Astronomy

What are the possible solutions to the Red Supergiant problem?

What are the possible solutions to the Red Supergiant problem?


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I have recently come across this so called "Red Supergiant problem" in the literature, a phrase that was coined by Stephen Smartt in 2009 in reference to why red supergiants with masses ∼16-30M⊙ have not been identified as progenitors of Type IIP supernovae.

As I understand it astronomers have observed red supergiants with masses between 8-25M⊙ but only those below ∼16M⊙ have been proven to have undergone a Type IIP supernovae. With the minimum mass required to produce a Wolf-Rayet star at least 25-30M⊙ these red supergiants arent massive enough to be evolving into WR stars either.

There could be systematic errors with the models or calculations and it is well known that calculating the mass of a supernova progenitor post-eruption can be quite a difficult task, but assuming these calculations are correct it leaves a big hole in our understanding of stellar evolution. So my question is, what can explain this gap?


Solutions to the Red Supergiant Problem can be either observational or physical, and to date, both types have been proposed. Recent data and improved computer modeling have both helped make the later stages of red supergiant evolution clearer, leading to possible solutions to the problem.

Walmswell & Eldridge (2012) suggested an observational solution, namely, that circumstellar dust resulting from mass loss via the strong stellar winds of a red supergiant could lead to strong extinction in some cases, making it hard or impossible to detect the resulting supernovae. Based on data collected on 18 supernovae, they found an upper limit for the mass of a Type IIP supernova progenitor (at 90% confidence) to be around $27 M_{odot}$, which would explain the dearth of detections of progenitors in the $16$-$30 M_{odot}$ range.

The observational difficulties are moot if there is a physical reason behind the problem, i.e. if there is some mechanism preventing these red supergiants from undergoing Type IIP supernovae. Yoon & Cantiello (2010) suggested that red supergiant stellar winds could be increased to "superwinds" by envelope pulsations. This would lead to periods of larger mass loss rates than expected, followed by periods of smaller mass loss rates. This would correspond to oscillating around states of instability due to the pulsations, and would be prominent only in red supergiants of masses greater than or equal to about $19M_{odot}$. The result could be drastic, with stars losing well over half their masses. The star could then evolve toward a Type Ib or Type IIb supernova.

Smartt et al. did consider the possibility of envelope loss in the paper you cited, but dismissed it as being unfeasible for a star in the mass range. However, it appears that they did not consider the superwinds induced by pulsations, making this solution feasible once more. It's also worth mentioning that the authors suggested direct collapse to black holes with at the most a very faint supernova; this idea had been explored in the past.


Mysterious hot spots observed in a cool red supergiant

Comparison between the red supergiant Antares and the Sun, shown as the tiny dot toward the upper right. The black circle is the size of the orbit of Mars. Arcturus is also included in the picture for size comparison. Credit: Wikipedia.

(Phys.org) —Astronomers have released a new image of the outer atmosphere of Betelgeuse – one of the nearest red supergiants to Earth – revealing the detailed structure of the matter being thrown off the star.

The new image, taken by the e-MERLIN radio telescope array operated from the Jodrell Bank Observatory in Cheshire, also shows regions of surprisingly hot gas in the star's outer atmosphere and a cooler arc of gas weighing almost as much as the Earth.

Betelgeuse is easily visible to the unaided eye as the bright, red star on the shoulder of Orion the Hunter. The star itself is huge – 1,000 times larger than our Sun – but at a distance of about 650 light years it still appears as a tiny dot in the sky, so special techniques combining telescopes in arrays are required to see details of the star and the region around it.

The new e-MERLIN image of Betelgeuse – published in the journal Monthly Notices of the Royal Astronomical Society, shows its atmosphere extends out to five times the size of the visual surface of the star. It reveals two hot spots within the outer atmosphere and a faint arc of cool gas even farther out beyond the radio surface of the star.

The hot spots are separated by roughly half the visual diameter of the star and have a temperature of about 4,000-5,000 Kelvin, much higher than the average temperature of the radio surface of the star (about 1,200 Kelvin) and even higher than the visual surface (3,600 Kelvin). The arc of cool gas lies almost 7.4 billion kilometres away from the star – about the same distance as the farthest Pluto gets from the Sun. It is estimated to have a mass almost two thirds that of the Earth and a temperature of about 150 Kelvin.

