Photometric amplitudes and phases of nonradial oscillation in rotating stars

Effects of rotational mode coupling on photometric parameters of stellar oscillations are studied. At moderate rotation rates, a strong coupling between modes of spherical harmonic degree, $\ell$, differing by 2 and of the same azimuthal order, $m$, takes place if the frequencies are close. This is a common situation amongst main sequence pulsators. Numerical results for a sequence of $\beta$ Cephei star models are reported for the two- and three-mode couplings. One consequence of mode coupling is that modes of higher degree should be considered in photometric mode identification. Modes with nominal degree $\ell>2$ acquire substantial $\ell\le2$ components and therefore are more likely to reach detectable amplitudes. Coupled mode positions in the amplitude ratio - phase difference diagrams, based on multicolour photometry, become both aspect- and $m$-dependent. Examples of the mode path in the diagram with varying aspect are given. The diagrams remain a useful tool for mode identification in rotating stars but the tool must be used with care.Comment: 9 pages, 8 figures, accepted for publication in A&

From the Heart of The Ghoul: C and N Abundances in the Corona of Algol B

Chandra Low Energy Transmission Grating Spectrograph observations of Algol have been used to determine the abundances of C and N in the secondary star for the first time. The analysis was performed relative to similar observations of an adopted "standard" star HR 1099. It is demonstrated that HR 1099 and Algol are coronal twins in many respects and that their X-ray spectra are very similar in nearly all details, except for the observed strengths of C and N lines. The H-like transitions of C and N in the coronae of Algol and HR 1099 demonstrate that the surface abundances of Algol B have been strongly modified by CN-processing, as shown earlier by Schmitt & Ness (2002). It is found that N is enhanced in Algol B by a factor of 3 compared to HR 1099. No C lines are detected in the Algol spectrum, indicating a C depletion relative to HR 1099 by a factor of 10 or more. These C and N abundances indicate that Algol B must have lost at least half of its initial mass, and are consistent with predictions of evolutionary models that include non-conservative mass transfer and angular momentum loss through magnetic activity. Little or no dredge-up of material subjected to CN-processing has occurred on the subgiant component of HR 1099. It is concluded that Fe is very likely depleted in the coronae of both Algol and HR 1099 relative to their photospheres by 0.5 dex, and C, N and O by 0.3 dex. Instead, Ne is enhanced by up to 0.5 dex.Comment: 17 pages, 4 figures, ApJ accepte

A photometric mode identification method, including an improved non-adiabatic treatment of the atmosphere

We present an improved version of the method of photometric mode identification of Heynderickx et al. (1994). Our new version is based on the inclusion of precise non-adiabatic eigenfunctions determined in the outer stellar atmosphere according to the formalism recently proposed by Dupret et al.(2002). Our improved photometric mode identification technique is therefore no longer dependent on ad hoc parameters for the non-adiabatic effects. It contains the complete physical conditions of the outer atmosphere of the star, provided that rotation does not play a key role. We apply our improved method to the two slowly pulsating B stars HD 74560 and HD 138764 and to the beta Cephei star EN (16) Lac. Besides identifying the degree l of the pulsating stars, our method is also a tool for improving the knowledge of stellar interiors and atmospheres, by imposing constraints on parameters such as the metallicity and the mixing-length parameter alpha (a procedure we label non-adiabatic asteroseismology).Comment: 10 pages, 9 figures Accepted for publication in Astronomy and Astrophysic

Effect of NLTE model atmospheres on photometric amplitudes and phases of early B-type pulsating stars

We study all possible sources of inaccuracy in theoretical values of the photometric observables, i.e. amplitude ratios and phase differences, of early B-type main sequence pulsators. Here, we discuss effects of parameters coming from both models of stellar atmospheres and linear nonadiabatic theory of stellar pulsation. In particular, we evaluate for the first time the effect of the departure from the LTE approximation. The atmospheric input comes from line-blanketed, LTE and NLTE plane-parallel, hydrostatic models. To compute the limb-darkening coefficients for NLTE models, we use the Least-Square Method taking into account the accuracy of the flux conservation. We present effects of NLTE atmospheres, chemical composition and opacities on theoretical values of the photometric observables of early B-type pulsators. To this end, we compute tables with the passband fluxes, flux derivatives over effective temperature and gravity as well as the non-linear limb-darkening coefficients in 12 most often used passbands, i.e. in the Str\"omgern system, $uvby$, and in the Johnson-Cousins-Glass system, $UBVRIJHK$. We make these tables public available at the Wroc{\l}aw HELAS Web page, http://helas.astro.uni.wroc.pl.Comment: 13 pages, 2 tables, 17 figues submitted to A&

Constraints on parameters of B-type pulsators from combined multicolour photometry and radial velocity data. I. β\beta Cephei stars

We analyze data on pulsation amplitudes and phases for two $\beta$ Cephei stars, $\delta$ Cet and $\nu$ Eri. Str\"omgren photometry and radial velocity measurements are used simultaneously to obtain constraints on mean parameters of the stars and identification of the excited modes. The inference about the radial mode order and mean star parameters is based on comparison of certain complex parameter, $f$, determined from data, with its counterpart derived from linear nonadiabatic modelling of stellar oscillations. The theoretical $f$ values are very sensitive to the adopted opacity data. In our modelling we rely on the data from OPAL and OP projects. Significant differences were found. New seismic models of $\nu$ Eri were constructed with both the OPAL and OP opacities.Comment: 11 pages, 13 figures, A&A in pres

