Fundamental parameters of bright Ap stars from wide-range energy distributions and advanced atmospheric models

Abstract

Aims. As a well-established procedure for the vast majority of normal main-sequence stars, determination of atmospheric and stellar parameters turns to be a challenging process in case of magnetic chemically peculiar stars. Inhomogeneous distribution of chemical elements and strong magnetic fields make most of the standard photometric and spectroscopic calibrations inapplicable for this class of stars. In this work we make use of available observed energy distributions calibrated to absolute units, stellar parallaxes, highresolution spectroscopic observations, and advanced stellar atmosphere models to derive parameters of three bright Ap stars: 33 Lib, γ Equ, and β CrB. Methods. Model atmospheres and fluxes were computed with the LLmodels code. Synth3 and Synthmag codes were used to compute profiles of individual spectral lines involved in abundance analysis. Results. For each of the stars, we construct a self-consistent atmospheric models assuming normal and depleted helium compositions and derive empirically stratification profiles of certain elements. The effective temperatures and surface gravities are found from the simultaneous fit to spectroscopic, photometric, and spectrophotometric observations calibrated to absolute units. We show that using advanced model atmospheres and accurate stellar parallaxes allows one to derive stellar radii with high accuracy, and these are consistent with those obtained from independent but more complicated interferometric observations.

Testing excitation models of rapidly oscillating Ap stars with interferometry

“…Accepting the global parameters derived by Shulyak et al (2009Shulyak et al ( , 2013, we find that HD 24712 is slightly beyond the red edge of the instability strip derived by Cunha (2002), meaning that in default polar models of this star no high frequencies are expected to be found unstable. If we ignore this fact for a moment and consider the scaling with radius of the envelop of growth rates (ignoring the small mass dependence expected in that scaling), we find, in addition, that the envelope would be centered around 2.0 mHz, for the radius considered, thus significantly out of place in relation to the observed frequencies.…”

K2 observations of the rapidly oscillating Ap star 33 Lib (HD 137949): new frequencies and unique non-linear interactions

“…We present a compilation of 33 Lib's properties in Table 1. The parameters shown in this table are mostly found in the work of Shulyak et al (2013) where a detailed study of several bright Ap stars was conducted. We note that Kochukhov & Bagnulo (2006) also presented parameters of many chemically peculiar stars, but derive their results through a homogeneous study, rather than from a star-by-star study which is preferable.…”

“…They found that, as with other roAp stars, the pulsation radial velocity (RV) amplitude varied with spectral region and line strength. Furthermore, Shulyak et al (2013) results using the Gaia DR2 parallax. Mkrtichian et al (2003) found at least one radial node in the upper atmosphere of 33 Lib; a conclusion drawn from the anti-phase relation between pulsations measured predominantly in Nd and Nd .…”

“…They argued that both this separation and twice its value are inconsistent with what would be expected for the large separation, given the star's global parameters. In fact, considering the parameters from Shulyak et al (2013) in Table 1 and the scaling of the large separation with the global parameters (see Sec. 1), we find that the expected ∆ν is ∼ 53 µHz.…”

Pulsation of magnetic stars

“…Surface magnetic field moduli of some roAp stars are plotted with respect to the dominant oscillation frequencies. The magnetic field strengths were taken from Shulyak et al (2013), Ryabchikova et al (2008), Mathys et al (1997), Hubrig et al (2005), Kochukhov et al (2004), Balona et al (2013).…”