Lead author Dr Anita Richards, from The University of Manchester, said that it was not yet clear why the hot spots are so hot. She said: "One possibility is that shock waves, caused either by the star pulsating or by convection in its outer layers, are compressing and heating the gas. Another is that the outer atmosphere is patchy and we are seeing through to hotter regions within. The arc of cool gas is thought to be the result of a period of increased mass loss from the star at some point in the last century but its relationship to structures like the hot spots, which lie much closer in, within the star's outer atmosphere, is unknown."

The mechanism by which supergiant stars like Betelgeuse lose matter into space is not well understood despite its key role in the lifecycle of matter, enriching the interstellar material from which future stars and planets will form. Detailed high-resolution studies of the regions around massive stars like the ones presented here are essential to improving our understanding.

Dr Richards, who is based in Manchester's School of Physics and Astronomy, added: "Betelgeuse produces a wind equivalent to losing the mass of the Earth every three years, enriched with the chemicals that will go into the next generation of star and planet formation. The full detail of how these cool, evolved stars launch their winds is one of the remaining big questions in stellar astronomy.

"This is the first direct image showing hot spots so far from the centre of the star. We are continuing radio and microwave observations to help decide which mechanisms are most important in driving the stellar wind and producing these hot spots. This won't just tell us how the elements that form the building blocks of life are being returned to space, it will also help determine how long it is before Betelgeuse explodes as a supernova."

Future observations planned with e-MERLIN and other arrays, including ALMA and VLA, will test whether the hotspots vary in concert due to pulsation, or show more complex variability due to convection. If it is possible to measure a rotation speed this will identify in which layer of the star they originate.


Pathways to Astronomy (5th Edition) Edit edition Solutions for Chapter 67 Problem 17QP: Betelgeuse is a red supergiant about 640 ly from Earth. Astronomers expect that within the next million years Betelgeuse will explode as a Type II supernova. Use Figure 67.4 to estimate how bright Betelgeuse will appear. Will it outshine the full Moon, which has a brightness of 3×10−3 watts/m2?FIGURE 67.4 Some examples of the light curves of supernovae of stars undergoing core collapse. The dashed red line shows the light curve of a Type Ia supernova for comparison. …

Betelgeuse is a red supergiant about 640 ly from Earth. Astronomers expect that within the next million years Betelgeuse will explode as a Type II supernova. Use Figure 67.4 to estimate how bright Betelgeuse will appear. Will it outshine the full Moon, which has a brightness of 3×10 −3 watts/m 2 ?

FIGURE 67.4 Some examples of the light curves of supernovae of stars undergoing core collapse. The dashed red line shows the light curve of a Type Ia supernova for comparison.


Scientists take the temperature of red supergiants

March 1 (UPI) -- Astronomers have developed a new technique for accurately measuring the surface temperature of red supergiants, voluminous stars that end their lives in supernova explosions.

Though the end stage of a red supergiant's life is well-documented, the lifecycle stages that proceed it aren't well understood -- part of the problem is that astronomers can't easily measure the surface temperatures of red supergiants.

As their name implies, red supergiants are very large stars. The average red supergiant boasts a mass nine times that of the sun. Though not the most massive or luminous in the cosmos, red supergiants are the most voluminous class of stars.

When their cores collapse at the end of their lives, red supergiants produce a tremendous explosion called a Type-II supernovae, which telescopes can spot from millions of light-years away.

When massive stars like red supergiants explode, their shredded stellar materials are flung into interstellar space. These supernova explosions seeded space with some of the elements that made life on Earth possible.

To study, model and predict how red supergiants arrive at the end of their lifecycle, astronomers need to measure the temperatures of these hyper giant stars.

Unfortunately, methods used to measure the temperatures of other types of stars yield inconsistent results for red supergiants, which are characterized by complex upper atmospheric structures.

"In order to measure the temperature of red supergiants, we needed to find a visible, or spectral, property that was not affected by their complex upper atmospheres," lead study author Daisuke Taniguchi said in a news release.

"Chemical signatures known as absorption lines were the ideal candidates, but there was no single line that revealed the temperature alone. However, by looking at the ratio of two different but related lines -- those of iron -- we found the ratio itself related to temperature. And it did so in a consistent and predictable way," said Taniguchi, an astronomy graduate student at the University of Tokyo.

