Using X-ray observations to explore the binary interaction in Eta Carinae     

Amit Kashi  Noam Soker 《Monthly notices of the Royal Astronomical Society》2009,397(3):1426-1434

We study the usage of the X-ray light curve, column density towards the hard X-ray source, and emission measure (density square times volume), of the massive binary system η Carinae to determine the orientation of its semimajor axis. The source of the hard X-ray emission is the shocked secondary wind. We argue that, by itself, the observed X-ray flux cannot teach us much about the orientation of the semimajor axis. Minor adjustment of some unknown parameters of the binary system allows to fit the X-ray light curve with almost any inclination angle and orientation. The column density and X-ray emission measure, on the other hand, impose strong constrains on the orientation. We improve our previous calculations and show that the column density is more compatible with an orientation where for most of the time the secondary – the hotter, less massive star – is behind the primary star. The secondary comes closer to the observer only for a short time near periastron passage. The 10-week X-ray deep minimum, which results from a large decrease in the emission measure, implies that the regular secondary wind is substantially suppressed during that period. This suppression is most likely resulted by accretion of mass from the dense wind of the primary luminous blue variable star. The accretion from the equatorial plane might lead to the formation of a polar outflow. We suggest that the polar outflow contributes to the soft X-ray emission during the X-ray minimum; the other source is the shocked secondary wind in the tail. The conclusion that accretion occurs at each periastron passage, every five and a half years, implies that accretion had occurred at a much higher rate during the Great Eruption of η Car in the 19th century. This has far reaching implications for major eruptions of luminous blue variable stars.  相似文献   

On the central core in MHD winds and jets     

V. S. Beskin  E. E. Nokhrina 《Monthly notices of the Royal Astronomical Society》2009,397(3):1486-1497

We analytically determine the structure of highly magnetized astrophysical jets at the origin in a region where the flow has been already collimated by an external medium, in both relativistic and non-relativistic regimes. We show that this can be achieved by solving a system of first-order ordinary differential equations that describe the transversal jet structure for a variety of external confining pressure profiles that collimate the jet to a near-cylindrical configuration. We obtain solutions for a central jet surrounded either by a self-similar wind or by an external pressure profile and derive the dependence of the velocity and the magnetic field strength along and across our jets. In particular, we find that the central core in a jet – the part of a flow with a nearly homogeneous magnetic field – must contain a poloidal field which is not much smaller than the critical value B _min. This allows us to determine the magnetic flux in a core which is much smaller than the total magnetic flux. We show that for such a small core flux the solutions with a magnetic field in a core much smaller than B _min are non-physical. For astrophysical objects the value of the critical magnetic field is quite large: 1 G for active galactic nuclei, 10¹⁰ G for gamma-ray bursts and 10⁻¹ G for young stellar objects. In a relativistic case for the core field greater than or of the order of B _min we show analytically that the plasma Lorentz factor must grow linearly with the cylindrical radius. For non-relativistic highly magnetized jets we propose that an oblique shock exists near the base of the jet so that the finite gas pressure plays an important role in force balance.  相似文献   

Magnetized relativistic jets and long-duration GRBs from magnetar spin-down during core-collapse supernovae     

N. Bucciantini  E. Quataert  B. D. Metzger  T. A. Thompson  J. Arons  L. Del Zanna 《Monthly notices of the Royal Astronomical Society》2009,396(4):2038-2050

