Abstract
To investigate the relationship between the far-ultraviolet (FUV) radiation and the abundance ratio of 13CO to C18O, we observed L1551 in 12CO (J=1-0), 13CO (J=1-0) and C18O (J=1-0) using the Nobeyama Radio Observatory (NRO) 45m telescope. L1551 is chosen because it is relatively isolated in the Taurus molecular cloud, providing an ideal environment for studying the variation of abundance ratios due to the penetration of the FUV photons. We derive the X(13CO)/X(C18O) value on the sub-parsec scales, which is in the range of ~3-27 with a mean value of 7.7+-2.9. 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) ~ 1-4mag), and decreases to the typical solar system value of 5.5 inside L1551 MC. The high X(13CO)/X(C18O) value 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, since it is uncertain but widely used in extragalactic studies. We found that X-factor is proportional to 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.