Numerous population of binary Cepheids with giant companions
A bold hypothesis that outliers above the period-luminosity relation of Classical Cepheids are binaries brings a discovery of 16 new binary Cepheids with giant companions in the LMC.
Cepheids with Giant Companions. I. Revealing a Numerous Population of Double-lined Binary Cepheids
B. Pilecki, G. Pietrzyński, R.I. Anderson, W. Gieren, M. Taormina, W. Narloch, N.R. Evans, J. Storm
Classical Cepheids, famous for their Period-Luminosity (P-L) relation, have been used as distance indicators for over 100 hundred years. Usually, outliers above the P-L relation are removed from the sample to improve the fit, but this time 41 outliers were targeted as candidates for binary Cepheids. A follow-up spectroscopic study performed on 18 candidates revealed that 16 of them are double-line spectroscopic binaries, tripling the number of known Cepheid binaries in the Large Magellanic Cloud.
P-L relation for reddening free Wesenheit index. Filled red circles show known eclipsing binaries with Cepheids (OGLE catalog numbers), while empty circles mark their brightness without the companion light. Our candidates for binary Cepheids are ≥ 50% brighter than an average Cepheid for its period. Note that their distribution parallel to the P-L relation advocates strongly against blending by a random star. Outlying points below the P-L relation may be binary type II Cepheids and Anomalous Cepheids.
Masses of classical Cepheids of 3 to 11 solar masses are predicted by theory, but those which are measured clump between 3.6 and 5 solar masses. As a result, their mass-luminosity relation is poorly constrained, impeding our understanding of basic stellar physics and the Leavitt Law. All Cepheids’ masses come from the analysis of 11 binary systems, including only 5 double-lined and well-suited for accurate dynamical mass determination. With newly discovered systems, the collection of Cepheids’ dynamical masses will be vastly enlarged, and this will allow us to better understand the physics of Classical Cepheids.
The full project dedicated to the LMC sample will take several years to be completed. Our preliminary results already have important implications for the interpretation of period-luminosity relations as we have now a strong clue that overbright Cepheids, that were often being rejected as P-L relation outliers, are just Cepheids with red, luminous (giant) companions. These conclusions can be likely extended to other pulsating stars with well-defined P-L relations like, for example, type II Cepheids or RR Lyrae stars.
