The Gas to Dust Ratio in Three Star Forming Regions
LV Zhang-pan;JIANG Bi-wei;LI Jun;Department of Astronomy,Beijing Normal University;
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 about100—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 → 0line 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,X_(CO)=2.0×10~(20) cm~(-2)·(K·km/s)~(-1),the gas to dust ratio N(H)/A_V is calculated,which is 25×10~(20),38×10~(20),and 55×10~(20) 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 R_v=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)/A_v 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 X_(CO) is discussed as well.
【Fund】： 国家自然科学基金项目(11373015 11533002);; 973项目(2014CB845702);; 中央高校基本科研业务费专项资助
【CateGory Index】： P152
【CateGory Index】： P152