183 research outputs found
Carbon-Enhanced Metal-Poor Stars, the Cosmic Microwave Background, and the Stellar IMF in the Early Universe
The characteristic mass of stars at early times may have been higher than
today owing to the cosmic microwave background (CMB). This study proposes that
(1) the testable predictions of this "CMB-IMF" hypothesis are an increase in
the fraction of carbon-enhanced metal-poor (CEMP) stars with declining
metallicity and an increase from younger to older populations at a single
metallicity (e.g. disk to halo), and (2) these signatures are already seen in
recent samples of CEMP stars and can be better tested with anticipated data.
The expected spatial variation may explain discrepancies of CEMP frequency
among published surveys. The ubiquity and time dependence of the CMB will
substantially alter the reconstruction of star formation histories in the Local
Group and early Universe.Comment: 7 pages emulateapj format, three figures, accepted for ApJ Letter
Carbon-Enhanced Hyper-metal-poor Stars and the Stellar IMF at Low Metallicity
The two known ``hyper-metal-poor'' (HMP) stars, HE0107-5240 and HE1327-2326,
have extremely high enhancements of the light elements C, N, and O relative to
Fe and appear to represent a statistically significant excess population
relative to the halo metallicity distribution extrapolated from [Fe/H] > -3.
This study weighs the available evidence for and against three hypothetical
origins for these stars: (1) that they formed from gas enriched by a primordial
``faint supernova'', (2) that they formed from gas enriched by core-collapse
supernovae and C-rich gas ejected in rotation-driven winds from massive stars,
and (3) that they formed as the low-mass secondaries in binary systems at Z ~
10^{-5.5} Zsun and acquired their light-element enhancements from an
intermediate-mass companion as it passed through an AGB phase. The observations
interpreted here, especially the depletion of lithium seen in HE1327-2326,
favor the binary mass-transfer hypothesis. If HE0107-5240 and HE1327-2326
formed in binary systems, the statistically significant absence of isolated
and/or C-normal stars at similar [Fe/H] implies that low-mass stars could form
at that metallicity, but that masses M ~< 1.4 Msun were disfavored in the IMF.
This result is also explained if the abundance-derived top-heavy IMF for
primordial stars persists to [Fe/H] ~ -5.5. This finding indicates that
low-mass star formation was possible at extremely low metallicity, and that the
typical stellar mass may have had a complex dependence on metallicity rather
than a sharp transition driven solely by gas cooling.Comment: 11 pages emulateapj text including three figures, accepted for
publication in ApJ v666 (Sept 2007). A companion paper to 0706.290
The Multiphase Intergalactic Medium towards PKS 2155-304
We study the cluster of H I and O VI absorption systems and the claimed
detection of O VIII absorption from the intergalactic medium at z ~ 0.0567,
associated with a group of galaxies toward the BL Lac object PKS 2155-304. As
measured by spectrographs on the Hubble Space Telescope, Far Ultraviolet
Spectroscopic Explorer, and Chandra, this system appears to contain gas at a
variety of temperatures. We analyze this multi-phase gas in a clumpy-infall
model. From the absence of C IV and Si III absorption in the Ly-alpha clouds,
we infer metallicities less than 2.5-10% of solar values. The only metals are
detected in two O VI absorption components, offset by +/- 400 km/s from the
group barycenter (cz ~ 16,600 km/s). The O VI components may signify "nearside"
and "backside" infall into the group potential well, which coincides with the
claimed O VIII absorption. If the claimed O VIII detection is real, our
analysis suggests that clusters of strong Ly-alpha and O VI absorbers,
associated with groups of galaxies, may be the "signposts" of shock-heated,
metal-enriched baryons. Through combined UV and X-ray spectra of H I and O VI,
O VII, and O VIII, one may be able to clarify the heating mechanism of this
multiphase gas.Comment: Accepted for ApJL (2003), 11 pages, 1 color figur
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
The Atomic to Molecular Transition in Galaxies. I: An Analytic Approximation for Photodissociation Fronts in Finite Clouds
In this series of papers we study the structure of the atomic to molecular
transition in the giant atomic-molecular complexes that are the repositories of
most molecular gas in galaxies, with the ultimate goal of attaining a better
understanding of what determines galaxies' molecular content. Here we derive an
approximate analytic solution for the structure of a photodissociation region
(PDR) in a cloud of finite size that is bathed in an external dissociating
radiation field. Our solution extends previous work, which with few exceptions
has been restricted to a one-dimensional treatment of the radiation field. We
show that our analytic results compare favorably to exact numerical
calculations in the one-dimensional limit. However, our more general geometry
provides a more realistic representation than a semi-infinite slab for
atomic-molecular complexes exposed to the interstellar radiation field,
particularly in environments such as low-metallicity dwarf galaxies where the
curvature and finite size of the atomic envelope cannot be neglected. For
clouds that are at least 20% molecular we obtain analytic expressions for the
molecular fraction in terms of properties of the gas and radiation field that
are accurate to tens of percent, while for clouds of lower molecular content we
obtain upper limits. As a side benefit, our analysis helps clarify when
self-shielding is the dominant process in H_2 formation, and under what
circumstances shielding by dust makes a significant contribution.Comment: 19 pages, 11 figures, emulateapj style, accepted to ApJ. Discussion
slightly changed from previous version, and some new analytic approximations
added. Underlying results unchange
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