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1.
We demonstrate a situation where the wavelength dependence of the intrinsic linear polarization of stellar radiation matches that of the interstellar linear polarization described by the Serkowski law. Such a situation can arise when the radiation from a star with a dipole magnetic field is scattered in a circumstellar plasma shell with a uniform electron density distribution. As a result, we have estimated the magnetic field strength at the photospheric phase of Supernova 1999gi. We show that the existence of intrinsic polarization in Galactic stars disguised as interstellar polarization is possible in principle. 相似文献
2.
The initially supersonic flow of the solar wind passes through a magnetic shock front where its velocity is supposed to be reduced to subsonic values. The location of this shock front is primarily determined by the energy density of the external interstellar magnetic field and the momentum density of the solar wind plasma. Interstellar hydrogen penetrating into the heliosphere undergoes charge exchange processes with the solar wind protons and ionization processes by the solar EUV radiation. This results in an extraction of momentum from the solar wind plasma. Changes of the geometry and the location of the shock front due to this interaction are studied in detail and it is shown that the distance of the magnetic shock front from the Sun decreases from 200 to 80 AU for an increase of the interstellar hydrogen density from 0.1 to 1.0 cm−3. The geometry of the shock front is essentially spherical with a pronounced embayment in the direction opposite to the approach of interstellar matter which depends very much on the temperature of the interstellar gas. Due to the energy loss by the interaction with neutral matter the solar wind plasma reduces its velocity with increasing distance from the Sun. This modifies Parker's solution of a constant solar wind velocity. 相似文献
3.
Using a well-known method for calculating the propagation of waves in an inhomogeneous medium, we have managed to reduce the
problem of wave propagation in pulsar magnetospheres to a system of two ordinary differential equations that allow the polarization
characteristics of the radio emission to be quantitatively described for any magnetic field structure and an arbitrary density
profile of the outflowing plasma. We confirm that for ordinary pulsars (period P ∼ 1 s, magnetic field B
0 ∼ 1012 G, particle production multiplicity parameter λ ∼ 104), the polarization is formed inside the light cylinder at a distance of the order of a thousand neutron star radii. For reasonable
magnetic field strengths and plasma densities on the emission propagation path, the degree of circular polarization is found
to be ∼5–20%, in good agreement with observations. 相似文献
4.
Pierre Bastien François Ménard Patrice Corporon Nadine Manset Frédérick Poidevin Gaspard Duchêne Jean-Louis Monin 《Astrophysics and Space Science》2004,292(1-4):427-433
We obtained linear polarization observations of 82 A/B-type stars in the young cluster NGC 6611, in order to probe the circumstellar material and to search for any evidence of intracluster or interstellar material that could also contribute to the polarization. We found linear polarization values that reach up to 14%. We consider the distribution of the polarization, its position angle, correlations with extinction and membership probability, polarization variability and wavelength distribution to identify the origin of the polarization toward NGC 6611. The polarization is found to be dominated by interstellar polarization, although some stars also have some circumstellar polarization. There is no evidence for intracluster dust. Rather, the dust must be located in a low density cloud toward the general line of sight to NGC 6611 and in front of it. The depth of that cloud along the line of sight increases slowly from the south–east to the north–west. The cloud is threaded by a very uniform magnetic field. 相似文献
5.
近红外偏振是研究恒星形成的有效工具.该文介绍了近红外偏振器的工作原理,然后分几个方面介绍了近红外偏振在恒星形成研究中的应用.红外反射云能很好地示踪年轻星天体及分子外流,通过分析偏振矢量的方法确定红外反射云的偏振对称中心,从而确定它的照亮源;偏振波长相关曲线包含了年轻星天体的星周物质的很多信息;年轻星的分子外流导致了红外反射云的形成,因此红外反射云的照亮源通常与年轻星天体成协,并是分子外流的驱动源;一些年轻星天体埋藏得很深,一般在近红外波段无法直接探测到,人们称之为深埋源,通过分析偏振矢量的方法可以找到深埋源;一般认为比较年轻的年轻星天体都是有尘埃盘的,尘埃盘的存在会导致它的偏振形态出现偏振盘,偏振盘町以用来研究尘埃盘;恒星形成区里成员星的偏振主要是由尘埃的二色性消光产生的,这样偏振方向会平行于致使尘埃排列的磁场的方向,从而能够揭示磁场的结构.最后进行了总结,并论述了中远红外偏振研究的优势和意义. 相似文献
6.
