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1.
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2.
High-resolution spectral observations of 123 B0–B5 stars in the main sequence evolutionary phase were obtained at two observatories, namely the McDonald Observatory (McDO) and the Crimean Astrophysical Observatory (CrAO). Accurate equivalent widths W of two Balmer lines, H β and H γ , and ten He  i lines were obtained for all the stars, as well as of the He  ii   λ 4686 line for the hottest ones. A careful analysis of the measured equivalent widths was performed. It is shown that there is a very good agreement between the W values derived from the McDO and CrAO spectra for 14 common stars. A comparison with published data leads to the conclusion that the W values measured earlier by some authors for strong He  i lines are very likely to be underestimated. Infrared photometric observations in the J , H , and K bands were performed for 70 programme stars. All these data will be used in other papers: in particular for the T eff and log  g determination and for the He, C, N and O abundance analyses.  相似文献   

3.
We investigate the role of classical microturbulence in the non-LTE He  i /He  ii line formation problem for luminous O-type stars. We find that the shapes and strengths of certain saturated He  i lines, in particular the triplets λλ4471, 4713 and the singlet λ4921, are sensitive to microturbulent velocities in excess of 5 km s−1. Weaker lines, including most of the He  i singlets, are effectively independent of this parameter, as are the Fowler series He  ii lines λλ4199, 4541, 5411. We show that this behaviour is due to interaction between direct line-broadening effects in the radiative transfer, and indirect changes in the atmospheric structure and the populations of absorbing states. Using an analysis of high-resolution, high signal-to-noise ratio observations of the O9.7 supergiant HD 152003 as an illustrative example, we show how the introduction of microturbulence in non-LTE models allows consistent fits to be obtained for all   blue-region He  i lines — including the strong triplets λλ4026, 4713, 4471 — at an assumed solar helium abundance, thereby offering a resolution to the problem of the 'generalized dilution effect' described by Voels et al. We argue that by extension this result may also have implications for the so-called 'helium discrepancy' identified in OB-type stars by Herrero et al.  相似文献   

4.
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5.
We model stellar differential rotation based on the mean-field theory of fluid dynamics. DR is mainly driven by Reynolds stress, which is anisotropic and has a non-diffusive component because the Coriolis force affects the convection pattern. Likewise, the convective heat transport is not strictly radial but slightly tilted towards the rotation axis, causing the polar caps to be slightly warmer than the equator. This drives a flow opposite to that caused by differential rotation and so allows the system to avoid the Taylor-Proudman state. Our model reproduces the rotation pattern in the solar convection zone and allows predictions for other stars with outer convection zones. The surface shear turns out to depend mainly on the spectral type and only weakly on the rotation rate. We present results for stars of spectral type F which show signs of very strong differential rotation in some cases. Stars just below the mass limit for outer convection zones have shallow convection zones with short convective turnover times. We find solar-type rotation and meridional flow patterns at much shorter rotation periods and horizontal shear much larger than on the solar surface, in agreement with recent observations. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

6.
Observations of rapidly rotating solar-like stars show a significant mixture of opposite-polarity magnetic fields within their polar regions. To explain these observations, models describing the surface transport of magnetic flux demand the presence of fast meridional flows. Here, we link subsurface and surface magnetic flux transport simulations to investigate (i) the impact of meridional circulations with peak velocities of  ≤125 m s−1  on the latitudinal eruption pattern of magnetic flux tubes and (ii) the influence of the resulting butterfly diagrams on polar magnetic field properties. Prior to their eruption, magnetic flux tubes with low field strengths and initial cross-sections below  ∼300 km  experience an enhanced poleward deflection through meridional flows (assumed to be polewards at the top of the convection zone and equatorwards at the bottom). In particular, flux tubes which originate between low and intermediate latitudes within the convective overshoot region are strongly affected. This latitude-dependent poleward deflection of erupting magnetic flux renders the wings of stellar butterfly diagrams distinctively convex. The subsequent evolution of the surface magnetic field shows that the increased number of newly emerging bipoles at higher latitudes promotes the intermingling of opposite polarities of polar magnetic fields. The associated magnetic flux densities are about 20 per cent higher than in the case disregarding the pre-eruptive deflection, which eases the necessity for fast meridional flows predicted by previous investigations. In order to reproduce the observed polar field properties, the rate of the meridional circulation has to be of the order of 100 m s−1, and the latitudinal range from which magnetic flux tubes originate at the base of the convective zone (≲50°) must be larger than in the solar case (≲35°).  相似文献   

