Nonradial oscillations in classical Cepheids: the problem revisited

C Mulet-Marquis,W Glatzel,C Winisdoerffer,I Baraffe

doi:10.1051/0004-6361:20066757

Abstract

Context. We analyse the presence of nonradial oscillations in Cepheids, a problem that has not been theoretically revised since the work of Dziembowski (1977, Acta Astron., 27, 95) and Osaki (1977, PASJ, 29, 235). Our analysis is motivated by a work of Moskalik et al. (2004, ASPC, 310, 498), which reports the detection of low-amplitude periodicities in a few Cepheids of the large Magellanic cloud. These newly discovered periodicities were interpreted as nonradial modes. Aims. Based on linear nonadiabatic stability analysis, our goal is to reanalyse the presence and stability of nonradial modes, taking into account improvement in the main input physics required for the modelling of Cepheids. Methods. We compare the results obtained from two different numerical methods used to solve the set of differential equations: a matrix method and the Ricatti method. Results. We show the limitation of the matrix method for finding low-order p-modes (l < 6), because of their dual character in evolved stars such as Cepheids. For higher order p-modes, we find excellent agreement between the two methods. Conclusions. No nonradial instability is found below l = 5, whereas many unstable nonradial modes exist for higher orders. We also find that nonradial modes remain unstable, even at hotter effective temperatures than the blue edge of the Cepheid instability strip, where no radial pulsations are expected.

Highlights

Cepheids are well-known radial pulsators, oscillating in the fundamental mode or the low overtones
We show the limitation of the matrix method for finding low-order p-modes (l < 6), because of their dual character in evolved stars such as Cepheids
No nonradial instability is found below l = 5, whereas many unstable nonradial modes exist for higher orders

Summary

Introduction

Cepheids are well-known radial pulsators, oscillating in the fundamental mode or the low overtones. Despite a very abundant literature analysing their radial pulsation properties, theoretically and observationally, only very few studies have been devoted to nonradial oscillations in these stars. Dziembowski (1971) was the first to study nonradial oscillations in Cepheids, based on a quasi-adiabatic linear stability analysis, and found that loworder modes (l ≤ 2) are stable. Following this study, Osaki (1977) and Dziembowski (1977) showed that higher degree nonradial modes (l ≥ 4) can become unstable. These analyses highlighted the difficulty of calculating low-order p-modes in such evolved stars. Since no observational evidence of nonradial modes in Cepheids was available at the time of these studies, no further theoretical analysis of the stability of nonradial modes was performed

Objectives

Results

Conclusion