362 research outputs found
An HST/WFPC Survey of Bright Young Clusters in M31. II. Photometry of Less Luminous Clusters in the Fields
We report on the properties of 89 low mass star clusters located in the
vicinity of luminous young clusters (blue globulars) in the disk of M31. 82 of
the clusters are newly detected. We have determined their integrated magnitudes
and colors, based on a series of Hubble Space Telescope Wide Field/Planetary
Camera 2 exposures in blue and red (HST filters F450W and F814W). The
integrated apparent magnitudes range from F450W = 17.5 to 22.5, and the colors
indicate a wide range of ages. Stellar color-magnitude diagrams for all
clusters were obtained and those with bright enough stars were fit to
theoretical isochrones to provide age estimates. The ages range from 12 Myr to
>500 Myr. Reddenings, which average E(F450 - F814) = 0.59 with a dispersion of
0.21 magnitudes, were derived from the main sequence fitting for those
clusters. Comparison of these ages and integrated colors with single population
theoretical models with solar abundances suggests a color offset of 0.085
magnitudes at the ages tested. Estimated ages for the remaining clusters are
based on their measured colors. The age-frequency diagram shows a steep decline
of number with age, with a large decrease in number per age interval between
the youngest and the oldest clusters detected.Comment: 20 pages, 9 figure
Nonequilibrium brittle fracture propagation: Steady state, oscillations and intermittency
A minimal model is constructed for two-dimensional fracture propagation. The
heterogeneous process zone is presumed to suppress stress relaxation rate,
leading to non-quasistatic behavior. Using the Yoffe solution, I construct and
solve a dynamical equation for the tip stress. I discuss a generic tip velocity
response to local stress and find that noise-free propagation is either at
steady state or oscillatory, depending only on one material parameter. Noise
gives rise to intermittency and quasi-periodicity. The theory explains the
velocity oscillations and the complicated behavior seen in polymeric and
amorphous brittle materials. I suggest experimental verifications and new
connections between velocity measurements and material properties.Comment: To appear in Phys. Rev. Lett., 6 pages, self-contained TeX file, 3
postscript figures upon request from author at [email protected] or
[email protected], http://cnls-www.lanl.gov/homepages/rafi/rafindex.htm
Hierarchical Structure Formation and Modes of Star Formation in Hickson Compact Group 31
The handful of low-mass, late-type galaxies that comprise Hickson Compact
Group 31 is in the midst of complex, ongoing gravitational interactions,
evocative of the process of hierarchical structure formation at higher
redshifts. With sensitive, multicolor Hubble Space Telescope imaging, we
characterize the large population of <10 Myr old star clusters that suffuse the
system. From the colors and luminosities of the young star clusters, we find
that the galaxies in HCG 31 follow the same universal scaling relations as
actively star-forming galaxies in the local Universe despite the unusual
compact group environment. Furthermore, the specific frequency of the globular
cluster system is consistent with the low end of galaxies of comparable masses
locally. This, combined with the large mass of neutral hydrogen and tight
constraints on the amount of intragroup light, indicate that the group is
undergoing its first epoch of interaction-induced star formation. In both the
main galaxies and the tidal-dwarf candidate, F, stellar complexes, which are
sensitive to the magnitude of disk turbulence, have both sizes and masses more
characteristic of z=1-2 galaxies. After subtracting the light from compact
sources, we find no evidence for an underlying old stellar population in F --
it appears to be a truly new structure. The low velocity dispersion of the
system components, available reservoir of HI, and current star formation rate
of ~10 solar masses per year, indicate that HCG31 is likely to both exhaust its
cold gas supply and merge within ~1 Gyr. We conclude that the end product will
be an isolated, X-ray-faint, low-mass elliptical.Comment: 24 pages, 14 figures (including low resolution versions of color
images), latex file prepared with emulateapj. Accepted for publication by the
Astronomical Journa
The Sparsest Clusters With O Stars
There is much debate on how high-mass star formation varies with environment,
and whether the sparsest star-forming environments are capable of forming
massive stars. To address this issue, we have observed eight apparently
isolated OB stars in the SMC using HST's Advanced Camera for Surveys. Five of
these objects appear as isolated stars, two of which are confirmed to be
runaways. The remaining three objects are found to exist in sparse clusters,
with <10 companion stars revealed, having masses of 1-4 solar mass. Stochastic
effects dominate in these sparse clusters, so we perform Monte Carlo
simulations to explore how our observations fit within the framework of
empirical, galactic cluster properties. We generate clusters using a simplistic
-2 power-law distribution for either the number of stars per cluster (N_*) or
cluster mass (M_cl). These clusters are then populated with stars randomly
chosen from a Kroupa IMF. We find that simulations with cluster lower-mass
limits of M_cl,lo >20 solar mass and N_*,lo >40 match best with observations of
SMC and Galactic OB star populations. We examine the mass ratio of the
second-most massive and most massive stars (m_max,2/m_max), finding that our
observations all exist below the 20th percentile of our simulated clusters.
However, all of our observed clusters lie within the parameter space spanned by
the simulated clusters, although some are in the lowest 5th percentile
frequency. These results suggest that clusters are built stochastically by
randomly sampling stars from a universal IMF with a fixed stellar upper-mass
limit. In particular, we see no evidence to suggest a m_max - M_cl relation.
Our results may be more consistent with core accretion models of star formation
than with competitive accretion models, and they are inconsistent with the
proposed steepening of the integrated galaxy IMF (IGIMF).Comment: 19 pages, 12 figures, accepted for publication in Ap
A gas gun based technique for studying the role of temperature in dynamic fracture and fragmentation
Phase-Field Model of Mode III Dynamic Fracture
We introduce a phenomenological continuum model for mode III dynamic fracture
that is based on the phase-field methodology used extensively to model
interfacial pattern formation. We couple a scalar field, which distinguishes
between ``broken'' and ``unbroken'' states of the system, to the displacement
field in a way that consistently includes both macroscopic elasticity and a
simple rotationally invariant short scale description of breaking. We report
two-dimensional simulations that yield steady-state crack motion in a strip
geometry above the Griffith threshold.Comment: submitted to PR
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