Water in the near-infrared spectrum of comet 8P/Tuttle

High-resolution spectra of comet 8P/Tuttle were obtained in the frequency range 3449.0–3462.2 cm⁻¹ on 2008 January 3 ut using CGS4 with echelle grating on United Kingdom Infrared Telescope. In addition to observing solar pumped fluorescent lines of H₂O, the long integration time (152 min on target) enabled eight weaker H₂O features to be assigned, most of which had not previously been identified in cometary spectra. These transitions, which are from higher energy upper states, are similar in character to the so-called SH lines recorded in the post Deep Impact spectrum of comet Tempel 1. We have identified certain characteristics that these lines have in common, and which in addition to helping to define this new class of cometary line give some clues to the physical processes involved in their production. Finally, we derive an H₂O rotational temperature of     and a water production rate of  (1.4 ± 0.3) × 10²⁸  molecules s⁻¹.     

Voigt profile fitting to quasar absorption lines: an analytic approximation to the Voigt–Hjerting function

The Voigt–Hjerting function is fundamental in order to correctly model the profiles of absorption lines imprinted on the spectra of bright background sources by intervening absorbing systems. In this work, we present a simple analytic approximation to this function in the context of absorption-line profiles of intergalactic H  i absorbers. Using basic calculus tools, we derive an analytic expression for the Voigt–Hjerting function that contains only fourth-order polynomial and Gaussian functions. In connection with the absorption coefficient of intergalactic neutral hydrogen, this approximation is suitable for modelling Voigt profiles with an accuracy of 10⁻⁴ or better for an arbitrary wavelength baseline, for column densities up to   N _H I= 10 ²² cm⁻²  , and for damping parameters   a ≲ 10⁻⁴  , that is, the entire range of parameters characteristic to all Lyman transitions arising in a variety of H  i absorbing systems such as Lyman α (Lyα) forest clouds, Lyman limit systems and damped Lyα systems. We hence present an approximation to the Voigt–Hjerting function that is both accurate and flexible to implement in various types of programming languages and machines, and with which Voigt profiles can be calculated in a reliable and very simple manner.     

On the presence of silicon and carbon in the pre-maximum spectrum of the Type Ia SN 1990N

The spectrum of the normal Type Ia SN 1990N observed very early on (14 days before B maximum) was analysed by Fisher et al., who showed that the large width and the unusual profile of the strong line near 6000 Å can be reproduced if the line is assumed to be due to C  ii 6578, 6583 Å and if carbon is located in a high-velocity shell. This line is one of the characterizing features of SNe Ia, and is usually thought to be due to Si  ii . A Monte Carlo spectrum synthesis code is used to investigate this suggestion further. The result is that if a standard explosion model is used, the mass enclosed in the shell at the required high velocity (25 000–35 000 km s⁻¹) is too small to give rise to a strong C  ii line. At the same time, removing silicon has a negative effect on the synthetic spectrum at other wavelengths, and removing carbon from the lower velocity regions near the photosphere makes it difficult to reproduce two weak lines which are naturally explained as C  ii , one of them being the line which Fisher et al. suggested is responsible for the strong 6000-Å feature. However, synthetic spectra confirm that although Si  ii can reproduce most of the observed 6000-Å line, the red wing of the line extends too far to be compatible with a Si  ii origin, and that the flat bottom of the line is also not easy to reproduce. The best fit is obtained for a normal SN Ia abundance mix at velocities near the photosphere (15 500–19 000 km s⁻¹) and an outer carbon–silicon shell beyond 20 000 km s⁻¹. This suggests that mixing is not complete in the outer ejecta of an SN Ia. Observations at even earlier epochs might reveal to what extent a carbon shell is unmixed.     

