2016ApJ...826..193L

We investigate the intrinsic abundance ratio of ¹³CO to C¹⁸O and the X-factor in L 1551 using the Nobeyama Radio Observatory (NRO) 45 m telescope. L 1551 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^′ ×40^′ with a resolution of ∼30^′′ , which make up maps with the highest spatial dynamical range to date. We derive the {X}_{{13}CO/{X}_{{C}^{18}Ø} 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 {A}_{V} (i.e., {A}_{V} ∼ 1-4 mag), and decreases to the typical solar system value of 5.5 inside L 1551 MC. The high {X}_{{13}CO/{X}_{{C}^{18}Ø} 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 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 the X-factor ∝{N}_{{
H}{2}}^{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.