13,044 research outputs found
Double Pion Photoproduction in Nuclei
The inclusive A(gamma,pi+ pi-)X reaction is studied theoretically. A sizeable
enhancement of the cross section is found, in comparison with the scaling of
the deuteron cross section (sigma_deuteron * A/2). This enhancement is due to
the modifications in the nuclear medium of the gamma N ----> pi pi N amplitude
and the pion dispersion relation. The enhancement is found to be bigger than
the one already observed in the (pi,pi pi) reaction in nuclei.Comment: 11 pages, 7 figures (figures available from authors); TeX, Version
3.141 [PD VMS 3.4/CERN 1.0
Relationship between the column density distribution and evolutionary class of molecular clouds as viewed by ATLASGAL
We present the first study of the relationship between the column density
distribution of molecular clouds within nearby Galactic spiral arms and their
evolutionary status as measured from their stellar content. We analyze a sample
of 195 molecular clouds located at distances below 5.5 kpc, identified from the
ATLASGAL 870 micron data. We define three evolutionary classes within this
sample: starless clumps, star-forming clouds with associated young stellar
objects, and clouds associated with HII regions. We find that the N(H2)
probability density functions (N-PDFs) of these three classes of objects are
clearly different: the N-PDFs of starless clumps are narrowest and close to
log-normal in shape, while star-forming clouds and HII regions exhibit a
power-law shape over a wide range of column densities and log-normal-like
components only at low column densities. We use the N-PDFs to estimate the
evolutionary time-scales of the three classes of objects based on a simple
analytic model from literature. Finally, we show that the integral of the
N-PDFs, the dense gas mass fraction, depends on the total mass of the regions
as measured by ATLASGAL: more massive clouds contain greater relative amounts
of dense gas across all evolutionary classes.Comment: Accepted for publication in A&A (25th June 15) 23 pages, 12 figures.
Additional appendix figures will appear in the journal version of this pape
Generating functional analysis of complex formation and dissociation in large protein interaction networks
We analyze large systems of interacting proteins, using techniques from the
non-equilibrium statistical mechanics of disordered many-particle systems.
Apart from protein production and removal, the most relevant microscopic
processes in the proteome are complex formation and dissociation, and the
microscopic degrees of freedom are the evolving concentrations of unbound
proteins (in multiple post-translational states) and of protein complexes. Here
we only include dimer-complexes, for mathematical simplicity, and we draw the
network that describes which proteins are reaction partners from an ensemble of
random graphs with an arbitrary degree distribution. We show how generating
functional analysis methods can be used successfully to derive closed equations
for dynamical order parameters, representing an exact macroscopic description
of the complex formation and dissociation dynamics in the infinite system
limit. We end this paper with a discussion of the possible routes towards
solving the nontrivial order parameter equations, either exactly (in specific
limits) or approximately.Comment: 14 pages, to be published in Proc of IW-SMI-2009 in Kyoto (Journal of
Phys Conference Series
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