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 13 CO to C 18 O 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 with a resolution of which make up maps with the highest spatial dynamical range to date. We derive the 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 (i.e., ∼ 1–4 mag), and decreases to the typical solar system value of 5.5 inside L 1551 MC. The high value at the boundary of the cloud is most likely due to the selective FUV photodissociation of C 18 O. 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 , which is consistent with previous simulations. Excluding the high density region, the average X-factor is similar to the Milky Way average value.