Discovery of the spectroscopic binary nature of six southern Cepheids

TL;DR

This study identifies six southern Cepheids as spectroscopic binaries, highlighting the high binary occurrence among Cepheids and analyzing period changes likely due to stellar evolution or binary effects.

Contribution

First spectroscopic binary identifications for six bright Galactic Cepheids, emphasizing the importance of binary considerations in Cepheid period-luminosity calibration.

Findings

V898 Cen has a large orbital radial velocity amplitude (>40 km/s).

AD Pup's pulsation period is increasing, likely due to stellar evolution.

Light-time effect may explain period variations in AD Pup.

Abstract

We present the analysis of photometric and spectroscopic data of six bright Galactic Cepheids: GH Carinae, V419 Centauri, V898 Centauri, AD Puppis, AY Sagittarii, and ST Velorum. Based on new radial velocity data (in some cases supplemented with earlier data available in the literature), these Cepheids have been found to be members in spectroscopic binary systems. V898 Cen turned out to have one of the largest orbital radial velocity amplitude (> 40 km/s) among the known binary Cepheids. The data are insufficient to determine the orbital periods nor other orbital elements for these new spectroscopic binaries. These discoveries corroborate the statement on the high frequency of occurrence of binaries among the classical Cepheids, a fact to be taken into account when calibrating the period-luminosity relationship for Cepheids. We have also compiled all available photometric data that revealed that the pulsation period of AD Pup, the longest period Cepheid in this sample, is continuously increasing with Delta P = 0.004567 d/century, likely to be caused by stellar evolution. The wave-like pattern superimposed on the parabolic O-C graph of AD Pup may well be caused by the light-time effect in the binary system. ST Vel also pulsates with a continuously increasing period. The other four Cepheids are characterised with stable pulsation periods in the last half century.