1. 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

2. 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: : : (from decay of 40K)
solar/ISM: ~ : ~ :
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

3. 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+

4. 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 ...

5. 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

6. 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

7. 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

8. 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 !!

9. 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

10. 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

11. 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.

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

13. 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.

14. 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, GHz