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~¢´¢ 40 40 with a resolution of~ 30 , which make up maps with the highest spatial dynamical range to date. We derive the X Xvalue 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 Xvalue 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 µN H 1.0 2 , which is consistent with previous simulations. Excluding the high density region, the average X-factor is similar to the Milky Way average value.

Mapping the 13CO/C18O abundance ratio in the massive star-forming region G29.96−0.02

“…The same was found by Shimajiri et al (2014) in most of the studied regions towards Orion-A, which are affected by OB stars. Kong et al (2015) observed a gradient in X 13/18 with the decreasing of the A V in the southeast of the California molecular cloud, while the same was observed in L 1551, an isolated nearby star forming region that does not contain OB stars and thus it is influenced only by the far-UV from the interstellar radiation (Lin et al 2016).…”

Wide-field 12CO ( ) and 13CO ( ) Observations toward the Aquila Rift and Serpens Molecular Cloud Complexes. I. Molecular Clouds and Their Physical Properties

“…For example, Pineda et al (2010) derived the X-factor of (1.6 ∼ 12) × 10 20 cm −2 K −1 km −1 s in the Taurus molecular cloud. For L1551, Lin et al (2016) measured the X CO = 1.08 × 10 20 cm −2 K −1 km −1 s.…”

Nobeyama 45 m mapping observations toward Orion A. II. Classification of cloud structures and variation of the 13CO/C18O abundance ratio due to far-UV radiation

“…Thus, the spatial variation of the 13 CO/C 18 O abundance ratio, X13 CO /X C 18 O , is expected to reveal the area where the FUV radiation affects the cloud structure and properties. The effect of selective photodissociation is observed in several star-forming regions (Bally & Langer 1982;Lada et al 1994;Kong et al 2015;Lin et al 2016;Paron et al 2018). Shimajiri et al (2014) revealed the distribution of X13 CO /X C 18 O toward the northern part of the Orion A GMC.…”