Meridith Joyce, an assistant professor in the University of Wyoming Department
of Astronomy and Physics, was part of a research team that studied the brightening
and dimming of a star known as Betelgeuse (pictured) and found the pulsing starlight
potentially comes from a companion star, dubbed “Betelbuddy.” The research was accepted
for publication in The Astrophysical Journal. (ALMA Telescope Photo)
The time until a star known as Betelgeuse explodes into a supernova has been debated
in literature by the scientific community. This was based on the brightening and dimming
of the star.
Now, recent research -- that included the work of a University of Wyoming faculty
member -- suggests the pulsing starlight potentially comes from a companion star,
dubbed “Betelbuddy.”
“This paper proposes a companion -- either a star or stellar-like object -- to Betelgeuse,
meaning Betelgeuse is a binary system. We call this alpha Orionis B (Betelgeuse is
properly known as Alpha Orionis), or ‘BetelBuddy’ colloquially,” says Meridith Joyce,
an assistant professor in UW’s Department of Physics and Astronomy. “The companion has never been detected observationally, but we know, from Betelgeuse’s
pulsational behavior, that it must exist.
“Based on orbital constraints, the companion must be below a certain mass, which we
say is about two times the mass of the sun,” she continues. “Based on statistical
arguments, it is most likely that this companion is a young star similar to the sun,
but it is possible that the companion is something more exotic, like a neutron star.
If it were a neutron star, that discovery would be huge.”
Joyce is co-author of a paper titled “A Buddy For Betelgeuse: Binarity As the Origin
of the Long Secondary Periods in An Orionis” that was accepted for publication in
The Astrophysical Journal, an open-access journal devoted to recent developments, discoveries and theories
in astronomy and astrophysics. The paper is expected to be published in the journal
near the end of November.
Jared Goldberg, a Flatiron Research Fellow at the Flatiron Institute’s Center for
Computational Astrophysics, is the paper’s lead author and who named the companion
star. László Molnár, an assistant researcher at Konkoly Observatory, which is part
of the HUN-REN Research Centre for Astronomy and Earth Sciences, located in Budapest,
Hungary, was another author of the paper.
Betelgeuse is a red giant star around 100,000 times the brightness of the sun and
more than 400 million times the sun’s volume. The star is nearing the end of its lifespan.
When Betelgeuse dies, the resulting explosion will be bright enough to see during
the day for weeks, according to the paper.
The research found Betelbuddy acts like a snowplow as it orbits Betelgeuse, pushing
light-blocking dust out of the way and temporarily making Betelgeuse -- the 10th brightest
star in the night sky -- seem brighter.
“There is a great deal of interest in stellar systems in which one or more of the
components is massive, because these systems are possible progenitors to exotic or
high-energy events,” Joyce says. “Betelgeuse is a massive star -- about 16 times the
mass of the sun -- and, while we think it is most likely that the companion is a ‘regular’
young star, it is possible that the companion is a neutron star.
“Betelgeuse is massive enough to undergo a supernova explosion and, when it does,
it will leave behind either a neutron star, which is more likely, or a black hole,
which is less likely at this mass,” she continues. “What would then remain is a neutron
star-neutron star binary system, the mergers of which generate gravitational waves.”
Astronomers can predict when Betelgeuse will die by effectively checking its pulse.
It’s a variable star, meaning it gets brighter and dimmer, pulsing much like a heartbeat.
In the case of Betelgeuse, there are two heartbeats: one that pulses on a timescale
a little longer than a year, and one that pulses on a timescale of about six years.
These heartbeats are deemed important because they are what scientists use to determine
when Betelgeuse will become a supernova. One of these heartbeats is Betelgeuse’s fundamental
mode, or its pattern of brightening and dimming intrinsic to the star itself. If the
star’s fundamental mode is its long-scale heartbeat, then Betelgeuse could be ready
to blow sooner than expected.
However, if its fundamental mode is its short-scale heartbeat, as several studies
suggest, then its longer heartbeat is a phenomenon called a long secondary period
(LSP). This longer brightening and dimming is caused by something external to the
star.
Scientists still don’t have a definitive answer as to what exactly causes LSPs, but
one leading theory is that they arise when a star has a companion that circles it
and barrels through the cosmic dust that is produced and expelled by the star. The
displaced dust alters how much starlight reaches Earth, changing the star’s apparent
brightness.
“Having demonstrated that this long period, known as an LSP, is driven by something
other than pressure variations in Betelgeuse’s outer layers, the question naturally
arises: ‘Then what drives the LSP?’” Joyce says.
During the study, Joyce performed the bulk of the analysis regarding alternative hypotheses
for the LSP signal. She investigated convection, strange modes, gravity modes, magnetism,
rotation, and LSP versus fundamental mode misidentification as possible explanations
for Betelgeuse’s 5.5-year or 2,100-day period. Each of these theories had at least
one fundamental flaw that prevented it from being a viable explanation for the LSP,
leaving binarity as the only possibility, Joyce says.
“Nothing else added up,” Goldberg says. “Basically, if there’s no Betelbuddy, then
that means there’s something way weirder going on; something impossible to explain
with current physics.”
“Without each of us considering this problem from very different angles -- László
as an expert in space-based observations and data analysis; Jared as someone who studies
and simulates massive stars; and myself as a 1D modeler -- the work wouldn’t have
been possible,” Joyce says. “I want to thank the Flatiron Center for Computational
Astrophysics, in particular, for creating an environment in which pulling together
such a diverse range of scientists is possible.”
The research team will attempt to take photos of the Betelbuddy with telescopes around
Dec. 6, a time when there will be a potential window of visibility.
“We need to confirm that Betelbuddy actually exists, since our result is based on
inference, not on direct detection,” Molnár says. “So, we’re working on observation
proposals now.”
The study was funded by grants from a European Commission Widening Fellowship and
the European Union’s Horizon 2020 research and innovation funding program.
