NO and c-C₃H₂ in the Outer Galaxy: High Molecular Abundances at <i>R</i> _GC &gt; 13 kpc

Abstract Observations of the J Ka, Kc = 4 1,4 → 3 0,3 , 4 0,4 → 3 1,3 , and 2 2,0 → 1 1,1 rotational lines of c-C 3 H 2 and the J = 3/2 → 1/2, Ω = 1/2 transition of NO (X 2 Π r ) were conducted at 2 mm toward 20 Galactic edge clouds with R GC = 10.8–23.5 kpc using the Arizona Radio Observatory 12 m telescope. The c-C 3 H 2 molecule was detected in all 20 objects in the sample, based typically on 2–3 transitions. NO, which exhibits a distinct pattern of lambda doubling and hyperfine splitting in the measured line, was identified in 16 clouds, with distances as far as R GC ∼ 23.5 kpc. While a 3 mm transition of c-C 3 H 2 had been observed before in some of the sample clouds, NO had not previously been detected at such large distances from the Galactic center. These new identifications double the number of molecular clouds known to contain NO. From a radiative transfer analysis, fractional abundances, relative to H 2 , were determined to be f (c-C 3 H 2 ) ∼ 0.5–39.8 × 10 −10 and f (NO) ∼ 0.2–21.2 × 10 −8 . These abundances are comparable to values observed in molecular clouds in the inner Galaxy (R GC &lt; 12 kpc). The abundances therefore appear to remain relatively constant with increasing galactocentric distance. These results suggest that elemental abundance gradients at R GC ≥ 15 kpc in C, N, and O are not as severe as predicted. They also indicate that gas-phase chemistry in the outer Galaxy is quite robust, strengthening the case for widening the extent of the Galactic habitable zone.