Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Interstellar Metastable Helium Absorption as a Probe of the Cosmic-Ray
  Ionization Rate   The ionization rate of interstellar material by cosmic rays has been a major
source of controversy, with different estimates varying by three orders of
magnitude. Observational constraints of this rate have all depended on
analyzing the chemistry of various molecules that are produced following
cosmic-ray ionization, and in many cases these analyses contain significant
uncertainties. Even in the simplest case (H3+) the derived ionization rate
depends on an (uncertain) estimate of the absorption path length. In this paper
we examine the feasibility of inferring the cosmic-ray ionization rate using
the 10830 A absorption line of metastable helium. Observations through the
diffuse clouds toward HD 183143 are presented, but yield only an upper limit on
the metastable helium column density. A thorough investigation of He+ chemistry
reveals that only a small fraction of He+ will recombine into the triplet state
and populate the metastable level. In addition, excitation to the triplet
manifold of helium by secondary electrons must be accounted for as it is the
dominant mechanism which produces He* in some environments. Incorporating these
various formation and destruction pathways, we derive new equations for the
steady state abundance of metastable helium. Using these equations in concert
with our observations, we find zeta_He < 1.2*10^-15 s^-1, an upper limit about
5 times larger than the ionization rate previously inferred for this sight line
using H3+. While observations of interstellar He* are extremely difficult at
present...

16
The categories of this paper are: stat
25
The categories of this paper are: patt-sol
4
The categories of this paper are: cond-mat
26
The categories of this paper are: bayes-an
1
The categories of this paper are: cs
37
The categories of this paper are: supr-con
35
The categories of this paper are: mtrl-th
14
The categories of this paper are: math-ph
33
The categories of this paper are: plasm-ph
7
The categories of this paper are: hep-th
19
The categories of this paper are: eess
22
The categories of this paper are: chao-dyn
0
The categories of this paper are: hep-ph
34
The categories of this paper are: chem-ph
23
The categories of this paper are: solv-int
27
The categories of this paper are: comp-gas