221 research outputs found
The Maximum Mass of Star Clusters
When an universal untruncated star cluster initial mass function (CIMF)
described by a power-law distribution is assumed, the mass of the most massive
star cluster in a galaxy (M_max) is the result of the size-of-sample (SoS)
effect. This implies a dependence of M_max on the total number of star clusters
(N). The SoS effect also implies that M_max within a cluster population
increases with equal logarithmic intervals of age. This is because the number
of clusters formed in logarithmic age intervals increases (assuming a constant
cluster formation rate). This effect has been observed in the SMC and LMC.
Based on the maximum pressure (P_int) inside molecular clouds, it has been
suggested that a physical maximum mass (M_max[phys]) should exist. The theory
predicts that M_max[phys] should be observable, i.e. lower than M_max that
follows from statistical arguments, in big galaxies with a high star formation
rate. We compare the SoS relations in the SMC and LMC with the ones in M51 and
model the integrated cluster luminosity function (CLF) for two cases: 1) M_max
is determined by the SoS effect and 2) M_max=M_max[phys]=constant. The observed
CLF of M51 and the comparison of the SoS relations with the SMC and LMC both
suggest that there exists a M_max[phys] of 5*10^5 M_sun in M51. The CLF of M51
looks very similar to the one observed in the ``Antennae'' galaxies. A direct
comparison with our model suggests that there M_max[phys]=2*10^6 M_sun.Comment: 4 pages, contribution to "Globular Clusters: Guides to Galaxies",
March 6th-10th, 200
Outskirts of Nearby Disk Galaxies: Star Formation and Stellar Populations
The properties and star formation processes in the far-outer disks of nearby
spiral and dwarf irregular galaxies are reviewed. The origin and structure of
the generally exponential profiles in stellar disks is considered to result
from cosmological infall combined with a non-linear star formation law and a
history of stellar migration and scattering from spirals, bars, and random
collisions with interstellar clouds. In both spirals and dwarfs, the far-outer
disks tend to be older, redder and thicker than the inner disks, with the
overall radial profiles suggesting inside-out star formation plus stellar
scattering in spirals, and outside-in star formation with a possible
contribution from scattering in dwarfs. Dwarf irregulars and the far-outer
parts of spirals both tend to be gas dominated, and the gas radial profile is
often non-exponential although still decreasing with radius. The ratio of
H-alpha to far-UV flux tends to decrease with lower surface brightness in these
regions, suggesting either a change in the initial stellar mass function or the
sampling of that function, or a possible loss of H-alpha photons.Comment: 20 pages, 8 figures, Invited review, Book chapter in "Outskirts of
Galaxies", Eds. J. H. Knapen, J. C. Lee and A. Gil de Paz, Astrophysics and
Space Science Library, Springer, in pres
High-precision determination of the critical exponents for the lambda-transition of 4He by improved high-temperature expansion
We determine the critical exponents for the XY universality class in three
dimensions, which is expected to describe the -transition in He.
They are obtained from the analysis of high-temperature series computed for a
two-component model. The parameter is fixed such that
the leading corrections to scaling vanish. We obtain ,
, . These estimates improve previous
theoretical determinations and agree with the more precise experimental results
for liquid Helium.Comment: 8 pages, revte
The starburst phenomenon from the optical/near-IR perspective
The optical/near-IR stellar continuum carries unique information about the
stellar population in a galaxy, its mass function and star-formation history.
Star-forming regions display rich emission-line spectra from which we can
derive the dust and gas distribution, map velocity fields, metallicities and
young massive stars and locate shocks and stellar winds. All this information
is very useful in the dissection of the starburst phenomenon. We discuss a few
of the advantages and limitations of observations in the optical/near-IR region
and focus on some results. Special attention is given to the role of
interactions and mergers and observations of the relatively dust-free starburst
dwarfs. In the future we expect new and refined diagnostic tools to provide us
with more detailed information about the IMF, strength and duration of the
burst and its triggering mechanisms.Comment: 6 pages, 3 figures, to appear in "Starbursts: from 30 Doradus to
Lyman Break Galaxies" 2005, eds. R. de Grijs and R. M. Gonzalez Delgado
(Kluwer
Efficiency of the dynamical mechanism
The most extreme starbursts occur in galaxy mergers, and it is now
acknowledged that dynamical triggering has a primary importance in star
formation. This triggering is due partly to the enhanced velocity dispersion
provided by gravitational instabilities, such as density waves and bars, but
mainly to the radial gas flows they drive, allowing large amounts of gas to
condense towards nuclear regions in a small time scale. Numerical simulations
with several gas phases, taking into account the feedback to regulate star
formation, have explored the various processes, using recipes like the Schmidt
law, moderated by the gas instability criterion. May be the most fundamental
parameter in starbursts is the availability of gas: this sheds light on the
amount of external gas accretion in galaxy evolution. The detailed mechanisms
governing gas infall in the inner parts of galaxy disks are discussed.Comment: 6 pages, 3 figures, to be published in "Starbursts - From 30 Doradus
to Lyman break galaxies", ed. R. de Grijs and R. Gonzalez-Delgad
Extension to order of the high-temperature expansions for the spin-1/2 Ising model on the simple-cubic and the body-centered-cubic lattices
Using a renormalized linked-cluster-expansion method, we have extended to
order the high-temperature series for the susceptibility
and the second-moment correlation length of the spin-1/2 Ising models on
the sc and the bcc lattices. A study of these expansions yields updated direct
estimates of universal parameters, such as exponents and amplitude ratios,
which characterize the critical behavior of and . Our best
estimates for the inverse critical temperatures are
and . For the
susceptibility exponent we get and for the correlation
length exponent we get .
