Author(s): , , , , ,
Institution(s): 1. ESO, 2. Johns Hopkins University, 3. Observatoire de Paris, 4. Universidad de Concepcion, 5. University of Geneva
Cepheid variable stars are important tools for stellar astrophysics and the extragalactic distance scale. Their variability is considered to be highly regular. Yet, four Cepheids were recently reported (Anderson 2014, A&A, 566, L10) to exhibit modulated radial velocity (RV) variability, i.e., the RV curves of some Cepheids change in shape and amplitude as a function of time. The long-period Cepheid l Carinae is one of these stars and carries high weight in the current calibration of the period-luminosity relation that renders Cepheids useful standard candles. No definitive explanation has been found so far for this modulated variability, which may be related to several effects, including pulsation-convection coupling and non-radial pulsations. Importantly, RV curve modulation can bias distances estimated via the Baade-Wesselink technique, which compares the angular and linear variability of Cepheid radii.
We performed interferometric time-series observations of l Carinae using ESO's VLT Interferometer in order to investigate whether the observed RV curve modulation has a clear counterpart in the angular radius variability and to better understand the physics behind this modulation. To this end, we observed l Carinae with the PIONIER instrument at three different epochs near maximum, minimum, and maximum radius, and inferred angular diameters with unprecedented precision. Contemporaneously, we monitored l Carinae with the CORALIE spectrograph mounted to the 1.2m Euler telescope at La Silla Observatory Chile, securing several observations nearly every night over the course of three months, building an extremely precise radial velocity curve.
Here, we present the current status of our analysis and report peculiar spectral line shape variability that appears to be mainly responsible for the modulated RV curves. We outline how this work aims at improving the accuracy of Baade Wesselink distances while also opening a new window into the analysis of Cepheid variability.