Spectroscopic properties of a two-dimensional time-dependent Cepheid model

Abstract

Context. Standard spectroscopic analyses of Cepheid variables are based on hydrostatic one-dimensional model atmospheres, with convection treated using various formulations of mixing-length theory. Aims. This paper aims to carry out an investigation of the validity of the quasi-static approximation in the context of pulsating stars. We check the adequacy of a two-dimensional time-dependent model of a Cepheid-like variable with focus on its spectroscopic properties. Methods. With the radiation-hydrodynamics code CO5BOLD, we construct a two-dimensional time-dependent envelope model of a Cepheid with T eff = 5600 K, log g = 2.0, solar metallicity, and a 2.8-day pulsation period. Subsequently, we perform extensive spectral syntheses of a set of artificial iron lines in local thermodynamic equilibrium. The set of lines allows us to systematically study effects of line strength, ionization stage, and excitation potential. Results. We evaluate the microturbulent velocity, line asymmetry, projection factor, and Doppler shifts. The microturbulent velocity, averaged over all lines, depends on the pulsational phase and varies between 1.5 and 2.7 km s −1 . The derived projection factor lies between 1.23 and 1.27, which agrees with observational results. The mean Doppler shift is non-zero and negative, -1 km s −1 , after averaging over several full periods and lines. This residual line-of-sight velocity (related to the "K-term") is primarily caused by horizontal inhomogeneities, and consequently we interpret it as the familiar convective blueshift ubiquitously present in non-pulsating late-type stars. Limited statistics prevent firm conclusions on the line asymmetries. Conclusions. Our two-dimensional model provides a reasonably accurate representation of the spectroscopic properties of a shortperiod Cepheid-like variable star. Some properties are primarily controlled by convective inhomogeneities rather than by the Cepheiddefining pulsations. Extended multi-dimensional modelling offers new insight into the nature of pulsating stars.

Atmospheric parameters of Cepheids from flux ratios with ATHOS: I. The temperature scale

“…The excitation equilibrium of Fe i and Fe ii lines is commonly used to derive the effective temperature of FGK stars, including in our own analyses of Cepheids (e.g., Lemasle et al 2007Lemasle et al , 2008Lemasle et al , 2017, under the assumptions of hydrostatic and local thermodynamic equilibrium (LTE). The first hypothesis obviously does not hold for pulsating variables and biases the results as recently verified for Cepheids by Vasilyev et al (2017Vasilyev et al ( , 2018. The LTE hypothesis does not hold either (e.g.…”

Atmospheric parameters of Cepheids from flux ratios with ATHOS

“…The excitation equilibrium of Fe I and Fe II lines is commonly used to derive the effective temperature of FGK stars, including in our own analyses of Cepheids (e.g., Lemasle et al 2007Lemasle et al , 2008Lemasle et al , 2017, under the assumptions of hydrostatic and local thermodynamic equilibrium (LTE). The first hypothesis obviously does not hold for pulsating variables and biases the results as recently verified for Cepheids by Vasilyev et al (2017Vasilyev et al ( , 2018. The LTE hypothesis does not hold either (e.g., Fuhrmann 1998;Thévenin & Idiart 1999) and this is also true for Cepheids (e.g., Vasilyev et al 2019).…”

Impact of Distance Determinations on Galactic Structure. I. Young and Intermediate-Age Tracers

“…This can be achieved by obtaining phase-resolved spectroscopy of well-understood Cepheids of different periods, which allows for empirical calibration of their variations in effective temperature, gravity and radial velocity. Recent works (Luck and Lambert, 2011) show that the metallicities of a given well-known star based on spectra at different epochs show a reasonable agreement with each other, but small scatters of about 0.1 dex may still be attributed to a systematic phase-dependent effect (see, e.g., the latest work in Vasilyev et al 2018).…”