3个典型恒星形成区的气尘比

摘要

Gas to Dust Ratio (GDR) is the mass ratio of interstellar gas to dust.It is widely adopted that gas to dust mass ratio in our Galaxy has a value of about 100-150.We choose three typical star forming regions to study gas to dust ratio:the Orion molecular cloud-a massive star forming region,the Taurus molecular cloud-a low-mass star forming region,and the Polaris molecular cloud-little or no star forming region.The mass of gas only takes into account the neutral gas because the amount of ionized gas is very small in molecular clouds,i.e.only atomic and molecular hydrogen.The column density of hydrogen is taken from the highresolution,high-sensitivity all-sky survey EBHIS (Effelsberg-Bonn HI survey).The CO J =1 → 0 line is used to trace the molecular hydrogen,since the spectral lines of molecular hydrogen which can be detected are rare.The intensity of CO J =1 → 0 line is taken from the Planck all-sky survey.The mass of dust is traced by interstellar extinction based on the 2MASS (Two Micron All Sky Survey) photometric database in the direction of anti-Galactic center.Adopting a constant conversion coefficient from the integrated intensity of CO line to the column density of molecular hydrogen,XCo =2.0 × 1020 cm-2 · (K.km/s)-1,the gas to dust ratio N(H)/Av is calculated,which is 25×1020,38×1020,and 55×1020 cm-2· mag-1 for Orion,Taurus,and Polaris molecular clouds,respectively.These values are significantly higher than previously obtained for the average situation of the Galaxy.Adopting the interstellar dust model with the ratio of total to selective extinction at V Rv =3.1 by WD01,the derived gas to dust mass ratio is 160,243,and 354 for the Orion,Taurus,and Polaris molecular clouds,respectively,which is apparently higher than 100-150 for the diffuse interstellar medium.On the other hand,the high N(H)/Av value may be explained by the growth of dust in molecular clouds because of either coagulation or accretion which can lead to the inefficiency of extinction in the V band.The situation of varying XCo is discussed as well.%“气尘比”(Gas to Dust Ratio,GDR)是星际气体与星际尘埃的质量之比.广泛认同的银河系气尘比值是100-150.气尘比值的大小不仅取决于星际环境,也与所考虑的尘埃成分相关.恒星形成区是恒星形成的致密分子云区域,不同的分子云,其GDR也可能不同于普遍采用的数值.此工作选择3个典型的恒星形成区进行气尘比的研究,它们分别是:大质量恒星形成活跃的猎户座(Orion)分子云,小质量恒星形成区的代表金牛座(Taurus)分子云,极少或者无恒星形成活动的Polaris分子云.对这3个天区的研究有利于了解不同辐射环境恒星形成区的气尘比变化.在此对CO谱线积分强度与氢分子柱密度之间的转换系数XCo取常数,以统计的方法计算了3个分子云的气尘比N(H)/Av,其值在Orion天区、Taurus天区和Polaris天区分别为25、38和55(单位:1020 cm-2.mag-1),明显高于之前人们给出的银河系平均值.根据星际尘埃模型,将N(H)/Av转换成气体尘埃的质量比.采用被广泛接受的WD01尘埃模型(V波段的选择性消光比Rv=3.1的情况),得到3个恒星形成区的气尘比分别为:160 (Orion分子云)、243 (Taurus分子云)、354 (Polaris分子云),显著高于普遍采用的弥漫星际介质中100-150的取值范围.恒星形成区的N(H)/Av值高于平均值的另外一个可能的原因是,恒星形成区的尘埃由于吸积或者碰撞增长变大,降低了V波段的单位质量消光效率,而不是气尘质量比本身的增加.