共查询到20条相似文献，搜索用时 15 毫秒
1.
J. A. Yates E. M. L. Humphreys A. M. S. Richards 《Astrophysics and Space Science》1997,251(1-2):285-288
The combination of a time-dependent spherically symmetric hydrodynamic model of stellar atmosphere pulsation and a radiation
transport code, which incorporates maser saturation theory, enabled us to synthesise maps and spectra of H2O maser emission from the circumstellar envelopes of long period variable stars.
The synthetic maps and spectra compare favourably with observed 22, 321 and 325 GHz H2O maser emission. As is observed in H2O maser regions the peak emission occurs between 3–8 stellar radii from the star. The calculated H2O maser regions are in conditions of nH2 = 106 − 108 cm−3, assuming a fractional abundance of 10−4; kinetic temperatures of 550–3000 K; dust ensemble temperatures of 500–1200 K and an accelerating velocity field. The IR
radiation field is explicitly included in the radiation transport model, incorporating the latest absorption efficiency data
for silicates from Draine. We reproduce the features seen in high angular resolution MERLIN spectral line datacubes. This
shows that a mass outflow model which extends the photosphere using pulsations and incorporates radiation pressure on silicate
based dust particles can produce the observed data on small (10-mas) angular scales.
This revised version was published online in September 2006 with corrections to the Cover Date. 相似文献
2.
Dinh-V-Trung † 《Monthly notices of the Royal Astronomical Society》2009,399(3):1495-1505
In this paper, we investigate the polarization property of the radiation amplified by astronomical masers in the presence of a strong magnetic field. Our model explicitly takes into account the broad-band nature of the radiation field and the interaction of the radiation with the maser transition J = 1–0 . The amplification of different realizations of the background continuum radiation by the maser is directly simulated and the Stokes parameters of the radiation field are then obtained by averaging over the ensemble of emerging maser radiation. For isotropic pumping and partially saturated masers, we find that the maser radiation is linearly polarized in two representative cases where the magnetic field B makes an angle θ= 30° and 90° to the maser axis. The linear polarization for maser radiation obtained in our simulations for both cases is in agreement with the results of the standard model. Furthermore, no instability during amplification is seen in our simulations. Therefore, we conclude that there is no problem with the previous numerical investigations of maser polarization in the unsaturated and partially saturated regime. 相似文献
3.
M. Szymczak 《Astrophysics and Space Science》1987,139(1):63-73
The interstellar UV field at 1565 Å is calculated around nearby OH/IR sources. The front-back asymmetry observed in the 1612 MHz maser line profile is well correlated with anisotropy of the interstellar UV flux. For some sources the spatial positions of the 1612 MHz masers are confined to the position angles for which stronger UV radiation occurs. These facts strongly support the theory of the photoproduction of OH from H2O induced by ambient interstellar UV photons penetrating the circumstellar shell. A simple model of the 1612 MHz maser with OH photoproduction suggests that the influence of the UV field on the observed maser profiles is governed by the mass loss rate and the relative abundances of OH and H2O molecules. 相似文献
4.
Leif Holmlid 《Astrophysics and Space Science》2006,305(1):91-98
The detailed processes giving maser line radiation from various molecules in space are not well understood, as can be seen from many recent detailed studies of maser line emission with high spatial and velocity resolution, and with polarization measurements. We now propose an improved maser mechanism based on amplification of the original molecular line emission by stimulated emission in Rydberg Matter (RM) clouds in HII regions, containing clusters H
N
and (H2)
N
. This mechanism will amplify the molecular lines, depending on the position, velocity, cluster size and state of excitation of the clusters in the RM cloud. RM will only support certain frequencies, corresponding to rotational transitions of the clusters. The bond lengths in the RM clusters are known within 1% from radio frequency emission measurements in the laboratory, and it is now shown that all the commonly studied maser lines agree well with stimulated emission transitions in several types of RM clusters simultaneously. This may explain the strongly varying intensities of neighboring or related maser lines, an important effect that is not well understood previously. It is also pointed out that the magnetic field due to RM is of the same order of magnitude as observed from the Zeeman splitting in maser lines; thus, the molecules that are the original sources of the lines may be embedded in the RM clouds, for example in dense HII regions that are likely to be RM regions. 相似文献
5.
