A research team led by Prof. Qian Shengbang and Ph.D. student Li Fuxing from the Chinese Academy of Sciences’ Yunnan Observatories discovered that the formation and evolution of massive binaries in the Milky Way and the Andromeda Galaxy (M31) may share the same mechanism. Their findings were published in the Royal Astronomical Society’s Monthly Notices and The Astrophysical Journal.
Massive binaries have at least one early-type star with the spectral type O, B. These binaries emit high-energy radiation, such as X-rays, and have the potential to form neutron stars or black holes. The semidetached binaries’ progenitors are detached binaries in which the original, more massive components evolve faster and fill their critical Roche lobes first, then transfer mass to their companions via case A evolution.
During this process, the system’s orbital period will be reduced while its mass ratio will be increased. This binary has the shortest orbital period when the system evolves to the critical state where the mass ratio equals one (twin binaries). Following this special stage, the binary’s mass ratio will be reversed with mass transfer from the less massive component to the more-massive component.
The researchers studied the evolution stage of V375 Cassiopeia (V375 Cas), a massive binary with two B-type components, in their study published in Monthly Notices of the Royal Astronomical Society.
They examined the light curves of V375 Cas and discovered that V375 Cas should go through a late case. A mass transfer from the less-massive to the more-massive component.
Meanwhile, statistics show that those massive semidetached binaries have a third body with varying periods. According to the H-R diagram, the massive binaries’ components are almost all main-sequence stars, and the evolutionary age of the secondary component is greater than that of the primary for V375 Cas. “V375 Cas is a hierarchical triple system in which a massive main-sequence star is accompanied by a massive semidetached mass-transfer binary based on the third light,” Li explained.
In M31, the researchers discovered two massive close binaries with twin components. M31 is the closest spiral galaxy to the Milky Way and the largest galaxy in the Local Group, with structure and metallicity similar to the Milky Way.
The photometric solutions from 437 eclipsing binaries are performed using the W-D method, and two twin binaries have been discovered. The first is a contact binary with a mass ratio of 0.974, while the second is a semidetached binary with a mass ratio of 0.924. This finding suggests that massive twin binaries are uncommon in M31.
The researchers discovered that these two massive twin binaries are at different evolutionary stages with similar mass ratios based on a study of orbital period changes by O-C diagrams and binary configurations (close to unity). The twin contact binary is on the verge of entering the critical evolutionary stage of the shortest period with rapid mass transfer. During the orbit decreasing phase with case A mass transfer, the semidetached binary has gone through that evolution stage and fails to form a contact binary.
The Astrophysical Journal published these findings. These two studies show that massive binaries can evolve in both the Milky Way and M31 and that these binaries, at a specific stage, provide an ideal testbed for evolutionary models of massive binaries.