475,192 research outputs found
Limits on the cosmic-ray ionization rate toward massive young stars
Recent models of the envelopes of seven massive protostars are used to
analyze observations of H3+ infrared absorption and H13CO+ submillimeter
emission lines toward these stars, and to constrain the cosmic-ray ionization
rate zeta. The H13CO+ gives best-fit values of zeta=(2.6+/- 1.8) x 10^-17 s^-1,
in good agreement with diffuse cloud models and with recent Voyager/Pioneer
data but factors of up to 7 lower than found from the H3+ data. No relation of
zeta with luminosity or total column density is found, so that local (X-ray)
ionization and shielding against cosmic rays appear unimportant for these
sources. The difference between the H3+ and H13CO+ results and the correlation
of N(H3+) with heliocentric distance suggest that intervening clouds contribute
significantly to the H3+ absorptions in the more distant regions. The most
likely absorbers are low-density (<~10^4 cm^-3) clouds with most carbon in
neutral form or in CO.Comment: To be published in A&A 358 (Letters); 4 pages including 3 figure
Universality class of nonequilibrium phase transitions with infinitely many-absorbing-states
We consider systems whose steady-states exhibit a nonequilibrium phase
transition from an active state to one -among an infinite number- absorbing
state, as some control parameter is varied across a threshold value. The pair
contact process, stochastic fixed-energy sandpiles, activated random walks and
many other cellular automata or reaction-diffusion processes are covered by our
analysis. We argue that the upper critical dimension below which anomalous
fluctuation driven scaling appears is d_c=6, in contrast to a widespread belief
(see Dickman cond-mat 0110043 for an overview). We provide the exponents
governing the critical behavior close to or at the transition point to first
order in a 6-d expansion.Comment: 4 pages, to appear in the Physical Review Letter
Infinitely-many absorbing-state nonequilibrium phase transitions
We present a general field-theoretic strategy to analyze three connected
families of continuous phase transitions which occur in nonequilibrium
steady-states. We focus on transitions taking place between an active state and
one absorbing state, when there exist an infinite number of such absorbing
states. In such transitions the order parameter is coupled to an auxiliary
field. Three situations arise according to whether the auxiliary field is
diffusive and conserved, static and conserved, or finally static and not
conserved.Comment: 7 pages, 2 .eps figures. To appear in the Brazilian Journal of
Physics (2003
Hot Molecular Cores and High-Mass Star Formation
This review covers hot cores in the context of high-mass star formation.
After giving an overview of chemical processes and diversity during high-mass
star formation, it reviews the `warm envelope' phase which probably precedes
the formation of hot cores. Some recent determinations of the cosmic-ray
ionization rate are discussed, as well as recent evidence for hot cores around
low-mass stars. Routes for future hot core research are outlined.Comment: 8 pages, 1 figure; to appear in the Proceeding of IAU Symposium 221,
Star Formation at High Angular Resolution, Editors M. Burton, R. Jayawardhana
& T. Bourke, Astronomical Society of the Pacifi
On certain multiple Bailey, Rogers and Dougall type summation formulas
A multidimensional generalization of Bailey's very-well-poised bilateral
basic hypergeometric summation formula and its Dougall type
hypergeometric degeneration for is studied. The multiple
Bailey sum amounts to an extension corresponding to the case of a nonreduced
root system of certain summation identities associated to the reduced root
systems that were recently conjectured by Aomoto and Ito and proved by
Macdonald. By truncation, we obtain multidimensional analogues of the
very-well-poised unilateral (basic) hypergeometric Rogers and
Dougall sums (both nonterminating and terminating). The terminating
sums may be used to arrive at product formulas for the norms of recently
introduced (-)Racah polynomials in several variables.Comment: 20 pages, LaTe
- …