近红外偏振观测在恒星形成研究中的应用

Near-infrared Polarimetry in Study of Star Formation Regions

近红外偏振是研究恒星形成的有效工具。该文介绍了近红外偏振器的工作原理,然后分几个方面介绍了近红外偏振在恒星形成研究中的应用。红外反射云能很好地示踪年轻星天体及分子外流,通过分析偏振矢量的方法确定红外反射云的偏振对称中心,从而确定它的照亮源;偏振波长相关曲线包含了年轻星天体的星周物质的很多信息;年轻星的分子外流导致了红外反射云的形成,因此红外反射云的照亮源通常与年轻星天体成协,并是分子外流的驱动源;一些年轻星天体埋藏得很深,一般在近红外波段无法直接探测到,人们称之为深埋源,通过分析偏振矢量的方法可以找到深埋源;一般认为比较年轻的年轻星天体都是有尘埃盘的,尘埃盘的存在会导致它的偏振形态出现偏振盘,偏振盘可以用来研究尘埃盘;恒星形成区里成员星的偏振主要是由尘埃的二色性消光产生的,这样偏振方向会平行于致使尘埃排列的磁场的方向,从而能够揭示磁场的结构。最后进行了总结,并论述了中远红外偏振研究的优势和意义。

Near-infrared polarization is an effective technique in star formation research. With near-infrared polarimetry astronomers have known a lot about star formation regions. This article describes several aspects of near-infrared polarization in star formation research, each one is very useful for our understanding of the formation process of protostars. A near-infrared polarimeter is needed to be installed upstream of the camera equipped for a telescope which is designed to obtain near-infrared polarization measurements. The near-infrared polarimeter mainly consists of an achromatic （1-2.5μ） half-wave plate and a polarizer located upstream of the camera. The achromatic half-wave plate is designed to be rotatable and sometimes retractable. The polarizer is always a cold wire grid fixed in a plane,e.g, the cold filter wheels of the camera. A calibration polarizer must be installed upstream of the achromatic half-wave plate for measuring the polarization efficiency of the polarimeter. From optical to near-infrared wavelength range, polarizations of YSOs are always wavelength dependent. Polarizations of YSOs are composed of intrinsic polarizations and interstellar polarizations. The intrinsic polarizations are mainly derived from scattering by particles around YSOs, while the interstellar polarizations are mainly derived from dichroic absorption by interstellar medium aligned by interstellar magnetic field. So the wavelength dependence of intrinsic polarizations are quite different from that of interstellar polarizations because of different polarization producing mechanism. Infrared reflectional nebulae larizations and centrosymmetric （IRN） is a kind of infrared nebulae that shows high degree popolarization vectors throughout the infrared nebulae. There is a close relationship between IRN and YSO. It is generally accepted that IRN is illuminated by YSO located at the center of IRN, and the illuminating source can be identified by measuring the centroid of IRN＇s polarization vectors. IRN arises naturally because of mass outflow of YSOs. It is generally believed that IRN is due to the radiation escaped into the polar regions above and below the optically thick circumstellar disk, the extension of IRN tends to be consistent with that of mass outflow. Thus IRN is a morphological tracer of the mass outflow. A number of YSOs which are deeply embedded in the molecular cloud and not detectable in the wavelengths less than 2μm are so-called deeply embedded sources （DESs）, which can be identified through near-infrared polarizations. H20 masers are always detected in the vicinity of deeply embedded sources while far-infrared and radio sources show bad correlations with DESs. It is generally believed that there are circumstellar disks around YSOs, especially for early stage YSOs. There are many YSOs whose near-infrared polarization vectors have showed socalled ＂polarization disks＂ in the vicinity of YSOs. Polarization disks are mainly derived from multiple-scattering or dichroic absorption by circumstellar particles in disks, and can trace disks well. There are close relationships between polarization vectors of YSOs and the magnetic field in star formation regions. If the polarization of a member star is mainly due to scattering, and the polarization vector will be perpendicular to the magnetic field in the molecular cloud, and if the polarization is mainly due to dichroic absorption, the polarization vector will be parallel to the magnetic field. Thus in a star formation region, the extensions of polarization vectors of all member stars will reveal the magnetic field in this region. Can reveal the structure of the magnetic field. Extinction is too large for some deeply embedded YSOs to be detected in near-infrared. Extending the wavelengths to middle-or far-infrared can avoid this problem. In MIR and FIR those deeply embedded YSOs can be detected directly without the analysis of the polarization vectors. In MIR and FIR polarization is perpendicular to magnetic field because of grain＇s thermal radiation. So the MIR and FIR polarizations can also reveal the magnetic field as well as near-infrared polarizations.