Massive Algol binaries
Eclipsing, double-lined spectroscopic binary systems caught during mass transfer- excellent systems to understand mass-transfer physics, tidal theory, and internal mixing processes.
Observed configuration of Algol binaries (figure above). The currently less massive star is larger in radius and transferring mass to the more massive star. This is the classical Algol paradox, whose solution comes from understanding the physics of the mass transfer phase (figure below). The initially more massive star evolves faster and fills its Roche lobe, initiating mass transfer on the thermal timescale. The orbital period shrinks initially until the two stars have the same mass. Once the mass donor is less massive, further mass loss from the donor expands the orbit and slows mass transfer. Now, the mass transfer is driven by the nuclear timescale radial expansion of the donor for the remainder of its main sequence lifetime. The nuclear timescale evolution of the mass transfer phase is key to being able to catch these systems in the Algol phase and also study the properties of the much more elusive thermal timescale mass transfer that precedes it.
The term “Algol” refers to its prototype star, Beta Persei, a bright multiple-star system in the Perseus constellation. It is one of the first stars recorded to show a variation in its brightness, dating back to 1244-1163 B.C. For all Batman enthusiasts, the word “Algol” is derived from the Arabic word “Ra’s al-Ghul”, or the head of the Demon. For a more extensive historical background, see the Wikipedia page.
These systems have short orbital periods (a few days) and can, more often than not, be observable as double-lined eclipsing binaries. Hence, they are ideal systems for studying stellar and binary physics as precise measurements of their component masses, radii, and effective temperatures are possible. About 50 massive Algol binaries have been observed across our Milky Way, the Large Magellanic Cloud, and the Small Magellanic Cloud combined. Through my ongoing projects and collaborations using the Gemini Observatory and the ESO Very Large Telescope, we are taking multi-epoch spectroscopic observations of another 40 and 70 massive eclipsing binary systems in the LMC and Milky Way, respectively. Beyond orbital periods and mass ratios, we will derive rotational velocities and abundances of the binary components for the first time for the entire sample, exponentially increasing the scientific return of studying these systems.
The above properties of Algol binaries provide a unique testbed for our theories of massive stellar and binary evolution, including the physics of binary interaction. I study the evolution of massive Algol binaries (Algols heavier than eight times the mass of the Sun) using detailed binary evolution models computed using the software MESA (Modules for Experiments in Stellar Astrophysics). By directly comparing binary model predictions with observed properties of massive Algols, I derive constraints on binary and stellar physics, including mass-transfer efficiency, internal mixing, and stellar winds.