For the study, Taniguchi and his research partners used an instrument called WINERED to capture the spectral properties of several red supergiants.

Using the spectral data, the astronomers were able to calculate the iron absorption lines and estimate the temperatures of the target stars.

Using precise distance measurements calculated by European Space Agency's Gaia space observatory, researchers established a consistent relationship between the distance, luminosity and temperature of red supergiants.

"We still have much to learn about supernovae and related objects and phenomena, but I think this research will help astronomers fill in some of the blanks," said Taniguchi.

"The giant star Betelgeuse -- on Orion's shoulder -- could go supernova in our lifetimes in 2019 and 2020 it dimmed unexpectedly. It would be fascinating if we were able to predict if and when it might go supernova. I hope our new technique contributes to this endeavor and more."


Why the Red Supergiant Star Betelgeuse Went Dark

Betelgeuse, in reference to the study, appears to have lost a lot of mass months before the Great Dimming occurred. During the pre-darkening of the star, it released a great volume of gas bubbles. This was because of the cold patch located in the southern hemisphere of the red giant. The drop of the star's temperature allows it to emit gas that transforms into dust. The process explains why the star is hardly visible from our perspective, clouding our view with a thick, dark cloud.

European Southern Observatory's Very Large Telescope (VLT) supported the study via the data gathered from the rare phenomenon. It provided details from the supergiant's surface for experts to track the changes in Betelgeuse's luminosity.

Compared to the Hubble Space Telescope, astronomers found the SPHERE instrument in the VLT to be more effective in calculating the structure and activities of the star. They captured a high-resolution view of the dust clouds hovering on the face of the red giant.

The records of the Great Dimming from 2019 and 2020 were compared, and it showed that the brightness of Betelgeuse is back to normal with the dark dust clouds already gone. Many studies will benefit from the activity exhibited by Betelgeuse.

Further research regarding the Great Dimming will help experts understand the burp theory, in which the dust emitted from the star could potentially form chunks of asteroids, planets, and possibly, life. The event will also add supporting facts to the mechanisms behind the supergiant's mass loss and, ultimately, predicting the future of the biggest stars in the universe, reports Gizmodo.

Check out more news and information on Space on Science Times.


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Scientists take the temperature of red supergiants

March 1 (UPI) &mdash Astronomers have developed a new technique for accurately measuring the surface temperature of red supergiants, voluminous stars that end their lives in supernova explosions.

Researchers described the technique in a new paper, forthcoming in the journal Monthly Notices of the Royal Astronomical Society.

Though the end stage of a red supergiant&rsquos life is well-documented, the lifecycle stages that proceed it aren&rsquot well understood &mdash part of the problem is that astronomers can&rsquot easily measure the surface temperatures of red supergiants.

As their name implies, red supergiants are very large stars. The average red supergiant boasts a mass nine times that of the sun. Though not the most massive or luminous in the cosmos, red supergiants are the most voluminous class of stars.

When their cores collapse at the end of their lives, red supergiants produce a tremendous explosion called a Type-II supernovae, which telescopes can spot from millions of light-years away.

When massive stars like red supergiants explode, their shredded stellar materials are flung into interstellar space. These supernova explosions seeded space with some of the elements that made life on Earth possible.

To study, model and predict how red supergiants arrive at the end of their lifecycle, astronomers need to measure the temperatures of these hyper giant stars.

Unfortunately, methods used to measure the temperatures of other types of stars yield inconsistent results for red supergiants, which are characterized by complex upper atmospheric structures.

&ldquoIn order to measure the temperature of red supergiants, we needed to find a visible, or spectral, property that was not affected by their complex upper atmospheres,&rdquo lead study author Daisuke Taniguchi said in a news release.

&ldquoChemical signatures known as absorption lines were the ideal candidates, but there was no single line that revealed the temperature alone. However, by looking at the ratio of two different but related lines &mdash those of iron &mdash we found the ratio itself related to temperature. And it did so in a consistent and predictable way,&rdquo said Taniguchi, an astronomy graduate student at the University of Tokyo.

For the study, Taniguchi and his research partners used an instrument called WINERED to capture the spectral properties of several red supergiants.

