The Birth of a Massive First-star Binary Sugimura, Kazuyuki; Matsumoto, Tomoaki; Hosokawa, Takashi ...
Astrophysical journal. Letters,
03/2020, Letnik:
892, Številka:
1
Journal Article
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We study the formation of massive Population III binary stars using a newly developed radiation hydrodynamics code with the adaptive mesh refinement and adaptive ray-tracing methods. We follow the ...evolution of a typical primordial star-forming cloud obtained from a cosmological hydrodynamics simulation. Several protostars form as a result of disk fragmentation and grow in mass by the gas accretion, which is finally quenched by the radiation feedback from the protostars. Our code enables us, for the first time, to consider the feedback by both the ionizing and dissociating radiation from the multiple protostars, which is essential for self-consistently determining their final masses. At the final step of the simulation, we observe a very wide ( 104 au) binary stellar system consisting of 60 and 70 M stars. One of the member stars also has two smaller mass (10 M ) companion stars orbiting at 200 and 800 au, making up a mini-triplet system. Our results suggest that massive binary or multiple systems are common among Population III stars.
ABSTRACT Gravitational collapse of a massive primordial gas cloud is thought to be a promising path for the formation of supermassive black holes in the early universe. We study conditions for the ...so-called direct collapse (DC) black hole formation in a fully cosmological context. We combine a semianalytic model of early galaxy formation with halo merger trees constructed from dark matter N-body simulations. We locate a total of 68 possible DC sites in a volume of on a side. We then perform hydrodynamics simulations for 42 selected halos to study in detail the evolution of the massive clouds within them. We find only two successful cases where the gas clouds rapidly collapse to form stars. In the other cases, gravitational collapse is prevented by the tidal force exerted by a nearby massive halo, which otherwise should serve as a radiation source necessary for DC. Ram pressure stripping disturbs the cloud approaching the source. In many cases, a DC halo and its nearby light source halo merge before the onset of cloud collapse. When the DC halo is assembled through major mergers, the gas density increases rapidly to trigger gravitational instability. Based on our cosmological simulations, we conclude that the event rate of DC is an order of magnitude smaller than reported in previous studies, although the absolute rate is still poorly constrained. It is necessary to follow the dynamical evolution of a DC cloud and its nearby halo(s) in order to determine the critical radiation flux for DC.
ABSTRACT The formation of supermassive stars (SMSs) via rapid mass accretion and their direct collapse into black holes (BHs) is a promising pathway for sowing seeds of supermassive BHs in the early ...universe. We calculate the evolution of rapidly accreting SMSs by solving the stellar structure equations including nuclear burning as well as general relativistic (GR) effects up to the onset of the collapse. We find that such SMSs have a less concentrated structure than a fully convective counterpart, which is often postulated for non-accreting ones. This effect stabilizes the stars against GR instability even above the classical upper mass limit 105 M derived for the fully convective stars. The accreting SMS begins to collapse at the higher mass with the higher accretion rate. The collapse occurs when the nuclear fuel is exhausted only for cases with . With , the star becomes GR unstable during the helium-burning stage at M 2-3.5 × 105 M . In an extreme case with 10 , the star does not collapse until the mass reaches 8.0 × 105 M , where it is still in the hydrogen-burning stage. We expect that BHs with roughly the same mass will be left behind after the collapse in all the cases.
