A curiously yellow pre-supernova star has caused astrophysicists to re-evaluate what's possible at the deaths of our Universe's most massive stars. The team describe the peculiar star and its resulting supernova in a new study published in Monthly Notices of the Royal Astronomical Society.
At the end of their lives, cool, yellow stars are typically shrouded in
hydrogen, which conceals the star's hot, blue interior. But this yellow star,
located 35 million light years from Earth in the Virgo galaxy cluster, was
mysteriously lacking this crucial hydrogen layer at the time of its explosion.
"We haven't seen this scenario before," said Charles
Kilpatrick, postdoctoral fellow at Northwestern University's Center for
Interdisciplinary Exploration and Research in Astrophysics (CIERA), who led the
study. "If a star explodes without hydrogen, it should be extremely blue
-- really, really hot. It's almost impossible for a star to be this cool
without having hydrogen in its outer layer. We looked at every single stellar
model that could explain a star like this, and every single model requires that
the star had hydrogen, which, from its supernova, we know it did not. It
stretches what's physically possible."
Kilpatrick is also a member of the Young Supernova Experiment, which
uses the Pan-STARRS telescope at Haleakala, Hawaii to catch supernovae right
after they explode. After the Young Supernova Experiment spotted supernova
2019yvr in the relatively nearby spiral galaxy NGC 4666, the team used deep
space images captured by NASA's Hubble Space Telescope, which fortunately
already observed this section of the sky two and a half years before the star
exploded.
"What massive stars do right before they explode is a big unsolved
mystery," Kilpatrick said. "It's rare to see this kind of star right
before it explodes into a supernova."
Hubble Space Telescope (HST) imaging showing the explosion site of 2019yvr from 2.5 years before its explosion. Upper left: the supernova itself is seen in an image from the Gemini-South telescope 72 days after it exploded. Lower left: a zoom in to the same site in the pre-explosion HST image, showing a single source that appears to be the progenitor star of 2019yvr [Credit: Charles Kilpatrick/Northwestern University] |
The Hubble images show the source of the supernova, a massive star
imaged just a couple of years before the explosion. Several months after the explosion
however, Kilpatrick and his team discovered that the material ejected in the
star's final explosion seemed to collide with a large mass of hydrogen. This
led the team to hypothesize that the progenitor star might have expelled the
hydrogen within a few years before its death.
"Astronomers have suspected that stars undergo violent eruptions
or death throes in the years before we see supernovae," Kilpatrick said.
"This star's discovery provides some of the most direct evidence ever
found that stars experience catastrophic eruptions, which cause them to lose
mass before an explosion. If the star was having these eruptions, then it
likely expelled its hydrogen several decades before it exploded."
In the new study, Kilpatrick's team also presents another possibility:
a less massive companion star might have stripped away hydrogen from the
supernova's progenitor star. However, the team will not be able to search for
the companion star until after the supernova's brightness fades, which could
take up to a decade.
"Unlike its normal behaviour right after it exploded, the hydrogen
interaction revealed it's kind of this oddball supernova," Kilpatrick
said. "But it's exceptional that we were able to find its progenitor star
in Hubble data. In four or five years, I think we will be able to learn more
about what happened."
Source: Royal Astronomical Society
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