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1.
We investigate the magnetic geometry of the active G8 dwarf ξ Bootis A (ξ Boo A), from spectropolarimetric observations obtained in 2003 with the MuSiCoS échelle spectropolarimeter at the Télescope Bernard Lyot (Observatoire du Pic du Midi, France). We repeatedly detect a photospheric magnetic field, with periodic variations consistent with rotational modulation. Circularly polarized (Stokes V) line profiles present a systematic asymmetry, showing up as an excess in amplitude and area of the blue lobe of the profiles. Direct modelling of Stokes V profiles suggests that the global magnetic field is composed of two main components, with an inclined dipole and a large-scale toroidal field. We derive a dipole intensity of about 40 G, with an inclination of 35° of the dipole with respect to the rotation axis. The toroidal field strength is of the order of 120 G. A noticeable evolution of the field geometry is observed over the 40 nights of our observation window and results in an increase in field strength and dipole inclination.
This study is the first step of a long-term monitoring of ξ Boo A and other active solar-type stars, with the aim of investigating secular fluctuations of stellar magnetic geometries induced by activity cycles.  相似文献   

2.
We used four coronal models to fit ROSAT data of a sample of late-type stars. The merits and demerits of each was discussed. We found a good correlation between the cornoal temperature so derived and the magnetic field strength. This indicates magnetic heating of the corona and provides a possible, indirect means of estimating the photospheric magnetic field.  相似文献   

3.
The interplay between accretion and ejection in the environment of young stellar objects (YSOs) is believed to be a crucial element in the star formation process. Since most of the properties of the models are set up in the first few AUs from the source (below the so-called Alfvèn surface), to validate and constrain the models observationally we need very high angular resolution. With HST (resolution ∼ 0.'1, i.e. about 14 AU in Taurus)we have been able to access the external border of the acceleration region, for jets in the Taurus-Auriga cloud. Here we see an onion-like kinematic structure in the first 200 AU of the flow, and indications for rotation around the symmetry axis for the resolved low/moderate velocity component. We have now planned observations with AMBER on the VLTI to investigate at 1 mas resolution (in J) the core of the central engine, down to 0.1 AU from the source. Here we describe a joint project by several Institutes in the AMBER consortium dedicated to the study of the morphology and detailed kinematics of a few selected targets. On one hand we will use the large UV coverage of the ATs to explore at medium spectral resolution the structure of the flow. On the other hand the large collection area of the UTs combined with the high resolution mode (R=10000) of AMBER will allow us to search for interesting kinematic features, among which signatures of rotation around the axis, that would constitute an important validation of the proposed models for the jet launching. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

4.
The optical pulsations in DQ Her are universally believed to be due to emission from the magnetic poles of the white dwarf. However, there is no way for a pulsation to be seen if the magnetic axis and the spin axis are aligned; whether the optical pulsation is seen directly from the magnetic poles or as a result of re-processing this beam from sides in an accretion disc, the magnetic and spin axes must be offset. This fact explains why the `oblique rotator' model has been adopted as a standard model for the DQ Her primary. In a recent paper, we have computed several axisymmetric models simulating the DQ Her white dwarf before its `turn-over' (where the term `turn-over' describes the process by which the magnetic axis gets inclining relative to the spin axis at a gradually increasing angle, the so-called `turn-over angle'). For such models, we have found that the moment of inertia along the rotation axis, I 33, is less than the moment(s) of inertia along the two other principal axes, I 11=I 22. The situation I 11>I 33 is known as `dynamical asymmetry', and can cause a spontaneous turn-over of the magnetic axis with respect to the rotation axis . Consequently, the DQ Her white dwarf is either an oblique rotator undergoing its turn-over phase, or it is already qalmostequal a `perpendicular rotator', i.e., its turn-over angle is almost equal to 90°. Assuming the first case, we study numerically the so-called `turn-over scenario', that is, a scenario on rotational evolution in which the turn-over phase is taken into account. We give emphasis on computations concerning the spin-down time rate of the DQ Her white dwarf due to turn-over (not to be confused with its spin-uptime rate due to accretion) for several possible values of the magnetic field. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

