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