In the core-accretion formation scenario of gas giants, most of the gas accreting onto a planet is processed through an accretion shock. In this series of papers we study this shock because it is key ...in setting the structure of the forming planet and thus its postformation luminosity, with dramatic observational consequences. We perform one-dimensional gray radiation-hydrodynamical simulations with nonequilibrium (two-temperature) radiation transport and up-to-date opacities. We survey the parameter space of accretion rate, planet mass, and planet radius and obtain postshock temperatures, pressures, and entropies, as well as global radiation efficiencies. We find that the shock temperature is usually given by the "free-streaming" limit. At low temperatures the dust opacity can make the shock hotter but not significantly so. We corroborate this with an original semianalytical derivation of . We also estimate the change in luminosity between the shock and the nebula. Neither nor the luminosity profile depend directly on the optical depth between the shock and the nebula. Rather, depends on the immediate preshock opacity, and the luminosity change on the equation of state. We find quite high immediate postshock entropies ( -20 ), which makes it seem unlikely that the shock can cool the planet. The global radiation efficiencies are high ( ), but the remainder of the total incoming energy, which is brought into the planet, exceeds the internal luminosity of classical cold starts by orders of magnitude. Overall, these findings suggest that warm or hot starts are more plausible.
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
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.
Abstract
The population of close-in super-Earths, with gas mass fractions of up to 10 per cent represents a challenge for planet formation theory: how did they avoid runaway gas accretion and ...collapsing to hot Jupiters despite their core masses being in the critical range of M
c ≃ 10 M⊕? Previous three-dimensional (3D) hydrodynamical simulations indicate that atmospheres of low-mass planets cannot be considered isolated from the protoplanetary disc, contrary to what is assumed in 1D-evolutionary calculations. This finding is referred to as the recycling hypothesis. In this paper, we investigate the recycling hypothesis for super-Earth planets, accounting for realistic 3D radiation hydrodynamics. Also, we conduct a direct comparison in terms of the evolution of the entropy between 1D and 3D geometries. We clearly see that 3D atmospheres maintain higher entropy: although gas in the atmosphere loses entropy through radiative cooling, the advection of high-entropy gas from the disc into the Bondi/Hill sphere slows down Kelvin–Helmholtz contraction, potentially arresting envelope growth at a sub-critical gas mass fraction. Recycling, therefore, operates vigorously, in line with results by previous studies. However, we also identify an ‘inner core’ – in size ≈25 per cent of the Bondi radius – where streamlines are more circular and entropies are much lower than in the outer atmosphere. Future studies at higher resolutions are needed to assess whether this region can become hydrodynamically isolated on long time-scales.
ABSTRACT We present radiation hydrodynamic simulations of collapsing protostellar cores with initial masses of 30, 100, and 200 M . We follow their gravitational collapse and the formation of a ...massive protostar and protostellar accretion disk. We employ a new hybrid radiative feedback method blending raytracing techniques with flux-limited diffusion for a more accurate treatment of the temperature and radiative force. In each case, the disk that forms becomes Toomre-unstable and develops spiral arms. This occurs between 0.35 and 0.55 freefall times and is accompanied by an increase in the accretion rate by a factor of 2-10. Although the disk becomes unstable, no other stars are formed. In the case of our 100 and 200 M simulations, the star becomes highly super-Eddington and begins to drive bipolar outflow cavities that expand outwards. These radiatively driven bubbles appear stable, and appear to be channeling gas back onto the protostellar accretion disk. Accretion proceeds strongly through the disk. After 81.4 kyr of evolution, our 30 M simulation shows a star with a mass of 5.48 M and a disk of mass 3.3 M , while our 100 M simulation forms a 28.8 M mass star with a 15.8 M disk over the course of 41.6 kyr, and our 200 M simulation forms a 43.7 M star with an 18 M disk in 21.9 kyr. In the absence of magnetic fields or other forms of feedback, the masses of the stars in our simulation do not appear to be limited by their own luminosities.