Constraints on stellar convection from multi-colour photometry of Delta Scuti stars

In Delta Scuti star models, the calculated amplitude ratios and phase differences for multi-colour photometry exhibit a strong dependence on convection. These observables are tools for determination of the spherical harmonic degree of the excited modes. The dependence on convection enters through the complex parameter f, which describes bolometric flux perturbation. We present a method of simultaneous determination of f and spherical harmonic degree from multi-colour data and apply it to three Delta Scuti stars. The method indeed works. Determination of the degree appears unique and the inferred f's are sufficiently accurate to yield a useful constraint on models of stellar convection. Furthermore, the method helps to refine stellar parameters, especially if the identified mode is radial.Comment: 9 pages, 12 figures, to appear in Astronomy and Astrophysic

Quantitative spectroscopy of close binary stars

The method of spectral disentangling has now created the opportunity for studying the chemical composition in previously inaccessible components of binary and multiple stars. This in turn makes it possible to trace their chemical evolution, a vital aspect in understanding the evolution of stellar systems. We review different ways to reconstruct individual spectra from eclipsing and non-eclipsing systems, and then concentrate on some recent applications to detached binaries with high-mass and intermediate-mass stars, and Algol-type mass-transfer systems.Comment: To appear in the Proceedings of IAU Symposium 282 'From Interacting Binaries to Exoplanets: Essential Modeling Tools

Massive pulsating stars observed by BRITE-Constellation. I. The triple system Beta Centauri (Agena)

This paper aims to precisely determine the masses and detect pulsation modes in the two massive components of Beta Cen with BRITE-Constellation photometry. In addition, seismic models for the components are considered and the effects of fast rotation are discussed. This is done to test the limitations of seismic modeling for this very difficult case. A simultaneous fit of visual and spectroscopic orbits is used to self-consistently derive the orbital parameters, and subsequently the masses, of the components. The derived masses are equal to 12.02 +/- 0.13 and 10.58 +/- 0.18 M_Sun. The parameters of the wider, A - B system, presently approaching periastron passage, are constrained. Analysis of the combined blue- and red-filter BRITE-Constellation photometric data of the system revealed the presence of 19 periodic terms, of which eight are likely g modes, nine are p modes, and the remaining two are combination terms. It cannot be excluded that one or two low-frequency terms are rotational frequencies. It is possible that both components of Beta Cen are Beta Cep/SPB hybrids. An attempt to use the apparent changes of frequency to distinguish which modes originate in which component did not succeed, but there is potential for using this method when more BRITE data become available. Agena seems to be one of very few rapidly rotating massive objects with rich p- and g-mode spectra, and precisely known masses. It can therefore be used to gain a better understanding of the excitation of pulsations in relatively rapidly rotating stars and their seismic modeling. Finally, this case illustrates the potential of BRITE-Constellation data for the detection of rich-frequency spectra of small-amplitude modes in massive pulsating stars.Comment: 17 pages (with Appendix), 15 figures, accepted for publication in A&

UVSat: a concept of an ultraviolet/optical photometric satellite

Time-series photometry from space in the ultraviolet can be presently done with only a few platforms, none of which is able to provide wide-field long-term high-cadence photometry. We present a concept of UVSat, a twin space telescope which will be capable to perform this kind of photometry, filling an observational niche. The satellite will host two telescopes, one for observations in the ultraviolet, the other for observations in the optical band. We also briefly show what science can be done with UVSat.Comment: 6 pages, 2 figures, accepted for publication in the Proceedings of the PAS (Proc. of the 2nd BRITE Science conference, Innsbruck

delta Ceti is not monoperiodic: seismic modeling of a beta Cephei star from MOST spacebased photometry

The beta Cephei star delta Ceti was considered one of the few monoperiodic variables in the class. Despite (or perhaps because of) its apparently simple oscillation spectrum, it has been challenging and controversial to identify this star's pulsation mode and constrain its physical parameters seismically. Broadband time-resolved photometry of delta Ceti spanning 18.7 days with a duty cycle of about 65% obtained by the MOST (Microvariability & Oscillations of STars) satellite -- the first scientific observations ever obtained by MOST -- reveals that the star is actually multiperiodic. Besides the well-known dominant frequency of f1 = 6.205886/d, we have discovered in the MOST data its first harmonic 2f1 and three other frequencies (f2 = 3.737/d, f3 = 3.673/d and f4 = 0.318/d), all detected with S/N > 4. In retrospect, f2 was also present in archival spectral line profile data but at lower S/N. We present seismic models whose modes match exactly the frequencies f1 and f2. Only one model falls within the common part of the error boxes of the star's observed surface gravity and effective temperature from photometry and spectroscopy. In this model, f1 is the radial (l = 0) first overtone and f2 is the g2 (l = 2, m = 0) mode. This model has a mass of 10.2+/-0.2 Msun and an age of 17.9+/-0.3 million years, making delta Ceti an evolved beta Cephei star. If f2 and f3 are rotationally split components of the same g2 mode, then the star's equatorial rotation velocity is either 27.6 km/s or half this value. Given its vsini of about 1 km/s, this implies we are seeing delta Ceti nearly pole-on.Comment: Accepted for publication in The Astrophysical Journal, 12 pages, 8 figures, 2 table