Close Binary Stars in Planetary Nebulae through Gaia EDR3

The aim of this work is to search for evidence of close binary stars associated with planetary nebulae (ionized stellar envelopes in expansion) by mining the astronomical archive of Gaia EDR3. For this task, using big data techniques, we selected a sample of central stars of planetary nebulae from almost 2000 million sources in an EDR3 database. Then, we analysed some of their parameters, which could provide clues about the presence of close binary systems, and we ran a statistical test to verify the results. Using this method, we concluded that red stars tend to show more affinity with close binarity than blue ones.

Most EL CVn systems are inner binaries of hierarchical triples

ABSTRACT In spite of their importance for modern astronomy, we do not fully understand how close binary stars containing at least one white dwarf form from main-sequence binary stars. The discovery of EL CVn binaries, close pre-white dwarfs with A/F main-sequence star companions, offers now the unique possibility to test models of close compact binary star formation. Binary evolution theories predict that these EL CVn stars descend from very close main-sequence binaries with orbital periods shorter than 3 d. If this is correct, nearly all EL CVn stars should be inner binaries of hierarchical triples because more than 95 per cent of very close main-sequence binaries (the alleged progenitor systems) are found to be hierarchical triples. We here present SPHERE/IRDIS observations of five EL CVn binaries, finding in all of them tertiary objects, as predicted. We conclude that EL CVn systems are inner binaries of hierarchical triples and indeed descend from very close main-sequence binaries that experience stable mass transfer.