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

TL;DR

This study investigates the abundance ratio and X-factor of CO isotopologues in the shielded L1551 cloud, revealing spatial variations influenced by FUV photodissociation and confirming the X-factor's proportionality to H2 column density.

Contribution

It provides the highest spatial resolution maps of CO isotopologue ratios in L1551 and links abundance variations to FUV effects, offering new insights into molecular cloud chemistry.

Findings

X(13CO)/X(C18O) varies from 3 to 27, averaging 8.0±2.8.

Abundance ratio peaks at low Av and decreases inside the cloud.

X-factor scales linearly with H2 column density.

Abstract

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.