5.
We present models of temperature distribution in the crust of a neutron star in the presence of a strong toroidal component superposed to the poloidal component of the magnetic field. The presence of such a toroidal field hinders heat flow toward the surface in a large part of the crust. As a result, the neutron star surface presents two warm regions surrounded by extended cold regions and has a thermal luminosity much lower than in the case the magnetic field is purely poloidal. We apply these models to calculate the thermal evolution of such neutron stars and show that the lowered photon luminosity naturally extends their life-time as detectable thermal X-ray sources. Work partially supported by UNAM-DGAPA grant #IN119306.  相似文献   

6.
It is well-known that the optical pulsations in DQ Her are due to emission from the magnetic poles of the white dwarf. As the white dwarf spins on its axis, the magnetic poles sweep into and out of the line of sight due to the fact that the magnetic axis and the spin axis are not aligned, that is, the DQ Her white dwarf is an `oblique rotator'. So, a central question is if an initially axisymmetric model simulating the DQ Her white dwarf before its `turn-over' (where the term `turn-over' describes the process by which the magnetic axis gets inclining relative to the spin axis at a progressively increasing angle, the so-called `turn-over angle') is indeed susceptible to turn-over. For the puprose of resolving this problem, we compute several axisymmetric models of the DQ Her white dwarf. Our results show that, for both the rotation periods proposed on the basis of the observational evidence regarding the optical pulsations of DQ Her (i.e.,71 s or 142 s), the moment of inertia along the rotation axis is less than the corresponding moment of inertia along the remaining two principal axes of the axisymmetric configuration, I 33 > I 11(=I 22). This is because toroidal magnetic field (tending to derive prolate equidensity surfaces) dominates over rotation (tending, in turn, to derive oblate equidensity surfaces), mainly in the interior of the star. The situation I 11 < I 33 is known as `dynamical asymmetry', and can cause a turn-over of the magnetic symmetry axis with respect to the rotation axis, eventually deriving a nonaxisymmetric configuration corresponding to the so-called `perpendicular rotator' with turn-over angle almost equal to 90°. In this view, our results explain why the DQ Her white dwarf is now an oblique rotator. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

7.
强磁场对非零温中子星壳层电子俘获反应的影响   总被引：6，自引：0，他引：6  
罗志全  彭秋和 《天文学报》1996,37(4):430-436
本文讨论了强磁场作用下非零温电子气体的化学势，分析了磁场作用下电子气体屏蔽势的变化；以核素３３Ｓ为例，讨论了不同温度下，磁场对电子俘获率的影响，结果表明：在足够低的温度和密度下，足够强的磁场使电子俘获率显著降低，而就中子星表面存在的磁场强度（１０９－１０１３Ｇ）而言，磁场对其电子俘获率几乎没有影响．  相似文献   

8.
近来我们在射电脉冲星的逆康普顿散射模型,致密星物理以及一些相关问题上做了下列几项工作：１） 对脉冲星物理中的一系列基本理论进行了较深入的研究,包括对裸奇异星比中子星更可能是射电脉冲星之起源的观点的论证。２） 进一步发展了射电脉冲星的逆康普顿散射（ＩＣＳ） 模型并完成了一系列相关工作。３） 进行了脉冲星的偏振分析和有关观测工作,并用其结果对银河系磁场和旋臂结构进行了分析。４） 研究了黑洞脉冲星双星系统的一些特性,为黑洞的搜寻工作作了初步准备。  相似文献   

9.
We review the basic theoretical elements leading to our current understanding of the role of magnetic fields in the process of star formation. In particular, we concentrate on: (i) the relevance of the mass-to-flux ratio for the stability of molecular clouds; (ii) the consequences of magnetic flux leakage for the evolution of cloud cores; (iii) the phase of anisotropic dynamical collapse following the formation of strongly peaked density distributions; (iv) the mechanism of magnetic braking as a possible solution to the angular momentum problem in star formation.  相似文献   

