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Register a new userTesting the forward approach in modelling beta Cephei pulsators: setting the stage
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The information on stellar parameters and on the stellar interior we can get by studying pulsating stars depends crucially on the available observational constraints: both seismic constraints precision and number of detected modes, identification, nature of the modes) and classical observations (photospheric abundances, effective temperature, luminosity, surface gravity). We consider the case of beta Cephei pulsators and, with the aim of estimating quantitatively how the available observational constraints determine the type and precision of our inferences, we set the stage for Hare&Hound exercises. In this contribution we present preliminary results for one simple case, where we assume as observed frequencies a subset of frequencies of a model and then evaluate a seismic merit function on a dense and extensive grid of models of B-type stars. We also compare the behaviour of chi^2 surfaces obtained with and without mode identification.
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We present results of a {bf comprehensive} asteroseismic modelling of the $beta$ Cephei variable $theta$ Ophiuchi. {bf We call these studies {it complex asteroseismology} because our goal is to reproduce both pulsational frequencies as well as corresponding values of a complex, nonadiabatic parameter, $f$, defined by the radiative flux perturbation.} To this end, we apply the method of simultaneous determination of the spherical harmonic degree, $ell$, of excited pulsational mode and the corresponding nonadiabatic $f$ parameter from combined multicolour photometry and radial velocity data. Using both the OP and OPAL opacity data, we find a family of seismic models which reproduce the radial and dipole centroid mode frequencies, as well as the $f$ parameter associated with the radial mode. Adding the nonadiabatic parameter to seismic modelling of the B-type main sequence pulsators yields very strong constraints on stellar opacities. In particular, only with one source of opacities it is possible to agree the empirical values of $f$ with their theoretical counterparts. Our results for $theta$ Oph point substantially to preference for the OPAL data.
We consider the impact of stochastic perturbations on otherwise coherent oscillations of classical pulsators. The resulting dynamics are modelled by a driven damped harmonic oscillator subject to either an external or an internal forcing and white noise velocity fluctuations. We characterize the phase and relative amplitude variations using analytical and numerical tools. When the forcing is internal the phase variation displays a random walk behaviour and a red noise power spectrum with a ragged erratic appearance. We determine the dependence of the root mean square phase and relative amplitude variations ($sigma_{Delta varphi}$ and $sigma_{Delta A/A}$, respectively) on the amplitude of the stochastic perturbations, the damping constant $eta$, and the total observation time $t_{rm obs}$ for this case, under the assumption that the relative amplitude variations remain small, showing that $sigma_{Delta varphi}$ increases with $t_{rm obs}^{1/2}$ becoming much larger than $sigma_{Delta A/A}$ for $t_{rm obs} gg eta^{-1}$. In the case of an external forcing the phase and relative amplitude variations remain of the same order, independent of the observing time. In the case of an internal forcing, we find that $sigma_{Delta varphi}$ does not depend on $eta$. Hence, the damping time cannot be inferred from fitting the power of the signal, as done for solar-like pulsators, but the amplitude of the stochastic perturbations may be constrained from the observations. Our results imply that, given sufficient time, the variation of the phase associated to the stochastic perturbations in internally driven classical pulsators will become sufficiently large to be probed observationally.
We present the results of a spectroscopic multisite campaign for the beta Cephei star 12 (DD) Lacertae. Our study is based on more than thousand high-resolution high S/N spectra gathered with 8 different telescopes in a time span of 11 months. In addition we make use of numerous archival spectroscopic measurements. We confirm 10 independent frequencies recently discovered from photometry, as well as harmonics and combination frequencies. In particular, the SPB-like g-mode with frequency 0.3428 1/d reported before is detected in our spectroscopy. We identify the four main modes as (l1,m1) = (1, 1), (l2,m2) = (0, 0), (l3,m3) = (1, 0) and (l4,m4) = (2, 1) for f1 = 5.178964 1/d, f2 = 5.334224 1/d, f3 = 5.066316 1/d and f4 = 5.490133 1/d, respectively. Our seismic modelling shows that f2 is likely the radial first overtone and that the core overshooting parameter alpha_ov is lower than 0.4 local pressure scale heights.
Only for very few beta Cephei stars has the behaviour of the magnetic field been studied over the rotation cycle. During the past two years we have obtained multi-epoch polarimetric spectra of the beta Cephei star V1449 Aql with SOFIN at the Nordic Optical Telescope to search for a rotation period and to constrain the geometry of the magnetic field. The mean longitudinal magnetic field is measured at 13 different epochs. The new measurements, together with the previous FORS1 measurements, have been used for the frequency analysis and the characterization of the magnetic field. V1449 Aql so far possesses the strongest longitudinal magnetic field of up to 700G among the beta Cephei stars. The resulting periodogram displays three dominant peaks with the highest peak at f=0.0720d^-1 corresponding to a period P=13.893d. The magnetic field geometry can likely be described by a centred dipole with a polar magnetic field strength B_d around 3kG and an inclination angle beta of the magnetic axis to the rotation axis of 76+-4deg. As of today, the strongest longitudinal magnetic fields are detected in the beta Cephei stars V1449 Aql and xi^1 CMa with large radial velocity amplitudes. Their peak-to-peak amplitudes reach ~90km/s and ~33km/s, respectively. Concluding, we briefly discuss the position of the currently known eight magnetic beta Cephei and candidate beta Cephei stars in the Hertzsprung-Russell (H-R) diagram.
We present new optical time-resolved photometry and medium-resolution spectroscopy of V2187 Cyg. We confirm its classification as a beta Cephei star based on sinusoidal light variations with a period of 0.2539 days and mean amplitudes of 0.037 and 0.042 magnitudes in i and V, respectively. We classified the spectrum of this star B2-3V with no evidence of variations in the profiles of its absorption lines in timescales of hours or days. The stellar spectrum is totally absent of emission lines. We detected unexpected faint radio continuum emission (between 0.4 and 0.8 mJy at 6-cm) showing a sinusoidal variation with a period of 12.8 days. The radio spectrum is thermal. We searched in the Very Large Array archive for radio continuum emission toward other 15 beta Cephei stars. None of these additional stars, some of them much closer to the Sun than V2187 Cyg, was detected, indicating that radio emission is extremely uncommon toward beta Cephei stars.
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