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Astronomers are going to extraordinary lengths in the quest to tot up the 'ordinary' matter in the Universe. The latest initiative has probed hot gas in intergalactic space by means of an X-ray lighthouse. [PUBLICATION ABSTRACT]

Bow-shocks are produced in the local interstellar medium by the passage of fast stars from the Galactic thin-disk and thick-disk populations with velocities \\(V_* = \\) 40-80 km/s. Stellar transits of local H I clouds occur every 3500-7000 yr on average and last between \\(10^4\\) and \\(10^5\\) yr. There could be 10-20 active bow shocks around low-mass stars inside clouds within 10-15 pc of the Sun. At local cloud distances of 3-10 pc, their turbulent wakes have transverse radial extents \\(R_{\\rm wake} \\approx\\) 10-300 AU, angular sizes 10-100 arcsec, and Lyman-alpha surface brightnesses of 2-8 Rayleighs in gas with total hydrogen density \\(n_H \\approx 0.1~{\\rm cm}^{-3}\\) and \\(V_* =\\) 40-80 km/s. These transit wakes may cover an area fraction \\(f_A \\approx (R_{\\rm wake}/R_{\\rm cl}) \\approx 10^{-3}\\) of local H I clouds and be detectable in IR (dust), UV (Lya, two-photon), or non-thermal radio emission. Turbulent heating in these wakes could produce the observed elevated rotational populations of H\\(_2\\) (\\(J \\geq 2\\)) and influence the endothermic formation of CH\\(^+\\) in diffuse interstellar gas at \\(T > 10^3\\) K.

In this investigation, we present an analysis of the metallicity distribution that pertains to neutral gas in the local Galactic interstellar medium (ISM). We derive relative ISM metallicities for a sample of 84 sight lines probing diffuse atomic and molecular gas within 4 kpc of the Sun. Our analysis is based, in large part, on column density measurements reported in the literature for 22 different elements that are commonly studied in interstellar clouds. We supplement the literature data with new column density determinations for certain key elements and for several individual sight lines important to our analysis. Our methodology involves comparing the relative gas-phase abundances of many different elements for a given sight line to simultaneously determine the strength of dust depletion in that direction and the overall metallicity offset. We find that many sight lines probe multiple distinct gas regions with different depletion properties, which complicates the metallicity analysis. Nevertheless, our results provide clear evidence that the dispersion in the metallicities of neutral interstellar clouds in the solar neighborhood is small (\\(\\)0.10 dex) and only slightly larger than the typical measurement uncertainties. We find no evidence for the existence of very low metallicity gas (as has recently been reported by De Cia et al.) along any of the 84 sight lines in our sample. Our results are consistent with a local Galactic ISM that is well mixed and chemically homogeneous.

We present new observations of the cosmic ultraviolet background (CUVB) at high Galactic latitudes (\\(|b| > 40^{\\circ}\\)), made using the Alice UV spectrograph on board the New Horizons spacecraft. These observations were taken at about 57 AU from the Sun, outside much of the foreground emission affecting previous missions, and allowed a new determination of the spectrum of the CUVB between 912 -- 1100~\\AA\\ and 1400 -- 1800~\\AA. We found a linear correlation between the CUVB and the Planck E(B~-~V) with offsets at zero-reddening of \\(221 \\pm 11\\) photon units at 1000~\\AA\\ and \\(264 \\pm 24\\) \\photu\\ at 1500~\\AA\\ (\\(4.4 \\pm 0.2\\) nW m\\(^{-2}\\) sr\\(^{-1}\\) at 1000~\\AA\\ and \\(5.3 \\pm 0.5\\) nW m\\(^{-2}\\) sr\\(^{-1}\\) at 1500~\\AA). The former is the first firm detection of the offset in the range 912 -- 1100 \\AA\\ while the latter result confirms previous results from \\galex, showing that there is little emission from the Solar System from 1400 -- 1800 \\AA. About half of the offset may be explained by known sources (the integrated light of unresolved galaxies, unresolved stars, emission from ionized gas, and two-photon emission from warm hydrogen in the halo) with the source of the remaining emission as yet unidentified. There is no detectable emission below the Lyman limit with an upper limit of \\(3.2 \\pm 3.0\\) photon units.

