- 3467 - STARS - Wolf-Rayet stars, Red Giants, and supernovae? With more advanced all-sky X-ray surveys on the way, our best bet is by conducting a more comprehensive suite of scientific investigations. That’s the only way we can truly learn what’s out there in the Universe. What will we find next?
----- 3467 - STARS - Wolf-Rayet stars, Red Giants, and supernovae?
- Finding giant stars and learning the ultimate fate of our Sun is using a new tool for discovery. This is a detailed study of the precise temperatures and sizes of 191 giant stars that was just competed. The study began in 1997 when a group of astronomers started making high-precision measurements of giant stars with the Palomar Observatory’s Testbed Interferometer (PTI).
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- The PTI was built as a test bed for the upcoming Keck Interferometer in Hawaii. The PTI closed down in 2008. After that, astronomers used telescopes at the Lowell Observatory to keep collecting more data.
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- Giant stars are different from main-sequence stars or dwarf stars. All of the hydrogen available for fusion in their cores is depleted and they’ve left the main sequence. Compared to a main-sequence star or dwarf star with the same temperature, a giant star will be more luminous and have a larger radius.
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- Giant stars can be between tens and thousands of times more luminous than the Sun and have radii a few hundred times greater than the Sun’s. Stars more luminous than giant stars are called “super giants” and “hyper giants“.
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- The temperature measurements are particularly precise and are two to four times more accurate than previous studies. This means that if you tell me what color a star is or if you tell me what type of star it is, I can tell you its temperature.
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- When astronomers find an exoplanet, nearly everything they can learn about it is in relation to the star it orbits. The mass and luminosity and size of the star are used to infer the characteristics of the planet, like its mass, size, and density. So the more accurate star measurements are, the more accurate planet measurements are.
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- Astronomers know that eventually our Sun will become a “red giant star” and will swell in size, engulfing Mercury and Venus, maybe Earth, too. The amount of the Sun swelling is unclear, with estimates ranging from 10 to 100 times its current size.
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- Another area of astronomy discoveries finds a new kind of Supernova. Normally a big star runs out of fuel, gravity collapses its core and BOOM! But astronomers have long thought at least one type of large star didn’t end with a supernova explosion. Known as “Wolf-Rayet stars“, they were thought to end with a quiet collapse of their core into a blackhole.
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- Wolf-Rayet stars are among the most massive stars known. They are at the end of their short lives, but rather than simply running out of fuel and exploding, they push out their outer layers with an extremely powerful stellar wind. This produces a surrounding nebula rich in ionized helium, carbon, and nitrogen, but almost no hydrogen.
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- The surface temperature of the remaining star can be over 200,000 Kelvin, making them the most luminous stars known. But because most of that light is in the ultraviolet range, they are not bright to the naked eye.
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- Even with the outer layers of a Wolf-Rayet star cast off, the central star is still much more massive than the Sun. It is only a matter of time before it becomes a supernova. No matter how far up the periodic table fusion occurs, it will eventually run out of fuel, leading to a core-collapse supernova.
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- We can see the spectra of elements within a supernova, and we’d never seen a spectrum that matched a Wolf-Rayet star. As our discovery of supernovae became commonplace, some astronomers began to wonder if Wolf-Rayet stars had a quiet death instead, and not supernova.
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- The idea was that the Wolf-Rayet star would cast off enough outer layers that the remaining core would eventually just collapse directly into a blackhole. No giant explosion needed. A silent death to a massive star.
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- At least some Wolf-Rayet stars do become supernovae. The team looked at the spectrum of a supernova “SN 2019hgp“, which was discovered by the Zwicky Transient Facility (ZTF). This supernova’s spectrum had bright emission light indicating the presence of carbon, oxygen, and neon, but not hydrogen or helium.
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- Astronomers found these particular emission lines weren’t caused by elements of the supernova directly. Instead, they were part of a nebula expanding away from the star at more than 1,500 kilometers / second.
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- The progenitor star was surrounded by a nebula rich in carbon, nitrogen, and neon, while lacking the lighter elements of hydrogen and helium. The expansion of the nebula must have been driven by strong stellar winds.
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- In 2018, a breakout supernova was discovered , “AT2018cow“, and was the first in a new class of “super luminous” transient events. AT2020mrf is unique, hundreds of times more luminous than the others. A central engine, like a magnetar or an actively accreting blackhole, is required to power this big of an explosion, which shows unique X-ray features.
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- The most common type of stellar cataclysm is a core-collapse supernova, where a massive star’s interior implodes, leading to a runaway fusion reaction and a tremendous explosion, where the energy emitted by the star can briefly shine billions of times brighter than a typical star.
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- It is the rarer types of stellar cataclysms, superluminous supernovae, hypernovae, tidal disruptions events, and even more exotic explosions, that can shine brighter than anything else we have observed. More than the more common supernova.
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- As they go through their life cycles, stars transform mass into energy through the process of nuclear fusion. By smashing light atomic nuclei together under tremendous pressures and temperatures, they can trigger the formation of heavier atomic nuclei.
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- If you were to put the total masses of the pre-fusion nuclei and the post-fusion nuclei on a scale, you’d find that the ones produced by fusion were slightly less massive than the ones that went into the reaction.
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- Where did that mass go? It gets transformed directly into energy through Einstein’s most famous equation: E = mc^2.
