分子外流源L1211中近红外HH天体的发现

报告分子外流源Ｌ１２１１ 中新发现的１２ 个近红外ＨＨ 天体．在Ｌ１２１１ 的～４’区域,取得了ＪＨＫ’宽带和Ｈ２ 　ｖ ＝ １０ 　Ｓ（１） 发射线窄带的近红外图像．分析表明,新探测到的许多红外源与浓密分子云物理成协,其中有２０ 多个源表现出典型的Ｔ Ｔａｕｒｉ 星、Ｈｅｒｂｉｇ Ａｅ／Ｂｅ星和原恒星的红外超．Ｌ１２１１ 中的ＩＲＡＳ点源没有被观测到,可能是更深地埋于分子云中．对红外观测和分子线观测消光估计的比较显示,小于２μｍ 的近红外观测还不足以揭示深埋于分子云的整个年轻星团．根据近红外ＨＨ 天体的形态分布和红外源性质,能进一步证认这些ＨＨ 天体的激发源．其结果表明,Ｌ１２１１ 分子云中具有多个红外源驱动的多个外流活动     

NGC7538的近红外偏振成像

介绍恒星形成区ＮＧＣ７５３８的Ｈ和Ｋｓ波段偏振成像的观测结果．近５′视场的偏振图像揭示了与亮红外源ＩＲＳ１１１成协的年轻星团和红外星云的复杂结构．ＩＲＳ１３星云的偏振图像表明星云的近红外辐射主要来自对深埋红外源ＩＲＳ１的反射;其形态可解释为颜色很红的高偏振的壳层包围着的低消光气泡．ＩＲＳ１周围的近红外的偏振强度在ＩＲＳ１双极分子外流方向上呈现出一较小的壳结构．ＩＲＳ４在近红外表现为双源,其中较红的源ＩＳＲ４ｂ周围观测到中心对称的偏振图像．ＩＲＳ５与亮星云成协,ＩＲＳ６则处于一组星的中心．在ＩＲＳ５和ＩＲＳ６周围的星云中没有观测到偏振．ＩＫＳ９照亮着两块星云,ＩＲＮ１和ＩＲＮ２．ＩＲＮ１星云有很好的锥形并包含一高偏振的喷流成分．云中较高偏振的部分对应较蓝的颜色,显示着其较大的视向张角的外流腔结构．与复杂的ＩＲＳ９区分子外流相比较,ＩＲＮ１星云与在ＮＥＳＷ方向的外流相符合．首次证认了ＩＲＳ１１近红外点源,并观测到其周围有一组星和很暗的星云物质     

原行星盘的研究进展

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。     

S255IR复合体的近红外偏振测量

介绍恒星形成区Ｓ２５５ ＩＲ 复合体的Ｈ 和Ｋｓ 波段成像偏振和对其中红外源ＩＲＳ１在１ ．２ － ４ ．２ μｍ 上的分光偏振测量．偏振图像揭示了Ｓ２５５ － ２ 核区的两类近红外辐射．以ＩＲＳ１ 双源为中心的一条亮云脊表现出高偏振,而周围延展云却基本上无偏振．偏振图样进一步显示出由ＩＲＳ１ａ 和１ｂ 分别照射的两块反射云．它们都呈双极结构,并且其轴向近于相互垂直．由ＩＲＳ１ｂ 照射的双极云位于方位角ＰＡ～６５°,与Ｈ２ 发射的双极喷流现象很好地符合．两个照射源都是深埋天体．ＩＲＳ１ｂ 和高密度拱星盘成协,其盘平面与双极云垂直．ＩＲＳ１ａ可能也与一未能分辨的盘结构成协,东西方向的盘同样与其成协反射云垂直．从ＩＲＳ１ 的分光偏振测量可辨认出３ μｍ 水冰吸收波段上的偏振超．在此吸收波段上还观测到了反转型的偏振角旋转     

IRAS 20231+3440区外流激发源的性质研究

给出红外点源IRAS 20231 3440附近恒星形成区近红外．JHK’和H2成像观测结果，以及与该IRAS点源成协的近红外点源IRS1的K波段分光观测结果．．JHK’观测显示该区域存在嵌埋的年轻星天体，H2窄波段观测揭示了若干个氢分子发射结点，其中有几个结点排列成线形，暗示分子氢喷流的存在．喷流的北部与已知观测的分子外流成协，表明二者之间存在联系．喷流的走向提示IRS1可能是其激发源，对IRS1的K波段分光观测给出了进一步的证据．从近红外、MSX及IRAS资料估计出IRS1的能谱分布，表明它是一个处于ClassI状态的中等质量的年轻星天体．     