For application to surveys of interstellar matter and Galactic structure, we compute new spectrophotometric distances to 139 OB stars frequently used as background targets for UV spectroscopy. Many of these stars have updated spectral types and digital photometry with reddening corrections from the Galactic O-Star (GOS) spectroscopic survey. We compare our new photometric distances to values used in previous IUE and FUSE surveys and to parallax distances derived from Gaia-DR2, after applying a standard (0.03 mas) offset from the quasar celestial reference frame. We find substantial differences between photometric and parallax distances (at d > 1.5 kpc) with increasing dispersion when parallax errors exceed 8%. Differences from previous surveys arise from new GOS stellar classifications, especially luminosity classes, and from reddening corrections. We apply our methods to two OB associations. For Perseus OB1 (nine O-stars) we find mean distances of \\(2.47\\pm0.57\\) kpc (Gaia parallax) and \\(2.99\\pm0.14\\) kpc (photometric) using a standard grid of absolute magnitudes (Bowen et al. 2008). For 29 O-stars in Car OB1 associated with Trumpler-16, Trumpler-14, Trumpler-15, and Collinder-228 star clusters, we find \\(2.87\\pm0.73\\) kpc (Gaia parallax) and \\(2.60\\pm0.28\\) kpc (photometric). Using an alternative grid of O-star absolute magnitudes (Martins et al. 2005) shifts these photometric distances 7% closer. Improving the distances to OB-stars will require attention to spectral types, photometry, reddening, binarity, and the grid of absolute magnitudes. We anticipate that future measurements in Gaia-DR3 will improve the precision of distances to massive star-forming regions in the Milky Way.

We report results from a FUSE survey of interstellar molecular hydrogen (H2) in the Galactic disk toward 139 O-type and early B-type stars at Galactic latitudes \\(|b| < 10^{\\circ}\\), with updated photometric and parallax distances. The H2 absorption is measured using the far-ultraviolet Lyman and Werner bands, including strong R(0), R(1), and P(1) lines from rotational levels \\(J = 0\\) and \\(J = 1\\) and excited states up to \\(J = 5\\) (sometimes \\(J = 6\\) and 7). For each sight line, we report column densities \\(N_{H2}\\), \\(N_{HI}\\), \\(N(J)\\), \\(N_H = N_{HI} + 2N_{H2}\\), and molecular fraction, \\(f_{H2} = 2N_{H2}/N_H\\). Our survey extends the 1977 Copernicus H2 survey up to \\(N_H \\sim 5\\times10^{21}\\) cm\\(^{-2}\\). The lowest rotational states have mean excitation temperatures and rms dispersions, \\(T_{01} = 88\\pm 20\\) K and \\(T_{02} = 77\\pm18\\) K, suggesting that J = 0,1,2 are coupled to the gas kinetic temperature. Populations of higher-J states exhibit mean excitation temperatures, \\(T_{24} = 237\\pm91\\) K and \\(T_{35} = 304\\pm108\\) K, produced primarily by UV radiative pumping. Correlations of \\(f_{H2}\\) with E(B-V) and N_H show a transition to \\(f_{H2} \\geq 0.1\\) at \\(N_ H \\geq 10^{21}\\) cm\\(^{-2}\\) and \\(E(B-V) > 0.2\\), interpreted with an analytic model of H2 formation-dissociation equilibrium and attenuation of the far-UV radiation field by self-shielding and dust opacity. Results of this disk survey are compared to previous FUSE studies of H2 in translucent clouds, at high Galactic latitudes, and in the Magellanic Clouds. Using updated distances to the target stars, we find average sight-line values \\(\\langle f_{H2} \\rangle \\geq 0.20\\) and \\(\\langle N_H/E(B-V) \\rangle = (6.07\\pm1.01)\\times10^{21}\\) cm\\(^{-2}\\) mag\\(^{-1}\\).

