Evolution of Angular Momentum Distribution during Star Formation

If the angular momentum of the molecular cloud core were conserved during the star formation process, a new-born star would rotate much faster than its fission speed. This constitutes the angular momentum problem of new-born stars. In this paper, the angular momentum transfer in the contraction of a rotating magnetized cloud is studied with axisymmetric MHD simulations. Owing to the large dynamic range covered by the nested-grid method, the structure of the cloud in the range from 10 AU to 0.1 pc is explored. First, the cloud experiences a run-away collapse, and a disk forms perpendicularly to the magnetic field, in which the central density increases greatly in a finite time-scale. In this phase, the specific angular momentum j of the disk decreases to $\simeq 1/3$ of the initial cloud. After the central density of the disk exceeds $\sim 10^{10}{\rm cm}^{-3}$, the infall on to the central object develops. In this accretion stage, the rotation motion and thus the toroidal magnetic field drive the outflow. The angular momentum of the central object is transferred efficiently by the outflow as well as the effect of the magnetic stress. In 7000 yr from the core formation, the specific angular momentum of the central $0.17M_\odot$ decreases a factor of 10^{-4} from the initial value (i.e. from $10^{20}{\rm cm^2 s^{-1}}$ to $10^{16}{\rm cm^2 s^{-1}}$).Comment: 15 pages, 2 figures, Astrophysical Journal Letters in pres

Monte-Carlo Simulations of Globular Cluster Evolution - I. Method and Test Calculations

We present a new parallel supercomputer implementation of the Monte-Carlo method for simulating the dynamical evolution of globular star clusters. Our method is based on a modified version of Henon's Monte-Carlo algorithm for solving the Fokker-Planck equation. Our code allows us to follow the evolution of a cluster containing up to 5x10^5 stars to core collapse in < 40 hours of computing time. In this paper we present the results of test calculations for clusters with equal-mass stars, starting from both Plummer and King model initial conditions. We consider isolated as well as tidally truncated clusters. Our results are compared to those obtained from approximate, self-similar analytic solutions, from direct numerical integrations of the Fokker-Planck equation, and from direct N-body integrations performed on a GRAPE-4 special-purpose computer with N=16384. In all cases we find excellent agreement with other methods, establishing our new code as a robust tool for the numerical study of globular cluster dynamics using a realistic number of stars.Comment: 35 pages, including 8 figures, submitted to ApJ. Revised versio

Shapes of Molecular Cloud Cores and the Filamentary Mode of Star Formation

Using recent dust continuum data, we generate the intrinsic ellipticity distribution of dense, starless molecular cloud cores. Under the hypothesis that the cores are all either oblate or prolate randomly-oriented spheroids, we show that a satisfactory fit to observations can be obtained with a gaussian prolate distribution having a mean intrinsic axis ratio of 0.54. Further, we show that correlations exist between the apparent axis ratio and both the peak intensity and total flux density of emission from the cores, the sign of which again favours the prolate hypothesis. The latter result shows that the mass of a given core depends on its intrinsic ellipticity. Monte Carlo simulations are performed to find the best-fit power law of this dependence. Finally, we show how these results are consistent with an evolutionary scenario leading from filamentary parent clouds to increasingly massive, condensed, and roughly spherical embedded cores.Comment: 16 pages, incl. 11 Postscript figures. Accepted by Ap

Dynamical Mass Estimates of Large-Scale Filaments in Redshift Surveys

We propose a new method to measure the mass of large-scale filaments in galaxy redshift surveys. The method is based on the fact that the mass per unit length of isothermal filaments depends only on their transverse velocity dispersion. Filaments that lie perpendicular to the line of sight may therefore have their mass per unit length measured from their thickness in redshift space. We present preliminary tests of the method and find that it predicts the mass per unit length of filaments in an N-body simulation to an accuracy of ~35%. Applying the method to a select region of the Perseus-Pisces supercluster yields a mass-to-light ratio M/L_B around 460h in solar units to within a factor of two. The method measures the mass-to-light ratio on length scales of up to 50h^(-1) Mpc and could thereby yield new information on the behavior of the dark matter on mass scales well beyond that of clusters of galaxies.Comment: 21 pages, LaTeX with 6 figures included. Submitted to Ap

