S252 HII区—分子云复合体中小尘埃的红外发射

THE INFRARED RADIATION FROM THE SMALL GRAINS IN THE DIFFUSE HII REGION S252

本文利用IRAS资料,详细研究了HII区一分子云复合体S252中12μm频带的辐射强度随辐射场的变化.结果表明,为了解释过量的短波辐射,必须涉及到非常小的尘埃(PAHs及其它小粒子)的非平衡发射过程.这些小尘埃发射总红外光度的36%,其大部分是在λ<25μm的频段辐射。PAHs在HII区中受到辐射场的强烈破坏,而稠密云的屏蔽作用使其相对少地受到影响.一个考虑了PAHs在HII区中的径向分布的模型可以满意地解释观测到的12μm辐射强度随空间的变化.

Using IRAS data, the excess emission at 12μm band of the S252 HII region has been studied in details. The results show that the infrared emission intensity at 12μm is larger by several orders of magnitude than the predictions of the standard dust distribution model. The ratios I_(12)/I_(25) and I_(12)/I_(100) increase with the increasing distance from the main exciting star. These behaviours can be explained by the non-equilibrium emission from the small grains (including PAHs and small graphite grains) and destruction effect of the strong radiation field to the PAHs in the region surrounding the early-type star. A simple model calculation for the 12μm emission has been given and is in agreement with observation. We also estimate that the contribution from the small grains in S252 is about 36% of the total diffuse infrared emission.