The VELOCE modulation zoo II. Humps and splitting patterns in spectral lines of classical Cepheids

Abstract

Line splitting in spectral lines is observed in various types of stars due to phenomena such as shocks, spectroscopic binaries, magnetic fields, spots, and non-radial modes. In pulsating stars, line splitting is often attributed to pulsation-induced shocks. However, this is rarely observed in classical Cepheids, with only a few reports, including X Sagittarii and BG Crucis, where it has been linked to atmospheric shocks. We investigate line splitting in X Sgr and BG Cru using spectroscopic time series, and search for similar phenomena in other classical Cepheids. High signal-to-noise cross-correlation function (CCF) time series from the VELOcities of CEpheids (VELOCE) project are analyzed. This dataset spans several years, allowing us to study the periodicities and evolution of CCF features. For X Sgr and BG Cru, we perform a detailed analysis of the individual components of the split CCFs. Additionally, we search for periodicities in CCF variations and examine other classical Cepheids for distortions resembling unresolved line splitting. We confirm line splitting in X Sgr and BG Cru, trace the features over time, and uncover the periodicity behind them. Several other Cepheids also exhibit CCF humps, suggesting unresolved or marginally resolved line splitting. We discuss the incidence and characteristics of these stars. The periodicity of line splitting in X Sgr and BG Cru differs significantly from the dominant pulsation period, ruling out pulsation-induced shocks. The periodicities are too short for rotation-related phenomena, suggesting non-radial modes as the most likely explanation, though their exact nature remains unknown. We also identify humps in six additional stars, indicating an incidence rate of 3% in the VELOCE sample.

15 pages, 20 figures, 5 tables, accepted for publication in A&A

References in corpus (10)

• Gaia Data Release 3: Summary of the content and survey properties
• Cosmic Distances Calibrated to 1% Precision with Gaia EDR3 Parallaxes and Hubble Space Telescope Photometry of 75 Milky Way Cepheids Confirm Tension with LambdaCDM
• A unifying explanation of complex frequency spectra of gamma Dor, SPB and Be stars: combination frequencies and highly non-sinusoidal light curves
• On the Enhancement of Mass Loss in Cepheids Due to Radial Pulsation
• On the effect of rotation on populations of classical Cepheids I. Predictions at solar metallicity
• Discovery of Cycle-to-cycle Modulated Spectral Line Variability and Velocity Gradients in Long-period Cepheids
• VELOcities of CEpheids (VELOCE) I. High-precision radial velocities of Cepheids
• Probing Polaris' Puzzling Radial Velocity Signals - Pulsational (In-)Stability, Orbital Motion, and Bisector Variations
• Precise Fourier parameters of Cepheid Radial Velocity Curves
• The VELOCE Modulation Zoo I. Spectroscopic detection of non-radial modes in the first-overtone Cepheids BG Crucis, QZ Normae, V0391 Normae, and V0411 Lacertae

Cited by in corpus (1)

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