1. AND HIGH H2 IONIZATION RATE
CENTRAL 300 PC OF THE GALAXY PROBED BY THE INFRARED SPECTRA OF H3+ AND CO I. PREDOMINANCE OF WARM AND DIFFUSE GAS
AND HIGH H2 IONIZATION RATE
Takeshi Oka, Department of Astronomy and Astrophysics
and Department of Chemistry, University of Chicago, Chicago, IL
Tom Geballe, Gemini Observatory, Hilo, HI
Miwa Goto, Centre for Astrochemical Studies,
Max Planck Institute of Extraterrestrial Physics, Garching, Germany
Tomonori Usuda, National Astronomical Observatory of Japan Nick Indriolo, Department of Astronomy,
University of Michigan, Ann Arbor, MI
71st International Symposium on Molecular Spectroscopy
University of Illinois Urbana, Champaign, June 22, 2016

2. The Central Molecular Zone (CMZ) The treasure house of H3+
Geballe, Phil. Trans. Roy. Soc. Lond. (2000)
D CMZ ~ 10-2DGalaxy
VCMZ ≤ 10-5 VGalaxy
NCMZ ~ 10-1NGalaxy
CMZ
300 pc
Mezger, Duschl, Zylka, A&AR 7, 289 (1996)

3. Bright dust-embedded stars with smooth continuum
GCS3-2 Oka et al. (2005)
8 stars Goto et al. (2008)
Geballe Oka (2010)

4. H3+ probes deep into the Central Molecular Zone
Depth meter

5. CMZ is not as opaque as previously thought
Morris & Serabyn, ARA&A 1996
N = n f R ≥ (104 cm-3)(0.1)(150 pc) = 4.6 × 1023 cm-2 ~ N(H2)
5 × 1019 cm-2 ~ N(CO)
Sgr E
Sgr C
Star
N(CO) % α
0.22
0.04 β
0.01 γ
0.08
0.02 δ
0.13
0.03
IRS21
0.05
IRS3
0.25
IRS16NE
0.15
IRS1W
0.28
0.06
NHS21
NHS22
NHS42
0.07
NHS25 η
GCS3-2
GCS4
FMM362
0.11 θ ι
2.7
0.54
Sgr A
Sgr B

6. Warm (T ~ 250 K) and Diffuse (n < 100 K) Gas
H3+
Thermometer
Densitometer

7. High cosmic ray ionization rate
H3+ production: H2 → H2+ + e̶ H2+ + H2 → H + H3+
𝑓 H 2 = 2𝑛( H 2 ) 𝑛(H) + 2𝑛( H 2 )
dilution: H2+ + H → H2 + H+　　　　
H3+ destruction: H3+ + e̶ → H + H + H, H2 + H
1.6×10-4
8×10-8 cm3 s-1 3
H3+
Dosimeter
(2‒6) ×1015 cm-2
< (2/3)2
>
(4 ‒ 12) ×105 cm s-1
For L = 150 pc ζ > (1 ‒ 3) × s-1
ζ = (1 ‒ 11) × s-1
Le Petit, Ruaud, Bron, Godard, Roueff, Languignon, Le Bourlot, A&A, 585 (2016)

8. High L ‒ high volume filling factor
High velocity dispersion
Saturation with ζ
Le Petit, Ruaud, Bron, Godard, Roueff, Languignon, Le Bourlot, A&A, 585 (2016)

9. Conclusions Corollary
Low volume filling factor of dense clouds f ≥ 0.1 → f < 0.01
Agrees with AV ~ 20 ‒ Cotera et al. (2000), Schultheis et al. (2009)
Predominance of warm (T ~ 250 K) diffuse (n < 100 cm-3) gas f ≥ 0.5
High cosmic ray ionization rate of H2 ζ ~ 5 × s-1
Corollary
Molecular mass in dense clouds ~ diffuse clouds ~ 107 M
X-ray emitting ultrahot (T ~ 108 K) does not exist

10. High Ionization Rate: ζ ~ 5 × 10=15 s-1
1.6×10-4
8×10-8 cm3 s-1 3
(2‒6) ×1015 cm-2
< (2/3)2
>
(4 ‒ 12) ×105 cm s-1
For L = 150 pc ζ > (1 ‒ 3) × s-1
From X-rays and γ-rays
ζ ~ 5 × s-1 Yusef-Zadeh, Muno, Wardle, Lis, ApJ 656, 847 (2007)
ζ ~ 1 × s Tatischeff, Decourschelle, Maurin, A&A 546, 88 (2012)
ζ ~ 5 × s Yusef-Zadeh, Hewitt, Wardle, Tatischeff, Robert, Cotton,
Uchiyama, Nobukawa, Tsuru, Heinke, Royster, ApJ