The VELOCE modulation zoo III. Detecting additional pulsation modes in optical spectra of classical Cepheids using semi-partial distance correlation periodograms

TL;DR

This study applies semi-partial distance correlation periodograms to high-precision spectroscopic data of classical Cepheids, successfully detecting known and new pulsation modes and line shape variations, enhancing asteroseismic analysis capabilities.

Contribution

The paper introduces the use of semi-partial distance correlation periodograms with CCF variants for detecting additional pulsation modes in Cepheids, improving sensitivity to line shape variations.

Findings

Detection of known radial mode in δ Cep confirming method validity.

Identification of new additional signals in BG Cru, X Sgr, and Polaris.

Enhanced detection sensitivity using median-subtracted CCFs for faint and low-amplitude stars.

Abstract

Known for their large amplitude radial pulsations, classical Cepheids are critical standard candles in astrophysics. However, they also exhibit various pulsational irregularities and additional signals that provide deeper insights into their structure and evolution. These signals appear in spectroscopic observations as shape deformations of the spectral lines. Using semi-partial distance correlation periodograms, we analyse high-precision spectroscopic data from the VELOCE project for four stars: $\delta$ Cep, BG Cru, X Sgr, and Polaris. For $\delta$ Cep, our control star, only the main radial mode is detected, confirming its stability and suitability as a benchmark for the method. In BG Cru, a strong additional signal at $\sim 3.01$ d is identified, likely linked to line splitting. X Sgr exhibits dominant additional signals, notably one at $\sim 12.31$ d, also associated with significant line splitting. Polaris reveals multiple low-frequency signals, with the most prominent candidate at $\sim 59.86$ d, which might be linked to the star's rotation period. We explore the semi-partial distance correlation periodograms by incorporating CCFs and their variants, such as the median-subtracted CCFs, which improves the sensitivity to variations in line shape. In particular, the latter enables the faithful detection of primary and additional signals present in the 1D spectra of fainter stars and low-amplitude pulsators. The semi-partial distance correlation periodograms demonstrate their utility for isolating signals associated with line shape variations; although, the analyses are complicated by the presence of artefact subharmonics and a visible low-frequency power increase for Polaris and BG Cru. This study underscores the method's potential for finding new and unexpected signals as well as detailed analyses of Cepheid pulsations and opens new pathways for asteroseismic investigations.