Ubiquitous Argonium, ArH+, in the Interstellar Medium P. Schilke, Holger S. P. Müller, C. Comito, Á. Sánchez-Monge, D. A. Neufeld, N. Indriolo, E. A. Bergin, D. C. Lis, M. Gerin, J. H. Black, M. G. Wolfire, J. C. Pearson, K. M. Menten, B. Winkel TF02, 69th ISMS, Urbana-Champaign, IL, USA, June 16–20, 2014

What is Argonium ? ArH+, 1Σ+, isoelectronic to HCl Formation: Ar+ + H2 → ArH+ + H Destruction (e.g.): ArH+ + H2 → Ar + H3+ Isotopic ratio: 36Ar : 38Ar : 40Ar terrestrial: 84.2 : 15.8 : 25018.8 (from decay of 40K) solar/ISM: ~84.6 : ~15.4 : 0.025 some basic information on formation and destruction required and useful. useful: mention additional obs. of OH+, but no details – except: nothing else. 36ArH+ toward Crab Nebula SNR: J = 1 – 0 & 2 – 1 in emission (w. OH+ N = 1 – 0); SPIRE/Herschel M. J. Barlow et al., Science 342 (2013) 1343

On the Spectroscopy of ArH+ 40ArH+; rotational spectroscopy: K. B. Laughlin et al., PRL 58 (1987) 996: J" = 1 J. M. Brown et al., JMSp 128 (1988) 587: J" = 2 – 6 D. J. Liu et al., JCP 87 (1987) 2442: J" = 20 – 24; v ≤ 4 (MIR) 40ArD+; rotational spectroscopy: W. C. Bowman et al., JCP 79 (1983) 2093: J" = 1 (+ 36ArD+ & 38ArD+ H. Odiashima et al., JMSp 195 (1999) 356: J" = 2 – 14 basic information from 40/1; info on 36 & 38 from Bowman; rovib info for balancing the vibrational corrections/parameters rovibrational spectroscopy: J. W. Brault & S. P. Davis, Phys. Sript. 25 (1982) 268: 40ArH+ J. W. C. Johns, JMSp 106 (1984) 124: 40ArH+, 40ArD+ R. R. Filueira & C. E. Blom, JMSp 127 (1988) 279: 36ArH+, 38ArH+ M. Cueto et al., ApJ 783 (2014) L5: 36ArH+, 38ArH+

ArH+ toward Sagittarius B2(M) – HIFI Line Survey (“toward“ is important here as will be seen later.) methanimine is almost negligible ….; absence of absorption at velocity associated with Sgr B2(M) as nice indication – and contrast with next slide. The observations are from the HEXOS GT program and involve line surveys of Sgr B2(M) & (N) as well as others ...

massive star-forming regions Absorption toward Sgr B2(M) massive star-forming regions as background sources with approximate origins Sagittarius B2(M) strong absorption at source velocity

Interstellar Chemistry of ArH+ I Ar+ + H2 → ArH+ + H exothermic (endo. e.g. for C+, N+) Ar+ + H2 → Ar + H + H+ endothermic (exo. e.g. for Ne+) ArH+ + H → Ar + H2+ endothermic ArH+ + hν ineffective (in contrast to HCl) ArH+ + e– very slow (in contrast to, e.g., NeH+) ArH+ + X → Ar + HX+ exothermic for H2, O, etc. first 5 in favor of ArH+, next one makes ArH+ avoid dense media; last one on cosmic ray ionization; ~ one order of magnitude IP(Ar) > IP(H); CRI(Ar) >> CRI(H) → ArH+ CRI rate indicator

Interstellar Chemistry of ArH+ II Highest ArH+ fraction: n(H2)/n(H) ≈ 10–3 – 10–4 one model; farther into the cloud, H2 incxreases and Ar+ and ArH+ decrease

Lots of H versus GC and Sgr B2(M), in part with considerable structure; some H2O+ at Sgr B2(M), ArH+ NOT; rather smooth curves wrt. HF: possibly considerable absorption from interarm gas, HF mainly from sprial arms clouds. High fraction of ArH+/H !!

38ArH+ and 36ArH+ in Sgr B2(M) 36ArH+ : 38ArH+ ≈ 5.5 5.33 is the terrestrial value; ISM: ~5.5. Absorption line is due to methanol ... CH3OH, 4–2 ← 3–1

Observing geometry PRISMAS Massive star forming regions as background sources for absorption spectroscopy – different molecular clouds along the sightlines Sgr B2(N) is similar to Sgr B2(M) – except: more emission lines ... NASA/SSC R. Monje

ArH+ in PRISMAS Sources green line simulation for one HFS component dashed line / gray area: velocity associated with continuum source itself. in many cases it looks like no ArH+ (or at least very little) toward continuum sources.

ArH+ ubiquitous toward W51e, but not directly toward the contiumuum source. Ansence of ArH+ at continuum source velocity not always as clear.

Conclusion ArH+ is a very good tracer of the very atomic diffuse ISM much better than atomic H (which also resides in much denser gas) is the molecule that avoids molecular clouds even better with OH+ and H2O+ tracing gas with n(H2)/n(H) ~ 0.1 ArH+ is a good probe into cosmic ray ionization rate instead of or together with OH+, H2O+, or H3+ see also: Schilke et al., Astron. Astrophys. 566 (2014) A29.

Outlook Observability with ALMA: – very difficult for galactic sources; proximity to H2O, 532 – 441 (620.7 GHz) cycle 2: B. Swinyard et al.: 36Ar/38Ar ratio in ArH+ toward Crab Nebula – better for extragalactic sources cycle 2: H. S. P. Müller et al.: search for 36ArH+ and 38ArH+ at moderate redshift Better with the Stratospheric Observatory For Infrared Astronomy: – option for a receiver on the German REceiver At Terahertz frequencies around H2O lines at 567, 621, ... 1113 GHz