Puzzling Frequencies in First Overtone Cepheids

TL;DR

This paper investigates unusual secondary frequencies in first overtone Cepheids, proposing high-degree f-modes as a potential explanation, and discusses the challenges related to their excitation and observational signatures.

Contribution

It introduces the hypothesis that high-degree f-modes explain the secondary variability in Cepheids, challenging traditional opacity-driven mode explanations.

Findings

High-degree f-modes (l=42-50) can reproduce observed period ratios.

The instability of these modes is driven by a non-opacity mechanism.

High intrinsic amplitudes may cause spectral line broadening issues.

Abstract

The OGLE project led to discovery of earlier unknown forms of multiperiodic pulsation in Cepheids. Often, the observed periods may be explained in terms of simultaneous excitation of two or rarely three radial modes. However, a secondary variability at about 0.6 of the dominant period, detected in a number of the first overtone (1O) pulsators inhabiting the Magellanic Clouds, seems to require a different explanation. After reviewing a possibility of explaining this signal in terms of radial and nonradial modes, I find that only unstable modes that may reproduce the observed period ratio are f-modes of high angular degrees (l=42-50). I discuss in detail the driving effect behind the instability and show that it is not the familiar opacity mechanism. Finally, I emphasize the main difficulty of this explanation, which requires high intrinsic amplitudes implying large broadening of spectral line.