arXiv subjects

Ralph E. Pudritz

Publications and source records attributed to Ralph E. Pudritz.

112 records · Page 7Linked to original sources

arXiv1999

Helical Fields and Filamentary Molecular Clouds

We study the equilibrium of pressure truncated, filamentary molecular clouds that are threaded by rather general helical magnetic fields. We first derive a new virial equation appropriate for magnetized filamentary clouds, which includes the effects of non-thermal motions and the turbulent pressure of the surrounding ISM. When compared with the data, we find that many filamentary clouds have a mass per unit length that is significantly reduced by the effects of external pressure, and that toroidal fields play a significant role in squeezing such clouds. We also develop exact numerical MHD models of filamentary molecular clouds with more general helical field configurations than have previously been considered. We also examine the effects of the equation of state by comparing ``isothermal'' filaments, with constant total (thermal plus turbulent) velocity dispersion, with equilibria constructed using a logatropic equation of state. We perform a Monte Carlo exploration of our parameter space to determine which choices of parameters result in models that agree with the available observational constraints. We find that both equations of state result in equilibria that agree with the observational results. Moreover, we find that models with helical fields have more realistic density profiles than either unmagnetized models or those with purely poloidal fields; we find that most isothermal models have density distributions that fall off as r^{-1.8} to r^{-2}, while logatropes have density profiles that range from r^{-1} to r^{-1.8}. We find that purely poloidal fields produce filaments with steep density gradients that not allowed by the observations.

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arXiv1996

Gravitational Collapse and Star Formation in Logotropic and Non-Isothermal Spheres

We present semi-analytical similarity solutions for the inside-out, expansion-wave collapse of initially virialized gas clouds with non-isothermal equations of state. Results are given for the family of negative-index polytropes, but we focus especially on the so-called logotrope, P/P_c=1+A ln(rho/rho_c). The formalism and interpretation of the present theory are extensions of those in Shu's (1977) standard model for accretion in self-gravitating isothermal spheres: a collapse front moves outwards into a cloud at rest, and the gas behind it falls back to a collapsed core, or protostar. The infalling material eventually enters free-fall, so that, at small radii, the density profiles and velocity fields have the same power-law forms in logotropic and isothermal spheres both. However, the accretion rate onto a protostar is not constant in a logotrope, but grows in proportion to t^3 during the expansion wave. Thus, low-mass stars are accreted over longer times, and high-mass stars over shorter times, than expected in isothermal clouds. This result has implications for the form and origin of the stellar IMF. We also find that infall velocities grow more slowly with time in a collapsing logotrope. These results lead to older inferred collapse ages for Class 0 protostars in general, and for the Bok globule B335 in particular.

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arXiv1996

A Model for the Internal Structure of Molecular Cloud Cores

We generalize the classic Bonnor-Ebert stability analysis of pressure-truncated, self-gravitating gas spheres, to include clouds with arbitrary equations of state. A virial-theorem analysis is also used to incorporate mean magnetic fields into such structures. The results are applied to giant molecular clouds (GMCs), and to individual dense cores, with an eye to accounting for recent observations of the internal velocity-dispersion profiles of the cores in particular. We argue that GMCs and massive cores are at or near their critical mass, and that in such a case the size-linewidth and mass-radius relations between them are only weakly dependent on their internal structures; any gas equation of state leads to essentially the same relations. We briefly consider the possibility that molecular clouds can be described by polytropic pressure-density relations (of either positive or negative index), but show that these are inconsistent with the apparent gravitational virial equilibrium, 2U + W = 0 of GMCs and of massive cores. This class of models would include clouds whose nonthermal support comes entirely from Alfven wave pressure. The simplest model consistent with all the salient features of GMCs and cores is a ``pure logotrope,'' in which P/P_c = 1 + A ln(rho/rho_c). Detailed comparisons with data are made to estimate the value of A, and an excellent fit to the observed dependence of velocity dispersion on radius in cores is obtained with A = 0.2.

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arXiv1995

On the Global Stability of Magnetized Accretion Disks. II. Vertical and Azimuthal Magnetic Fields

We investigate the global stability of a differentially rotating fluid shell threaded by vertical and azimuthal magnetic fields to linear, axisymmetric perturbations. This system, which models a thick accretion disk in the vicinity of its midplane, is susceptible to the Velikhov-Chandrasekhar (VC) instability in the absence of the azimuthal field. In most cases, the azimuthal field tends to stabilize the VC instability, although strong fields (Alfvén speed of order the characteristic rotational speed in our incompressible model) are required for complete stabilization. Stability diagrams are constructed, indicating critical values of the two fields for instability. We find an additional strong field instability that arises when the azimuthal Alfvén speed exceeds the characteristic rotational speed. This instability, in the case of a freely bounded configuration, has certain similarities to the sausage instability for interpenetrating fields in plasma physics, and may be important for very massive disks or filamentary molecular clouds. An application to the L1641 region in Orion A is briefly discussed. Finally, we find that the effect of a radially varying vertical field (without an azimuthal field) is mainly stabilizing.

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