On the exact shape of the horizontal profile of a topographically rectified tidal flow     

L. R. M. Maas  J. T. F. Zimmerman  N. M. Temme 《地球物理与天体物理流体动力学》2013,107(2):105-129

Abstract

Starting from the nonlinear shallow water equations of a homogeneous rotating fluid we derive the equation describing the evolution of vorticity by a fluctuating bottom topography of small amplitude, using a multiple scale expansion in a small parameter, which is the topographic length scale relative to the tidal wave length. The exact response functions of residual vorticity for a sinusoidal bottom topography are compared with those obtained by a primitive perturbation series and by harmonic truncation, showing the former to be invalid for small topographic length scales and the latter to be only a fair approximation for vorticity produced by planetary vortex stretching. In deriving the exact shape of the horizontal residual velocity profile at a step-like break in the bottom topography, it is shown that the Lagrangian profile only exists in a strip having the width of the amplitude of the tidal excursion at both sides of the break, and that it vanishes outside that interval. Moreover, in the limit of small amplitude topography at least, it vanishes altogether for the generation mechanism by means of planetary vortex stretching. The Eulerian profile is shown to extend over twice the interval of the Lagrangian profile both for production by vortex stretching and by differential bottom friction. These finite intervals over which the residual velocity profiles exist for a step-like topography are not reproduced by harmonic truncation of the basic equation. This method gives exponentially decaying profiles, indicating spurious horizontal diffusion of vorticity. In terms of orders of magnitude, the method of harmonic truncation is reliable for residual velocity produced by vortex stretching but it overestimates the residual velocity produced by differential bottom friction by a factor 2.  相似文献   

Hidden subluminous stars among the FAUST UV sources towards Ophiuchus     

Liliana Formiggini  Noah Brosch  Elchanan Almoznino  Stuart Bowyer  Michael Lampton 《Monthly notices of the Royal Astronomical Society》2002,332(2):441-455

We present results of an analysis of a UV image in the direction of Ophiuchus, obtained with the FAUST instrument. The image contains 228 UV sources. Most of these are identified as normal early-type stars through correlations with catalogued objects. For the first time in this project we identify UV sources as such stars by selecting suitable candidates in crowded fields as the bluest objects in colour–colour diagrams using observations from the Wise Observatory. These candidates are then studied using low-resolution spectroscopy, which allows the determination of spectral types to an accuracy of about one-half class, for 60 stars.
Synthetic photometry of spectral data is performed in order to predict the expected UV emission, on the basis of the photometric information. These results are used along with the Hipparcos /Tycho information, to search for subluminous stars. The comparison of the predicted emission with the FAUST measured magnitudes allows us to select 12 stars as highly probable evolved hot stars. High signal-to-noise spectra are obtained for nine of these stars, and Balmer line profiles are compared with the prediction of atmosphere models and with the spectrum of real stellar atmospheres. Among the nine candidates, six are classified as previously unrecognized sdB stars, and two as white dwarfs. Our result indicates that indeed more bright subluminous stars are still unrecognized in the existing samples.  相似文献   

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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Cyclic and secular variation in the temperatures and radii of extreme helium stars     

C. Simon Jeffery  Rhaana L. C. Starling  †  Philip W. Hill ‡  Don Pollacco 《Monthly notices of the Royal Astronomical Society》2001,321(1):111-130

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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   

The contribution of faint blue galaxies to the submillimetre counts and background     

Geoff S. Busswell  Tom Shanks 《Monthly notices of the Royal Astronomical Society》2001,323(1):67-74