The evolution of protostellar outflow is investigated with resistive magneto-hydrodynamic nested-grid simulations that cover a wide range of spatial scales (∼1 au-1 pc). We follow cloud evolution ...from the pre-stellar core stage until the infalling envelope dissipates long after the protostar formation. We also calculate protostellar evolution to derive protostellar luminosity with time-dependent mass accretion through a circumstellar disc. The protostellar outflow is driven by the first core prior to protostar formation and is directly driven by the circumstellar disc after protostar formation. The opening angle of the outflow is large in the Class 0 stage. A large fraction of the cloud mass is ejected in this stage, which reduces the star formation efficiency to ∼50 per cent. After the outflow breaks out from the natal cloud, the outflow collimation is gradually improved in the Class I stage. The head of the outflow travels more than ∼105 au in ∼105 yr. The outflow momentum, energy and mass derived in our calculations agree well with observations. In addition, our simulations show the same correlations among outflow momentum flux, protostellar luminosity and envelope mass as those in observations. These correlations differ between Class 0 and I stages, which are explained by different evolutionary stages of the outflow; in the Class 0 stage, the outflow is powered by the accreting mass and acquires its momentum from the infalling envelope; in the Class I stage, the outflow enters the momentum-driven snow-plough phase. Our results suggest that protostellar outflow should determine the final stellar mass and significantly affect the early evolution of low-mass protostars.
Protoplanetary disks are thought to have lifetimes of several million yr in the solar neighborhood, but recent observations suggest that the disk lifetimes are shorter in a low-metallicity ...environment. We perform a suite of radiation hydrodynamics simulations of photoevaporating protoplanetary disks to study their long-term evolution of ∼10,000 yr and the metallicity dependence of mass-loss rates. Our simulations follow hydrodynamics, extreme and far-ultraviolet (FUV) radiative transfer, and nonequilibrium chemistry in a self-consistent manner. Dust-grain temperatures are also calculated consistently by solving the radiative transfer of the stellar irradiation and grain (re-)emission. We vary the disk metallicity over a wide range of 10 − 4 Z ≤ Z ≤ 10 Z . The photoevaporation rate is lower with higher metallicity in the range of 10 − 1 Z Z 10 Z , because dust shielding effectively prevents FUV photons from penetrating and heating the dense regions of the disk. The photoevaporation rate sharply declines at even lower metallicities in 10 − 2 Z Z 10 − 1 Z , because FUV photoelectric heating becomes less effective than dust-gas collisional cooling. The temperature in the neutral region decreases, and photoevaporative flows are excited only in an outer region of the disk. At 10 − 4 Z ≤ Z 10 − 2 Z , H i photoionization heating acts as a dominant gas heating process and drives photoevaporative flows with a roughly constant rate. The typical disk lifetime is shorter at Z = 0.3 Z than at Z = Z , being consistent with recent observations of the extreme outer galaxy.
We perform a suite of radiation hydrodynamics simulations of photoevaporating disks, varying the metallicity in a wide range of . We follow the disk evolution for over ∼5000 years by solving ...hydrodynamics, radiative transfer, and nonequilibrium chemistry. Our chemistry model is updated from the first paper of this series by adding X-ray ionization and heating. We study the metallicity dependence of the disk photoevaporation rate and examine the importance of X-ray radiation. In the fiducial case with solar metallicity, including the X-ray effects does not significantly increase the photoevaporation rate when compared to the case with ultraviolet (UV) radiation only. At subsolar metallicities in the range of , the photoevaporation rate increases as metallicity decreases owing to the reduced opacity of the disk medium. The result is consistent with the observational trend that disk lifetimes are shorter in low metallicity environments. In contrast, the photoevaporation rate decreases at even lower metallicities of , because dust-gas collisional cooling remains efficient compared to far-UV photoelectric heating whose efficiency depends on metallicity. The net cooling in the interior of the disk suppresses the photoevaporation. However, adding X-ray radiation significantly increases the photoevaporation rate, especially at . Although the X-ray radiation itself does not drive strong photoevaporative flows, X-rays penetrate deep into the neutral region in the disk, increase the ionization degree there, and reduce positive charges of grains. Consequently, the effect of photoelectric heating by far-UV radiation is strengthened by the X-rays and enhances the disk photoevaporation.
