The Behavior of the Paschen and Calcium Triplet Lines in Cepheid Variables II: The 16-day Variable X Cygni

Abstract

Cepheid variable stars calibrate the cosmic distance ladder and exhibit large-amplitude pulsations that provide interesting hydrodynamical insights into stellar atmospheres. Here, we investigate velocities of different spectral line species (ionization levels, elements) to provide new detail into the complex velocity structure of the particularly large-amplitude Cepheid X Cygni, P puls =16.3 d. This is done against the backdrop of studies aimed at understanding the projection factor for Baade-Wesselink-type distance methods, as well as the timeseries velocimetry being gathered by the Gaia satellite. We analyze and compare velocities measured by cross-correlation using thousands of optical metallic lines, as well as individual line velocities of hydrogen (Balmer and Paschen lines), oxygen, ionized silicon, potassium, sodium, and ionized calcium. We compare individual line velocity curves to a reference velocity curve based on cross-correlation and discuss observed differences as a function of pulsation phase. This purely empirical approach yields detailed insights into the dependence of certain lines on NLTE effects, shocks, and phase lags in velocity space, which can be used as strong constraints for hydrodynamical modeling. Of particular importance is the velocity curve of the Ca II IR triplet, which dominates the Gaia radial velocities of F, G, and K type variable stars.

Pulsating chromosphere of classical Cepheids

“…First, we observe a double absorption profile centered on 0 km s −1 velocity. The same features were recently observed by Wallerstein et al (2019) who published Ca IR profiles of X Cygni with a good phase coverage. We interpret this feature as a Schwarzschild mechanism (Schwarzschild 1952).…”

“…8 could appear between metallic lines (which are formed at a great optical depth) and Ca IR when observing with the RVS instrument. This de-synchronization between metallic and Ca IR lines was already observed by Wallerstein et al (2015Wallerstein et al ( , 2019 for Cepheids δ Cep and X Cyg. Further investigations are necessary to quantify these differences and calibrate, if necessary, the projection factor to be used together with the RVS measurements of Cepheid RVs in the Baade-Wesselink method of distance determination (Nardetto et al 2017).…”

“…In addition, Ca IR lines were recently used to measure radial velocities (RVs) in Cepheids' atmospheres (Wallerstein et al 2015(Wallerstein et al , 2019. Indeed, as stated by Wallerstein et al (2019), since the Radial Velocity Spectrometer (RVS) of ESA Gaia survey observes in the same wavelength range (Munari 1999;Sartoretti et al 2018;Katz et al 2019), it is of prime importance to understand the dynamic of these lines for Cepheid RV measurement accuracy, in particular in the context of the use of the Baade-Wesselink method for extragalactic distance scale measurement. The present paper is structured as follows.…”

Pulsating chromosphere of classical Cepheids. Calcium infrared triplet and H$α$ profile variations

“…First, we observe a double absorption profile centered on 0 km/s velocity. The same features were recently observed by Wallerstein et al (2019) who published Ca IR profiles of X Cygni with a good phase coverage. We interpret this feature as a Schwarzschild mechanism (Schwarzschild 1952).…”

“…In addition, Ca IR lines were recently used to measure radial velocities (RVs) in Cepheids' atmospheres (Wallerstein et al 2015(Wallerstein et al , 2019. Indeed, as stated by Wallerstein et al (2019), since the Radial Velocity Spectrometer (RVS) of ESA Gaia survey observes in the same wavelength range (Munari 1999;Sartoretti et al 2018;Katz et al 2019), it is of prime importance to understand the dynamic of these lines for Cepheid RV measurement accuracy, in particular in the context of the use of the Baade-Wesselink method for extragalactic distance scale measurement. The paper is structured as follows.…”