The ratio of the critical amplitudes of above and below the critical
temperature is estimated to be . The analogous ratio for
is estimated to be . For the correction-to-scaling
amplitude ratio we obtain .Comment: Misprints corrected, 8 pages, latex, no figure
25th-order high-temperature expansion results for three-dimensional Ising-like systems on the simple cubic lattice
25th-order high-temperature series are computed for a general
nearest-neighbor three-dimensional Ising model with arbitrary potential on the
simple cubic lattice. In particular, we consider three improved potentials
characterized by suppressed leading scaling corrections. Critical exponents are
extracted from high-temperature series specialized to improved potentials,
obtaining , , ,
, , . Moreover, biased
analyses of the 25th-order series of the standard Ising model provide the
estimate for the exponent associated with the leading scaling
corrections. By the same technique, we study the small-magnetization expansion
of the Helmholtz free energy. The results are then applied to the construction
of parametric representations of the critical equation of state, using a
systematic approach based on a global stationarity condition. Accurate
estimates of several universal amplitude ratios are also presented.Comment: 40 pages, 15 figure
Star Formation on Galactic Scales: Empirical Laws
Empirical star formation laws from the last 20 years are reviewed with a
comparison to simulations. The current form in main galaxy disks has a linear
relationship between the star formation rate per unit area and the molecular
cloud mass per unit area with a timescale for molecular gas conversion of about
2 Gyr. The local ratio of molecular mass to atomic mass scales nearly linearly
with pressure, as determined from the weight of the gas layer in the galaxy. In
the outer parts of galaxies and in dwarf irregular galaxies, the disk can be
dominated by atomic hydrogen and the star formation rate per unit area becomes
directly proportional to the total gas mass per unit area, with a consumption
time of about 100 Gyr. The importance of a threshold for gravitational
instabilities is not clear. Observations suggest such a threshold is not always
important, while simulations generally show that it is. The threshold is
difficult to evaluate because it is sensitive to magnetic and viscous forces,
the presence of spiral waves and other local effects, and the equation of
state.Comment: 16 pages, 2 figures, Ecole Evry Schatzman 2010: Star Formation in the
Local Universe. Lecture 1 of
Post-Newtonian Gravitational Radiation
1 Introduction 2 Multipole Decomposition 3 Source Multipole Moments 4
Post-Minkowskian Approximation 5 Radiative Multipole Moments 6 Post-Newtonian
Approximation 7 Point-Particles 8 ConclusionComment: 46 pages, in Einstein's Field Equations and Their Physical
Implications, B. Schmidt (Ed.), Lecture Notes in Physics, Springe
Variation of the IMF
(abridged) The {stellar IMF} has been found to be essentially invariant.
While some apparent differences are seen, the uncertainties inherent to this
game do not allow a firm conclusion to be made that the IMF varies
systematically with conditions. The IMF integrated over entire galaxies,
however, is another matter. Chemical and photometric properties of various
galaxies do hint at {galaxial IMFs} being steeper than the stellar IMF, as is
also deduced from direct star-count analysis in the MW. These results are
sensitive to the modelling of stellar populations and to corrections for
stellar evolution, and are thus also uncertain. However, by realising that
galaxies are made from dissolving star clusters, star clusters being viewed as
{the fundamental building blocks of galaxies}, the result is found that
galaxial IMFs must be significantly steeper than the stellar IMF, because the
former results from a folding of the latter with the star-cluster mass
function. Furthermore, this notion leads to the important insight that galaxial
IMFs must vary with galaxy mass, and that the galaxial IMF is a strongly
varying function of the star-formation history for galaxies that have assembled
only a small mass in stars. Cosmological implications of this are discussed.Comment: 13 pages, to appear in IMFat50: The Initial Mass Function 50 years
later, ed: E. Corbelli, F. Palla, and H. Zinnecker, Kluwer Academic
Publishers; a meeting held at the Abbazia di Spineto, Tuscany, Italy -- May
16-20, 200
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