878 research outputs found
Galactic porosity and a star formation threshold for the escape of ionising radiation from galaxies
The spatial distribution of star formation within galaxies strongly affects
the resulting feedback processes. Previous work has considered the case of a
single, concentrated nuclear starburst, and also that of distributed single
supernovae (SNe). Here, we consider ISM structuring by SNe originating in
spatially distributed clusters having a cluster membership spectrum given by
the observed HII region luminosity function. We show that in this case, the
volume of HI cleared per SN is considerably greater than in either of the two
cases considered hitherto.
We derive a simple relationship between the ``porosity'' of the ISM and the
star formation rate (SFR), and deduce a critical SFR_crit, at which the ISM
porosity is unity. This critical value describes the case in which the SN
mechanical energy output over a timescale t_e is comparable with the ISM
``thermal'' energy contained in random motions; t_e is the duration of SN
mechanical input per superbubble. This condition also defines a critical gas
consumption timescale t_exh, which for a Salpeter IMF and random velocities of
\simeq 10 km s-1 is roughly 10e10 years.
We draw a link between porosity and the escape of ionising radiation from
galaxies, arguing that high escape fractions are expected if SFR >~ SFR_crit.
The Lyman Break Galaxies, which are presumably subject to infall on a timescale
< t_exh, meet this criterion, as is consistent with the significant leakage of
ionising photons inferred in these systems. We suggest the utility of this
simple parameterisation of escape fraction in terms of SFR for semi-empirical
models of galaxy formation and evolution and for modeling mechanical and
chemical feedback effects.Comment: Accepted to MNRAS. 11 pages, 1 figure; uses mn2e.cls (included
The Superbubble Size Distribution in the Interstellar Medium of Galaxies
We use the standard, adiabatic shell evolution to predict the size
distribution N(R) for populations of OB superbubbles in a uniform ISM. We
derive N(R) for simple cases of superbubble creation rate and mechanical
luminosity function (MLF). For R < the characteristic radius R_e, N(R) is
dominated by stalled objects, while for R>R_e it is dominated by growing
objects. We also briefly investigate N(R) resulting from momentum-conserving
shell evolution. We predict a peak in N(R) corresponding to individual SNRs. To
estimate the MLF, we also examine evolutionary effects on the HII region
luminosity function (HII LF), finding that for nebular luminosity fading as a
power law in time, there is a minimum observed slope for the HII LFs.
Comparison with the largely complete HI hole catalog for the SMC shows
surprising agreement in the predicted and observed slope of N(R), suggesting
that no other fundamental process is needed to explain the size distribution of
shells in the SMC. Further comparison with largely incomplete HI data for M31,
M33, and Holmberg II is also encouraging. We present expressions for the ISM
porosity parameters, and estimate that they are substantially <1 for all of the
galaxies except Holmberg II. Most of these galaxies therefore may not be
strongly dominated by a hot interstellar component. However, porosity results
for the Galaxy remain inconclusive.Comment: 25 pages, MN latex, 4 figures. MNRAS accepted. Complete abstract and
preprint also available at http://ast.cam.ac.uk/~oey/oeypubs.htm
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
Do O-stars form in isolation?
Around 4% of O-stars are observed in apparent isolation, with no associated
cluster, and no indication of having been ejected from a nearby cluster. We
define an isolated O-star as a star > 17.5 M_\odot in a cluster with total mass
10 M_\odot) stars. We show that
the fraction of apparently isolated O-stars is reproduced when stars are
sampled (randomly) from a standard initial mass function and a standard cluster
mass function of the form N(M) \propto M^-2.
This result is difficult to reconcile with the idea that there is a
fundamental relationship between the mass of a cluster and the mass of the most
massive star in that cluster. We suggest that such a relationship is a typical
result of star formation in clusters, and that `isolated O-stars' are low-mass
clusters in which massive stars have been able to form.Comment: 6 pages, 5 figures, MNRAS in pres
Statistical Confirmation of a Stellar Upper Mass Limit
We derive the expectation value for the maximum stellar mass (m_max) in an
ensemble of N stars, as a function of the IMF upper-mass cutoff (m_up) and N.
We statistically demonstrate that the upper IMF of the local massive star
census observed thus far in the Milky Way and Magellanic Clouds clearly
exhibits a universal upper mass cutoff around 120 - 200 M_sun for a Salpeter
IMF, although the result is more ambiguous for a steeper IMF.Comment: PDF, 5 pages, 4 figures. Accepted to the Astrophysical Journal
Letter
The star formation process in the Magellanic Clouds
The Magellanic Clouds offer unique opportunities to study star formation both
on the global scales of an interacting system of gas-rich galaxies, as well as
on the scales of individual star-forming clouds. The interstellar media of the
Small and Large Magellanic Clouds and their connecting bridge, span a range in
(low) metallicities and gas density. This allows us to study star formation
near the critical density and gain an understanding of how tidal dwarfs might
form; the low metallicity of the SMC in particular is typical of galaxies
during the early phases of their assembly, and studies of star formation in the
SMC provide a stepping stone to understand star formation at high redshift
where these processes can not be directly observed. In this review, I introduce
the different environments encountered in the Magellanic System and compare
these with the Schmidt-Kennicutt law and the predicted efficiencies of various
chemo-physical processes. I then concentrate on three aspects that are of
particular importance: the chemistry of the embedded stages of star formation,
the Initial Mass Function, and feedback effects from massive stars and its
ability to trigger further star formation.Comment: 12pages, 5figures, invited review at the IAUS 256, The Magellanic
System: Stars, Gas, and Galaxies, eds. Jacco van Loon, Joana Oliveir
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