Further images of α Persei G dwarfs     

J.R. Barnes  A. Collier Cameron  D.J. James  D. Steeghs 《Monthly notices of the Royal Astronomical Society》2001,326(3):1057-1066

We present two images of intermediate and low axial inclination G dwarfs (AP 149 and AP 193) in the young open cluster α Persei, and compare these with previous images of intermediate and high axial inclination objects in this cluster. All stars show starspots at high latitudes, with one star exhibiting a strong polar spot. Although low-latitude features are found on all stars to some degree, the detection of spots on AP 193 is marginal. The apparent difference in starspot morphology from one object to the next is probably the result of a stellar magnetic cycle, although the exact effect on the starspot distribution throughout a cycle is unknown.
Polar spots are found in many Doppler images of rapidly rotating cool stars. In the past, their existence has been called into question, and it has been suggested that they could be the manifestations of NLTE (e.g. chromospheric filling in of line profiles) effects rather than real photospheric features. We assume the polar spots to be real photospheric features, and conclude that the flat-bottomed nature of the profile shape can be attributed to photospheric polar spots. The degree of truncation of the profile depends not only on spot size and strength, but also on the effective foreshortening of the polar region, a function of axial inclination.
H α is in emission on AP 149 which shows a double peak at most phases. The time-series of the profile shows an s-wave pattern as the position of these peaks changes throughout the rotation cycle. We attribute this to coronal clouds located above the stellar surface in synchronous orbit. A maximum-entropy tomogram is derived revealing four distinct emission regions located near and above the corotation radius.  相似文献   

A new exact method for line radiative transfer   总被引：1，自引：0，他引：1  

Moshe Elitzur  Andrés Asensio Ramos 《Monthly notices of the Royal Astronomical Society》2006,365(3):779-791

We present a new method, the coupled escape probability (CEP), for exact calculation of line emission from multi-level systems, solving only algebraic equations for the level populations. The CEP formulation of the classical two-level problem is a set of linear equations , and we uncover an exact analytic expression for the emission from two-level optically thick sources that holds as long as they are in the 'effectively thin' regime. In a comparative study of a number of standard problems, the CEP method outperformed the leading line transfer methods by substantial margins.
The algebraic equations employed by our new method are already incorporated in numerous codes based on the escape probability approximation. All that is required for an exact solution with these existing codes is to augment the expression for the escape probability with simple zone-coupling terms. As an application, we find that standard escape probability calculations generally produce the correct cooling emission by the C  ii 158-μm line but not by the ³P lines of O  i .  相似文献   

Wind signatures in the X-ray emission-line profiles of the late-O supergiant ζ Orionis     

David H. Cohen  Maurice A. Leutenegger  Kevin T. Grizzard  Catherine L. Reed  Roban H. Kramer  Stanley P. Owocki 《Monthly notices of the Royal Astronomical Society》2006,368(4):1905-1916

X-ray line-profile analysis has proved to be the most direct diagnostic of the kinematics and spatial distribution of the very hot plasma around O stars. The Doppler-broadened line profiles provide information about the velocity distribution of the hot plasma, while the wavelength-dependent attenuation across a line profile provides information about the absorption to the hot plasma, thus providing a strong constraint on its physical location. In this paper, we apply several analysis techniques to the emission lines in the Chandra High Energy Transmission Grating Spectrometer (HETGS) spectrum of the late-O supergiant ζ Ori (O9.7 Ib), including the fitting of a simple line-profile model. We show that there is distinct evidence for blueshifts and profile asymmetry, as well as broadening in the X-ray emission lines of ζ Ori. These are the observational hallmarks of a wind-shock X-ray source, and the results for ζ Ori are very similar to those for the earlier O star, ζ Pup, which we have previously shown to be well fit by the same wind-shock line-profile model. The more subtle effects on the line-profile morphologies in ζ Ori, as compared to ζ Pup, are consistent with the somewhat lower density wind in this later O supergiant. In both stars, the wind optical depths required to explain the mildly asymmetric X-ray line profiles imply reductions in the effective opacity of nearly an order of magnitude, which may be explained by some combination of mass-loss rate reduction and large-scale clumping, with its associated porosity-based effects on radiation transfer. In the context of the recent reanalysis of the helium-like line intensity ratios in both ζ Ori and ζ Pup, and also in light of recent work questioning the published mass-loss rates in OB stars, these new results indicate that the X-ray emission from ζ Ori can be understood within the framework of the standard wind-shock scenario for hot stars.  相似文献   