Since collective plasma behavior may determine important transport processes (e.g., plasma diffusion across a magnetic field) in certain cosmic environments, it is important to delineate the parameter space in which weakly ionized cosmic gases may be characterized as plasmas. In this short note, we do so. First, we use values for the ionization fraction given in the literature, wherein the ionization is generally assumed to be due primarily to ionization by cosmic rays. We also discuss an additional mechanism for ionization in such environments, namely, the photoelectric emission of electrons from cosmic dust grains in an interstellar FUV radiation field. Simple estimates suggest that under certain conditions this mechanism may dominate cosmic ray ionization, and possibly also the photoionization of metal atoms by the interstellar FUV field, and thereby lead to an enhanced ionization level. 相似文献
7.
M. A. Walker L. V. E. Koopmans D. R. Stinebring W. van Straten 《Monthly notices of the Royal Astronomical Society》2008,388(3):1214-1222
The dynamic spectrum of a radio pulsar is an in-line digital hologram of the ionized interstellar medium. It has previously been demonstrated that such holograms permit image reconstruction, in the sense that one can determine an approximation to the complex electric field values as a function of Doppler shift and delay, but to date the quality of the reconstructions has been poor. Here we report a substantial improvement in the method which we have achieved by simultaneous optimization of the thousands of coefficients that describe the electric field. For our test spectrum of PSR B0834+06 we find that the model provides an accurate representation of the data over the full 63 dB dynamic range of the observations: residual differences between model and data are noise like. The advent of interstellar holography enables detailed quantitative investigation of the interstellar radio-wave propagation paths for a given pulsar at each epoch of observation. We illustrate this using our test data which show the scattering material to be structured and highly anisotropic. The temporal response of the medium exhibits a scattering tail which extends to beyond 100 μs , and the centroid of the pulse at this frequency and this epoch of observation is delayed by approximately 15 μs as a result of multipath propagation in the interstellar medium. 相似文献
8.
9.
L. Kühn 《Astronomische Nachrichten》1972,294(4):159-174
The region around ξ Persei is qualified for an extended investigation of the interstellar matter in a limited volume because of its position in the Galaxy and the high brightness of ξ Persei. The basis for this analysis were spectra of high dispersion, 21 cm line profiles, photoelectric UBV measurements, measurements of star light polarization, and a photographic three colour photometry. The total numbers of absorbers per cm2 for Ca II and Na I were evaluated by the dublet ratio method. The adaption of observations to models for the distribution of the interstellar gas yielded the abundances of Ca and Na. Ca is deficient by about a factor 150. The total number of H I atoms per cm2 followed from a gaussian analysis of 21 cm lines. The results of the analysis can be best described by a model consisting of a dense H I layer with a density of 20 to 140 cm−3 situated 10 to 30 pc in front of ξ Persei. The interstellar reddening is not uniform in this region; the extension of this cloud in the line of sight is about 100 pc. The structure analysis of reddening provided micro-scales of 0.4 and 4.5 pc for the fluctuations in density of interstellar absorbing matter. On the assumption that the interstellar dust consists of silicate grains the density ratio between interstellar gas and dust amounts to nearly 35. The total mass of the interstellar matter in the analysed region is ≤ 1100 z.dstR;⊙. The direction and strength of the magnetic field were estimated from the star light polarization; according to that the field strength is about 8 · 10−6G. It is likely that the interstellar matter around ξ Persei is connected with the evolution of the association II Persei. 相似文献
10.
Sachin Kaothekar 《Journal of Astrophysics and Astronomy》2016,37(3):23
The effects of finite ion Larmor radius (FLR) corrections, Hall current and radiative heat-loss function on the thermal instability of an infinite homogeneous, viscous plasma incorporating the effects of finite electrical resistivity, thermal conductivity and permeability for star formation in interstellar medium have been investigated. A general dispersion relation is derived using the normal mode analysis method with the help of relevant linearized perturbation equations of the problem. The wave propagation is discussed for longitudinal and transverse directions to the external magnetic field and the conditions of modified thermal instabilities and stabilities are discussed in different cases. We find that the thermal instability criterion gets modified into radiative instability criterion. The finite electrical resistivity removes the effect of magnetic field and the viscosity of the medium removes the effect of FLR from the condition of radiative instability. The Hall parameter affects only the longitudinal mode of propagation and it has no effect on the transverse mode of propagation. Numerical calculation shows stabilizing effect of viscosity, heat-loss function and FLR corrections, and destabilizing effect of finite resistivity and permeability on the thermal instability. The outcome of the problem discussed the formation of star in the interstellar medium. 相似文献
11.
