2019ApJ...875...62Y

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 x 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 x 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.