1. Abundance and Distribution of the HNCS/HSCN isomer pair
In Sagittarius B2
Gilles Adande, Lucy Ziurys, DeWayne Halfen
Department of Chemistry, Department of Astronomy, Steward Observatory and Arizona Radio Observatory, University of Arizona
Donghui Quan, Eric Herbst
The chemical physics program, Departments of Chemistry, Astronomy
and Physics, The Ohio State University, Columbus

2. metastable isomers • ISM convenient “laboratory” for transient species
E (cm-1)
21825
• ISM convenient “laboratory” for
transient species
• Understanding interstellar chemistry
formation pathways, branching ratios etc…
• Tracers of ISM physical conditions
HSCN
4127
HNCS
• HNCO/HOCN in cold gas
• Large organic molecules only in hot cores
• What about HNCS/HSCN ?

3. ARO 12 m 12 m at Kitt Peak • ALMA type Band 3 receiver (84-116 GHz)
Species
Frequency
(MHz)
Transition
J′Ka′,Kc′  JKa,Kc
Sgr B2
HSCN
80,8  70,7
90,9  80,8
HNCS
TMC-1
70,7  60,6
80,8  70,8
12 m at Kitt Peak
• ALMA type Band 3 receiver
( GHz)
• 3 mm low (68-90 GHz)

4. Mapping Observations of Sgr B2
Hot cores
n(H2) < 104 cm-3
T < 20 K
n(H2) ~ 107 cm-3
T ~ K
~ 15 pc
Approximate beam size ~ 1’
Sgr B2 (M)
n(H2) ~ 105 cm-3
T ~ K
Hot ring
T ~ K

5. HNCS: J = 80,8  70, Sgr B2
CCS
Δδ (arcmin)
Δα (arcmin)

6. HSCN: J = 80,8  70, Sgr B2
Δδ (arcmin)
Δα (arcmin)

7. • Consistent lineshapes
90,9  80,8
• Extended emission
• Not hot core molecule
• Consistent lineshapes
• Gas phase processes
major contribution

8. HNCS
HSCN

9. Abundances • LTE approximation • Optically thin • Trot = 20 K
• Average abundance of 2 transitions consistent to 10%
HSCN: 1.2×1012 – 1.3×1013 cm-2 across the cloud
HNCS: 6.3×1012 – 4.3×1013 cm-2

10. HNCS/HSCN ratio • ~ 1.7 - 10.1 across Sgr B2Da Dd
HNCS/HSCN
Sgr B2 1 -2
4.9
3.7 -1
5.3
3.5
2.6
4.6
3.3
2.5
3.1
1.7
2.8
3.4 2
8.5
5.8
5.9 3
10.1
7.3
HNCS/HSCN ratio
• ~ across Sgr B2
• Smaller toward hot cores
• Higher in the envelope

11. Observations in TMC-1
Kaifu et al, 2004

12. • 1st detection of HNCS in dark clouds
• T ~ 10K
• Column density
~ 7 – 8 × 1011 cm-2
• HNCS/HSCN ratio
HNCS
~ 1.3
HSCN

13. HNCSH+ + e-  HNCS / HSCN + H
Formation of HNCS/HSCN ?
• Accretion on grains
• Gas Phase ion-molecule formation
NCS+ + H2  HNCS+ + H H H
HNCS+ + H2  HNCSH+ / H2NCS+ + H
NCS
HNCSH+ + e-  HNCS / HSCN + H
HSCN
HNCS
H2NCS+ + e-  HNCS + H
Thermal + non thermal
desorption
• Destruction with ions and neutrals

14. Detailed gas-grain model
• 3 phase model: warm
envelope of hot cores
• 1 phase model: cold
clouds, cold halo

15. Conclusions • In Sgr B2 • In TMC-1
• Extended, consistent HNCS and HSCN emission
• HNCS/HSCN ~ 1 near hot cores
• HNCS/HSCN up to 10 in moderately dense gas
• In TMC-1
• HNCS/HSCN ~ 1 in dark clouds
• Gas phase process major contribution
• Possible gas grain influence
• HNCSH+, reaction rate etc… await measurement

16. Acknowledgements • Professor Lucy Ziurys • Dr. DeWayne Halfen
• Dr. Donghui Quan
• Professor Eric Herbst
• Dr. Emmy Tenenbaum, Lindsay Zack, Jessica Dodd, Ming Sun
Matthew Bucchino, Jie Min, Robin Pulliam, Brent Harris
• Funding: NASA-NSF