Abstract
Recent multiwavelength observations suggest that inner parts of protoplanetary disks (PPDs) have shorter lifetimes for heavier host stars. Since PPDs around high-mass stars are irradiated by ...strong ultraviolet radiation, photoevaporation may provide an explanation for the observed trend. We perform radiation hydrodynamics simulations of photoevaporation of PPDs for a wide range of host star mass of
M
*
= 0.5–7.0
M
⊙
. We derive disk mass-loss rate
M
̇
, which has strong stellar dependence as
M
̇
≈
7.30
×
10
−
9
(
M
*
/
M
⊙
)
2
M
⊙
yr
−
1
. The absolute value of
M
̇
scales with the adopted far-ultraviolet and X-ray luminosities. We derive the surface mass-loss rates and provide polynomial function fits to them. We also develop a semianalytic model that well reproduces the derived mass-loss rates. The estimated inner-disk lifetime decreases as the host star mass increases, in agreement with the observational trend. We thus argue that photoevaporation is a major physical mechanism for PPD dispersal for a wide range of the stellar mass and can account for the observed stellar mass dependence of the inner-disk lifetime.
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 3D radiation hydrodynamics simulations of photoevaporation of molecular gas clumps illuminated by external massive stars. We study the fate of solar-mass clumps and derive their ...lifetimes by varying the gas metallicity over a range of . Our simulations incorporate radiation transfer of far- and extreme-ultraviolet photons and follow atomic/molecular line cooling and dust-gas collisional cooling. Nonequilibrium chemistry is coupled with the radiative transfer and hydrodynamics in a self-consistent manner. We show that radiation-driven shocks compress gas clumps to have a volume that is set by the pressure equilibrium with the hot ambient gas. Radiative cooling enables metal-rich clumps to condense and have small surface areas where photoevaporative flows are launched. For our fiducial setup with an O-type star at a distance of 0.1 pc, the resulting photoevaporation rate is as small as for metal-rich clumps, but it is larger for metal-poor clumps that have larger surface areas. The clumps are continuously accelerated away from the radiation source by the so-called rocket effect and can travel over ∼1 pc within the lifetime. We also study the photoevaporation of clumps in a photodissociation region. Photoelectric heating is inefficient for metal-poor clumps that contain a smaller amount of grains, and thus they survive for over 105 yr. We conclude that the gas metallicity strongly affects the clump lifetime and thus determines the strength of feedback from massive stars in star-forming regions.
Abstract
Ultraviolet and X-rays from radiation sources disperse nearby gas clumps by driving winds due to heating associated with the photochemical processes. This dispersal process, ...photoevaporation, constrains the lifetimes of the parental bodies of stars and planets. To understand this process in a universal picture, we develop an analytical model that describes the fundamental physics in the vicinity of the wind-launching region. The model explicitly links the density and velocity of photoevaporative winds at the launch points to the radiation flux reaching the wind-launching base, using a jump condition. The model gives a natural boundary condition for the wind-emanating points. We compare the analytical model with the results of radiation hydrodynamic simulations, where a protoplanetary disk is irradiated by the stellar extreme-ultraviolet, and confirm good agreement of the base density and velocity, and radial profiles of mass-loss rates. We expect that our analytical model is applicable to other objects subject to photoevaporation not only by extreme-ultraviolet but by far-ultraviolet/X-rays with suitable modifications. Future self-consistent numerical studies can test the applicability.
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.
The density distribution of the intergalactic medium is an uncertain but highly important issue in the study of cosmic reionization. It is expected that there are abundant gas clouds hosted by ...low-mass "minihalos" in the early universe, which act as photon sinks until being photoevaporated by the emerging ultraviolet background (UVB) radiation. We perform a suite of radiation-hydrodynamic simulations to study the photoevaporation of minihalos. Our simulations follow hydrodynamics, nonequilibrium chemistry, and the associated cooling processes in a self-consistent manner. We conduct a parametric study by considering a wide range of gas metallicities (0 Z ≤ Z ≤ 10−3 Z ), halo mass (103 M ≤ M ≤ 108 M ), UVB intensity (0.01 ≤ J21 ≤ 1), and turn-on redshift of ionizing sources (10 ≤ zIN ≤ 20). We show that small halos are evaporated in a few tens of millions of years, whereas larger mass halos survive 10 times longer. The gas mass evolution of a minihalo can be characterized by a scaling parameter that is given by a combination of the halo mass, background radiation intensity, and redshift. Efficient radiative cooling in metal-enriched halos induces fast condensation of the gas to form a dense, self-shielded core. The cold, dense core can become gravitationally unstable in halos with high metallicities. Early metal enrichment may allow star formation in minihalos during cosmic reionization.
