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Ultra-high spectral resolution observations of time-varying interstellar absorption towards κ Vel are reported, using the Ultra-High Resolution Facility on the Anglo-Australian Telescope. Detections of interstellar Ca i, Ca ii, K i, Na i and CH are obtained, whilst an upper limit on the column density is reported for C2. The results show continued increases in column densities of K i and Ca i since observations ∼4 yr earlier, as the transverse motion of the star carried it ∼10 au perpendicular to the line of sight. Line profile models are fitted to the spectra and two main narrow components (A and B) are identified for all species except CH. The column density N(K i) is found to have increased by 82+ 10− 9 per cent between 1994 and 2006, whilst N(Ca i) is found to have increased by 32 ± 5 per cent over the shorter period of 2002–2006. The line widths are used to constrain the kinetic temperature to Tk, A < 671+ 18− 17 K and Tk, B < 114+ 15− 14 K. Electron densities are determined from the Ca i/Ca ii ratio, which in turn place lower limits on the total number density of nA ≳ 7 × 103 cm−3 and nB ≳ 2 × 104 cm−3. Calcium depletions are estimated from the Ca i/K i ratio. Comparison with the chemical models of Bell et al. confirms the high number density, with n = 5 × 104 cm−3 for the best-fitting model. The first measurements of diffuse interstellar bands (DIBs) towards this star are made at two epochs, but only an upper limit of ≲ 40 per cent is placed on their variation over ∼9 yr. The DIBs are unusually weak for the measured E(B − V) and appear to exhibit similar behaviour to that seen in Orion. The ratio of equivalent widths of the λ5780 to λ5797 DIBs is amongst the highest known, which may indicate that the carrier of λ5797 is more sensitive to ultraviolet radiation than to local density. [ABSTRACT FROM AUTHOR]

The multiphase interstellar medium (ISM) is highly structured, on scales from the size of the Solar System to that of a galaxy. In particular, small-scale structures are difficult to study and hence are poorly understood. We used the multiplex capabilities of the AAOmega spectrograph at the Anglo-Australian Telescope to create a half-square-degree map of the neutral and low-ionized ISM in front of the nearby (∼5 kpc), most massive Galactic globular cluster, ω Centauri. Its redshifted, metal-poor and hot horizontal branch stars probe the medium-strong Ca ii K and Na i D2 line absorption, and weak absorption in the λ5780 and λ5797 diffuse interstellar bands (DIBs), on scales around a parsec. The kinematical and thermodynamical picture emerging from these data is that we predominantly probe the warm neutral medium and weakly ionized medium of the Galactic Disc–Halo interface, ∼0.3–1 kpc above the mid-plane. A comparison with Spitzer Space Telescope 24 μm and DIRBE/ IRAS maps of the warm and cold dust emission confirms that both Na i and Ca ii trace the overall column density of the warm neutral and weakly ionized medium. Clear signatures are seen of the depletion of calcium atoms from the gas phase into dust grains. Curiously, the coarse DIRBE/ IRAS map is a more reliable representation of the relative reddening between sightlines than the Na i and Ca ii absorption-line measurements, most likely because the latter are sensitive to fluctuations in the local ionization conditions. The behaviour of the DIBs is consistent with the λ5780 band being stronger than the λ5797 band in regions where the ultraviolet radiation level is relatively high, as in the Disc–Halo interface. This region corresponds to a σ-type cloud in which Ca i and small diatomic molecules such as CH and CN are usually absent. In all, our maps and simple analytical model calculations show in unprecedented detail that small-scale density and/or ionization structures exist in the extra-planar gas of a spiral galaxy. [ABSTRACT FROM AUTHOR]