2M17091769+3127589: A Mass-transfer Binary with an Extreme Mass Ratio

Abstract

Abstract We present the orbital solution of a peculiar double-lined spectroscopic and eclipsing binary system, 2M17091769+3127589. This solution was obtained by a simultaneous fit of both APOGEE radial velocities and TESS and ASAS-SN light curves to determine masses and radii. This system consists of an M = 0.256 − 0.006 + 0.010 M ⊙, R = 3.961 − 0.032 + 0.049 R ⊙ red giant and a hotter M = 1.518 − 0.031 + 0.057 M ⊙, R = 2.608 − 0.321 + 0.034 R ⊙ subgiant. Modeling with the MESA evolutionary codes indicates that the system likely formed 5.26 Gyrs ago, with a M = 1.2 M ⊙ primary that is now the system’s red giant and a M = 1.11 M ⊙ secondary that is now a more massive subgiant. Due to Roche-lobe overflow as the primary ascends the red giant branch, the more evolved “primary” (i.e., originally the more massive star of the pair) is now only one sixth as massive as the “secondary.” Such a difference between the initial and the current mass ratio is one of the most extreme detected so far. Evolutionary modeling suggests the system is still engaged in mass transfer, at a rate of M ̇ ∼ 10 − 9 M ⊙/yr, and it provides an example of a less evolved precursor to some of the systems that consist of white dwarfs and blue stragglers.