Examination of thermal plasma data obtained by low-altitude satellite measurements indicates that the intersection of the cusp in the dayside magnetosphere with the topside ionosphere creates a distinct plasma geometry at low altitudes. This region consists of one or two plasma discontinuities with steep boundaries. As a result of the plasma structuring in the cusp which commonly takes place in the winter hemisphere, the propagation of compressional surface MHD waves is supported. This point is illustrated by an analysis of the polarization state of compressional surface MHD waves propagating along a plasma layer with thickness a and ambient magnetic field B0 parallel to the interfaces. The results obtained are applicable to the case of a single interface, which is derived in the limit a → ∞. In the general case the polarization of the compressional surface MHD waves in the plane transverse to the magnetic field B0 is elliptical. This feature of the polarization state of the compressional surface modes does not follow from the former analysis by Edwin and Roberts (1982, Solar Phys. 76, 239) for a magnetic slab, because the disturbance components parallel to the interfaces and perpendicular to the magnetic field B0 have not been examined. Although the absence of these components does not prove to be essential for deriving the exact dispersion equation for arbitrary wave directions of the surface modes, they must be included when considering polarization states. The surface mode polarization in the plasma layer changes its sense three times: at interfaces X = 0 and X = a and in the middle plane X = a/2. For the symmetrical (sausage) mode the wave disturbance component bn transverse (normal) to the interfaces becomes zero in the middle plane; for the asymmetrical (kink) mode, the component bt parallel to the interfaces and transverse to the ambient magnetic field is zeroed in the same plane. For a moving observer such as a satellite the polarization patterns which might be recorded change, depending on the velocity of the observer and the angles at which the layered cusp is traversed. An essential feature in the polarization of the compressional surface MHD modes is the presence of jumps in the magnetic disturbance component bt at the interfaces. These jumps disappear only for propagation along the ambient magnetic field. In this particular case the component bt vanishes and then the surface modes are undistinguishable from the body modes. 相似文献
12.
We demonstrate a new way of studying interplanetary magnetic field—Ground State Alignment (GSA). Instead of sending thousands of space probes, GSA allows magnetic mapping with any ground telescope facilities equipped with spectropolarimeter. The polarization of spectral lines that are pumped by the anisotropic radiation from the Sun is influenced by the magnetic realignment, which happens for magnetic field (<1 G). As a result, the linear polarization becomes an excellent tracer of the embedded magnetic field. The method is illustrated by our synthetic observations of the Jupiter’s Io and comet Halley. Polarization at each point was constructed according to the local magnetic field detected by spacecrafts. Both spatial and temporal variations of turbulent magnetic field can be traced with this technique as well. The influence of magnetic field on the polarization of scattered light is discussed in detail. For remote regions like the IBEX ribbons discovered at the boundary of interstellar medium, GSA provides a unique diagnostics of magnetic field. 相似文献
13.
We calculate the polarization of the radiation from an optically thick accretion disk with a vertical averaged magnetic field.
The polarization arises from the scattering of light by free electrons in a magnetized disk plasma. The Faraday rotation of
the polarization plane during the propagation of a photon in a medium with a magnetic field is considered as the main effect.
We discuss various models of optically thick accretion disks with a vertical averaged magnetic field. Our main goal is to
derive simple asymptotic formulas for the polarization of radiation in the case where the Faraday rotation angle Ψ ≫ 1 at
the Thomson optical depth τ = 1. The results of our calculations allow the magnetic field strength in the region of the marginally
stable orbit near a black hole to be estimated from polarimetric observations, including X-ray observations expected in the
future. Since the polarization spectrum of the radiation strongly depends on the accretion disk model, a realistic physical
model of the accretion disk can be determined from data on the polarization of its radiation. 相似文献
14.
N. L. Jabir S. R. Jabbar S. A. H. Salih Q. S. Majeed 《Astrophysics and Space Science》1986,123(2):351-362
It is shown that available optical and ultraviolet data relating to the interstellar dust are elegantly explained on the basis of a modified microbal grain model. A model comprised of two biologically derived components, modified under interstellar conditions. Data on interstellar extinction, albedo and polarization may be accounted for by this model. 相似文献
15.
Nobuki Kawashima Susumu Sasaki Osamu Kaneko Akira Yamori Ryo Okamura Shoichi Okamura 《Planetary and Space Science》1978,26(4):367-373
A controlled active experiment was performed by a plasma gun on-board a rocket to study the plasma stream across the magnetic field in space. A mother and daughter system was employed. An 8 kV 0.8 μF (25 J) capacitor bank on the daughter rocket was discharged every 12 s and about 1016 ion and electron pairs were ejected. A plasma signal was detected on the electron temperature probe. The propagation speed of the plasma stream is estimated to be 106 cm/s, considerably lower than that obtained in the prelaunch test. The amplitude of the plasma signal decays as the inverse square of the mutual distance between the mother and daughter rockets and this decay is much slower than the free streaming. The importance of the geomagnetic field on the plasma propagation in space is discussed. 相似文献
16.
