- 4141 - SUPERNOVA - how exploding stars evolve? The Pinwheel Galaxy, Messier 101 was observed on May 21, 2023, just four days after the light from the supernova “2023ixf “. It was the nearest supernova since 2014, 21 million light years from Earth.
-------------- 4141 - SUPERNOVA - how exploding stars evolve?
- These observations of the Pinwheel Galaxy
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 ejecting some of the material.
So when the supernova explodes, either the shock wave or the ultraviolet
radiation causes the stuff to glow.
Astronomers can then get some indication of the shape and extent of the
circumstellar material.
-
- This 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.
-
- The explosion was 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.
-
- 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.
-
- 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.
-
- Lick Observatory's telescopes on top of
Mount Hamilton near San Jose were critical to the astronomers' efforts to
assemble a complete picture of the supernova. The Kast spectrograph on the
Shane 120-inch telescope is able to switch quickly from a normal spectrometer
to a spectropolarimeter, which allowed
measurements of both the spectrum and its polarization.
-
- 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 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.
-
- 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. 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,
so-called shock ionization.
-
- 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.
-
- 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.
-
- 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.
-
-
Septemebr 5, 2023 SUPERNOVA - how
exploding stars evolve? 4141
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