a few robust results that stand out from them. The issues related to some particular groups of neutron stars originated from different channels of stellar evolution are shown. Our conclusions are that last centurys paradigm that there a single, $1.4 M_{odot}$ scale is too simple. A bimodal or even more complex distribution is actually present. It is confirmed that some neutron stars have masses of $sim 2 M_{odot}$, and, while there is still no firm conclusion on the maximum and minimum values produced in nature, the field has entered a mature stage in which all these and related questions can soon be given an answer.
The observational consequences of the merger scenario for massive star formation are explored and contrasted with the gradual accumulation of mass by accretion. Protostellar mergers may produce high luminosity infrared flares lasting years to centuri
es followed by a luminosity decline on the Kelvin-Helmholtz time-scale of the merger product. Mergers may be surrounded by thick tori of expanding debris, impulsive wide-angle outflows, and shock induced maser and radio continuum emission. Collision products are expected to have fast stellar rotation and a large multiplicity fraction. Close encounters or mergers will produce circumstellar debris disks with an orientation that differs form that of a pre-existing disk. The extremely rare merger of two stars close to the upper-mass end of the IMF may be a possible pathway to hypernova generated gamma-ray bursters. While accretional growth can lead to the formation of massive stars in isolation or in loose clusters, mergers can only occur in high-density cluster environments. It is proposed that the outflow emerging from the OMC1 core in the Orion molecular cloud was produced by a protostellar merger that released between $10^{48}$ to $10^{49}$ ergs less than a thousand years ago.
We perform hydrodynamical simulations of neutron-star mergers for a large sample of temperature-dependent, nuclear equations of state, and determine the threshold mass above which the merger remnant promptly collapses to form a black hole. We find th
at, depending on the equation of state, the threshold mass is larger than the maximum mass of a non-rotating star in isolation by between 30 and 70 per cent. Our simulations also show that the ratio between the threshold mass and maximum mass is tightly correlated with the compactness of the non-rotating maximum-mass configuration. We speculate on how this relation can be used to derive constraints on neutron-star properties from future observations.
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