7.
Towards an understanding of the Of?p star HD 191612: optical spectroscopy   总被引：1，自引：0，他引：1  
We present extensive optical spectroscopy of the early-type magnetic star HD 191612 (O6.5f?pe–O8fp). The Balmer and He  i lines show strongly variable emission which is highly reproducible on a well-determined 538-d period. He  ii absorptions and metal lines (including many selective emission lines but excluding He  ii λ4686 Å emission) are essentially constant in line strength, but are variable in velocity, establishing a double-lined binary orbit with   P orb= 1542 d, e = 0.45  . We conduct a model-atmosphere analysis of the spectrum, and find that the system is consistent with a ∼O8 giant with a ∼B1 main-sequence secondary. Since the periodic 538-d changes are unrelated to orbital motion, rotational modulation of a magnetically constrained plasma is strongly favoured as the most likely underlying 'clock'. An upper limit on the equatorial rotation is consistent with this hypothesis, but is too weak to provide a strong constraint.  相似文献   

8.
We present simultaneous X-ray ( RXTE ) and optical (ULTRACAM) narrow-band (Bowen blend/He  ii and nearby continuum) observations of Sco X-1 at 2–10 Hz time resolution. We find that the Bowen/He  ii emission lags the X-ray light curves with a light traveltime of     s which is consistent with reprocessing in the companion star. The echo from the donor is detected at orbital phase ∼0.5 when Sco X-1 is at the top of the flaring branch (FB). Evidence of echoes is also seen at the bottom of the FB but with time-lags of 5–10 s which are consistent with reprocessing in an accretion disc with a radial temperature profile. We discuss the implication of our results for the orbital parameters of Sco X-1.  相似文献   

9.
We present time-resolved spectroscopy of the soft X-ray transient XTE J2123–058 in outburst. A useful spectral coverage of 3700–6700 Å was achieved spanning two orbits of the binary, with single-epoch coverage extending to ∼9000 Å. The optical spectrum approximates a steep blue power law, consistent with emission on the Rayleigh–Jeans tail of a hot blackbody spectrum. The strongest spectral lines are He  ii 4686 Å and C  iii /N  iii 4640 Å (Bowen blend) in emission. Their relative strengths suggest that XTE J2123–058 was formed in the Galactic plane, not in the halo. Other weak emission lines of He  ii and C  iv are present, and Balmer lines show a complex structure, blended with He  ii . He  ii 4686-Å profiles show a complex multiple S-wave structure, with the strongest component appearing at low velocities in the lower-left quadrant of a Doppler tomogram. H α shows transient absorption between phases 0.35 and 0.55. Both of these effects appear to be analogous to similar behaviour in SW Sex type cataclysmic variables. We therefore consider whether the spectral line behaviour of XTE J2123–058 can be explained by the same models invoked for those systems.  相似文献   

10.
We study the reionization of He  ii by quasars using a numerical approach that combines 3D radiative transfer calculations with cosmological hydrodynamical simulations. Sources producing the ionizing radiation are selected according to an empirical quasar luminosity function, and are assigned luminosities according to their intrinsic masses. The free parameters associated with this procedure are (1) a universal source lifetime, (2) a minimum mass cut-off, (3) a minimum luminosity cut-off, (4) a solid angle specifying the extent to which radiation is beamed, and (5) a tail-end spectral index for the radiative energy distribution of the sources. We present models in which these parameters are varied, and examine characteristics of the resultant reionization process that distinguish the various cases. In addition, we extract artificial spectra from the simulations and quantify statistical properties of the spectral features in each model.
We find that the most important factor affecting the evolution of He  ii reionization is the cumulative number of ionizing photons that are produced by the sources. Comparisons between He  ii opacities measured observationally and those obtained by our analysis reveal that the available ranges in plausible values for the parameters provide enough leeway to provide a satisfactory match. However, one property common to all our calculations is that the epoch of He  ii reionization must have occurred at a redshift in the range  3≲ z ≲4  . If so, future observational programmes will be able to directly trace the details of the ionization history of helium and to probe the low-density phase of the intergalactic medium during this phase of the evolution of the Universe.  相似文献   

11.
We present the results of an analysis of ultraviolet observations of T Tauri stars (TTs). By analysing emission measures taken from the literature, we derive rates of ionizing photons from the chromospheres of five classical TTs in the range  ∼1041–1044  photon s−1, although these values are subject to large uncertainties. We propose that the He  ii /C  iv line ratio can be used as a reddening-independent indicator of the hardness of the ultraviolet spectrum emitted by TTs. By studying this line ratio in a much larger sample of objects, we find evidence for an ionizing flux which does not decrease, and may even increase, as TTs evolve. This implies that a significant fraction of the ionizing flux from TTs is not powered by the accretion of disc material on to the central object, and we discuss the significance of this result and its implications for models of disc evolution. The presence of a significant ionizing flux in the later stages of circumstellar disc evolution provides an important new constraint on disc photoevaporation models.  相似文献   

