Ubiquitous argonium (ArH⁺) in the diffuse interstellar medium: A molecular tracer of almost purely atomic gas

Abstract

Aims. 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 confidence of the assignment to ArH + is discussed, and the column densities are determined toward several lines of sight. The results are then discussed in the framework of chemical models, with the aim of explaining the observed column densities. Methods. We fitted the spectral lines with multiple velocity components, and determined column densities from the line-to-continuum ratio. The column densities of ArH + were compared to those of other species, tracing interstellar medium (ISM) components with different H 2 abundances. We constructed chemical models that take UV radiation and cosmic ray ionization into account. , and HF column densities promises to be a faithful tracer of the distribution of the H 2 fractional abundance by providing unique information on a poorly known phase in the cycle of interstellar matter and on 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 characterizing the ISM. Paradoxically, the ArH + molecule is a better tracer of almost purely atomic hydrogen gas than H itself, since H can also be present in gas with a significant molecular content, but ArH + singles out gas that is >99.9% atomic.

Detection of extragalactic argonium, ArH+, toward PKS 1830−211

“…Such an analysis is currently under way for PKS 1830−211 and will be published separately. Schilke et al (2014) and Schilke et al (2013).…”

“…The newly detected hydride species are SH + (Menten et al 2011), OH + (Wyrowski et al 2010), H 2 O + (Ossenkopf et al 2010), H 2 Cl + , HCl + (De Luca et al 2012), SH , and, most recently, ArH + (Barlow et al 2013;Schilke et al 2014). OH + and H 2 O + were also observed extensively in extragalactic sources (van der Werf et al 2010;Weiß et al 2010;González-Alfonso et al 2013).…”

“…More recently, Schilke et al (2014) reported on their early detections of the ArH + J = 1−0 transition in absorption toward six bright continuum sources using Herschel-HIFI. The absorption patterns were unique in their appearances; ArH + showed up in all velocity components associated with diffuse foreground molecular clouds, but was conspicuously absent at velocities related to the sources themselves.…”

“…Although ArH + abhors molecular clouds, it does rely upon a small amount of H 2 for its existence. Moreover, with its specificity to the ISM with a molecular fraction of 10 −3 , it is a much better tracer of the almost purely atomic ISM than the 21 cm hyperfine structure line of atomic hydrogen, because the latter is also seen in the ISM with a molecular fraction of 0.01 or even larger than 0.5 (Schilke et al 2014). Argonium may also be used to infer ζ in combination with other tracers.…”

Quantum astrochemical spectroscopy

“…In any case, ArH 1 is now known to be relatively abundant in the ISM [105,106] and is even used as a tracer of nearly pure atomic gas. [107] Françoise Pauzat, Yves Ellinger, and coworkers have been utilizing quantum chemical tools to analyze noble gas compounds related to astrochemistry for many years. [108][109][110][111] However, the advent of ArH 1 everywhere in the ISM has created a new impetus for this study.…”