599 research outputs found
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
An Ionization Cone in the Dwarf Starburst Galaxy NGC 5253
There are few observational constraints on how the escape of ionizing photons
from starburst galaxies depends on galactic parameters. Here, we report on the
first major detection of an ionization cone in NGC 5253, a nearby starburst
galaxy. This high-excitation feature is identified by mapping the emission-line
ratios in the galaxy using [S III] lambda 9069, [S II] lambda 6716, and H_alpha
narrow-band images from the Maryland-Magellan Tunable Filter at Las Campanas
Observatory. The ionization cone appears optically thin, which is suggestive of
the escape of ionizing photons. The cone morphology is narrow with an estimated
solid angle covering just 3% of 4pi steradians, and the young, massive clusters
of the nuclear starburst can easily generate the radiation required to ionize
the cone. Although less likely, we cannot rule out the possibility of an
obscured AGN source. An echelle spectrum along the minor axis shows complex
kinematics that are consistent with outflow activity. The narrow morphology of
the ionization cone supports the scenario that an orientation bias contributes
to the difficulty in detecting Lyman continuum emission from starbursts and
Lyman break galaxies.Comment: 5 pages, 4 figures, Accepted to ApJ Letter
Mapping Lyman Continuum escape in Tololo 1247-232
Low redshift, spatially resolved Lyman continuum (LyC) emitters allow us to
clarify the processes for LyC escape from these starburst galaxies. We use
Hubble Space Telescope (HST) WFC3 and ACS imaging of the confirmed low-redshift
LyC emitter Tol 1247-232 to study the ionization structure of the gas and its
relation to the ionizing star clusters. We perform ionization parameter mapping
(IPM) using [O III]4959, 5007 and [O II]3727 imaging as the high- and
low-ionization tracers, revealing broad, large-scale, optically thin regions
originating from the center, and reaching the outskirts of the galaxy,
consistent with LyC escape. We carry out stellar population synthesis modeling
of the 26 brightest clusters using our HST photometry. Combining these data
with the nebular photometry, we find a global LyC escape fraction of f_esc =
0.12, with uncertainties also consistent with zero escape and with all measured
f_esc values for this galaxy. Our analysis suggests that, similar to other
candidate LyC emitters, a two-stage starburst has taken place in this galaxy,
with a 12 Myr old, massive, central cluster likely having pre-cleared regions
in and around the center, and the second generation of 2 - 4 Myr old clusters
dominating the current ionization, including some escape from the galaxy.Comment: Accepted for publication in Ap
Sigma Coordinate Pressure Gradient Errors and the Seamount Problem
In a recent paper by Mellor et al., it was found that, in two-dimensional (x, z) applications with finite horizontal viscosity and zero diffusivity, the velocity error, associated with the evaluation of horizontal density or pressure gradients on a sigma coordinate grid, prognostically disappeared, leaving behind a small and physically insignificant distortion in the density field. The initial error is numerically consistent in that it decreases as the square of the grid increment size. In this paper, we label this error as a sigma error of the first kind. In three-dimensional applications, the authors have encountered an error that did not disappear and that has not been understood by us or, apparently, others. This is a vorticity error that is labeled a sigma error of the second kind and is a subject of this paper. Although it does not prognostically disappear, it seems to be tolerably small. To evaluate these numerical errors, the authors have adopted the seamount problem initiated by Beckman and Haidvogel. It represents a stringent test case, as evidenced by their paper, wherein the model is initialized with horizontal isopycnals, zero velocity, and no forcing; then, any velocities that develop must be considered errors. Two appendices are important adjuncts to the paper, the first providing theoretical confirmation and understanding of the numerical results, and the second delving into additional errors related to horizontal or isosigma diffusion. It is, however, shown that satisfactory numerical solutions are obtained with zero diffusivity
The Pressure Gradient Conundrum of Sigma Coordinate Ocean Models
It is shown that the differencing scheme cited here, though conventional, is not hydrostatically inconsistent; the sigma coordinate, pressure gradient error decreases with the square of the vertical and horizontal grid size. Furthermore, it is shown that the pressure gradient error is advectively eliminated after a long time integration. At the other extreme, it is shown that diagnostic calculations of the North Atlantic Ocean using rather coarse resolution, and where the temperature and salinity and the pressure gradient error are held constant, do not exhibit significant differences when compared to a calculation where horizontal pressure gradients are computed on z-level coordinates. Finally, a way of canceling the error ab initio is suggested
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