Our current understanding of the spatial and temporal distribution of interplan etary neutral hydrogen is currently limited to a comparison of Lyman-σ photometric data with predictions of the solar backscattered radiation using theoretical models. In this paper, how the uncertainties in current model calculations could be reduced through the future use of polarization measurements made from interplanetary spacecraft is investigated. In particular, inquiry into how a mapping of the degree of linear polarization made from a spacecraft at various locations in the Solar System can improve knowledge of the interstellar wind parameters, number density, temperature, and velocity, is made. A polarization measurement can, in principle, be made with very high precision. In this regard, being a relative quantity, a polarization measurement can be made independent of instrumental calibration and long-term sensitivity degredation. Furthermore, the sky distribution of both intensity and polarization has been calculated using a variety of models for the neutral hydrogen. It is found that the polarization distribution over the sky is quite different from that of the intensity distribution. It is also showed that the maximum degree of polarization of the Laymam-σ line increases with heliocentric distance of the spacecraft, varying from 0 up to ～ 18% at 20 AU. 相似文献
17.
The plasma flow in the vicinity of the heliopause stagnation point in the presence of the H atom flow is studied. The plasma at both sides of the heliopause is considered to be a single fluid. The back reaction of the plasma flow on the H atom flow is neglected, and the density, temperature and velocity of the H atom flow are taken to be constant. The solution describing the plasma flow is obtained in the form of power series expansions with respect to the radial distance from the symmetry axis. The main conclusion made on the basis of the obtained solution is that the heliopause is not the surface of discontinuity anymore. Rather, it is the surface separating the flows of the solar wind and interstellar medium with all plasma parameters continuous at this surface. 相似文献
18.
We present Australia Telescope Compact Array (ATCA) observations of the supernova remnant (SNR) G296.8–00.3. A 1.3-GHz continuum image shows the remnant to have a complex multi-shelled appearance, with an unusual rectangular strip running through its centre. H I absorption yields a kinematic distance to the remnant of 9.6 ± 0.6 kpc, from which we estimate an age in the range (2–10) × 103 yr. The ATCA's continuum mode allows a measurement of the Faraday rotation across the band, from which we derive a mean rotation measure towards the SNR of 430 rad m−2 . We consider possible explanations for the morphology of G296.8–00.3, and conclude that either it has a biannular structure, as might be produced through interaction with an asymmetric progenitor wind, or its appearance is caused by the effects of the surrounding interstellar medium. We argue that the adjacent pulsar J1157–6224 is at a similar distance to the SNR, but that a physical association is highly unlikely. The pulsar is the only detectable source in the field in circular polarization, suggesting a method for finding pulsars during aperture synthesis. 相似文献
19.
S. Y. Kim 《Astrophysics and Space Science》1991,180(1):59-74
A one-dimensional electrostatic particle simulation of plasma streaming perpendicular to a magnetic field with nonperiodic boundary condition has been carried out. When a bulk of plasma in injected across an ambient magnetic field, a stream of neutral plasma, consisting of equal numbers of ions and electrons, polarizes, and the resulting polarization electric field gives rise to the penetration of plasma across the magnetic field so that the integrity of plasma maintains. Computer simulation demonstrates the properties of cross-field propagation of plasma stream in a magnetic field with different plasma parameters. 相似文献
20.
W. G. Roberge Glenn E. Ciolek 《Monthly notices of the Royal Astronomical Society》2007,382(2):717-732
We study the physics of wave propagation in a weakly ionized plasma, as it applies to the formation of multifluid, magnetohydrodynamics (MHD) shock waves. We model the plasma as separate charged and neutral fluids which are coupled by ion–neutral friction. At times much less than the ion–neutral drag time, the fluids are decoupled and so evolve independently. At later times, the evolution is determined by the large inertial mismatch between the charged and neutral particles. The neutral flow continues to evolve independently; the charged flow is driven by and slaved to the neutral flow by friction. We calculate this driven flow analytically by considering the special but realistic case where the charged fluid obeys linearized equations of motion. We carry out an extensive analysis of linear, driven, MHD waves. The physics of driven MHD waves is embodied in certain Green functions which describe wave propagation on short time-scales, ambipolar diffusion on long time-scales and transitional behaviour at intermediate times. By way of illustration, we give an approximate solution for the formation of a multifluid shock during the collision of two identical interstellar clouds. The collision produces forward and reverse J shocks in the neutral fluid and a transient in the charged fluid. The latter rapidly evolves into a pair of magnetic precursors on the J shocks, wherein the ions undergo force-free motion and the magnetic field grows monotonically with time. The flow appears to be self-similar at the time when linear analysis ceases to be valid. 相似文献