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arXiv 1706.04570astro-ph.EPastro-ph.GAastro-ph.SR
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紫外辐射下原行星盘光蒸发的辐射流体力学模拟:金属丰度依赖性

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

Riouhei Nakatani, Takashi Hosokawa, Naoki Yoshida, Hideko Nomura, Rolf Kuiper

摘要:

原行星盘被认为在太阳附近有几百万年的寿命,但最近的观测表明,在低金属含量的环境中,盘的寿命更短。利用辐射流体力学方法模拟了原行星盘的光蒸发过程,研究了盘的结构及其$\sim 10000$年的长期演化,以及质量损失率与金属丰度的关系.我们的模拟遵循流体力学,极端和远紫外辐射传输,和非平衡化学在一个自洽的方式。通过求解恒星辐射和尘埃颗粒(再)发射的辐射传输,一致地计算了尘埃颗粒的温度。我们在很宽的$10^{-4}~ Z_\odot \leq Z \leq 10 ~Z_\odot$范围内改变盘气体金属丰度。在$10^{-1} \,Z_\odot \lesssim Z \lesssim 10 \,Z_\odot$范围内,金属丰度越高,光蒸发率越低,因为灰尘屏蔽有效地防止了远紫外(FUV)光子穿透并加热盘的致密区域。光蒸发率急剧下降,甚至更低的金属含量在$10^{-2} \,Z_\odot \lesssim Z \lesssim 10^{-1}\,Z_\odot$,因为FUV光电加热变得不如有效的尘埃气体碰撞冷却。中性区域的温度降低,光蒸发流仅在盘的外部区域被激发。在$10^{-4}\,Z_\odot \leq Z \lesssim 10^{-2}\,Z_\odot$,HI光电离加热作为一个主要的气体加热过程,并驱动光蒸发流大致恒定的速率。在$Z=0.3~Z_\odot$上的典型盘寿命比在$Z = Z_\odot$上的短,这与最近对极端外星系的观测结果一致。

英文摘要:

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.

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