3,379 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 Massive Stars through Stellar Collisions
In this review, I present the case for how massive stars may form through
stellar collisions. This mechanism requires very high stellar densities, up to
4 orders of magnitude higher than are observed in the cores of dense young
clusters. In this model, the required stellar densities arise due to gas
accretion onto stars in the cluster core, including the precursers of the
massive stars. This forces the core to contract until the stellar densities are
sufficiently high for collisions to occur. Gas accretion is also likely to play
a major role in determining the distribution of stellar masses in the cluster
as well as the observed mass segregation. One of the main advantages of this
mechanism is that it explicitly relies on the cluster environment in order to
produce the massive stars. It is thus in a position to explain the relation
between clustered and massive star formation which is not an obvious outcome of
an isolated accretion mechanism. A recent numerical simulation supports this
model as the cluster core increases its density by during gas accretion.
Approximately 15 stellar collisions occur (with au) in the cluster
core, making a significant contribution to the mass of the most massive star.Comment: 14 pages, 7 figures. in The earliest phase of massive star formation.
ASP Conference Series, P. Crowther E
The high-mass stellar IMF in different environments
The massive-star IMF is found to be invariable. However, integrated IMFs
probably depend on galactic mass.Comment: Invited talk at JD05, IAU General Assembly XVI, Pragu
Competitive Accretion in Clusters and the IMF
Observations have revealed that most stars are born in clusters. As these
clusters typically contain more mass in gas than in stars, accretion can play
an important role in determining the final stellar masses. Numerical
simulations of gas accretion in stellar clusters have found that the stars
compete for the available reservoir of gas. The accretion rates are highly
nonuniform and are determined primarily by each star's position in the cluster.
Stars in the centre accrete more gas, resulting in initial mass segregation.
This competitive accretion naturally results in a mass spectrum and is
potentially the dominant mechanism for producing the initial mass function.
Furthermore, accretion on to the core of a cluster forces it to shrink, which
may result in formation of massive stars through collisions.Comment: Proc. of 33rd ESLAB Symp. "Star Formation from the Small to the Large
Scale" (F. Favato, A.A. Laas & A. Wilson Eds, ESA SP-445, 2000). 10 pages,
incl. 5 figure
An investigation of spherical blast waves and detonation waves in a rocket combustion chamber
Spherical blast waves and detonation waves in rocket combustion chambe
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
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