THE CHEMISTRY OF INTERSTELLAR OH{sup +}, H{sub 2}O{sup +}, AND H{sub 3}O{sup +}: INFERRING THE COSMIC-RAY IONIZATION RATES FROM OBSERVATIONS OF MOLECULAR IONS
- SETI Institute, Mountain View, CA 94043-5203 (United States)
- Department of Physics and Astronomy, San Jose State University, San Jose, CA 95192-0106 (United States)
- Department of Physics and Astronomy, The Johns Hopkins University, Baltimore, MD 21218 (United States)
- Department of Astronomy, University of Maryland, College Park, MD 20742 (United States)
- Departamento de Astrofisica, Centro de Astrobiologia (CSIC-INTA), 28850 Madrid (Spain)
We model the production of OH{sup +}, H{sub 2}O{sup +}, and H{sub 3}O{sup +} in interstellar clouds, using a steady-state photodissociation region code that treats the freezeout of gas species, grain surface chemistry, and desorption of ices from grains. The code includes polycyclic aromatic hydrocarbons (PAHs), which have important effects on the chemistry. All three ions generally have two peaks in abundance as a function of depth into the cloud, one at A{sub V} {approx}< 1 and one at A{sub V} {approx} 3-8, the exact values depending on the ratio of incident ultraviolet flux to gas density. For relatively low values of the incident far-ultraviolet flux on the cloud ({chi} {approx}< 1000; {chi} = 1 = local interstellar value), the columns of OH{sup +} and H{sub 2}O{sup +} scale roughly as the cosmic-ray primary ionization rate {zeta}{sub crp} divided by the hydrogen nucleus density n. The H{sub 3}O{sup +} column is dominated by the second peak, and we show that if PAHs are present, N(H{sub 3}O{sup +}) {approx}4 Multiplication-Sign 10{sup 13} cm{sup -2} independent of {zeta}{sub crp} or n. If there are no PAHs or very small grains at the second peak, N(H{sub 3}O{sup +}) can attain such columns only if low-ionization potential metals are heavily depleted. We also model diffuse and translucent clouds in the interstellar medium, and show how observations of N(OH{sup +})/N(H) and N(OH{sup +})/N(H{sub 2}O{sup +}) can be used to estimate {zeta}{sub crp}/n, {chi}/n and A{sub V} in them. We compare our models to Herschel observations of these two ions, and estimate {zeta}{sub crp} {approx}4-6 Multiplication-Sign 10{sup -16}(n/100 cm{sup -3}) s{sup -1} and {chi}/n = 0.03 cm{sup 3} for diffuse foreground clouds toward W49N.
- OSTI ID:
- 22039244
- Journal Information:
- Astrophysical Journal, Vol. 754, Issue 2; Other Information: Country of input: International Atomic Energy Agency (IAEA); ISSN 0004-637X
- Country of Publication:
- United States
- Language:
- English
Similar Records
PDR MODEL MAPPING OF PHYSICAL CONDITIONS VIA SPITZER/IRS SPECTROSCOPY OF H{sub 2}: THEORETICAL SUCCESS TOWARD NGC 2023-SOUTH
A NEW DETERMINATION OF THE BINDING ENERGY OF ATOMIC OXYGEN ON DUST GRAIN SURFACES: EXPERIMENTAL RESULTS AND SIMULATIONS
Related Subjects
COSMOLOGY AND ASTRONOMY
ASTROPHYSICS
COMPARATIVE EVALUATIONS
COMPUTERIZED SIMULATION
COSMIC RADIATION
COSMOCHEMISTRY
FAR ULTRAVIOLET RADIATION
GALAXIES
GALAXY NUCLEI
HYDROGEN
HYDROXIDES
ICE
INTERSTELLAR GRAINS
INTERSTELLAR SPACE
IONIZATION
IONIZATION POTENTIAL
METALS
OXONIUM IONS
PHOTOLYSIS
POLYCYCLIC AROMATIC HYDROCARBONS