We use ideal axisymmetric relativistic magnetohydrodynamic simulations to calculate the spin-down of a newly formed millisecond,   B ∼ 10¹⁵ G  , magnetar and its interaction with the surrounding stellar envelope during a core-collapse supernova (SN) explosion. The mass, angular momentum and rotational energy lost by the neutron star are determined self-consistently given the thermal properties of the cooling neutron star's atmosphere and the wind's interaction with the surrounding star. The magnetar drives a relativistic magnetized wind into a cavity created by the outgoing SN shock. For high spin-down powers  (∼10⁵¹–10⁵² erg s⁻¹)  , the magnetar wind is superfast at almost all latitudes, while for lower spin-down powers  (∼10⁵⁰ erg s⁻¹)  , the wind is subfast but still super-Alfvénic. In all cases, the rates at which the neutron star loses mass, angular momentum and energy are very similar to the corresponding free wind values (≲30 per cent differences), in spite of the causal contact between the neutron star and the stellar envelope. In addition, in all cases that we consider, the magnetar drives a collimated  (∼5–10°)  relativistic jet out along the rotation axis of the star. Nearly all of the spin-down power of the neutron star escapes via this polar jet, rather than being transferred to the more spherical SN explosion. The properties of this relativistic jet and its expected late-time evolution in the magnetar model are broadly consistent with observations of long duration gamma-ray bursts (GRBs) and their associated broad-lined Type Ic SN.  相似文献   

Linearly Organized Turbulence Structures Observed Over a Suburban Area by Dual-Doppler Lidar   总被引：1，自引：1，他引：0  

Rob Newsom  Ron Calhoun  David Ligon  Jerry Allwine 《Boundary-Layer Meteorology》2008,127(1):111-130

Dual-Doppler lidar observations are used to investigate the structure and evolution of surface-layer flow over a suburban area. The observations were made during the Joint Urban 2003 (JU2003) field experiment in Oklahoma City, U.S.A. in the summer of 2003. This study focuses specifically on a 10-h sequence of scan data beginning shortly after noon local time on 7 July 2003. During this period two coherent Doppler lidars performed overlapping low elevation angle sector scans upwind and south of Oklahoma City’s central business district. Radial velocity data from the two lidars are processed to reveal the structure and evolution of the horizontal velocity field in the surface layer throughout the afternoon and during the evening transition period. The retrieved velocity fields clearly show a tendency for turbulence structures to be elongated in the direction of the mean flow throughout the entire 10-h study period. In order to quantify the observed anisotropy and its dependence on stability, integral length scales are estimated directly from the spatially resolved velocity retrievals. As the flow became more stably stratified the characteristic cross-stream dimension of the linear structures decreased. The streamwise component was consistently more anisotropic than the cross-stream component, and both velocity components exhibited maximum anisotropy under neutral conditions. The ratio of the streamwise to cross-stream length scale was estimated to be about eight for the streamwise component, and four for the cross-stream component under neutral conditions.  相似文献   

Filament generation off the Strait of Gibraltar in response to Gap winds     

lvaro Peliz  Ana Teles-Machado  Patrick Marchesiello  Jesus Dubert  Jesus Garcia Lafuente 《Dynamics of Atmospheres and Oceans》2009,46(1-4):36-45

We present a case study of the generation of a cold filament rooted off the southwestern edge of the Strait of Gibraltar (Atlantic side) during the summer of 2000. The event is successfully simulated using high-resolution atmospheric and oceanic numerical models. It is shown that a sharp filament may develop oceanwards with little modification of the Atlantic inflow into the Mediterranean, contrary to usual expectations. The filament is essentially driven by the surface layer response to Gap winds occurring during Levanter conditions. The easterly wind funnelling in the Strait generates a strong wind jet and intense wind curl which impacts the oceanic surface layer through Ekman pumping and mixing processes. The generation and fate of the filament is very similar to the Gulf of Tehuantepec case, where strong Gap wind events produce asymmetric deformation and erosion of the thermocline that tends to favour anticyclonic mesoscale circulations. Our observations and model results from both realistic and idealized experiments suggest that similar phenomena are present in the Gulf of Cadiz, but they are altered by the persisting Atlantic inflow, so that the response to Gap winds is not as dramatic.  相似文献   

