ALMA observations of layered structures due to CO selective dissociation in the $\rho$ 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 $\rho$ Ophiuchi A photo-dissociation region (PDR) obtained with ALMA. Layered structures of the three CO isotopologues with an angular separation of 10 arcsec = 6.6$\times$10${}^{-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 in a small angular scale of 4 arcsec = 2.6$\times$10${}^{-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 $\rho$ Ophiuchi A PDR, and that the $\rho$ Ophiuchi A PDR has a plane-parallel structure.

arXiv comment14 pages, 6 figures, ApJ accepted

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Cited by in corpus (3)

• Study of the 13CO/C18O abundance ratio towards the filamentary infrared dark cloud IRDC 34.43+0.24
• Chemical compositions in the vicinity of protostars in Ophiuchus
• ALMA view of the $\rho$ Ophiuchi A PDR with a 360-au beam: the [CI] emission originates from the plane-parallel PDR and extended gas