ABSTRACT We report interstellar silicon (Si) depletion and dust-phase column densities of Si along 131 Galactic sight lines using archival observations. The data were corrected for differences in the assumed oscillator strength. This is a much larger sample than previous studies but confirms the majority of results, which state that the depletion of Si is correlated with the average density of hydrogen along the line of sight ( 〈 n ( H ) 〉 ) as well as the fraction of hydrogen in molecular form ( f (H 2 )). We also find that the linear part of the extinction curve is independent of Si depletion. Si depletion is correlated with the bump strength ( c 3 / R V ) and the FUV curvature ( c 4 / R V ) suggesting that silicon plays a significant role in both the 2175 Å bump and the FUV rise.

The balloon-borne Solar Disk Sextant (SDS) experiment has measured the angular size of the Sun on seven occasions spanning the years 1992 to 2011. The solar half-diameter - observed in a 100 nm wide passband centred at 615 nm - is found to vary over that period by up to 200 mas, while the typical estimated uncertainty of each measure is 20 mas. The diameter variation is not in phase with the solar activity cycle; thus, the measured diameter variation cannot be explained as an observational artefact of surface activity. Other possible instrument-related explanations for the observed variation are considered but found unlikely, leading us to conclude that the variation is real. The SDS is described here in detail, as is the complete analysis procedure necessary to calibrate the instrument and allow comparison of diameter measures across decades.

We report previously undetermined interstellar gas and dust-phase carbon abundances along 15 Galactic sight lines based on archival data of the strong 1334.5323 angstrom transition observed with the Space Telescope Imaging Spectrograph. These are combined with previously reported carbon measurements along six sight lines to produce a complete sample of interstellar C II measurements determined with the 1334 angstrom transition. Our data set includes a variety of Galactic disk environments characterized by different extinctions and samples paths ranging over three orders of magnitude in average density of hydrogen (< n(H)>). Our data support the idea that dust, specifically carbon-based grains, are processed in the neutral interstellar medium. We, however, do not find that the abundance of carbon in dust or the grain-size distribution is related to the strength of the 2175 angstrom bump. This is surprising, given that many current models have polycyclic aromatic hydrocarbons as the bump-producing dust.

Carbon is arguably the most important element in the interstellar medium, yet its abundance in gas and dust is poorly understood due to a paucity of data. We explore the possibility of substantially increasing our knowledge of interstellar carbon by applying and assessing a new method for determining the column density of the dominant ion of interstellar carbon in diffuse neutral lines of sight. The method relies on profile fitting of the strong transition of C II at 1334 angstrom in spectra continuum normalized with stellar models. We apply our method to six sight lines for which the carbon abundance has previously been determined with a weak intersystem absorption transition. Our strong- line method consistently shows a significantly lower gas-phase C abundance than the measurements from the weak lines. This result implies that more carbon could reside in dust than was previously thought. This has implications for dust models, which often suffer from a lack of sufficient carbon to plausibly explain extinction. There is no immediately clear explanation for the difference found between the strong-and weak-line C II determinations, but there are indications that the results from the method presented here have advantages over the weak-line column densities. If this is the case, then the reported oscillator strength for the C II transition at 2325 angstrom may be too small. Our findings further suggest that damping wings modeled with a single absorption component may not produce accurate abundances. This problem could affect a large number of H I abundances determined through absorption line analysis that are reported in the literature.

We present interstellar C II (1334.5323 angstrom) and C II* (1335.7077 angstrom) column density measurements along 14 Galactic sight-lines. These sight-lines sample a variety of Galactic disk environments and include paths that range nearly two orders of magnitude in average hydrogen densities (< n(H)>) along the lines of sight. Five of the sight-lines show super-solar gas phase abundances of carbon. Our results show that the excess carbon along these sight-lines may result from different mechanisms taking place in the regions associated with these stars.

We present interstellar C II (1334.5323 \AA) and C II* (1335.7077 \AA) column density measurements along 14 Galactic sight-lines. These sight-lines sample a variety of Galactic disk environments and include paths that range nearly two orders of magnitude in average hydrogen densities ( ) along the lines of sight. Five of the sight-lines show super-Solar gas phase abundances of carbon. Our results show that the excess carbon along these sight-lines may result from different mechanisms taking place in the regions associated with these stars.

