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Riouhei Nakatani 仲谷崚平

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Riouhei Nakatani 仲谷崚平 Doctorate Dissertation 博士論文 Photoevaporation of Protoplanetary Disks and Molecular Cloud Cores in Star-Forming Regions （星形成領域中の原始惑星系円盤および分子雲コアの光蒸発） A Dissertation Submitted for Degree of Doctor of Philosophy December 2018 平成 30 年 12 月 博士（理学）申請 Department of Physics, Graduate School of Science, The University of Tokyo 東京大学大学院理学系研究科物理学専攻 Riouhei Nakatani 仲谷崚平 ABSTRACT Galaxies contain a large amount of gas in the form of molecules. The molecules shape gaseous clumps called molecular clouds. They are the parental bodies of stars, and star formation is initiated by the gravitational collapse of the clouds. A protostar forms in the collapsing core, and, at the same time, a gaseous disk surrounds the star. The circumstellar disk is composed of gas and a small amount of solid particles (dust) and is the birthplace of planets, and thus it is named a protoplanetary disk (PPD). The PPD coevolves with the central star and eventually disperses. A young stellar system is left behind as a remnant. Recent observations have revealed that planets commonly exist around solar-type stars, but many of them have different characters from those of our Solar System's planets. Although the origin of the variety is not yet clear, it can be attributed to some environmental factors that affect the formation and evolutionary processes of stellar systems. In fact, the occurrence of Jupiter-like planets (Jovian planets) is observationally known to decrease with the host star mass and with the host star metallicity, i.e. the amount of heavy elements. Similarly, the observations of the outer Galaxy, where metallicity is low, have proposed a short dispersal time of PPDs. The origins of these trends are also poorly understood, but it explicitly shows that the formation and evolutionary processes of stellar systems are affected by their forming environments. Studying the processes occurring in diverse environments is a necessary step to understand the origins of such varieties and trends, and, ultimately, to construct a universal picture of stellar system formation and evolution. In this thesis, our main interest is in star and planet formation/evolution in various metallicity environments. Metallicity is one of the important quantities that has the potential to characterize the location of star-forming regions and the period of star-forming activities. For instance, metal- licity decreases as going further from the galactic center in the Milky Way, and more importantly, metallicity is increased as a whole, as galaxies evolve from their primordial states. Hence, consid- ering metallicity variation can give essential implications to study the formation and evolution of stellar systems since the birth of galaxies. This is, in turn, indispensable to study the evolution of galaxies, which are an important constituent of the universe. To this end, in this thesis, we investigate the dispersal processes of PPDs and molecular clouds in star-forming regions with a wide variety of metallicity. PPDs are irradiated by the central stars with their intense ultraviolet (UV) and X-ray radiation. It heats disk surfaces and drives evaporative flows. The dispersal process is termed as photoevaporation and can disperse PPDs within a few million years, which is consistent with the observationally-estimated dispersal time. The disk lifetime limits the available time to form planets, especially for Jovian planets, within PPDs, and is a significantly relevant quantity to determine initial configurations of primordial planetary systems. Hence, the disk lifetime is a particularly important parameter in the context of planet formation. We investigate photoevaporation of PPDs with various metallicities to estimate their lifetimes and to derive, if any, their metallicity dependence. Similarly, in star-forming regions around massive stars, the star's intense radiation drives photoevaporation from the surfaces of molecular clouds and reduces the cloud mass. This negatively affects star formation around massive stars. On the other hand, the intense radiation is known to drive a strong shock in the interior of clouds. The central density of the clouds is increased to strengthen self-gravity. This process positively affects star formation in star-forming regions nearby massive stars. Whether positive or negative, these processes due to the existence of nearby massive stars contribute to set the efficiency of star formation in the star- forming regions. These processes may be particularly important in high-redshift galaxies and the early time of the Milky Way, where massive stars are considered to be preferentially formed. We study photoevaporation of molecular clouds with various metallicities to discuss the influences of massive stars on the efficiency of star formation. First, we perform radiation hydrodynamics simulations of photoevaporating PPDs irradiated by far-UV (FUV; 6 eV . hν ≤ 13:6 eV) and extreme-UV (EUV; hν > 13:6 eV) from the central young star. We solve nonequilibrium chemistry in a self-consistent manner. Dust temperatures are also self-consistently determined by solving radiation transfer for stellar irradiation and dust (re- −4 )emission. We vary the disk metallicity Z in a wide range of 10 Z⊙ ≤ Z ≤ 10 Z⊙. We find that FUV photoelectric heating drives dense, neutral photoevaporative flows, and the FUV heating and dust-gas collisional cooling regulate the resulting mass-loss rates. The FUV-driven neutral flows −8 −1 yield a mass-loss rate of the order of ∼ 10 M⊙ yr at solar metallicity. The mass-loss rate −1 increases as metallicity decreases for 10 Z⊙ . Z . 