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1.
日冕是太阳大气活动的关键区域,是日地空间天气的源头.受观测限制,对日冕低层大气等离子体结构和磁场状态的研究非常欠缺,国际上对于可见光波段日冕低层大气的亮度分层研究很少.利用丽江日冕仪YOGIS(Yunnan Green-line Imaging System)的日冕绿线(FeⅩⅣ5303?)观测资料,对内日冕区域(1.03R☉-1.25R☉,R☉表示太阳半径)亮结构及其中冕环进行了有效的强度衰减分析.对亮结构的强度在太阳径向高度上进行了指数衰减拟合,比较这些拟合结果发现所得到的静态内冕环的衰减指数在一固定值附近.然后将比较明显的冕环提取出来,通过对不同高度的绿线强度进行指数拟合,得出的衰减指数与亮结构中也比较相近,这对进一步研究日冕中的各项物理参数演化提供了参考. 相似文献
2.
Bruce G. Elmegreen 《Earth, Moon, and Planets》1978,19(2):159-161
Carbon monoxide observations of neutral matter near W3 provide an excellent example of the formative process for O stars. As in other regions where O stars are forming, the young stars in W3 are located in dense cloud fragments that are at the edge of an extended molecular cloud complex. This peculiar formation site is simply explained in terms of a million year old shock that probably surrounds the expanding H II region IC1805. The contrast between this situation for W3 and what is found in other star-forming regions where only low mass stars occur is emphasized.Paper presented at the conference on Protostars and Planets, held at the Planetary Science Institute, University of Arizona, Tucson, Arizona, between January 3 and 7, 1978. 相似文献
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4.
本文根据球状星团所特有的金属丰度特征,利用星族综合方法,探讨了球状星团诸恒星的形成史。研究表明,这些恒星不可能通过恒星形成率和初始质量函数均不随时间变化的单一恒星形成模式产生。原初云通过恒星演化而得到金属丰度污染的过程和多数恒星的形成过程必须分为两个不同的阶段。 球状星团得到金属丰度污染的过程中,若恒星形成具有通常的初始质量函数,则其恒星形成率必须较低,且初始质量函数不能太陡,从而使污染过程中只形成数量较少的低质量恒星,以保证单个球状星团内金属丰度的均匀性。另一种可能性是污染阶段有非常特殊的初始质量函数,只形成大质量的恒星,从而除提供适量污染外不留下任何痕迹。 多数恒星应是在原初云不同部位得到适当污染后通过局域的短暂的爆发性恒星形成(星暴过程)产生。本文进一步探讨了在Fan和Rees球状星团形成模型的框架下,两相介质中温云块相互碰撞造成星暴过程的可能性。 相似文献
5.
恒星作为宇宙的基础组成元素,其形成过程一直是天文学中的重要研究对象。人们已基本了解中小质量恒星(质量小于8M⊙)的形成和演化过程;受到数量少、嵌埋深、演化快和反馈剧烈等因素的影响,大质量恒星(质量大于8M⊙)的形成过程依然谜团重重。介绍了小质量恒星形成的基本理论,以及吸积盘、竞争吸积、并合三种主流的大质量恒星形成模型;回顾了以往使用红外或射电望远镜对大质量恒星形成区的观测和分析,以及现阶段使用多波段巡天观测手段对大质量恒星形成区的研究成果;着重介绍了目前公认的大质量恒星形成的示踪物——分子外向流的理论和观测现状,以及大质量外向流的优秀候选体——绿色延展天体的发现、理论及观测研究情况。最后,对大质量恒星形成的理论和观测研究进行了总结和展望。 相似文献
6.
Jay A. Frogel 《Astrophysics and Space Science》1999,265(1-4):303-309
Observations of the stellar content of the bulge of the Milky Way can provide critical guidelines for the interpretation of
observations of distant galaxies, in particular for understanding their stellar content and evolution. In this brief overview
I will first highlight some recent work directed towards measuring the history of star formation and the chemical composition
of the central few parsecs of the Galaxy. These observations point to an episodic history of star formation in the central
region with several bursts having occurred over the past few 100 Myr (e.g. Blum et al., 1996b). High resolution spectroscopic observations by Ramírez et al. (1998) of luminous M stars in this region yield a near solar value for [Fe/H] from direct measurements of iron lines. Then
I will present some results from an ongoing program by my colleagues and myself the objective of which is the delineation
of the star formation and chemical enrichment histories of the central 100 parsecs of the Galaxy, the `inner bulge'. From
new photometric data we have concluded that there is a small increase in mean [Fe/H] from Baade's Window to the Galactic Center
and deduce a near solar value for stars in the central region. For radial distances greater than 1° from the Galactic Center we fail to find a measurable population of stars that are significantly younger than those in Baade's
Window. Within 1° we find a number of luminous M giants that most likely are the result of a star formation episode not more than one or two
Gyr ago.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
7.
