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

Abstract

We analyze 12 CO( J  = 2−1), 13 CO( J  = 2−1), C 18 O ( 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 12 CO, 13 CO, and C 18 O). We estimate the spatial variations of X ( 13 CO)/ X (C 18 O) 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 ( 12 CO(2−1))/ I ( 13 CO(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.