10 Z⊙, because the amount of dust, which is −2 −1 the main absorber of FUV, is reduced. For 10 Z⊙ . Z . 10 Z⊙, dust-gas collisional cooling becomes efficient compared to FUV heating. This suppresses the FUV-driven photoevaporation, and the resulting mass-loss rates decrease with decreasing metallicity. In the low-metallicity extent −4 −2 of 10 Z⊙ . Z . 10 Z⊙, EUV-driven ionized flows dominantly contribute to the mass loss. Since the main absorber of EUV is hydrogen, the photoevaporation rates are roughly constant with −9 −1 ∼ 10 M⊙ yr . The metallicity dependence of the estimated lifetimes is consistent with those of the observational lifetimes. It directly shows that FUV photoevaporation can be a cause for the observational trend in PPD lifetimes. Next, we incorporate X-ray (0:1 keV . hν . 10 keV) effects into our simulations of photoevapo- rating PPDs and study the influences of X-ray on photoevaporation and lifetimes. The effectiveness of X-ray on photoevaporation has remained a matter of debate, and for the first time, we directly examine it with radiation hydrodynamics simulations with taking into account the spectral energy distribution of X-ray irradiation. The disk metallicity is varied again to investigate the metallicity dependence of photoevaporation caused by both UV and X-ray. Our results show that X-ray heating is not efficient to drive the dense, neutral photoevaporative flows as FUV, but X-ray ionization can strengthen FUV heating by ionizing the neutral regions of PPDs. The metallicity dependence of photoevaporation rates is again largely regulated by FUV heating and dust-gas collisional cooling, and thus the trend is similar to that in the case of UV photoevaporation. However, the strength- −2:5 −2 ening effect of X-ray boosts the photoevaporation rates in 10 Z⊙ . Z . 10 Z⊙, where FUV hardly drives the neutral photoevaporative flows without the X-ray effect. The resulting photoe- −8 −7 −1 vaporation rates are of the order of 10 {10 M⊙ yr and decreases with decreasing metallicity. −3 At Z . 10 Z⊙, the neutral photoevaporative flows are hardly driven, and the resulting mass-loss rates are mainly contributed from the EUV-driven flows. The mass-loss rates are roughly constant without FUV heating, because the metallicity-independent EUV-driven flows set the mass-loss rates in this case. The estimated lifetimes are even more consistent with the observations when the X-ray effects are taken into account in the UV photoevaporation model. Although X-ray photoevaporation has been suggested as a key mechanism to explain the metallicity dependence of the observational lifetimes in a few previous studies that employ a simplified method, our direct comparison based on the self-consistent radiation hydrodynamics simulations indicate that X-ray is ineffective to drive a strong photoevaporation or to directly cause a metallicity-dependent trend in the lifetimes. Finally, we investigate the evolution of low-mass molecular clouds in star-forming regions with a −3 wide variety of metallicities. We consider molecular cloud cores with 10 Z⊙ ≤ Z ≤ 1 Z⊙ exposed to UV field of nearby massive stars in order to derive their lifetimes and discuss star formation efficiency around the massive stars. We perform 3D radiation hydrodynamics simulations, including relevant thermochemical reactions. Again, we simultaneously solve hydrodynamics, radiative transfer, and nonequilibrium chemistry in a self-consistent manner. In our simulations, we observe a strong shock −1 driven by the intense EUV irradiation in the interior of the cores with Z & 10 Z⊙. The shock and hot ambient gas compress the cores to form a cometary structure. The cores survive for ∼ 105 yr. −2 Low-metallicity cores (Z . 10 Z⊙) lack coolants inside, and hence keep high temperatures after compression by the shock. The cores are short-lived and have lifetimes of the order of 104 yr. We find that the free fall time, which is an important measure to quantify the collapsing time of cores to form stars, is sufficiently shortened in the higher-metallicity cores to initiate gravitational collapse before completely dispersed. We also study photoevaporation of molecular cloud cores in photodissociation regions (PDRs), where EUV photons are hardly present. It is shown that FUV heating also compresses cores and form a cometary structure for Z & 1 Z⊙, but the cores −0:5 with Z . 10 Z⊙ expand rather than shrinking. This makes the free fall time longer and thus −1 suppresses star formation.
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