Nine Cepheid-Cepheid binaries confirmed spectroscopically
Newly discovered nine binary systems consisting of two Cepheids shed more light on the evolution of multiple systems, and reveal their merger origin.
Cepheids with giant companions. II. Spectroscopic confirmation of nine new double-lined binary systems composed of two Cepheids
B. Pilecki, I.B. Thompson, F. Espinoza-Arancibia, G. Hajdu, W. Gieren, M. Taormina, G. Pietrzyński, W. Narloch, G. Bono, A. Gallenne, P. Kervella, P. Wielgórski, B. Zgirski, D. Graczyk, P. Karczmarek, N.R. Evans
Binary Cepheids with giant companions are already rare and hard to find, but binary Cepheids with another Cepheid companions are extremely difficult to discover, monitor, and analyze. That hard work pays off, because Cepheid-Cepheid binaries provide the best means to study the physical properties of these variables, which allows us to learn in greater detail about their evolution in multiple systems, and possibly to reveal their origin.
Up to date, only one such system has been identified, in the Large Magellanic Cloud (LMC).
We started a spectroscopic monitoring campaign of nine unresolved pairs of Cepheids from the OGLE catalog: two in the LMC, five in the Small Magellanic Cloud (SMC), and another two in the Milky Way (MW). This campaign resulted in the spectroscopic confirmation of the binarity of all nine targets, called binary double (BIND) Cepheids. The SMC and MW objects are the first found in SB2 systems composed of giants in their host galaxies.
The newly confirmed nine BIND Cepheids have orbital periods ranging from 2 to 18 years. We show the first empirical evidence that typical period-luminosity relations (PLRs) are rather binary Cepheid PLRs, as they include light of the companion.
Some of the pulsation period ratios of BIND Cepheids do not agree with those expected for pairs of Cepheids of the same age — this suggest a merger origin of at least one component of the systems.
