原行星盘的研究进展

Progress in the Study of Protoplanetary Disks

原行星盘是环绕在年轻星天体(如T Tauri型星,HAe/Be星)周围的气体尘埃盘,是具有初始角动量的分子云核在塌缩形成恒星过程中的自然结果,是行星系统的起源地。原行星盘研究不仅是恒星形成理论的重要组成部分,而且是行星形成理论的基础。首先介绍了盘的形成与演化规律;然后介绍了年轻星天体的能谱分布,盘的模型和参数(质量吸积率、质量、尺度、温度、寿命);随后讨论了尘埃颗粒在盘中生长的观测证据以及行星在盘中形成的大致过程;最后对原行星盘研究的现状和未来做了总结与展望。

Protoplanetary disk is a rotating circumstellar disk of gas and dust surrounding a newly formed star,such as a T Tauri star or Herbig Ae/Be star.It constitutes an integral part of the formation of protostars,being a natural result of the collapse of protostellar cores which have initial angular momentum.At the beginning of the paper,we give a brief outline of how stars form in molecular clouds. Evidence for the existence of protoplanetary disks around young stars,both from theoretical modeling and observations in wavelengths ranging from optical to millimeter,are presented in Section 2. In spite of major developments in spatially resolved observations of the protoplanetary disks, much of our knowledge of their structure is derived from modeling the spectral energy distributions （SEDs） of young stellar objects.The third part of the paper focuses on the SEDs and disk models. Disk models have evolved from geometrically thin and optically thick disks,through geometrically flared and optically thick disks to radiative transfer disk models in which the disk temperature and vertical structure are computed self-consistently.Physical disk models,such as CG97 and DDN01 models,can interpret not only the SEDs of most young stellar objects,for example the silicate emission features present in the spectra of many T Tauri disks,but also spatially resolved data. We summarize the derived disk parameters in Section 4,such as mass accretion rate,mass, size,temperature and lifetime.These parameters can help us to understand the structure and evolution of protoplanetary disks. It is generally believed that particle growth in protoplanetary disks will lead to planet formation. Observational evidence for dust growth has been accumulating,such as a lower dust emissivity index for dust in protoplanetary disks compared to interstellar medium dust.We discuss these evidence in Section 5. Finally,we briefly summarize the unanswered questions and prospects of protoplanetary study. To date there is no unified model of protoplanetary disks that can interpret all of the observational data consistently.In observations only a small number of disks around nearby T Tauri and luminous HAe/Be stars have been spatially resolved in a handful of molecular lines.The chemical structure of protoplanetary disks is still unclear.With great improvements in their spatial resolution and sensitivity,the next generation of telescopes will gradually uncover the remaining mysteries of protoplanetary disks.