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1.
Hsien Shang 《Astrophysics and Space Science》2007,311(1-3):25-34
We review status of theoretical development for jets and molecular outflows from young stellar objects. A particular framework
for explaining these phenomena is one based on the X-wind theory in an environment of magnetized collapsing molecular cloud
cores. The magnetized gravitational collapse follows the standard picture of isolated low-mass star formation, from quasi-static
evolution of the parent molecular cloud cores. The outflow phenomena operate throughout the early evolution of young stars
as a result of star-disk interaction. We discuss emission mechanisms of jets and formation of molecular outflows in this general
framework. The general theoretical framework provides room for self-consistent interpretations for recent observations. Jets
and outflows are integral part of earliest evolution of young stellar objects. 相似文献
2.
Non-degenerate stars of essentially all spectral classes are soft X-ray sources. Their X-ray spectra have been important in
constraining physical processes that heat plasma in stellar environments to temperatures exceeding one million degrees. Low-mass
stars on the cooler part of the main sequence and their pre-main sequence predecessors define the dominant stellar population
in the galaxy by number. Their X-ray spectra are reminiscent, in the broadest sense, of X-ray spectra from the solar corona.
The Sun itself as a typical example of a main-sequence cool star has been a pivotal testbed for physical models to be applied
to cool stars. X-ray emission from cool stars is indeed ascribed to magnetically trapped hot gas analogous to the solar coronal
plasma, although plasma parameters such as temperature, density, and element abundances vary widely. Coronal structure, its
thermal stratification and geometric extent can also be interpreted based on various spectral diagnostics. New features have
been identified in pre-main sequence stars; some of these may be related to accretion shocks on the stellar surface, fluorescence
on circumstellar disks due to X-ray irradiation, or shock heating in stellar outflows. Massive, hot stars clearly dominate
the interaction with the galactic interstellar medium: they are the main sources of ionizing radiation, mechanical energy
and chemical enrichment in galaxies. High-energy emission permits to probe some of the most important processes at work in
these stars, and put constraints on their most peculiar feature: the stellar wind. Medium and high- resolution spectroscopy
have shed new light on these objects as well. Here, we review recent advances in our understanding of cool and hot stars through
the study of X-ray spectra, in particular high-resolution spectra now available from XMM-Newton and Chandra. We address issues related to coronal structure, flares, the composition of coronal plasma, X-ray production in accretion
streams and outflows, X-rays from single OB-type stars, massive binaries, magnetic hot objects and evolved WR stars. 相似文献
3.
J. M. De Buizer 《Monthly notices of the Royal Astronomical Society》2003,341(1):277-298
The results of a survey searching for outflows using near-infrared imaging are presented. Targets were chosen from a compiled list of massive young stellar objects associated with methanol masers in linear distributions. Presently, it is a widely held belief that these methanol masers are found in (and delineate) circumstellar accretion discs around massive stars. If this scenario is correct, one way to test the disc hypothesis is to search for outflows perpendicular to the methanol maser distributions. The main objective of the survey was to obtain wide-field near-infrared images of the sites of linearly distributed methanol masers using a narrow-band 2.12-μm filter. This filter is centred on the H2 v = 1–0 S(1) line; a shock diagnostic that has been shown to successfully trace CO outflows from young stellar objects. 28 sources in total were imaged of which 18 sources display H2 emission. Of these, only two sources showed emission found to be dominantly perpendicular to the methanol maser distribution. Surprisingly, the H2 emission in these fields is not distributed randomly, but instead the majority of sources are found to have H2 emission dominantly parallel to their distribution of methanol masers. These results seriously question the hypothesis that methanol masers exist in circumstellar discs. The possibility that linearly distributed methanol masers are instead directly associated with outflows is discussed. 相似文献
4.
5.
Bringfried Stecklum Helmut Meusinger Dirk Froebrich 《Astrophysics and Space Science》2007,311(1-3):63-67
Herbig-Haro objects (HHOs) are caused by outflows from young objects. Since the outflow relies on mass accretion from a circumstellar
disk, it indicates ongoing growth. Recent results of infrared observations yielded evidence for disks around brown dwarfs.
This suggests that at least a certain fraction of brown dwarfs forms like stars. Thus, young sub-stellar objects might cause
HHOs as well. We present selected results of a general survey for HHOs based on DSS-II plates and CCD images taken with the
Tautenburg Schmidt telescope. Numerous young objects could be identified due to their association with newly detected HHOs.
