Betelgeuse may have an invisible companion

The bright red supergiant Betelgeuse has long been a familiar sight to stargazers, winking with a red glow from the shoulder of Orion the Hunter. But it’s also an increasingly popular target for professional astronomers because of its scientific quirks, from pulsations to mysterious dimming events.

Now, astronomers believe they may have found a key clue to Betelgeuse’s strange behavior: two independent studies posted on arXiv The preprint server claims that Betelgeuse is actually part of a binary system with a previously unknown companion star.

This second proposed star is much smaller than Betelgeuse and so far remains unseen; both teams deduced its presence based on the way its gravity gently pulls Betelgeuse back and forth across the sky. But if astronomers can confirm the star with observations, it could explain Betelgeuse’s longest pulsation, which has puzzled astronomers for decades.

Playing the long game

Betelgeuse’s strange behavior comes in the form of brightness variations. The star pulsates in brightness not in one regular beat, but in a series of overlapping rhythms, with periods as short as a few hundred days and as long as thousands of days. One of them is directly related to the time Betelgeuse has left before it inevitably explodes. Most of the others are tones or harmonics related to that supernova time bomb. Which has been a matter of disagreement among researchers, but most agree that a 416-day cycle is the so-called fundamental mode of the star, the one that most directly portends its demise, and the shorter modes are its notions.

But the longest cycle, which ends every 2,170 days, has remained a puzzle of its own. There are some researcher which I believe is the star’s fundamental mode – and if true, would indicate that Betelgeuse is twice as large as generally believed and poised to go supernova at any moment. But most researchers disagree, hypothesizing that this longest cycle is something stranger and more subtle.

In addition, for several months from the end of 2019 to 2020, Betelgeuse suddenly faded completely out of cycle, surprising astronomers and eventually leading them to assume that the star must have blown off a giant dust cloud. Moreover, it prompted a new series of investigations into the known star, including the latest development, which he arrived at independently in two different studies currently on arXiv preprint server, which Betelgeuse probably has a companion.

A companion star was first proposed over a century ago as a possible explanation for Betelgeuse’s periodic dimming and brightening. But that hypothesis fell out of favor as astronomers learned more about the life cycles of massive stars. It became clear that Betelgeuse has reached a stage of life where it physically pulsates on its own, expanding and contracting in a cycle. These pulsations are released from clouds of dust and gas at the surface of the star into an envelope of material that surrounds the star.

However, these intrinsic pulsations generally occur over periods of hundreds of days or shorter, so they do not explain the much slower pulsations found in Betelgeuse and other similar stars. These long secondary periods (LSPs) typically take thousands of days to complete a brightening and dimming cycle.

Low-mass companion stars or large planets could explain some of these LSPs because they transit or occult their central star and block some of its light. But finding them, especially around super-luminous variable stars with extended atmospheres like Betelgeuse, is a steep challenge, so their existence is largely conjecture rather than observed fact.

Searching through history

Key to both studies was the comparison of Betelgeuse’s LSP with astrometric and radial velocity data. Both reveal how Betelgeuse moves gently across the sky—perfectly as if pulled by an unseen companion.

A team led by Jared Goldberg of the Center for Computational Astrophysics at the Flatiron Institute in New York used astrometric data from the European Space Agency’s Gaia space telescope and painstakingly addressed every other possibility for Betelgeuse’s LSP by poking holes in each theory until one remained: a low-mass companion star that orbits every 2,170 days. their paper has been posted to arXiv August 17 and has been accepted for publication in The Astrophysical Journal.

Morgan MacLeod of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, led a second research group, which used radial velocity data from a century of observations as well as astrometric data – measurements of Betelgeuse’s position – to to reveal the star’s minute changes. in the sky They posted their discovery on arXiv just one month later on September 17.

Both teams strongly agree on the facts of the invisible companion. It can’t be much more than the mass of the Sun, making it only one-twentieth the mass of Betelgeuse. And it orbits Betelgeuse at a distance slightly less than Saturn’s average distance from the Sun—roughly the equivalent of one Betelgeuse width away from the surface of the star itself.

But Betelgeuse provided yet another twist to the story.

“Before our work,” Goldberg tells him Astronomy“a leading theory for the behavior of the LSP was a companion with a dusty tail obscuring the star. But because of the radial velocity, it must be the other way around – the dust blocks the star when the companion is not in view! So not an eclipse, but an anti-eclipse. Or rather, the dust eclipses the star, but the dust is where the companion is not.”

MacLeod’s team reached the same conclusion. Instead of the companion blocking Betelgeuse’s light and causing a dip in brightness, the companion was apparently clearing dust like a snowplow, making Betelgeuse’s light shine a little brighter during the eclipse.

This could mesh perfectly with the dust puff explanation for the 2020 Great Eclipse, Goldberg’s team proposes in their paper. The reduction occurred near the LSP minimum, when the theoretical companion would have been behind Betelgeuse. This would put its L3 Lagrange point, a strange orbital of gravitational stability, directly between Betelgeuse and Earth viewers, possibly driving the dust cloud along our line of sight and obscuring the star’s light.

A shot in the dark

The next obvious step is also the most difficult yet – spotting the companion. According to Goldberg, this “may not even be possible with current tools. Part of it depends on how lucky we are and the properties of the companion.” Whether the companion is accreting material from Betelgeuse and how hot the companion is would produce different signals that could tell the team about the companion, as well as whether it is harder or easier to spot.

“But,” he continued, “it’s easier now that we know when to look: if we’re right about this, the companion will pass Betelgeuse’s limb close to December 6 this year; at this point they will be maximally separated (from our point of view), and that configuration … gives us the best chance of distinguishing the companion from the star.”

He calls their upcoming remarks, which have already been approved, “a bit long. Detecting something ~100,000 times fainter than the thing next to it is hard!”

But Betelgeuse was always full of surprises. Maybe this time, the improbability will work in our favor.