An enhanced cosmic-ray flux towards zeta Persei inferred from a laboratory study of the H3+-e- recombination rate.
The H3+ molecular ion plays a fundamental role in interstellar chemistry, as it initiates a network of chemical reactions that produce many molecules. In dense interstellar clouds, the H3+ abundance is understood using a simple chemical model, from which observations of H3+ yield valuable estimates of cloud path length, density and temperature. But observations of diffuse clouds have suggested that H3+ is considerably more abundant than expected from the chemical models. Models of diffuse clouds have, however, been hampered by the uncertain values of three key parameters: the rate of H3+ destruction by electrons (e-), the electron fraction, and the cosmic-ray ionization rate. Here we report a direct experimental measurement of the H3+ destruction rate under nearly interstellar conditions. We also report the observation of H3+ in a diffuse cloud (towards Persei) where the electron fraction is already known. From these, we find that the cosmic-ray ionization rate along this line of sight is 40 times faster than previously assumed. If such a high cosmic-ray flux is ubiquitous in diffuse clouds, the discrepancy between chemical models and the previous observations of H3+ can be resolved.
McCall
B J
BJ
Department of Chemistry, Department of Astronomy, University of California at Berkeley, Berkeley, California 94720, USA. bjmccall@astro.berkeley.edu
Huneycutt
A J
AJ
Saykally
R J
RJ
Geballe
T R
TR
Djuric
N
N
Dunn
G H
GH
Semaniak
J
J
Novotny
O
O
Al-Khalili
A
A
Ehlerding
A
A
Hellberg
F
F
Kalhori
S
S
Neau
A
A
Thomas
R
R
Osterdahl
F
F
Larsson
M
M