Grant M. Kennedy Scott J. Kenyon Benjamin C. Bromley 《Astrophysics and Space Science》2007,311(1-3):9-13
Planets result from a series of processes within a circumstellar disk. Evidence comes from the near planar orbits in the Solar System and other planetary systems, observations of newly formed disks around young stars, and debris disks around main-sequence stars. As planet-hunting techniques improve, we approach the ability to detect systems like the Solar System, and place ourselves in context with planetary systems in general. Along the way, new classes of planets with unexpected characteristics are discovered. One of the most recent classes contains super Earth-mass planets orbiting a few AU from low-mass stars. In this contribution, we outline a semi-analytic model for planet formation during the pre-main sequence contraction phase of a low-mass star. As the star contracts, the “snow line”, which separates regions of rocky planet formation from regions of icy planet formation, moves inward. This process enables rapid formation of icy protoplanets that collide and merge into super-Earths before the star reaches the main sequence. The masses and orbits of these super-Earths are consistent with super-Earths detected in recent microlensing experiments. 相似文献
8.
Multicolor photometric and polarimetric observations of the eclipsing binary EK Cep at the Crimean Astrophysical Observatory
in 1995 and 2006–2007 are reported. Polarimetric observations were made of stars in the neighborhood of EK Cep. It is shown
that the observed linear polarization of EK Cep is determined by a variable circumstellar constituent, as well as by the interstellar
component. Various possible mechanisms for formation of the intrinsic polarization of binary stars are discussed.
Translated from Astrofizika, Vol. 52, No. 1, pp. 117–126 (February 2009). 相似文献
9.
I. I. Romanyuk 《Astrophysical Bulletin》2017,72(3):286-304
We have analyzed more than 90 papers in the area “Magnetic fields and physical parameters of chemically peculiar and related stars,” published mainly in 2016. The main results of the period under survey are as follows. The search for new magnetic stars continued.Many measurements weremade at the 6-m BTA telescope of the SAO RAS, new data on stellar magnetism in the OrionOB1 association were obtained. A systematic study ofmagnetic fields of stars with large anomalies in the energy distribution in the continuum was started. New data on ultra-slowmagnetic rotators—chemically peculiar stars with rotation periods of years and decades are obtained. Successful observations on the search for new magnetic stars are performed among the objects of the southern sky in Chile at the FORS2 VLT spectropolarimeter. A new direction was developed, namely, the study of binarymagnetic stars. Depending on the mass–distance ratio between the components, interaction with the magnetosphere and, possibly, magnetic braking may occur. The study of the details of this process is important for the theory of formation of stellar magnetic fields. The search for large-scale, but weak magnetic fields (magnitude of unities and tens of G) in non-CP stars is ongoing. Such fields are found in Am stars. No fields were found in the classical Be stars. Cool stars of various types were studied in detail. They manifested magnetic fields of a complex structure. Their mapping was performed, changes in the topology of the field were found at timescales of several years. Spectral and photometric variability was studied. Dozens of new potentially magnetic stars are discovered as a result of the ASAS-3, SuperWASP, Stereo and Kepler surveys. High-accuracy observations of rapidly oscillating stars were performed with the BRITE nanosatellite.Work continued on the studies of magnetic and photometric variability of white dwarfs. Finally, an overview of several papers on exoplanets, related with the subject of our study is presented. 相似文献
10.
H.R.M. Magee P.L. Dufton F.P. Keenan W.R.J. Rolleston D. Kilkenny D. O'Donoghue C. Koen R.S. Stobie 《Monthly notices of the Royal Astronomical Society》2001,324(3):747-756
We present high-resolution spectroscopic observations of 21 B-type stars, selected from the Edinburgh–Cape Blue Object Survey. Model atmosphere analyses confirm that 14 of these stars are young, main-sequence B-type objects with Population I chemical compositions. The remaining seven are found to be evolved objects, including subdwarfs, horizontal branch and post-AGB objects. A kinematical analysis shows that all 14 young main-sequence stars could have formed in the disc and subsequently been ejected into the halo. These results are combined with the analysis of a previous subsample of stars taken from the Survey. Of the complete sample, 31 have been found to be young, main-sequence objects, with formation in the disc, and subsequent ejection into the halo, again being found to be a plausible scenario. 相似文献
11.
