247,096 research outputs found
Two Stellar Mass Functions Combined into One by the Random Sampling Model of the IMF
The turnover in the stellar initial mass function (IMF) at low mass suggests
the presence of two independent mass functions that combine in different ways
above and below a characteristic mass given by the thermal Jeans mass in the
cloud. In the random sampling model introduced earlier, the Salpeter IMF at
intermediate to high mass follows primarily from the hierarchical structure of
interstellar clouds, which is sampled by various star formation processes and
converted into stars at the local dynamical rate. This power law part is
independent of the details of star formation inside each clump and therefore
has a universal character. The flat part of the IMF at low mass is proposed
here to result from a second, unrelated, physical process that determines only
the probability distribution function for final star mass inside a clump of a
given mass, and is independent of both this clump mass and the overall cloud
structure. Both processes operate for all potentially unstable clumps in a
cloud, regardless of mass, but only the first shows up above the thermal Jeans
mass, and only the second shows up below this mass. Analytical and stochastic
models of the IMF that are based on the uniform application of these two
functions for all masses reproduce the observations well.Comment: 4 pages, 2 figures, MNRAS pink pages in press 199
Simple and Efficient Local Codes for Distributed Stable Network Construction
In this work, we study protocols so that populations of distributed processes
can construct networks. In order to highlight the basic principles of
distributed network construction we keep the model minimal in all respects. In
particular, we assume finite-state processes that all begin from the same
initial state and all execute the same protocol (i.e. the system is
homogeneous). Moreover, we assume pairwise interactions between the processes
that are scheduled by an adversary. The only constraint on the adversary
scheduler is that it must be fair. In order to allow processes to construct
networks, we let them activate and deactivate their pairwise connections. When
two processes interact, the protocol takes as input the states of the processes
and the state of the their connection and updates all of them. Initially all
connections are inactive and the goal is for the processes, after interacting
and activating/deactivating connections for a while, to end up with a desired
stable network. We give protocols (optimal in some cases) and lower bounds for
several basic network construction problems such as spanning line, spanning
ring, spanning star, and regular network. We provide proofs of correctness for
all of our protocols and analyze the expected time to convergence of most of
them under a uniform random scheduler that selects the next pair of interacting
processes uniformly at random from all such pairs. Finally, we prove several
universality results by presenting generic protocols that are capable of
simulating a Turing Machine (TM) and exploiting it in order to construct a
large class of networks.Comment: 43 pages, 7 figure
Star Formation from Galaxies to Globules
The empirical laws of star formation suggest that galactic-scale gravity is
involved, but they do not identify the actual triggering mechanisms for
clusters in the final stages. Many other triggering processes satisfy the
empirical laws too, including turbulence compression and expanding shell
collapse. The self-similar nature of the gas and associated young stars
suggests that turbulence is more directly involved, but the small scale
morphology of gas around most embedded clusters does not look like a random
turbulent flow. Most clusters look triggered by other nearby stars. Such a
prominent local influence makes it difficult to understand the universality of
the Kennicutt and Schmidt laws on galactic scales. A unified view of
multi-scale star formation avoids most of these problems. Ambient self-gravity
produces spiral arms and drives much of the turbulence that leads to
self-similar structures, while localized energy input from existing clusters
and field supernovae triggers new clusters in pre-existing clouds. The
hierarchical structure in the gas made by turbulence ensures that the
triggering time scales with size, giving the Schmidt law over a wide range of
scales and the size-duration correlation for young star fields. The efficiency
of star formation is determined by the fraction of the gas above a critical
density of around 10^5 m(H2)/cc. Star formation is saturated to its largest
possible value given the fractal nature of the interstellar medium.Comment: accepted for ApJ, 42 pages, Dannie Heineman prize lecture, January
200
On turbulent fragmentation and the origin of the stellar IMF
Two varieties of the universal stellar initial mass function (IMF) viz., the
Kroupa and the Chabrier IMF, have emerged over the last decade to explain the
observed distribution of stellar masses. The possibility of the universal
nature of the stellar IMF leads us to the interesting prospect of a universal
mode of star-formation. It is well-known that turbulent fragmentation of gas in
the interstellar medium produces a lognormal distribution of density which is
further reflected by the mass-function for clumps at low and intermediate
masses. Stars condense out of unstable clumps through a complex interplay
between a number of dynamic processes which must be accounted for when tracing
the origin of the stellar IMF. In the present work, applying the theory of
gravitational fragmentation we first derive the mass function (MF) for clumps.
Then a core mass function (CMF) is derived by allowing the clumps to fragment,
having subjected each one to a random choice of gas temperature. Finally, the
stellar IMF is derived by applying a random core-to-star conversion efficiency,
, in the range of 5%-15% to each CMF. We obtain a power-law IMF that
has exponents within the error-bars on the Kropua IMF. This derived IMF is
preceded by a similar core mass function which suggests, gravoturbulent
fragmentation plays a key role in assembling necessary conditions that relate
the two mass-functions. In this sense the star-formation process, at least at
low redshifts where gas cooling is efficient, is likely to be universal. We
argue that the observed knee in the CMF and the stellar IMF may alternatively
be interpreted in terms of the characteristic temperature at which gas in
potential star-forming clouds is likely to be found. Our results also show that
turbulence in star-forming clouds is probably driven on large spatial scales
with a power-spectrum steeper than Kolmogorov-type.Comment: 10 pages, 5 figures; To appear in New Astronomy; Figure numbers
corrected in this versio
From rings to bulges: evidence for rapid secular galaxy evolution at z~2 from integral field spectroscopy in the SINS survey
We present Ha integral field spectroscopy of well resolved, UV/optically
selected z~2 star-forming galaxies as part of the SINS survey with SINFONI on
the ESO VLT. Our laser guide star adaptive optics and good seeing data show the
presence of turbulent rotating star forming rings/disks, plus central
bulge/inner disk components, whose mass fractions relative to total dynamical
mass appears to scale with [NII]/Ha flux ratio and star formation age. We
propose that the buildup of the central disks and bulges of massive galaxies at
z~2 can be driven by the early secular evolution of gas-rich proto-disks. High
redshift disks exhibit large random motions. This turbulence may in part be
stirred up by the release of gravitational energy in the rapid cold accretion
flows along the filaments of the cosmic web. As a result dynamical friction and
viscous processes proceed on a time scale of <1 Gyr, at least an order of
magnitude faster than in z~0 disk galaxies. Early secular evolution thus drives
gas and stars into the central regions and can build up exponential disks and
massive bulges, even without major mergers. Secular evolution along with
increased efficiency of star formation at high surface densities may also help
to account for the short time scales of the stellar buildup observed in massive
galaxies at z~2.Comment: accepted Astrophysical Journal, main July 8 200
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