Steven R. Federman

We present the results of an effort to derive interstellar gas-phase C ii abundances along the lines of sight toward ρ Oph A and B. Our analysis is based on high-resolution near-ultraviolet and far-ultraviolet archival spectra acquired with the Space Telescope Imaging Spectrograph on the Hubble Space Telescope. Column densities of C ii are derived both from fits to the weak C ii] λ2325 intersystem transition and from fits to the damping wings of the strong C ii λ1334 line. We find that the results from the weak-line and strong-line determinations agree with each other remarkably well for both sight lines, demonstrating the reliability of the f-values of the C ii transitions. Furthermore, the gas-phase C abundance that we obtain for ρ Oph A is in very good agreement with previous determinations of interstellar C abundances from measurements of the weak C ii] λ2325 line. By demonstrating the reliability of the damping wing fitting technique for the C ii λ1334 line, our analysis opens the door to future surveys of interstellar C abundances using the same methodology.

Aims . The runaway star AE Aur (HD 34078) is currently crossing the molecular cloud associated with the IC 405 nebula. Previous observations have revealed large time variations of CH ⁺ and CH absorption lines in AE Aur spectra, as the line of sight drifts through the foreground gas at a rate of about 20 au/year. We wish to extend the study of these changes over a longer interval, 2003-2023, and determine their properties (e.g. amplitude, timescales) with greater accuracy to obtain constraints on the processes involved in the production of CH ⁺ and CH. Methods . We have obtained new high resolution spectra for AE Aur and for ζ Per, a bright reddened star that we used as a reference to check the instrumental stability. We focused not only on the strength of the lines (equivalent widths) but also on line widths, line centroids, and full velocity distributions in order to more thoroughly characterise the time variations of absorption lines. We also include CN in our analysis of variations in addition to CH ⁺ and CH. Results . The large and complex CH ⁺ and CH column density variations (by up to −35 and −20% respectively, in our measurements) on scales ranging from a few au to about 400 au are confirmed and appear consistent with those observed by other authors. A significant decrease of CH ⁺ and CH line widths (by −19 and −13%) was also detected, together with a change in the CH ⁺ centroid velocity of ≈-1 km s ⁻¹ . Finally, wings extending ±12 km s ⁻¹ around CH ⁺ and CH line centre are clearly present. We show that most of the H ₂ gas towards AE Aur (about 70%) is associated with two dense globulettes previously imaged in CO and located close to the star. Despite the large amount of highly excited H ₂ , CH ⁺ production in the photodissociation region (PDR) facing the star is far too small to account for the observed CH ⁺ column density. In line with a recently published model, we propose that CH ⁺ is produced in a thin layer at the interface between a dense photoevaporating globulette and the ambient warm neutral medium (WNM) gas. The observed time variations were used to set constraints on the thickness of this CH ⁺ -rich shell (≈ 7 × 10 ⁻³ pc) and on its density structure (sharp decrease of n (CH ⁺ ) outwards). Such a picture appears to provide a simple and coherent description of both the CH ⁺ and CH line profiles and their time variations towards AE Aur.

Context. Recent spectroscopic measurements have revealed absorption from higher rotational levels in C₂ than previous observations. These improvements are accompanied by the availability of updated radiative and collisional data. Aims. We revisit the density and radiation field intensity diagnostics provided by the observations of many rotational levels of interstellar C₂ and extensive molecular information. Methods. We built an excitation model of C₂ without spatial structure, including levels up to J = 34 where updated radiative and collisional excitation data are introduced as well as excitation by chemical formation. Results. We confirm the importance of the recent collisional excitation rate coefficients of C₂ by molecular H₂. We show that the new higher level observations cannot be explained by the standard balance between collisional excitation and radiative transitions. We propose that chemical excitation at formation provides a plausible mechanism to explain the observed high excitation ofC₂. In addition, it allows us to lift the degeneracy of the density over radiation field strength parameter in the excitation model. Conclusions. A 0D model remains limited and it is highly desirable to use a full Photon Dominated Region (PDR) model, which includes all excitation processes introduced here and full chemical and thermal balance.

