Identification of pulsational modes in rotating slowly pulsating B-type stars

Abstract

Knowledge of the geometry of pulsational modes is a prerequisite for seismic modelling of stars. In the case of slowly pulsating B-type (SPB) pulsators, the simple zero-rotation approach so far used for mode identification is usually not valid because pulsational frequencies are often of the order of the rotational frequency. Moreover, this approach allows us to determine only the spherical harmonic degree, , while the azimuthal order, m, is beyond its reach. On the other hand, because of the density of oscillation spectra of SPB stars, knowledge of m is indispensable if one wants to assign the radial order, n, to the observed frequency peaks. Including the effects of rotation via the traditional approximation, we perform identification of the mode angular numbers ( , m) for 31 SPB stars with available multicolour time series photometry. Simultaneously, constraints on the rotational velocity, V rot , and the inclination angle, i, are determined assuming uniform rotation and a constant value of V rot sin i. Dependence of the results on the adopted model is tested using HD 21071 as an example. Despite some model uncertainties and limitations of the method, our studies show the correct approach to identifying the low-frequency oscillation modes.

The magnetic early B-type stars I: magnetometry and rotation

“…The light curve is dominated by pulsations, so the Hipparcos photometry could not be used to determine Prot. However, it is worth noting that there is no power in the Bz periodogram at the pulsation periods of 1.23 d and 1.25 d (Szewczuk & Daszyńska-Daszkiewicz 2015). The FAP of this peak is 7 × 10 −4 , subtstantially smaller than the minimum FAP in the Nz period spectrum of 0.12.…”

Domains of pulsational instability of low-frequency modes in rotating upper main sequence stars

“…Recently, Szewczuk & Daszyńska-Daszkiewicz (2015) compiled the parameters of SPB stars. The observed parameters of the SPB stars in their list are consistent in terms of both the position on the H-R diagram and the excited frequencies with the calculations presented in the present paper.…”

“…However, Dziembowski et al (2007) showed that the whole oscillation spectrum of the star can be explained solely by g modes. Subsequently, Szewczuk & Daszyńska-Daszkiewicz (2015) carried out mode identification for 31 SPB stars with available multicolour ground-based photometry and found that scarcely two frequencies observed in two stars may be associated, but with rather low probability, with r modes. Nevertheless, we think that in the era of high-precision space photometry, detection of r modes is only a question of time, hence the motivation to consider them.…”

Magnetic OB[A] Stars with TESS: probing their Evolutionary and Rotational properties (MOBSTER) – I. First-light observations of known magnetic B and A stars

“…The photometric variations are difficult to reconcile with non-radial pulsations and mode identification has proven to be arduous (De Cat et al 2005). The detected frequency lies within the (large) range of possible rotational frequencies based on its projected rotational velocity and inferred radius (Szewczuk & Daszyńska-Daszkiewicz 2015), and could therefore rather be the rotational period.…”