ALMA Observations of Layered Structures due to CO Selective Dissociation in the ρ Ophiuchi A Plane-parallel PDR

Yamagishi, M.; Hara, C.; Kawabe, R.; Nakamura, F.; Kamazaki, T.; Takekoshi, T.; Shimajiri, Y.; Nomura, H.; Takakuwa, S.; Di Francesco, J.

We analyze 12CO(J = 2−1), 13CO(J = 2−1), C18O (J = 2−1), and 1.3 mm continuum maps of the ρ Ophiuchi A photodissociation region (PDR) obtained with ALMA. Layered structures of the three CO isotopologues with an angular separation of 10″ = 6.6 × 10−3 pc = 1400 au are clearly detected around the Be star, S1 (i.e., each front of emission shifts from the near to far side in order of 12CO, 13CO, and C18O). We estimate the spatial variations of X(13CO)/X(C18O) abundance ratios, and find that the abundance ratio is as high as 40 near the emission front, and decreases to the typical value in the solar system of 5.5 on a small angular scale of 4″ = 2.6 × 10−3 pc = 560 au. We also find that the I(12CO(2−1))/I(13CO(2−1)) intensity ratio is very high (>21) in the flat-spectrum young stellar object, GY-51, located in the PDR. The enhancement of the ratios indicates that the UV radiation significantly affects the CO isotopologues via selective dissociation in the overall ρ Ophiuchi A PDR, and that the ρ Ophiuchi A PDR has a plane-parallel structure. 

DOI: 10.3847/1538-4357/ab0d80