85 research outputs found
The Ionizing Efficiency of the First Stars
We investigate whether a single population of first stars could have
influenced both the metal enrichment and reionization of the high-redshift
intergalactic medium (IGM), by calculating the generated ionizing radiation per
unit metal yield as a function of the metallicity of stellar populations. We
examine the relation between the ionizing radiation and carbon created by the
first stars, since the evidence for the widespread enrichment of the IGM at
redshifts about 3-4 comes from the detection of C IV absorption. We find
that the number of ionizing photons per baryon generated in association with
the detected IGM metallicity may well exceed that required for a late hydrogen
reionization at of about 6, by up to a factor of 10-20 for metal-free stars
in a present-day initial mass function (IMF). This would be in agreement with
similar indications from recent observations of the microwave background and
the high- IGM. In addition, the contribution from intermediate-mass stars to
the total metal yield, neglected in past works, substantially impacts such
calculations. Lastly, a top-heavy IMF is not necessarily preferred as a more
efficient high- source of ionizing radiation, based on nucleosynthetic
arguments in association with a given level of IGM enrichment.Comment: 5 pages, 1 figure. Accepted for publication in ApJLetters, v. 594
(Sept. 1, 2003); minor revisions, results unchange
Observational Signatures of the First Stars
At present, there are several feasible observational probes of the first
stars in the universe. Here, we examine the constraints on early stellar
activity from the metallicity of the high-redshift Ly- clouds, from the
effects of stellar ionizing photons on reionization and the cosmic microwave
background (CMB), and from the implications of gravitational microlensing
results for the presence of stellar remnants in galactic halos. We also discuss
whether the above signatures are consistent with each other, i.e., if they
reflect the same population of stars.Comment: 3 pages, to appear in the conference proceedings of "Cosmic
Evolution", Institut d'Astrophysique de Paris, November, 200
Type I X-ray Bursts at Low Accretion Rates
Neutron stars, with their strong surface gravity, have interestingly short
timescales for the sedimentation of heavy elements. Recent observations of
unstable thermonuclear burning (observed as X-ray bursts) on the surfaces of
slowly accreting neutron stars ( of the Eddington rate) motivate us to
examine how sedimentation of CNO isotopes affects the ignition of these bursts.
We further estimate the burst development using a simple one-zone model with a
full reaction network. We report a region of mass accretion rates for weak H
flashes. Such flashes can lead to a large reservoir of He, the unstable burning
of which may explain some observed long bursts (duration s).Comment: 6 pages, 2 figures, submitted to the proceedings of the conference
"The Multicoloured Landscape of Compact Objects and Their Explosive
Origins'', 2006 June 11--24, Cefalu, Sicily (Italy), to be published by AI
Na-22 decay gamma rays from classical novae
NASA Grant NAG 5-1565 has provided support for a program of theoretical research in nuclear astrophysics and related areas, focusing upon the possibility of detecting gamma rays from nearby novae. Particular attention has been given to the evaluation of the theoretical expectations for gamma ray emission from four possible sources: (1) the positron decays of the unstable CNO and fluorine isotopes that are transported to the surface regions of the envelope in the earliest stages of the outbursts; (2) Be-7 decay gamma rays, (3) Na-22 decay gamma rays released in the later stages of the outbursts; and (4) Al-26 decay gamma rays from novae and their possible contribution to Galactic emission. The critical questions of (1) the frequency of occurrence of ONeMg-enriched novae; (2) the expected Galactic distribution of the novae that produce 26Al; and (3) the nature of the observed soft X-ray emission from classical novae, have also been addressed. Considerable progress in research has been achieved on many of these fronts. Brief summaries of the results of several research projects are presented
On relative supernova rates and nucleosynthesis roles
It is shown that the Ni-56-Fe-56 observed in SN 1987A argues that core collapse supernovae may be responsible for more that 50 percent of the iron in the galaxy. Furthermore it is argued that the time averaged rate of thermonuclear driven Type I supernovae may be at least an order of magnitude lower than the average rate of core collapse supernovae. The present low rate of Type II supernovae (below their time averaged rate of approx. 1/10 yr) is either because the past rate was much higher because many core collapse supernovae are dim like SN 1987A. However, even in this latter case they are only an order of magnitude dimmer that normal Type II's due to the contribution of Ni-56 decay to the light curve
A Simple Model for r-Process Scatter and Halo Evolution
Recent observations of heavy elements produced by rapid neutron capture
(r-process) in the halo have shown a striking and unexpected behavior: within a
single star, the relative abundances of r-process elements heavier than Eu are
the same as the same as those of solar system matter, while across stars with
similar metallicity Fe/H, the r/Fe ratio varies over two orders of magnitude.
In this paper we present a simple analytic model which describes a star's
abundances in terms of its ``ancestry,'' i.e., the number of nucleosynthesis
events (e.g., supernova explosions) which contributed to the star's
composition. This model leads to a very simple analytic expression for the
abundance scatter versus Fe/H, which is in good agreement with the data and
with more sophisticated numerical models. We investigate two classes of
scenarios for r-process nucleosynthesis, one in which r-process synthesis
events occur in only \sim 4% of supernovae but iron synthesis is ubiquitous,
and one in which iron nucleosynthesis occurs in only about 9% of supernovae.
(the Wasserburg- Qian model). We find that the predictions in these scenarios
are similar for [Fe/H] \ga -2.5, but that these models can be readily
distinguished observationally by measuring the dispersion in r/Fe at [Fe/H] \la
-3.Comment: AASTeX, 21 pages, includes 4 figure
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