大质量恒星形成区和恒星SiO脉泽的谱线观测研究
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摘要
本篇论文利用中国紫金山天文台青海观测站的13.7米口径毫米波望远镜和美国国立射电天文台的甚长基线干涉阵(VLBA)对大质量恒星形成区的CO分子和恒星SiO脉泽进行了谱线的观测与研究。
大质量恒星形成的研究目前是一个十分活跃的领域。人们对大质量恒星形成的了解相对于小质量星是十分贫乏的。对于大质量星形成过程目前主要有两个观点：类似于小质量星形成的吸积方式和由中、小质量原恒星并和形成方式。如果大质量星是通过吸积方式形成，那么大质量恒星形成过程可分为几个演化阶段：巨分子云→星前核→热核→UC HⅡ区。因此通过比较不同演化阶段的分子云核与外流的物理性质，会帮助我们更好地理解大质量恒星的形成过程。我们利用紫金山天文台青海站的13.7 m毫米波望远镜，对处在不同环境下的九个大质量恒星形成区进行了~(12)CO(J=1-0)，~(13)CO (J=1-0)和C~(18)O (J=1-0)的成图观测。样本中7个源探测到了~(13)CO云核，5个源探测到C~(18)O云核。只探测到2个源有明显的外流结构，且伴有C~(18)O云核及水脉泽。首次在HⅡ区S152／S153内探测到可能与红外源IRAS 22566+5828成协的~(12)CO分子外流。并采用典型的LTE方法给出了这些云核与外流的物理参量。观测结果表明只有当恒星形成演化到一定的阶段，使得气体密度足够高时，才会形成C~(18)O云核，而同时出现C~(18)O云核和H_2O脉泽时形成外流的可能性极高。
天体脉泽作为一种极端的非热平衡现象，为我们提供了研究一些特殊的天文环境，特别是小尺度环境的物理和动力学条件的最好工具。谱线甚长基线干涉(VLBI)技术在脉泽的观测中得到了广泛的应用，它为研究脉泽源的空间分布、视尺度以及运动学提供了有力的手段。我们在文中介绍了目前关于脉泽的最新研究进展和应用领域，同时也总结了谱线VLBI基本原理及其数据处理流程。
我们利用VLBA在1999年4-5月份期间对M-型半规则变星VX Sgr拱星包层中的43 GHzυ=1，J=1-0 SiO脉泽进行了三个历元的观测。这些高分辩的VLBA观测揭示了在时间间隔为1个月的观测时间里，VX Sgr的SiO脉泽的分布几乎没有发生明显的变化，都呈相似的环型结构，其半径为3个恒星半径(R_*)，这个结果与晚型星SiO脉泽分布于恒星表面2-4 R_*的典型值相一致。另外，与前期VLBI观测结果相比，SiO脉泽分布的整体形态发生了明显的变化，SiO脉泽的主要辐射区域从1992和1994年的西南方向变化到我们观测时(1999)的东北方向，这暗示着在从1992／1994年到1999年的～5-7年的时间里，VX Sgr的主要质量损失流的方向也从西南方向变化到了东北方向。通过对脉泽“spot”的两点相关函数分析，我们得到在两个尺度范围0.03-0.25 mas和0.5-20 mas上，SiO脉泽“spot”有很强的成团性。幂率谱的中断处0.25 mas表明由脉泽“spot”形成脉泽“feature”的角直径为0.5 mas。通过比较成图流量密度比(即VLBI的互相关流量密度与单天线的流量密度的比值)，发现在我们观测时的SiO脉泽大小明显要小于1992年的观测结果。这可能与由恒星活动性引起的SiO脉泽的突然爆发现象有关。通过对42个在三个历元上都存在的脉泽“feature”的“脉泽对运动尺度”(即每对脉泽间在不同历元上距离的差值)分析，我们得到在假定VX Sgr距离为1.7 kpc时，SiO脉泽壳层在以4 km s~(-1)的速度内落收缩。这个内落运动速度值基本与声速同一量级，很容易通过引力加速拱星尘埃壳层的物质来获得。利用SiO脉泽自行的统计视差分析，我们估算VX Sgr的距离为1.58±0.15 kpc。这个距离值与利用水脉泽自行测定的距离值是一致的。在此距离上VX Sgr可以被证认为是红超巨星。
In this dissertation, we report spectral line observations of CO molecularof massive star forming regions and circumstellar SiO masers, using the 13.7 mmillimeter wave telescope at Qinghai station of Purple Mountain Observatory,China and Very Long Baseline Array (VLBA) of National Radio AstronomyObservatory, USA, respectively.