Jiong Liu Ye-Fei Yuan Xiao-Long Deng Center for Astrophysics University of Science Technology of China Hefei 《中国天文和天体物理学报》2007,7(2):304-316
Synchro-curvature radiation describes the emission from a relativistic charged par- ticle which is moving and spiralling in a curved magnetic field. We investigate the maser emission for synchro-curvature radiation including drift of the guiding center of the radiating electron. It is shown that under some conditions the absorption coefficient can be negative, so maser can happen. These conditions are different from those needed for maser emission of curvature radiation including drift of the charged particles. We point out that our results, in- cluding the emissivity, can reduce to these of curvature radiation. Previously it was found that synchro-curvature radiation can not generate maser in vacuum, but we argue that synchro- curvature radiation including drift can generate maser even in vacuum. We discuss the possi- bilities of the potential applications of the synchro-curvature maser in modeling gamma ray bursts and pulsars. 相似文献
6.
Rudolf A. Treumann 《Astronomy and Astrophysics Review》2006,13(4):229-315
The electron–cyclotron maser is a process that generates coherent radiation from plasma. In the last two decades, it has gained increasing attention as a dominant mechanism of producing high-power radiation in natural high-temperature magnetized plasmas. Originally proposed as a somewhat exotic idea and subsequently applied to include non-relativistic plasmas, the electron–cyclotron maser was considered as an alternative to turbulent though coherent wave–wave interaction which results in radio emission. However, when it was recognized that weak relativistic corrections had to be taken into account in the radiation process, the importance of the electron–cyclotron maser rose to the recognition it deserves. Here we review the theory and application of the electron–cyclotron maser to the directly accessible plasmas in our immediate terrestrial and planetary environments. In situ access to the radiating plasmas has turned out to be crucial in identifying the conditions under which the electron–cyclotron maser mechanism is working. Under extreme astrophysical conditions, radiation from plasmas may provide a major energy loss; however, for generating the powerful radiation in which the electron–cyclotron maser mechanism is capable, the plasma must be in a state where release of susceptible amounts of energy in the form of radiation is favorable. Such conditions are realized when the plasma is unable to digest the available free energy that is imposed from outside and stored in its particle distribution. The lack of dissipative processes is a common property of collisionless plasmas. When, in addition, the plasma density becomes so low that the amount of free energy per particle is large, direct emission becomes favorable. This can be expressed as negative absorption of the plasma which, like in conventional masers, leads to coherent emission even though no quantum correlations are involved. The physical basis of this formal analogy between a quantum maser and the electron–cyclotron maser is that in the electron–cyclotron maser the free-space radiation modes can be amplified directly. Several models have been proposed for such a process. The most famous one is the so-called loss-cone maser. However, as argued in this review, the loss-cone maser is rather inefficient. Available in situ measurements indicate that the loss-cone maser plays only a minor role. Instead, the main source for any strong electron–cyclotron maser is found in the presence of a magnetic-field-aligned electric potential drop which has several effects: (1) it dilutes the local plasma to such an extent that the plasma enters the regime in which the electron–cyclotron maser becomes effective; (2) it generates energetic relativistic electron beams and field-aligned currents; (3) it deforms, together with the magnetic mirror force, the electron distribution function, thereby mimicking a high energy level sufficiently far above the Maxwellian ground state of an equilibrium plasma; (4) it favors emission in the free-space RX mode in a direction roughly perpendicular to the ambient magnetic field; (5) this emission is the most intense, since it implies the coherent resonant contribution of a maximum number of electrons in the distribution function to the radiation (i.e., to the generation of negative absorption); (6) it generates a large number of electron holes via the two-stream instability, and ion holes via the current-driven ion-acoustic instability which manifest themselves as subtle fine structures moving across the radiation spectrum and being typical for the electron–cyclotron maser emission process. These fine structures can thus be taken as the ultimate identifier of the electron–cyclotron maser. The auroral kilometric radiation of Earth is taken here as the paradigm for other manifestations of intense radio emissions such as the radiation from other planets in the solar system, from exoplanets, the Sun and other astrophysical objects. 相似文献
7.
In recent years radiation has been observed from planets, Sun and stars that is best explained by the cyclotron maser instability; in fact, all celestial bodies that might feasibly emit and be detected by their cyclotron maser radiation have been detected. Here we review those observations, the developments in the theory, the recent work on the effiency of energy transfer by cyclotron maser radiation, and some recent and future observations that might demonstrate whether the mechanism is energetically important in solar and stellar flares.
相似文献8.
9.
氢原子钟用吸附泵调研报告 总被引：1，自引：0，他引：1
杨浩 《中国科学院上海天文台年刊》2006,(1):100-106
阐述了吸附泵的工作原理与结构特点;介绍了国内外吸附泵在氢原子钟上的使用情况;提出了吸附泵在上海天文台氢原子钟上的使用建议。 相似文献
10.
Interstellar H2O and OH masers associated with massive star-forming regions can be classified into three morphological types: isolated H2O masers; isolated OH masers; and spatially overlapping OH/H2O maser groups. In a large sample of star-forming regions the total number of maser groups of each type is approximately equal.