We perform radiation hydrodynamics simulations to study the structure and evolution of a photoevaporating protoplanetary disk. Ultraviolet and X-ray radiation from the host star heats the disk ...surface, where H2 pumping also operates efficiently. We run a set of simulations in which we varied the number of dust grains or the dust-to-gas mass ratio, which determines the relative importance between photoelectric heating and H2 pumping. We show that H2 pumping and X-ray heating contribute more strongly to the mass loss of the disk when the dust-to-gas mass ratio is D≤10−3. The disk mass-loss rate decreases with a lower dust amount, but remains around 10−10−11M⊙yr−1. In these dust-deficient disks, H2 pumping enhances photoevaporation from the inner disk region and shapes the disk mass-loss profile. We thus argue that the late-stage disk evolution is affected by the ultraviolet H2 pumping effect. The mass-loss rates derived from our simulations can be used in the study of long-term disk evolution.
Abstract
Debris disks are classically considered to be gas-less systems, but recent (sub)millimeter observations have detected tens of those with rich gas content. The origin of the gas component ...remains unclear, but it could be protoplanetary remnants and/or secondary products from large bodies. In order to be protoplanetary in origin, the gas component of the parental protoplanetary disk is required to survive for
≳
10
Myr
. However, previous models predict
≲
10
Myr
lifetimes because of efficient photoevaporation at the late stage of disk evolution. We investigate photoevaporation of gas-rich, optically-thin disks around intermediate-mass stars at a late stage of the disk evolution. The evolved system is modeled like those devoid of small grains (
≲
4
μ
m
). We find that grain depletion reduces photoelectric heating so that far-ultraviolet photoevaporation is not excited. Extreme-ultraviolet (EUV) photoevaporation is dominant and yields a mass-loss rate of the order of
1
×
10
−
11
(
Φ
EUV
/
10
38
s
−
1
)
1
/
2
M
⊙
yr
−
1
, where
Φ
EUV
is the EUV emission rate of the host star. The estimated gas–disk lifetimes are
∼
100
(
M
disk
/
10
−
3
M
⊙
)
(
Φ
EUV
/
10
38
s
−
1
)
1
/
2
Myr
and depend on the “initial” disk mass at the point small grains have been depleted in the system. We show that the gas component can survive for a much longer time around A-type stars than lower-mass (F-, G-, K-type) stars owing to their atypical low EUV (and X-ray) luminosities. This trend is consistent with the higher frequency of gas-rich debris disks around A-type stars, implying the possibility of the gas component being protoplanetary remnants.
Astrophysical fluid flow studies often encompass a wide range of physical processes to account for the complexity of the system under consideration. In addition to gravity, a proper treatment of ...thermodynamic processes via continuum radiation transport and/or photoionization is becoming the state of the art. We present a major update of our continuum radiation transport module, Makemake, and a newly developed module for photoionization, Sedna, coupled to the magnetohydrodynamics code PLUTO. These extensions are currently not publicly available; access can be granted on a case-by-case basis. We explain the theoretical background of the equations solved, elaborate on the numerical layout, and present a comprehensive test suite for radiation-ionization hydrodynamics. The grid-based radiation and ionization modules support static one-dimensional, two-dimensional, and three-dimensional grids in Cartesian, cylindrical, and spherical coordinates. Each module splits the radiation field into two components, one originating directly from a point source-solved using a ray-tracing scheme-and a diffuse component-solved with a three-dimensional flux-limited diffusion (FLD) solver. The FLD solver for the continuum radiation transport makes use of either the equilibrium one-temperature approach or the linearization two-temperature approach. The FLD solver for the photoionization module enables accounting for the temporal evolution of the radiation field from direct recombination of free electrons into hydrogen's ground state as an alternative to on-the-spot approximation. A brief overview of completed and ongoing scientific studies is given to explicitly illustrate the multipurpose nature of the numerical framework presented.
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