The study of massive star formation is very active at present. The under-standings of forming processes of massive stars are not clear relative to that oflow mass star formation. Now there are two models regarding the massive starformation: infall and accretion model as does the low-mass star formation, andthe collision or merging of low- or intermediate-mass stars model. If massive starscould form via accretion, their forming processes could be separated into follow-ing different phases: giant molecular clouds→prestellar cores→hot cores→UC HⅡregions. Therefore, a direct comparison of the physical properties of molecularcores and outflows at different phases would be helpful to better understand theevolution of high-mass star formation. The ~(12)CO (J=1-0), ~(13)CO (J=1-0) andC~(18)O (J=1-0) emissions in 9 massive star forming regions, which are believed tobe at different stages of massive star formation, were mapped with the 13.7 mmillimeter wave telescope at Qinghai Station of Purple Mountain Observatory.Of the observed 9 sources, ~(13)CO cores were detected in seven of them, and C~(18)Ocores in five of them. And only two sources associated with C~(18)O cores and H_2Omasers showed the extended structures and strong outflows. This is the firstdetection of outflow associated with IRAS 22566+5828 in the observing filed ofS152/S153. The physical parameters of cores and outflows for these sources,derived from Local Thermal Equilibrium (LTE) analysis, are presented. Theseresults suggest that the C~(18)O cores will appear only when the gas density is highenough, and the probability to have an outflow is very high when the clumpsshow the C~(18)O core and H_2O maser simultaneously.
As an extreme nonthermal equilibrium phenomena, astronomical masers provide the best tool for us to study physical and dynamical conditions for somepeculiar astronomical circumstance, especially for the smaller scale circumstance.Spectral line very long baseline interferometry (VLBI) technology being widelyapplied in the observations of astronomical masers offers a powerful means tostudy the spatial distributions, apparent size and kinematics of masers. We givean introduction to the newest research progress and its application in astro-physics, and summarize the fundamental theory as well as the data reduction ofthe spectral line VLBI in this dissertation.
The 43 GHzυ=1, J=1-0 SiO maser emission toward M-type semi-regularvariable star VX Sagittarii (VX Sgr) was observed at 3 epochs during 1999 April-May using the VLBA. Our observations confirmed a persistent ringlike structureof SiO masers with a projected radius of about 3 stellar radii (R_*), which isconsistent with the typical 2-4 R, for late-type stars. Moreover, by comparingwith the previous VLBI observations, we find that the overall morphology haschanged significantly with the majority of masers appearing in North-East (NE)of the star in 1999, compared to that lying to the South-West (SW) directionin 1992 and 1994, suggesting that the direction of the major mass loss has alsochanged from the SW to NE over～5-7 years from 1992/1994 to 1999. The two-point correlation function of spots shows the maser spots are strongly clusteredon scales of 0.03-0.25 mas and 0.5-20 mas. The break of the power-law at 0.25 massuggests an angular diameter of 0.5 mas for clustering of spots to make a feature.We also find that the apparent size of maser features in 1999 is distinctly smallerthan that observed in 1992, by comparing their fractions of total power imaged.This may be related to stellar activity that caused a large SiO flare during ourobservations. Analysis of "pairwise separation" of 42 matched features appearingin all the three epochs suggests that the maser shell contracts toward VX Sgrwith a velocity of about 4 km s~(-1) at a distance of 1.7 kpc to VX Sgr. Such avelocity is on the order of the sound speed, and can be easily explained by thegravitational infall of material from the circumstellar dust shell. We estimate adistance to VX Sgr of 1.58±0.15 kpc with statistical parallax analysis for SiOmasers proper motions, which is consistent with that based on proper motionsof H_2O masers, at which VX Sgr is indeed a red supergiant.