年轻星天体喷流的近红外成像观测进展

分子氢的红外振动发射线是显现年轻星质量外流的重要谱线之一。自Gautier等人1976年在猎户座发现年轻星质量外流的分子氢发射开始,人们在银河系内几乎所有的恒星形成区都发现了这种线发射。研究表明,分子氢发射与年轻星周围的其它活动现象（如分子外流和光学喷流）之间有着非常密切的联系。红外和光学喷流代表了年轻星剧烈活动的两个侧面,是喷流与周围介质相互作用强弱不同的表现,这种作用还拖带周围介质,产生分子外流,光学、红外喷流和分子外流组成了恒星形成区壮观的景象,它们是恒星形成活动的重要标志。随着红外探测技术的飞速发展,对年轻星外流活动现象的观测越来越丰富的详细,使人们对这种现象的本质越来越了解。在20世纪90年代NICMOS等大阵列红外探测器投入使用后,红外成像观测有了长足的进步。目前已在70个左右的区域里发现了H2发射,这一数字还在迅速增加,今后的研究主要可能向两个方向发展。其一是高分辨观测,进一步了解H2发射的结构以及与光学喷流和分子外流之间的关系;其二是天观测,了解银河系内的恒星形成H2区发射的大尺度结构和恒星形成的统计分布规律。     

分子云磁场与尘埃导致的偏振

磁场对分子云及其中的恒星的形成和演化起到重要的作用.分子云磁场的探测方法主要是谱线塞曼效应、尘埃热辐射的偏振,以及谱线的线偏振观测.利用谱线的塞曼效应可以直接测量视线方向的磁场强度.尘埃热辐射偏振可以有效地示踪磁场方向在天球上的分布.分子云内部的磁场会受到不同物理过程的影响.高分辨率观测可以研究磁场扰动的细节,低分辨率观测可以得到分子云甚至银河系大尺度磁场的宏观信息.只有多波段的观测才能全面地认识分子云磁场与各种物理过程的联系.该文对分子云尘埃热辐射偏振的观测情况做了调研,总结了分子云大尺度磁场的研究现状和发展前景.     

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.     

恒星形成时期强远红外源的红外发射与赫罗图

本文从IRAS点源星表（PSC）中选择了一批流量只F100μ≥500Jy，且属于正在形成或早期演化阶段的强远红外源，利用北京天文台兴隆观测站的1.26m红外望远镜，对这批源进行了J、H、K波段的近红外观测，得到了12个源的第一次J、H、K测光资料．结合外台站的红外观测资料和IRAS点源星表数据，我们对强远红外源的红外发射进行了修正的黑体光球模型、双壳层尘埃发射模型以及详细的辐射转移等模型的拟合，得到了这些源的一系列重要的物理参量，如中心星的等效光球温度，近红外消光以及包层的尘埃温度和密度随半径变化的幂律等．本文利用红外导出的中心星光球温度和光度，得到了与这些强远红外源成协的大质量年轻天体在赫罗图上的位置和演化特征．我们发现，对质量一定的大质量年轻星，沿着向主序演化的方向，红外谱在12μ－60μ间的斜率S有逐渐增大的趋势；而对演化到同一时间线的星，随着质量的增大，S也有增大的趋势．本文分析了这一性质．并提出了利用红外资料研究赫罗图的方法，它是一种研究深埋于分子云和尘埃包层的大质量年轻星早期演化的有效方法．     

恒星形成区GGD12-15的近红外观测

给出恒星形成区GGD12-15的宽波段JHK和窄波段H2v=1-0S(1)近红外成像观测．观测图像揭示了致密的年轻红外星团和与红外源成协的红外星云，并发现了以H2发射结表征的星团外流活动．大多数红外点源在光学波段不可见；对76颗红外点源的JHK′测光结果显示，有32颗具有红外超，其中5颗表现原恒星特征，表明分子云中的恒星形成活动很活跃．以B8光谱型作为大质量星分界，由色星等图估计出大质量星所占星团比例为-10％～26％．GGD12—15星团的K′星等分布的峰值位于15.0mag，并在13．0mag-16．0mag平坦分布；[H—K′]色分布的峰值出现在-0．7mag，在此以上更红的星团成分占70％．在GGD12-15区新发现的氢分子发射结集中在星团中心领域，其空间分布明显与剧烈的恒星形成活动相关；有5个发射结位于分子外流的中心区域，暗示其激发可能与分子外流同源．     

