Accretion in Gravitationally Contracting Clouds

Accretion flow in a contracting magnetized isothermal cloud was studied using magnetohydrodynamical simulations and a nested grid technique. First, the interstellar magnetized cloud experiences a ``runaway collapse'' phase, in which the central density increases drastically within a finite time scale. Finally, it enters an accretion phase, in which inflowing matter accretes onto a central high-density disk or a new-born star. We found that the accretion rate reaches (4 -- 40) $\times c_s^3/G$, where $c_s$ and $G$ represent the isothermal sound speed and the gravitational constant, respectively. This is much larger than the standard accretion rate of $0.975c_s^3/G$ for a hydrostatic isothermal spherical cloud (Shu 1977, AAA19.065.044). Due to the effect of an extra infall velocity achieved in the runaway phase ($\sim 2 c_s$), the accretion rate is boosted. This rate declines with time in contrast to Shu's solution, but keeps \gtsim 2.5 c_s^3/G. The observed gas infall rate around proto-stars such as L1551 IRS 5 and HL Tau is also discussed.Comment: 10 pages, latex using PASJ style file (available from http://www.tenmon.or.jp/pasj/index-e.html), postscripted text and figures are available from http://quasar.ed.niigata-u.ac.jp/docs/Papers/mag3ps.tg

Collapse and Fragmentation of Cylindrical Magnetized Clouds. II. Simulation with Nested Grid Scheme

Fragmentation process in a cylindrical magnetized cloud is studied with the nested grid method. The nested grid scheme use 15 levels of grids with different spatial resolution overlaid subsequently, which enables us to trace the evolution from the molecular cloud density $\sim 100 {\rm cm}^{-3}$ to that of the protostellar disk $\sim 10^{10} {\rm cm} ^{-3}$ or more. Fluctuation with small amplitude grows by the gravita- tional instability. It forms a disk perpendicular to the magnetic fields which runs in the direction parallel to the major axis of the cloud. Matter accrets on to the disk mainly flowing along the magnetic fields and this makes the column density increase. The radial inflow, whose velocity is slower than that perpendicular to the disk, is driven by the increase of the gravity. While the equation of state is isothermal and magnetic fields are perfectly coupled with the matter, which is realized in the density range of $\rho \lesssim 10^{10} {\rm cm}^{-3}$, never stops the contraction. The structure of the contracting disk reaches that of a singular solution as the density and the column density obey $\rho(r)\propto r^{-2}$ and $\sigma(r) \propto r^{-1}$, respectively. The magnetic field strength on the mid-plane is proportional to $\rho(r)^{1/2}$ and further that of the center ($B_c$) evolves as proportional to the square root of the gas density ($\propto \rho_c^{1/2}$). It is shown that isothermal clouds experience ``run-away'' collapses. The evolution after the equation of state becomes hard is also discussed.Comment: 12 pages without figures, AASTEX, submitted to ApJ. Postscript version with figures is available from http://quasar.ed.niigata-u.ac.jp/docs/Papers/mag2.ps.g

Collapse and Fragmentation of Magnetized Cylindrical Clouds

Gravitational collapse of the cylindrical elongated cloud is studied by numerical magnetohydrodynamical simulations. In the infinitely long cloud in hydrostatic configuration, small perturbations grow by the gravitational instability. The most unstable mode indicated by a linear perturbation theory grows selectively even from a white noise. The growth rate agrees with that calculated by the linear theory. First, the density-enhanced region has an elongated shape, i.e., prolate spheroidal shape. As the collapse proceeds, the high-density fragment begins to contract mainly along the symmetry axis. Finally, a spherical core is formed in the non-magnetized cloud. In contrast, an oblate spheroidal dense disk is formed in a cloud in which the magnetic pressure is nearly equal to the thermal one. The radial size of the disk becomes proportional to the initial characteristic density scale-height in the r-direction. As the collapse proceeds, a slowly contracting dense part is formed (approximately < 10% in mass) inside of the fast contracting disk. And this is separated from other part of the disk whose inflow velocity is accelerated as reaching the center of the core. From arguments on the Jeans mass and the magnetic critical mass, it is concluded that the fragments formed in a cylindrical elongated cloud can not be supported against the self- gravity and it will eventually collapse.Comment: 20 pages, figures available upon request, LaTeX, NIGAST040

Origin of Molecular Outflow Determined from Thermal Dust Polarization

The observational expectation of polarization measurements of thermal dust radiation is investigated to find information on molecular outflows based on magnetohydrodynamical (MHD) and radiation transfer simulations. There are two major proposed models for the driving of molecular outflows: (1) molecular gas is accelerated by a magnetic pressure gradient or magnetocentrifugal wind mechanism before the magnetic field and molecular gas are decoupled, (2) the linear momentum of a highly collimated jet is transferred to the ambient molecular gas. In order to distinguish between these two models, it is crucial to observe the configuration of the magnetic field. An observation of a toroidal magnetic field is strong evidence that the first of the models is appropriate. In this paper, we calculated the polarization distribution of thermal dust radiation due to the alignment of dust grains along the magnetic field using molecular outflow data calculated by two-dimensional axisymmetric MHD simulations. An asymmetric distribution around the z-axis is characteristic for magnetic fields composed of both poloidal and toroidal components. We determined that the outflow has a low polarization degree compared with the envelope and that the envelope and outflow have different polarization directions (B-vector), namely, the magnetic field within the envelope is parallel to the global magnetic field lines while the magnetic field of the outflow is perpendicular to it. Thus we have demonstrated that the point-symmetric (rather than axisymmetric) distributions of low polarization regions indicate that molecular outflows are likely to be magnetically driven. Observations of this polarization distribution with tools such as ALMA would confirm the origin of the molecular outflow.Comment: Erratum (PASJ 63, June issue) was reflecte

Evolution of magnetized, rotating, isothermal clouds

Molecular cloud cores, in which star formation process now proceeds, are often found with elongated shape. This suggests that the core collapsed preferentially along the direction parallel to the global magnetic field and/or parallel to the cloud's initial angular momentum. Actually the magnetic field strength in the cloud has been measured recently with the Zeeman splitting (Goodman et al. 1989). The authors indicate the magnetic field of 10 to 30 mu G exists in the cloud. Further, the observation of polarization in the near IR from background stars shows that the magnetic field runs perpendicularly to the major axis of the cloud (Tamura et al. 1987). As for the angular momentum, the rotation rate of 0.2 to 6 kms s(exp -1) pc(exp -1) is reported (Goldsmith and Arquilla 1984) in 16 dark cloud regions. If the cloud collapses from the diffuse cloud with density n approx. 1 cm (exp -3) with strictly conserving the angular momentum which was shared from the galactic rotation, the rotation rate of the cloud will be x mega sub j = const approx. 3(n/1000 cm exp -3) exp 2/3 km s(exp -1) pc (exp -1) (Mouschovias 1987). The evolution of the rotating magnetized cloud is discussed here