The Intrinsic Abundance Ratio and X-factor of CO Isotopologues in L 1551 Shielded from FUV Photodissociation

Abstract

We investigate the intrinsic abundance ratio of ¹³CO to C¹⁸O 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 ∼ 40^\prime × 40^\prime with a resolution of ∼ 30^\prime\prime , which make up maps with the highest spatial dynamical range to date. We derive the {X}_{¹³{CO}}/{X}_{{{C}}¹⁸{{O}}} 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 {A}_{{V}} (I.e., {A}_{{V}} ∼ 1-4 mag), and decreases to the typical solar system value of 5.5 inside L 1551 MC. The high {X}_{¹³{CO}}/{X}_{{{C}}¹⁸{{O}}} 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 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 the X-factor \propto {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.