A Reexamination of Phosphorus and Chlorine Depletions in the Diffuse Interstellar Medium*

Abstract

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 .

The Distribution of Metallicities in the Local Galactic Interstellar Medium*

“…If the column density measurements pertain to just a single line of sight, then the sight line is also indicated. It is important to note that, while over 40 references are listed in Table 3, approximately two- (2011( , 2018( , 2023( ), and Jenkins (2019. Another 12% of the measurements are newly derived column densities obtained in this work (Section 3.2).…”

“…Later, Ritchey et al (2018), adopting the Jenkins (2009) methodology, derived depletion parameters for B, Ga, As, Cd, Sn, and Pb and reevaluated the depletion parameters for O, Ge, and Kr using larger samples of elemental abundance measurements. Most recently, Ritchey et al (2023) redetermined the values of the depletion parameters for the elements P and Cl, both to account for recent updates in the oscillator strengths of the relevant P II and Cl I transitions and to correct for a deficiency in the Cl analysis presented in Jenkins (2009). In the original analysis, Jenkins (2009) considered only Cl II column densities in deriving total Cl abundances.…”

“…However, Cl is unique among most other elements commonly studied in the ISM in that either its neutral or its singly ionized form can dominate the total abundance depending on the amount of molecular hydrogen present along the line of sight (e.g., Jura & York 1978). Ritchey et al (2023) included column densities of both Cl I and Cl II in their derivations of total Cl abundances and Cl depletion factors.…”

“…In compiling our database of high-quality column density measurements from the literature, we focus on the following elements: B, C, N, O, Mg, Si, P, Cl, Ti, Cr, Mn, Fe, Ni, Cu, Zn, Ga, Ge, As, Kr, Cd, Sn, and Pb. All 22 of these elements have been analyzed in a way consistent with the methodology devised by Jenkins (2009), and all have values of the depletion parameters A X , B X , and z X tabulated in the literature (Jenkins 2009;Ritchey et al 2018Ritchey et al , 2023. 8 The inclusion of as many different elements as possible in our least-squares linear fits helps to ensure that the relative ISM metallicities we derive are robust.…”

“…Many investigations of elemental abundances based on observations of interstellar absorption lines fall into one of two categories. Some studies focus on one or several elements and obtain abundances for a large, diverse set of interstellar sight lines (e.g., Cartledge et al 2004Cartledge et al , 2006Ritchey et al 2011Ritchey et al , 2018Ritchey et al , 2023Jenkins 2019). Others focus on a single line of sight and derive abundances for as many atomic (and molecular) species as possible (e.g., Sonnentrucker et al 2002Sonnentrucker et al , 2003Welty et al 2020).…”