23,311 research outputs found
Copernicus observations of C I and CO in diffuse interstellar clouds
Copernicus was used to observe absorption lines of C I in its ground state and excited fine structure levels and CO toward 29 stars. We use the C I data to infer densities and pressures within the observed clouds, and because our results are of higher precision than previous work, much more precise estimates of the physical conditions in clouds are obtained. In agreement with previous work, the interstellar thermal pressure appears to be variable, with most clouds having values of p/k between 1000/cu cm K and 10,000/cu cm K, but there are some clouds with p/k as high as 100,000/cu cm K. Our results are consistent with the view that the interstellar thermal pressure is so variable that the gas undergoes continuous dynamic evolution. Our observations provide useful constraints on the physical processes on the surfaces of grains. In particular, we find that grains are efficient catalysts of interstellar H2 in the sense that at least half of the hydrogen atoms that strike grains come off as part of H2. Results place strong constraints on models for the formation and destruction of interstellar CO. In many clouds, an order of magnitude less CO than predicted in some models was found
A Near-Solar Metallicity, Nitrogen-Deficient Lyman Limit Absorber Associated with two S0 Galaxies
From UV spectra of the bright quasar PHL 1811 recorded by FUSE and the E140M
configuration on STIS, we have determined the abundances of various atomic
species in a Lyman limit system at z = 0.0809 with log N(H I) = 17.98.
Considerably more hydrogen may be in ionized form, since the abundances of C
II, Si II, S II and Fe II are very large compared to that of O I, when compared
to their respective solar abundance ratios. Our determination [O/H] = -0.19 in
the H I-bearing gas indicates that the chemical enrichment of the gas is
unusually high for an extragalactic QSO absorption system. However, this same
material has an unusually low abundance of nitrogen, [N/O] < -0.59, indicating
that there may not have been enough time during this enrichment for secondary
nitrogen to arise from low and intermediate mass stars. In an earlier
investigation we found two galaxies at nearly the same redshift as this
absorption system and displaced by 34 and 87 kpc from the line of sight. An
r-band image recorded by the ACS on HST indicates these are S0 galaxies. One or
both of these galaxies may be the source of the gas, which might have been
expelled in a fast wind, by tidal stripping, or by ram-pressure stripping.
Subtraction of the ACS point-spread function from the image of the QSO reveals
the presence of a face-on spiral galaxy under the glare of the quasar; although
it is possible that this galaxy may be responsible for the Lyman limit
absorption, the exact alignment of the QSO with the center of the galaxy
suggests that the spiral is the quasar host.Comment: 74 pages, 14 figures; to be published in the Astrophysical Journal
(Part 1) May 1, 2005 issue. A version of the paper with figures of better
quality may be found at http://www.astro.princeton.edu/~ebj/PHL1811_paper.ps
(postscript) or http://www.astro.princeton.edu/~ebj/PHL1811_paper.pdf (pdf
A Comparison of Absorption and Emission Line Abundances in the Nearby Damped Lyman-alpha Galaxy SBS 1543+593
We have used the Space Telescope Imaging Spectrograph (STIS) aboard HST to
measure a sulfur abundance of [S/H] = -0.41 +/-0.06 in the interstellar medium
(ISM) of the nearby damped Lyman-alpha (DLA) absorbing galaxy SBS 1543+593. A
direct comparison between this QSO absorption line abundance on the one hand,
and abundances measured from HII region emission line diagnostics on the other,
yield the same result: the abundance of sulfur in the neutral ISM is in good
agreement with that of oxygen measured in an HII region 3 kpc away. Our result
contrasts with those of other recent studies which have claimed
order-of-magnitude differences between HI (absorption) and HII (emission)
region abundances. We also derive a nickel abundance of [Ni/H] < -0.81, some
three times less than that of sulfur, and suggest that the depletion is due to
dust, although we cannot rule out an over-abundance of alpha-elements as the
cause of the lower metallicity. It is possible that our measure of [S/H] is
over-estimated if some SII arises in ionized gas; adopting a plausible star
formation rate for the galaxy along the line of sight, and a measurement of the
CII* 1335.7 absorption line detected from SBS 1543+593, we determine that the
metallicity is unlikely to be smaller than we derive by more than 0.25 dex. We
estimate that the cooling rate of the cool neutral medium is log [l_c (ergs
s^{-1} H atom^{-1})] = -27.0, the same value as that seen in the high redshift
DLA population.Comment: 31 pages; accepted for publication in the Ap
Analysis of General Power Counting Rules in Effective Field Theory
We derive the general counting rules for a quantum effective field theory
(EFT) in dimensions. The rules are valid for strongly and weakly
coupled theories, and predict that all kinetic energy terms are canonically
normalized. They determine the energy dependence of scattering cross sections
in the range of validity of the EFT expansion. We show that the size of cross
sections is controlled by the power counting of EFT, not by chiral
counting, even for chiral perturbation theory (PT). The relation between
and is generalized to dimensions. We show that the
naive dimensional analysis counting is related to counting. The
EFT counting rules are applied to PT, low-energy weak interactions,
Standard Model EFT and the non-trivial case of Higgs EFT.Comment: V2: more details and examples added; version published in journal. 17
pages, 4 figures, 2 table
Revealing the Warm-Hot Intergalactic Medium with OVI Absorption
Hydrodynamic simulations of growth of cosmic structure suggest that 30-50% of
the total baryons at z=0 may be in a warm-hot intergalactic medium (WHIM) with
temperatures ~10^5-10^7K. The O VI \lambda \lambda 1032, 1038 absorption line
doublet in the FUV portion of QSO spectra provides an important probe of this
gas. Utilizing recent hydrodynamic simulations, it is found that there should
be ~5 O VI absorption lines per unit redshift with equivalent widths >= 35 mA,
decreasing rapidly to ~0.5 per unit redshift at >= 350 mA. About 10% of the
total baryonic matter or 20-30% of the WHIM is expected to be in the O VI
absorption line systems with equivalent width >= 20 mA; the remaining WHIM gas
may be too hot or have too low metallicity to be detected in O VI. We find that
the simulation results agree well with observations with regard to the line
abundance and total mass contained in these systems. Some of the O VI systems
are collisionally ionized and some are photoionized, but most of the mass is in
the collisionally ionized systems. We show that the gas that produces the O VI
absorption lines does not reside in virialized regions such as galaxies,
groups, or clusters of galaxies, but rather has an overdensity of 10-40 times
the average density. These regions form a somewhat connected network of
filaments. The typical metallicity of these regions is 0.1-0.3Zsun.Comment: accepted to ApJ Letters; full color Figure 1 may be obtained at
http://astro.princeton.edu/~cen/PROJECTS/p2/p2.html (at the bottom of the
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