481 research outputs found
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
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
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
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
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