Population synthesis of Galactic subdwarf B stars

I briefly review the method of population synthesis of binary stars and discuss the preliminary results of a study of the Galactic population of subdwarf B stars. In particular I focus on the formation of (apparently) single sdB stars and their relation to (apparently) single helium-core white dwarfs. I discuss the merits of mergers of two helium white dwarfs and interactions with sub-stellar companions for explaining these single objects. A preliminary conclusion is that the current observations suggest both mechanisms may contribute, but that the helium white dwarfs are likely formed in majority from interactions with sub-stellar companions.     

Charged particles’ areas of three-dimensional motions in a system of two magnetic stars

We study the parametric evolution of the regions where three-dimensional motions of a charged particle are allowed in the combined electromagnetic field produced by two rotating magnetic stars. We discuss the changes in the topology of the zero-velocity surfaces, as well as in the trapping regions of the particle motion for various values of the dipoles’ magnetic moments.     

Effect of Coriolis force on the shapes of rotating stars and stars in binary systems

In this paper an attempt has been made to determine the effect of Coriolis force on the shapes of Roche equipotential surfaces of rotating stars and stars in binary systems. Equations of Roche equipotential surfaces have been obtained for rotating and binary stars which take into account the effects of Coriolis force besides the centrifugal and gravitational forces. Shapes of Roche equipotentials and values of Roche limits are obtained for different values of angular velocity of rotation for rotating stars and for different values of mass ratios for the binary stars. The obtained results have been compared with the corresponding results in which the effect of Coriolis force has not been considered.     

Spectroscopic mode identification of main-sequence non-radially pulsating stars

We are undertaking an extensive observational campaign of a number of non-radially pulsating stars using the high-resolution HERCULES spectrograph on the 1.0-m telescope at the Mt John University Observatory. This is part of a large world-wide multi-site campaign to improve mode-identification techniques in non-radially pulsating stars, particularly for g-mode pulsators. This paper outlines our campaign and presents preliminary results for one γ Doradus star, HD 40745, and one β Cephei star, HD 61068. We have used a representative cross-correlation line-profile technique presented by Wright in 2008 to extract line profiles and these have then been analyzed using the FAMIAS package due to Zima published in 2006 to derive a spectroscopic mode identification.     

Asteroseismology and the degeneracy of model solutions for subgiant stars

Stellar evolutionary models are essential for the determination of stellar masses and ages. Several parameters can be used to characterize the structure of subgiant stars. Amongst others we can find: the helium and metal abundances, mixing length and core overshooting. In many cases, these parameters are scaled taking into account the solar values. Yet, the universality of such scalings has been put to question. As shown here, by allowing some freedom to these parameters we end up with a degeneracy of model solutions. This puts serious limitations on the determination of mass and age. Asteroseismology may provide a valuable help to break such a degeneracy. This is illustrated with an application to the subgiant star μ Her.     

The formation of hot subdwarf stars and its implications for the UV-upturn phenomenon of elliptical galaxies

In this paper, we review the formation scenario for field hot subdwarf stars and extreme horizontal branch stars in globular clusters and discuss how the scenario helps us to understand the UV-upturn phenomenon of elliptical galaxies. It is widely accepted that field hot subdwarf stars originate from binary evolution via the following three channels, common envelope evolution channel for hot subdwarf binaries with short orbital periods, stable Roche lobe overflow channel for hot subdwarf binaries with long orbital periods, and the double helium white dwarf merger channel for single hot subdwarfs. Such a scenario can also explain the lack of binarity of extreme horizontal branch stars in globular clusters. We have applied, in an a priori way, the scenario to the study of UV-upturn phenomenon of elliptical galaxies via an evolutionary population synthesis approach and found that the UV-upturn can be naturally explained. This has major implications for understanding the evolution of UV-upturn and elliptical galaxies in general. In particular, it implies that the UV-upturn is not a sign of age, as had been postulated previously, and should not be strongly dependent on the metallicity of the population, but exists universally from dwarf ellipticals to giant ellipticals. The above a priori UV-upturn model is supported by recent GALEX observations and has been applied to naturally explain the colours of both dwarf ellipticals and giant ellipticals without the requirement of dichotomy between their stellar population properties.     

