Seismic modelling of the $\beta\,$Cep star HD\,180642 (V1449\,Aql)

TL;DR

This study models the seismic behavior of the $eta$ Cep star HD 180642 using observational data, finding models that fit most observed oscillations and revealing insights into its rotation and mode coupling, but not supporting solar-like oscillations.

Contribution

The paper demonstrates that current stellar models can explain most seismic properties of HD 180642, with specific constraints on mass, overshooting, and mode excitation, and clarifies the nature of high-frequency power excess.

Findings

Models fit observed oscillation properties within narrow mass range

Rotation period estimated at about 13 days

High-frequency power due to mode coupling rather than solar-like oscillations

Abstract

We present modelling of the $\beta\,$Cep star HD\,180642 based on its observational properties deduced from CoRoT and ground-based photometry as well as from time-resolved spectroscopy. We investigate whether present-day state-of-the-art models are able to explain the full seismic behaviour of this star, which has extended observational constraints for this type of pulsator. We constructed a dedicated database of stellar models and their oscillation modes tuned to fit the dominant radial mode frequency of HD\,180642, by means of varying the hydrogen content, metallicity, mass, age, and core overshooting parameter. We compared the seismic properties of these models with those observed. We find models that are able to explain the numerous observed oscillation properties of the star, for a narrow range in mass of 11.4--11.8\,M$_\odot$ and no or very mild overshooting (with up to 0.05 local pressure scale heights), except for an excitation problem of the $\ell=3$, p$_1$ mode. We deduce a rotation period of about 13\,d, which is fully compatible with recent magnetic field measurements. The seismic models do not support the earlier claim of solar-like oscillations in the star. We instead ascribe the power excess at high frequency to non-linear resonant mode coupling between the high-amplitude radial fundamental mode and several of the low-order pressure modes. We report a discrepancy between the seismic and spectroscopic gravity at the $2.5\sigma$ level.