We present observations of interstellar Na i and Ca ii absorption lines toward HD 47240, which probes shocked gas in the Monoceros Loop supernova remnant. The observations were obtained at five different epochs spanning the time period from 2017 January to 2024 September. Time variable Na i and Ca ii absorption is observed in two high-velocity components at +63 and +73 km s⁻¹. These are the same high-velocity features discussed by C. Dirks & D. M. Meyer, who found significant variations in Na i column densities between 2006 January and 2014 January. We discuss our new observations of temporal variability in the context of these and other previous observations of the high-velocity gas toward HD 47240. We speculate that the column density variations are a result of small-scale structure in the post-shock flow passing in front of the background star.

Context. Recent spectroscopic measurements have revealed absorption from higher rotational levels in C$_2$ than previous observations. These improvements are accompanied by the availability of updated radiative and collisional data. Aims. We revisit the density and radiation field intensity diagnostics provided by the observations of many rotational levels of inter- stellar C$_2$ and extensive molecular information. Methods. We built an excitation model of C2 without spatial structure, including levels up to J= 34 where updated radiative and collisional excitation data are introduced as well as excitation by chemical formation. Results. We confirm the importance of the recent collisional excitation rate coefficients of C$_2$ by molecular H$_2$. We show that the new higher level observations cannot be explained by the standard balance between collisional excitation and radiative transitions. We propose that chemical excitation at formation provides a plausible mechanism to explain the observed high excitation of C$_2$. In addition, it allows us to lift the degeneracy of the density over radiation field strength parameter in the excitation model. Conclusions. A 0D model remains limited and it is highly desirable to use a full Photon Dominated Region (PDR) model, which includes all excitation processes introduced here and full chemical and thermal balance.

We present the results of a sensitive search for high-velocity gas in interstellar absorption lines associated with the Cygnus Loop supernova remnant (SNR). We examine high-resolution high signal-to-noise ratio optical spectra of six stars in the Cygnus Loop region with distances greater than ∼700 pc. All stars show low-velocity Na i and Ca ii absorption. However, only one star, HD 198301, exhibits high-velocity Ca ii absorption components, at velocities of +62, +82, and +96 km s⁻¹. The distance to this star of ∼870 pc helps to constrain the distance to the receding edge of the Cygnus Loop’s expanding shock front. One of our targets, HD 335334, was previously thought to exhibit high positive and high negative velocity interstellar Na i and Ca ii absorption. This was one factor leading Fesen et al. to derive a distance to the Cygnus Loop of 725 ± 15 pc. However, we find that HD 335334 is in fact a double-line spectroscopic binary and shows no evidence of high-velocity interstellar absorption. As such, the distance to HD 335334 cannot be used to constrain the distance to the Cygnus Loop. Our detection of Ca ii absorption approaching 100 km s⁻¹ toward HD 198301 is the first conclusive detection of high-velocity absorption from a low-ionization species associated with the Cygnus Loop SNR. A large jump in the Na i column density toward BD+31 4218, a star located beyond the northwestern boundary of the Cygnus Loop, helps to constrain the distance to a large molecular cloud complex with which the Cygnus Loop is evidently interacting.

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 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 a comprehensive examination of interstellar P and Cl abundances based on an analysis of archival spectra acquired with the Space Telescope Imaging Spectrograph of the Hubble Space Telescope and the Far Ultraviolet Spectroscopic Explorer. Column densities of P ii, Cl i, and Cl ii are determined for a combined sample of 107 sight lines probing diffuse atomic and molecular gas in the local Galactic interstellar medium. We reevaluate the nearly linear relationship between the column densities of Cl i and H₂, which arises from the rapid conversion of Cl⁺ to Cl⁰ in regions where H₂ is abundant. Using the observed total gas-phase P and Cl abundances, we derive depletion parameters for these elements, adopting the methodology of Jenkins. We find that both P and Cl are essentially undepleted along sight lines showing the lowest overall depletions. Increasingly severe depletions of P are seen along molecule-rich sight lines. In contrast, gas-phase Cl abundances show no systematic variation with molecular hydrogen fraction. However, enhanced Cl (and P) depletion rates are found for a subset of sight lines showing elevated levels of Cl ionization. An analysis of neutral chlorine fractions yields estimates for the amount of atomic hydrogen associated with the H₂-bearing gas in each direction. These results indicate that the molecular fraction in the H₂-bearing gas is at least 10% for all sight lines with log N ( H 2 ) ≳ 18 and that the gas is essentially fully molecular at log N ( H 2 ) ≈ 21 .

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.