Formation and Collapse of Nonaxisymmetric Protostellar Cores in Planar Magnetic Interstellar Clouds: Formulation of the Problem and Linear Analysis

We formulate the problem of the formation and collapse of nonaxisymmetric protostellar cores in weakly ionized, self-gravitating, magnetic molecular clouds. In our formulation, molecular clouds are approximated as isothermal, thin (but with finite thickness) sheets. We present the governing dynamical equations for the multifluid system of neutral gas and ions, including ambipolar diffusion, and also a self-consistent treatment of thermal pressure, gravitational, and magnetic (pressure and tension) forces. The dimensionless free parameters characterizing model clouds are discussed. The response of cloud models to linear perturbations is also examined, with particular emphasis on length and time scales for the growth of gravitational instability in magnetically subcritical and supercritical clouds. We investigate their dependence on a cloud's initial mass-to-magnetic-flux ratio (normalized to the critical value for collapse), the dimensionless initial neutral-ion collision time, and also the relative external pressure exerted on a model cloud. Among our results, we find that nearly-critical model clouds have significantly larger characteristic instability lengthscales than do more distinctly sub- or supercritical models. Another result is that the effect of a greater external pressure is to reduce the critical lengthscale for instability. Numerical simulations showing the evolution of model clouds during the linear regime of evolution are also presented, and compared to the results of the dispersion analysis. They are found to be in agreement with the dispersion results, and confirm the dependence of the characteristic length and time scales on parameters such as the initial mass-to-flux ratio and relative external pressure.Comment: 30 pages, 7 figures Accepted by Ap

Radiative transfer models of non-spherical prestellar cores

We present 2D Monte Carlo radiative transfer simulations of prestellar cores. We consider two types of asymmetry: disk-like asymmetry, in which the core is denser towards the equatorial plane than towards the poles; and axial asymmetry, in which the core is denser towards the south pole than the north pole. We limit our treatment to cores with mild asymmetries, which are exposed directly to the interstellar radiation field or are embedded inside molecular clouds. The isophotal maps of a core depend strongly on the viewing angle. Maps at wavelengths longer than the peak of the SED (e.g. 850 micron) essentially trace the column-density. Thus, for instance, cores with disk-like asymmetry appear elongated when mapped at 850 micron from close to the equatorial plane. However, at wavelengths near the peak of the SED (e.g. 200 micron), the emissivity is more strongly dependent on the temperature, and therefore, at particular viewing angles, there are characteristic features which reflect a more complicated convolution of the density and temperature fields within the core. These characteristic features are on scales 1/5 to 1/3 of the overall core size, and so high resolution observations are needed to observe them. They are also weaker if the core is embedded in a molecular cloud (because the range of temperature within the core is then smaller), and so high sensitivity is needed to detect them. Herschel, to be launched in 2007, will in principle provide the necessary resolution and sensitivity at 170 to 250 micron.Comment: 16 pages, 22 figures, accepted by A&A, also available (with high resolution figures) at http://www.astro.cf.ac.uk/pub/Dimitrios.Stamatellos/publications

A Two-Fluid Method for Ambipolar Diffusion

We present a semi-implicit method for isothermal two-fluid ion-neutral ambipolar drift that is second-order accurate in space and time. The method has been implemented in the RIEMANN code for astrophysical fluid dynamics. We present four test problems that show the method works and correctly tracks the propagation of MHD waves and the structure of two-fluid C-shocks. The accurate propagation of MHD waves in the two-fluid approximation is shown to be a stringent test of the algorithm. We demonstrate that highly accurate methods are required in order to properly capture the MHD wave behaviour in the presence of ion-neutral friction.Comment: 29 pages, 16 figures, accepted to MNRA

The dust temperature distribution in prestellar cores

We have computed the dust temperature distribution to be expected in a pre-protostellar core in the phase prior to the onset of gravitational instability. We have done this in the approximation that the heating of the dust grains is solely due to the attenuated external radiation field and that the core is optically thin to its own radiation. This permits us to consider non spherically symmetric geometries. We predict the intensity distributions of our model cores at millimeter and sub-millimeter wavelengths and compare with observations of the well studied object L1544. We have also developed an analytical approximation for the temperature at the center of spherically symmetric cores and we compare this with the numerical calculations. Our results show (in agreement with Evans et al. 2001) that the temperatures in the nuclei of cores of high visual extinction (> 30 magnitudes) are reduced to values of below ~8 K or roughly half of the surface temperature. This has the consequence that maps at wavelengths shortward of 1.3 mm see predominantly the low density exterior of pre-protostellar cores. It is extremely difficult to deduce the true density distribution from such maps alone. We have computed the intensity distribution expected on the basis of the models of Ciolek & Basu (2000) and compared with the observations of L1544. The agreement is good with a preference for higher inclinations (37 degrees instead of 16) than that adopted by Ciolek & Basu (2000). We find that a simple extension of the analytic approximation allows a reasonably accurate calculation of the dust temperature as a function of radius in cores with density distributions approximating those expected for Bonnor-Ebert spheres and suggest that this may be a useful tool for future calculations of the gas temperature in such cores.Comment: 14 latex pages, 10 ps figures, A&A accepte

