1. The Interstellar Detection of HSCN in Sgr B2(N)
DeWayne T. Halfen, L. M. Ziurys
Departments of Chemistry and Astronomy, Steward Observatory, Arizona Radio Observatory, University of Arizona
M. C. McCarthy, C. Gottlieb, P. Thaddeus
Harvard-Smithsonian Center for Astrophysics, and School of Engineering & Applied Sciences, Harvard University
June 24, 2010

2. Isomer Chemistry in the ISM
Some isomer pairs now found in the interstellar medium (ISM)
By examining isomer ratio, insight into formation mechanism
HCN and HNC found in many dense molecular clouds and circumstellar envelopes (Hirota et al. 1998; Ziurys & Turner 1986)
HNC – 5600 K higher in energy than HCN
Yet HNC/HCN isomer ratio ranges from – 4.4
Both species created via electronic recombination of HCNH+
Larger isomers also observed
Methyl formate, acetic acid, glycolaldehyde (C2H4O2)
Methyl formate dominant species, but unclear three species formed by common mechanism

3. HNCS and HSCN Isomers HNCS known, abundant interstellar molecule
Detected by Frerking et al. (1979) towards Sgr B2(OH)
HSCN metastable isomer of HNCS
Lies 3000 K higher in energy than HNCS
Rotational spectrum of HSCN recently measured in Harvard laboratory (Brünken et al. 2009)
18 transitions recorded from GHz

4. Astronomical Search for HSCN
Spectrum of HSCN searched for towards Sgr B2(N)
Giant molecular cloud near center of the Milky Way
Examined our spectral-line survey data taken with the Arizona Radio Observatory (ARO) 12m on Kitt Peak
Observations at 2 and 3 mm ( GHz)
HNCS transitions also studied for comparison
KP 12m

5. 3 mm data taken with ALMA Band 3 receiver (84-116 GHz)
Uses new ALMA-type sideband-separating (2SB) mixers
Order of magnitude improvement in receiver sensitivity, bandwidth, and stability
Raw data only need simple baseline taken out
RAW data
HSCN
ALMA Band 3
mixer

6. Eight transitions of HSCN detected towards Sgr B2(N)
Almost linear species: only Ka = 0 transitions are strong, favorable lines
Eight transitions of HSCN detected towards Sgr B2(N) H C N S H C N S

7. 5 clean transitions, 1 blended, 2 contaminated
HSCN Sgr B2(N)
HSCN Sgr B2(N)
Eight transitions of HSCN observed
5 clean transitions, 3 others contaminated
Halfen et al. 2009
5 clean transitions, 1 blended, 2 contaminated

8. 6 transitions of HNCS also present in survey
HNCS Sgr B2(N)
6 transitions of HNCS also present in survey
2 clean lines, rest blended with emission from other species
Lines from HNCO, HOCN also detected
Source velocity and line width consistent with HSCN and HNCS
HNCS

9. Intensities from 50-110 mK, consistent VLSR, DV1/2
Freq. (MHz)
J′Ka,Kc  J″Ka,Kc
Eu(K)
qb(″)
TR*(K)
DV1/2(km/s)
VLSR(km/s)
Comment
HSCN
60,6  50,5
11.5 91
0.069 ± 0.014
23.5 ± 4.4
69.7 ± 4.4
Good match
70,7  60,6
15.4 78
0.073 ± 0.006
20.1 ± 3.7
64.8 ± 3.7
80,8  70,7
19.8 68
0.109 ± 0.003
16.1 ± 3.3
66.6 ± 3.3
90,9  80,8
24.7 61
0.063 ± 0.004
19.6 ± 2.9
66.3 ± 2.9
100,10  90,9
30.2 55
0.074 ± 0.007
20.5 ± 2.6
67.2 ± 2.6
120,12  110,11
42.9 46
0.050 ± 0.004
20.0b
66.0b
Partially Resolved
130,13  120,12
50.1 42
~0.07
No discernible peak
140,14  130,13
57.8 39
HNCS
15.7 77
~0.08
Blended with HC3N
20.2 67
0.077 ± 0.003
17.8 ± 3.2
68.3 ± 3.2
25.3 60
0.114 ± 0.003
20.4 ± 2.8
63.8 ± 2.8
43.9 45
~0.04
Edge of 0.10 K line
51.2 41
~0.09
Edge of 0.13 K line
59.1 38 …
Contaminated
Intensities from mK, consistent VLSR, DV1/2

10. f(X/H2) = 1.1 x 10-11
f(X/H2) = 4.5 x 10-12

11. HNCSH+ + e-  HNCS or HSCN + H
Formation in Sgr B2(N)
Self-consistent data set taken with same telescope
Compare the chemistry of HNCS and HSCN
HSCN only factor of 3 less abundant
Samples cold gas in this region
Both formed in cold gas via ion-molecule chemistry from a common precursor: HNCSH+
HNCSH+ + e-  HNCS or HSCN + H
Not formed on grains or through shocks
HNCSH+ not yet studied in laboratory

12. Comparison to HNCO and HOCN
Spectra of HNCO and HOCN also present in Sgr B2(N) data
HOCN lies 12,300 K above HNCO
HNCO: f(X/H2) = 5.7 x 10-10
HOCN: f(X/H2) = 3.2 x 10-12
HNCO/HOCN = 180
Formed from HNCOH+ / H2NCO+ ?
O/S ratio ~ 10 from NO/NS, CH3OH/CH3SH, H2CO/H2CS
HNCO/HNCS = 52
Suggests the ratio is due to formation mechanism, not overall ratio in Sgr B2(N)

13. Future Directions
Conducted mapping observations across Sgr B2 and searched for HNCS and HSCN in TMC-1 (see RF02)
Search for these species in cold and warm sources
Confirm formation mechanism

14. Acknowledgements
Ziurys Group
Harvard Group
NSF CCI