72 research outputs found
Book Review: James Martin, S.J., A Jesuit Guide to (Almost) Everything: A Spirituality for Real Life
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
A Cosmic Milestone: Constraints from Metal-Poor Halo Stars on the Cosmological Reionization Epoch
Theoretical studies and current observations of the high-redshift
intergalactic medium (IGM) indicate that at least two cosmic transitions occur
by the time the universe reaches gas metallicities of about of solar
values. These are the cosmological reionization of the IGM, and the transition
from a primordial to present-day mode of star formation. We quantify this
relation through new calculations of the ionizing radiation produced in
association with the elements carbon, oxygen and silicon observed in Galactic
metal-poor halo stars, which are likely second-generation objects formed in the
wake of primordial supernovae. We demonstrate that sufficient ionizing photons
per baryon are created by enrichment levels of [Fe/H] of about -3 in the
environment of metal-poor halo stars to provide the optical depth in the cosmic
microwave background of about 0.1 detected by . We show, on a star by
star basis, that a genuine cosmic milestone in IGM ionization and star
formation mode occurred at metallicities of to solar in
these halo stars. This provides an important link in the chain of evidence for
metal-free first stars having dominated the process of reionization by redshift
6. We conclude that many of the Fe-poor halo stars formed close to the end of
or soon after cosmological reionization, making them the ideal probe of the
physical conditions under which the transition from first- to second-generation
star formation happened in primordial galaxies.Comment: ApJ Letters, in press. Minor revisions to reflect new 3-year WMAP
data, results unchange
The Escape Fraction of Ionizing Radiation from Galaxies
The escape of ionizing radiation from galaxies plays a critical role in the
evolution of gas in galaxies, and the heating and ionization history of the
intergalactic medium. We present semi-analytic calculations of the escape
fraction of ionizing radiation for both hydrogen and helium from galaxies
ranging from primordial systems to disk-type galaxies that are not heavily
dust-obscured. We consider variations in the galaxy density profile, source
type, location, and spectrum, and gas overdensity/distribution factors. For
sufficiently hard first-light sources, the helium ionization fronts closely
track or advance beyond that of hydrogen. Key new results in this work include
calculations of the escape fractions for He I and He II ionizing radiation, and
the impact of partial ionization from X-rays from early AGN or stellar clusters
on the escape fractions from galaxy halos. When factoring in
frequency-dependent effects, we find that X-rays play an important role in
boosting the escape fractions for both hydrogen and helium, but especially for
He II. We briefly discuss the implications of these results for recent
observations of the He II reionization epoch at low redshifts, as well as the
UV data and emission-line signatures from early galaxies anticipated from
future satellite missions.Comment: 43 pages, 9 figures, accepted in ApJ, comments welcom
Perspectives on the Indigenous Worldviews in Informal Science Education Conference
The chronic underrepresentation of Native and indigenous peoples in STEM fields (Fig. 1) has been a longstanding issue in the United States, despite concentrated efforts by many local and national groups, including the Society for Advancement of Chicanos/Hispanics and Native Americans in Science (SACNAS) and the American Indian Science and Engineering Society (AISES) to address it. Here we report on the conference on Indigenous Worldviews in Informal Science Education (I-WISE), convened in Albuquerque, NM, on Sept. 2-5, 2015. We share what we learned on the commonalities and differences in perspectives between indigenous knowledge (IK) and Western science; summarize the role that IK is already playing in scientific fields, ranging from astrophysics to medicine to climate change; and describe how IK can help science education and research be more sustainable, inclusive, and respectful to all peoples
Detecting the Transition From Pop III to Pop II Stars
We discuss the cosmological significance of the transition from the Pop III
to Pop II mode of star formation in the early universe, and when and how it may
occur in primordial galaxies. Observations that could detect this transition
include those of element abundances in metal-poor Galactic halo stars, and of
the helium reionization and associated heating of the intergalactic medium. We
suggest that gamma-ray bursts may be a better probe of the end of the
first-stars epoch than of Pop III stars.Comment: 10 pages, 3 figures; to appear in New Astronomy Reviews as
proceedings of "First Light and Reionization Workshop", eds. A. Cooray & E.
Barton, Irvine, CA, May 19-21, 200
Evolving Spectra of Pop III Stars: Consequences for Cosmological Reionization
We examine the significance of the first metal-free stars (Pop III) for the
cosmological reionization of HI and HeII. These stars have unusually hard
spectra, with the integrated ionizing photon rates from a Pop III stellar
cluster for HI and HeII being 1.6 and times stronger respectively than
those from a Pop II cluster. For the currently favored cosmology, we find that
Pop III stars alone can reionize HI and HeII at redshifts, of about 9 (4.7)
and 5.1 (0.7) for continuous (instantaneous) modes of star formation. More
realistic scenarios involving combinations of Pop III and Pop II stellar
spectra yield similar results for hydrogen. Helium never reionizes completely
in these cases; the ionization fraction of HeIII reaches a maximum of about 60
percent at of about 5.6 if Pop III star formation lasts for yr.
Future data on HI reionization can test the amount of small-scale power
available to the formation of the first objects, and provide a constraint on
values of less than or about 0.7. Since current UV observations
indicate an epoch of reionization for HeII at of about 3, HeII may reionize
more than once. Measurements of the HeII Gunn-Peterson effect in the
intergalactic medium at redshifts exceeding about 3 may reveal the significance
of Pop III stars for HeII reionization, particularly in void regions that may
contain relic ionization from early Pop III stellar activity.Comment: 29 pages, 4 figures. Accepted for publication in Ap.J. (Feb. 20, 2003
issue; v. 584); minor revisions, results unchange
Where Did the First Generation of Stars Form in the Universe
The first generation of stars in the universe are expected to contain no heavy elements beyond helium, and are considered âmetal-freeâ Population III stars (or Pop III). These metal-free stars in the early universe are predicted to have hard ionizing photon spectra and unique element yields from their supernovae, leaving signatures through the reionization of the intergalactic medium and the metal enrichment of gas in the early universe. Here, we examine the metal abundances in a variety of systems in the nearby universe, from very metal-poor Galactic halo stars to ultra-faint dwarf spheroidal galaxies, and compare them with the latest theoretical models of massive stars with and without rotation. We find new abundance trends of interest in a variety of individual elements spanning metallicity values of [Fe/H] from about -2 to -5. We also compare our results with the abundances found in the very metal-deficient nearby dwarf irregular galaxy Leo P, which was recently discovered in the Arecibo ALFALFA survey. We comment on the similarities and differences between abundance trends in gas-rich dwarf galaxy systems like Leo P versus gas-poor ones like the ultra-faint dwarf spheroidals, and on the possibility of such systems hosting populations of the first stars
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