THE INTRINSIC ABUNDANCE RATIO AND X-FACTOR OF CO ISOTOPOLOGUES IN L 1551 SHIELDED FROM FUV PHOTODISSOCIATION

We investigate the intrinsic abundance ratio of (CO)-C-13 to (CO)-O-18 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 similar to 40' x 40' with a resolution of similar to 30 '', which make up maps with the highest spatial dynamical range to date. We derive the X-13CO/X-C18O value on the sub-parsec scales in the range of similar to 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 similar to 1-4 mag), and decreases to the typical solar system value of 5.5 inside L 1551 MC. The high X-13CO/X-C18O value at the boundary of the cloud is most likely due to the selective FUV photodissociation of (CO)-O-18. This is in contrast with Orion-A where internal OB stars keep the abundance ratio at a high level, greater than similar to 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 alpha 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.