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Register a new userConstraints on opacities from complex asteroseismology of B-type pulsators: the $beta$ Cephei star $theta$ Ophiuchi
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Daszynska-Daszkiewicz Jadwiga
Publication date
2009
fields
Physics
and research's language is
English
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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.
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We present examples of an extended asteroseismic modelling in which we aim at fitting not only pulsational frequencies but also certain complex parameter related to each frequency. This kind of studies, called textbf{complex asteroseismology}, has been successfully applied to a few main sequence B-type pulsators and provided, in particular, plausible constraints on textbf{stellar opacities}. Here, we briefly describe our results for three early B-type stars.
We undertake another attempt towards seismic modelling of the most intensive studied main sequence pulsators of the early B spectral type, $ u$ Eridani. Our analysis is extended by a requirement of fitting not only pulsational frequencies but also the complex amplitude of the bolometric flux variation, $f$, related to each mode frequency. This approach, called {it complex asteroseismology}, provides a unique test of stellar parameters, atmospheres and opacities. In particular, the concordance of the empirical and theoretical values of $f$ we obtained for the high-order g mode opens a new gate in seismic studies of the main-sequence hybrid pulsators. The most intriguing and challenging result is that whereas an agreement of the theoretical and empirical values of $f$ for the radial mode can be achieved only with the OPAL data, a preference for the OP tables is derived from the analysis of the high-order gravity mode.
We analyse time-series observations from the BRITE-Constellation of the well known $beta$ Cephei type star $theta$ Ophiuchi. Seven previously known frequencies were confirmed and nineteen new frequency peaks were detected. In particular, high-order g modes, typical for the SPB (Slowly Pulsating B-type star) pulsators, are uncovered. These low-frequency modes are also obtained from the 7-year SMEI light curve. If g modes are associated with the primary component of $theta$ Oph, then our discovery allows, as in the case of other hybrid pulsators, to infer more comprehensive information on the internal structure. To this aim we perform in-depth seismic studies involving simultaneous fitting of mode frequencies, reproducing mode instability and adjusting the relative amplitude of the bolometric flux variations. To explain the mode instability in the observed frequency range a significant increase of the mean opacity in the vicinity of the $Z$-bump is needed. Moreover, constraints on mass, overshooting from the convective core and rotation are derived. If the low-frequency modes come from the speckle B5 companion then taking into account the effects of rotation is enough to explain the pulsational mode instability.
We present a seismic study of the beta Cephei star theta Ophiuchi. Our analysis is based on the observation of one radial mode, one rotationally split l = 1 triplet and three components of a rotationally split l = 2 quintuplet for which the m-values were well identified by spectroscopy. We identify the radial mode as fundamental, the triplet as p_1 and the quintuplet as g_1. Our NLTE abundance analysis results in a metallicity and CNO abundances in full agreement with the most recent updated solar values. With X in [0.71,0.7211] and Z in [0.009,0.015], and using the Asplund et al. (2005) mixture but with a Ne abundance about 0.3 dex larger (Cunha et al. 2006), the matching of the three independent modes, enables us to deduce constrained ranges for the mass (M = 8.2 +/- 0.3 Msun) and central hydrogen abundance (X_c = 0.38 +/- 0.02) of theta Oph and to prove the occurrence of core overshooting (alpha_ov = 0.44 +/- 0.07). We also derive an equatorial rotation velocity of 29 +/- 7 km/s. Moreover, we show that the observed non-equidistance of the l=1 triplet can be reproduced by second order effects of rotation. Finally, we show that the observed rotational splitting of two modes cannot rule out a rigid rotation model.
We present a comprehensive seismic study of the three pulsating stars of $beta$ Cep/SPB type: $ u$ Eridani, 12 Lacertae and $gamma$ Pegasi. Models with the modified mean opacity profile are constructed in order to account for both the observed frequency range and the values of some individual frequencies. To decrease the number of possible solutions, we make use of the non-adiabatic parameter $f$, whose value is very sensitive to subphotospheric layers where pulsations are driven. This complex seismic modelling show the need for a significant modification of the opacity profile.
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