INVESTIGATING THE COSMIC-RAY IONIZATION RATE IN THE GALACTIC DIFFUSE INTERSTELLAR MEDIUM THROUGH OBSERVATIONS OF H+3

By N. Indriolo, B. McCall. 2011

227 citations recorded by the source corpus.

Observations of H+3 in the Galactic diffuse interstellar medium have led to various surprising results, including the conclusion that the cosmic-ray ionization rate (ζ2) is about one order of magnitude larger than previously thought. The present survey expands the sample of diffuse cloud sight lines with H+3 observations to 50, with detections in 21 of those. Ionization rates inferred from these observations are in the range (1.7 ± 1.3) × 10−16 s−1 < ζ2 < (10.6 ± 8.2) × 10−16 s−1 with a mean value of ζ2 = (3.5+5.3−3.0) × 10−16 s−1. Upper limits (3σ) derived from non-detections of H+3 are as low as ζ2 < 0.4 × 10−16 s−1. These low upper limits, in combination with the wide range of inferred cosmic-ray ionization rates, indicate variations in ζ2 between different diffuse cloud sight lines. A study of ζ2 versus NH (total hydrogen column density) shows that the two parameters are not correlated for diffuse molecular cloud sight lines, but that the ionization rate decreases when NH increases to values typical of dense molecular clouds. Both the difference in ionization rates between diffuse and dense clouds and the variation of ζ2 among diffuse cloud sight lines are likely the result of particle propagation effects. The lower ionization rate in dense clouds is due to the inability of low-energy (few MeV) protons to penetrate such regions, while the ionization rate in diffuse clouds is controlled by the proximity of the observed cloud to a site of particle acceleration.

Subfields: Astronomy and Astrophysics, Atomic and Molecular Physics, and Optics

View Paper: https://www.semanticscholar.org/paper/b8b0984038b07cdef6a589ff2fa944a9fb63a7e2

How researchers see this paper

The researchers whose trusted networks rank it highest, strongest advocate first. Every researcher gets a different order.

In Astronomy and Astrophysics

In Atomic and Molecular Physics, and Optics

Citations in the corpus

HERSCHEL SURVEY OF GALACTIC OH+, H2O+, AND H3O+: PROBING THE MOLECULAR HYDROGEN FRACTION AND COSMIC-RAY IONIZATION RATE (2014, methodology): “Shown in the top panel of Figure 24 is the distribution of ionization rates in diffuse molecular clouds found by Indriolo & McCall (2012) using observations of H+3 .”
EXCLUSION OF COSMIC RAYS IN PROTOPLANETARY DISKS: STELLAR AND MAGNETIC EFFECTS (2013, methodology): “…ISM CR ionization rate ζCR has been studied using various observational and theoretical techniques (e.g., using molecular ion emission such as HCO+ and H+3 , see discussion and references in Indriolo & McCall 2012) as well as towards sources spanning vastly different physical environments.”
The Nine Lives of Cosmic Rays in Galaxies (2015, background): “The identification of H+3 in diffuse molecular gas (Geballe et al. 1999, Indriolo et al. 2007, Indriolo & McCall 2012, McCall et al. 1998, 2002, 2003) as well as in dense clouds (Brittain et al. 2004, Gibb et al. 2010, Kulesa 2002, McCall et al. 1999) meant that the cosmic ray ionization rate could…”
Physical Processes in the Interstellar Medium (2014, methodology): “One of these involves the reaction of atomic oxygen with H+3 : O + H+3 →OH + + H2.”
Photodissociation and photoionisation of atoms and molecules of astrophysical interest (2017, background): “…OH+ and H2O+ are interpreted with updated H+3 dissociative recombination rate coefficients generally and require a higher rate in diffuse clouds, 1 to 4 × 10−15 s−1 H2−1 (Indriolo & McCall 2012; Rimmer et al. 2012; Vaupré et al. 2014; Indriolo et al. 2015), and higher still in the Galactic centre.”