25,624 research outputs found
The [?/Fe] ratios of very metal-poor stars within the integrated galactic initial mass function theory
The aim of this paper is to quantify the amplitude of the predicted plateau in [α/Fe] ratios associated with the most metal-poor stars of a galaxy. We assume that the initial mass function (IMF) in galaxies is steeper if the star formation rate (SFR) is low – as per the integrated galactic initial mass function (IGIMF) theory. A variant of the theory, in which the IGIMF depends upon the metallicity of the parent galaxy, is also considered. The IGIMF theory predicts low [α/Fe] plateaus in dwarf galaxies, characterized by small SFRs. The [α/Fe] plateau is up to 0.7 dex lower than the corresponding plateau of the Milky Way. For a universal IMF one should expect instead that the [α/Fe] plateau is the same for all the galaxies, irrespective of their masses or SFRs. Assuming a strong dependence of the IMF on the metallicity of the parent galaxy, dwarf galaxies can show values of the [α/Fe] plateau similar to those of the Milky Way, and almost independent of the SFR. The [Mg/Fe] ratios of the most metal-poor stars in dwarf galaxies satellites of the Milky Way can be reproduced either if we consider metallicity-dependent IMFs or if the early SFRs of these galaxies were larger than we presently think. Present and future observations of dwarf galaxies can help disentangle between these different IGIMF formulations
On Dwarf Galaxies as the Source of Intracluster Gas
Recent observational evidence for steep dwarf galaxy luminosity functions in
several rich clusters has led to speculation that their precursors may be the
source of the majority of gas and metals inferred from intracluster medium
(ICM) x-ray observations. Their deposition into the ICM is presumed to occur
through early supernovae-driven winds, the resultant systems reflecting the
photometric and chemical properties of the low luminosity dwarf spheroidals and
ellipticals we observe locally. We consider this scenario, utilising a
self-consistent model for spheroidal photo-chemical evolution and gas ejection
via galactic superwinds. Insisting that post-wind dwarfs obey the observed
colour-luminosity-metallicity relations, we conclude that the bulk of the ICM
gas and metals does not originate within their precursors.Comment: 43 pages, 8 figures, LaTeX, also available at
http://msowww.anu.edu.au/~gibson/publications.html, to appear in ApJ, Vol
473, 1997, in pres
Three-body structure of the system with coupling
The structure of the three-body system, which has been observed
recently by the HypHI collaboration, is investigated taking coupling explicitly into account. The and interactions employed in
this work reproduce the binding energies of H, H
and He. We do not find any bound state, which
contradicts the interpretation of the data reported by the HypHI collaboration.Comment: To be publsihed in PRC as a Rapid communicatio
Model Dependence of the 2H Electric Dipole Moment
Background: Direct measurement of the electric dipole moment (EDM) of the
neutron lies in the future; measurement of a nuclear EDM may well come first.
The deuteron is one nucleus for which exact model calculations are feasible.
Purpose: We explore the model dependence of deuteron EDM calculations. Methods:
Using a separable potential formulation of the Hamiltonian, we examine the
sensitivity of the deuteron EDM to variation in the nucleon-nucleon
interaction. We write the EDM as the sum of two terms, the first depending on
the target wave function with plane-wave intermediate states, and the second
depending on intermediate multiple scattering in the 3P1 channel, the latter
being sensitive to the off-shell behavior of the 3P1 amplitude. Results: We
compare the full calculation with the plane-wave approximation result, examine
the tensor force contribution to the model results, and explore the effect of
short range repulsion found in realistic, contemporary potential models of the
deuteron. Conclusions: Because one-pion exchange dominates the EDM calculation,
separable potential model calculations will provide an adequate description of
the 2H EDM until such time as a better than 10% measurement is obtained.Comment: 21 pages, 2 figures, submitted to Physical Review
Importance of Baryon-Baryon Coupling in Hypernuclei
The coupling in --hypernuclei and coupling in --hypernuclei produce novel
physics not observed in the conventional, nonstrange sector. Effects of
conversion in H are reviewed.
The role of coupling suppression in the
--hypernuclei due to Pauli blocking is highlighted, and the
implications for the structure of B are explored.
Suppression of conversion in He is hypothesized as the reason that the
matrix element is small. Measurement of H is
proposed to investigate the full interaction. The
implication for analog states is discussed.Comment: 17 pages LATEX, 1 figure uuencoded postscrip
Modeling the chemical evolution of Omega Centauri using three-dimensional hydrodynamical simulations
We present a hydrodynamical and chemical model for the globular cluster Omega
Cen, under the assumption that it is the remnant of an ancient dwarf spheroidal
galaxy (dSph), the bulk of which was disrupted and accreted by our Galaxy ~10
Gyr ago. We highlight the very different roles played by Type II and Type Ia
supernovae (SNe) in the chemical enrichment of the inner regions of the
putative parent dSph. While the SNe II pollute the interstellar medium rather
uniformly, the SNe Ia ejecta may remain confined inside dense pockets of gas as
long as succesive SNe II explosions spread them out. Stars forming in such
pockets have lower alpha-to-iron ratios than the stars forming elsewhere. Owing
to the inhomogeneous pollution by SNe Ia, the metal distribution of the stars
in the central region differs substantially from that of the main population of
the dwarf galaxy, and resembles that observed in Omega Cen. This inhomogeneous
mixing is also responsible for a radial segregation of iron-rich stars with
depleted [alpha/Fe] ratios, as observed in some dSphs. Assuming a star
formation history of ~1.5 Gyr, our model succeeds in reproducing both the iron
and calcium distributions observed in Omega Cen and the main features observed
in the empirical alpha/Fe versus Fe/H plane. Finally, our model reproduces the
overall spread of the color-magnitude diagram, but fails in reproducing the
morphology of the SGB-a and the double morphology of the main sequence.
However, the inhomogeneous pollution reduces (but does not eliminate) the need
for a significantly enhanced helium abundance to explain the anomalous position
of the blue main sequence. Further models taking into account the dynamical
interaction of the parent dwarf galaxy with the Milky Way and the effect of AGB
pollution will be required.Comment: 15 pages, 13 figures. MNRAS accepte
Constraints on Early Nucleosynthesis from the Abundance Pattern of a Damped Ly-alpha System at z = 2.626
We have investigated chemical evolution in the young universe by analysing
the detailed chemical enrichment pattern of a metal-rich galaxy at high
redshift. The recent detection of over 20 elements in the gas-phase of a damped
Lyman-alpha absorber (DLA) at z = 2.626 represents an exciting new avenue for
exploring early nucleosynthesis. Given a strict upper age of ~2.5 Gyr and a
gas-phase metallicity about one third solar, we have shown the DLA abundance
pattern to be consistent with the predictions of a chemical evolution model in
which the interstellar enrichment is dominated by massive stars with a small
contribution from Type Ia supernovae. Discrepancies between the empirical data
and the models are used to highlight outstanding issues in nucleosynthesis
theory, including a tendency for Type II supernovae models to overestimate the
magnitude of the "odd-even" effect at subsolar metallicities. Our results
suggest a possible need for supplemental sources of magnesium and zinc, beyond
that provided by massive stars.Comment: 12 pages, 7 figs. Accepted for publication in ApJ (The Astrophysical
Journal
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