Lyman α radiative transfer in a multiphase medium     

Matthew Hansen  S. Peng Oh 《Monthly notices of the Royal Astronomical Society》2006,367(3):979-1002

Hydrogen Lyman α (Lyα) is our primary emission-line window into high-redshift galaxies. Despite an extensive literature, Lyα radiative transfer in the most realistic case of a dusty, multiphase medium has received surprisingly little detailed theoretical attention. We investigate Lyα resonant scattering through an ensemble of dusty, moving, optically thick gas clumps. We treat each clump as a scattering particle and use Monte Carlo simulations of surface scattering to quantify continuum and Lyα surface scattering angles, absorption probabilities, and frequency redistribution, as a function of the gas dust content. This atomistic approach speeds up the simulations by many orders of magnitude, making possible calculations which are otherwise intractable. Our fitting formulae can be readily adapted for fast radiative transfer in numerical simulations. With these surface scattering results, we develop an analytic framework for estimating escape fractions and line widths as a function of gas geometry, motion, and dust content. Our simple analytic model shows good agreement with full Monte Carlo simulations. We show that the key geometric parameter is the average number of surface scatters for escape in the absence of absorption,     , and we provide fitting formulae for several geometries of astrophysical interest. We consider the following two interesting applications. (i) Equivalent widths ( EWs ). Lyα can preferentially escape from a dusty multiphase interstellar medium if most of the dust lies in cold neutral clouds, which Lyα photons cannot penetrate. This might explain the anomalously high EWs sometimes seen in high-redshift/submillimetre sources. (ii) Multiphase galactic outflows . We show the characteristic profile is asymmetric with a broad red tail, and relate the profile features to the outflow speed and gas geometry. Many future applications are envisaged.  相似文献   

Averaging spectral shapes     

Piotr Lubi&#;ski 《Monthly notices of the Royal Astronomical Society》2004,350(2):596-608

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The role of non-thermal electrons in the hydrogen and calcium lines of stellar flares     

M.D. Ding  C. Fang 《Monthly notices of the Royal Astronomical Society》2001,326(3):943-949

There is observational evidence showing that stellar and solar flares occur with a similar circumstance, although the former are usually much more energetic. It is expected that the bombardment by high-energy electrons is one of the chief heating processes of the flaring atmosphere. In this paper we study how a precipitating electron beam can influence the line profiles of Ly α , H α , Ca  ii K and λ 8542. We use a model atmosphere of a dMe star and make non-LTE computations taking into account the non-thermal collisional rates owing to the electron beam. The results show that the four lines can be enhanced to different extents. The relative enhancement increases with increasing formation height of the lines. Varying the energy flux of the electron beam has different effects on the four lines. The wings of Ly α and H α become increasingly broad with the beam flux; change of the Ca  ii K and λ 8542 lines, however, is most significant in the line centre. Varying the electron energy (i.e. the low-energy cut-off for a power-law beam) has a great influence on the Ly α line, but little on the H α and Ca  ii lines. An electron beam of higher energy precipitates deeper, thus producing less enhancement of the Ly α line. The Ly α /H α flux ratio is thus sensitive to the electron energy.  相似文献   

A two-zone model for the broad iron line emission in MCG–6-30-15     

R. Misra 《Monthly notices of the Royal Astronomical Society》2001,320(4):445-450

We reanalyse the ASCA and BeppoSAX data of MCG–6-30-15, using a double-zone model for the iron line profile. In this model, the X-ray source is located around ≈10 Schwarzschild radii and the regions interior and exterior to the X-ray source produce the line emission. We find that this model fits the data with a similar reduced χ ² to the standard single-zone model. Thus we show that the presence of a broad iron line feature does not necessarily require that the X-ray source be located close to the last stable orbit or in the disc rotation axis.
Within the framework of this model, the best-fitting inclination angle of the source     for the intermediate-intensity ASCA data set is compatible with that determined by earlier modelling of optical lines. The observed variability of the line profile with intensity can be explained as variations of the X-ray source size. That several active galactic nuclei with broad lines have the peak centroid near 6.4 keV can be explained under certain conditions.
We also show that the simultaneous broad-band observations of this source by BeppoSAX rule out the Comptonization model which was an alternative to the standard inner-disc one. We thereby strengthen the case that line broadening occurs as a result of the strong gravitational influence of a black hole.  相似文献   