Orbital elements of the mild and strong barium stars formed through a wind accretion scenario

Taking account of the metallicity dependence of the s-process nucleosynthesis in the AGB stars, we adopted the wind accretion model with the condition of total angular momentum conservation and used the Monte-Carlo method to study the variations and the distributions of the orbital elements of the mild and strong Ba stars. The calculated results show that the level of heavy-element overabundance in a Ba star depends on the orbital period. Since there is a strong dependence of s-process yields on the initial stellar metallicity of the AGB star and a strong increase of the s-process yields in AGB stars with decreasing metallicity, the calculated results strongly suggest that the initial metallicity of the Ba star systems is another important parameter for the level of heavy-element overabundance in a Ba star. The strong Ba stars generally have lower metallicities than mild Ba stars. The masses of AGB progenitor and Ba star are other two parameters which also have some impact on the heavy-element overabundance in the Ba star.     

The atomic physics underlying the spectroscopic analysis of massive stars and supernovae

We have developed a radiative transfer code, cmfgen, which allows us to model the spectra of massive stars and supernovae. Using cmfgen we can derive fundamental parameters such as effective temperatures and surface gravities, derive abundances, and place constraints on stellar wind properties. The last of these is important since all massive stars are losing mass via a stellar wind that is driven from the star by radiation pressure, and this mass loss can substantially influence the spectral appearance and evolution of the star. Recently we have extended cmfgen to allow us to undertake time-dependent radiative transfer calculations of supernovae. Such calculations will be used to place constraints on the supernova progenitor, to place constraints on the supernova explosion and nucleosynthesis, and to derive distances using a physical approach called the “Expanding Photosphere Method”. We describe the assumptions underlying the code and the atomic processes involved. A crucial ingredient in the code is the atomic data. For the modeling we require accurate transition wavelengths, oscillator strengths, photoionization cross-sections, collision strengths, autoionization rates, and charge exchange rates for virtually all species up to, and including, cobalt. Presently, the available atomic data varies substantially in both quantity and quality.     

AGB star intershell abundances inferred from UV spectra of extremely hot post-AGB stars

The hydrogen-deficiency in extremely hot post-AGB stars of spectral class PG1159 is probably caused by a (very) late helium-shell flash or a AGB final thermal pulse that consumes the hydrogen envelope, exposing the usually-hidden intershell region. Thus, the photospheric element abundances of these stars allow us to draw conclusions about details of nuclear burning and mixing processes in the precursor AGB stars. We compare predicted element abundances to those determined by quantitative spectral analyses performed with advanced non-LTE model atmospheres. A good qualitative and quantitative agreement is found for many species (He, C, N, O, Ne, F, Si, Ar) but discrepancies for others (P, S, Fe) point at shortcomings in stellar evolution models for AGB stars. Almost all of the chemical trace elements in these hot stars can only be identified in the UV spectral range. The Far Ultraviolet Spectroscopic Explorer and the Hubble Space Telescope played a crucial role for this research.     

Follow-up observations of β-pic-like stars

Following the discovery, by IRAS, of the dust disc around Vega and three other main sequence stars, searches have been made for other candidates. The-Pic-like candidates have 12µm excesses and 100µm fluxes (unlike the Vega-like candidates), so they can be further investigated using ground-based techniques. Data are presented here, comprising 10µm spectroscopy and sub-mm observations, for several candidates from the Walker & Wolstencroft list, showing that the stars have silicate dust, and optically thick dust discs even at 1300µm.     