In order to account for these statistics we propose a pumping scheme based on a broadband radiative pump which produces inverted
populations of both OH and H2O masers by a process involving predissociation and dissociation of H2O. This scheme overcomes some drawbacks of earlier radiative pumping models, and may account for the association of OH and
H2O masers in massive star forming regions. 相似文献
11.
It has been argued that the loss-cone-driven electron cyclotron maser instability can account for the properties of millisecond microwave spike bursts observed during some solar flares. However, as it propagates outward from the corona, maser radiation undergoes gyroresonance absorption when its frequency is a harmonic of the local electron-cyclotron frequency. Existing analytical models using slab geometries predict that this absorption should be sufficiently strong to prevent the radiation from being seen at the observed levels, except under highly restrictive conditions or for unrealistic plasma parameters. A more comprehensive analysis is presented here to determine if and when maser radiation can escape to produce microwave spike bursts. This analysis employs numerical raytracing and incorporates propagation and absorption of fundamental maser emission in a realistic model of a coronal flux loop. It is found that ranges of physical conditions do exist under which maser radiation can escape to an observer and that these conditions are much more limiting for fundamental emission in the extraordinary ()-mode than in the ordinary (o)-mode. Detailed investigation implies that escaping radiation in the -mode is highly directional and chiefly observable toward the center of the solar disk, while escapingo-mode radiation is found to emerge from the corona over a much wider range of directions, with some cases corresponding to radiation observable near the solar limb. 相似文献
12.
猎户座中SiO脉泽分布的新模型 总被引：1，自引：0，他引：1
我们研究了Orion-IRc2SiO脉泽饱和辐射谱线轮廓，找到了很好的拟和谱线轮廓的函数．在观测结果中发现，Orion－IRc2SiO脉泽饱和辐射谱线轮廓中存在着若干峰，为了解释这一现象，我们提出了一个新的分布模型，即多重分离的旋转膨胀盘壳模型.计算结果表明，这个新模型，不仅可以解释脉泽的非饱和辐射谱，而且可以解释脉泽的饱和辐射谱. 相似文献
13.
In recent years radiation has been observed from planets, Sun and stars that is best explained by the cyclotron maser instability; in fact, all celestial bodies that might feasibly emit and be detected by their cyclotron maser radiation have been detected. Here we review those observations, the developments in the theory, the recent work on the effiency of energy transfer by cyclotron maser radiation, and some recent and future observations that might demonstrate whether the mechanism is energetically important in solar and stellar flares.This work was supported in part by NASA's Solar Heliospheric Physics and Solar Terrestrial Theory Programs under grants NSG-7287 and NAGW-91 to the University of Colorado. The numerical simulations were performed on the Cray XMP at the San Diego Supercomputer Center which is funded by the National Science Foundation. 相似文献
14.
Generation of auroral kilometric radiation (AKR) in the auroral acceleration region is studied. It is shown that auroral kilometric radiation can be generated by the backscattered electrons trapped in the acceleration region via a cyclotron maser process. The parallel electric field in the acceleration region is required to be distributed over 1–2 RE. The observed AKR frequency spectrum can be used to estimate the altitude range of the auroral acceleration region. The altitudes of the lower and upper boundaries of the acceleration region determined from the AKR data are respectively ～2000 and ～9000 km. 相似文献
15.
The electron-cyclotron maser is believed to be the source of microwave spike bursts often observed during solar and stellar flares. Partial absorption of this radiation as it propagates through the corona can produce plasma heating and soft X-ray emission over an extended region. In this paper, the propagation and absorption of the maser radiation during solar flares are examined through linear theory and electro-magnetic particle simulations. It is shown using linear theory that strong absorption of the radiation should occur as it propagates towards the second harmonic layer where the magnetic field is half as strong as in the emission region. Only radiation propagating nearly parallel to the magnetic field in a low-temperature plasma may be able to escape under certain, limited conditions. Finite temperature effects can cause radiation propagating nearly perpendicular to the magnetic field to refract, causing enhanced absorption. Particle simulations are then used to evaluate the nonlinear response of the plasma as the maser radiation propagates through the absorption layer. It is shown that some of the maser radiation is able to escape through a process of absorption below the second harmonic of the local gyrofrequency and re-emission above it. The fraction able to escape is much higher than that predicted by linear theory, although the amount of escaping energy is only a small fraction of the incident energy. The bulk of incident energy goes into the perpendicular heating of the ambient electrons, with the rate of energy absorption showing no signs of leveling off during the simulations. This indicates that the absorption layer does not become optically thin after continuous heating by the maser radiation. A few electrons are accelerated to several tens of keVs as a result of the heating. 相似文献
16.
