RV Tauri variable is a rare form of stellar binary in which a pair of stars—one nearing the end of its life—revolves inside an expansive disk of dust. Astronomers have now painted their most optimized picture yet of this stellar binary.
The 130-year dataset of these variables covers the broadest range of light obtained to date, for one of these systems, from X-rays to radio.
There are only about 300 known RV Tauri variables in the Milky Way galaxy. We focused our study on the second brightest, named U Monocerotis, which is now the first of these systems from which X-rays have been detected.
Laura Vega, Doctoral Recipient, Vanderbilt University
An article explaining these results, headed by Vega, was recently published in The Astrophysical Journal.
Called U Mon for short, the system is located about 3,600 light-years away in the constellation Monoceros. The two stars of this system orbit one another around every six and a half years on an orbit tipped approximately 75° from a human’s standpoint.
U Mon’s primary star—an old yellow supergiant—has a mass that is about twice that of the Sun’s but has expanded to 100 times the size of the Sun.
This primary star routinely expands and contracts because of a tug of war between temperature and pressure in its atmosphere. And such pulsations generate a foreseeable change in brightness with alternating shallow and deep dips in light, which is a characteristic of RV Tauri systems. While researchers do not know much about the companion star, they do believe that it is of almost the same mass and relatively younger than the primary star.
The cool disk surrounding both stars is made up of dust and gas discharged by the primary star as it evolved. Vega’s research team used radio observations from the Submillimeter Array located on Maunakea, Hawai’i, and determined that the disk measures about 51 billion miles, or 82 billion km, across.
This binary disk circles within a core gap which, according to the researchers, is similar to the distance existing between both stars at their highest separation, when they are around 40 million miles, or 870 million km, apart.
When the two stars are located farthest from one other, they are more or less aligned with the humans’ line of sight. The binary disk partly conceals the primary star and produces another foreseeable variation in the brightness of the system.
According to Vega and her collaborators, this is the point when one or both stars interact with the inner edge of the disk, siphoning off dust and gas streams. The researchers suggested that the gas is funneled into its own disk by the companion star, and this disk becomes warm and produces an X-ray-emitting outflow of gas. Such a model may explain the X-rays identified by the European Space Agency’s XMM-Newton satellite in 2016.
The XMM observations make U Mon the first RV Tauri variable detected in X-rays. It’s exciting to see ground- and space-based multiwavelength measurements come together to give us new insights into a long-studied system.
Kim Weaver, Astrophysicist, Goddard Space Flight Center in Greenbelt, NASA
Weaver is also a project scientist from XMM U.S.
During their study of U Mon, Vega’s research team also integrated the visible light observed for 130 years. The archives of the American Association of Variable Star Observers (AAVSO) provided the earliest available measurement of the system, obtained on December 25, 1888.
AAVSO is an international network of professional and amateur astronomers headquartered in Cambridge, Massachusetts. It offered more historical measurements that ranged from the mid-1940s to the present time.
In addition to these measurements, the team also utilized the archived images compiled by the Digital Access to a Sky Century @ Harvard (DASCH)—a program at the Harvard College Observatory in Cambridge devoted to digitizing astronomical pictures from glass photographic plates created by ground-based telescopes from the 1880s to 1990s.
The light from U Mon tends to fluctuate because the disk partly conceals it every 6.5 years or so, and also due to the pulsation of the primary star.
The integrated DASCH and AAVSO data-enabled Vega and her collaborators to detect an even longer cycle, in which the brightness of the system increases and decreases around every 60 years. The researchers believe that a clump or warp in the disk—situated around as far from the binary as Neptune is from the Sun—induces this additional change as it revolves.
Vega finished her examination of the U Mon system as a NASA Harriett G. Jenkins Predoctoral Fellow—a program financially supported by the Minority University Research and Education Project of NASA Office of STEM Engagement.
For her doctoral dissertation, Laura used this historical dataset to detect a characteristic that would otherwise appear only once in an astronomer’s career. It’s a testament to how our knowledge of the universe builds over time.
Rodolfo Montez Jr, Study Co-Author and Astrophysicist, Center for Astrophysics
Montez Jr also works in Cambridge.
Keivan Stassun, the co-author of the study, an expert in star formation, and Vega’s doctoral advisor at Vanderbilt University, observed that this evolved system shares several behaviors and features with the recently developed binaries.
For example, both systems are integrated into disks of dust and gas, pull material from those disks, and create outflows of gas. And the disks, in both cases, can create clumps or warps. In the case of young binaries, those may probably signal the beginnings of the formation of planets.
“We still have questions about the feature in U Mon’s disk, which may be answered by future radio observations. But otherwise, many of the same characteristics are there. It’s fascinating how closely these two binary life stages mirror each other,” concluded Stassun.
Scientists Build a Detailed Image of U Mon Binary
Two stars orbit each other within an enormous dusty disk in the U Monocerotis system, illustrated here. When the stars are farthest from each other, they funnel material from the disk’s inner edge. At this time, the primary star is slightly obscured by the disk from our perspective. The primary star, a yellow supergiant, expands and contracts. The smaller secondary star is thought to maintain its own disk of material, which likely powers an outflow of gas that emits X-rays. Video Credit: NASA’s Goddard Space Flight Center/Chris Smith (USRA/GESTAR).
Vega, L. D., et al. (2021) Multiwavelength Observations of the RV Tauri Variable System U Monocerotis: Long-term Variability Phenomena That Can Be Explained by Binary Interactions with a Circumbinary Disk. The Astrophysical Journal. doi.org/10.3847/1538-4357/abe302.