Analysis of the X-ray emission from OB stars Ⅲ: low-resolution spectra of OB stars

This paper is the third in our series of papers devoted to the investigation of X-ray emission from OB stars.In our two previous papers,we study the high-resolution X-ray spectra of 32 O stars and 25 B stars to investigate the correlations between the properties of X-ray emission and stellar parameters.We checked if the X-ray hardness and post-shock plasma temperature grow with increasing stellar magnetic field,mass loss rate and terminal wind velocity.Our previous analysis of high-resolution spectra showed that the correlations are weak or even absent.In the present paper,we analyzed low-resolution X-ray spectra,using model-independent X-ray hardness values for checking the above mentioned dependencies.We establish that X-ray luminosities L_X weakly depend on the stellar magnetic field.At the same time,Lx ∝ M~(0.5) and L_X ∝ E_(kin)~(0.5),where M is the mass loss rate and E_(kin) is the kinetic energy of the wind.The X-ray luminosities decrease with growing magnetic confinement parameter η.We also argue that there is an additional(probably non-thermal) component contributed to the stellar X-ray emission.     

EXOTIME: searching for planets around pulsating subdwarf B stars

In 2007, a companion with planetary mass was found around the pulsating subdwarf B star V391 Pegasi with the timing method, indicating that a previously undiscovered population of substellar companions to apparently single subdwarf B stars might exist. Following this serendipitous discovery, the EXOTIME (http://www.na.astro.it/~silvotti/exotime/) monitoring program has been set up to follow the pulsations of a number of selected rapidly pulsating subdwarf B stars on time scales of several years with two immediate observational goals:

1. determine $\dot{P}$ of the pulsational periods P
2. search for signatures of substellar companions in O–C residuals due to periodic light travel time variations, which would be tracking the central star’s companion-induced wobble around the centre of mass

These sets of data should therefore, at the same time, on the one hand be useful to provide extra constraints for classical asteroseismological exercises from the $\dot{P}$ (comparison with “local” evolutionary models), and on the other hand allow one to investigate the preceding evolution of a target in terms of possible “binary” evolution by extending the otherwise unsuccessful search for companions to potentially very low masses. While timing pulsations may be an observationally expensive method to search for companions, it samples a different range of orbital parameters, inaccessible through orbital photometric effects or the radial velocity method: the latter favours massive close-in companions, whereas the timing method becomes increasingly more sensitive toward wider separations. In this paper we report on the status of the on-going observations and coherence analysis for two of the currently five targets, revealing very well-behaved pulsational characteristics in HS?0444+0458, while showing HS?0702+6043 to be more complex than previously thought.     

Hydrogen-line profiles of six λ Bootis stars

Based on high resolution spectroscopic observations hydrogen-line profiles of six Bootis stars: HD 31295, HD 101108, HD 105058, HD 106223, HD 142703 and HD 183324 are presented. Peculiar line profiles with weak cores and broad but shallow wings are found for four of the stars. Influence of metallicity parameter [Z/H] and effective temperature on the appearance of the observed hydrogen lines is discussed.     

Infrared spectral variations of three Mira variable carbon stars with the 11.3 μm SiC feature

The spectral variations of three Mira variable carbon stars, V CrB, T Dra and V Cyg in the infrared are investigated based on ISO SWS data. It is found that either continua or molecular/dust features were variable with time in the infrared for these carbon stars during one and a half year observations. When stars were brighter the infrared continuum spectra became blue while stars were fainter the infrared continuum spectra became red. In addition, during spectral variations there were the correlation between the 3.05 μm HCN+C₂H₂ and the 5.2 μm C₃ molecular band strengths and the anti-correlation between the 3.05 μm HCN+C₂H₂ molecular band strengths and 13.7 μm C₂H₂ band strengths while during variations the 11.3 μm SiC dust emission strengths were not clearly changed.     

