Line-profile variations of stochastically excited oscillations in four evolved stars

TL;DR

This study improves spectroscopic line-profile analysis to identify azimuthal modes and surface rotation in four evolved stars with solar-like oscillations, enhancing understanding of stellar oscillation modes and rotation.

Contribution

An improved spectroscopic analysis method that accurately determines azimuthal order and surface rotational frequency in evolved stars with solar-like oscillations.

Findings

Azimuthal order of oscillations identified with ~1 accuracy.

Surface rotational frequency and inclination angle derived.

Line-profile variations differ across frequencies, providing additional insights.

Abstract

Since solar-like oscillations were first detected in red-giant stars, the presence of non-radial oscillation modes has been debated. Spectroscopic line-profile analysis was used in the first attempt to perform mode identification, which revealed that non-radial modes are observable. Despite the fact that the presence of non-radial modes could be confirmed, the degree or azimuthal order could not be uniquely identified. Here we present an improvement to this first spectroscopic line-profile analysis. Aims: We aim to study line-profile variations of stochastically excited solar-like oscillations in four evolved stars to derive the azimuthal order of the observed mode and the surface rotational frequency. Methods: Spectroscopic line-profile analysis is applied to cross-correlation functions, using the Fourier Parameter Fit method on the amplitude and phase distributions across the profiles. Results: For four evolved stars, beta Hydri (G2IV), epsilon Ophiuchi (G9.5III), eta Serpentis (K0III) and delta Eridani (K0IV) the line-profile variations reveal the azimuthal order of the oscillations with an accuracy of ~1. Furthermore, our analysis reveals the projected rotational velocity and the inclination angle. From these parameters we obtain the surface rotational frequency. Conclusions: We conclude that line-profile variations of cross-correlation functions behave differently for different frequencies and that they provide additional information in terms of the surface rotational frequency and azimuthal order.