Cepheids with giant companions. II. Spectroscopic confirmation of nine new double-lined binary systems composed of two Cepheids
Astronomy & Astrophysics(2024)
摘要
Binary Cepheids with giant companions are crucial for studying the physical
properties of Cepheid variables, providing the best means to measure their
masses. Systems composed of two Cepheids are even more important but to date,
only one such system in the Large Magellanic Cloud (LMC) was known. Our current
aim is to increase the number of these systems tenfold and provide their basic
characteristics. The final goal is to obtain the physical properties of the
component Cepheids, including their masses and radii, and to learn about their
evolution in the multiple systems, also revealing their origin. We started a
spectroscopic monitoring of nine unresolved pairs of Cepheids from the OGLE
catalog, to check if they are gravitationally bound. Two of these so-called
double Cepheids are located in the LMC, five in the Small Magellanic Cloud
(SMC), and two in the Milky Way (MW). We report the spectroscopic detection of
binarity of all 9 of these double Cepheids with orbital periods from 2 to 18
years. This increases the number of known binary double (BIND) Cepheids from 1
to 10 and triples the number of all confirmed double-lined binary (SB2)
Cepheids. For five BIND Cepheids disentangled pulsational light curves of the
components show anti-correlated phase shifts due to orbital motion. We show the
first empirical evidence that typical period-luminosity relations (PLRs) are
rather binary Cepheid PLRs that include the companion's light. The statistics
of pulsation period ratios of BIND Cepheids do not agree with those expected
for pairs of the same-age Cepheids. These ratios together with the mass ratios
far from unity suggest merger-origin of at least one component for about half
of the systems. The SMC and MW objects are the first found in SB2 systems
composed of giants in their host galaxies. The Milky Way BIND Cepheids are also
the closest such systems, being located at about 11 and 26 kpc.
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