Interstellar Medium Absorption Profile Spectrograph Observations of Interstellar Neutral Argon and the Implications for Partially Ionized Gas

We use the absorption features from the resonance transitions of neutral argon at 1048 and 1066 Å to determine interstellar argon abundances or their lower limits toward nine early-type stars. These features were observed with the interstellar medium absorption profile spectrograph (IMAPS) along sight lines with low reddening and low fractional abundances of molecular hydrogen. Four of the sight lines in our sample have reliably measured interstellar Ar abundances, of which three also have good determinations of N(H I). We find that the interstellar Ar is below its solar and B-star abundance with respect to hydrogen toward ζ Pup, γ2 Vel, and β Cen A with (logarithmic) reduction factors D = -0.37 ± 0.09 dex, D = -0.18 ± 0.10 dex, and D = -0.61 ± 0.12 dex, respectively. While Ar can condense onto the surfaces of dust grains in the interiors of dense clouds, it is unlikely that argon atoms are depleted by this process in the low-density lines of sight considered in this study. Instead, we propose that the relatively large photoionization cross section of Ar makes it much easier to hide in its ionized form than H. In regions that are about half ionized, this effect can lower Ar I/H I by -0.11 to -0.96 dex, depending on the energy of the photoionizing radiation and its intensity divided by the local electron density. We apply this interpretation for the condition of the gas in front of β Cen A, which shows the largest deficiency of Ar. Also, we determine the expected magnitudes of the differential ionizations for He, N, O, Ne, and Ar in the partly ionized, warm gas in the local cloud around our solar system. For the local cloud and others that can be probed by UV studies, the observed Ar I-to-H I ratio may be a good discriminant between two possible alternatives, collisional ionization or photoionization, for explaining the existence of partly ionized regions.