12.
Following a brief overview of the two main approaches to investigate the interaction between magnetic fields and convective flows near the solar surface layers by numerical simulation, namely idealized model problems and ‘realistic’ large‐eddy simulations, we present first results obtained with a newly developed MHD code. The first example concerns the realistic simulation of the magnetic field dynamics in a solar plage region while the second example demonstrates small‐scale dynamo action in idealized compressible convection.  相似文献   

13.
The low-density hydrogen and helium in the intergalactic medium (IGM) probed by quasi-stellar object (QSO) absorption lines is sensitive to the amplitude and spectral shape of the metagalactic ultraviolet (UV) background. We use realistic H  i and He  ii Lyα forest spectra, constructed from state-of-the-art hydrodynamical simulations of a Λ cold dark matter (ΛCDM) universe to confirm the reliability of using line profile fitting techniques to infer the ratio of the metagalactic H  i and He  ii ionization rates. We further show that the large spatial variations and the anticorrelation with H  i absorber density observed in the ratio of the measured He  ii to H  i column densities can be explained in a model where the H  i ionization rate is dominated by the combined UV emission from young star-forming galaxies and QSOs and the He  ii ionization rate is dominated by emission from QSOs only. In such a model the large fluctuations in the column density ratio are due to the small number of QSOs expected to contribute at any given point to the He  ii ionization rate. A significant contribution to UV emission at the He  ii photoelectric edge from hot gas in galaxies and galaxy groups would decrease the expected fluctuations in the column density ratio. Consequently, this model appears difficult to reconcile with the large increase in He  ii opacity fluctuations towards higher redshift. Our results further strengthen previous suggestions that observed He  ii Lyα forest spectra at z ∼ 2–3.5 probe the tail end of the reionization of He  ii by QSOs.  相似文献   

14.
The eccentricities of the barium stars   总被引：3，自引：0，他引：3  
We investigate the eccentricities of barium (Ba  ii ) stars formed via a stellar wind accretion model. We carry out a series of Monte Carlo simulations using a rapid binary evolution algorithm, which incorporates full tidal evolution, mass loss and accretion, and nucleosynthesis and dredge-up on the thermally pulsing asymptotic giant branch. We follow the enhancement of barium in the envelope of the accreting main-sequence companion and dilution into its convective envelope once the star ascends the giant branch.
The observed eccentricities of Ba  ii stars are significantly smaller than those of an equivalent set of normal red giants but are nevertheless non-zero. We show that such a distribution of eccentricities is consistent with a wind accretion model for Ba  ii star production with weak viscous tidal dissipation in the convective envelopes of giant stars. We successfully model the distribution of orbital periods and the number of observed Ba  ii stars. The actual distribution of eccentricities is quite sensitive to the strength of the tides, so that we are able to confirm that this strength is close to, but less than, what is expected theoretically and found with alternative observational tests. Two systems – one very short-period but eccentric, and one long-period and highly eccentric – still lie outside the envelope of our models, and so require a more exotic formation mechanism. All our models, even those which were a good fit to the observed distributions, overproduced the number of high-period barium stars, a problem that could not be solved by some combination of the three parameters: tidal strength, tidal enhancement and wind accretion efficiency.  相似文献   

15.
Magnetic activity signatures in the atmosphere of active stars can be used to place constrains on the underlying processes of flux transport and dynamo operation in its convective envelope. The ‘solar paradigm’ for magnetic activity suggests that the magnetic field is amplified and stored at the base of the convection zone. Once a critical field strength is exceeded, perturbations initiate the onset of instabilities and the growth of magnetic flux loops, which rise through the convection zone, emerge at the stellar surface, and eventually lead to the formation of starspots and active regions. In close binaries, the proximity of the companion star breaks the rotational symmetry. Although the magnitude of tidal distortions is rather small, non‐linear MHD simulations have nevertheless shown in the case of main‐sequence binary components that they can cause non‐uniform surface distributions of flux tube eruptions. The present work extends the investigation to post‐mainsequence components to explore the specific influence of the stellar structure on the surface pattern of erupting flux tubes. In contrast to the case of main‐sequence components, where the consistency between simulation results and observations supports the presumption of a solar‐like dynamo mechanism, the numerical results here do not recover the starspot properties frequently observed on evolved binary components. This aspect points out an insufficiency of the applied flux tube model and leads to the conclusion that additional flux transport and possibly amplification mechanisms have to be taken into account. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

