Ubiquitous argonium (ArH+) in the diffuse interstellar medium: A molecular tracer of almost purely atomic gas

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Table 2

Reaction list.

Reaction	Assumed rate or rate coefficient	Notes

Ar + CR → Ar⁺ + e	(10 + 3.85φ)ζ_p(H)	(1)
	10^-9 cm³ s^-1	(2)
	8 × 10^-10exp(−6400 K /T) cm³ s^-1	(3)
Ar⁺ + e → Ar + hν	3.7 × 10^-12 (T/ 300 K)^-0.651cm³ s^-1	(4)
Ar⁺ + PAH⁻ → Ar + PAH	6.8 × 10^-8 (T/ 300 K)^-0.5 cm³ s^-1	(5)
Ar⁺ + PAH → Ar + PAH⁺	5.9 × 10^-9 cm³ s^-1	(5)
Ar⁺ + H₂ → ArH⁺ + H	8.4 × 10^-10 (T/ 300 K)^0.16 cm³ s^-1	(6)
	8 × 10^-10 cm³ s^-1	(3)
ArH⁺ + CO → Ar + HCO⁺	1.25 × 10^-9 cm³ s^-1	(3)
ArH⁺ + O → Ar + OH⁺	8 × 10^-10 cm³ s^-1	(7)
ArH⁺ + C → Ar + CH⁺	8 × 10^-10 cm³ s^-1	(7)
ArH⁺ + e → Ar + H	< 10^-9 cm³ s^-1	(8)
ArH⁺ + hν → Ar⁺ + H	1.0 × 10^-11χ_UVf_As^-1	(9)

Notes.

⁽¹⁾

Kingdon (1965), Jenkins (2013); φ is the number of secondary ionizations of H per primary cosmic ray ionization: we adopt the fit given by Dalgarno et al. (1999).

⁽²⁾

Estimate.

⁽³⁾

Villinger et al. (1982).

⁽⁴⁾

Shull & van Steenberg (1982).

⁽⁵⁾

Hollenbach et al. (2012), scaled by reduced mass^-0.5.

⁽⁶⁾

Rebrion et al. (1989); Bedford & Smith (1990).

⁽⁷⁾

Assumed equal to the rate for reaction with H₂.

⁽⁸⁾

Mitchell et al. (2005b).

⁽⁹⁾

Unshielded rate based on theoretical cross-sections of Alexseyev et al. (2007). Attenuation factor f_A = [ E₂(3.6A_V) + E₂(3.6 [ A_V(tot) − A_V ] ) ] / 2, where E₂ is an exponential integral. (Based on attenuation factor obtained by NW09 for photoionization of Cl.)

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