IMAPS 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 to determine interstellar 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. We find that the interstellar Ar I is below its solar and B-star abundance with respect to hydrogen toward zeta Pup, 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 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 beta 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.Comment: 36 pages, 3 figure

Seeing the History of the Earth in the Cliffs at Møn: The Interaction between Landscape Painting and Geology in Denmark in the First Half of the 19th Century

During the first part of the nineteenth century, geologists developed a history of the earth so different from that accepted in previous centuries that it encouraged a rethinking of the relationship between man and nature. In this article I will argue that painters followed these changes closely and that some of them let the narratives and images of geology inform the way they depicted nature. In arguing my point, I will focus on images and descriptions of the chalk cliffs on the Danish island of Møn by both geologists and painters. I will follow the scientific advances in geology by referring to the texts and images of Søren Abildgaard, Henrich Steffens, Johan Georg Forchhammer, and Christopher Puggaard, and discuss how their changing theories correspond with paintings of the cliffs by four artists: Christopher Wilhelm Eckersberg, Frederik Sødring, Louis Gurlitt, and Peter Christian Skovgaard