ALMA observations of layered structures due to CO selective dissociation in the $rho$ Ophiuchi A plane-parallel PDR. (arXiv:1903.02192v1 [astro-ph.GA])
<a href="http://arxiv.org/find/astro-ph/1/au:+Yamagishi_M/0/1/0/all/0/1">M. Yamagishi</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Hara_C/0/1/0/all/0/1">C. Hara</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Kawabe_R/0/1/0/all/0/1">R. Kawabe</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Nakamura_F/0/1/0/all/0/1">F. Nakamura</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Kamazaki_T/0/1/0/all/0/1">T. Kamazaki</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Takekoshi_T/0/1/0/all/0/1">T. Takekoshi</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Shimajiri_Y/0/1/0/all/0/1">Y. Shimajiri</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Nomura_H/0/1/0/all/0/1">H. Nomura</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Takakuwa_S/0/1/0/all/0/1">S. Takakuwa</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Francesco_J/0/1/0/all/0/1">J. Di Francesco</a>
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.
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.
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