649 research outputs found
Superbubble Activity in Star-Forming Galaxies
Mechanical feedback from massive stars, primarily from supernovae, can
dominate ISM structuring and phase balance, thereby profoundly affecting
galactic evolutionary processes. Our understanding of mechanical feedback is
based on the adiabatic, wind-driven bubble model, applied on size scales
ranging over three decades. Tests of the model, and our consequent
understanding of feedback, are reviewed. While the model is broadly successful,
critical unknowns still prevent a comprehensive understanding of the
consequences of feedback.Comment: 10 pp., uses kluwer.cls. Invited Review to appear in "From
Observations to Self-Consistent Modelling of the ISM in Galaxies, eds. M. A.
de Avillez & D. Breitschwerdt, JENAM 2002 WS-ISM, (Kluwer) Ap&SS, in pres
The number and metallicities of the most metal-poor stars
Simple, one-zone models for inhomogeneous chemical evolution of the Galactic
halo are used to predict the number fraction of zero-metallicity, Population
III stars, which currently is empirically estimated at < 4e-4. These analytic
models minimize the number of free parameters, highlighting the most
fundamental constraints on halo evolution. There are disagreements of at least
an order of magnitude between observations and predictions in limiting cases
for both homogeneous Simple Model and Simple Inhomogeneous Model (SIM). Hence,
this demonstrates a quantitative, unambiguous discrepancy in the observed and
expected fraction of Population III stars. We explore how the metallicity
distribution of the parent enrichment events f(z_0) drives the SIM and
predictions for the Population III fraction. The SIM shows that the
previously-identified "high halo" and "low halo" populations are consistent
with a continuous evolutionary progression, and therefore may not necessarily
be physically distinct populations. Possible evolutionary scenarios for halo
evolution are discussed within the SIM's simplistic one-zone paradigm.
The values of z_0 depend strongly on metal dispersal processes, thus we
investigate interstellar mixing and mass transport, for the first time
explicitly incorporating this into a semi-analytic chemical evolution model.
Diffusion is found to be inefficient for all phases, including the hot phase,
of the interstellar medium (ISM): relevant diffusion lengths are 2 - 4 orders
of magnitude smaller than corresponding length scales for turbulent mixing.
Rough relations for dispersal processes are given for multiphase ISM. These
suggest that the expected low-metallicity threshold above zero is consistent
with the currently observed limit.Comment: Accepted to MNRAS. 14 pp, 4 figures; uses mn2e.cls (included
Exploring the Milky Way stellar disk. A detailed elemental abundance study of 714 F and G dwarf stars in the Solar neighbourhood
ABRIDGED: METHODS: We have conducted a high-resolution spectroscopic study of
714 F and G dwarf and subgiant stars in the Solar neighbourhood. The star
sample has been kinematically selected to trace the Galactic thin and thick
disks to their extremes...... The determination of stellar parameters and
elemental abundances is based on a standard 1-D LTE analysis using equivalent
width measurements in high-resolution (R=40000-110000) and high signal-to-noise
(S/N=150-300) spectra obtained with..... RESULTS: .... Our data show that there
is an old and alpha-enhanced disk population, and a younger and less
alpha-enhanced disk population. While they overlap greatly in metallicity
between -0.7<[Fe/H]<+0.1, they show a bimodal distribution in [alpha/Fe]. This
bimodality becomes even clearer if stars where stellar parameters and
abundances show larger uncertainties (Teff<5400 K) are discarded, showing that
it is important to constrain the data set to a narrow range in the stellar
parameters if small differences between stellar populations are to be revealed.
We furthermore find that the alpha-enhanced population has orbital parameters
placing the stellar birthplaces in the inner Galactic disk while the low-alpha
stars mainly come from the outer Galactic disk........... We furthermore have
discovered that a standard 1-D, LTE analysis, utilising ionisation and
excitation balance of Fe I and Fe II lines produces a flat lower main sequence.
As the exact cause for this effect is unclear we chose to apply an empirical
correction. Turn-off, and more evolved, stars, appears to be un-affected.Comment: 28 pages, accepted for publication in Astronomy and Astrophysics,
this version includes 11 more stars and has been modified after referee's
comment
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
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