Non-radial modes in classical Cepheids. What to look for in spectroscopy?

TL;DR

This paper explores how spectroscopic observations can detect non-radial pulsation modes in classical Cepheids, which are suggested by recent photometric data and models, by simulating line profile time series under various observational conditions.

Contribution

It demonstrates the potential of spectroscopic time series analysis to identify low-amplitude non-radial modes in Cepheids, providing guidelines for observational strategies.

Findings

High sampling and signal-to-noise ratios are crucial for detection.

Detection depends on the azimuthal order and inclination of the modes.

Simulations help optimize spectroscopic campaigns for mode identification.

Abstract

Recent photometric observations of first-overtone classical Cepheids and RR Lyrae stars have led to the discovery of additional frequencies showing a characteristic period ratio of 0.60-0.65 with the main pulsation mode. In a promising model proposed by Dziembowski (2016), these signals are suggested to be due to the excitation of non-radial modes with degrees 7, 8 and 9 (Cepheids) or 8 and 9 (RR Lyrae). Such modes usually have low amplitudes in photometric data. Spectroscopic time series offer an unexplored and promising way forward. We simulated time series of synthetic line profiles for a representative first-overtone classical Cepheid model and added a low-amplitude non-radial mode. We studied sets of spectra with dense sampling and without noise, so-called 'perfect' cases, as well as more realistic samplings and signal-to-noise levels. Besides the first-overtone mode and the non-radial mode, also the harmonics of both modes and combination signals were often detected, but a sufficiently high sampling and signal-to-noise ratio prove essential. The amplitudes of the non-radial mode and its harmonic depend on the azimuthal order $m$. The inclination is also an important factor determining the detectability of the non-radial mode and/or its harmonic. We compared the results obtained for the predicted high degrees with those for lower-degree modes. Finally, we studied the sampling requirements for detecting the non-radial mode. Our findings can be used to plan a spectroscopic observing campaign tailored to uncover the nature of these mysterious modes.