年轻星天体的质量

质量是恒星最基本、最重要的物理量之一,它在恒星形成和演化过程中起着至关重要的作用.年轻星天体的形成机制以及早期演化过程是彻底解决恒星形成问题的关键所在,而年轻星天体形成过程中的物质下落、质量吸积以及外流等物理过程都与年轻星天体的质量密切相关.该文介绍了确定年轻星天体质量的一些常用方法,包括利用开普勒定律的动力学方法、赫罗图方法、年轻星天体脉动方法和磁层吸积方法等.并对这些方法的适用范围及测量精度进行了评述,还综述了各种方法的研究进展及未来可能的发展.

The Mass of Young Stellar Objects

Young stellar objects (YSOs) evolve from molecular cloud cores through protostars and pre-main-sequence(PMS) to zero-aged main-sequence (ZAMS) stars. During this process,their masses, which are the most fundamental parameter, play a very important role. The masses of YSOs determine almost everything about their birth, life, and death.Four fundamental methods of estimating the mass of YSOs are introduced in the paper(classi-fied as Dynamic, Hertzsprung-Russell (H-R) diagram, Pulsation, and Magnetospheric accretion).The most direct method to determine stellar mass is through the analysis of dynamical param-eters. This can be accomplished by measuring the periods of orbital components, the radial velocities of unresolved companions in binary system, or the kinematics of a circumstellar disk in a single star. The second one is to plot the objects onto the H-R diagram and compare their positions with theoretical evolutionary tracks. Their positions depend on their temperatures and luminosities (or their colors and magnitudes) that are determined by their spectral types and extinctions and/or photometry. The third one to estimate the mass of Herbig Ae/Be star is through the pulsational instability strip, which is defined by the analysis of photometric variabil- ity. Finally, the velocity of the azcreting material can be used to measure the stellar potential energy, which, combined with the stellar radius, yields the mass of a PMS star.These methods can be used to constrain the mass of YSOs, but there are still some prob- lems. For the first method, although the determined mass is more precise than other methods, there would be errors in dynamical parameters due to observational uncertainty. Moreover, the sample of such objects is small; the errors of second method come from the uncertainties of the theoretical evolutionary tracks and the uncertainties of temperature and luminosity due to the errors of the measured spectral type and extinction (or the uncertainties of color and magnitude of YSOs due to the errors of extinction and photometry); for the third method, the boundaries of the pulsational instability strip are not precise enough due to the unclear intrinsic properties of YSOs; for the last one, there is a complex motion of the accreting material, its physical velocities may not be truly free-fall.With the development of new observational technology, the observational accuracy will be im- proved,and the samples of YSOs will be enlarged. This will greatly improve our understanding of the formation and evolution of YSOs. And the mass of YSOs will be more accurately determined.