Near-infrared imaging polarimetry of young stellar objects in ρ Ophiuchi

The results of a near-infrared ( J H K L _P) imaging linear polarimetry survey of 20 young stellar objects (YSOs) in ρ Ophiuchi are presented. The majority of the sources are unresolved, with K -band polarizations, P _K < 6 per cent. Several objects are associated with extended reflection nebulae. These objects have centrosymmetric vector patterns with polarization discs over their cores; maximum polarizations of P _K > 20 per cent are seen over their envelopes. Correlations are observed between the degree of core polarization and the evolutionary status inferred from the spectral energy distribution. K -band core polarizations >6 per cent are only observed in Class I YSOs.
A 3D Monte Carlo model with oblate grains aligned with a magnetic field is used to investigate the flux distributions and polarization structures of three of the ρ Oph YSOs with extended nebulae. A ρ∝ r ^−1.5 power law for the density is applied throughout the envelopes. The large-scale centrosymmetric polarization structures are due to scattering. However, the polarization structure in the bright core of the nebula appears to require dichroic extinction by aligned non-spherical dust grains. The position angle indicates a toroidal magnetic field in the inner part of the envelope. Since the measured polarizations attributed to dichroic extinction are usually ≤10 per cent, the grains must either be nearly spherical or very weakly aligned. The higher polarizations observed in the outer parts of the reflection nebulae require that the dust grains responsible for scattering have maximum grain sizes 1.05 μm.     

Evolution of Magnetic fields in Bok Globules?

We study the influence and structure of the magnetic field in the early phases of low-mass star formation using polarization maps of Bok globules at a wavelength of 850 μm, obtained with the Submillimeter Common-User Bolometer Array (SCUBA) at the James Clerk Maxwell Telescope (JCMT). We discuss observations of the following sources: CB 26—a small globule with a nearly dispersed dense core and a young and large circumstellar disk, CB 54—a large globule with a massive dense core and a deeply embedded young stellar cluster, and B 335, CB 230, and CB 244—three nearby, relatively isolated small globules with low-mass protostellar cores. We find strongly aligned polarization vectors in the case of CB 26, B 335, and CB 230, whereas the vector orientations in the case of CB 54 and CB 244 are more or less randomly distributed. The degree of polarization, amounting to several percent, was found to decrease toward the center in each source. Assuming dichroic emission by aligned non-spherical grains as the polarization mechanism, where the magnetic field plays a role in the alignment process, we derive magnetic field strengths and structures from the observed polarization patterns. We compare the magnetic field topology with the morphology and outflow directions of the globules. In the Class 0 sources B 335, CB 230, and CB 244, the magnetic field is oriented almost perpendicular to the ouflow direction. In contrast, the inclination between outflow axis and magnetic field direction is much more moderate (36°) in the more evolved Class I source CB26.     

Can the intrinsic polarization of stellar radiation correspond to the Serkowski law?

We demonstrate a situation where the wavelength dependence of the intrinsic linear polarization of stellar radiation matches that of the interstellar linear polarization described by the Serkowski law. Such a situation can arise when the radiation from a star with a dipole magnetic field is scattered in a circumstellar plasma shell with a uniform electron density distribution. As a result, we have estimated the magnetic field strength at the photospheric phase of Supernova 1999gi. We show that the existence of intrinsic polarization in Galactic stars disguised as interstellar polarization is possible in principle.     

The compact nebulae cn1 and cn2, coupled to infrared stellar clusters

This is a study of the compact nebulae cn1 and cn2 situated in the extended, bright nebula S235. ¹²CO observations reveal the presence of blue and red outflows (i.e., a bipolar outflow) from the molecular cloud in which these nebulae are embedded. cn1 and cn2 are shown to be coupled to IR clusters of young stars, some of which have dust disks or envelopes (these are so-called young stellar objects, YSOs), with the YSOs grouped around the center of the clusters. cn1 is coupled to the infrared point source IRAS05377 + 3548, whose IR colors are close to those of T Tau stars. A chain of objects emerging from S235 (which clearly implies they are coupled in terms of evolution) is studied. These are the compact nebulae S235 A, B, and C, and the Herbig-Haro objects GGD5 and GGD6. A group of IR stars associated with GGD6 is also studied.     

Axisymmetry in protoplanetary nebulae: using imaging polarimetry to investigate envelope structure

We use ground-based imaging polarimetry to detect and image the dusty circumstellar envelopes of a sample of protoplanetary nebulae (PPNe) at near-infrared wavelengths. This technique allows the scattered light from the faint envelope to be separated from the glare of the bright central star, and is particularly well suited to this class of object. We detect extended (up to 9-arcsec diameter) circumstellar envelopes around 15 out of 16 sources with a range of morphologies including bipolars and shells. The distribution of scattered light in combination with its polarization (up to 40 per cent) provides unambiguous evidence for axisymmetry in 14 objects, showing this to be a common trait of PPNe. We suggest that the range of observed envelope morphologies results from the development of an axisymmetric dust distribution during the superwind phase at the end of the AGB. We identify shells seen in polarized light with scattering from these superwind dust distributions, which allows us to provide constraints on the duration of the superwind phase. In one object (IRAS 19475+3119) the circumstellar envelope has a two-armed spiral structure, which we suggest results from the interaction of the mass-losing star with a binary companion.     

