大质量年轻星体的红外谱研究

A Study of the Spectral Energy Distribution of Massive Young Stellar Objects in the Infrared

本文对一批中心星光度大于１０３Ｌ⊙，质量大于７～８Ｍ⊙的大质量年轻星体（ＹＳＯｓ）进行了研究．利用近几年对这批源进行的ＪＨＫ波段的观测以及其它台站的红外测光和ＩＲＡＳ点源资料，得到了这批源的红外能谱．根据它们在７－２３μｍ间的低分辨率光谱（ＬＲＳ），确定了其尘埃粒子的类型，并将其分成三类：（１）具有９．７μｍ的硅酸盐特征；（２）具有１１．３μｍ的ＰＡＨ特征；（３）特征不明显或谱噪音较大．本文采用球对称、各向同性、吸收和散射同时考虑的辐射转移模型，对１２个大质量年轻星体进行了红外能谱拟合，得到了这些源的一系列重要的物理参量，如：中心星的光度和表面有效温度、特征波长上的光深以及包层的尘埃温度和密度随半径变化的幂律等．模型拟合的结果显示：（１）在大部分尘埃包层中，尘埃温度随尘埃包层半径的分布为Ｔｄ（ｒ）∝ｒ－０．４±０．０３，它与假设尘埃的吸收／发射效率Ｑ（ν）＝Ｑ０（νν）β，β＝１时的理论预期一致．（２）尘埃密度随尘埃包层半径的分布为ｎｄ（ｒ）∝ｒ－２．０～－１．５，该关系式间接反映了恒星形成早期，分子包层从等温塌缩到自由下落的过渡．

A study of the infrared spectral energy distribution (SED) of massive young stellar objects (YSOs) has been made. Most of them are either in the stage of star formation or early stellar evolution. Their infrared luminosity is higher than 10 3L ⊙ and the masses are larger than 7～8 M ⊙. Using JHK photometry obtained with the 1.26 m telescope at the Xinglong station in recent years, additional infrared data taken from other observatories and IRAS PSC data, we got the SED of twelve massive YSOs. Several possible dust grainmodels were considered according to the IRAS LRS data. It can be divided into three classes: the first is associated with the silicate feature at 9.7μm, the second is associated with the PAH feature at 11.3μm, and the third shows either little distinctive feature or rather noisy spectra. Adopting a spherical, isotropic radiative transfer model (including absorption and scattering), we made model fitting for the observed infrared spectral distribution of 12 massive YSOs. Based on detailed model fitting, many physical parameters of dust envelopes associated with these YSOs are obtained, such as the effective temperature and luminosity of the central star, the optical depth at characteristic wavelength and the distribution of dust temperature and density with the radius of the dust envelope. Our model fitting shows that: (1) In most envelopes, the dust temperature distribution with the radius is as T d(r) ∝r -0.4±0.03 . It is in agreement with the theoretical prediction of T d(r) when the absorption/emission efficiency of dust grains Q(ν) equals Q 0 (νν 0) β , where β= 1. (2) The dust density distribution with the radius is as n d (r) ∝r -2.0 ～-1.5 . If the abundance of dust relative to that of gas is constant with r, it may reflect the transition of a molecular envelope from isothermal to free fall collapse in the early stages of star formation.