In some cases the luminosity is consistent with very low-mass stars or close to sub-stellar values. This holds for L1415-IRS
and a few infrared sources embedded in other dark clouds (e.g., GF9, BHR111). The question on the minimum mass for outflow
activity is addressed. 相似文献
6.
The observational approach to the early stages of stellar evolution has been applied to some problems relating to the formation and dissipation of stellar associations, the origin of OB field stars, and low-mass star formation in OB associations. The OB field stars ejected from parent associations are older on the average than the OB stars in the associations. The average duration of active OB-star formation in associations is evaluated. It is suggested that, under the conditions in OB associations, low-mass stars may be formed from dense protostellar objects.Translated fromAstrofizika, Vol. 39, No. 3, pp. 393–406, July–September, 1996. 相似文献
7.
We present a brief overview of the theory of stellar winds with a strong emphasis on the radiation-driven outflows from massive
stars. The resulting implications for the evolution and fate of massive stars are also discussed. Furthermore, we relate the
effects of mass loss to the angular momentum evolution, which is particularly relevant for the production of long and soft
gamma-ray bursts. Mass-loss rates are not only a function of the metallicity, but are also found to depend on temperature,
particularly in the region of the bi-stability jump at 21 000 Kelvin. We highlight the role of the bi-stability jump for Luminous
Blue Variable (LBV) stars, and discuss suggestions that LBVs might be direct progenitors of supernovae. We emphasize that
radiation-driven wind studies rely heavily on the input opacity data and linelists, and that these are thus of fundamental
importance to both the mass-loss predictions themselves, as well as to our overall understanding of the lives and deaths of
massive stars. 相似文献
8.
An observational review is provided of the properties of accretion disks around young stars. It concerns the primordial disks of intermediate- and high-mass young stellar objects in embedded and optically revealed phases. The properties were derived from spatially resolved observations and, therefore, predominantly obtained with interferometric means, either in the radio/(sub)millimeter or in the optical/infrared wavelength regions. We make summaries and comparisons of the physical properties, kinematics, and dynamics of these circumstellar structures and delineate trends where possible. Amongst others, we report on a quadratic trend of mass accretion rates with mass from T Tauri stars to the highest mass young stellar objects and on the systematic difference in mass infall and accretion rates. 相似文献
9.
L. Kaper L.E. Ellerbroek B.B. Ochsendorf R.N. Caballero Pouroutidou 《Astronomische Nachrichten》2011,332(3):232-237
The birth process and (early) evolution of massive stars is still poorly understood. Massive stars are rare, their birthplaces are hidden from view and their formation timescale is short. So far, our physical knowledge of these young massive stars has been derived from near‐IR imaging and spectroscopy, revealing populations of young OB‐type stars, some still surrounded by a (remnant?) accretion disk, others apparently “normal” main sequence stars powering H II regions. The most important spectral features of OB‐type stars are, however, located in the UV and optical range. With VLT/X‐shooter it is possible to extend the spectral coverage of these young massive stars into the optical range, to better determine their photospheric properties, to study the onset of the stellar wind, and to characterize the physical structure of the circumstellar disk (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
10.
W.-R. Hamann 《Astrophysics and Space Science》2010,329(1-2):151-158
Stellar winds appear as a persistent feature of hot stars, irrespective of their wide range of different luminosities, masses, and chemical composition. Among the massive stars, the Wolf–Rayet types show considerably stronger mass loss than the O stars. Among hot low-mass stars, stellar winds are seen at central stars of planetary nebulae, where again the hydrogen-deficient stars show much stronger winds than those central stars with “normal” composition. We also studied mass-loss from a few extreme helium stars and sdOs. Their mass-loss rate roughly follows the same proportionality with luminosity to the power 1.5 as the massive O stars. This relation roughly marks a lower limit for the mass loss from hot stars of all kinds, and provides evidence that radiation pressure on spectral lines is the basic mechanism at work. For certain classes of stars the mass-loss rates lie significantly above this relation, for reasons that are not yet fully understood. Mass loss from low-mass stars may affect their evolution, by reducing the envelope mass, and can easily prevent diffusion from establishing atmospheric abundance patterns. In close binary systems, their winds can feed the accretion onto a companion. 相似文献
11.