16.
Previous studies using observations made at low spatial and spectral resolution showed that the resonance lines of He  i and He  ii are anomalously strong in the quiet Sun when compared with other transition region lines formed at similar temperatures. Here, the higher spatial and spectral resolution provided by the Coronal Diagnostic Spectrometer ( cds ) instrument on board the Solar and Heliospheric Observatory ( SOHO ) is used to re-examine the behaviour of the He  i and He  ii lines and other transition region lines, in quiet regions near Sun centre. Supergranulation cell boundaries and cell interiors are examined separately. Near-simultaneous observations with the sumer instrument provide information on the lower transition region and the electron pressure. While the lines of He  i and He  ii have a common behaviour, as do the other transition region lines, the behaviour of the helium lines relative to the other transition region lines is significantly different. The emission measure distributions that account for all transition region lines, except those of helium, fail to produce sufficient emission in the He  i and He  ii resonance lines by around an order of magnitude, in both supergranulation cell boundary and cell interior regions. The electron pressure appears to be higher in the cell interiors than in the average cell boundaries, although the uncertainties are large. While the VAL-D model gives a closer match to the He  i 584.3-Å line, it does not successfully reproduce other transition region lines.  相似文献   

17.
An attempt is made to infer the structure of the solar convection zone from observedp-mode frequencies of solar oscillations. The differential asymptotic inversion technique is used to find the sound speed in the solar envelope. It is found that envelope models which use the Canuto-Mazzitelli (CM) formulation for calculating the convective flux give significantly better agreement with observations than models constructed using the mixing length formalism. This inference can be drawn from both the scaled frequency differences and the sound speed difference. The sound speed in the CM envelope model is within 0.2% of that in the Sun except in the region withr > 0.99R ⊙. The envelope models are extended below the convection zone, to find some evidence for the gravitational settling of helium beneath the base of the convection zone. It turns out that for models with a steep composition gradient below the convection zone, the convection zone depth has to be increased by about 6 Mm in order to get agreement with helioseismic observations.  相似文献   

18.
1 INTRODUCTIONThe mixing length theory (MLT) for stellar convection originally developed by Vitense(1953, 1958) has been the most popularly used local convection theory in the studies of stellarstructure and evolution. The theory was later modified and revised by many investigators,who suggested some different expressions. In fact, MLT is not a real hydrodynamic theory,rather, it is a simple "ballistic" theory which traces the motion of imaginary convective elements. In reality j stell…  相似文献   

19.
The spectroscopic variability of Arcturus hints at cyclic activity cycle and differential rotation. This could provide a test of current theoretical models of solar and stellar dynamos. To examine the applicability of current models of the flux transport dynamo to Arcturus, we compute a mean‐field model for its internal rotation, meridional flow, and convective heat transport in the convective envelope. We then compare the conditions for dynamo action with those on the Sun. We find solar‐type surface rotation with about 1/10th of the shear found on the solar surface. The rotation rate increases monotonically with depth at all latitudes throughout the whole convection zone. In the lower part of the convection zone the horizontal shear vanishes and there is a strong radial gradient. The surface meridional flow has maximum speed of 170 m/s and is directed towards the equator at high and towards the poles at low latitudes. Turbulent magnetic diffusivity is of the order 1015–1016 cm2/s. The conditions on Arcturus are not favorable for a circulation‐dominated dynamo (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

20.
In solar extreme ultraviolet spectra the He  i and He  ii resonance lines show unusual behaviour and have anomalously high intensities compared with other transition region lines. The formation of the helium resonance lines is investigated through extensive non-local thermal equilibrium radiative transfer calculations. The model atmospheres of Vernazza, Avrett & Loeser are found to provide reasonable matches to the helium resonance line intensities but significantly overestimate the intensities of other transition region lines. New model atmospheres have been developed from emission measure distributions derived by Macpherson & Jordan, which are consistent with SOHO observations of transition region lines other than those of helium. These models fail to reproduce the observed helium resonance line intensities by significant factors. The possibility that non-Maxwellian electron distributions in the transition region might lead to increased collisional excitation rates in the helium lines is studied. Collisional excitation and ionization rates are recomputed for distribution functions with power-law suprathermal tails that may form by the transport of fast electrons from high-temperature regions. Enhancements of the helium resonance line intensities are found, but many of the predictions of the models regarding line ratios are inconsistent with observations. These results suggest that any such departures from Maxwellian electron distributions are not responsible for the helium resonance line intensities.  相似文献   

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