Gravitational Collapse of Filamentary Magnetized Molecular Clouds

We develop models for the self-similar collapse of magnetized isothermal cylinders. We find solutions for the case of a fluid with a constant toroidal flux-to-mass ratio (Gamma_phi=constant) and the case of a fluid with a constant gas to magnetic pressure ratio (beta=constant). In both cases, we find that a low magnetization results in density profiles that behave as rho ~ r^{-4} at large radii, and at high magnetization we find density profiles that behave as rho ~ r^{-2}. This density behaviour is the same as for hydrostatic filamentary structures, suggesting that density measurements alone cannot distinguish between hydrostatic and collapsing filaments--velocity measurements are required. Our solutions show that the self-similar radial velocity behaves as v_r ~ r during the collapse phase, and that unlike collapsing self-similar spheres, there is no subsequent accretion (i.e. expansion-wave) phase. We also examine the fragmentation properties of these cylinders, and find that in both cases, the presence of a toroidal field acts to strengthen the cylinder against fragmentation. Finally, the collapse time scales in our models are shorter than the fragmentation time scales. Thus, we anticipate that highly collapsed filaments can form before they are broken into pieces by gravitational fragmentation.Comment: 20 pages, 4 figures, accepted to Ap

A Genetic Algorithm-Based Exploration of Three Filament Models: A Case for the Magnetic Support of the G11.11-0.12 Infrared-Dark Cloud

The G11.11-0.12 infrared-dark cloud has a filamentary appearance, both in extinction against the diffuse infrared emission of the Galactic plane and in emission at 850 microns. We use a novel computational technique based on an advanced genetic algorithm to explore thoroughly 3 different models of self-gravitating, pressure truncated filaments and to constrain their parameters. Specifically, the models tested are the non-magnetic Ostriker (1964) model, a generalized version of the magnetic Stodolkiewicz (1963) model, and the magnetic Fiege & Pudritz (2000) model. Previous results showed that G11.11-0.12 has a much steeper r^{-4} radial density profile than other filaments, where the density varies approximately as r^{-2}, and that this steep density profile is consistent with the Ostriker (1964) model. We present a more complete analysis that shows that the radial structure of G11.11-0.12 is consistent with regimes of each of these models. All of the magnetic models that agree with the data are threaded by a dominant poloidal magnetic field, and most have dynamically significant fields. Thus, G11.11-0.12 is an excellent candidate for radial support by a magnetic field that is predominantly poloidal. We predict the polarization patterns expected for both magnetic models and show that the two magnetic models produce different polarization patterns that should be distingished by observations.Comment: To appear in Ap.J. Dec. 1 edition, volume 616. 40 pages and 42 figures. Figures are severely reduced to satisfy astro-ph size limits. A version with higher quality figures is available by contacting the first autho

Monte Carlo Simulations of Globular Cluster Evolution. IV. Direct Integration of Strong Interactions

We study the dynamical evolution of globular clusters containing populations of primordial binaries, using our newly updated Monte Carlo cluster evolution code with the inclusion of direct integration of binary scattering interactions. We describe the modifications we have made to the code, as well as improvements we have made to the core Monte Carlo method. We present several test calculations to verify the validity of the new code, and perform many comparisons with previous analytical and numerical work in the literature. We simulate the evolution of a large grid of models, with a wide range of initial cluster profiles, and with binary fractions ranging from 0 to 1, and compare with observations of Galactic globular clusters. We find that our code yields very good agreement with direct N-body simulations of clusters with primordial binaries, but yields some results that differ significantly from other approximate methods. Notably, the direct integration of binary interactions reduces their energy generation rate relative to the simple recipes used in Paper III, and yields smaller core radii. Our results for the structural parameters of clusters during the binary-burning phase are now in the tail of the range of parameters for observed clusters, implying that either clusters are born significantly more or less centrally concentrated than has been previously considered, or that there are additional physical processes beyond two-body relaxation and binary interactions that affect the structural characteristics of clusters.Comment: Accepted for publication in ApJ; 17 pages, 19 figures; changes to reflect accepted versio

