1,183 research outputs found
Gravitational fragmentation and the formation of brown dwarfs in stellar clusters
We investigate the formation of brown dwarfs and very low-mass stars through
the gravitational fragmentation of infalling gas into stellar clusters. The
gravitational potential of a forming stellar cluster provides the focus that
attracts gas from the surrounding molecular cloud. Structures present in the
gas grow, forming filaments flowing into the cluster centre. These filaments
attain high gas densities due to the combination of the cluster potential and
local self-gravity. The resultant Jeans masses are low, allowing the formation
of very low-mass fragments. The tidal shear and high velocity dispersion
present in the cluster preclude any subsequent accretion thus resulting in the
formation of brown dwarfs or very low-mass stars. Ejections are not required as
the brown dwarfs enter the cluster with high relative velocities, suggesting
that their disc and binary properties should be similar to that of low-mass
stars. This mechanism requires the presence of a strong gravitational potential
due to the stellar cluster implying that brown dwarf formation should be more
frequent in stellar clusters than in distributed populations of young stars.
Brown dwarfs formed in isolation would require another formation mechanism such
as due to turbulent fragmentation.Comment: 8 pages, 7 figures. MNRAS, in pres
Massive star formation: Nurture, not nature
We investigate the physical processes which lead to the formation of massive
stars. Using a numerical simulation of the formation of a stellar cluster from
a turbulent molecular cloud, we evaluate the relevant contributions of
fragmentation and competitive accretion in determining the masses of the more
massive stars. We find no correlation between the final mass of a massive star,
and the mass of the clump from which it forms. Instead, we find that the bulk
of the mass of massive stars comes from subsequent competitive accretion in a
clustered environment. In fact, the majority of this mass infalls onto a
pre-existing stellar cluster. Furthermore, the mass of the most massive star in
a system increases as the system grows in numbers of stars and in total mass.
This arises as the infalling gas is accompanied by newly formed stars,
resulting in a larger cluster around a more massive star. High-mass stars gain
mass as they gain companions, implying a direct causal relationship between the
cluster formation process, and the formation of higher-mass stars therein.Comment: 8 pages, accepted for publication in MNRAS. Version including hi-res
colour postscript figure available at
http://star-www.st-and.ac.uk/~sgv/ps/massnurt.ps.g
The formation of close binary systems
A viable solution to the origin of close binary systems, unaccounted for in
recent theories, is presented. Fragmentation, occurring at the end of the
secondary collapse phase (during which molecular hydrogen is dissociating), can
form binary systems with separations less than 1 au. Two fragmentation modes
are found to occur after the collapse is halted. The first consists of the
fragmentation of a protostellar disc due to rotational instabilities in a
protostellar core, involving both an and an mode. This
fragmentation mechanism is found to be insensitive to the initial density
distribution: it can occur in both centrally condensed and uniform initial
conditions. The second fragmentation mode involves the formation of a rapidly
rotating core at the end of the collapse phase which is unstable to the
axisymmetric perturbations. This core bounces into a ring which quickly
fragments into several components. The binary systems thus formed contain less
than 1 per cent of a solar mass and therefore will need to accrete most of
their final mass if they are to form a binary star system. Their orbital
properties will thus be determined by the properties of the accreted matter.Comment: 6 pages, uuencoded compressed postscript file (containing 2 figures
The Origin of the Initial Mass Function and Its Dependence on the Mean Jeans Mass in Molecular Clouds
We investigate the dependence of stellar properties on the mean thermal Jeans
mass in molecular clouds. We compare the results from the two largest
hydrodynamical simulations of star formation to resolve the fragmentation
process down to the opacity limit, the first of which was reported by Bate,
Bonnell & Bromm. The initial conditions of the two calculations are identical
except for the radii of the clouds, which are chosen so that the mean densities
and mean thermal Jeans masses of the clouds differ by factors of nine and
three, respectively. We find that the denser cloud, with the lower mean thermal
Jeans mass, produces a higher proportion of brown dwarfs and has a lower
characteristic (median) mass of the stars and brown dwarfs. This dependence of
the initial mass function (IMF) on the density of the cloud may explain the
observation that the Taurus star-forming region appears to be deficient in
brown dwarfs when compared with the Orion Trapezium cluster. The new
calculation also produces wide binaries (separations >20 AU), one of which is a
wide binary brown dwarf system. Based on the hydrodynamical calculations, we
develop a simple accretion/ejection model for the origin of the IMF. In the
model, all stars and brown dwarfs begin with the same mass (set by the opacity
limit for fragmentation) and grow in mass until their accretion is terminated
stochastically by their ejection from the cloud through dynamically
interactions. The model predicts that the main variation of the IMF in
different star-forming environments should be in the location of the peak (due
to variations in the mean thermal Jeans mass of the cloud) and in the
substellar regime. However, the slope of the IMF at high-masses may depend on
the dispersion in the accretion rates of protostars.Comment: 22 pages, 14 figures, accepted for publication in MNRAS. Paper with
high-resolution figures and animations available from
http://www.astro.ex.ac.uk/people/mbate/ Replacement removes inconsistent
definitions of base 10 logarithm
The Formation of Close Binary Systems by Dynamical Interactions and Orbital Decay
We present results from the first hydrodynamical star formation calculation
to demonstrate that close binary stellar systems (separations \lsim 10 AU)
need not be formed directly by fragmentation. Instead, a high frequency of
close binaries can be produced through a combination of dynamical interactions
in unstable multiple systems and the orbital decay of initially wider binaries.