The chemical enrichment in the galactic halo is studied, on the basis of the numerical model developed in Paper I, with paricular attention to the overabundances of O and light elements with respect to Fe shown by metal poor stars. Some representative nucleosynthesis pictures for stars of both Population I and Population II are considered and their yields are compared with observations of relative abundances in the Sun and in the halo, to identify the possible reasons of the observed compositional differences. It is found that solar elemental ratios can be reproduced if intermediate mass stars are allowed to give some contribution to the production of Fe by type-I supernovae, while the ratios of abundances observed in the halo are more similar to the relative yields produced by massive stars. These features are shared by all the nucleosynthesis schemes which have been considered. Using the best model of Paper I, we show that the steep star formation induced by the collapse has a decisive effect in maintaining the overabundances of light elements during the whole evolution of the halo. The relevance of this conclusion is discussed also in the light of a possible interpretation of the differences between the two abundance scales for globular clusters. 相似文献
12.
G. H. A. Roelofs G. Nelemans P. J. Groot 《Monthly notices of the Royal Astronomical Society》2007,382(2):685-692
The AM Canum Venaticorum (AM CVn) stars are rare interacting white dwarf binaries, whose formation and evolution are still poorly known. The Sloan Digital Sky Survey provides, for the first time, a sample of six AM CVn stars (out of a total population of 18) that are sufficiently homogeneous that we can start to study the population in some detail.
We use the Sloan sample to 'calibrate' theoretical population synthesis models for the space density of AM CVn stars. We consider optimistic and pessimistic models for different theoretical formation channels, which yield predictions for the local space density that are more than two orders of magnitude apart. When calibrated with the observations, all models give a local space density ρ0 = 1–3 × 10−6 pc−3 , which is lower than expected.
We discuss the implications for the formation of AM CVn stars, and conclude that at least one of the dominant formation channels (the double-degenerate channel) has to be suppressed relative to the optimistic models. In the framework of the current models this suggests that the mass transfer between white dwarfs usually cannot be stabilized. We furthermore discuss evolutionary effects that have so far not been considered in population synthesis models, but which could be of influence for the observed population. We finish by remarking that, with our lower space density, the expected number of Galactic AM CVn stars resolvable by gravitational-wave detectors like the Laser Interferometer Space Antenna ( LISA ) should be lowered from current estimates, to about 1000 for a mission duration of 1 yr. 相似文献
We use the Sloan sample to 'calibrate' theoretical population synthesis models for the space density of AM CVn stars. We consider optimistic and pessimistic models for different theoretical formation channels, which yield predictions for the local space density that are more than two orders of magnitude apart. When calibrated with the observations, all models give a local space density ρ
We discuss the implications for the formation of AM CVn stars, and conclude that at least one of the dominant formation channels (the double-degenerate channel) has to be suppressed relative to the optimistic models. In the framework of the current models this suggests that the mass transfer between white dwarfs usually cannot be stabilized. We furthermore discuss evolutionary effects that have so far not been considered in population synthesis models, but which could be of influence for the observed population. We finish by remarking that, with our lower space density, the expected number of Galactic AM CVn stars resolvable by gravitational-wave detectors like the Laser Interferometer Space Antenna ( LISA ) should be lowered from current estimates, to about 1000 for a mission duration of 1 yr. 相似文献
13.
Since their formation in the outflows of evolved stars, materials suffer in space deep chemical and physical modifications. Most abundant elements (C, N, O, Mg, Si, S and Fe) are present in dust as refractory chemical species. Among them silicates are one of the main constituents. Spectroscopic observations in various astronomical environments have shown that magnesium rich silicates are present both in amorphous and in crystalline form. An accurate interpretation of these observations requires studies on the formation of silicate dust in the atmospheres of giant stars and their evolution in the interstellar medium until their inclusion in protoplanetary disks.Many theoretical works have described the chemical and physical evolution of solids in space and their link to observable optical properties. Laboratory studies of cosmic dust analogues are needed to investigate these processes experimentally.In this work, experiments aimed at simulating the formation of silicates in space are presented. In particular, the laser ablation technique is used to produce amorphous silicates with various Si-Mg-Fe content. The analysis of their thermal evolution is presented. 相似文献
14.
I. I. Romanyuk 《Astrophysical Bulletin》2016,71(3):314-329
We present an analytical survey of key publications concerned with the study of stellar magnetism published in 2015. We considered about 80 publications, the most significant from our point of view, presented brief reviews of them, and made generalizations. The paper considers: instruments, techniques of observations and analysis; large-scale magnetic fields of OBA stars on the Main Sequence (MS) (formation and evolution, field topology, search for new magnetic stars including the projects MiMeS, BOB, and BinaMIcS and observations with the Russian 6-m telescope, rotation and chemical abundance analysis of magnetic CP stars); magnetic fields, chemical abundance and variability of stars related to peculiar, primarily, active cool stars, solar-type stars and white dwarfs; multiple magnetic stars including interferometry data, exoplanets in a system of magnetic stars. We make a conclusion that the accuracy of magnetic field measurements has grown due to universal application of the multilinear method of observations especially with high-resolution spectropolarimeters. Usage of Zeeman-Doppler imaging technique (ZDI) when analyzing the obtained data allows us to confidently search and measure fields of complex topology of the order of 10 Gs. For the first time, a magnetic field has been detected for post-AGB stars and some other types of objects. 相似文献
15.
