Scientists Build a Detailed Image of U Mon Binary

Astronomers have painted their best picture yet of an RV Tauri variable, a rare type of stellar binary where two stars – one approaching the end of its life – orbit within a sprawling disk of dust. Their 130-year dataset spans the widest range of light yet collected for one of these systems, from radio to X-rays.

The system, U Monocerotis or U Mon for short, lies around 3,600 light-years away in the constellation Monoceros. Its two stars circle each other about every six and a half years on an orbit tipped about 75 degrees from our perspective.

The primary star, an elderly yellow supergiant, has around twice the Sun’s mass but has billowed to 100 times the Sun’s size. A tug of war between pressure and temperature in its atmosphere causes it to regularly expand and contract, and these pulsations create predictable brightness changes with alternating deep and shallow dips in light – a hallmark of RV Tauri systems. Scientists know less about the companion star, but they think it’s of similar mass and much younger than the primary.

The cool disk around both stars is composed of gas and dust ejected by the primary star as it evolved. Astronomers estimate that the disk is around 51 billion miles (82 billion kilometers) across. The binary orbits inside a central gap that they think is comparable to the distance between the two stars at their maximum separation, when they're 540 million miles (870 million kilometers) apart.

When the stars are farthest from each other, they’re roughly aligned with our line of sight. The disk partially obscures the primary and creates another predictable fluctuation in the system’s light. Scientists think this is when one or both stars interact with the disk’s inner edge, siphoning off streams of gas and dust. They suggest that the companion star funnels the gas into its own disk, which heats up and generates an X-ray-emitting outflow of gas. This model could explain X-rays detected in 2016 by the European Space Agency’s XMM-Newton satellite which made U Mon the first RV Tauri observed in X-rays.

The research team also used visible light measurements from the archives of the American Association of Variable Star Observers (AAVSO) and the Digital Access to a Sky Century @ Harvard (DASCH), both headquartered in Cambridge, Massachusetts.

U Mon’s light varies both because the primary star pulsates and because the disk partially obscures it every 6.5 years or so. The combined AAVSO and DASCH data of U Mon – which spans over 130 years – allowed the astronomers to spot an even longer cycle, where the system’s brightness rises and falls about every 60 years. They think a warp or clump in the disk, located about as far from the binary as Neptune is from the Sun, causes this extra variation as it orbits.

See https://www.nasa.gov/feature/goddard/2021/scientists-sketch-aged-star-system-using-over-a-century-of-observations/