為了研究遠紫外輻射(FUV)與13CO/C18O豐度比(X(13CO)/X(C18O))的關係，我們使用野邊山宇宙電波觀測所(NRO)的45公尺電波望遠鏡來觀測L1551區域的12CO(J=1-0)、13CO(J=1-0)及C18O(J=1-0)譜線。選擇L1551為觀測目標乃是由於其在金牛座分子雲(Taurus molecular cloud)中相對孤立，對研究起因於FUV光子的穿透而導致的豐度比變化提供了理想的環境。我們導出在次秒差距尺度上的X(13CO)/X(C18O)，其值大約介於3到27之間，平均值為7.7+-2.9。與從赫雪爾(Herschel)太空望遠鏡的觀測資料導出的V波段消光值(A(V))比較，我們發現X(13CO)/X(C18O)的最大值發生在低的A(V)(A(V)~1-4mag)，而X(13CO)/X(C18O)會在L1551 MC的內部下降到太陽系的典型值5.5。其中高的X(13CO)/X(C18O)極可能是由C18O的選擇性FUV光解離作用(selective FUV photodissociation of C18O)所造成。以上現象可與獵戶座A分子雲(Orion-A molecular cloud)對比。獵戶座A分子雲內部的OB型恆星使得其X(13CO)/X(C18O)可高達~10左右。此外，我們也研究了X-factor，因其有不確定性但也常運用於河外星系的研究。我們發現在L1551的X-factor正比於N(H_2)^1.0，這與文獻中的數值模擬結果相符。而除高密度區域外，平均的X-factor也相近銀河系的平均值。

To investigate the relationship between the far-ultraviolet (FUV) radiation and the abundance ratio of 13CO to C18O, we observed L1551 in 12CO (J=1-0), 13CO (J=1-0) and C18O (J=1-0) using the Nobeyama Radio Observatory (NRO) 45m telescope. L1551 is chosen because it is relatively isolated in the Taurus molecular cloud,
providing an ideal environment for studying the variation of abundance ratios due to the penetration of the FUV photons. We derive the X(13CO)/X(C18O) value on the sub-parsec scales, which is in the range of ~3-27 with a mean value of 7.7+-2.9.
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) ~ 1-4mag), and decreases to the typical solar system value of 5.5 inside L1551 MC.
The high X(13CO)/X(C18O) value 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, since it is uncertain but widely used in extragalactic studies.
We found that X-factor is proportional to 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.

Abstract i
摘要 ii
Contents iii
List of Tables iv
List of Figures v
1 Introduction 1
2 Observations and data reduction 3
2.1 NRO 45m observations . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.2 Herschel column density and dust temperature maps . . . . . . . . 4
3 Results 7
3.1 12CO (J=1{0) emission line . . . . . . . . . . . . . . . . . . . . . . 7
3.1.1 Distribution of the 12CO (J=1{0) emission line . . . . . . . 7
3.1.2 Velocity structure of the 12CO (J=1{0) emission line . . . . 7
3.2 13CO (J=1{0) emission line . . . . . . . . . . . . . . . . . . . . . . 9
3.2.1 Distribution of the 13CO (J=1{0) emission line . . . . . . . 9
3.2.2 Velocity structure of the 13CO (J=1{0) emission line . . . . 9
3.3 C18O (J=1{0) emission line . . . . . . . . . . . . . . . . . . . . . . 10
3.3.1 Distribution of the C18O (J=1{0) emission line . . . . . . . 10
3.3.2 Velocity structure of the C18O (J=1{0) emission line . . . . 10
3.4 Column densities of the 13CO and C18O gas and abundance ratio
of 13CO to C18O . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
4 Discussion 14
4.1 Selective photodissociation of C18O . . . . . . . . . . . . . . . . . . 14
4.2 In
uence of the excitation temperature . . . . . . . . . . . . . . . . 16
4.3 Comparison between the L 1551 molecular cloud and the Orion-A
giant molecular cloud . . . . . . . . . . . . . . . . . . . . . . . . . . 18
4.4 CO-to-H2 conversion factor across the L 1551 molecular cloud . . . 19
5 Summary 22
Appendix: Relationship between CO isotopes and H2 column density 24

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