1,109 research outputs found
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&
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
Massive star formation via high accretion rates and early disk-driven outflows
We present an investigation of massive star formation that results from the
gravitational collapse of massive, magnetized molecular cloud cores. We
investigate this by means of highly resolved, numerical simulations of initial
magnetized Bonnor-Ebert-Spheres that undergo collapse and cooling. By comparing
three different cases - an isothermal collapse, a collapse with radiative
cooling, and a magnetized collapse - we show that massive stars assemble
quickly with mass accretion rates exceeding 10^-3 Msol/yr. We confirm that the
mass accretion during the collapsing phase is much more efficient than
predicted by selfsimilar collapse solutions, i.e. dM/dt ~ c^3/G. We find that
during protostellar assembly the mass accretion reaches 20 - 100 c^3/G.
Furthermore, we determined the self-consistent structure of bipolar outflows
that are produced in our three dimensional magnetized collapse simulations.
These outflows produce cavities out of which radiation pressure can be
released, thereby reducing the limitations on the final mass of massive stars
formed by gravitational collapse. Moreover, we argue that the extraction of
angular momentum by disk-threaded magnetic fields and/or by the appearance of
bars with spiral arms significantly enhance the mass accretion rate, thereby
helping the massive protostar to assemble more quickly.Comment: 22 pages, 12 figures, aastex style, accepted for publication in ApJ,
see http://www.ita.uni-heidelberg.de/~banerjee/publications/MassiveStars.pdf
for high resolution figure
Starburst-driven Starbursts in the Heart of Ultraluminous Infrared Galaxies
There is increasing evidence for the presence of blue super star clusters in
the central regions of ultraluminous infrared galaxies like Arp 220.
Ultraluminous galaxies are thought to be triggered by galaxy mergers, and it
has often been argued that these super star clusters may form during violent
collisions between gas clouds in the final phase of the mergers. We now
investigate another set of models which differ from previous ones in that the
formation of the super star clusters is linked directly to the very intense
starburst occurring at the very center of the galaxy. Firstly we show that a
scenario in which the super star clusters form in material compressed by shock
waves originating from the central starburst is implausible because the objects
so produced are much smaller than the observed star clusters in Arp 220. We
then investigate a scenario (based on the Shlosman-Noguchi model) in which the
infalling dense gas disk is unstable gravitationally and collapses to form
massive gaseous clumps. Since these clumps are exposed to the external high
pressure driven by the superwind (a blast wave driven by a collective effect of
a large number of supernovae in the very core of the galaxy), they can collapse
and then massive star formation may be induced in them. The objects produced in
this kind of collapse have properties consistent with those of the observed
super star clusters in the center of Arp 220.Comment: 13 pages, 1 figure, ApJ (Letters) in pres
Why do starless cores appear more flattened than protostellar cores?
We evaluate the intrinsic three dimensional shapes of molecular cores, by
analysing their projected shapes. We use the recent catalogue of molecular line
observations of Jijina et al. and model the data by the method originally
devised for elliptical galaxies. Our analysis broadly supports the conclusion
of Jones et al. that molecular cores are better represented by triaxial
intrinsic shapes (ellipsoids) than biaxial intrinsic shapes (spheroids).
However, we find that the best fit to all of the data is obtained with more
extreme axial ratios () than those derived by Jones et al.
More surprisingly, we find that starless cores have more extreme axial ratios
than protostellar cores -- starless cores appear more `flattened'. This is the
opposite of what would be expected from modeling the freefall collapse of
triaxial ellipsoids. The collapse of starless cores would be expected to
proceed most swiftly along the shortest axis - as has been predicted by every
modeller since Zel'dovich - which should produce more flattened cores around
protostars, the opposite of what is seen.Comment: 7 pages, 3 figure
J plots: a new method for characterizing structures in the interstellar medium
Large-scale surveys have brought about a revolution in astronomy. To analyse the resulting wealth of data, we need automated tools to identify, classify, and quantify the important underlying structures. We present here a method for classifying and quantifying a pixelated structure, based on its principal moments of inertia. The method enables us to automatically detect, and objectively compare, centrally condensed cores, elongated filaments, and hollow rings. We illustrate the method by applying it to (i) observations of surface density from Hi-GAL, and (ii) simulations of filament growth in a turbulent medium. We limit the discussion here to 2D data; in a future paper, we will extend the method to 3D data
Interactions between brown-dwarf binaries and Sun-like stars
Several mechanisms have been proposed for the formation of brown dwarfs, but
there is as yet no consensus as to which -- if any -- are operative in nature.
Any theory of brown dwarf formation must explain the observed statistics of
brown dwarfs. These statistics are limited by selection effects, but they are
becoming increasingly discriminating. In particular, it appears (a) that brown
dwarfs that are secondaries to Sun-like stars tend to be on wide orbits, a\ga
100\,{\rm AU} (the Brown Dwarf Desert), and (b) that these brown dwarfs have a
significantly higher chance of being in a close (a\la 10\,{\rm AU}) binary
system with another brown dwarf than do brown dwarfs in the field. This then
raises the issue of whether these brown dwarfs have formed {\it in situ}, i.e.
by fragmentation of a circumstellar disc; or have formed elsewhere and
subsequently been captured. We present numerical simulations of the purely
gravitational interaction between a close brown-dwarf binary and a Sun-like
star. These simulations demonstrate that such interactions have a negligible
chance () of leading to the close brown-dwarf binary being captured by
the Sun-like star. Making the interactions dissipative by invoking the
hydrodynamic effects of attendant discs might alter this conclusion. However,
in order to explain the above statistics, this dissipation would have to favour
the capture of brown-dwarf binaries over single brown-dwarfs, and we present
arguments why this is unlikely. The simplest inference is that most brown-dwarf
binaries -- and therefore possibly also most single brown dwarfs -- form by
fragmentation of circumstellar discs around Sun-like protostars, with some of
them subsequently being ejected into the field.Comment: 10 pages, 8 figures, Accepted for publication in Astrophysics and
Space Scienc
Spatial-Distance Cues Influence Economic Decision-Making in a Social Context
Social distance (i.e., the degree of closeness to another person) affects the
way humans perceive and respond to fairness during financial negotiations.
Feeling close to someone enhances the acceptance of monetary offers. Here, we
explored whether this effect also extends to the spatial domain. Specifically,
using an iterated version of the Ultimatum Game in a within-subject design, we
investigated whether different visual spatial distance-cues result in
different rates of acceptance of otherwise identical monetary offers. Study 1
found that participants accepted significantly more offers when they were cued
with spatial closeness than when they were cued with spatial distance. Study 2
replicated this effect using identical procedures but different spatial-
distance cues in an independent sample. Importantly, our results could not be
explained by feelings of social closeness. Our results demonstrate that mere
perceptions of spatial closeness produce analogous–but independent–effects to
those of social closeness
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