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- When AT2018cow was first seen, it appeared simply as a rapidly brightening, high-temperature event: like a supernova, but with some unusual features to it. Some of those features include:
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----------------------------- The copious detection of iron
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----------------------------- An extremely luminous brightening in ultraviolet wavelengths
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----------------------------- Ten times the intrinsic brightness of a normal supernova
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----------------------------- Brightness across all wavelengths of light, from X-ray down to the radio
----------------------------- Evidence that it was surrounded by very dense material, with a tremendously fast shockwave moving through it
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- The one explanation for the event AT2018cow is a cocooned supernova that undergoes a shock breakout. The long-term nature of the X-rays suggested that a remnant was left behind to power it, eliminating that as a potential explanation.
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- With that realization, the pieces fell into place. If there were a cocoon of gas surrounding a star that was reaching the end of its life, then:
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-------------------- An initial supernova would shock the surrounding cocoon
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-------------------- The material would heat to exceedingly high temperatures
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-------------------- The injected energy would cause a “breakout” event, creating the extreme brightness, the rapid increase in luminosity, and the ultra-fast shock wave
the remnant of the supernova, like a neutron star, would continue to inject energy for long periods of time after the initial explosion
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- This new class of objects are now known not exclusively as “Cow” class objects, but rather as FBOTs: “Fast Blue Optical Transients“. What makes an explosion a Fast Blue Optical Transient? There must be a rapid increase in brightness; that’s the “fast” part.
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- You have to have lots of energy in the ultraviolet portion of the spectrum; that’s the “blue” part.
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- It has to have a large brightness increase in the visible light portion of the spectrum; that’s the “optical” part.
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- And it needs to have a time variation in its overall energy output, where it rises, increases to a maximum, and then decreases and fades away; that’s the “transient” part.
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- The 48-inch Samuel Oschin Telescope at Mt. Palomar is where the Zwicky Transient Facility (ZTF) takes its data from. Even though it’s only a 48″ telescope, its wide field of view and rapid observing speed allows it to discover optical changes in the night sky that practically every other observatory cannot find.
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- The X-ray telescope known as the Spektrum-Roentgen-Gamma (SRG) telescope is unique among all the X-ray observatories we have operating today for numerous reasons, but the most spectacular is that it is the only one that plans on imaging the entire sky numerous times over.
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- The Spektrum-Roentgen-Gamma telescope completed its first full survey of the sky in June, 2020, and quickly embarked on its second sweep, of a planned eight sweeps. The whole point of surveying the sky over and over again is to look for changes, as they signify an astronomical event of interest.
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- In July, 2020, right at the start of that second sweep, something fascinating emerged; an entirely new source of X-ray light, where none had been previously just six months prior, had not only emerged, but was incredibly bright.
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- The original “Cow” event, AT2018cow had a large and significant X-ray brightness for a supernova. It was 20 times brighter in that X-ray light. These events had a substantial but erratic variability in their X-ray brightness, varying rapidly on timescales of less than a day.
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- 35 days before the SRG telescope found the remarkable X-ray brightening, an optical brightening had occurred, just as it had for other FBOT events, including the Cow. It possessed other features:
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---------------------------- A very high temperature of around 20,000 K
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---------------------------- Significant emission features that indicate a very high speed, of around 10% the speed of light (much faster than a normal supernova’s of 2-3% the speed of light)
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--------------------------- A bright set of radio emissions belongs to a very small, low-mass, dwarf galaxy: one with a mass of only 100 million stars, or less than 0.1% the mass of our Milky Way.
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- This event, AT2020mrf, is now the fifth event to meet all the criteria for an FBOT, and somehow all five of them have occurred in dwarf galaxies that are forming new stars.
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- Some 328 days after the explosion began, NASA’s Chandra X-ray telescope pointed its eyes at this object 2 billion light-years away. Within its first six hours of observation, Chandra saw 29 individual X-ray photons coming from this one object: a remarkably large number.
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- Over a second six-hour observation window, it discovered another 10 X-ray photons. Those two observations, taken nearly a year after the initial explosion occurred, indicate a number of remarkable facts:
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- The X-ray flux coming from this object must be absolutely enormous; some 200 times as luminous in X-ray light as AT2018cow was at a comparable time in its evolution.
The X-rays make it the most luminous Cow-like supernova ever seen in the X-ray.
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- It demonstrates the diversity of Fast Blue Optical Transients, while still supporting the cocooned-supernova breakout model of FBOTs.
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- It shows that even a full year after the supposed supernova first occurred, the rapid X-ray variability on a timescale of 1 day or less still remains.
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- The only way the X-ray flux could remain this large this long after a supernova explosion is if it’s powered by a still-active central engine, which might be either an accreting blackhole or an extremely rapidly spinning, highly magnetized neutron star: a millisecond magnetar.
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- When stars are on the path to going supernova, they expel large amounts of material and then, when the core implodes, the injected energy has to propagate through that material, shocking it, rebounding, delaying the arrival of the initial light by hours.
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- But with these FBOTs, or Cow-like events, the central cores of those ripped-apart stars are rapidly exposed with the surrounding debris cleared away. Nobody knows why.
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- They’re only found in star-forming regions around dwarf galaxies and we don’t understand why that’s the case. And even though AT2020mrf looks very similar to the original Cow, AT2018cow, in optical wavelengths, it’s scores to hundreds of times intrinsically brighter in the X-rays.
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- With more advanced all-sky X-ray surveys on the way, our best bet is by conducting a more comprehensive suite of scientific investigations. That’s the only way we can truly learn what’s out there in the Universe. What will we find next?
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February 18, 2022 STARS - Wolf-Rayet stars, Red Giants, and supernovae? 3467
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----------------------------- Monday, February 21, 2022 ---------------------------
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