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  • 1
    Electronic Resource
    Electronic Resource
    Palo Alto, Calif. : Annual Reviews
    ISSN: 0066-4146
    Source: Annual Reviews Electronic Back Volume Collection 1932-2001ff
    Topics: Physics
    Type of Medium: Electronic Resource
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  • 2
    Publication Date: 2015-02-08
    Description: We perform a large set of cosmological simulations of early structure formation and follow the formation and evolution of 1540 star-forming gas clouds to derive the mass distribution of primordial stars. The star formation in our cosmological simulations is characterized by two distinct populations, the so-called Population III.1 stars and primordial stars formed under the influence of far-ultraviolet (FUV) radiation (Population III.2 D stars). In this work, we determine the stellar masses by using the dependences on the physical properties of star-forming cloud and/or the external photodissociating intensity from nearby primordial stars, which are derived from the results of 2D radiation hydrodynamic simulations of protostellar feedback. The characteristic mass of the Pop III stars is found to be a few hundred solar masses at z  ~ 25, and it gradually shifts to lower masses with decreasing redshift. At high redshifts z  〉 20, about half of the star-forming gas clouds are exposed to intense FUV radiation and thus give birth to massive Pop III.2 D stars. However, the local FUV radiation by nearby Pop III stars becomes weaker at lower redshifts, when typical Pop III stars have smaller masses and the mean physical separation between the stars becomes large owing to cosmic expansion. Therefore, at z  〈 20, a large fraction of the primordial gas clouds host Pop III.1 stars. At z   15, the Pop III.1 stars are formed in relatively cool gas clouds due to efficient radiative cooling by H 2 and HD molecules; such stars have masses of a few 10 M . Since the stellar evolution and the final fate are determined by the stellar mass, Pop III stars formed at different epochs play different roles in the early universe.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
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  • 3
    Publication Date: 2014-09-18
    Description: The ratio of mass and magnetic flux determines the relative importance of magnetic and gravitational forces in the evolution of molecular clouds and their cores. Its measurement is thus central in discriminating between different theories of core formation and evolution. Here, we discuss the effect of chemical depletion on measurements of the mass-to-flux ratio using the same molecule (OH) both for Zeeman measurements of the magnetic field and the determination of the mass of the region. The uncertainties entering through the OH abundance in determining separately the magnetic field and the mass of a region have been recognized in the literature. It has been proposed however that, when comparing two regions of the same cloud, the abundance will in both cases be the same. We show that this assumption is invalid. We demonstrate that when comparing regions with different densities, the effect of OH depletion, in measuring changes of the mass-to-flux ratio between different parts of the same cloud can even reverse the direction of the underlying trends (for example, the mass-to-flux ratio may appear to decrease as we move to higher density regions). The systematic errors enter primarily through the inadequate estimation of the mass of the region.
    Print ISSN: 1745-3925
    Electronic ISSN: 1745-3933
    Topics: Physics
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  • 4
    Publication Date: 2016-04-23
    Description: The formation of supermassive stars (SMSs) is a potential pathway to seed supermassive black holes in the early universe. A critical issue for forming SMSs is stellar UV feedback, which may limit the stellar mass growth via accretion. In this paper, we study the evolution of an accreting SMS and its UV emissivity with realistic variable accretion from a circumstellar disc. First we conduct a 2D hydrodynamical simulation to follow the protostellar accretion until the stellar mass exceeds 10 4 M . The disc fragments by gravitational instability, creating many clumps that migrate inward to fall on to the star. The resulting accretion history is highly time-dependent: short episodic accretion bursts are followed by longer quiescent phases. We show that the disc for the direct collapse model is more unstable and generates greater variability than normal Pop III cases. Next, we conduct a stellar evolution calculation using the obtained accretion history. Our results show that, regardless of the variable accretion, the stellar radius monotonically increases with almost constant effective temperature at T eff ~= 5000 K as the stellar mass increases. The resulting UV feedback is too weak to hinder accretion due to the low flux of stellar UV photons. The insensitivity of stellar evolution to variable accretion is attributed to the fact that time-scales of variability, 10 3 yr, are too short to affect the stellar structure. We argue that this evolution will continue until the SMS collapses to produce a black hole by the general relativistic instability after the mass reaches 10 5 M .
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
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  • 5
    Publication Date: 2015-07-10
    Description: Recent observations show that supermassive black holes (BHs) with ~10 9 M exist at redshift z   6. A promising formation channel is the so-called direct collapse model, which posits that a massive seed BH forms through gravitational collapse of a ~10 5 M supermassive star (SMS). We study the evolution of such an SMS growing by rapid mass accretion. In particular, we examine the impact of time-dependent mass accretion of repeating burst and quiescent phases expected to occur with a self-gravitating circumstellar disc. We show that protostars growing via episodic accretion can substantially contract during the quiescent phases, in contrast to the case of constant mass accretion, whereby the star expands roughly monotonically. The stellar effective temperature and ionizing photon emissivity increase accordingly, which can cause strong ionizing feedback and halt the mass accretion. With a fixed duration of the quiescent phase t q , this contraction occurs in early evolutionary phases, i.e. for M *   10 3 M with t q ~= 10 3  yr. For later epochs and larger masses but the same t q , contraction is negligible even during quiescent phases. With larger t q , however, the star continues to contract during quiescent phases even for the higher stellar masses. We show that this behaviour is well understood by comparing the interval time and the thermal relaxation time for a bloated surface layer. We conclude that the feedback becomes effective, if t q   10 3 yr, which is possible in an accretion disc forming in the direct collapse model.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
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  • 6
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    In:  Other Sources
    Publication Date: 2018-06-08
    Type: Ionized Gaseous Nebulae; Mexico City; Mexico
    Format: text
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  • 7
    Publication Date: 2018-06-08
    Description: The evolution and appearance of protostellar disks can be significantly altered by their UV environment. We follow the evolution after the external UV radiation source has been turned on.
    Keywords: Astrophysics
    Type: Astrophysical Journal
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  • 8
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    In:  Other Sources
    Publication Date: 2018-06-08
    Description: Although massive stars play a critical role in the production of turbulent energy in the ISM, in the formation and destruction of molecular clouds, and ultimately in the dynamical and chemodynamical evolution of galaxies, our understanding of the sequence of events which leads to their formation is still rather limited.
    Keywords: Astrophysics
    Type: Max-Planck Institute for Astronomy, Star Formation 2000; Heidelberg; Germany
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  • 9
    Publication Date: 2018-06-08
    Keywords: Astronomy
    Type: 201st Meeting of the American Astronomical Society; Seattle, WA; United States
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  • 10
    Publication Date: 2018-06-08
    Description: We investigate the interaction of an external UV radiation field with protostellar disks of low-mass stars using 2D radiation hydrodynamical simulations.
    Type: The Interaction of Stars with their Environment; Budapest; Hungary
    Format: text
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