10.
X射线双星中的回旋共振散射吸收特征(即回旋吸收线特征)是直接测量中子星磁场的工具。回旋吸收线表现为X射线能谱中多阶吸收特征。截至目前,已在30多颗源中探测到该现象,其能量范围为10-80 keV,对应的磁场强度范围为10^7-10^9 T。随着X射线探测技术的进步,回旋吸收线观测及理论研究也迅速发展,包括谐频和基频回旋吸收线能量之比、回旋吸收线形状的复杂性、回旋吸收线形态参数间的相关性、回旋吸收线能量与光度的关系、回旋吸收线的脉冲相位解析谱及回旋吸收线能量的长时标演化等。未来,人们将通过对回旋吸收线的研究,在探测高磁场中子星,以及在探究中子星磁场结构和吸积柱物理等方面取得更多成果。  相似文献   

11.
Lines of equal optical thickness are calculated for cold plasma jets propagating in the simplest magnetic configuration. The paraxial approximation is used in the case of jets directed along the axis of a magnetic dipole. The results explain the apparently convergent as well as divergent shapes of the jets in a divergent magnetic configuration.  相似文献   

12.
Magnetic clouds (MCs) are a subset of interplanetary coronal mass ejections (ICMEs) which exhibit signatures consistent with a magnetic flux rope structure. Techniques for reconstructing flux rope orientation from single-point in situ observations typically assume the flux rope is locally cylindrical, e.g., minimum variance analysis (MVA) and force-free flux rope (FFFR) fitting. In this study, we outline a non-cylindrical magnetic flux rope model, in which the flux rope radius and axial curvature can both vary along the length of the axis. This model is not necessarily intended to represent the global structure of MCs, but it can be used to quantify the error in MC reconstruction resulting from the cylindrical approximation. When the local flux rope axis is approximately perpendicular to the heliocentric radial direction, which is also the effective spacecraft trajectory through a magnetic cloud, the error in using cylindrical reconstruction methods is relatively small (≈ 10∘). However, as the local axis orientation becomes increasingly aligned with the radial direction, the spacecraft trajectory may pass close to the axis at two separate locations. This results in a magnetic field time series which deviates significantly from encounters with a force-free flux rope, and consequently the error in the axis orientation derived from cylindrical reconstructions can be as much as 90∘. Such two-axis encounters can result in an apparent ‘double flux rope’ signature in the magnetic field time series, sometimes observed in spacecraft data. Analysing each axis encounter independently produces reasonably accurate axis orientations with MVA, but larger errors with FFFR fitting.  相似文献   

13.
We present a geometric study of the radio and γ-ray pulsar B1055−52 based on recent observations at the Parkes radio telescope. We conclude that the pulsar's magnetic axis is inclined at an angle of 75° to its rotation axis and that both its radio main pulse and interpulse are emitted at the same height above their respective poles. This height is unlikely to be higher or much lower than 700 km, a typical value for radio pulsars.
It is argued that the radio interpulse arises from emission formed on open fieldlines close to the magnetic axis which do not pass through the magnetosphere's null (zero-charge) surface. However, the main pulse emission must originate from fieldlines lying well outside the polar cap boundary beyond the null surface, and farther away from the magnetic axis than those of the outer gap region where the single γ-ray peak is generated. This casts doubt on the common assumption that all pulsars have closed, quiescent, corotating regions stretching to the light cylinder.  相似文献   

14.
Magnetohydrodynamics waves and instabilities in rotating, self-gravitating, anisotropic and collision-less plasma were investigated. The general dispersion relation was obtained using standard mode analysis by constructing the linearized set of equations. The wave mode solutions and stability properties of the dispersion relations are discussed in the propagations transverse and parallel to the magnetic field. These special cases are discussed considering the axis of rotation to be in transverse and along the magnetic field. In the case of propagation transverse to the magnetic field with axis of rotation parallel to the magnetic field, we derived the dispersion relation modified by rotation and self-gravitation. In the case of propagation parallel to the magnetic field with axis of rotation perpendicular to the magnetic field, we obtained two separate modes affected by rotation and self-gravitation. This indicates that the Slow mode and fire hose instability are not affected by rotation. Numerical analysis was performed for oblique propagation to show the effect of rotation and self-gravitation. It is found that rotation has an effect of reducing the value of the phase speeds on the fast and Alfven wave modes, but self-gravitation affect only on the Slow modes, thereby reducing the phase speed compare to the ideal magneto hydrodynamic (MHD) case.  相似文献   

