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arXiv · 1605.03467

The intrinsic abundance ratio and X-factor of CO isotopologues in L1551 shielded from FUV photodissociation

Sheng-Jun Lin·Yoshito Shimajiri·Chihomi Hara·Shih-Ping Lai·Fumitaka Nakamura·Koji Sugitani·Ryohei Kawabe·Yoshimi Kitamura·Atsushi Yoshida·Hidefumi Tatei·Toshiya Akashi·Aya E. Higuchi·Takashi Tsukagoshi
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Abstract

L1551 is chosen because it is relatively isolated in the Taurus molecular cloud shielded from FUV photons, providing an ideal environment for studying the target properties. Our observations cover ~40'x40' with resolution ~30", which are the maps with highest spatial dynamical range to date. We derive the X(13CO)/X(C18O) value on the sub-parsec scales in the range of ~3-27 with a mean value of 8.0+-2.8. Comparing to the visual extinction map derived from the Herschel observations, we found that the abundance ratio reaches its maximum at low Av (i.e., Av ~ 1-4mag), and decreases to the typical solar system value of 5.5 inside L1551 MC. The high X(13CO)/X(C18O) value at the boundary of the cloud is most likely due to the selective FUV photodissociation of C18O. This is in contrast with Orion-A where its internal OB stars keep the abundance ratio at a high level greater than ~10. In addition, we explore the variation of the X-factor, because it is an uncertain but widely used quantity in extragalactic studies. We found that X-factor is proportional to N(H2)^1.0 which is consistent with previous simulations. Excluding the high density region, the average X-factor is similar to the Milky Way average value.

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Sheng-Jun Lin, Yoshito Shimajiri, Chihomi Hara, Shih-Ping Lai, Fumitaka Nakamura, Koji Sugitani, Ryohei Kawabe, Yoshimi Kitamura, Atsushi Yoshida, Hidefumi Tatei, Toshiya Akashi, Aya E. Higuchi, Takashi Tsukagoshi. 2016-05-11. The intrinsic abundance ratio and X-factor of CO isotopologues in L1551 shielded from FUV photodissociation. https://doi.org/10.3847/0004-637x/826/2/193

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