[Paper Review] New light on the driving mechanism in roAp stars. Part I. Effects of metallicity

S. Théado, M. A. Dupret+2 more|Nov 21, 2008

Stellar, planetary, and galactic studies1 references13 citations

TL;DR

This study investigates how metallicity variations affect the pulsation driving mechanism in roAp stars using stellar evolution and non-adiabatic oscillation models. It reveals that increased opacity in the driving region can reduce pulsation growth rates—an 'inverse $κ$-mechanism'—explaining why theoretical instability strips are less sensitive to metallicity than expected, despite strong opacity changes.

ABSTRACT

Observations suggest that a relationship exists between the driving mechanism of roAp star pulsations and the heavy element distribution in these stars. We attempt to study the effects of local and global metallicity variations on the excitation mechanism of high order p-modes in A star models. We developed stellar evolutionary models to describe magnetic A stars with different global metallicity or local metal accumulation profiles. These models were computed with CLES ("Code Liègeois d'évolution stellaire"), and the stability of our models was assessed with the non-adiabatic oscillation code MAD. Our models reproduce the blue edge of the roAp star instability strip, but generate a red edge hotter than the observed one, regardless of metallicity. Surprisingly, we find that an increase in opacity inside the driving region can produce a lower amount of driving, which we refer to as the "inverse $κ$-mechanism".

Motivation & Objective

• To understand how global and local metallicity variations influence the excitation of high-order p-modes in roAp stars.
• To test whether the observed instability strip of roAp stars can be reproduced by models with varying metallicity and opacity structure.
• To investigate the role of heavy elements (especially iron) in shaping the opacity bumps responsible for pulsation driving.
• To resolve the discrepancy between theoretical instability strips (too hot on the red edge) and observations by analyzing the non-adiabatic driving mechanism in detail.
• To explore the qualitative differences between the driving mechanism in roAp stars and the classical $κ$-mechanism in other pulsators.

Proposed method

• Stellar evolution models were computed using the CLES code for magnetic A stars with different global metallicities ([Fe/H] = -0.89, 0.00, 0.83) and local metal enrichment profiles.
• Non-adiabatic oscillation analysis was performed using the MAD code to assess the stability and growth rates of p-modes.
• The location of the last pressure node in the eigenfunctions was tracked as a key determinant of driving efficiency.
• Opacity structure and temperature gradients were analyzed in the region around log T ≈ 4.1, where the main opacity bump for driving occurs.
• The effect of increasing opacity on the temperature gradient and density structure was modeled to assess its impact on the $δ P/P$ amplitude.
• Comparisons were made between models with varying metallicity and iron abundance to isolate the role of heavy elements in the driving region.

Experimental results

Research questions

• RQ1 How does increasing global metallicity affect the location and width of the theoretical instability strip in roAp stars?
• RQ2 What is the role of local metal accumulation (e.g., from radiative levitation and gravitational settling) in modifying the pulsation driving mechanism?
• RQ3 Why does the theoretical red edge of the instability strip remain too hot compared to observations, despite changes in metallicity?
• RQ4 How does opacity variation in the driving region affect the growth rate of p-modes, and does it follow the classical $κ$-mechanism?
• RQ5 Can the observed insensitivity of the instability strip to metallicity changes be explained by a non-classical driving mechanism?

Key findings

• Theoretical models reproduce the blue edge of the roAp instability strip but predict a red edge that is too hot, failing to account for the coolest observed roAp stars.
• An increase in opacity within the driving region leads to a reduction in the driving amplitude—a phenomenon termed the 'inverse $κ$-mechanism', contrary to the classical $κ$-mechanism.
• The location of the last node of the eigenfunction $|δ P/P|$ is critical for efficient driving, and it must lie between the two main opacity bumps near log T ≈ 4.3.
• Changes in the structure of the stellar surface layers affect driving less than changes in the eigenfunction shape, explaining the weak sensitivity of the instability strip to metallicity.
• The contribution of metals (especially iron) to the opacity bump at log T ≈ 4.1 is significant and cannot be neglected in driving mechanism models.
• The driving efficiency is more sensitive to the phase and location of the eigenfunction nodes than to the absolute magnitude of the opacity bump, which explains the limited impact of metallicity on the instability strip.