  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.