It is demonstrated here that the passage of luminous stars through the Oort Cloud over the age of the solar system will subject all comets to heating episodes up to at least 16 K, and a sizeable fraction to 30 K. Stochastic supernovae have even more striking effects during monthlong events; conservative estimates of the supernovae are in the Galactic disk suggest that most comets have been heated to 45 K, and a fraction to about 60 K. These results imply that comets are not fully pristine relics of solar system formation.

We analyze the sources of free electrons that produce the large dispersion measures, DM \\(\\approx 300-1600\\) (in units cm\\(^{-3}\\) pc), observed toward fast radio bursts (FRBs). Individual galaxies typically produce DM \\(\\sim 25-60\\) cm\\(^{-3}\\) pc from ionized gas in their disk, disk-halo interface, and circumgalactic medium. Toward an FRB source at redshift \\(z\\), a homogeneous IGM containing a fraction \\(f_{\\rm IGM}\\) of cosmological baryons will produce DM \\(= (935~{\\rm cm}^{-3}~{\\rm pc}) f_{\\rm IGM} \\, h_{70}^{-1} I(z)\\), where \\(I(z) = (2/3 \\Omega_m)[ \\{ \\Omega_m(1+z)^3 + \\Omega_{\\Lambda} \\}^{1/2} - 1 ]\\). A structured IGM of photoionized Ly-alpha absorbers in the cosmic web produces similar dispersion, modeled from the observed distribution, \\(f_b(N,z)\\), of H I (Lya-forest) absorbers in column density and redshift with ionization corrections and scaling relations from cosmological simulations. An analytic formula for DM(\\(z\\)) applied to observed FRB dispersions suggests that \\(z_{\\rm FRB} \\approx 0.2-1.5\\) for an IGM containing a significant baryon fraction, \\(f_{\\rm IGM} = 0.6\\pm0.1\\). Future surveys of the statistical distribution, DM(\\(z)\\), of FRBs identified with specific galaxies and redshifts, can be used to calibrate the IGM baryon fraction and distribution of Ly-alpha absorbers. Fluctuations in DM at the level \\(\\pm10\\) cm\\(^{-3}\\) pc will arise from filaments and voids in the cosmic web.

We present a blind search for doublet intergalactic metal absorption with a method dubbed `agnostic stacking'. Using a forward-modelling framework we combine this with direct detections in the literature to measure the overall metal population. We apply this novel approach to the search for NeVIII absorption in a set of 26 high-quality COS spectra. We probe to an unprecedented low limit of log N\\(>\\)12.3 at 0.47\\(\\leq z \\leq\\)1.34 over a pathlength \\(\\Delta\\)z = 7.36. This method selects apparent absorption without requiring knowledge of its source. Stacking this mixed population dilutes doublet features in composite spectra in a deterministic manner, allowing us to measure the proportion corresponding to NeVIII absorption. We stack potential NeVIII absorption in two regimes: absorption too weak to be significant in direct line studies (12.3 \\(<\\) log N \\(<\\) 13.7), and strong absorbers (log N \\(>\\) 13.7). We do not detect NeVIII absorption in either regime. Combining our measurements with direct detections, we find that the NeVIII population is reproduced with a power law column density distribution function with slope \\(\\beta = -1.86 \\substack{+0.18 \\\ -0.26}\\) and normalisation log \\(f_{13.7} = -13.99 \\substack{+0.20 \\\ -0.23}\\), leading to an incidence rate of strong NeVIII absorbers \\(dn/dz =1.38 \\substack{+0.97 \\\ -0.82}\\). We infer a cosmic mass density for NeVIII gas with 12.3 \\(<\\) log N \\(<\\) 15.0 of \\(\\Omega _{NeVIII} = 2.2 \\substack{+1.6 \\\ _-1.2} \\times 10^{-8}\\), a value significantly lower that than predicted by recent simulations. We translate this density into an estimate of the baryon density \\(\\Omega _{b} \\approx 1.8 \\times 10^{-3}\\), constituting 4\\% of the total baryonic mass.