RUI: A Detailed Study of the Circumstellar Environment Around Neaby A-Type Stars: Possible Planet Construction Zones

摘要张启平AST98-19737这项研究项目的目的是评估我们太阳系附近“Beta Pictoris现象”的频率，并通过对Beta Pictoris类星系的详细光谱研究来更深入地了解“行星系统的形成过程”。多年来，天文学家们一直在寻找太阳系外的其他行星。他们特别想研究正在经历行星形成过程的系统，以便他们能够洞察我们自己的太阳系的起源。最好的候选者是Beta Pictoris，它是附近的一颗A型恒星，周围环绕着一个巨大的尘埃盘和非常动态的星际气体环境。有人提出，Beta Pictoris系统中恒星周围气体吸收组分的短期变化可能是由于彗星大小的天体蒸发所致。还有人提出，围绕Beta Pictoris的巨大尘埃盘可能类似于我们的柯伊伯带，柯伊伯带是一个超过海王星轨道的区域，现在被认为是短周期彗星的来源。对附近A型恒星的星周环境进行了详细的光谱研究。该计划的恒星是年轻的A型恒星，可以作为实验室，研究行星盘清理阶段的气体吸积。目标是确定这些恒星周围的碎片圆盘的特征。在距离太阳25秒以内的62颗A星中，对类似Beta Pictoris系统的天空体积有限的搜索已经完成。初步搜索包括两项独立的研究：一项是利用IRAS卫星数据获得关于星际尘埃的数据，另一项是利用高分辨率可见光和紫外光谱获得关于星际气体的数据。研究发现，超过18%的恒星都有星际尘埃，并发现了十多颗含有星际气体的恒星。对这些恒星的进一步监测显示，其中至少有四颗恒星具有类似Beta Pictoris的可变光谱特征，表明存在气体流入。这四颗恒星中的一颗被确定为具有可变的星际气体特征，并且在IRAS弱源调查探测极限时没有尘埃特征。将开展更多关于类Beta Pictoris系统的研究，为我们理解行星系统的形成提供重要的约束。分析高信噪比和高分辨率的可见光和紫外波长光谱将识别出更多附近有类似Beta Pictoris的恒星周气体的A星。通过监测在高分辨率的钙离子吸收光谱和钠离子吸收光谱中可观察到的时间和变星周吸收线分量，将对所有类Beta Pictoris系统的动力学进行研究。这项研究将有助于更好地理解星际气体和尘埃之间的相互关系。最终，将编制一份关于恒星周围尘埃和气体属性的地图集；即质量、密度、温度分布、速度场和附近所有A型恒星的化学梯度。这些物理条件，特别是类Beta Pictoris系统中的物理条件，将为吸积和行星系统形成的模型提供有价值的约束。这份地图集将是第一次使用IRAS、HST、IUE卫星数据以及地面数据，对附近所有A型恒星周围的星际尘埃和气体的性质进行全面汇编。该地图集将通过期刊出版物和地面数据(在基特峰国家天文台、Cerro Tololo国际天文台、麦克唐纳天文台和Mt.斯特罗姆洛天文台)将通过互联网提供。

Abstract K. P. Chang AST 98-19737The objectives of this research project are to assess the frequency of the "Beta Pictoris phenomenon" in our solar neighborhood, and to gain more insight into the "planetary system formation process" through detailed spectroscopic studies of Beta Pictoris-like systems.For years, astronomers have been searching for other planets outside our Solar System. They especially want to study systems that are undergoing the process of planet formation so that they can gain insight into the origin of our own Solar System. The best candidate is Beta Pictoris, a nearby A type star surrounded by a huge dust disk and a very dynamic circumstellar gas environment. It has been suggested that the short-term variations of circumstellar gas absorption components in the Beta Pictoris system could be due to the evaporation of comet-size bodies. It also has been suggested that the huge dust disk around Beta Pictoris could be an analog of our Kuiper Belt, a region past the orbit of the planet Neptune that is now considered to be the source of short-period comets.A detailed, spectroscopic study of the circumstellar environments of nearby A-type stars has been carried out. The program stars are young A-type stars that could serve as laboratories for the study of gas accretion during the clearing phase of planetary disks. The goal has been to characterize the debris disks around these stars. A sky volume-limited search for Beta Pictoris-like systems among the 62 A-stars within 25 parsecs of the Sun has been completed. The preliminary search consisted of two independent studies: one to obtain data on circumstellar dust using the IRAS satellite data and the second one for data on the circumstellar gas using high-resolution visual and ultraviolet spectra. It was found that more than 18% of the stars have circumstellar dust, and more than a dozen stars with circumstellar gas have been identified. Further monitoring of the stars has revealed that at least four of these have Beta Pictoris-like variable spectral signatures indicating gaseous infall. One of the four stars is identified to have variable circumstellar gas features and has no dust signature at the IRAS Faint Source Survey detection limit. More studies of Beta Pictoris-like systems will be carried out to provide important constraints for our understanding of planetary system formation.Analyzing high signal-to-noise and high-resolution visual and ultraviolet wavelength spectra will identify more nearby A-stars that have circumstellar gas similar to Beta Pictoris. A study of the dynamics in all the Beta Pictoris-like systems will be carried out by monitoring the temporal and variable circumstellar absorption line components observable in high-resolution Calcium II K and Sodium I D wavelength spectra. This study will lead to a better understanding of the inter-relationship between the circumstellar gas and dust. Ultimately, an atlas will be compiled of the circumstellar dust and gas properties; namely mass, density, temperature distributions, velocity fields, and chemical gradients for all the nearby A-type stars. These physical conditions, especially those in the Beta Pictoris-like systems, will provide valuable constraints for models of accretion and planetary system formation. This atlas will be the first thorough compilation of the properties of circumstellar dust and gas around all the nearby A-type stars using data from the IRAS, HST, IUE satellite data, in addition to ground-based data. The atlas will be available to the general astronomical community through journal publications, and the ground-based data (150 plus nights of observations obtained at Kitt Peak National Astronomy Observatory, Cerro Tololo International Observatory, McDonald Observatory, and Mt. Stromlo Observatory) will be made available via the Internet.