2,075 research outputs found
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
: Characterising the structure of young star clusters
Many young star clusters appear to be fractal, i.e. they appear to be
concentrated in a nested hierarchy of clusters within clusters. We present a
new algorithm for statistically analysing the distribution of stars to quantify
the level of sub-structure. We suggest that, even at the simplest level, the
internal structure of a fractal cluster requires the specification of three
parameters. (i) The 3D fractal dimension, , measures the extent to
which the clusters on one level of the nested hierarchy fill the volume of
their parent cluster. (ii) The number of levels, , reflects the
finite ratio between the linear size of the large root-cluster at the top of
the hierarchy, and the smallest leaf-clusters at the bottom of the hierarchy.
(iii) The volume-density scaling exponent, measures the factor by which the excess density, , in a structure of scale , exceeds that of the background formed by
larger structures; it is similar, but not exactly equivalent, to the exponent
in Larson's scaling relation between density and size for molecular clouds. We
describe an algorithm which can be used to constrain the values of and apply this method to artificial and observed
clusters. We show that this algorithm is able to reliably describe the three
dimensional structure of an artificial star cluster from the two dimensional
projection, and quantify the varied structures observed in real and simulated
clusters.Comment: Accepted by MNRA
Star Formation triggered by cloud-cloud collisions
We present the results of SPH simulations in which two clouds, each having
mass and radius
, collide head-on at relative velocities of
. There is a clear trend with increasing . At low
, star formation starts later, and the shock-compressed
layer breaks up into an array of predominantly radial filaments; stars condense
out of these filaments and fall, together with residual gas, towards the centre
of the layer, to form a single large- cluster, which then evolves by
competitive accretion, producing one or two very massive protostars and a
diaspora of ejected (mainly low-mass) protostars; the pattern of filaments is
reminiscent of the hub and spokes systems identified recently by observers. At
high , star formation occurs sooner and the
shock-compressed layer breaks up into a network of filaments; the pattern of
filaments here is more like a spider's web, with several small- clusters
forming independently of one another, in cores at the intersections of
filaments, and since each core only spawns a small number of protostars, there
are fewer ejections of protostars. As the relative velocity is increased, the
{\it mean} protostellar mass increases, but the {\it maximum} protostellar mass
and the width of the mass function both decrease. We use a Minimal Spanning
Tree to analyse the spatial distributions of protostars formed at different
relative velocities.Comment: 10 pages, 11 figure
On the evolution of the density pdf in strongly self-gravitating systems
The time evolution of the probability density function (PDF) of the mass
density is formulated and solved for systems in free-fall using a simple
appoximate function for the collapse of a sphere. We demonstrate that a
pressure-free collapse results in a power-law tail on the high-density side of
the PDF. The slope quickly asymptotes to the functional form
for the (volume-weighted) PDF and
for the corresponding mass-weighted
distribution. From the simple approximation of the PDF we derive analytic
descriptions for mass accretion, finding that dynamically quiet systems with
narrow density PDFs lead to retarded star formation and low star formation
rates. Conversely, strong turbulent motions that broaden the PDF accelerate the
collapse causing a bursting mode of star formation. Finally, we compare our
theoretical work with observations. The measured star formation rates are
consistent with our model during the early phases of the collapse. Comparison
of observed column density PDFs with those derived from our model suggests that
observed star-forming cores are roughly in free-fall.Comment: accepted for publication, 13 page
Simulating star formation in molecular cloud cores IV. The role of turbulence and thermodynamics
We perform SPH simulations of the collapse and fragmentation of low-mass
cores having different initial levels of turbulence
(alpha_turb=0.05,0.10,0.25). We use a new treatment of the energy equation
which captures the transport of cooling radiation against opacity due to both
dust and gas (including the effects of dust sublimation, molecules, and H^-
ions). We also perform comparison simulations using a standard barotropic
equation of state. We find that -- when compared with the barotropic equation
of state -- our more realistic treatment of the energy equation results in more
protostellar objects being formed, and a higher proportion of brown dwarfs; the
multiplicity frequency is essentially unchanged, but the multiple systems tend
to have shorter periods (by a factor ~3), higher eccentricities, and higher
mass ratios. The reason for this is that small fragments are able to cool more
effectively with the new treatment, as compared with the barotropic equation of
state. We find that the process of fragmentation is often bimodal. The first
protostar to form is usually, at the end, the most massive, i.e. the primary.