15.
Based on the spectropolarimetric data obtained at the 6-m telescope, a study of the magnetic field and physical parameters of the magnetic He-weak star HD 35298 was performed. A comparison of the results of magnetic field measurements by various methods has been carried out. The star’s magnetic field varies in the range from ?3 to +3 kG. The field geometry is explained in terms of the oblique rotator model. The obtained magnetic field variation curve can be described by a central dipole with the dipole axis inclined to the axis of rotation by β = 60°, and the magnetic field strength at the pole of B p = 11.5 kG. The data on the variability of spectral lines of some metals are presented, allowing to make an assumption that the stellar surface is heavily spotted.  相似文献   

16.
We examine the effect of magnetic fields on star cluster formation by performing simulations following the self-gravitating collapse of a turbulent molecular cloud to form stars in ideal magnetohydrodynamics. The collapse of the cloud is computed for global mass-to-flux ratios of  ∞, 20, 10, 5  and 3, i.e. using both weak and strong magnetic fields. Whilst even at very low strengths the magnetic field is able to significantly influence the star formation process, for magnetic fields with plasma  β < 1  the results are substantially different to the hydrodynamic case. In these cases we find large-scale magnetically supported voids imprinted in the cloud structure; anisotropic turbulent motions and column density striations aligned with the magnetic field lines, both of which have recently been observed in the Taurus molecular cloud. We also find strongly suppressed accretion in the magnetized runs, leading to up to a 75 per cent reduction in the amount of mass converted into stars over the course of the calculations and a more quiescent mode of star formation. There is also some indication that the relative formation efficiency of brown dwarfs is lower in the strongly magnetized runs due to a reduction in the importance of protostellar ejections.  相似文献   

17.
This is a preliminary study of the star HD 35502. Its magnetic field has been measured in different phases of its period. Preliminary values of the magnetic field parameters have been obtained based on a central quadrupole model. The effective magnetic field Be varies over 0-5000 G, the average surface magnetic field ranges over 6300-6700 G, the field at the poles is Bp=7000 G, and the angle between the quadrupole axis and the axis of rotation is β = 80o. As a first approximation, the surface helium is concentrated around the (negative) pole and for τ > 1 its abundance is reduced by approximately 2-4 dex, which confirms the hypothesis of helium diffusion under the action of gravitation and wind in a stable atmosphere. The chemical elements Si and Cr are concentrated in four spots on the magnetic equator between the magnetic poles, or in a ring coincident with the magnetic equator; precisely which is not clear at present.  相似文献   

18.
We present N -body simulations of dissolving star clusters close to Galactic Centres. For this purpose, we developed a new N -body program called nbody6gc based on Aarseth's series of N -body codes. We describe the algorithm in detail. We report about the density wave phenomenon in the tidal arms which has been recently explained by Küpper, Macleod & Heggie. Standing waves develop in the tidal arms. The wave knots or clumps develop at the position, where the emerging tidal arm hits the potential wall of the effective potential and is reflected. The escaping stars move through the wave knots further into the tidal arms. We show the consistency of the positions of the wave knots with the theory in Just et al. We also demonstrate a simple method to study the properties of tidal arms. By solving many eigenvalue problems along the tidal arms, we numerically construct a one-dimensional coordinate system whose direction is always along a principal axis of the local tensor of inertia. Along this coordinate system, physical quantities can be evaluated. The half-mass or dissolution times of our models are almost independent of the particle number which indicates that two-body relaxation is not the dominant mechanism leading to the dissolution. This may be a typical situation for many young star clusters. We propose a classification scheme which sheds light on the dissolution mechanism.  相似文献   

19.
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   

20.
Numerical calculations of the electron cap shape of a rapidly rotating neutron star with a strong magnetic field have been provided. It is supposed that the magnetic dipole axis is aligned, and ejection of positive charged particles from the star's surface absent. The total charge of the star has been calculated. Estimation of the character of charged-particle motion in the electromagnetic field of the star has been obtained. It is shown that two streams of charged particles escape from the star surface. The electron stream moves along the magnetic axis. The electron stream is enveloped by proton-positron stream, which is generated by returning hard electrons accelerated by electromagnetic field of the star.  相似文献   

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