Abstract
ABSTRACT KIC 8264293 is a fast-rotating B-type pulsator observed by Kepler satellite. Its photometric variability is mainly due to pulsations in high-order g modes. Besides, we detected a weak Hα emission. Thus, the second source of variability is the fluctuation in a disc around the star. The pulsational spectrum of KIC 8264293 reveals a frequency grouping and period spacing pattern. Here, we present the thorough seismic analysis of the star based on these features. Taking into account the position of the star in the HR diagram and fitting the 14 frequencies that form the period spacing, we constrain the internal structure of the star. We conclude that the star barely left the zero-age main sequence and the best seismic model has $M = 3.54\, \mathrm{M}_{\odot }$, $V_\mathrm{rot}=248\, \mathrm{km\, s}^{-1}$, and Z = 0.0112. We found the upper limit on the mixing at the edge of the convective core, with the overshooting parameter up to fov = 0.03. On the other hand, we were not able to constrain the envelope mixing for the star. To excite the modes in the observed frequency range, we had to modify the opacity data. Our best seismic model with an opacity increase by 100 per cent at the ‘nickel’ bump log T = 5.46 explains the whole instability. KIC 8264293 is the unique, very young star pulsating in high-order g modes with the Be feature. However, it is not obvious whether the source of this circumstellar matter is the ejection of mass from the underlying star or whether the star has retained its protostellar disc.
Highlights
The oscillation spectra of β Cephei stars are dominated by the low-order p/g modes (e.g. Stankov & Handler 2005) whereas frequencies of Slowly Pulsating B-type (SPB) stars are associated with high-order g modes (Waelkens 1991)
In the age of space observations, it is difficult to talk about these classes of stars separately because in most B-type main sequence stars, if not all, variability is detected both in the p-mode as well as gmode regime (e.g. Jerzykiewicz et al 2005; Handler et al 2006; Chapellier et al 2006; Balona et al 2011)
Because the classical Be star are too old to have retained the protostellar disk, the source of this circumstellar matter must be from the ejection of mass from the underlying star
Summary
Pulsations are very common among stars of the B spectral type. The two mostly studied classes of B-type pulsators are β Cephei stars and Slowly Pulsating B-type (SPB) stars. These quasi- spaced in period structures consist of modes with consecutive radial orders, n, of the same harmonic degree, l, and azimuthal order, m (e.g. Tassoul 1980; Dziembowski et al 1993; Bouabid et al 2013; Szewczuk & Daszyńska-Daszkiewicz 2017) Such characteristic patterns in oscillation spectrum allow for mode identification. Their observational discovery had to wait until the era of space borne high precision high cadence photometric times series (e.g. Degroote et al 2012; Pápics et al 2014, 2015, 2017; Szewczuk & Daszyńska-Daszkiewicz 2018; Szewczuk et al 2021; Pedersen et al 2021) Another problem in seismic modeling of B-type stars is the fairly common problem with the excitation of high-order g modes (e.g., Pamyatnykh et al 2004).
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