Detection of HOC[TSUP]+[/TSUP] toward the Orion Bar and M17-SW: Enhanced Abundances in Photon-dominated Regions?

A. J. Apponi, T. C. Pesch, L. M. Ziurys
1999 Astrophysical Journal  
New measurements of HOC ϩ emission have been carried out toward several photon-dominated regions using the National Radio Astronomy Observatory 12 m telescope. Toward the Orion bar, the , , and J ϭ 1 r 0 2 r 1 transitions of HOC ϩ at 89, 179, and 268 GHz were detected, as well as the line in M17-SW. 3 r 2 J ϭ 1 r 0 In conjunction with observations of HC 18 O ϩ , [HCO ϩ ]/[HOC ϩ ] abundance ratios have been established in these objects. Toward M17-SW, a ratio of ∼2260 was found. In the Orion
more » ... d. In the Orion bar, on the other hand, this ratio was determined to be , both from and measurements-the lowest ratio ϩ ϩ [HCO ]/[HOC ] ∼ 270 J ϭ 1 r 0 J ϭ 2 r 1 thus far found in the Galaxy, and remarkably different from the value of 1800 found in nearby OMC-1. Moreover, the fractional abundance of HOC ϩ was found to be in this source, the highest value yet observed Ϫ12 f տ 7 # 10 for this molecular ion. The Orion bar observations suggest that the abundance of HOC ϩ is enhanced relative to HCO ϩ because of the unique chemistry of photon-dominated regions.
doi:10.1086/312096 fatcat:ewub2rsfk5fs7f55ybd2uwhcki