Cepheids - a family of stars whose luminosity varies over time - are used to measure distances in the universe. The role of Cepheids as distance calibrators has brought them attention from researchers for more than a century. While it was thought that nearly everything was known about the prototype of Cepheids, named Delta Cephei, a team of researchers at the University of Geneva (UNIGE), the Johns Hopkins University, and the European Space Agency (ESA), have now discovered that this star is not alone, but that it has a hidden companion -- a revelation published in the Astrophysical Journal.
New research shows Delta Cephei is, in fact, a double star, made up of a Cepheid-type variable star and a companion, which had thus far escaped detection, probably because of its low luminosity. Yet, pairs of stars, referred to as binaries, complicate the calibration of the period-luminosity relationship, and can bias the measurement of distances. This is a surprising discovery, since Delta Cephei is one of the most studied stars, of which we thought we knew almost everything.
"We were shocked: despite all the attention Delta Cephei was given over the years, we were lacking an essential piece of information," said Richard Anderson, researcher at UNIGE at the time of the discovery, first author of the article and now researcher at Johns Hopkins University, according to the press release.
According to the scientists, the data collected in the framework of ESA's Gaia space mission will enable Delta Cephei's orbit to be precisely measured. The presence of the companion must consequently be taken into account when the Gaia team determines the Delta Cephei's distance.
"Although our study does not challenge the cosmic distance ladder as a whole, improving the precision of every one of its rungs will eventually benefit cosmology," explained Anderson, according to the press release. "This discovery reminds us that something is always to be learned. If even one of the closest Cepheids is keeping such secrets, who knows what we will discover about the ones furthest from us!"
This study is likely to inspire further research aiming to better understand the evolution of Delta Cephei, since binarity is an essential property to consider for interpreting the evolution of a star. Studying the evolution of Cepheids is particularly interesting, since it helps to improve the understanding of the structure and evolution of stars in general. "We are waiting for the results from new measurements taken with the Hermes spectrograph and the observations from Gaia. These will allow us to precisely trace the possibly turbulent past of Delta Cephei," the astronomers said, according to the press release. "It is a fascinating adventure!"
Reference:
Revealing δ Cephei's Secret Companion and Intriguing Past, R. I. Anderson, J. Sahlmann, B. Holl, L. Eyer, L. Palaversa, N. Mowlavi, M. Suveges & M. Roelens, 2015, to appear in the Astrophysical Journal. On Arxiv: arxiv.org/abs/1503.04116
