Ubiquitous argonium (ArH${}^{+}$) in the diffuse interstellar medium -- a molecular tracer of almost purely atomic gas

Abstract

We describe the assignment of a previously unidentified interstellar absorption line to ArH${}^{+}$ and discuss its relevance in the context of hydride absorption in diffuse gas with a low H${}_2$ fraction. The column densities along several lines of sight are determined and discussd in the framework of chemical models. The column densities of ArH${}^{+}$ are compared to those of other species, tracing interstellar medium (ISM) components with different H${}_2$ abundances. Chemical models are constructed, taking UV radiation and cosmic ray ionization into account. Due to the detection of two isotopologues, ${}^{36}$ArH${}^{+}$ and ${}^{38}$ArH${}^{+}$, we are confident about the carrier assignment to ArH${}^{+}$. NeH${}^{+}$ is not detected with a limit of [NeH${}^{+}$]/[ArH${}^{+}$] $\le$ 0.1. The derived column densities agree well with the predictions of chemical models. ArH${}^{+}$ is a unique tracer of gas with a fractional H${}_2$ abundance of $10^{-4}-10^{-3}$ and shows little correlation with H${}_2$O${}^{+}$, which traces gas with a fractional H${}_2$ abundance of $\approx$0.1. A careful analysis of variations in the ArH${}^{+}$, OH${}^{+}$, H${}_2$O${}^{+}$ and HF column densities promises to be a faithful tracer of the distribution of the H${}_2$ fractional abundance, providing unique information on a poorly known phase in the cycle of interstellar matter, its transition from atomic diffuse gas to dense molecular gas traced by CO emission. Abundances of these species put strong observational constraints upon magnetohydrodynamical (MHD) simulations of the interstellar medium, and potentially could evolve into a tool to characterize the ISM. Paradoxically, the ArH${}^{+}$ molecule is a better tracer of \new{almost} purely atomic hydrogen gas than H{\sc i} itself, since H{\sc i} can also be present in gas with a significant molecular content, but ArH${}^{+}$ singles out gas that is $>99.9$\% atomic.

12 pages, 11 figures, Astronomy and Astrophysics, accepted revised version: previous version had column density of SgrB2 sources wrong

References in corpus (10)

• Trigonometric Parallaxes of Massive Star Forming Regions: VI. Galactic Structure, Fundamental Parameters and Non-Circular Motions
• Early Science with SOFIA, the Stratospheric Observatory for Infrared Astronomy
• GREAT: the SOFIA high-frequency heterodyne instrument
• Gas flow models in the Milky Way embedded bars
• Models of turbulent dissipation regions in the diffuse interstellar medium
• Modeling and Analysis Generic Interface for eXternal numerical codes (MAGIX)
• A simple prescription for simulating and characterizing gravitational arcs
• The Fractional Ionization of the Warm Neutral Interstellar Medium
• Structural Variation of Molecular Gas in the Sagittarius Arm and Inter-Arm Regions
• Widespread Rotationally-Hot Hydronium Ion in the Galactic Interstellar Medium

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• Modelling the ArH${}^{+}$ emission from the Crab Nebula
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• Detection of CH+, SH+, and their 13C- and 34S- isotopologues toward PKS1830-211
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• Revised spectroscopic parameters of SH${}^{+}$ from ALMA and IRAM 30 m observations
• The HIFI spectral survey of AFGL 2591 (CHESS). II. Summary of the survey
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