Using the spectral data, the astronomers were able to calculate the iron absorption lines and estimate the temperatures of the target stars.

Using precise distance measurements calculated by European Space Agency&rsquos Gaia space observatory, researchers established a consistent relationship between the distance, luminosity and temperature of red supergiants.

&ldquoWe still have much to learn about supernovae and related objects and phenomena, but I think this research will help astronomers fill in some of the blanks,&rdquo said Taniguchi.

&ldquoThe giant star Betelgeuse &mdash on Orion&rsquos shoulder &mdash could go supernova in our lifetimes in 2019 and 2020 it dimmed unexpectedly. It would be fascinating if we were able to predict if and when it might go supernova. I hope our new technique contributes to this endeavor and more.&rdquo


Supergiant Atmosphere of Antares Revealed by Radio Telescopes

An international team of astronomers has created the most detailed map yet of the atmosphere of the red supergiant star Antares. The unprecedented sensitivity and resolution of both the Atacama Large Millimeter/submillimeter Array (ALMA) and the National Science Foundation’s Karl G. Jansky Very Large Array (VLA) revealed the size and temperature of Antares’ atmosphere from just above the star’s surface, throughout its chromosphere, and all the way out to the wind region.

Red supergiant stars, like Antares and its more well-known cousin Betelgeuse, are huge, relatively cold stars at the end of their lifetime. They are on their way to run out of fuel, collapse, and become supernovae. Through their vast stellar winds, they launch heavy elements into space, thereby playing an important role in providing the essential building blocks for life in the universe. But it is a mystery how these enormous winds are launched. A detailed study of the atmosphere of Antares, the closest supergiant star to Earth, provides a crucial step towards an answer.

The ALMA and VLA map of Antares is the most detailed radio map yet of any star, other than the Sun. ALMA observed Antares close to its surface (its optical photosphere) in shorter wavelengths, and the longer wavelengths observed by the VLA revealed the star’s atmosphere further out. As seen in visible light, Antares’ diameter is approximately 700 times larger than the Sun. But when ALMA and the VLA revealed its atmosphere in radio light, the supergiant turned out to be even more gigantic.

“The size of a star can vary dramatically depending on what wavelength of light it is observed with,” explained Eamon O’Gorman of the Dublin Institute for Advanced Studies in Ireland and lead author of the study published in the June 16 edition of the journal Astronomy & Astrophysics. “The longer wavelengths of the VLA revealed the supergiant’s atmosphere out to nearly 12 times the star’s radius.”

The radio telescopes measured the temperature of most of the gas and plasma in Antares’ atmosphere. Most noticeable was the temperature in the chromosphere. This is the region above the star’s surface that is heated up by magnetic fields and shock waves created by the vigorous roiling convection at the stellar surface – much like the bubbling motion in a pot of boiling water. Not much is known about chromospheres, and this is the first time that this region has been detected in radio waves.

Thanks to ALMA and the VLA, the scientists discovered that the star’s chromosphere extends out to 2.5 times the star’s radius (our Sun’s chromosphere is only 1/200th of its radius). They also found that the temperature of the chromosphere is lower than previous optical and ultraviolet observations have suggested. The temperature peaks at 3,500 degrees Celsius (6,400 degrees Fahrenheit), after which it gradually decreases. As a comparison, the Sun’s chromosphere reaches temperatures of almost 20,000 degrees Celsius.

“We found that the chromosphere is ‘lukewarm’ rather than hot, in stellar temperatures,” said O’Gorman. “The difference can be explained because our radio measurements are a sensitive thermometer for most of the gas and plasma in the star’s atmosphere, whereas past optical and ultraviolet observations were only sensitive to very hot gas and plasma.”

“We think that red supergiant stars, such as Antares and Betelgeuse, have an inhomogeneous atmosphere,” said co-author Keiichi Ohnaka of the Universidad Católica del Norte in Chile who previously observed Antares’ atmosphere in infrared light. “Imagine that their atmospheres are a painting made out of many dots of different colors, representing different temperatures. Most of the painting contains dots of the lukewarm gas that radio telescopes can see, but there are also cold dots that only infrared telescopes can see, and hot dots that UV telescopes see. At the moment we can’t observe these dots individually, but we want to try that in future studies.”

