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

We use the absorption features from neutral argon at 1048 and 1066 A todetermine interstellar abundances or their lower limits toward nine early-typestars. These features were observed with the Interstellar Medium AbsorptionProfile Spectrograph (IMAPS) along sight lines with low reddening and lowfractional abundances of molecular hydrogen. We find that the interstellar Ar Iis below its solar and B-star abundance with respect to hydrogen toward zetaPup, gamma2 Vel and beta Cen A with (logarithmic) reduction factors-0.37+/-0.09, -0.18+/-0.10, and -0.61+/-0.12 dex, respectively. While Ar can condense onto the surfaces of dust grains in the interiors ofdense clouds, it is unlikely that argon atoms are depleted by this process inthe low-density lines of sight considered in this study. Instead, we proposethat the relatively large photoionization cross section of Ar makes it mucheasier to hide in its ionized form than H. In regions that are about halfionized, this effect can lower Ar I/H I by -0.11 to -0.96 dex, depending on theenergy of the photoionizing radiation and its intensity divided by the localelectron density. We apply this interpretation for the condition of the gas infront of beta Cen A, which shows the largest deficiency of Ar. Also, wedetermine 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 solarsystem. For the local cloud and others that can be probed by UV studies, theobserved Ar I to H I ratio may be a good discriminant between two possiblealternatives, collisional ionization or photoionization, for explaining theexistence of partly ionized regions.Comment: 36 pages, 3 figure