132 research outputs found
Proposal For A Market-Based Solution to Airport Delays
With the clamor rising over airport delays and with both the Congress and the Administration considering remedies, this paper advocates the use of market mechanisms, specifically slot auctions, to promote efficient usage of airport capacity, reduce airport delays, and, more generally, promote competition.
Forming a Primordial Star in a Relic HII Region
There has been considerable theoretical debate over whether photoionization
and supernova feedback from the first Population III stars facilitate or
suppress the formation of the next generation of stars. We present results from
an Eulerian adaptive mesh refinement simulation demonstrating the formation of
a primordial star within a region ionized by an earlier nearby star. Despite
the higher temperatures of the ionized gas and its flow out of the dark matter
potential wells, this second star formed within 23 million years of its
neighbor's death. The enhanced electron fraction within the HII region
catalyzes rapid molecular hydrogen formation that leads to faster cooling in
the subsequent star forming halos than in the first halos. This "second
generation" primordial protostar has a much lower accretion rate because,
unlike the first protostar, it forms in a rotationally supported disk of
approx. 10-100 solar masses. This is primarily due to the much higher angular
momentum of the halo in which the second star forms. In contrast to previously
published scenarios, such configurations may allow binaries or multiple systems
of lower mass stars to form. These first high resolution calculations offer
insight into the impact of feedback upon subsequent populations of stars and
clearly demonstrate how primordial chemistry promotes the formation of
subsequent generations of stars even in the presence of the entropy injected by
the first stars into the IGM.Comment: 4 pages, 2 figures. Some revisions, including enhanced discussion of
angular momentum issues. Asrophysical Journal, accepte
Radiation Hydrodynamical Evolution of Primordial H II Regions
We simulate the ionization environment of z ~ 20 luminous objects formed
within the framework of the current CDM cosmology and compute their UV escape
fraction. These objects are likely single very massive stars that are copious
UV emitters. We present analytical estimates as well as one--dimensional
radiation hydrodynamical calculations of the evolution of these first HII
regions in the universe. The initially D--type ionization front evolves to
become R--type within yrs at a distance pc. This
ionization front then completely overruns the halo, accelerating an expanding
shell of gas outward to velocities in excess of 30 km s, about ten times
the escape velocity of the confining dark matter halo. We find that the
evolution of the HII region depends only weakly on the assumed stellar ionizing
luminosities. Consequently, most of the gas surrounding the first stars will
leave the dark halo whether or not the stars produce supernovae. If they form
the first massive seed black holes these are unlikely to accrete within a
Hubble time after they formed until they are incorporated into larger dark
matter halos that contain more gas. Because these I--fronts exit the halo on
timescales much shorter than the stars' main sequence lifetimes their host
halos have UV escape fractions of , fixing an important parameter
for theoretical studies of cosmological hydrogen reionization.Comment: 10 pages, 8 figures, in emulateapj5 format, revised version submitted
to Ap
The Birth of a Galaxy: Primordial Metal Enrichment and Stellar Populations
By definition, Population III stars are metal-free, and their protostellar
collapse is driven by molecular hydrogen cooling in the gas-phase, leading to
large characteristic masses. Population II stars with lower characteristic
masses form when the star-forming gas reaches a critical metallicity of 10^{-6}
- 10^{-3.5} Z_\odot. We present an adaptive mesh refinement radiation
hydrodynamics simulation that follows the transition from Population III to II
star formation. The maximum spatial resolution of 1 comoving parsec allows for
individual molecular clouds to be well-resolved and their stellar associations
to be studied in detail. We model stellar radiative feedback with adaptive ray
tracing. A top-heavy initial mass function for the Population III stars is
considered, resulting in a plausible distribution of pair-instability
supernovae and associated metal enrichment. We find that the gas fraction
recovers from 5 percent to nearly the cosmic fraction in halos with merger
histories rich in halos above 10^7 solar masses. A single pair-instability
supernova is sufficient to enrich the host halo to a metallicity floor of
10^{-3} Z_\odot and to transition to Population II star formation. This
provides a natural explanation for the observed floor on damped Lyman alpha
(DLA) systems metallicities reported in the literature, which is of this order.
We find that stellar metallicities do not necessarily trace stellar ages, as
mergers of halos with established stellar populations can create superpositions
of t-Z evolutionary tracks. A bimodal metallicity distribution is created after
a starburst occurs when the halo can cool efficiently through atomic line
cooling.Comment: 11 pages, 7 figures; replaced with accepted version to ApJ;
additional movies and images can be found at
http://www.astro.princeton.edu/~jwise/research/GalaxyBirth.htm
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