arXiv subjects

Riouhei Nakatani

Publications and source records attributed to Riouhei Nakatani.

22 records · Page 2Linked to original sources

arXiv2019

Super-Eddington accretion of dusty gas onto seed black holes: metallicity-dependent efficiency of mass growth

The super-Eddington accretion onto intermediate seed BHs is a potential formation mode of supermassive black holes exceeding $10^9~M_\odot$ in the early universe. We here investigate how such rapid accretion may occur with finite amounts of heavy elements contained in the gas and dust. In our 1D radiation-hydrodynamics simulations, the radiative transfer is solved for both the direct UV lights emitted by an accretion disk and the diffuse IR lights thermally emitted by dust grains. Our results show that the radiative force by the IR lights causes a strong feedback to regulate the mass accretion. The resulting mean accretion rate is lower with the higher metallicity, and there is the critical metallicity $Z \sim 10^{-2}~Z_\odot$, above which the super-Eddington accretion is prevented by the radiation pressure of the IR lights. With this taken into account, we examine if the dusty super-Eddington accretion occurs in young galaxies using a simple model. We show that a sufficient number of galaxies at $z \gtrsim 10$ can be such potential sites if BHs accrete the cold dense gas with $T \sim 10^2$ K, approximately the thermal equilibrium value at $Z = 10^{-2}~Z_\odot$. We argue that the efficiency of the BH growth via the rapid accretion depends on the metallicity, and that the metallicity slightly lower than $10^{-2}~Z_\odot$ provides a chance for the most efficient growth.

astro-ph.GA↗

arXiv2018

Photoevaporation of Molecular Gas Clumps Illuminated by External Massive Stars: Clump Lifetimes and Metallicity Dependence

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 with varying the gas metallicity over a range of $10^{-3} \,Z_\odot \leq Z \leq Z_\odot $. Our simulations incorporate radiation transfer of far ultraviolet (FUV) and extreme ultraviolet (EUV) 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 to have small surface areas, where photoevaporative flows are launched. For our fiducial set-up with an O-type star at a distance of 0.1 parsec, the resulting photoevaporation rate is as small as $\sim 10^{-5} M_{\odot}/{\rm yr}$ for metal-rich clumps, but 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 $\sim$1 parsec within the lifetime. We also study photoevaporation of clumps in a photo-dissociation region. Photoelectric heating is inefficient for metal-poor clumps that contain a smaller amount of grains, and thus they survive for over $10^5$ years. 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.

astro-ph.GA↗

arXiv2018

Radiation hydrodynamics simulations of photoevaporation of protoplanetary disks II: Metallicity dependence of UV and X-ray photoevaporation

We perform a suite of radiation hydrodynamics simulations of photoevaporating disks with varying the metallicity in a wide range of $10^{-3} \, Z_\odot \leq Z \leq 10^{0.5} \, Z_\odot $. We follow the disk evolution for over $\sim 5000$ years by solving hydrodynamics, radiative transfer, and non-equilibrium 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 ultra-violet (UV) radiation only. At sub-solar metallicities in the range of $Z \gtrsim 10^{-1.5} \, Z_\odot $, 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. Contrastingly, the photoevaporation rate decreases at even lower metallicities of $Z \lesssim 10^{-1.5} \, Z_\odot $, 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 $Z \sim 10^{-2}\, Z_\odot$. 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.

astro-ph.EP↗

arXiv2018

Radiation Hydrodynamics Simulations of Photoevaporation of Protoplanetary Disks by Ultra Violet Radiation: Metallicity Dependence

Protoplanetary disks are thought to have lifetimes of several million years 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 photoevaporation of protoplanetary disks to study the disk structure and its long-term evolution of $\sim 10000$ years, and the metallicity dependence of mass-loss rate. Our simulations follow hydrodynamics, extreme and far ultra-violet radiative transfer, and non-equilibrium 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 gas metallicity over a wide range of $10^{-4}~ Z_\odot \leq Z \leq 10 ~Z_\odot$. The photoevaporation rate is lower with higher metallicity in the range of $10^{-1} \,Z_\odot \lesssim Z \lesssim 10 \,Z_\odot$, because dust shielding effectively prevents far-ultra violet (FUV) photons from penetrating into and heating the dense regions of the disk. The photoevaporation rate sharply declines at even lower metallicities in $10^{-2} \,Z_\odot \lesssim Z \lesssim 10^{-1}\,Z_\odot$, 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 $10^{-4}\,Z_\odot \leq Z \lesssim 10^{-2}\,Z_\odot$, HI photoionization heating acts as a dominant gas heating process and drives photoevaporative flows with roughly a constant rate. The typical disk lifetime is shorter at $Z=0.3~Z_\odot$ than at $Z = Z_\odot$, being consistent with recent observations of the extreme outer galaxy.

astro-ph.EP↗