冬季黄渤海大风天气与渔船风损统计分析   总被引：8，自引：1，他引：7  

尹尽勇  刘涛  张增海  赵伟 《气象》2009,35(6):90-95

选用1987-2006年NECP 500hPa高度场及海平面气压场日平均再分析资料及沿海岸基、石油平台、海岛及船舶实时观测资料,统计分析了冬季造成黄渤海海域8级和10级以上大风天气过程;选用中国渔船安全分析报告1999-2005年风灾船损事故资料,分析了大风和船损灾害的关系.结果显示:(1)冬季影响黄渤海的冷空气过程主要是来自西北路径,占全部冷空气过程的50.4%.(2)造成海上8级以上大风的冷空气过程的天气类型主要是小槽发展型112次,占全部冷空气过程的50.0%;其次是横槽转竖型61次,占全部冷空气过程的27.2%.(3)近20年冬季冷空气大风过程次数呈下降趋势;(4)风灾事故占渔船全损事故的51.85%,冬半年突发性的冷空气大风是导致木质渔船出现风灾事故的主要原因.  相似文献   

两种导风方法在台风路径分析中的 应用比较研究   总被引：2，自引：1，他引：1  

李峰  王振会  官莉  河惠清 《气象科学》2009,29(3):330-334

基于云导风使用的最大相关法（已经广泛应用）和TCFM法（傅立叶相位分析法和最大相关法相结合）两方法的原理,开发了利用FY-2C静止气象卫星30min间隔卫星云图资料获得云迹风的可视化导风实验系统。利用该系统处理2005年夏季麦莎、卡努、龙王等3个台风期间的卫星云图,对得到的云迹风进行台风路径反演分析。结果表明,TCFM导风方法得到的结果优于最大相关法。  相似文献   

Numerical study of a downslope windstorm in Northwestern Greece     

I. Koletsis  K. Lagouvardos  V. Kotroni  A. Bartzokas 《Atmospheric Research》2009,94(2):178-193

This paper describes the leeside wind storm of 25–26 March 1998, the most intense wind storm of the last decade in Northwestern Greece. This wind storm produced wind gusts of  30 m s^− 1 that resulted in tree uprooting, roof damaging, electric power network disruption and flooding in the lake-side areas of Ioannina city in Northwestern Greece. With the aim to identify the role of Mountain Mitsikeli near the city of Ioannina on the windstorm and to investigate the physical mechanisms responsible for such orographically induced weather events, numerical simulations with MM5 model have been performed. The model results showed that a resolution of 2-km resolution is necessary in order to reproduce the localized character of the wind storm. The analysis revealed that a synergistic combination of the cross-barrier northeasterly flow, the stable layer above the mountain top and the presence of a critical level, led to the intensification of the lee side winds during the studied wind event. Sensitivity experiments with modified topography, further supported the important role of mountain Mitsikeli that stands as an isolated obstacle, on the modification of the wind field during the observed windstorm.  相似文献   

Accreting white dwarfs as supersoft X‐ray sources     

M. Kato 《Astronomische Nachrichten》2010,331(2):140-145

I review various phenomena associated with mass‐accreting white dwarfs (WDs) in the view of supersoft X‐ray sources. When the mass‐accretion rate is low (Ṁ_acc < a few × 10^–7 M⊙yr^–1), hydrogen nuclear burning is unstable and nova outbursts occur. A nova is a transient supersoft X‐ray source (SSS) in its later phase which timescale depends strongly on the WD mass. The X‐ray turn on/off time is a good indicator of the WD mass. At an intermediate mass‐accretion rate an accreting WD becomes a persistent SSS with steady hydrogen burning. For a higher mass‐accretion rate, the WD undergoes “accretion wind evolution” in which the WD accretes matter from the equatorial plane and loses mass by optically thick winds from the other directions. Two SSS, namely RX J0513‐6951 and V Sge, are corresponding objects to this accretion wind evolution. We can specify mass increasing WDs from light‐curve analysis based on the optically thick wind theory using multiwavelength observational data including optical, IR, and supersoft X‐rays. Mass estimates of individual objects give important information for the binary evolution scenario of type Ia supernovae (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

The study of proto-magnetar winds     

Yan-Jun Chen  Ye-Fei Yuan 《中国天文和天体物理学报》2010,10(3)

The velocity profiles and properties of proto-magnetar winds are investigated.It is found that the corotation of wind matter with magnetic field lines significantly affects r-process nucleosynthesis and could lead to long duration γ-ray bursts and hyper-energetic supernovae.  相似文献