Optical and submillimetre observations of Bok globules -- tracing the magnetic field from low to high density

We present optical and submillimetre polarimetry data of the Bok globule CB3 and optical polarimetry data of the Bok globule CB246. We use each set of polarimetry data to infer the B-field orientation in each of the clouds. The optical data can only be used in the low density, low extinction edge regions of clouds. The submillimetre data can only be used in the high column-density, central regions of the clouds. It has previously been found that near-infrared polarisation mapping of background stars does not accurately trace the magnetic field in dense cloud regions. This may be due to a lack of aligned grains in dense regions. We test this by comparing the field orientations measured by our two independent polarimetry methods. We find that the field orientation deduced from the optical data matches up well with the orientation estimated from the submillimetre data. We therefore claim that both methods are accurately tracing the same magnetic field in CB3. Hence, in this case, there must be significant numbers of aligned dust grains in the high density region, and they do indeed trace the magnetic field in the submillimetre. We find an offset of 40$\pm$14 degrees between the magnetic field orientation and the short axis of the globule. This is consistent with the mean value of 31$\pm$3 degrees found in our previous work on prestellar cores, even though CB3 is a protostellar core. Taken together, the six prestellar cores that we have now studied in this way show a mean offset between magnetic field orientation and core short axis of $\sim30\pm$3 degrees, in apparent contradiction with some models of magnetically dominated star formation.Comment: 8 pages, 3 figures, accepted for publication in MNRA

Thermal instability in ionized plasma

We study magnetothermal instability in the ionized plasmas including the effects of Ohmic, ambipolar and Hall diffusion. Magnetic field in the single fluid approximation does not allow transverse thermal condensations, however, non-ideal effects highly diminish the stabilizing role of the magnetic field in thermally unstable plasmas. Therefore, enhanced growth rate of thermal condensation modes in the presence of the diffusion mechanisms speed up the rate of structure formation.Comment: Accepted for publication in Astrophysics & Space Scienc

Submillimeter Studies of Prestellar Cores and Protostars: Probing the Initial Conditions for Protostellar Collapse

Improving our understanding of the initial conditions and earliest stages of protostellar collapse is crucial to gain insight into the origin of stellar masses, multiple systems, and protoplanetary disks. Observationally, there are two complementary approaches to this problem: (1) studying the structure and kinematics of prestellar cores observed prior to protostar formation, and (2) studying the structure of young (e.g. Class 0) accreting protostars observed soon after point mass formation. We discuss recent advances made in this area thanks to (sub)millimeter mapping observations with large single-dish telescopes and interferometers. In particular, we argue that the beginning of protostellar collapse is much more violent in cluster-forming clouds than in regions of distributed star formation. Major breakthroughs are expected in this field from future large submillimeter instruments such as Herschel and ALMA.Comment: 12 pages, 9 figures, to appear in the proceedings of the conference "Chemistry as a Diagnostic of Star Formation" (C.L. Curry & M. Fich eds.

Externally Fed Accretion onto Protostars

The asymmetric molecular emission lines from dense cores reveal slow, inward motion in the clouds' outer regions. This motion is present both before and after the formation of a central star. Motivated by these observations, we revisit the classic problem of steady, spherical accretion of gas onto a gravitating point mass, but now include self-gravity of the gas and impose a finite, subsonic velocity as the outer boundary condition. We find that the accretion rate onto the protostar is lower than values obtained for isolated, collapsing clouds, by a factor that is the Mach number of the outer flow. Moreover, the region of infall surrounding the protostar spreads out more slowly, at a speed close to the subsonic, incoming velocity. Our calculation, while highly idealized, provides insight into two longstanding problems -- the surprisingly low accretion luminosities of even the most deeply embedded stellar sources, and the failure so far to detect spatially extended, supersonic infall within their parent dense cores. Indeed, the observed subsonic contraction in the outer regions of dense cores following star formation appears to rule out a purely hydrodynamic origin for these clouds.Comment: accepted by MNRA

Simulating star formation in molecular cores II. The effects of different levels of turbulence

(Abridged) We explore, by means of a large ensemble of SPH simulations, how the level of turbulence affects the collapse and fragmentation of a star-forming core. All our simulated cores have the same, except that we vary (a) the initial level of turbulence (as measured by the ratio of turbulent to gravitational energy, $\alpha_{\rm turb} \equiv U_{\rm turb}/|\Omega| = 0, 0.01, 0.025, 0.05, 0.10 {\rm and} 0.25$) and (b), for fixed $\alpha_{\rm turb}$, the details of the initial turbulent velocity field (so as to obtain good statistics). A low level of turbulence ($\alpha_{\rm turb} \sim 0.05$) suffices to produce multiple systems. As $\alpha_{\rm turb}$ is increased, the number of objects formed and the companion frequency both increase. The mass function is bimodal, with a flat low-mass segment representing single objects ejected from the core before they can accrete much, and a Gaussian high-mass segment representing objects which because they remain in the core grow by accretion and tend to pair up in multiple systems.Comment: 15 pages, 8 figures. In press in A&