大质量恒星形成的研究目前是一个十分活跃的领域。人们对大质量恒星形成的了解相对于小质量星是十分贫乏的。对于大质量星形成过程目前主要有两个观点：类似于小质量星形成的吸积方式和由中、小质量原恒星并和形成方式。如果大质量星是通过吸积方式形成，那么大质量恒星形成过程可分为几个演化阶段：巨分子云→星前核→热核→UC HⅡ区。因此通过比较不同演化阶段的分子云核与外流的物理性质，会帮助我们更好地理解大质量恒星的形成过程。我们利用紫金山天文台青海站的13.7 m毫米波望远镜，对处在不同环境下的九个大质量恒星形成区进行了~(12)CO(J=1-0)，~(13)CO (J=1-0)和C~(18)O (J=1-0)的成图观测。样本中7个源探测到了~(13)CO云核，5个源探测到C~(18)O云核。只探测到2个源有明显的外流结构，且伴有C~(18)O云核及水脉泽。首次在HⅡ区S152／S153内探测到可能与红外源IRAS 22566+5828成协的~(12)CO分子外流。并采用典型的LTE方法给出了这些云核与外流的物理参量。观测结果表明只有当恒星形成演化到一定的阶段，使得气体密度足够高时，才会形成C~(18)O云核，而同时出现C~(18)O云核和H_2O脉泽时形成外流的可能性极高。
天体脉泽作为一种极端的非热平衡现象，为我们提供了研究一些特殊的天文环境，特别是小尺度环境的物理和动力学条件的最好工具。谱线甚长基线干涉(VLBI)技术在脉泽的观测中得到了广泛的应用，它为研究脉泽源的空间分布、视尺度以及运动学提供了有力的手段。我们在文中介绍了目前关于脉泽的最新研究进展和应用领域，同时也总结了谱线VLBI基本原理及其数据处理流程。
我们利用VLBA在1999年4-5月份期间对M-型半规则变星VX Sgr拱星包层中的43 GHzυ=1，J=1-0 SiO脉泽进行了三个历元的观测。这些高分辩的VLBA观测揭示了在时间间隔为1个月的观测时间里，VX Sgr的SiO脉泽的分布几乎没有发生明显的变化，都呈相似的环型结构，其半径为3个恒星半径(R_*)，这个结果与晚型星SiO脉泽分布于恒星表面2-4 R_*的典型值相一致。另外，与前期VLBI观测结果相比，SiO脉泽分布的整体形态发生了明显的变化，SiO脉泽的主要辐射区域从1992和1994年的西南方向变化到我们观测时(1999)的东北方向，这暗示着在从1992／1994年到1999年的～5-7年的时间里，VX Sgr的主要质量损失流的方向也从西南方向变化到了东北方向。通过对脉泽“spot”的两点相关函数分析，我们得到在两个尺度范围0.03-0.25 mas和0.5-20 mas上，SiO脉泽“spot”有很强的成团性。幂率谱的中断处0.25 mas表明由脉泽“spot”形成脉泽“feature”的角直径为0.5 mas。通过比较成图流量密度比(即VLBI的互相关流量密度与单天线的流量密度的比值)，发现在我们观测时的SiO脉泽大小明显要小于1992年的观测结果。这可能与由恒星活动性引起的SiO脉泽的突然爆发现象有关。通过对42个在三个历元上都存在的脉泽“feature”的“脉泽对运动尺度”(即每对脉泽间在不同历元上距离的差值)分析，我们得到在假定VX Sgr距离为1.7 kpc时，SiO脉泽壳层在以4 km s~(-1)的速度内落收缩。这个内落运动速度值基本与声速同一量级，很容易通过引力加速拱星尘埃壳层的物质来获得。利用SiO脉泽自行的统计视差分析，我们估算VX Sgr的距离为1.58±0.15 kpc。这个距离值与利用水脉泽自行测定的距离值是一致的。在此距离上VX Sgr可以被证认为是红超巨星。
In this dissertation, we report spectral line observations of CO molecularof massive star forming regions and circumstellar SiO masers, using the 13.7 mmillimeter wave telescope at Qinghai station of Purple Mountain Observatory,China and Very Long Baseline Array (VLBA) of National Radio AstronomyObservatory, USA, respectively.
