论文标题
Co-to-h $ _2 $转换和分子云中的光谱柱密度：$ x _ {\ rm co} $ factor的vribility factor。
CO-to-H$_2$ Conversion and Spectral Column Density in Molecular Clouds: The Vriability of $X_{\rm CO}$ Factor
论文作者
论文摘要
我们使用Nobeyama 45-m望远镜分析了银河飞机CO调查，我们将使用当前转换因子$ x _ {\ rm co} $计算的h $ _2 $的光谱柱密度（SCD）与$^{12} $ CO线相比，用于$^{13} $ CO LINE M16和W43区域。在这里，SCD由$ dn _ {\ rm H_2}/dv $与$ n _ {\ rm h_2} $和$ v $分别为列密度和径向速度。发现$ x _ {\ rm co} $方法大大低估了云或区域中scd超过临界值的h $ _2 $密度（$ \ sim 3 \ sim 3 \ times 10^{21}} \ [{\ rm h_2 \ rm h_2 \ cm^cm^cm^cm^cm^{ - 2} { - }但是在较低的SCD区域中过高估计。我们指出，实际的Co-to-H $ _2 $转换因子随h $ _2 $圆柱密度或共线强度而变化：它在分子云的内部和不透明部分增加，而低密度包膜的降低。但是，就当前的$ x _ {\ rm co} $而言，与集成的$^{12} $ co强度相结合，在整个云上平均，它可以通过LTE使用$^{13} $ CO强度计算得出的值一致的值。基于分析，我们提出了一个新的co-to-htwo转换关系，$ n _ {\ rm h_2}^*= \ int x _ {\ rm co}^*（t _ {\ rm b}^*（\ rm b}） b}/t _ {\ rm b}^*）^βX_ {\ rm co} $是修改的光谱转换因子，作为亮度温度的函数，$ t _ {\ rm b} $，$ {12} $ co（$ j = 1-0 $）的$ {12} $ co（ B}^*= 12-16 $ K是通过拟合观察到的数据获得的经验常数。该公式可以校正低/高CO线强度下的柱密度的过度/低，并且适用于$ t _ {\ rm B} \ ge 1 $ k（数据中的RMS噪声）从封装到核心尺度的核心（分辨率）的分子云（RMS噪声）。 （完整分辨率副本可在http://www.ioa.s.u.s.u-tokyo.ac.jp/~sofue/news/2020_mn_xco12co12co13_fugin.pdf获得）
Analyzing the Galactic plane CO survey with the Nobeyama 45-m telescope, we compared the spectral column density (SCD) of H$_2$ calculated for $^{12}$CO line using the current conversion factor $X_{\rm CO}$ to that for $^{13}$CO line under LTE in M16 and W43 regions. Here, SCD is defined by $dN_{\rm H_2}/dv$ with $N_{\rm H_2}$ and $v$ being the column density and radial velocity, respectively. It is found that the $X_{\rm CO}$ method significantly under-estimates the H$_2$ density in a cloud or region, where SCD exceeds a critical value ($ \sim 3\times 10^{21}\ [{\rm H_2 \ cm^{-2} \ (km \ s^{-1})^{-1}}]$), but over-estimates in lower SCD regions. We point out that the actual CO-to-H$_2$ conversion factor varies with the H$_2$ column density or with the CO-line intensity: It increases in the inner and opaque parts of molecular clouds, whereas it decreases in the low-density envelopes. However, in so far as the current $X_{\rm CO}$ is used combined with the integrated $^{12}$CO intensity averaged over an entire cloud, it yields a consistent value with that calculated using the $^{13}$CO intensity by LTE. Based on the analysis, we propose a new CO-to-\Htwo conversion relation, $N_{\rm H_2}^* = \int X_{\rm CO}^*(T_{\rm B}) T_{\rm B} dv$, where $X_{\rm CO}^*=(T_{\rm B}/T_{\rm B}^*)^βX_{\rm CO}$ is the modified spectral conversion factor as a function of the brightness temperature, $T_{\rm B}$, of the ${12}$CO ($J=1-0$) line, and $β\sim 1-2$ and $T_{\rm B}^*=12-16$ K are empirical constants obtained by fitting to the observed data. The formula corrects for the over/under estimation of the column density at low/high-CO line intensities, and is applicable to molecular clouds with $T_{\rm B} \ge 1$ K (rms noise in the data) from envelope to cores at sub-parsec scales (resolution). (Full resolution copy available at http://www.ioa.s.u-tokyo.ac.jp/~sofue/news/2020_mn_Xco12co13_fugin.pdf)
