- 4150 - SUPERNOVA - turns on in 2023? Supernova explodes. Closest supernova in a decade reveals how exploding stars evolve. The Pinwheel Galaxy, or Messier 101, on May 21, 2023, sees supernova four days after the light from the supernova 2023ixf reached Earth.
-------------- 4150 - SUPERNOVA - turns on in 2023?
- Astronomers were eager to observe the
nearest supernova since 2014, a mere 21 million light years from Earth.
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- These observations were the earliest-ever
measurements of polarized light from a supernova, showing more clearly the
evolving shape of a stellar explosion. The polarization of light from distant
sources like supernovae provides the best information on the geometry of the
object emitting the light, even for events that cannot be spatially resolved.
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- Some stars prior to exploding go through
undulations, behavior that gently ejects some of the material, so that when the supernova explodes, either
the shock wave or the ultraviolet radiation causes the stuff to glow.
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- The spectropolarimetry data told a story in
line with current scenarios for the final years of a red supergiant star about
10 to 20 times more massive than our sun.
Energy from the explosion lit up clouds of gas that the star shed over
the previous few years; the ejecta then punched through this gas, initially
perpendicular to the bulk of the circumstellar material; and finally, the
ejecta engulfed the surrounding gas and evolved into a rapidly expanding but
symmetric cloud of debris.
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- The explosion, a “Type II supernova”
resulting from the collapse of the iron core of a massive star, presumably left
behind a dense neutron star or a black hole. Such supernovae are used as
calibratable candles to measure the distances to distant galaxies and map the
cosmos.
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- Astronomers analyzed the data to
reconstruct the pre- and post-explosion history of the star, and found evidence
that it had shed gas for the previous three to six years before collapsing and
exploding. The amount of gas shed or ejected before the explosion could have
been 5% of its total mass, enough to create a dense cloud of material through
which the supernova ejecta had to plow.
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- The improved understanding of how Type II
supernovae evolve could help refine their use as distance measures in the
expanding universe. In the world of
Type II supernovae, it's very rare to have basically every wavelength detected,
from hard X-rays to soft X-rays to ultraviolet. to optical, near-infrared,
radio, millimeter.
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- The big-picture question here is we want to
connect how a star lives with how a star dies.
Then figure out how to fit it all together to understand this particular
object. The polarization of light
emitted by an object, the orientation of the electric field of the
electromagnetic wave, carries information about the shape of the object. Light
from a spherically symmetric cloud, for example, would be unpolarized because
the electric fields symmetrically cancel. Light from an elongated object,
however, would produce a nonzero polarization.
-
- While polarimetry measurements of
supernovae have been going on for more than three decades, few are close
enough, and thus bright enough, for such measurements. And no other supernova
has been observed as early as 1.4 days after the explosion, as with SN 2023ixf.
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-The most
exciting thing is that this supernova shows a very high continuum polarization,
nearly 1%, at early times. That sounds
like a small number, but it's actually a huge deviation from spherical
symmetry. Based on the changing
intensity and direction of polarization, the researchers were able to identify
three distinct phases in the evolution of the exploding star.
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- Between one and three days after the
explosion, the light was dominated by emission from the circumstellar medium,
perhaps a disk of material or lopsided blob of gas shed earlier by the star.
This was due to ionization of the surrounding gas by ultraviolet and X-ray
light from the explosion and by stellar material plowing through the gas,
called shock ionization.
-
- The light that we're seeing is from some
kind of non-spherical circumstellar medium that is confined to somewhere around
30 A.U. An astronomical unit (AU), the
average distance between Earth and our sun, is 93 million miles.
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- At 3.5 days, the polarization quickly
dropped by half, and then a day later shifted by nearly 70 degrees, implying an
abrupt change in the geometry of the explosion. They interpret this moment, 4.6
days after explosion, as the time when the ejecta from the exploding star broke
out from the dense circumstellar material.
-
- Essentially, it engulfs the circumstellar
material, and you get this peanut-shaped geometry. The intuition there is that the material in
the equatorial plane is denser, and the ejecta get slowed down, and the path of
least resistance will be toward the axis where there's less circumstellar
material. That's why you get this peanut shape aligned with the preferential
axis through which it explodes.
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- The polarization remained unchanged between
days 5 and 14 after the explosion, implying that the expanding ejecta had
overwhelmed the densest region of surrounding gas, allowing emission from the
ejecta to dominate over light from shock ionization.
- The spectroscopic evolution saw emissions
from the gas surrounding the star about a day after the explosion, likely
produced as the ejecta slammed into the circumstellar medium and produced
ionizing radiation that caused the surrounding gas to emit light.
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- Spectroscopic measurements of the light
from this shock ionization showed emission lines from hydrogen, helium, carbon
and nitrogen, which is typical of core-collapse supernovae.
-
- The emissions produced by shock ionization
continued for about eight days, after which it decreased, indicating that the
shock wave had moved into a less dense area of space with little gas to ionize
and reemit.
-
- Astronomers have looked at archival images
of the Pinwheel Galaxy and found several occasions when the progenitor star
brightened in the years before the explosion, suggesting that the red
supergiant repeatedly sloughed off gas. This is consistent with observations of
ejecta from the explosion plowing through this gas, estimating a density about
1,000 times less than implied by the pre-explosion undulations.
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- This is a very special situation where we
know what the progenitor was doing before because we saw it slowly oscillating,
and we have all the probes in place to try to reconstruct the geometry of the
circumstellar medium. We know for a fact
that it cannot be spherical. By putting together the radiant X-ray we will be
able to have a complete picture of the explosion.
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-
September 12,
2023 SUPERNOVA
- turns on in 2023? 4150
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