The study of massive star formation is very active at present. The under-standings of forming processes of massive stars are not clear relative to that oflow mass star formation. Now there are two models regarding the massive starformation: infall and accretion model as does the low-mass star formation, andthe collision or merging of low- or intermediate-mass stars model. If massive starscould form via accretion, their forming processes could be separated into follow-ing different phases: giant molecular clouds→prestellar cores→hot cores→UC HⅡregions. Therefore, a direct comparison of the physical properties of molecularcores and outflows at different phases would be helpful to better understand theevolution of high-mass star formation. The ~(12)CO (J=1-0), ~(13)CO (J=1-0) andC~(18)O (J=1-0) emissions in 9 massive star forming regions, which are believed tobe at different stages of massive star formation, were mapped with the 13.7 mmillimeter wave telescope at Qinghai Station of Purple Mountain Observatory.Of the observed 9 sources, ~(13)CO cores were detected in seven of them, and C~(18)Ocores in five of them. And only two sources associated with C~(18)O cores and H_2Omasers showed the extended structures and strong outflows. This is the firstdetection of outflow associated with IRAS 22566+5828 in the observing filed ofS152/S153. The physical parameters of cores and outflows for these sources,derived from Local Thermal Equilibrium (LTE) analysis, are presented. Theseresults suggest that the C~(18)O cores will appear only when the gas density is highenough, and the probability to have an outflow is very high when the clumpsshow the C~(18)O core and H_2O maser simultaneously.
As an extreme nonthermal equilibrium phenomena, astronomical masers provide the best tool for us to study physical and dynamical conditions for somepeculiar astronomical circumstance, especially for the smaller scale circumstance.Spectral line very long baseline interferometry (VLBI) technology being widelyapplied in the observations of astronomical masers offers a powerful means tostudy the spatial distributions, apparent size and kinematics of masers. We givean introduction to the newest research progress and its application in astro-physics, and summarize the fundamental theory as well as the data reduction ofthe spectral line VLBI in this dissertation.
The 43 GHzυ=1, J=1-0 SiO maser emission toward M-type semi-regularvariable star VX Sagittarii (VX Sgr) was observed at 3 epochs during 1999 April-May using the VLBA. Our observations confirmed a persistent ringlike structureof SiO masers with a projected radius of about 3 stellar radii (R_*), which isconsistent with the typical 2-4 R, for late-type stars. Moreover, by comparingwith the previous VLBI observations, we find that the overall morphology haschanged significantly with the majority of masers appearing in North-East (NE)of the star in 1999, compared to that lying to the South-West (SW) directionin 1992 and 1994, suggesting that the direction of the major mass loss has alsochanged from the SW to NE over～5-7 years from 1992/1994 to 1999. The two-point correlation function of spots shows the maser spots are strongly clusteredon scales of 0.03-0.25 mas and 0.5-20 mas. The break of the power-law at 0.25 massuggests an angular diameter of 0.5 mas for clustering of spots to make a feature.We also find that the apparent size of maser features in 1999 is distinctly smallerthan that observed in 1992, by comparing their fractions of total power imaged.This may be related to stellar activity that caused a large SiO flare during ourobservations. Analysis of "pairwise separation" of 42 matched features appearingin all the three epochs suggests that the maser shell contracts toward VX Sgrwith a velocity of about 4 km s~(-1) at a distance of 1.7 kpc to VX Sgr. Such avelocity is on the order of the sound speed, and can be easily explained by thegravitational infall of material from the circumstellar dust shell. We estimate adistance to VX Sgr of 1.58±0.15 kpc with statistical parallax analysis for SiOmasers proper motions, which is consistent with that based on proper motionsof H_2O masers, at which VX Sgr is indeed a red supergiant.
引文
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