The Gunn–Peterson effect and the Lyman alpha forest

We show that spatial correlations in a stochastic large-scale velocity field in an otherwise smooth intergalactic medium (homogeneous comoving density) superposed on the general Hubble flow may cause a 'line-like' structure in QSO spectra similar to the population of unsaturated Lyα forest lines which usually are attributed to individual clouds with 10¹¹ ≲ N _{H i}  5 × 10¹³ cm⁻². Therefore there is no clear observational distinction between a diffuse intergalactic medium and discrete intergalactic clouds. It follows that the H  i density in the diffuse intergalactic medium might be substantially underestimated if it is determined from the observed intensity distribution near the apparent continuum in high-resolution spectra of QSOs. Our tentative estimate implies a diffuse neutral hydrogen opacity τ_GP ∼ 0.3 at z  ∼ 3 and a current baryon density Ω_IGM ≃ 0.08, assuming a Hubble constant H ₀ = 70 km s⁻¹ Mpc⁻¹.     

Analytical solutions for saturated P cygni type profiles II: General case

As in the first part of the present study (Sapar et al., 2002) we use the β-law for velocity of stellar wind and the Sobolev approximation for radiative transfer. Here we have succeeded to derive general and relatively simple analytical formulae in elementary functions for saturated P Cygni type line profiles if parameter 2β is arbitrary positive integer (in the first part we studied the cases 2β ≤ 4). The four terms obtained describe contributions to the line profile due to isotropic and anisotropic parts of optical thickness in the source function of the light-scattering layer followed by isotropic and anisotropic parts of multiple scattering. The limits of acceptability of the Sobolev approximation for β-law have been discussed and specified. This revised version was published online in July 2006 with corrections to the Cover Date.     

On the detection of artefacts in spectro-astrometry

We demonstrate that artificial bipolar structure can be detected using spectro-astrometry when the point spread function (PSF) of a point source suffers distortion in a relatively wide slit. Spectro-astrometry is a technique which allows us to probe the spatial structure of astronomical sources on milliarcsec (mas) scales making it possible to detect close binaries and to study the geometry and kinematics of outflowing gas on scales much smaller than the seeing or the diffraction limit of the telescope. It is demonstrated that a distorted PSF, caused by tracking errors of the telescope or unstable active optics during an exposure, can induce artificial signals which may be misinterpreted as a real spectro-astrometric signal. Using simulations, we show that these may be minimized by using a narrow slit relative to the seeing. Spectra should be obtained at antiparallel slit position angles (e.g. 0° and 180°) for comparison in order to allow artificial signatures to be identified.     

On bare carbon chain anions as possible carriers of the diffuse interstellar features

The recent hypothesis that some of the diffuse interstellar bands originate in bare carbon chains such as             and     is tested using echelle spectra covering the whole range of possible (i.e. found in the laboratory) transitions of these molecules. Most of the expected features are below the level of detection in astrophysical spectra, and even those observed show intensity ratios that are different from those in the laboratory. Thus the hypothesis is very unlikely to be true.     

内日冕的绿线强度分布研究

日冕是太阳大气活动的关键区域, 是日地空间天气的源头. 受观测限制, 对日冕低层大气等离子体结构和磁场状态的研究非常欠缺, 国际上对于可见光波段日冕低层大气的亮度分层研究很少. 利用丽江日冕仪YOGIS (Yunnan Green-line Imaging System)的日冕绿线($\rm Fe_{\Rmnum{14     

Measurements of spectral lines observed with Hinode/EIS as implications for coronal heating

We have measured the line widths and nonthermal velocities in 12 solar regions using high resolution EUV data taken by Hinode/EIS. We find that there exists a positive correlation between the intensity and nonthermal velocity for the Fe XII emission line as well as some other lines. The correlation coefficients decrease from the disk center to the limb. However, the nonthermal velocities of a particular spectral line do not vary much in different regions, so they are considered isotropic. In particular, we find that for a coronal loop structure, the largest widths and nonthermal velocities occur at the footpoints, where outflows appear. Based on these observational results, we discuss several physical processes responsible for coronal heating.     

