751 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
The Early Formation, Evolution and Age of the Neutron-Capture Elements in the Early Galaxy
Abundance observations indicate the presence of rapid-neutron capture (i.e.,
r-process) elements in old Galactic halo and globular cluster stars. These
observations demonstrate that the earliest generations of stars in the Galaxy,
responsible for neutron-capture synthesis and the progenitors of the halo
stars, were rapidly evolving. Abundance comparisons among several halo stars
show that the heaviest neutron-capture elements (including Ba and heavier) are
consistent with a scaled solar system r-process abundance distribution, while
the lighter such elements do not conform to the solar pattern. These
comparisons suggest two r-process sites or at least two different sets of
astrophysical conditions. The large star-to-star scatter observed in the
neutron-capture/iron ratios at low metallicities -- which disappears with
increasing [Fe/H] -- suggests an early, chemically unmixed and inhomogeneous
Galaxy. The stellar abundances indicate a change from the r-process to the slow
neutron capture (i.e., s-) process at higher metallicities in the Galaxy. The
detection of thorium in halo and globular cluster stars offers a promising,
independent age-dating technique that can put lower limits on the age of the
Galaxy.Comment: 6 pages, 3 figures; To appear in the proceedings of the 20th Texas
Symposium on Relativistic Astrophysics, J. C. Wheeler & H. Martel (eds.
MESA and NuGrid Simulations of Classical Nova Outbursts and Nucleosynthesis
Classical novae are the results of surface thermonuclear explosions of
hydrogen accreted by white dwarfs (WDs) from their low-mass main-sequence or
red-giant binary companions. Chemical composition analysis of their ejecta
shows that nova outbursts occur on both carbon-oxygen (CO) and more massive
oxygen-neon (ONe) WDs, and that there is cross-boundary mixing between the
accreted envelope and underlying WD. We demonstrate that the state-of-the-art
stellar evolution code MESA and post-processing nucleosynthesis tools of NuGrid
can successfully be used for modeling of CO and ONe nova outbursts and
nucleosynthesis. The convective boundary mixing (CBM) in our 1D numerical
simulations is implemented using a diffusion coefficient that is exponentially
decreasing with a distance below the bottom of the convective envelope. We show
that this prescription produces maximum temperature evolution profiles and
nucleosynthesis yields in good agreement with those obtained using the commonly
adopted 1D nova model in which the CBM is mimicked by assuming that the
accreted envelope has been pre-mixed with WD's material. In a previous paper,
we have found that 3He can be produced in situ in solar-composition envelopes
accreted with slow rates (dM/dt < 1e-10 M_sun/yr) by cold (T_WD < 1d7 K) CO
WDs, and that convection is triggered by 3He burning before the nova outburst
in this case. Here, we confirm this result for ONe novae. Additionally, we find
that the interplay between the 3He production and destruction in the
solar-composition envelope accreted with an intermediate rate, e.g. dM/dt =
1e-10 M_sun/yr, by the 1.15 M_sun ONe WD with a relatively high initial central
temperature, e.g. T_WD = 15e6 K, leads to the formation of a thick radiative
buffer zone that separates the bottom of the convective envelope from the WD
surface.Comment: 6 pages, 4 figures, STELLA NOVAE: FUTURE AND PAST DECADES Conference
Proceedings, Submitted to ASP Conference Serie
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
Nuclear Chronometers
Observations of metal-poor Galactic halo stars indicate that the abundance
pattern of the (heaviest) neutron-capture elements is consistent with the
scaled solar system r-process abundances. Utilizing the radioactive (r-process)
element thorium, age determinations have been made for several of these same
stars, placing constraints on both Galactic and cosmological age estimates.Comment: 6 pages, 2 figures. To appear in the Proceedings of ``Cosmic
Evolution'
Neutron-Capture Element Trends in the Halo
In a brief review of abundances neutron-capture elements (Z > ~30) in
metal-poor halo stars, attention is called to their star-to-star scatter, the
dominance of r-process synthesis at lowest metallicities, the puzzle of the
lighter members of this element group, and the possibility of a better
r-/s-process discriminant.Comment: 6 pages, 2 figures. To appear in the Proceedings of ``Cosmic
Evolution'
CNO abundances and hydrodynamic models of the Nova outbursts. 4: Comparison with observations
A variety of observations of novae are discussed in light of theoretical models. It is proposed that the nearly constant bolometric luminosity of FH Ser originates in the non-degenerate hydrogen-burning region at the bottom of the hydrogen-rich envelope which remains after the primary ejection. The shift of the wavelength of peak emission from the visual to shortward of the ultraviolet is caused by the decrease of the photospheric radius of the remnant envelope as the bolometric luminosity stays nearly constant. The oscillations in the light curve of GK Per during the transition stage can be explained by a pulsation of the remnant envelope when it is the size of the Roche lobe. The CNO over-abundances in novae reported by various observers are strongly suggestive of this nova mechanism. Finally, the implications of the upper limits of C-13 and N-15 in DQ Her are discussed
- …