A significant discovery has provided new insights into our understanding of classical Cepheid stars. These pulsating giants and supergiants are well known for their established relationship between pulsation period and luminosity, making them essential tools for measuring extragalactic distances. They also play a key role in testing stellar evolution and pulsation theory models. Despite their predicted mass range between 3 and 13 times the mass of our Sun, the measured masses of Cepheids so far have all clustered around 4 solar masses.
Recently, a fascinating discovery was made of a spectroscopic binary system featuring a double-mode Cepheid, OGLE-LMC-CEP-1347 (Pilecki et al. 2022). This system is remarkable for having the shortest orbital period (59 days) among all known binary Cepheids, and even more intriguing is that the Cepheid is almost twice as massive as its companion star. This hints that the system was a triple system before, and the Cepheid might result from two lower-mass stars merging into one. The unique characteristics of this system make it an essential candidate for mass determination.
In a recent article, “A novel q-PED method: precise physical properties of a merger-origin binary Cepheid OGLE-LMC-CEP-1347” published in the prestigious Astrophysical Journal Letters, Filipe Espinoza-Arancibia and Bogumił Pilecki from the Nicolaus Copernicus Astronomical Center in Warsaw introduced the q-PED method. This approach combines observed mass ratio (q), pulsation (P), evolutionary models (E), and known distance (D), along with multi-band photometry, to determine the physical properties of OGLE-LMC-CEP-1347 and its companion.
The results showed that the Cepheid has a mass of 3.4 solar masses and a radius of 13.6 solar radius, while its companion has a mass of 1.9 solar masses and a radius of 12.5 solar radius. Remarkably, the Cepheid mass obtained is lower than any measured Cepheid mass before. The companion is a low-mass star, fainter and redder than the Cepheid, although, interestingly, of similar size. The apparent age difference of nearly 1 billion years between the two stars supports the merger origin scenario. This also implies that the actual age of the Cepheid would be 1.09 billion years, indicating that a significant fraction of Cepheids may be much older than typically assumed. These results led to the conclusion that the most likely scenario is that the system was once a triple, and the Cepheid we see today is the product of a past merger between two lower-mass stars of an inner binary.
The method used in this work has major implications. By offering a new and precise way to measure the properties of binary Cepheid systems, the q-PED method could help refine our understanding of how Cepheids work and how they fit into the larger puzzle of stellar evolution.
Figure 1. Artistic representation of the initial (triple system on the left) and the current state of the binary Cepheid. Being a merger, the Cepheid is almost two times more massive but the companion is more evolutionarily advanced and thus of similar size.
Text: Filipe Espinoza-Arancibia.
This project is supported by the Polish National Science Center (grant SONATA BIS 2020/38/E/ST9/00486).