Suhardja D. Wiramihardja Tomokazu Kogure Makoto Nakano Shigeomi Yoshida Ken'Ichi Tatematsu 《Astrophysics and Space Science》1986,118(1-2):311-315
A survey of emission-line stars was carried out in the CMa star formation region, based on spectral observations with the Kiso Schmidt telescope. In total, 128 emission-line stars were detected, ranging fromV=6 to 15, and the majority are found withV between 11 and 14. A comparison with the existing catalogues shows that some emission-line stars are common with ours, while some exhibit no detectable emission line on our plates, suggesting the variability of emission-line strength. The two-colour (U–B, B–V) diagram is also shown for the detected stars.Paper presented at the IAU Third-Asian-Pacific Regional Meeting, held in Kyoto, Japan, between 30 September–6 October, 1984.On leave from the Bosscha Observatory, Institute of Technology Bandung, Indonesia. 相似文献
16.
Stephen Skinner Manuel Güdel Werner Schmutz Svetozar Zhekov 《Astrophysics and Space Science》2006,304(1-4):97-99
We present an overview of recent X-ray observations of Wolf-Rayet (WR) stars with XMM-Newton and Chandra. These observations are aimed at determining the differences in X-ray properties between massive WR + OB binary systems and putatively single WR stars. A new XMM spectrum of the nearby WN8 + OB binary WR 147 shows hard absorbed X-ray emission (including the Fe Kα line complex), characteristic of colliding wind shock sources. In contrast, sensitive observations of four of the closest known single WC (carbon-rich) WR stars have yielded only non-detections. These results tentatively suggest that single WC stars are X-ray quiet. The presence of a companion may thus be an essential factor in elevating the X-ray emission of WC + OB stars to detectable levels. 相似文献
17.
A. K. Pandey K. Upadhyay K. Ogura Ram Sagar V. Mohan H. Mito H. C. Bhatt B. C. Bhatt 《Monthly notices of the Royal Astronomical Society》2005,358(4):1290-1308
UBVRI CCD photometry in a wide field around two young open clusters, NGC 663 and 654, has been carried out. Hα and polarimetric observations for the cluster NGC 654 have also been obtained. We use the photometric data to construct colour–colour and colour–magnitude diagrams, from which we can investigate the reddening, age, mass and evolutionary states of the stellar contents of the these clusters. The reddening across the cluster regions is found to be variable. There is evidence for anomalous reddening law in both clusters; however, more infrared and polarimetric data are needed to conclude about the reddening law. Both clusters are situated at about a distance of 2.4 kpc. Star formation in both clusters is found to be a continuous process. In the case of NGC 663, star formation seems to have taken place sequentially, in the sense that formation of low-mass stars precedes the formation of most massive stars. Whereas, in the case of NGC 654, formation of low-mass stars did not cease after the formation of most massive stars in the cluster. 相似文献
18.
We present velocity observations, obtained with the E. W. Fick Observatory 0.6 m telescope, of the 7th magnitudeK1 star HR 7112. Our observations show that HR 7112 is a double-line spectroscopic binary system composed of almost equal mass
stars with a period of 215.6 days. The orbit has an eccentricitye = 0.142 and the stars orbit each other with a mean semi-major axis distance of approximately 0.4sin(i) AU. The masses of
the stars are consistent with the classification as giant stars. Tidal effects are known to circularise the orbits of giant
stars and this process is apparently underway in this system. 相似文献
19.
根据贫金属星个别重元素丰度的观测值和太阳系重核素的ｒ过程和ｓ过程分量的丰度分布,计算了贫金属星ＨＤ２０３６０８和ＨＤ２１１９９８的重元素丰度并与观测进行了比较。结果表明,太阳系纯ｒ过程和ｓ过程元素丰度均不能拟合出这二颗样品星的丰度观测值,而应同时考虑ｒ过程和ｓ过程的贡献 相似文献
20.
Neill Reid 《Monthly notices of the Royal Astronomical Society》2003,342(3):837-850
We present high-resolution echelle spectroscopy of 20 stars in 16 systems catalogued as members of the TW Hydrae association, and 16 stars identified as possible new members. We have calibrated the range of coronal and chromospheric activity expected for such young stars as a function of spectral type by combining our observations with literature data for field and open cluster stars. We also compute space motions for TWA members and candidate members with proper motion measurements, using two techniques to estimate distances to stars lacking direct trigonometric parallax measurements. The mean space motion of the four TWA members with known parallaxes is ( U , V , W : −10.0, −17.8, −4.6) km s−1 . 14 of the candidates have properties inconsistent with cluster membership; the remaining two are potential new members, although further observations are required to confirm this possibility. 相似文献