The coronal structure of Speedy Mic – I. A densely packed prominence system beyond corotation     

N. J. Dunstone  J. R. Barnes  A. Collier Cameron  M. Jardine 《Monthly notices of the Royal Astronomical Society》2006,365(2):530-538

We present new observations of the prominence system on the K3 dwarf Speedy Mic (BO Mic, HD 197890). Using an improved technique to track the absorption features in Hα we find a very active prominence system with approximately 10 prominences on the observable hemisphere per rotation. From a total of 25 prominences, we find an average axial distance of  (2.85 ± 0.54) R _*  which is twice the corotation radius above the stellar surface. We discuss the consequences of these observations on the nature of the supporting magnetic structures. Two consecutive nights, with complete phase coverage, combined with a further night after a three-night gap allow us to study the evolution of the prominence system on two different time-scales. Several of the prominences have counterparts at similar phases on consecutive nights. During this interval, many prominences show evidence for evolution in their heights and phases of observation. Five nights (13 rotation cycles) later, we recover many prominences at approximately the same phases. Whilst individual prominences change axial distances or appear/reappear from night-to-night, the underlying prominence supporting structures appear to be stable over as many as 13 stellar rotations.  相似文献   

Atomic oscillator strengths in the spectral domain of Gaia     

L. Bigot  F. Thévenin 《Monthly notices of the Royal Astronomical Society》2006,372(2):609-614

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A high-accuracy computed water line list     

R. J. Barber  J. Tennyson  G. J. Harris  R. N. Tolchenov 《Monthly notices of the Royal Astronomical Society》2006,368(3):1087-1094

A computed list of  H₂¹⁶O  infrared transition frequencies and intensities is presented. The list, BT2, was produced using a discrete variable representation two-step approach for solving the rotation–vibration nuclear motions. It is the most complete water line list in existence, comprising over 500 million transitions (65 per cent more than any other list) and it is also the most accurate (over 90 per cent of all known experimental energy levels are within 0.3 cm⁻¹ of the BT2 values). Its accuracy has been confirmed by extensive testing against astronomical and laboratory data.
The line list has been used to identify individual water lines in a variety of objects including comets, sunspots, a brown dwarf and the nova-like object V838 Mon. Comparison of the observed intensities with those generated by BT2 enables water abundances and temperatures to be derived for these objects. The line list can also be used to provide an opacity for models of the atmospheres of M dwarf stars and assign previously unknown water lines in laboratory spectra.  相似文献   

Time-resolved ultraviolet spectroscopy of the compact interacting binary QU Car     

L. E. Hartley  J. E. Drew  K. S. Long 《Monthly notices of the Royal Astronomical Society》2002,336(3):808-816

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高精度时间间隔测量技术与方法   总被引：31，自引：0，他引：31  

张延  黄佩诚 《天文学进展》2006,24(1):1-15

时间间隔测量技术,尤其是高精度的时间间隔测量技术意义重大。不论是电信通讯、芯片设计等工程实践,还是原子物理、天文观测实验等理论研究,以及激光测距、卫星定位等航天军事技术,都离不开高精度的时间间隔测量技术。经过几十年的研究,目前已经有多种测量方法。在阐明插值原理之后,依次介绍了直接计数法、扩展法、时间幅度转换法、游标法、抽头延迟线法和差分延迟线法等主要方法。然后对影响测量的几个因素,包括非线性和不定态作了讨论。针对这些不良影响,还探讨了一些提高精度和稳定度的方法,比如非线性校正、PLL(Phase Lock Loop)和DLL (Delay Lock Loop)技术。最后,对时间间隔测量技术的前景作了展望。  相似文献   

A far-infrared molecular and atomic line survey of the Orion KL region     

M. R. Lerate  M. J. Barlow  B. M. Swinyard  J. R. Goicoechea  J. Cernicharo  T. W. Grundy  T. L. Lim  E. T. Polehampton  J.-P. Baluteau  S. Viti  J. Yates 《Monthly notices of the Royal Astronomical Society》2006,370(2):597-628

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The scattering of Lyman-series photons in the intergalactic medium     

Steven R. Furlanetto  Jonathan R. Pritchard 《Monthly notices of the Royal Astronomical Society》2006,372(3):1093-1103

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