User:ZaperaWiki44/sandbox/Quasi-star



A quasi-star (QS), or quasistar (also called black hole star), is a hypothetical type of extremely massive and luminous star that may have existed early in the history of the Universe. Unlike modern stars, which are powered by nuclear fusion in their cores, a quasi-star's energy would come from material falling into a black hole at its core.

Description
A quasi-star would have resulted from the core of a large protostar collapsing into a black hole, where the outer layers of the protostar are massive enough to absorb the resulting burst of energy without being blown away or falling into the black hole, as occurs with modern supernovae. These intermediate-mass black holes have been suggested as the progenitors of modern supermassive black holes such as the one in the center of the Galaxy.

Formation
Formation of quasi-stars could only happen early in the development of the Universe, before hydrogen and helium were contaminated by heavier elements; thus, they may have been very massive Population III stars, although some may have existed even prior to the formation of the first "normal" stars.

Quasi-stars may have formed via monolithic collapse of atomic-cooling dark matter halos with a viral temperature over $10,000 K$, drawing in enormous amounts of gas via gravity.

This can produce supermassive stars with over tens of thousands of solar masses.

The formation of a quasi-star depends if whether the infall of gas is high enough to prevent a thermal equilibrium to be established in the central hydrostatic core of the atomic-cooling halo. Otherwise, the supermassive star would instead collapse to become a direct collapse black hole.

(over 1 billion solar masses; for reference, some small galaxies have only 5 million solar masses)

The inner core of the supermassive star forms a stellar-mass black hole with an initial mass between in a radiation pressure-supported envelope.

Once the black hole had formed at the core of the protostar with a part of it becoming a luminous central accretion disk, it would continue generating a large amount of radiant energy from the infall of stellar material. This constant outburst of energy would counteract the force of gravity, creating an equilibrium similar to the one that supports modern fusion-based stars and causing the outer layers of the quasi-star to expand and cool down.

The interior black hole may then continue accreting from the stellar envelope, while the accretion rate of the quasi-star will only be limited by the Eddington rate corresponding to the total mass of the configuration.

Models from a 2023 paper, although this study did not follow the collapse of supermassive stars to late times, predict that it is unlikely that X-rays from the central black hole could halt the collapse of modelled stars (with final masses of $e7 solar mass$) because of large infall velocities that enclose most of the stellar mass so it will go into the black hole soon after birth. This would thus prevent the formation of a quasi-star that could create black hole seeds of up to a million solar masses, and instead become straight direct collapse black holes born with the mass at which their progenitors die.

Lifespan
Depending on models, quasi-stars would have had a short maximum lifespan, approximately 7 million years, during which the core black hole would have grown to about 1000 - 10,000 solar mass.

Mass
Depending on models, a quasi-star would have been at least 1,000. almost 10 million solar masses at this point, but can be as high as up to about 100 million solar masses as the upper mass limit.

Mass loss
Extreme mass loss, as high

Temperature
At the time of their formation, quasi-stars are predicted to have had surface temperatures higher than 10000 K.

Luminosity
In the model, the luminosity is approximately equal to the Ed-dington luminosity at the boundary of the innermost convective layer

At these temperatures, and with theses bolometric luminosities, each quasi-star would be about as luminous as a small galaxy.

Radius
They have estimated radii of $3.5 solar mass$. Because quasi-stars cool over time, they also grow larger in radius.

At these temperatures, and with a radius of approximately 800 thousand times that of the Sun (as large as the Solar System).

Such stars would dwarf Mu Cephei, VV Cephei A, and VY Canis Majoris, three among the largest known modern stars.

Accretion disk
The black hole's accretion disk would be geometrically thick, advective disk with a very high accretion rate.

Outflows
The production of a jet may be mediated by rotation of poloidal magnetic fields in the BH and/or disk magnetosphere, similar to that occurring in AGN and GRBs. Such fields can effectively be transported from the outer regions to the center only by geometrically thick accretion flows (Cao 2011; McKinney et al. 2012; Tchekhovskoy et al. 2012).

The jets produce gamma rays in the reconfinement shocks formed within 0.01-1 rQS, i.e. 1015 − 1017 cm. As such, researches also proposed that quasi-star jets may be accounted for a large fraction of unidentified gamma-ray sources located at high latitudes, in which most of them are considered to be extragalactic. Like blazars, they would produce nonthermal spectra at lower energies (optical-IR) dominated by the synchrotron mechanism and in the gamma-ray band by the inverse-Compton process. However, they can be distinguished from blazars depending on the ratio of the gamma-ray to the IR components and the presence of broad emission lines. Most of BL Lacertae objects are expected to have a low ratio and no broad emission lines.

Evolution
As a quasi-star cools over time, its outer envelope would become transparent, until further cooling to a limiting temperature of roughly 5000 - 4000 K for Population III opacities or lower if metal-enriched, as low as roughly $3,030 solar radius$ for a solar metallicity (e.g Population I stars). The limiting temperature would mark the end of the quasi-star's life since there is no hydrostatic equilibrium at or below this limiting temperature. The object would then quickly dissipate by radiation pressure, leaving behind the intermediate mass black hole.

Ball
A quasi-star with an initial mass $3,000 K$ begins its life with an effective temperature of $10,000 solar mass$, a luminosity of $14,300 K$ and radius of $3.48 solar luminosity$ with a $3,030 solar radius$ central black hole accreting at $5 solar mass$.

The second feature, apparent in all but the first density profile in Fig. 1, is the density inversion in the outer layers. It appears once the photospheric temperature Tsurf drops below about 8000 K. From then, the surface opacity increases owing to hydrogen recombination.

Before the end of the evolution by 3.7 million years, these properties, along with the black hole's accretion rate, achieve their local limits, reaching $solar mass$ and $4,490 K$ with a luminosity of $40,700 solar radius$ and a black hole accretion rate of $6.05 solar luminosity$. By 4.23 million years, the quasi-star's evolution terminates as the black hole reaches a final mass $3.7 solar mass$ with a cavity mass of $1,194 solar mass$, but with a decreased accretion rate $3,360 solar mass$, with quasi-star's properties being $3.53 solar mass$ and $4,510 K$ with a luminosity of $39,600 solar radius$ before hydrostatic equilibrium breaks down.

The physical reason for this upper limit remains elusive but we have made some progress in understanding it using a modified version of the Lane-Emden equation (see Section 4). The existence of the limit is certainly robust as it is does not depend on the total mass of the quasi-star over at least two orders of magnitude (see Section 5.4) nor on whether the envelope mass changes in time (see Section 5.3).

Subsequent evolution
The material within the cavity, thus the Bondi radius of the black hole, would be already still moving towards the black hole, presumably accreting at its Eddington-limited rate.

Fiacconi
In another model, quasi-stars with a relatively low mass might be able to form a central accretion disc and reach an equilibrium configuration but last shorter, for only few thousands of years before the accretion luminosity unbinds the surrounding envelope, with outflows then suppress the growth of the central black hole. This would result rather a mass for the black hole for $5.81 solar luminosity$.