Polarization in the young cluster NGC 6611: circumstellar, interstellar, or ... both?

We obtained linear polarization observations of 82 A/B-type stars in the young cluster NGC 6611, in order to probe the circumstellar material and to search for any evidence of intracluster or interstellar material that could also contribute to the polarization. We found linear polarization values that reach up to 14%. We consider the distribution of the polarization, its position angle, correlations with extinction and membership probability, polarization variability and wavelength distribution to identify the origin of the polarization toward NGC 6611. The polarization is found to be dominated by interstellar polarization, although some stars also have some circumstellar polarization. There is no evidence for intracluster dust. Rather, the dust must be located in a low density cloud toward the general line of sight to NGC 6611 and in front of it. The depth of that cloud along the line of sight increases slowly from the south–east to the north–west. The cloud is threaded by a very uniform magnetic field.     

Galactic Meteoroid Background

The density of the Galactic meteoroid background formed by the loss of large dust grains escaped from circumstellar disks during the formation of a star is estimated. This density is shown to be substantially higher than the local density of meteoroids that escaped from dusty stellar disks as a result of two-star collisions. The flux of meteors of the Galactic background near the Earth is calculated for a given latitude of the observing site taking into account the velocity distribution of dust particles and the motion of particles in the solar gravity field. The expected rate of the Galactic background meteor events at the AMOR radar latitude is lower by at least a factor of 30 than the reported frequency of interstellar meteor events that are supposedly registered in the AMOR experiment.     

On the angular momentum in star formation

We discuss the rotation of interstellar clouds which are in a stage immediately before star formation. Cloud collisions seem to be the principal cause of the observed rotation of interstellar clouds. The rotational motion of the clouds is strongly influenced by turbulence.Theories dealing with the resolution of the angular momentum problem in star formation are classified into five major groups. We develop the old idea that the angular momentum of an interstellar cloud passes during star formation into the angular momentum of double star systems and/or circumstellar clouds.It is suggested that a rotating gas cloud contracts into a ring-like structure which fragments into self-gravitating subcondensations. By collisions and gas accretion these subcondensations accrete into binary systems surrounded by circumstellar clouds. Using some rough approximations we find analytical expressions for the semi-major axis of the binary system and for the density of the circumstellar clouds as a function of the initial density and of the initial angular velocity of an interstellar cloud. The obtained values are well within the observational limits.     

Observations of Magnetic Fields on T Tauri Stars

We present measurements of magnetic field strength and geometry on the surfaces of T Tauri stars (TTS) with and without circumstellar disks. We use these measurements to argue that magnetospheric accretion models should not assume that a fixed fraction of the stellar surface contains magnetic field lines that couple with the disk. We predict the fractional area of accretion footpoints, using magnetospheric accretion models and assuming field strength is roughly constant for all TTS. Analysis of Zeeman broadened infrared line profiles shows that individual TTS each have a distribution of surface magnetic field strengths extending up to 6 kG. Averaging over this distribution yields mean magnetic field strengths of 1-3 kG for all TTS, regardless of whether the star is surrounded by a disk. These strong magnetic fields suggest that magnetic pressure dominates gas pressure in TTS photospheres, indicating the need for new model atmospheres. The He I 5876 Å emission line in TTS can be strongly polarized, so that magnetic field lines at the footpoints of accretion have uniform polarity. The circular polarization signal appears to be rotationally modulated, implying that accretion and perhaps the magnetosphere are not axisymmetric. Time series spectropolarimetry is fitted reasonably well by a simple model with one magnetic spot on the surface of a rotating star. On the other hand, spectropolarimetry of photospheric absorption lines rules out a global dipolar field at the stellar surface for at least some TTS.     

Near-infrared imaging polarimetry of dusty young stars

We have carried out JHK polarimetric observations of 11 dusty young stars, by using the polarimeter module IRPOL2 with the near-infrared camera UIST on the 3.8-m United Kingdom Infrared Telescope (UKIRT). Our sample targeted systems for which UKIRT-resolvable discs had been predicted by model fits to their spectral energy distributions. Our observations have confirmed the presence of extended polarized emission around TW Hya and around HD 169142. HD 150193 and HD 142666 show the largest polarization values among our sample, but no extended structure was resolved. By combining our observations with Hubble Space Telescope (HST) coronographic data from the literature, we derive the J - and H -band intrinsic polarization radial dependences of the disc of TW Hya. We find the polarizing efficiency of the disc is higher at H than at J , and we confirm that the J - and H -band percentage polarizations are reasonably constant with radius in the region between 0.9 and 1.3 arcsec from the star. We find that the objects for which we have detected extended polarizations are those for which previous modelling has suggested the presence of flared discs, which are predicted to be brighter than flat discs and thus would be easier to detect polarimetrically.