Erratum: The spectrum of BAL QSO 03 + 308: intrinsic variability and line-locking stability     

E.Y. Vilkoviskij  M.J. Irwin 《Monthly notices of the Royal Astronomical Society》2001,326(1):416-416

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Testing the line-driven disc wind model: time-resolved ultraviolet spectroscopy of IX Vel and V3885 Sgr     

L. E. Hartley  J. E. Drew  K. S. Long  C. Knigge  D. Proga 《Monthly notices of the Royal Astronomical Society》2002,332(1):127-143

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Extremely weak reflection features in the X-ray spectrum of XTE J1118+480: possible evidence for X-ray-emitting jets?     

J. M. Miller  D. R. Ballantyne  A. C. Fabian  W. H. G. Lewin 《Monthly notices of the Royal Astronomical Society》2002,335(4):865-870

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Profiles of blue and infrared diffuse interstellar bands     

G. A. Galazutdinov  F. A. Musaev  M. R. Schmidt  J. Kre&#;owski 《Monthly notices of the Royal Astronomical Society》2001,323(2):293-300

The paper presents a survey of profiles of reasonably strong diffuse interstellar bands (DIBs) based on the extensive set of high-resolution spectra acquired with the aid of echelle spectrographs installed at the 2-m Terskol, 2-m Pic du Midi and 1-m SAO telescopes. The surveyed diffuse interstellar bands cover the spectral ranges of blue and near-infrared, i.e the DIBs not surveyed by Krełowski & Schmidt . The possible modifications caused by stellar and telluric lines are discussed. The very broad features such as 4430 are not discussed because the shapes of their profiles, extracted from echelle spectra, are very uncertain. The signal-to-noise (S/N) ratios of the spectra are not high enough to enable discussion of the profiles of numerous weak interstellar features discovered recently.  相似文献   

Modelling line profiles in infalling cores     

J.M.C. Rawlings  J.A. Yates 《Monthly notices of the Royal Astronomical Society》2001,326(4):1423-1430

Results are presented from a model of molecular line formation in collapsing star-forming cores. The study includes, for the first time, a self-consistent chemical and dynamical model which is then directly coupled to an appropriate radiative transfer model. The assumptions of chemical uniformity or simple monotonic variations within such cores are shown to be unacceptable. The results show that the abundance variations and the line profiles are highly sensitive to the assumed values of the free parameters in the chemical model. Extreme caution is therefore advised in the quantitative analysis of emission-line profiles from infall sources. The implied degeneracy can be overcome by multiple line-of-sight observations of many species and transitions.  相似文献   

Further constraints on variation of the fine-structure constant from alkali-doublet QSO absorption lines     

M.T. Murphy  J.K. Webb  V.V. Flambaum  J.X. Prochaska  A.M. Wolfe 《Monthly notices of the Royal Astronomical Society》2001,327(4):1237-1243

Comparison of quasar (QSO) absorption-line spectra with laboratory spectra provides a precise probe for variability of the fine-structure constant, α , over cosmological time-scales. We constrain variation in α in 21 Keck/HIRES Si  iv absorption systems using the alkali-doublet (AD) method in which changes in α are related to changes in the doublet spacing. The precision obtained with the AD method has been increased by a factor of 3:     . We also analyse potential systematic errors in this result. Finally, we compare the AD method with the many-multiplet method, which has achieved an order of magnitude greater precision, and we discuss the future of the AD method.  相似文献