Infrared study of infrared carbon stars based on 2MASS,IRAS and ISO SWS data

For this paper, we collected all infrared carbon stars (IRCSs) known so far from the literature and identified the 2MASS counterparts of all IRCSs. Using 2MASS, IRAS and ISO SWS data, we investigate the infrared properties of IRCSs. We find that the infrared colors and temperatures of IRCSs—not only in the IRAS region but also in the near infrared—are between those for visual carbon stars and extreme carbon stars. The results in this paper strongly support the suggestion that the sequence of visual carbon stars → infrared carbon stars → extreme carbon stars is the evolutionary sequence in the AGB phase for carbon-rich stars. In addition, using the ISO SWS data, we find that an evolutionary sequence also exists within the IRCS stage.     

UVbYβ electrophotometry of the RR Lyrae stars

The behaviour of the spectral metallicity parameters S of the RR Lyrae stars in the course of light change is considered. Electrophotometry results of 9 stars of the given class inUVbY are presented. Analysis of the data obtained shows that the variables for which variation of S with the light phase was established do not show any real variations of metallicity indexm ₁.     

Hβ luminosities of planetary nebula central stars

The determination of the luminosities of planetary nebula central stars from H nebular fluxes is investigated. A correlation is obtained with the luminosities derived from independent stellar parameters. An average scaling factor is determined for H luminosities of optically thick nebulae, as well as correlations of this parameter with the Zanstra He II and H I temperatures.     

Prediction of the Maximum Amplitude and Timing of Sunspot Cycle 24

Precursor techniques, in particular those using geomagnetic indices, often are used in the prediction of the maximum amplitude for a sunspot cycle. Here, the year 2008 is taken as being the sunspot minimum year for cycle 24. Based on the average aa index value for the year of the sunspot minimum and the preceding four years, we estimate the expected annual maximum amplitude for cycle 24 to be about 92.8±19.6 (1-sigma accuracy), indicating a somewhat weaker cycle 24 as compared to cycles 21 – 23. Presuming a smoothed monthly mean sunspot number minimum in August 2008, a smoothed monthly mean sunspot number maximum is expected about October 2012±4 months (1-sigma accuracy).     

Rapid variations of Hα emission strength of eight Be stars

Rapid observations of H profiles of eight Be stars have been obtained and it is found that rapid (in a time-scale of 4 to 6 minutes) and irregular variations of total emission strength of H line are present in these stars. It has been suggested that these variations may be due to the material circulation in the envelope of Be stars.     

Oscillatory Response of the 3D Solar Atmosphere to the Leakage of Photospheric Motion

The direct propagation of acoustic waves, driven harmonically at the solar photosphere, into the three-dimensional solar atmosphere is examined numerically in the framework of ideal magnetohydrodynamics. It is of particular interest to study the leakage of 5-minute global solar acoustic oscillations into the upper, gravitationally stratified and magnetised atmosphere, where the modelled solar atmosphere possesses realistic temperature and density stratification. This work aims to complement and bring further into the 3D domain our previous efforts (by Erdélyi et al., 2007, Astron. Astrophys. 467, 1299) on the leakage of photospheric motions and running magnetic-field-aligned waves excited by these global oscillations. The constructed model atmosphere, most suitable perhaps for quiet Sun regions, is a VAL IIIC derivative in which a uniform magnetic field is embedded. The response of the atmosphere to a range of periodic velocity drivers is numerically investigated in the hydrodynamic and magnetohydrodynamic approximations. Among others the following results are discussed in detail: i) High-frequency waves are shown to propagate from the lower atmosphere across the transition region, experiencing relatively low reflection, and transmitting most of their energy into the corona; ii) the thin transition region becomes a wave guide for horizontally propagating surface waves for a wide range of driver periods, and particularly at those periods that support chromospheric standing waves; iii) the magnetic field acts as a waveguide for both high- and low-frequency waves originating from the photosphere and propagating through the transition region into the solar corona. Electronic Supplementary Material  The online version of this article () contains supplementary material, which is available to authorized users.