We present the results of a detailed investigation into the physical conditions in interstellar material interacting with the supernova remnant (SNR) IC 443. Our analysis is based on a comprehensive examination of high-resolution far-ultraviolet spectra obtained with the Space Telescope Imaging Spectrograph onboard the Hubble Space Telescope of two stars behind IC 443. One of our targets (HD 43582) probes gas along the entire line of sight through the SNR, while the other (HD 254755) samples material located ahead of the primary supernova shock front. We identify low-velocity quiescent gas in both directions and find that the densities and temperatures in these components are typical of diffuse atomic and molecular clouds. Numerous high-velocity components are observed in the absorption profiles of neutral and singly ionized atomic species toward HD 43582. These components exhibit a combination of greatly enhanced thermal pressures and significantly reduced dust-grain depletions. We interpret this material as cooling gas in a recombination zone far downstream from shocks driven into neutral gas clumps. The pressures derived for a group of ionized gas components at high positive velocity toward HD 43582 are lower than those of the other shocked components, pointing to pressure inhomogeneities across the remnant. A strong, very high velocity component near −620 km s−1 is seen in the absorption profiles of highly ionized species toward HD 43582. The velocity of this material is consistent with the range of shock velocities implied by observations of soft thermal X-ray emission from IC 443. Moderately high velocity gas toward HD 254755 may represent shocked material from a separate foreground SNR.

Using visible, radio, microwave, and sub-mm data, we study several lines of sight toward stars generally closer than 1 kpc on a component-by-component basis. We derive the component structure seen in absorption at visible wavelengths from Ca II, Ca I, K I, CH, CH + , and CN and compare it to emission from H I, CO and its isotopologues, and C + from the GOT C+ survey. The correspondence between components in emission and absorption help create a more unified picture of diffuse atomic and molecular gas in the interstellar medium. We also discuss how these tracers are related to the CO-dark H 2 gas probed by C + emission and discuss the kinematic connections among the species observed.• Diffuse Atomic Clouds (Warm Neutral Medium): Components seen in H I only; the densities and column densities are too low to produce C + emission above the GOT C+ sensitivity limit. (n H ≤ 1 cm −3 )• Diffuse Atomic Clouds (Cold Neutral Medium): Components seen in H I, and/or C + , with 12 CO and 13 CO emission well below the detection limit, with the assumption that this H I is associated with the envelope around denser colder gas. Here, some of the C + emission comes from H I and some from CO-dark H 2 gas. (1 cm −3 ≤ n H ≤ 300 cm −3 )• CO-dark H 2 Gas: Components seen in H I, and/or C + , but with no 12 CO and 13 CO emission above the detection limit. (n H ≈ 300 cm −3 )• Molecular Clouds: Components seen in H I, and/or C + , 12 CO, and 13 CO emission, with the distinction that clouds with 13 CO are denser. (300 cm −3 ≤ n H ≤ 1000 cm −3 )Studies of atomic and molecular absorption and emission in the diffuse ISM have their own seemingly separate paradigms. However, measurements of atomic and molecular absorption at visible and UV wavelengths provide complementary data on some of the environments seen in the GOT C+ survey. By associating probes seen at optical and UV wavelengths with the results from the GOT C+ survey, we can pursue an integrated study of the diffuse ISM, incorporating data from radio to UV wavelengths. We also note how these tracers are related to the CO-dark H 2 gas being probed by C + emission, focusing on the kinematic connections among the species. The outline of this first paper is as follows. Section 2 describes the observations. Section 3 provides the results of our survey, while Section 4 discusses connections between the various probes. The last section presents our conclusions. OBSERVATIONS The DataSeventeen sight lines were chosen by examining the SIMBAD database (Wenger et al. 2000); only GOT C+ pointings accessible to the Northern Hemisphere were considered. The background targets in our survey had to be relatively bright (B, V ≤ 10) A and B stars with amounts of extinction per kpc typical for directions with diffuse molecular gas (such as that seen toward ζ Oph). The lines of sight had to be within 30 arcminutes of the Herschel pointings. Visual extinctions were derived by assuming A V = 3.1 E (B -V ). Distances were obtained by spectroscopic parallax as many stars did not have definitive Hipparcos or GAI...

We study four lines of sight that probe the transition from diffuse molecular gas to molecular cloud material in Taurus. Measurements of atomic and molecular absorption are used to infer the distribution of species and the physical conditions toward stars behind the Taurus Molecular Cloud (TMC). New high-resolution spectra at visible and near-IR wavelengths of interstellar Ca ii, Ca i, K i, CH, CH+, C2, CN, and CO toward HD 28975 and HD 29647 are combined with data at visible wavelengths and published CO results from ultraviolet measurements for HD 27778 and HD 30122. Gas densities and temperatures are inferred from C2, CN, and CO excitation and CN chemistry. Our results for HD 29647 are noteworthy because the CO column density is 1018 cm−2 while C2 and CO excitation reveals a temperature of 10 K and a density of ∼1000 cm−3, more like conditions found in dark molecular clouds. Similar results arise from our chemical analysis for CN through reactions involving observations of CH, C2, and NH. Enhanced potassium depletion and a reduced CH/H2 column density ratio also suggest the presence of a dark cloud. The directions toward HD 27778 and HD 30122 probe molecule-rich diffuse clouds, which can be considered CO-dark gas, while the sight line toward HD 28975 represents an intermediate case. Maps of dust temperature help refine the description of the material along the four sight lines and provide an estimate of the distance between HD 29647 and a clump in the TMC. An appendix provides results for the direction toward HD 26571; this star also probes diffuse molecular gas.

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