503 research outputs found

    Trapping of the HII and Photodissociation Region in a Radially Stratified Molecular Cloud

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    We study the expansion of the ionization and dissociation fronts (DFs) in a radially stratified molecular cloud, whose density distribution is represented as n(r) \propto r^-w. We focus on cases with w \leq 1.5, when the ionization front is ``trapped'' in the cloud and expands with the preceding shock front. The simultaneous evolution of the outer photodissociation region (PDR) is examined in detail. First, we analytically probe the time evolution of the column densities of the shell and envelope outside the HII region, which are key physical quantities for the shielding of dissociating photons. Next, we perform numerical calculations, and study how the thermal/chemical structure of the outer PDR changes with different density gradients. We apply our numerical model to the Galactic HII region, Sharpless 219 (Sh219). The time evolution of the column densities of the shell and outer envelope depends on w, and qualitatively changes across w = 1. In the cloud with w < 1, the shell column density increases as the HII region expands. The DFs are finally trapped in the shell, and the molecular gas gradually accumulates in the shell. The molecular shell and envelope surround the HII region. With w > 1, on the other hand, the shell column density initially increases, but finally decreases. The column density of the outer envelope also quickly decreases as the HII region swells up. It becomes easier and easier for the dissociating photons to penetrate the shell and envelope. The PDR broadly extends around the trapped HII region. A model with w = 1.5 successfully explains the observational properties of Sh219. Our model suggests that a density-bounded PDR surrounds the photon-bounded HII region in Sh219.Comment: 9 pages, 8 figures, accepted for publication in A&

    Radiation Transfer of Models of Massive Star Formation. III. The Evolutionary Sequence

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    We present radiation transfer (RT) simulations of evolutionary sequences of massive protostars forming from massive dense cores in environments of high surface densities. The protostellar evolution is calculated with a detailed multi-zone model, with the accretion rate regulated by feedback from an evolving disk-wind outflow cavity. Disk and envelope evolutions are calculated self-consistently. In this framework, an evolutionary track is determined by three environmental initial conditions: the initial core mass M_c, the mean surface density of the ambient star-forming clump Sigma_cl, and the rotational-to-gravitational energy ratio of the initial core, beta_c. Evolutionary sequences with various M_c, Sigma_cl, beta_c are constructed. We find that in a fiducial model with M_c=60Msun, Sigma_cl=1 g/cm^2 and beta_c=0.02, the final star formation efficiency >~0.43. For each evolutionary track, RT simulations are performed at selected stages, with temperature profiles, SEDs, and images produced. At a given stage the envelope temperature is highly dependent on Sigma_cl, but only weakly dependent on M_c. The SED and MIR images depend sensitively on the evolving outflow cavity, which gradually wides as the protostar grows. The fluxes at <~100 microns increase dramatically, and the far-IR peaks move to shorter wavelengths. We find that, despite scatter caused by different M_c, Sigma_cl, beta, and inclinations, sources at a given evolutionary stage appear in similar regions on color-color diagrams, especially when using colors at >~ 70 microns, where the scatter due to the inclination is minimized, implying that such diagrams can be useful diagnostic tools of evolutionary stages of massive protostars. We discuss how intensity profiles along or perpendicular to the outflow axis are affected by environmental conditions and source evolution.Comment: 28 pages, 26 figures. Accepted for publication in Ap

    Supersonic Gas Streams Enhance the Formation of Massive Black Holes in the Early Universe

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    The origin of super-massive black holes in the early universe remains poorly understood.Gravitational collapse of a massive primordial gas cloud is a promising initial process,but theoretical studies have difficulty growing the black hole fast enough.We report numerical simulations of early black hole formation starting from realistic cosmological conditions.Supersonic gas motions left over from the Big Bang prevent early gas cloud formation until rapid gas condensation is triggered in a proto-galactic halo. A protostar is formed in the dense, turbulent gas cloud, and it grows by sporadic mass accretion until it acquires 34,000 solar masses.The massive star ends its life with a catastrophic collapse to leave a black hole -- a promising seed for the formation of a monstrous black hole.Comment: Published in Science, combined with updated SOM, additional images and movies are available at http://www-utap.phys.s.u-tokyo.ac.jp/naoki.yoshida/Blackhole/0929e.htm