However, frequently a disc-like structure subsequently forms round this
primary, and then, once it has accumulated sufficient mass, quickly fragments
to produce several secondaries. We believe that this delayed fragmentation of a
disc-like structure is likely to be an important source of very low-mass
hydrogen-burning stars and brown dwarfs.Comment: 14 pages, 8 figures. Accepted for publication by A&
Filamentary fragmentation in a turbulent medium
We present the results of smoothed particle hydrodynamic simulations
investigating the evolution and fragmentation of filaments that are accreting
from a turbulent medium. We show that the presence of turbulence, and the
resulting inhomogeneities in the accretion flow, play a significant role in the
fragmentation process. Filaments which experience a weakly turbulent accretion
flow fragment in a two-tier hierarchical fashion, similar to the fragmentation
pattern seen in the Orion Integral Shaped Filament. Increasing the energy in
the turbulent velocity field results in more sub-structure within the
filaments, and one sees a shift from gravity-dominated fragmentation to
turbulence-dominated fragmentation. The sub-structure formed in the filaments
is elongated and roughly parallel to the longitudinal axis of the filament,
similar to the fibres seen in observations of Taurus, and suggests that the
fray and fragment scenario is a possible mechanism for the production of
fibres. We show that the formation of these fibre-like structures is linked to
the vorticity of the velocity field inside the filament and the filament's
accretion from an inhomogeneous medium. Moreover, we find that accretion is
able to drive and sustain roughly sonic levels of turbulence inside the
filaments, but is not able to prevent radial collapse once the filaments become
supercritical. However, the supercritical filaments which contain fibre-like
structures do not collapse radially, suggesting that fibrous filaments may not
necessarily become radially unstable once they reach the critical line-density.Comment: (Accepted for publication in MNRAS
Evidence that widespread star formation may be underway in G0.253+016, "The Brick"
Image cubes of differential column density as a function of dust temperature
are constructed for Galactic Centre molecular cloud G0.253+0.016 ("The Brick")
using the recently described PPMAP procedure. The input data consist of
continuum images from the Herschel Space Telescope in the wavelength range
70-500 m, supplemented by previously published interferometric data at 1.3
mm wavelength. While the bulk of the dust in the molecular cloud is consistent
with being heated externally by the local interstellar radiation field, our
image cube shows the presence, near one edge of the cloud, of a filamentary
structure whose temperature profile suggests internal heating. The structure
appears as a cool ( K) tadpole-like feature, pc in length, in
which is embedded a thin spine of much hotter ( 40-50 K) material. We
interpret these findings in terms of a cool filament whose hot central region
is undergoing gravitational collapse and fragmentation to form a line of
protostars. If confirmed, this would represent the first evidence of widespread
star formation having started within this cloud.Comment: 5 pages, 4 figures; accepted for publication in MNRAS Letter
How to identify the youngest protostars
We study the transition from a prestellar core to a Class 0 protostar, using
SPH to simulate the dynamical evolution, and a Monte Carlo radiative transfer
code to generate the SED and isophotal maps. For a prestellar core illuminated
by the standard interstellar radiation field, the luminosity is low and the SED
peaks at ~190 micron. Once a protostar has formed, the luminosity rises (due to
a growing contribution from accretion onto the protostar) and the peak of the
SED shifts to shorter wavelengths (~80-100 micron). However, by the end of the
Class 0 phase, the accretion rate is falling, the luminosity has decreased, and
the peak of the SED shifts back towards longer wavelengths (90-150 micron). In
our simulations, the density of material around the protostar remains
sufficiently high well into the Class 0 phase that the protostar only becomes
visible in the NIR if it is displaced from the centre dynamically. Raw submm/mm
maps of Class 0 protostars tend to be much more centrally condensed than those
of prestellar cores. However, when convolved with a typical telescope beam, the
difference in central concentration is less marked, although the Class 0
protostars appear more circular. Our results suggest that, if a core is deemed
to be prestellar on the basis of having no associated IRAS source, no cm radio
emission, and no outflow, but it has a circular appearance and an SED which
peaks at wavelengths below ~170 micron, it may well contain a very young Class
0 protostar.Comment: Accepted by A&A (avaliable with high-res images at
http://carina.astro.cf.ac.uk/pub/Dimitrios.Stamatellos/publications
- …