Via a study of the evolutionary tracks of 3∼10 M stars on the Hertzsprung-Russell diagram, the variations of the energy, density, temperature at the peak of helium-shell burning, ratio of surface luminosity of helium shell to stellar surface luminosity as well as the stellar radius are analyzed. Then the demarcation point of medium-mass stars in the evolution from early AGB stars to thermally pulsing AGB stars on the HR diagram is determined, and for 119 carbon stars our analysis agrees rather well with observation. At the same time the following is suggested. After arriving at this demarcation point in stellar evolution, in the formula of the loss of stellar wind material it is probably needed to introduce a quantity which is not concerned with the surface luminosity, but it dominates the formation of super stellar wind. On this basis and via the analysis of the structure and evolution of 5 M stars as well as the rate of mass loss of stellar wind, it is found that the effect of turbulent pressure on the mass loss of stellar wind in the stage of thermally pulsing AGB stars is rather great, hence the turbulent pressure of thermally pulsing AGB stars cannot be overlooked. Furthermore, the physical factors which possibly affect the matter loss of the stellar winds of thermally pulsing AGB stars are suggested. 相似文献
12.
对于大质量的形成，由于区域的遥远和结构的复杂以及过程的特殊，研究相对迟缓，对可能形成大质量得的云核和大质量年轻星体的活动，进行多波段搜寻和研究，取得了相应的进展。 相似文献
13.
E. J. Delgado-Donate C. J. Clarke M. R. Bate 《Monthly notices of the Royal Astronomical Society》2003,342(3):926-938
We present results from high-resolution hydrodynamical simulations that explore the effects of small-scale clustering in star-forming regions. A large ensemble of small- N clusters with five stellar seeds have been modelled and the resulting properties of stars and brown dwarfs statistically derived and compared with observational data.
Close dynamical interactions between the protostars and competitive accretion driven by the cloud collapse are shown to produce a distribution of final masses that is bimodal, with most of the mass residing in the binary components. When convolved with a suitable core mass function, the final distribution of masses resembles the observed initial mass function, in both the stellar and substellar regimes. Binaries and single stars are found to constitute two kinematically distinct populations, with about half of the singles attaining velocities ≥2 km s−1 , which might deprive low-mass star-forming regions of their lightest members in a few crossing times. The eccentricity distribution of binaries and multiples is found to follow a distribution similar to that of observed long-period (uncircularized) binaries.
The results obtained support a mechanism in which a significant fraction of brown dwarfs form under similar circumstances as those of normal stars but are ejected from the common envelope of unstable multiple systems before their masses exceed the hydrogen burning limit. We predict that many close binary stars should have wide brown dwarf companions. Brown dwarfs, and, in general, very low-mass stars, would be rare as pure binary companions. The binary fraction should be a decreasing function of primary mass, with low-mass or substellar primaries being scarce. Where such binaries exist, they are expected either to be close enough (semimajor axis ∼10 au) to survive strong interactions with more massive binaries or to be born in very small molecular cloud cores. 相似文献
Close dynamical interactions between the protostars and competitive accretion driven by the cloud collapse are shown to produce a distribution of final masses that is bimodal, with most of the mass residing in the binary components. When convolved with a suitable core mass function, the final distribution of masses resembles the observed initial mass function, in both the stellar and substellar regimes. Binaries and single stars are found to constitute two kinematically distinct populations, with about half of the singles attaining velocities ≥2 km s
The results obtained support a mechanism in which a significant fraction of brown dwarfs form under similar circumstances as those of normal stars but are ejected from the common envelope of unstable multiple systems before their masses exceed the hydrogen burning limit. We predict that many close binary stars should have wide brown dwarf companions. Brown dwarfs, and, in general, very low-mass stars, would be rare as pure binary companions. The binary fraction should be a decreasing function of primary mass, with low-mass or substellar primaries being scarce. Where such binaries exist, they are expected either to be close enough (semimajor axis ∼10 au) to survive strong interactions with more massive binaries or to be born in very small molecular cloud cores. 相似文献
14.
T. Montmerle 《Astronomische Nachrichten》2008,329(2):210-213
Star-forming regions have been observed in X-rays since the first generation of satellites in the late 70s. They are very rich in magnetically-controlled X-ray phenomena: stellar flares and star-disk interactions in hundreds of T Tauri stars, confined winds in massive stars, etc. More recently, in a few low-mass stars, X-ray evidence has been found for accretion shocks. Even if it is not dominant, when it is found the influence of the circumstellar environment on X-ray emission gives precious clues on the magnetic structure in the vicinity of young stars. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
15.
