Abstract
Context. It has recently been shown that the infrared (IR) emission of Cepheids, constant over the pulsation cycle, might be due to a pulsating shell of ionized gas with a radius of about 15% of that of the star radius, which could be attributed to the chromospheric activity of Cepheids. Aims. The aim of this paper is to investigate the dynamical structure of the chromosphere of Cepheids along the pulsation cycle and to quantify its size. Methods. We present Hα and calcium near-infrared triplet (Ca IR) profile variations using high-resolution spectroscopy with the UVES spectrograph of a sample of 24 Cepheids with a good period coverage from ≈3 to 60 days. After a qualitative analysis of the spectral line profiles, we quantified the Van Hoof effect (velocity gradient between the Hα and Ca IR) as a function of the period of the Cepheids. We then used the Schwarzschild mechanism (a line doubling due to a shock wave) to quantify the size of the chromosphere. Results. We find a significant Van Hoof effect for Cepheids with a period larger than P = 10 days. In particular, Hα lines are delayed with a velocity gradient up to Δv ≈ 30 km s−1 compared to Ca IR. By studying the shocks, we find that the size of the chromosphere of long-period Cepheids is of at least ≈50% of the stellar radius, which is consistent at first order with the size of the shell made of ionized gas previously found from the analysis of IR excess. Last, for most of the long-period Cepheids in the sample, we report a motionless absorption feature in the Hα line that we attribute to a circumstellar envelope that surrounds the chromosphere. Conclusions. Analyzing the Ca IR lines of Cepheids is of importance to potentially unbias the period–luminosity relation from their IR excess, particularly in the context of forthcoming observations of radial velocity measurements from the Radial Velocity Spectrometer on board Gaia, which could be sensitive to their chromosphere.
Highlights
Cepheids are milestones of the extragalactic distance scale since their period and luminosity are correlated, which is known as the Leavitt law (Leavitt 1908) and is referred to as the period– luminosity (PL) relation
Analyzing the calcium near-infrared triplet (Ca IR) lines of Cepheids is of importance to potentially unbias the period–luminosity relation from their IR excess, in the context of forthcoming observations of radial velocity measurements from the Radial Velocity Spectrometer on board Gaia, which could be sensitive to their chromosphere
To study the line profile variations, we focus on the core of each line, we consider the same radial velocities (RVs) window ranging from −200 to 200 km s−1 and centered on the line rest wavelength corrected by the star center-of-mass velocity
Summary
Cepheids are milestones of the extragalactic distance scale since their period and luminosity are correlated, which is known as the Leavitt law (Leavitt 1908) and is referred to as the period– luminosity (PL) relation These variable stars have provided among the most essential advances in the history of astronomy from the discovery of galaxies to the expansion of the Universe (Hubble 1926, 1929). The discovery of the accelerated expansion of the Universe (Riess et al 1998) has demonstrated the central importance of Cepheids in modern astronomy Uncertainties on both zero point and slope of the PL relation are today one of the largest contributors to the error on the extragalactic distance ladder and on the determination of H0, the Hubble-Lemaître constant (Riess et al 2019). While CSE emission is explained by dust emission in some cases (Gallenne et al 2012, 2013; Groenewegen 2020), it fails to reproduce the IR excess in other studies (Schmidt 2015)
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