Abstract
We study the early growth of massive seed black holes (BHs) via accretion in protogalactic nuclei where the stellar bulge component is assembled, performing axisymmetric two-dimensional ...radiation hydrodynamical simulations. We find that when a seed BH with
M
•
∼ 10
5
M
⊙
is embedded in dense metal-poor gas (
Z
= 0.01
Z
⊙
) with a density of ≳ 100 cm
−3
and bulge stars with a total mass of
M
⋆
≳ 100
M
•
, a massive gaseous disk feeds the BH efficiently at rates of ≳ 0.3–1
M
⊙
yr
−1
, and the BH mass increases nearly tenfold within ∼2 Myr. This rapid accretion phase lasts until a good fraction of the gas bounded within the bulge accretes onto the BH, although the feeding rate is regulated owing to strong outflows driven by ionizing radiation emitted from the accreting BH. The transient growing mode can be triggered for seed BHs formed in massive dark-matter halos with masses of ≳ 10
9
M
⊙
at
z
∼ 15–20 (the virial temperature is
T
vir
≃ 10
5
K). The host halos are heavier and rarer than those of typical first galaxies, but are more likely to end up in quasar hosts by
z
≃ 6. This mechanism naturally yields a mass ratio of
M
•
/
M
⋆
> 0.01 higher than the value seen in the local universe. The existence of such overmassive BHs provides us with a unique opportunity to detect highly accreting seed BHs at
z
∼ 15 with AB magnitude of
m
AB
∼ 26–29 mag at 2
μ
m (rest frame 10 eV) by the upcoming observations by the James Webb Space Telescope and Nancy Grace Roman Space Telescope.
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.
Ring structures are observed through (sub)millimeter dust continuum emission in various circumstellar disks from the early stages of class 0 and I to the late stage of class II young stellar objects ...(YSOs). In this paper, we study one of the possible scenarios for such ring formation, which is the coagulation of dust aggregates in the early stage. The dust grains grow in an inside-out manner because the growth timescale is roughly proportional to the orbital period. The boundary of the dust evolution can be regarded as the growth front, where the growth time is comparable to the disk age. Using radiative transfer calculations based on the dust coagulation model, we find that the growth front can be observed as a ring structure because the dust surface density changes sharply at this position. Furthermore, we confirm that the observed ring positions in YSOs with an age of 1 Myr are consistent with the growth front. The growth front could be important in creating the ring structure in particular for the early stage of disk evolution, such as class 0 and I sources.
Context. In recent years hydrodynamical (HD) models have become important to describe the gas kinematics in protoplanetary disks, especially in combination with models of photoevaporation and/or ...magnetically driven winds. Our aim is to investigate how vertical shear instability (VSI) could influence the thermally driven winds on the surface of protoplanetary disks. Aims. In this first part of the project, we focus on diagnosing the conditions of the VSI at the highest numerical resolution ever recorded, and suggest at what resolution per scale height we obtain convergence. At the same time, we want to investigate the vertical extent of VSI activity. Finally, we determine the regions where extreme UV (EUV), far-UV (FUV), and X-ray photons are dominant in the disk. Methods. We perform global HD simulations using the PLUTO code. We adopt a global isothermal accretion disk setup, 2.5D (2 dimensions, 3 components) which covers a radial domain from 0.5 to 5.0 and an approximately full meridional extension. Our simulation runs cover a resolution from 12 to 203 cells per scale height. Results. We determine 50 cells per scale height to be the lower limit to resolve the VSI. For higher resolutions, ≥50 cells per scale height, we observe the convergence for the saturation level of the kinetic energy. We are also able to identify the growth of the “body” modes, with higher growth rate for higher resolution. Full energy saturation and a turbulent steady state is reached after 70 local orbits. We determine the location of the EUV heated region defined by Σr = 1019 cm−2 to be at HR ~ 9.7 and the FUV–X-ray heated boundary layer defined by Σr = 1022 cm−2 to be at HR ~ 6.2, making it necessary to introduce a hot atmosphere. For the first time we report the presence of small-scale vortices in the r − Z plane between the characteristic layers of large-scale vertical velocity motions. Such vortices could lead to dust concentration, promoting grain growth. Our results highlight the importance of combining photoevaporation processes in the future high-resolution studies of turbulence and accretion processes in disks.
PDF
