Both Stars in This Binary System Have Accretion Disks Around Them

Stars exhibit all sorts of behaviors as they evolve. Small red dwarfs smolder for billions or even trillions of years. Massive stars burn hot and bright but don’t last long. And then of course there are supernovae.

Some other stars go through a period of intense flaring when young, and those young flaring stars have caught the attention of astronomers. A team of researchers are using the Atacama Large Millimeter/sub-millimeter Array (ALMA) to try to understand the youthful flaring. Their new study might have found the cause, and might have helped answer a long-standing problem in astronomy.

The type of star in question are FU Orionis stars (FU Ori). FU Orionis is both a type of star, and also a specific star in the constellation Orion. The type is named after the specific star, which was the first of its kind seen flaring in 1937.

FU Ori stars are young stars that aren’t on the main sequence yet, and haven’t acquired all of their mass. They can flare by several orders of magnitude in only a single year. These flaring episodes can last decades, and researchers think the activity is caused by increased accretion in the star’s youth. Scientists think that during the flaring, the star can acquire a significant amount of its final mass.

“Episodic accretion and its implications for star and planet formation are not well understood.”

Perez et. al. 2020

Now a team of researchers are studying FU Ori stars more closely. Sebastien Perez at the University of Santiago, Chile, led the study. Their new paper is titled “Resolving the FU Orionis System with ALMA: Interacting Twin Disks?” It’s published in The Astrophysical Journal.

Scientists want to know what’s behind this accretion and associated flaring. Do only some stars experience it? Or is it a stage that all or most stars go through? How long does it last; does it happen only once in a star’s lifetime; why does it end?

This artist’s concept shows a young stellar object and the whirling accretion disk surrounding it. NASA/JPL-Caltech

Young proto-stars are less luminous than expected according to our understanding of stellar formation. That’s known as the “luminosity problem” in astronomy, and scientists have been wrestling with that problem for a long time. If young stars were accreting at a regular rate, they should be more luminous. If all young stars exhibit the flaring activity seen in FU Ori stars, it could explain this missing luminosity. Astronomers have wondered for some time if mass accretion in these young forming stars might not be constant, and if that might explain the luminosity problem.

“Episodic accretion and its implications for star and planet formation are not well understood,” the authors say in their paper. “Several physical processes have been proposed to explain such dramatic accretion events. The most favored mechanisms include disk fragmentation and the subsequent inward migration of the fragments, gravitational instability, and magneto-rotational instabilities among others.” 

The archetypal FU Ori star is its namesake, FU Orionis, in the constellation Orion. It was observed flaring in 1937, and its magnitude increased from 16.5 to 9.6. Astronomers thought it was the only one of its kind, until others were observed.

FU Orionis is in the constellation of Orion. It’s not marked in this image, but it’s up and to the right of Betelgeuse. Image Credit: By IAU and Sky & Telescope magazine (Roger Sinnott & Rick Fienberg) – [1], CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=15407823

FU Orionis is actually two stars, each surrounded by its own accretion disk. They’re in Orion, about 1360 light years away. Perez and the team of researchers took a close look at the system with ALMA, the first step to understanding the binary pair’s flaring behaviour.

ALMA revealed two accretion disks, one around each star. The scientists used observations and models to conclude that each of the disks is about 11 Astronomical Units in radius, which is small but comparable to other proto-stellar disks. The pair of disks are separated by about 250 Astronomical Units.

ALMA continuum observations show the dust of the two disks surrounding the binary stars of FU Orionis. Each disk is about 11 AU in radius. [Pérez et al. 2020]

Key to understanding the flaring activity in these stars is the movement, or kinematics, of their disks. As the team studied the disks, they found that each one is skewed and asymmetrical. They think that might be caused by some sort of flyby by another star. It could also be caused by interactions between the disks themselves. Either of those could cause the episodic accretion and flaring.

Artist’s impression of a young star throwing a temper tantrum as it suddenly increases its accretion rate and flares. [Caltech/T. Pyle (IPAC)]

The team also found evidence of a long, arcing stream of gas between the disks. That stream strengthens the argument that the disks are interacting. As they say in their paper, “The emission revealing disk rotation also appears asymmetric and skewed, suggesting the disks are subject to interaction in the form of a flyby.”  

The authors also point to an alternative to the disk-disk interaction that another team of researchers proposed. “Here, the capture of a cloudlet or cloud fragment also leads to arc-shaped reflection nebulae <the arc of gas connecting the disks.> The capture of this cloud fragment also replenishes the disk allowing for a fresh supply of material to maintain the high accretion rate.”

The study doesn’t answer the missing luminosity question once and for all. But by using ALMA to get a close look at the FU Ori binary pair, the team of scientists have advanced our understanding of episodic accretion and flaring. There are other binary pairs of FU Ori stars, and they’ll be targets for future study.

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Evan Gough

View Comments

  • To get a sense of the distances involved, this is a nice reference (also using ALMA, as well as VLA): https://arxiv.org/pdf/2001.04997.pdf .

    Figure 11 is their accreting protostar model synthesis, where a typical collapsing low-mass star destined molecular cloud starts out at ~ 5,000 AU, before forming an opaque and later hydrostatic core and at the disk forming stage has contracted to ~ 500 AU before the protostar ignites.

    I guess the binaries here at ~ 250 AU distances fits these clouds comfortably. Likely interacting stars as well as disks could explain the latter's relatively small radius.

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