In the ALMA and VLA data, astronomers for the first time saw a clear distinction between the chromosphere and the region where winds start to form. In the VLA image, a huge wind is visible, ejected from Antares and lit up by its smaller but hotter companion star Antares B.

“When I was a student, I dreamt of having data like this,” said co-author Graham Harper of the University of Colorado, Boulder. “Knowing the actual sizes and temperatures of the atmospheric zones gives us a clue of how these huge winds start to form and how much mass is being ejected.”

“Our innate understanding of the night sky is that stars are just points of light. The fact we can map the atmospheres of these supergiant stars in detail, is a true testament to technological advances in interferometry. These tour de force observations bring the universe close, right into our own backyard,” said Chris Carilli of the National Radio Astronomy Observatory, who was involved in the first observations of Betelgeuse at multiple radio wavelengths with the VLA in 1998.

The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.

Media contact:
Iris Nijman
NRAO News and Public Information Manager
[email protected]

This research was presented in a paper titled “ALMA and VLA reveal the lukewarm chromospheres of the nearby red supergiants Antares and Betelgeuse,” by E. O’Gorman et al., appearing in the journal Astronomy & Astrophysics. www.aanda.org/10.1051/0004-6361/202037756

The Atacama Large Millimeter/submillimeter Array (ALMA), an international astronomy facility, is a partnership of the European Organisation for Astronomical Research in the Southern Hemisphere (ESO), the U.S. National Science Foundation (NSF) and the National Institutes of Natural Sciences (NINS) of Japan in cooperation with the Republic of Chile. ALMA is funded by ESO on behalf of its Member States, by NSF in cooperation with the National Research Council of Canada (NRC) and the Ministry of Science and Technology (MOST) and by NINS in cooperation with the Academia Sinica (AS) in Taiwan and the Korea Astronomy and Space Science Institute (KASI).

ALMA construction and operations are led by ESO on behalf of its Member States by the National Radio Astronomy Observatory (NRAO), managed by Associated Universities, Inc. (AUI), on behalf of North America and by the National Astronomical Observatory of Japan (NAOJ) on behalf of East Asia. The Joint ALMA Observatory (JAO) provides the unified leadership and management of the construction, commissioning and operation of ALMA.


Astronomers Reveal Best View Yet Of Red Supergiant Star Antares' Atmosphere

Antares, the brightest star in the constellation Scorpio, is a red supergiant about 550 light-years from Earth. It is 700 times larger than our Sun in visible light and 12 times more massive, and we now have the best map of its atmosphere. In fact, it's the best radio map of any star other than the Sun.

Red supergiants are huge cold stars at the end of their life, spewing out heavy elements into space via vast stellar winds. It's unknown how these huge winds are launched so astronomers looked at Antares, the closest supergiant star to Earth.

The exceptional achievement is reported in Astronomy & Astrophysics, and was possible thanks to the combined use of two world-class observatories. The National Science Foundation’s Karl G. Jansky Very Large Array (VLA) looked at the external part of Antares' atmosphere, even highlighting the stellar wind released by this gigantic star. The Atacama Large Millimeter/submillimeter Array (ALMA) in Chile went in closer, studying the atmosphere just around its surface.

Radio images of Antares. ALMA observed Antares close to its surface in shorter wavelengths, and the longer wavelengths observed by the VLA revealed the star’s atmosphere further out. In the VLA image a huge wind is visible on the right, ejected from Antares and lit up by its smaller but hotter companion star Antares B. ALMA (ESO/NAOJ/NRAO), E. O’Gorman NRAO/AUI/NSF, S. Dagnello

The observations revealed how large the atmosphere actually is. The region known as the chromosphere, which was targeted by ALMA, has a radius 2.5 times of Antares and is cooler than previous studies had suggested, with a temperature around 3,500°C (6,300°F). The VLA observations show how far out the outer layers of this star stretch.

“The size of a star can vary dramatically depending on what wavelength of light it is observed with,” lead author Eamon O’Gorman of the Dublin Institute for Advanced Studies explained in a statement. “The longer wavelengths of the VLA revealed the supergiant’s atmosphere out to nearly 12 times the star’s radius.”