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
16.
Shaolan Bi 《Astrophysics and Space Science》1993,206(2):179-190
The detailed evolution of low-mass main-sequence stars (M < 1M
) with a compact companion is studied. For angular momentum loss associated with magnetic braking it is found that about 10–11–10–12
M
yr–1 in stellar wind loss would be required. This wind is 102–103 times stronger than the solar wind, so we believe here magnetic stellar wind is insufficient. It is well known that there is mass outflow in low-mass close binary systems. We believe here that these outflows are centrifugal driven winds from the outer parts of the accretion disks. The winds extract angular momentum from these systems and therefore drive secular evolution. Disk winds are preferred to winds from the secondary, because of the lower disk surface gravity. 相似文献
17.
Recent observational studies of the chemical composition of circumstellar matter around both high- and low-mass young stellar objects are reviewed. The molecular abundances are found to be a strong function of evolutionary state, but not of system mass or luminosity. The data are discussed with reference to recent theoretical models. 相似文献
18.
We present high-resolution observations made with the Very Large Array (VLA) in its A configuration at frequencies between 5 and 43 GHz of a sample of five massive young stellar objects (YSOs): Lk Hα101, NGC 2024-IRS2, S106-IR, W75N and S140-IRS1. The resolution varied from 0.04 arcsec (at 43 GHz) to 0.5 arcsec (at 5 GHz), corresponding to a linear resolution as high as 17 au for our nearest source. A MERLIN observation of S106-IR at 23 GHz with 0.03-arcsec resolution is also presented. S106-IR and S140-IRS1 are elongated at 43 GHz perpendicular to their large-scale bipolar outflows. This confirms the equatorial wind picture for these sources seen previously in MERLIN 5-GHz observations. The other sources are marginally resolved at 43 GHz. The spectral indices we derive for the sources in our sample range from +0.2 to +0.8, generally consistent with ionized stellar winds. We have modelled our sources as uniform, isothermal spherical winds, with Lk Hα101 and NGC 2024-IRS2 yielding the best fits. However, in all cases our fits give wind temperatures of only 2000–5000 K, much less than the effective temperatures of main-sequence stars of the same luminosity, a result which is likely due to the clumpy nature of the winds. 相似文献
19.
J. M. Oliveira R. D. Jeffries J. Th. van Loon 《Monthly notices of the Royal Astronomical Society》2009,392(3):1034-1050
NGC 6611 is the massive young cluster (2–3 Myr) that ionizes the Eagle Nebula. We present very deep photometric observations of the central region of NGC 6611 obtained with the Hubble Space Telescope and the following filters: ACS/WFC F775W and F850LP and NIC2 F110W and F160W, loosely equivalent to ground-based IZJH filters. This survey reaches down to I ∼ 26 mag . We construct the initial mass function (IMF) from ∼1.5 M⊙ well into the brown dwarf regime (down to ∼0.02 M⊙ ). We have detected 30–35 brown dwarf candidates in this sample. The low-mass IMF is combined with a higher-mass IMF constructed from the ground-based catalogue from Oliveira et al. We compare the final IMF with those of well-studied star-forming regions: we find that the IMF of NGC 6611 more closely resembles that of the low-mass star-forming region in Taurus than that of the more massive Orion Nebula Cluster. We conclude that there seems to be no severe environmental effect in the IMF due to the proximity of the massive stars in NGC 6611. 相似文献
20.
恒星形成于分子云环境中。近30多年的观测研究使得天文学家对小质量恒星的形成有了相对明确的认识：小质量恒星通过坍缩、吸积和外向流的路标而形成。至于大质量恒星，其形成过程还存在着许多不确定因素，现有的观测证据表明：大质量恒星也可能通过坍缩、吸积和外向流的路标来形成，但也不排除在星团中通过中小质量恒星聚合而成的因素。大质量恒星形成与致密电离氢区(UCHII)成协较好，而与大质量恒星形成区成协的分子云环境中，既有大质量恒星也有小质量恒星形成。综述了恒星形成各个阶段的观测结果和研究现状以及成协的天体物理环境情况。未来的观测和研究重点在于，大质量恒星形成以及星团环境中的恒星形成。 相似文献