谱线形成深度理论及其应用（Ⅲ）：太阳矢量磁场结构的推导

叙述了谱线形态深度理论在推导太阳矢量磁场空间结构上的应用，列举了有典型意义的工作，指出了用响应函数比有贡献函数导出的结果更好的原因以有产的局限性，作为对比，也举例了说明了不依赖于这种理论来推导矢量磁场结构的方法。     

Modelling the H Lyman lines in evolved late-type stars

The importance of partial redistribution (PRD) in the modelling of the Lyman α and Lyman β emission lines of hydrogen in stellar atmospheres is examined using simple atmospheric models of a range of late-type stars. These models represent the subgiant Procyon (F5 IV–V), and the two giants β Gem (K0 III) and α Tau (K5 III). These stars are selected to span a wide range of surface gravities: 1.25< log  g <4.00 . The calculations are performed using the computer code multi with the modifications made by Hubeny & Lites. It is found that PRD effects are highly significant, both in the direct prediction of the Lyman line profiles and in the application of hydrostatic equilibrium to calculate the atmospheric electron density in static atmospheric models.     

Mode Identification from Line Profiles using the Direct Fitting Technique

The method of moments and the direct fitting method are the onlyspectroscopic methods of mode identification which allow a determination ofall pulsational parameters. The pulsation parameters are required to predictthe light amplitude and phase which can be important discriminants in modeidentification. The direct fitting method has several advantages over themethod of moments. It is not restricted to low spherical harmonic degree or form of the eigenfunction and is not sensitive to the placement of the continuum. In the last few years the method has been applied to several different types of stars. We briefly describe the method and give someexamples of its application.     

色球耀斑Hα谱线轮廓不对称性研究

Hα谱线轮廓的不对称性是色球耀斑光谱观测中的重要特征,也是耀斑动力学过程的重要观测事实之一．以紫金山天文台太阳光谱仪的观测资料为依据,给出Hα谱线不对称性的典型轮廓．在考虑氢原子非热激发、电离的作用下,经验性地计算了不同大气模型下谱线的不对称性特征,并在此基础上,对观测谱线进行半经验的研究．结果表明, 色球区的向下运动能够产生Hα谱线的红、蓝不对称性,并可以再生具体耀斑的谱线不对称性特征．此外,不仅非热粒子的能流、谱指数大小以及速度场所处的高度对谱线轮廓有影响,耀斑大气的背景模型对谱线的轮廓也有一定的影响．     

An ultra-high-resolution study of the interstellar medium towards Orion

We report ultra-high-resolution observations     of Na  i , Ca  ii , K  i , CH and CH⁺ for interstellar sightlines towards 12 bright stars in Orion. These data enable the detection of many more absorption components than previously recognized, providing a more accurate perspective on the absorbing medium. This is especially so for the line of sight to the Orion nebula, a region not previously studied at very high resolution. Model fits have been constructed for the absorption-line profiles, providing estimates for the column density, velocity dispersion and central velocity for each constituent velocity component. A comparison between the absorption occurring in sightlines with small angular separations has been used, along with comparisons with other studies, to estimate the line-of-sight velocity structures. Comparisons with earlier studies have also revealed temporal variability in the absorption-line profile of ζ Ori, highlighting the presence of small-scale spatial structure in the interstellar medium on scales of ≈10 au. Where absorption from both Na⁰ and K⁰ is observed for a particular cloud, a comparison of the velocity dispersions measured for each of these species provides rigorous limits on both the kinetic temperature and turbulent velocity prevailing in each cloud. Our results indicate the turbulent motions to be subsonic in each case.     abundance ratios are derived for individual clouds, providing an indication of their physical state.     

An Analysis of Average Pulsar Profiles and A Study of the ρ-P relation of Pulsars

Using the method of Gaussian Fit Separation of Average Profile (GF-SAP), we re-examine the average profiles of nine pulsars at several frequencies, ranging from 408-1642 MHz. This method enables us to obtain the number of components for each pulsar, and the parameters for each component, the width, position and amplitude. The ρ-P relation for the inner cone and outer cone are studied separately, and the results are, respectively, ρ = p-0.51±0.05 and ρ= p-0.42±0.06 The results can be interpreted as a confirmation of the double-cone structure of pulsar emission beams. The altitudes of emission region, and the radius-to-frequency-map (RFM) are also examined; for the outer cone, we obtained γ(v) ∝v-0.19±0.09.