摘要
观测表明银河系内存在为数众多的星际泡,泡壁的二维投影亦称壳层,太阳系便位于称之为本地泡的一个泡结构之内。随着越来越多星际泡的发现和确认,人们对泡的研究渐趋深入,包括泡结构的多种统计性质,星际泡的若干可能的形成机制,以及泡与恒星形成和演化的关系等。从上述诸方面介绍了银河系泡结构的多波段研究进展。
Bubbles are a ubiquitous feature of the interstellar medium (ISM) on a wide range of mass and size scales, and the study of them gives information about the stellar winds that produce them and the structure and physical properties of the ambient ISM. An interstellar bubble is composed of an expending thin shell with cool matter of relatively high density, which is the outer boundary of the bubble, and a cavity with very diffuse and hot material bounded by the shell. Bubbles with large size are also referred to as superbubbles. In 1980's it was confirmed that our solar system is in an old bubble, which is referredto as the Local Bubble (LB). The LB was formed by injecting the energy of about 104~ J into the ISM surrounding the Sun (1 - 1.5) x 107 a ago, and this energy injecting event was caused by a burst of supernova activity that involved some 14-20 massive OB stars in a nearby young association. The LB extends to about 200 pc in the Galactic plane and 600 pc perpendicular to it, but is inclined some 20~ relative to the direction of the Galactic pole. Bubble structure discovered by 21 cm radio observations is usually referred to as HI shell. Since 1979 more and more HI shells have been found and collected in the literature, where important parameters used to describe the morphological and physical features of HI shell are listed, such as the LSR velocity, the angular size, the galactocentric and heliocentric distances, mass, expansion velocity, the density of the ambient ISM, etc. It is shown from statistical analyses that: (1) HI shells are concentrated towards the Galactic plane and their radial distribution follows an exponential law with a scale length of about 3 kpc; (2) there are no shells younger than 1 Ma and only a few shells with ages older than some 50 Ma; (3) the size distribution of shells follows a power law with an index of -2.1; (4) there are many low-energy shells, the majority of which are small with radii 〈100 pc. However, all of large shells are not the most energetic structures. Infrared observations can be used to probe the spatial distribution of the ISM com- posed of dust and gas, including the various morphologies of bubbles, which are bright at mid-infrared wavelengths. Among others, the observations and studies completed in 2006 2007 are the most successful, from which some 600 infrared dust bubbles are found. Besides, similar observations in far-infrared wavelengths are also made, from which more than 460 bubbles are discovered, and corresponding studies are completed. Based upon the pub- lished data of these bubbles, statistical analyses are made and some interesting results and conclusions are obtained. Several possible physical processes have been used to explain the origin of bubbles: (1) energy release from massive stars (winds and supernova explosions), which is the most probable mechanism of bubble formation, especially at low galactic latitude; (2) high velocity clouds with a large amount of energy infalling into the Galactic disc of constant density; (3) supersonic turbulence, which may be important in the control of star formation and can also govern the structure of the ISM above the Galactic plane; (4) Gamma-ray burst explosions, which can deposit enough energy into the ambient ISM to create large-size shells. No matter how bubbles are formed, a sequential and very important effect is that expanding shells can eventually trigger bursts of star formation because of the onset of gravitational instability.
出处
《天文学进展》
CSCD
北大核心
2012年第2期186-201,共16页
Progress In Astronomy
基金
国家自然科学基金(10773020
10778003
10833055)
关键词
银河系
泡
壳层
恒星形成
Galaxy
bubble
shell
star formation