2,283 research outputs found
Observation of Infrared and Radio Lines of Molecules toward GL2591 and Comparison to Physical and Chemical Models
We have observed rovibrational transitions of acetylene and HCN near 13
microns in absorption toward GL2591. We also observed rotational lines of CS,
HCN, H2CO, and HCO+. The combined data are analyzed in terms of models with a
cloud envelope with density gradients and discrete regions of hot, dense gas,
probably near the infrared source. The abundance of HCN is enhanced by a factor
of 400 in the gas producing the infrared absorption, in agreement with chemical
models which involve depletion of molecules onto grains and subsequent
sublimation when temperatures are raised.Comment: 34 pages, postscript with 14 postscript figure files, uuencoded
compressed and tar'ed; unpacks self with csh. In case of problems, contact
[email protected]
GUT-Scale Primordial Black Holes: Consequences and Constraints
A population of very light primordial black holes which evaporate before
nucleosynthesis begins is unconstrained unless the decaying black holes leave
stable relics. We show that gravitons Hawking radiated from these black holes
would source a substantial stochastic background of high frequency
gravititational waves ( Hz or more) in the present universe. These
black holes may lead to a transient period of matter dominated expansion. In
this case the primordial universe could be temporarily dominated by large
clusters of "Hawking stars" and the resulting gravitational wave spectrum is
independent of the initial number density of primordial black holes.Comment: 4 pages; grey body factors included in graviton emission
calculations, and a couple of references added, but the conclusions are
unchanged. v3 Minor changes to references and wording; final versio
Cosmological Constraints on a Dynamical Electron Mass
Motivated by recent astrophysical observations of quasar absorption systems,
we formulate a simple theory where the electron to proton mass ratio is allowed to vary in space-time. In such a minimal theory only
the electron mass varies, with and kept constant. We find
that changes in will be driven by the electronic energy density after
the electron mass threshold is crossed. Particle production in this scenario is
negligible. The cosmological constraints imposed by recent astronomical
observations are very weak, due to the low mass density in electrons. Unlike in
similar theories for spacetime variation of the fine structure constant, the
observational constraints on variations in imposed by the weak
equivalence principle are much more stringent constraints than those from
quasar spectra. Any time-variation in the electron-proton mass ratio must be
less than one part in since redshifts This is more than
one thousand times smaller than current spectroscopic sensitivities can
achieve. Astronomically observable variations in the electron-proton must
therefore arise directly from effects induced by varying fine structure
'constant' or by processes associated with internal proton structure. We also
place a new upper bound of on any large-scale spatial
variation of that is compatible with the isotropy of the microwave
background radiation.Comment: New bounds from weak equivalence principle experiments added,
conclusions modifie
The evolution of pebble size and shape in space and time
We propose a mathematical model which suggests that the two main geological
observations about shingle beaches, i.e. the emergence of predominant pebble
size ratios and strong segregation by size are interrelated. Our model is a
based on a system of ODEs called the box equations, describing the evolution of
pebble ratios. We derive these ODEs as a heuristic approximation of Bloore's
PDE describing collisional abrasion. While representing a radical
simplification of the latter, our system admits the inclusion of additional
terms related to frictional abrasion. We show that nontrivial attractors
(corresponding to predominant pebble size ratios) only exist in the presence of
friction. By interpreting our equations as a Markov process, we illustrate by
direct simulation that these attractors may only stabilized by the ongoing
segregation process.Comment: 22 pages, 8 figure
Metal Abundances of Intermediate-Redshift AGN: Evidence for a Population of Lower-Metallicty Seyfert 2 Galaxies at z = 0.3-0.4
We derive oxygen abundances for two samples of Seyfert 2 (Sy2) active
galactic nuclei (AGN) selected from the KPNO International Spectroscopic Survey
(KISS). The two samples from KISS include 17 intermediate-redshift (0.29 < z <
0.42) Sy2s detected via their [O III] lines, and 35 low-redshift (z < 0.1),
Halpha-detected Sy2s. The primary goal of this work is to explore whether the
metallicity distribution of these two samples changes with redshift. To
determine the oxygen abundances of the KISS galaxies, we use Cloudy to create a
large number of photoionization model grids by varying the temperature of the
accretion disk, the ratio of X-ray to UV continuum light, the ionization
parameter, the hydrogen density, and the metallicity of the narrow-line region
clouds. We link the results of these models to the observed [O III]/H-beta and
[N II]/H-alpha emission-line ratios of the KISS sample on the BPT diagram,
interpolating across the model grids to derive metallicity. The two redshift
samples overlap substantially in terms of derived metal abundances, but we find
that some of the intermediate-redshift Sy2 galaxies possess lower abundances
than their local universe counterparts. Our analysis provides evidence for
modest levels of chemical evolution (0.18 +/- 0.06 dex) over 3-4 Gyrs of
look-back time. We compare our results to other AGN abundance derivation
methods from the literature.Comment: 17 pages, 12 figures; accepted for publication in the Astrophysical
Journa
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