180 research outputs found

    Formation and Collapse of Quiescent Cloud Cores Induced by Dynamic Compressions

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    (Abridged) We present numerical hydrodynamical simulations of the formation, evolution and gravitational collapse of isothermal molecular cloud cores. A compressive wave is set up in a constant sub-Jeans density distribution of radius r = 1 pc. As the wave travels through the simulation grid, a shock-bounded spherical shell is formed. The inner shock of this shell reaches and bounces off the center, leaving behind a central core with an initially almost uniform density distribution, surrounded by an envelope consisting of the material in the shock-bounded shell, with a power-law density profile that at late times approaches a logarithmic slope of -2 even in non-collapsing cases. The resulting density structure resembles a quiescent core of radius < 0.1 pc, with a Bonnor-Ebert-like (BE-like) profile, although it has significant dynamical differences: it is initially non-self-gravitating and confined by the ram pressure of the infalling material, and consequently, growing continuously in mass and size. With the appropriate parameters, the core mass eventually reaches an effective Jeans mass, at which time the core begins to collapse. Thus, there is necessarily a time delay between the appearance of the core and the onset of its collapse, but this is not due to the dissipation of its internal turbulence as it is often believed. These results suggest that pre-stellar cores may approximate Bonnor-Ebert structures which are however of variable mass and may or may not experience gravitational collapse, in qualitative agreement with the large observed frequency of cores with BE-like profiles.Comment: Accepted for publication in ApJ. Associated mpeg files can be found in http://www.astrosmo.unam.mx/~g.gomez/publica.htm

    IUE observations of Fe 2 galaxies

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    Repeated observations of the Seyfert 1 galaxies I Zw 1 and II Zw 136, which have very strong Fe II emission lines in the optical region, were made at low resolution with the IUE Satellite. The ultraviolet spectra are very similar: both are variable and show broad emission features of Fe II (especially the UV multiplets 1, 33, 60, 62, and 63) as well as the emission lines usually strong in Seyferts and quasars. The data strongly support the hypothesis that the optical Fe II emission lines are primarily due to collisional excitation and that resonance fluorescence makes only a minor contribution to the excitation of these lines

    Growth of Perturbation in Gravitational Collapse and Accretion

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    When a self-gravitating spherical gas cloud collapses or accretes onto a central mass, the inner region of the cloud develops a density profile ρ∝r−3/2\rho\propto r^{-3/2} and the velocity approaches free-fall. We show that in this region, nonspherical perturbations grow with decreasing radius. In the linear regime, the tangential velocity perturbation increases as r−1r^{-1}, while the Lagrangian density perturbation, Δρ/ρ\Delta\rho/\rho, grows as r−1/2r^{-1/2}. Faster growth occurs if the central collapsed object maintains a finite multiple moment, in which case Δρ/ρ\Delta\rho/\rho increases as r−lr^{-l}, where ll specifies the angular degree of the perturbation. These scaling relations are different from those obtained for the collapse of a homogeneous cloud. Our numerical calculations indicate that nonspherical perturbations are damped in the subsonic region, and that they grow and approach the asymptotic scalings in the supersonic region. The implications of our results to asymmetric supernova collapse and to black hole accretion are briefly discussed.Comment: 23 pages including 6 ps figures; Minor changes and update; To appear in ApJ, 200

    Thermal and Fragmentation Properties of Star-forming Clouds in Low-metallicity Environments

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    The thermal and chemical evolution of star-forming clouds is studied for different gas metallicities, Z, using the model of Omukai (2000), updated to include deuterium chemistry and the effects of cosmic microwave background (CMB) radiation. HD-line cooling dominates the thermal balance of clouds when Z \~ 10^{-5}-10^{-3} Z_sun and density ~10^{5} cm^{-3}. Early on, CMB radiation prevents the gas temperature to fall below T_CMB, although this hardly alters the cloud thermal evolution in low-metallicity gas. From the derived temperature evolution, we assess cloud/core fragmentation as a function of metallicity from linear perturbation theory, which requires that the core elongation E := (b-a)/a > E_NL ~ 1, where a (b) is the short (long) core axis length. The fragment mass is given by the thermal Jeans mass at E = E_NL. Given these assumptions and the initial (gaussian) distribution of E we compute the fragment mass distribution as a function of metallicity. We find that: (i) For Z=0, all fragments are very massive, > 10^{3}M_sun, consistently with previous studies; (ii) for Z>10^{-6} Z_sun a few clumps go through an additional high density (> 10^{10} cm^{-3}) fragmentation phase driven by dust-cooling, leading to low-mass fragments; (iii) The mass fraction in low-mass fragments is initially very small, but at Z ~ 10^{-5}Z_sun it becomes dominant and continues to grow as Z is increased; (iv) as a result of the two fragmentation modes, a bimodal mass distribution emerges in 0.01 0.1Z_sun, the two peaks merge into a singly-peaked mass function which might be regarded as the precursor of the ordinary Salpeter-like IMF.Comment: 38 pages, 16 figures, ApJ in pres