Orbital decay may occur due to gas accretion and/or the interaction of a binary
with its circumbinary disc. These three mechanisms avoid the problems
associated with the fragmentation of optically-thick gas to form close systems
directly. They also result in a preference for close binaries to have roughly
equal-mass components because dynamical exchange interactions and the accretion
of gas with high specific angular momentum drive mass ratios towards unity.
Furthermore, due to the importance of dynamical interactions, we find that
stars with greater masses ought to have a higher frequency of close companions,
and that many close binaries ought to have wide companions. These properties
are in good agreement with the results of observational surveys.Comment: Published in MNRAS, 10 pages, 6 figures (5 degraded). Paper with
high-resolution figures and animations available from
http://www.astro.ex.ac.uk/people/mbat
The efficiency of star formation in clustered and distributed regions
We investigate the formation of both clustered and distributed populations of
young stars in a single molecular cloud. We present a numerical simulation of a
10,000 solar mass elongated, turbulent, molecular cloud and the formation of
over 2500 stars. The stars form both in stellar clusters and in a distributed
mode which is determined by the local gravitational binding of the cloud. A
density gradient along the major axis of the cloud produces bound regions that
form stellar clusters and unbound regions that form a more distributed
population. The initial mass function also depends on the local gravitational
binding of the cloud with bound regions forming full IMFs whereas in the
unbound, distributed regions the stellar masses cluster around the local Jeans
mass and lack both the high-mass and the low-mass stars. The overall efficiency
of star formation is ~ 15 % in the cloud when the calculation is terminated,
but varies from less than 1 % in the the regions of distributed star formation
to ~ 40 % in regions containing large stellar clusters. Considering that large
scale surveys are likely to catch clouds at all evolutionary stages, estimates
of the (time-averaged) star formation efficiency for the giant molecular cloud
reported here is only ~ 4 %. This would lead to the erroneous conclusion of
'slow' star formation when in fact it is occurring on a dynamical timescale.Comment: 9 pages, 8 figures, MNRAS in pres
The Formation Mechanism of Brown Dwarfs
We present results from the first hydrodynamical star formation calculation
to demonstrate that brown dwarfs are a natural and frequent product of the
collapse and fragmentation of a turbulent molecular cloud. The brown dwarfs
form via the fragmentation of dense molecular gas in unstable multiple systems
and are ejected from the dense gas before they have been able to accrete to
stellar masses. Thus, they can be viewed as `failed stars'. Approximately three
quarters of the brown dwarfs form in gravitationally-unstable circumstellar
discs while the remainder form in collapsing filaments of molecular gas. These
formation mechanisms are very efficient, producing roughly the same number of
brown dwarfs as stars, in agreement with recent observations. However, because
close dynamical interactions are involved in their formation, we find a very
low frequency of binary brown dwarf systems (\lsim 5%) and that those binary
brown dwarf systems that do exist must be close \lsim 10 AU. Similarly, we
find that young brown dwarfs with large circumstellar discs (radii \gsim 10
AU) are rare (%).Comment: 5 pages, 2 GIF figures, postscript with figures available at
http://www.astro.ex.ac.uk/people/mbat
The hierarchical formation of a stellar cluster
Recent surveys of star forming regions have shown that most stars, and
probably all massive stars, are born in dense stellar clusters. The mechanism
by which a molecular cloud fragments to form several hundred to thousands of
individual stars has remained elusive. Here, we use a numerical simulation to
follow the fragmentation of a turbulent molecular cloud and the subsequent
formation and early evolution of a stellar cluster containing more than 400
stars. We show that the stellar cluster forms through the hierarchical
fragmentation of a turbulent molecular cloud. This leads to the formation of
many small subclusters which interact and merge to form the final stellar
cluster. The hierarchical nature of the cluster formation has serious
implications in terms of the properties of the new-born stars. The higher
number-density of stars in subclusters, compared to a more uniform distribution
arising from a monolithic formation, results in closer and more frequent
dynamical interactions. Such close interactions can truncate circumstellar
discs, harden existing binaries, and potentially liberate a population of
planets. We estimate that at least one-third of all stars, and most massive
stars, suffer such disruptive interactions.Comment: 6 pages, 4 figures, accepted for publication in MNRAS. Version
including hi-res colour postscript figure available at
http://star-www.st-and.ac.uk/~sgv/ps/clufhier.ps.g
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