S. Goedhart V. Minier M. J. Gaylard D. J. van der Walt 《Monthly notices of the Royal Astronomical Society》2005,356(3):839-848
The class II methanol maser source G9.62+0.20E undergoes periodic flares at both 6.7 and 12.2 GHz. The flare starting in 2001 October was observed at seven epochs over three months using the Very Long Baseline Array (VLBA) at 12.2 GHz. High angular resolution images (beam size ∼1.7 × 0.6 mas ) were obtained, enabling us to observe changes in 16 individual maser components. It was found that while existing maser spots increased in flux density, no new spots developed and no changes in morphology were observed. This rules out any mechanism which disturbs the masing region itself, implying that the flares are caused by a change in either the seed or pump photon levels. A time delay of one to two weeks was observed between groups of maser features. These delays can be explained by light travel time between maser groups. The regularity of the flares can possibly be explained by a binary system. 相似文献
17.
H2O maser emissions have been found in external galaxies for more than 30 years. Main sciences associated with extragalactic H2O masers can be summarized roughly into three parts: maser emission itself, AGN sciences and cosmology exploration. Our work in this field focusses on two projects: X-ray data analysis of individual maser source using X-ray penetrability to explore maser host obscured AGN; multi-wavelength statistical properties of the whole published H2O maser sample. Here their nuclear radio properties were investigated in detail, based on their 6-cm and 20-cm radio observation data. Comparing the radio properties between maser-detected sources and non-detected sources at similar distance scale, we find that: (1) maser host galaxies tend to have higher nuclear radio luminosity; (2) the spectral index of both samples is comparable (～0.6), within the error ranges. In addition, for AGN-maser sources, the isotropic maser luminosity tends to increase with rising radio luminosity. Thus we propose the nuclear radio luminosity as one good indicator for searching AGN-masers in the future. 相似文献
18.
本文介绍了氢钟钛合金钟罩的设计制作以及机械泵─分子泵前级真空机组的改进设计．由于钛是一种崭新的工程金属，用钛合金制作的真空钟罩由于采用了较正确的工艺手段，具有强度高、重量轻、在真空中放气率低、无磁、外表美观和经济实惠等优点．而前级真空机组，由于采用了不锈钢波纹管及卡箍结构，无粗大的真空橡皮管，也改善了氢钟的真空条件．钛合金钟罩及改进型前级真空机组的应用对氢钟钛离子泵的启动及系统真空度的提高起了一定的作用． 相似文献
19.
A fully relativistic electron maser is proposed for the explanation of certain non-thermal solar and stellar radio bursts. This mechanism (maser synchrotron) is based on a gyroresonant interaction between waves and electrons of high energies and uses the free energy contained in an electronic distribution function that peaks for energies around 1 MeV.By a calculation of the growth rates of the three electromagnetic modes, we show that the X-mode prevails for values of
p/
cup to 2 or 3. This result is very different from the standard cyclotron maser case where such values of
p/
clead to quench the X-mode growth. Hence, the synchrotron maser instability appears to be a direct and efficient amplification process for considerably larger physical conditions than the cyclotron maser. In addition, the radiation, emitted over the second gyroharmonic, freely propagates without a strong reabsorbtion at the 2
clayer. All these points can constitute major advantages of this mechanism in an astrophysical context.Proceedings of the Second CESRA Workshop on Particle Acceleration and Trapping in Solar Flares, held at Aubigny-sur-Nère (France), 23–26 June, 1986. 相似文献
20.
Ye Xu Xing-Wu Zheng Dong-Rong JiangNational Astronomical Observatories Chinese Academy of Sciences Urumqi xuye@nju.edu.cnShanghai Astronomical Observatory Chinese Academy of Sciences Shanghai Department of Astronomy Nanjing University Nanjing 《中国天文和天体物理学报》2003,3(1):49-68
We present a statistical analysis of 482 6.7 GHz methanol maser sources from the available literature, on their maser emission and the characteristics of their associated infrared sources. On the color-color diagram, more than 70% of the objects fall within a very small region (0.57 ≤ [25 - 12] ≤ 1.30 and 1.30 ≤[60 - 12] ≤ 2.50). This suggests that 6.7 GHz methanol maser emission occurs only within a very short evolutionary phase during the earliest stage of star formation. The velocity ranges of the masers belong to two main groups: one from 1 to 10 km s^-1, and one from about 11 to 20 km s^-1. These velocity ranges indicate that the masers are probably associated with both disks and outflows. The correlations between the maser and infrared flux densities, and between the maser and infrared luminosities, suggest that far-infrared radiation is a possible pumping mechanism for the masers which most probably originate from some outer molecular envelopes or disks. 相似文献