3,112 research outputs found
Monte Carlo Radiative Transfer in Embedded Prestellar Cores
We implement a Monte Carlo radiative transfer method, that uses a large
number of monochromatic luminosity packets to represent the radiation
transported through a system. These packets are injected into the system and
interact stochastically with it. We test our code against various benchmark
calculations and determine the number of packets required to obtain accurate
results under different circumstances. We then use this method to study cores
that are directly exposed to the interstellar radiation field (non-embedded
cores) and find similar results with previous studies. We also explore a large
number of models of cores that are embedded in the centre of a molecular cloud.
Our study indicates that the temperature profiles in embedded cores are less
steep than those in non-embedded cores. Deeply embedded cores (ambient cloud
with visual extinction larger than 15-25) are almost isothermal at around 7-8
K. The temperature inside cores surrounded by an ambient cloud of even moderate
thickness (Av~5) is less than 12 K, which is lower than previous studies have
assumed. Thus, previous mass calculations of embedded cores (for example in the
rho Ophiuchi protocluster), based on mm continuum observations, may
underestimate core masses by up to a factor of 2. Our study shows that the best
wavelength region to observe embedded cores is between 400 and 500 microns,
where the core is quite distinct from the background. We also predict that very
sensitive observations (~1-3 MJy/sr) at 170-200 microns can be used to estimate
how deeply a core is embedded in its parent molecular cloud. The upcoming
HERSCHEL mission (ESA, 2007) will, in principle, be able to detect these
features and test our models.Comment: 15 pages, 18 figures, accepted by A&
Are the majority of Sun-like stars single?
It has recently been suggested that, in the field, of Sun-like
stars ()
are single. We argue here that this suggestion may be incorrect, since it
appears to be based on the multiplicity frequency of systems with Sun-like
primaries, and therefore takes no account of Sun-like stars that are secondary
(or higher-order) components in multiple systems. When these components are
included in the reckoning, it seems likely that only of Sun-like
stars are single. This estimate is based on a model in which the system mass
function has the form proposed by Chabrier, with a power-law Salpeter extension
to high masses; there is a flat distribution of mass ratios; and the
probability that a system of mass is a binary is for , for , and for . The constants in
this last relation are chosen so that the model also reproduces the observed
variation of multiplicity frequency with primary mass. However, the more
qualitative conclusion, that a minority of Sun-like stars are single, holds up
for virtually all reasonable values of the model parameters. Parenthetically,
it is still likely that the majority of {\it all} stars in the field are
single, but that is because most M Dwarfs probably are single.Comment: 6 pages. Accepted by MNRA
High-resolution simulations of clump-clump collisions using SPH with Particle Splitting
We investigate, by means of numerical simulations, the phenomenology of star
formation triggered by low-velocity collisions between low-mass molecular
clumps. The simulations are performed using an SPH code which satisfies the
Jeans condition by invoking On-the-Fly Particle Splitting. Clumps are modelled
as stable truncated (non-singular) isothermal, i.e. Bonnor-Ebert, spheres.
Collisions are characterised by M_0 (clump mass), b (offset parameter, i.e.
ratio of impact parameter to clump radius), and M (Mach Number, i.e. ratio of
collision velocity to effective post-shock sound speed). The gas subscribes to
a barotropic equation of state, which is intended to capture (i) the scaling of
pre-collision internal velocity dispersion with clump mass, (ii) post-shock
radiative cooling, and (iii) adiabatic heating in optically thick protostellar
fragments. The efficiency of star formation is found to vary between 10% and
30% in the different collisions studied and it appears to increase with
decreasing M_0, and/or decreasing b, and/or increasing M. For b<0.5 collisions
produce shock compressed layers which fragment into filaments. Protostellar
objects then condense out of the filaments and accrete from them. The resulting
accretion rates are high, 1 to 5 x 10^{-5} M_sun yr^{-1}, for the first 1 to 3
x 10^4 yrs. The densities in the filaments, n >~ 5 x 10^5 cm^{-3}, are
sufficient that they could be mapped in NH_3 or CS line radiation, in nearby
star formation regions.Comment: Accepted for publication in MNRAS; 21 pages; 25 figures. Four figures
are provided separately in reduced jpg format due to their large original ps
size: click on "PostScript" to have direct access to the 4 jpg figures; full
size ps files for these 4 figures can be found at
http://www.aip.de/People/skitsionas/papers
An empirical model for protostellar collapse
We propose a new analytic model for the initial conditions of protostellar
collapse in relatively isolated regions of star formation. The model is
non-magnetic, and is based on a Plummer-like radial density profile as its
initial condition. It fits: the observed density profiles of pre-stellar cores
and Class 0 protostars; recent observations in pre-stellar cores of roughly
constant contraction velocities over a wide range of radii; and the lifetimes
and accretion rates derived for Class 0 and Class I protostars. However, the
model is very simple, having in effect only 2 free parameters, and so should
provide a useful framework for interpreting observations of pre-stellar cores
and protostars, and for calculations of radiation transport and time-dependent
chemistry. As an example, we model the pre-stellar core L1544.Comment: To appear in Astrophysical Journal, Jan 20th, 2001. 18 pages incl. 3
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