Carbon is arguably the most important element in dust, yet its abundance in grains is poorly understood due to a paucity of data The primary source of interstellar neutral-cloud carbon abundances has been the weak C II] 2325 angstrom feature, a measurement of which requires high resolution and signal-to-noise ultraviolet spectra The stringent data requirements have yielded only 13 abundance measurements in neutral interstellar regions Since merely seven of these sight lines have measured extinction curves, relating the carbon abundance in grains to specific extinction characteristics is nearly impossible For that reason we have been testing the possibility of obtaining interstellar C II abundances from the strong 1334 angstrom transition, of which there are more observations Our preliminary results show that careful analysis of this line is able yield reliable interstellar gas-phase carbon abundances Our tests have used the translucent-sight-line sample of Sofia et al (2004) For these six sight lines, the strong-line method consistently shows a lower gas-phase C abundance than the measurements from the weak lines This implies that more carbon resides in dust than was previously thought, which may solve the carbon crisis We are extending our sample to include all of the STIS archived data for sight lines with measured interstellar hydrogen abundances

We present an analysis of Kr I 1236 line measurements from 50 sight lines in the Hubble Space Telescope Space Telescope Imaging Spectrograph and Goddard High Resolution Spectrograph data archives that have sufficiently high resolution and signal-to-noise ratio to permit reliable krypton-to-hydrogen abundance ratio determinations. The distribution of Kr/H ratios in this sample is consistent with a single value for the ISM within 5900 pc of the Sun, log(Kr/H) = -9.02+/-0.02, apart from a rough annulus from between about 600 and 2500 pc distant. The Kr/H ratio toward stars within this annulus is elevated by approximately 0.11 dex, similar to previously noted elevations of O/H and Cu/H gas-phase abundances beyond about 800 pc. A significant drop in the gas-phase N/O ratio in the same region suggests that this is an artifact of nucleosynthetic history. Since the physical scale of the annulus' inner edge is comparable to the radius of the Gould Belt and the outer limit of heliocentric distances where the D/H abundance ratio is highly variable, these phenomena may be related to the Gould Belt's origins.

Aims. In order to gain new insight into the unidentified identity of the diffuse interstellar band (DIB) carriers, this paper describes research into possible links between the shape of the interstellar extinction curve ( including the 2175 angstrom bump and far-UV rise), the presence or absence of DIBs, and physical and chemical conditions of the diffuse interstellar medium ( gas and dust) in the Small Magellanic Cloud (SMC).Methods. We searched for DIB absorption features in VLT/UVES spectra of early-type stars in the SMC whose reddened lines-of-sight probe the diffuse interstellar medium of the SMC. Apparent column density profiles of interstellar atomic species (Na I, KI, Ca II and Ti II) are constructed to provide information on the distribution and conditions of the interstellar gas.Results. The characteristics of eight DIBs detected toward the SMC wing target AzV 456 are studied and upper limits are derived for the DIB equivalent widths toward the SMC stars AzV 398, AzV 214, AzV 18, AzV 65 and Sk 191. The amount of reddening is derived for these SMC sightlines, and, using R-V and the Hi column density, converted into a gas-to-dust ratio. From the atomic column density ratios we infer an indication of the strength of the interstellar radiation field, the titanium depletion level and a relative measure of turbulence/ quiescence. The presence or absence of DIBs appears to be related to the shape of the extinction curve, in particular with respect to the presence or absence of the 2175 angstrom feature. Our measurements indicate that the DIB characteristics depend on the local physical conditions and chemical composition of the interstellar medium of the SMC, which apparently determine the rate of formation (and/or) destruction of the DIB carriers. The UV radiation field ( via photoionisation and photo-destruction) and the metallicity (i.e. carbon abundance) are important factors in determining diffuse band strengths which can differ greatly both between and within galaxies.

We report interstellar FeII and SiII column densities toward six translucent sight lines (A_V >~ 1) observed with the Space Telescope Imaging Spectrograph (STIS). The abundances were determined from the absorption of SiII] at 2335 Angstroms, and several weak Fe transitions including the first reported detections of the 2234 Angstrom line. We derive an empirical f-value for the FeII 2234 Angstrom transition of log(f lambda) = -1.54 +/- 0.05. The observed sight lines sample a variety of extinction characteristics as indicated by their R_V values, which range from 2.6 - 5.8. The dust-phase abundances of both Si and Fe are positively correlated with the small-grain population (effective radii smaller than a few hundred micron) toward the targets. The physical conditions along the sight lines suggest that this relationship may be due to differences in the survival of small particles in some interstellar environments. The chemical composition of the small grains could either resemble dust mantles or be silicate rich.