Artist impression of the atmosphere of Antares. As seen with the naked eye (up until the photosphere), Antares is around 700 times larger than our sun, big enough to fill the solar system beyond the orbit of Mars (Solar System scale shown for comparison). But ALMA and VLA showed that its atmosphere, including the lower and upper chromosphere and wind zones, reaches out 12 times farther than that. NRAO/AUI/NSF, S. Dagnello

Previous observations of the star were conducted in UV and visible light. That approach tends to pick up the hottest bits of plasma in and around a star. Radio and submillimeter instruments are sensitive to cooler material and for this reason, they have been able to see the atmosphere extending further out.

“We think that red supergiant stars, such as Antares and Betelgeuse, have an inhomogeneous atmosphere,” said co-author Keiichi Ohnaka of the Universidad Católica del Norte in Chile, who previously observed Antares’ atmosphere in infrared light. “Imagine that their atmospheres are a painting made out of many dots of different colors, representing different temperatures. Most of the painting contains dots of the lukewarm gas that radio telescopes can see, but there are also cold dots that only infrared telescopes can see, and hot dots that UV telescopes see. At the moment we can’t observe these dots individually, but we want to try that in future studies.”

The work also shows a clear distinction between the chromosphere and the region where the stellar wind is accelerated. The VLA observation also caught some of that wind being illuminated by Antares B, which has a mass about 7 times of the Sun and 5 times its radius.


Hubble offers explanation for dramatic dimming of Betelgeuse

An artist’s impression of an eruption on on the surface of the red supergiant Betelgeuse. As the hot, dense material moved away, it cooled and formed a vast dust cloud that would have partially obscured the giant star, blocking light and causing it

In October 2019, the red supergiant Betelgeuse, the brilliant red star in Orion’s left shoulder, suddenly started dimming. By February 2020, the giant star had dimmed to less than a third of its normal brightness, prompting widespread astronomical interest and internet speculation on the possible cause.

Now, using ultraviolet observations by the Hubble Space Telescope, researchers say the dimming likely was caused by an enormous eruption that ejected a vast cloud of superheated material that then cooled, forming a dust cloud that blocked out light from about a quarter of of Betelgeuse’s surface.

“With Hubble, we see the material as it left the star’s visible surface and moved out through the atmosphere, before the dust formed that caused the star to appear to dim,” said Andrea Dupree, associate director of the Harvard-Smithsonian Center for Astrophysics. “We could see the effect of a dense, hot region in the southeast part of the star moving outward.”

“This material was two to four times more luminous than the star’s normal brightness,” she continued. “And then, about a month later, the southern hemisphere of Betelgeuse dimmed conspicuously as the star grew fainter. We think it is possible that a dark cloud resulted from the outflow that Hubble detected. Only Hubble gives us this evidence of what led up to the dimming.”

Betelgeuse is a massive supergiant nearing the end of its life, so large that its surface would extend beyond the orbit of Jupiter if it were at the center of Earth’s solar system. It naturally pulses, brightens and dims, in a 240-day cycle.

Another visualisation of Betelgeuse in the aftermath of an eruption of hot gas that resulted in a huge dust cloud blocking light from the giant star as viewed from Earth. This image is based on observations by the European Southern Observatory’s Very Large Telescope and the SPHERE instrument. Image: ESO, ESA/Hubble, M. Kornmesser

When it eventually finally runs out of nuclear fuel, the giant star will no longer be able to counteract the inward crush of gravity and the core will collapse, triggering a supernova explosion.

Dupree’s team began monitoring Betelgeuse early last year as part of a three-year study to learn more about variations in the star’s outer atmosphere. Spectra measuring strongly ionised magnesium indicated super-heated material moving at some 200,000 miles per hour passing from the star’s surface into the outer atmosphere.

After moving millions of miles away, the material cooled enough to form dust, effectively blocking out starlight as viewed from Earth.

Dupree said she does not know what triggered the eruption, although it may be related to the star’s pulsation cycle. Some astronomers have suggested the sudden dimming could be a pre-supernova event.

“No one knows what a star does right before it goes supernova, because it’s never been observed,” Dupree said. “Astronomers have sampled stars maybe a year ahead of them going supernova, but not within days or weeks before it happened. But the chance of the star going supernova anytime soon is pretty small.”