Pulsation models for the roAp star HD 134214

Abstract

Precise time-series photometry with the MOST satellite has led to identification of 10 pulsation frequencies in the rapidly oscillating Ap (roAp) star HD 134214. We have fitted the observed frequencies with theoretical frequencies of axisymmetric modes in a grid of stellar models with dipole magnetic fields. We find that, among models with a standard composition of $(X,Z) = (0.70,0.02)$ and with suppressed convection, eigenfrequencies of a $1.65\,{\rm M}_\odot$ model with $\log T_{\rm eff} = 3.858$ and a polar magnetic field strength of 4.1kG agree best with the observed frequencies. We identify the observed pulsation frequency with the largest amplitude as a deformed dipole ($\ell = 1$) mode, and the four next-largest-amplitude frequencies as deformed $\ell = 2$ modes. These modes have a radial quasi-node in the outermost atmospheric layers ($\tau \sim 10^{-3}$). Although the model frequencies agree roughly with observed ones, they are all above the acoustic cut-off frequency for the model atmosphere and hence are predicted to be damped. The excitation mechanism for the pulsations of HD 134214 is not clear, but further investigation of these modes may be a probe of the atmospheric structure in this magnetic chemically peculiar star.Comment: 9 pages, 6 figures; accepted for publication in MNRA

Asteroseismology can reveal strong internal magnetic fields in red giant stars

“…Finally, many papers (such as (39) and references therein) have examined the effect of magnetic fields on the acoustic oscillations of rapidly oscillating Ap stars. In this case, the magnetic field strongly affects the acoustic waves only near the surface of the star where the magnetic pressure becomes comparable to the gas pressure.…”

Testing excitation models of rapidly oscillating Ap stars with interferometry

“…Presently there are about 45 roAp stars known with oscillation periods varying between a little more than 20 minutes (Alentiev et al 2012;Elkin et al 2005) and a little less than 6 minutes (Saio et al 2012;Kreidl & Kurtz 1986). There is some observational indication that the roAp stars with long pulsation periods are relatively evolved, i.e., that they are approaching the terminal age main sequence.…”

Investigation of Rare-Earth Elements in the Atmosphere of the roAp Star HD 134214: Nd II, Nd III, and Gd II Lines

“…The effective temperature of the star was found to be logT eff = 3.849 ± 0.018 in 2000 [12], logT eff = 3.858 ± 0.012 in 2006 [13], T eff = 7315 (logT eff = 3.864) in 2008 [11], and logT eff = 3.858 in 2012 [5]. The calculations were performed using Kurtz's model atmosphere [14] with the parameters T eff = 7250 K, logg = 4.45, and 1 t km/sec.…”

“…HD 134214 is a magnetic, rapidly oscillating, chemically peculiar star [4,5]. For roAp stars the amplitude of the light oscillations, just as the intensity of the magnetic field, in some cases can be modulated by the rotation of the star.…”