3,802 research outputs found
Thermal photon production in high-energy nuclear collisions
We use a boost-invariant one-dimensional (cylindrically symmetric) fluid
dynamics code to calculate thermal photon production in the central rapidity
region of S+Au and Pb+Pb collisions at SPS energy ( GeV/nucleon).
We assume that the hot matter is in thermal equilibrium throughout the
expansion, but consider deviations from chemical equilibrium in the high
temperature (deconfined) phase. We use equations of state with a first-order
phase transition between a massless pion gas and quark gluon plasma, with
transition temperatures in the range MeV.Comment: revised, now includes a_1 contribution. revtex, 10 pages plus 4
figures (uuencoded postscript
Measuring hadron properties at finite temperature
We estimate the numbers and mass spectra of observed lepton and kaon pairs
produced from meson decays in the central rapidity region of an Au+Au
collision at lab energy 11.6 GeV/nucleon. The following effects are considered:
possible mass shifts, thermal broadening due to collisions with hadronic
resonances, and superheating of the resonance gas. Changes in the dilepton mass
spectrum may be seen, but changes in the dikaon spectrum are too small to be
detectable.Comment: 9 pages (revtex), 3 figures (uuencoded postscript
Tightly Correlated HI and FUV Emission in the Outskirts of M83
We compare sensitive HI data from The HI Nearby Galaxy Survey (THINGS) and
deep far UV (FUV) data from GALEX in the outer disk of M83. The FUV and HI maps
show a stunning spatial correlation out to almost 4 optical radii (r25),
roughly the extent of our maps. This underscores that HI traces the gas
reservoir for outer disk star formation and it implies that massive (at least
low level) star formation proceeds almost everywhere HI is observed. Whereas
the average FUV intensity decreases steadily with increasing radius before
leveling off at ~1.7 r25, the decline in HI surface density is more subtle. Low
HI columns (<2 M_solar/pc^2) contribute most of the mass in the outer disk,
which is not the case within r25. The time for star formation to consume the
available HI, inferred from the ratio of HI to FUV intensity, rises with
increasing radius before leveling off at ~100 Gyr, i.e., many Hubble times,
near ~1.7 r25. Assuming the relatively short H2 depletion times observed in the
inner parts of galaxies hold in outer disks, the conversion of HI into bound,
molecular clouds seems to limit star formation in outer galaxy disks. The long
consumption times suggest that most of the extended HI observed in M83 will not
be consumed by in situ star formation. However, even these low star formation
rates are enough to expect moderate chemical enrichment in a closed outer disk.Comment: Accepted for Publication in ApJ
The Starburst Nature of Lyman-Break Galaxies: Testing UV Extinction with X-rays
We derive the bolometric to X-ray correlation for a local sample of normal
and starburst galaxies and use it, in combination with several UV reddening
schemes, to predict the 2--8 keV X-ray luminosity for a sample of 24
Lyman-break galaxies in the HDF/CDF-N. We find that the mean X-ray luminosity,
as predicted from the Meurer UV reddening relation for starburst galaxies,
agrees extremely well with the Brandt stacking analysis. This provides
additional evidence that Lyman-break galaxies can be considered as scaled-up
local starbursts and that the locally derived starburst UV reddening relation
may be a reasonable tool for estimating the UV extinction at high redshift. Our
analysis shows that the Lyman-break sample can not have far-IR to far-UV flux
ratios similar to nearby ULIGs, as this would predict a mean X-ray luminosity
100 times larger than observed, as well as far-IR luminosities large enough to
be detected in the sub-mm. We calculate the UV reddening expected from the
Calzetti effective starburst attenuation curve and the radiative transfer
models of Witt & Gordon for low metallicity dust in a shell geometry with
homogeneous or clumpy dust distributions and find that all are consistent with
the observed X-ray emission. Finally, we show that the mean X-ray luminosity of
the sample would be under predicted by a factor of 6 if the the far-UV is
unattenuated by dust.Comment: 7 pages, 3 figures. Accepted for publication in A
GALEX and Optical Light Curves of EF Eridanus During a Low State: the Puzzling Source of UV Light
Low state optical photometry of EF Eri during an extended low accretion state
combined with GALEX near and far UV time-resolved photometry reveals a source
of UV flux that is much larger than the underlying 9500K white dwarf, and that
is highly modulated on the orbital period. The near UV and optical light curves
can be modeled with a 20,000K spot but no spot model can explain both the large
amplitude FUV variations and the SED. The limitations of limb darkening,
cyclotron and magnetic white dwarf models in explaining the observations are
discussed.Comment: 10 pages, 4 figures to be published in ApJ Letter
A truncated antenna mutant of Chlamydomonas reinhardtii can produce more hydrogen than the parental strain
Photoproduction of H(2) gas was examined in the Chlamydomonas reinhardtii tla1 strain, CC-4169, containing a truncated light-harvesting antenna, along with its parental CC-425 strain. Although enhanced photosynthetic performance of truncated antenna algae has been demonstrated previously (Polle etal. Planta 2003; 217:49-59), improved H(2) photoproduction has yet to be reported. Preliminary experiments showed that sulfur-deprived, suspension cultures of the tla1 mutant could not establish anaerobiosis in a photobioreactor, and thus, could not photoproduce H(2) gas under conditions typical for the sulfur-deprived wild-type cells (Kosourov et al. Biotech Bioeng 2002; 78:731-40). However, they did produce H(2) gas when deprived of sulfur and phosphorus after immobilization within thin (similar to 300 mu m) alginate films. These films were monitored for long-term H(2) photoproduction activity under light intensities ranging from 19 to 350 mu E m(-2) s(-1) PAR. Both the tla1 mutant and the CC-425 parental strain produced H(2) gas for over 250 h under all light conditions tested. Relative to the parental strain, the CC-4169 mutant had lower maximum specific rates of H(2) production at low and medium light intensities (19 and 184 mu E m(-2) s(-1)), but it exhibited a 4-times higher maximum specific rate at 285 mu E m(-2) s(-1) and an 8.5-times higher rate at 350 mu E M(-2) s(-1) when immobilized at approximately the same cell density as the parental strain. As a result, the CC-4169 strain accumulated almost 4-times more H(2) than CC-425 at 285 mu E M(-2) s(-1) and over 6-times more at 350 mu E M(-2) s(-1) during 250-h experiments. These results are the first demonstration that truncating light-harvesting antennae in algal cells can increase the efficiency of H(2) photoproduction in mass culture at high light intensity. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved
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