Observations in the submillimetre (submm) waveband have recently revealed a new population of luminous sources. These are proposed to lie at high redshift and to be optically faint because of their high intrinsic dust obscuration. The presence of dust has been previously invoked in optical galaxy count models which use the Bruzual & Charlot evolution models with an exponential τ =9 Gyr star formation rate (SFR) for spirals, and these fit the count data well from U to K . We now show that by using either a 1/ λ or Calzetti absorption law for the dust and re-distributing the evolved spiral galaxy ultraviolet (UV) radiation into the far-infrared (FIR), these models can account for all of the 'faint' ( 1 mJy) 850-μm galaxy counts, but fail to fit 'bright' ( 2 mJy) sources, indicating that another explanation for the submm counts may apply at brighter fluxes, e.g., quasi-stellar objects (QSOs) or ultraluminous infrared galaxies (ULIRGs). We find that the main contribution to the faint, submm number counts is in the redshift range 0.5< z <3, peaking at z ≈1.8. The above model, using either dust law, can also explain a significant proportion of the extragalactic background at 850 μm, as well as producing a reasonable fit to the bright 60-μm IRAS counts.  相似文献   

FUSE observations of V592 Cas: the continuing enigma of orbitally modulated spectral signatures of accretion disc winds     

R. K. Prinja  C. Knigge  D. K. Witherick  K. S. Long  G. Brammer 《Monthly notices of the Royal Astronomical Society》2004,355(1):137-142

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Accurate laboratory ultraviolet wavelengths for quasar absorption-line constraints on varying fundamental constants     

M. Aldenius  S. Johansson  M. T. Murphy 《Monthly notices of the Royal Astronomical Society》2006,370(1):444-452

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Multiwavelength study of X-ray selected star-forming galaxies within the Chandra Deep Field-South     

Daniel Rosa-González  Denis Burgarella  Kirpal Nandra  Daniel Kunth  Elena Terlevich †  Roberto Terlevich † 《Monthly notices of the Royal Astronomical Society》2007,379(1):357-369

We have combined multiwavelength observations of a selected sample of star-forming galaxies with galaxy evolution models in order to compare the results obtained for different star formation rate (SFR) tracers and to study the effect that the evolution of the star-forming regions has on them. We also aimed at obtaining a better understanding of the corrections due to extinction and nuclear activity on the derivation of the SFR. We selected the sample from Chandra data for the well studied region Chandra Deep Field -South (CDFS) and chose the objects that also have ultraviolet (UV) and infrared (IR) data from Galaxy Evolution Explorer ( GALEX ) and Great Observatories Origins Deep Survey (GOODS) Spitzer , respectively.
Our main finding is that there is good agreement between the extinction corrected SFR(UV) and the SFR(X), and we confirm the use of X-ray luminosities as a trustful tracer of recent star formation activity. Nevertheless, at SFR(UV) larger than about  5 M_⊙ yr⁻¹  there are several galaxies with an excess of SFR(X) suggesting the presence of an obscured active galactic nucleus (AGN) not detected in the optical spectra. We conclude that the IR luminosity is driven by recent star formation even in those galaxies where the SFR(X) is an order of magnitude higher than the SFR(UV) and therefore may harbour an AGN. One object shows SFR(X) much lower than expected based on the SFR(UV); this SFR(X) 'deficit' may be due to an early transient phase before most of the massive X-ray binaries were formed. An X-ray deficit could be used to select extremely young bursts in an early phase just after the explosion of the first supernovae associated with massive stars and before the onset of massive X-ray binaries.  相似文献   

Ultraviolet variability of HD 62001 – The central star of the missing nebula V-V 1-7     

N. Kameswara Rao  D. P. Gilra 《Journal of Astrophysics and Astronomy》1983,4(1):59-63

IUE observations of HD 62001 do not confirm the ultraviolet variability of 0.15 mag seen from the ANS observations. These intriguing variations in ANS observations seem to be caused by a nearby (∼ 70 arcsec away) B star getting in and out of the field of view of the telescope, which had a jitter of 20 arcsec. There seems to be no evidence for the presence of a hot companion, either from the energy distribution or from UV variability. However, visual light variations are present. This, coupled with the radial velocity variations, indicates a binary nature probably of Algol type. Based on observations obtained with IUE satellite at the Villafranca Satellite Tracking Station of the European Space Agency.  相似文献