ABSTRACT
Coalescence of intermediate-mass black holes (IMBHs) as a result of the migration toward galactic centres via dynamical friction may contribute to the formation of supermassive BHs. Here we ...reinvestigate the gaseous dynamical friction, which was claimed to be inefficient with radiative feedback from BHs in literature, by performing 3D radiation-hydrodynamics simulations that solve the flow structure in the vicinity of BHs. We consider a 104-M⊙ BH moving at the velocity vflow through the homogeneous medium with metallicity Z in the range of 0–0.1 Z⊙ and density n∞. We show that, if n∞ ≲ 106 cm−3 and vflow ≲ 60 km s−1, the BH is accelerated forward because of the gravitational pull from a dense shell ahead of an ionized bubble around the BH, regardless of the value of Z. If n∞ ≳ 106 cm−3, however, our simulation shows the opposite result. The ionized bubble and associating shell temporarily appear, but immediately go downstream with significant ram pressure of the flow. They eventually converge into a massive downstream wake, which gravitationally drags the BH backward. The BH decelerates over the time-scale of ∼0.01 Myr, much shorter than the dynamical time-scale in galactic discs. Our results suggest that IMBHs that encounter the dense clouds rapidly migrate toward galactic centres, where they possibly coalescence with others.
Abstract
Discovery of high-redshift (z > 6) supermassive black holes (BHs) may indicate that the rapid (or super-Eddington) gas accretion has aided their quick growth. Here, we study such rapid ...accretion of the primordial gas on to intermediate-mass (102–105 M⊙) BHs under anisotropic radiation feedback. We perform two-dimensional radiation hydrodynamics simulations that solve the flow structure across the Bondi radius, from far outside of the Bondi radius down to a central part that is larger than a circum-BH accretion disc. The radiation from the unresolved circum-BH disc is analytically modelled considering self-shadowing effect. We show that the flow settles into a steady state, where the flow structure consists of two distinct parts: (1) bipolar ionized outflowing regions, where the gas is pushed outward by thermal gas pressure and super-Eddington radiation pressure, and (2) an equatorial neutral inflowing region, where the gas falls towards the central BH without affected by radiation feedback. The resulting accretion rate is much higher than that in the case of isotropic radiation, far exceeding the Eddington-limited rate to reach a value slightly lower than the Bondi one. The opening angle of the equatorial inflowing region is determined by the luminosity and directional dependence of the central radiation. We find that photoevaporation from its surfaces set the critical opening angle of about 10° below which the accretion to the BH is quenched. We suggest that the shadowing effect allows even stellar-remnant BHs to grow rapidly enough to become high-redshift supermassive BHs.
Abstract
We perform the first three-dimensional radiation hydrodynamical simulations that investigate the growth of intermediate-mass BHs (IMBHs) embedded in massive self-gravitating, dusty nuclear ...accretion disks. We explore the dependence of mass accretion efficiency on the gas metallicity
Z
and mass injection at super-Eddington accretion rates from the outer galactic disk
M
̇
in
, and we find that the central BH can be fed at rates exceeding the Eddington rate only when the dusty disk becomes sufficiently optically thick to ionizing radiation. In this case, mass outflows from the disk owing to photoevaporation are suppressed, and thus a large fraction (≳40%) of the mass injection rate can feed the central BH. The conditions are expressed as
M
̇
in
>
2.2
×
10
−
1
M
⊙
yr
−
1
(
1
+
Z
/
10
−
2
Z
⊙
)
−
1
(
c
s
/
10
km
s
−
1
)
, where
c
s
is the sound speed in the gaseous disk. With increasing numerical resolution, vigorous disk fragmentation reduces the disk surface density, and dynamical heating by formed clumps makes the disk geometrically thicker. As a result, the photoevaporative mass-loss rate rises and thus the critical injection rate increases for fixed metallicity. This process enables super-Eddington growth of BHs until the BH mass reaches
M
BH
∼
10
7
–
8
M
⊙
, depending on the properties of the host dark-matter halo and metal-enrichment history. In the assembly of protogalaxies, seed BHs that form in overdense regions with a mass variance of 3–4
σ
at
z
∼ 15–20 are able to undergo short periods of rapid growth and transit into the Eddington-limited growth phase afterward to be supermassive BHs observed at
z
> 6–7.
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