Radiative transfer and the energy equation in SPH simulations of star formation

We introduce and test a new and highly efficient method for treating the thermal and radiative effects influencing the energy equation in SPH simulations of star formation. The method uses the density, temperature and gravitational potential of each particle to estimate a mean optical depth, which then regulates the particle's heating and cooling. The method captures -- at minimal computational cost -- the effects of (i) the rotational and vibrational degrees of freedom of H2, H2 dissociation, H0 ionisation, (ii) opacity changes due to ice mantle melting, sublimation of dust, molecular lines, H-, bound-free and free-free processes and electron scattering; (iv) external irradiation; and (v) thermal inertia. The new algorithm reproduces the results of previous authors and/or known analytic solutions. The computational cost is comparable to a standard SPH simulation with a simple barotropic equation of state. The method is easy to implement, can be applied to both particle- and grid-based codes, and handles optical depths 0<tau<10^{11}.Comment: Submitted to A&A, recommended for publicatio

Role of dipolar interactions in a system of Ni nanoparticles studied by magnetic susceptibility measurements

The role of dipolar interactions among Ni nanoparticles (NP) embedded in an amorphous SiO2/C matrix with different concentrations has been studied performing ac magnetic susceptibility Chi_ac measurements. For very diluted samples, with Ni concentrations < 4 wt % Ni or very weak dipolar interactions, the data are well described by the Neel-Arrhenius law. Increasing Ni concentration to values up to 12.8 wt % Ni results in changes in the Neel-Arrhenius behavior, the dipolar interactions become important, and need to be considered to describe the magnetic response of the NPs system. We have found no evidence of a spin-glasslike behavior in our Ni NP systems even when dipolar interactions are clearly present.Comment: 7 pages, 5 figures, 3 table

Increase in the magnitude of the energy barrier distribution in Ni nanoparticles due to dipolar interactions

The energy barrier distribution Eb of five samples with different concentrations x of Ni nanoparticles using scaling plots from ac magnetic susceptibility data has been determined. The scaling of the imaginary part of the susceptibility Chi"(nu, T) vs. Tln(t/tau_0) remains valid for all samples, which display Ni nanoparticles with similar shape and size. The mean value increases appreciably with increasing x, or more appropriately with increasing dipolar interactions between Ni nanoparticles. We argue that such an increase in constitutes a powerful tool for quality control in magnetic recording media technology where the dipolar interaction plays an important role.Comment: 3 pages, 3 figures, 1 tabl

Shock fragmentation model for gravitational collapse

A cloud of gas collapsing under gravity will fragment. We present a new theory for this process, in which layers shocked gas fragment due to their gravitational instability. Our model explains why angular momentum does not inhibit the collapse process. The theory predicts that the fragmentation process produces objects which are significantly smaller than most stars, implying that accretion onto the fragments plays an essential role in determining the initial masses of stars. This prediction is also consistent with the hypothesis that planets can be produced by gravitational collapse.Comment: 22 pages, 3 figure

Spectroscopic Evidence for Gas Infall in GF9-2

We present spectroscopic evidence for infall motion of gas in the natal cloud core harboring an extremely young low-mass protostar GF9-2. We previously discussed that the ongoing collapse of the GF9-2 core has agreement with the Larson-Penston-Hunter (LPH) theoretical solution for the gravitational collapse of a core (Furuya et al.; paper I). To discuss the gas infall on firmer ground, we have carried out On-The-Fly mapping observations of the HCO+ (1--0) line using the Nobeyama 45m telescope equipped with the 25 Beam Array Receiver System. Furthermore, we observed the HCN (1--0) line with the 45m telescope, and the HCO+ (3--2) line with the Caltech Submillimeter Observatory 10.4 m telescope. The optically thick HCO+ and HCN lines show blueskewed profiles whose deepest absorptions are seen at the peak velocity of optically thin lines, i.e., the systemic velocity of the cloud (paper I), indicating the presence of gas infall toward the central protostar. We compared the observed HCO+ line profiles with model ones by solving the radiative transfer in the core under LTE assumption.We found that the core gas has a constant infall velocity of ~0.5 km/s in the central region, leading to a mass accretion rate of 2.5x10^{-5} Msun/yr. Consequently, we confirm that the gas infall in the GF9-2 core is consistent with the LPH solution.Comment: 13 pages, 5 figure, full resolution version of the figures are available at http://subarutelescope.org/staff/rsf/publication.htm

Collimated jets from the first core

We have performed Smoothed Particle Magnetohydrodynamics (SPMHD) simulations demonstrating the production of collimated jets during collapse of 1 solar mass molecular cloud cores to form the `first hydrostatic core' in low mass star formation. Recently a number of candidate first core objects have been observed, including L1448 IRS2E, L1451-mm and Per Bolo 58, although it is not yet clear that these are first hydrostatic cores. Recent observations of Per Bolo 58 in particular appear to show collimated, bipolar outflows which are inconsistent with previous theoretical expectations. We show that low mass first cores can indeed produce tightly collimated jets (opening angles <~ 10 degrees) with speeds of ~2-7 km/s, consistent with some of the observed candidates. We have also demonstrated, for the first time, that such phenomena can be successfully captured in SPMHD simulations.Comment: 5 pages, 4 figures, accepted to MNRAS Letters. Movies at http://users.monash.edu.au/~dprice/pubs/jet

Warm Extended Dense Gas Lurking At The Heart Of A Cold Collapsing Dense Core

In order to investigate when and how the birth of a protostellar core occurs, we made survey observations of four well-studied dense cores in the Taurus molecular cloud using CO transitions in submillimeter bands. We report here the detection of unexpectedly warm (~ 30 - 70 K), extended (radius of ~ 2400 AU), dense (a few times 10^{5} cm^{-3}) gas at the heart of one of the dense cores, L1521F (MC27), within the cold dynamically collapsing components. We argue that the detected warm, extended, dense gas may originate from shock regions caused by collisions between the dynamically collapsing components and outflowing/rotating components within the dense core. We propose a new stage of star formation, "warm-in-cold core stage (WICCS)", i.e., the cold collapsing envelope encases the warm extended dense gas at the center due to the formation of a protostellar core. WICCS would constitutes a missing link in evolution between a cold quiescent starless core and a young protostar in class 0 stage that has a large-scale bipolar outflow.Comment: Accepted for publication in The Astrophysical Journal Letter

On Exact Polytropic Equilibria of Self-Gravitating Gaseous and Radiative Systems: Their Application to Molecular Cloud Condensation

We propose a novel mathematical method to construct an exact polytropic sphere in self-gravitating hydrostatic equilibrium, improving the non-linear Poisson equation. The central boundary condition for the present equation requires a ratio of gas pressure to total one at the centre, which is uniquely identified by the whole mass and molecular weight of the system. The special solution derived from the Lane-Emden equation can be reproduced. This scheme is now available for modelling the molecular cloud cores in interstellar media. The mass-radius relation of the first core is found to be consistent with the recent results of radiation hydrodynamic simulations.Comment: 5 pages, 3 figures, 1 table. Accepted for publication in MNRA