On the Mass of Population III Stars

Performing 1D hydrodynamical calculations coupled with non-equilibrium processes for H2 formation, we pursue the thermal and dynamical evolution of filamentary primordial clouds and attempt to make an estimate on the mass of population III stars. It is found that, almost independent of initial conditions, a filamentary cloud continues to collapse nearly isothermally due to H_2 cooling until the cloud becomes optically thick against the H_2 lines. During the collapse the cloud structure separates into two parts, i.e., a denser spindle and a diffuse envelope. The spindle contracts quasi-statically, and thus the line mass of the spindle keeps a characteristic value determined solely by the temperature ($\sim 800$ K). Applying a linear theory, we find that the spindle is unstable against fragmentation during the collapse. The wavelength of the fastest growing perturbation lessens as the collapse proceeds. Consequently, successive fragmentation could occur. When the central density exceeds $n_c \sim 10^{10-11} cm^{-3}$, the successive fragmentation may cease since the cloud becomes opaque against the H_2 lines and the collapse decelerates appreciably. The mass of the first star is then expected to be typically $\sim 3 M_\odot$, which may grow up to $\sim 16 M_\odot$ by accreting the diffuse envelope. Thus, the first-generation stars are anticipated to be massive but not supermassive.Comment: 23 pages, 6 figures, accepted by ApJ (April 10

Collapse of Rotating Magnetized Molecular Cloud Cores and Mass Outflows

Collapse of the rotating magnetized molecular cloud core is studied with the axisymmetric magnetohydrodynamical (MHD) simulations. Due to the change of the equation of state of the interstellar gas, the molecular cloud cores experience several different phases as collapse proce eds. In the isothermal run-away collapse ($n \lesssim 10^{10}{\rm H_2 cm}^{-3}$), a pseudo-disk is formed and it continues to contract till the opaque core is fo rmed at the center. In this disk, a number of MHD fast and slow shock pairs appear running parallelly to the disk. After the equation of state becomes hard, an adiabatic core is formed, which is separated from the isothermal contracting pseudo-disk by the accretion shock front facing radially outwards. By the effect of the magnetic tension, the angular momentum is transferred from the disk mid-plane to the surface. The gas with excess angular momentum near the surface is finally ejected, which explains the molecular bipolar outflow. Two types of outflows are observed. When the poloidal magnetic field is strong (magnetic energy is comparable to the thermal one), a U-shaped outflow is formed in which fast moving gas is confined to the wall whose shape looks like a capit al letter U. The other is the turbulent outflow in which magnetic field lines and velocity fi elds are randomly oriented. In this case, turbulent gas moves out almost perpendicularly from the disk. The continuous mass accretion leads to the quasistatic contraction of the first core. A second collapse due to dissociation of H$_2$ in the first core follows. Finally another quasistatic core is again formed by atomic hydrogen (the second core). It is found that another outflow is ejected around the second atomic core, which seems to correspond to the optical jets or the fast neutral winds.Comment: submitted to Ap

Parker Instability in a Self-Gravitating Magnetized Gas Disk: I. Linear Stability Analysis

To be a formation mechanism of such large-scale structures as giant molecular clouds (GMCs) and HI superclouds, the classical Parker instability driven by external gravity has to overcome three major obstacles: The convective motion accompanying the instability generates thin sheets than large condensations. The degree of density enhancement achieved by the instability is too low to make dense interstellar clouds. The time and the length scales of the instability are significantly longer and larger than the estimated formation time and the observed mean separation of the GMCs, respectively. This paper examines whether a replacement of the driving agent from the external to the self gravity might remove these obstacles by activating the gravitational instability in the Galactic ISM disk. The self gravity can suppress the convective motions, and a cooperative action of the Jeans and the Parker instabilities can remove all the obstacles confronting the classical version of the Parker instability. The mass and mean separation of the structures resulting from the odd-parity undular mode solution are shown to agree better with the HI superclouds than with the GMCs. We briefly discuss how inclusions of the external gravity and cosmic rays would modify behaviors of the odd-parity undular mode solution.Comment: 53 pages, 21 figure