Friday, September 17, 2021

Dense Molecular Clouds in the Center of Milky Way are Unable to Form Stars

Composite image of Hubble Space Telescope (red) mapping hot ionized gas, and ALMA (blue and purple) detail of the much colder molecular gas of Sgr A* circumnuclear disk. Credit: Dong, H. et al 2011 – ESA/Hubble | Hsieh, P.-Y. et al. – N. Lira – ALMA (EOS/NAOJ/NRAO)

Composite animation of Hubble Space Telescope (red) mapping hot ionized gas, and ALMA (blue and purple) detail of the much colder molecular gas of Sgr A* circumnuclear disk. Credit:Dong, H. et al 2011 – ESA/Hubble | Hsieh, P.-Y. et al. – N. Lira – ALMA (EOS/NAOJ/NRAO)

Clumps of molecular gas overlaid on Sgr A* circumnuclear disk as seen by ALMA in the CS(7-6) line. Yellow circles are thin clumps that are going to be shredded by the gravitational force of supermassive black hole Sgr A*. Green circles are dense enough to survive the tidal shredding but are not able to form stars. Purple/pink circles have the needed density to form stars, but no star formation has been observed. Credit: Hsieh, P.-Y. et al. – ALMA (EOS/NAOJ/NRAO)

New observations with the Atacama Large Millimeter/submillimeter Array (ALMA) allowed astronomers to map in exquisite detail the ring of dense molecular gas rotating around the supermassive black hole in Sgr A* at the center of our galaxy. In that ring, also known as a circumnuclear disk, they found thousands of dense gas clumps but, surprisingly, no active star formation: either the tidal stress of the black hole or some other mechanism prevents the clumps from collapsing into new stars.

Every large galaxy has a central supermassive black hole that dominates and is fed by nearby molecular gas. In many galaxies, there are also bright nuclear star clusters. Since molecular gas is the material that supplies black holes and forms stars, the research team wanted to know how much gas is available to form stars and how much is going to feed the supermassive black hole. Sgr A* is the closest supermassive black hole to us. The first challenge of star formation in the vicinity of the Galactic Center is avoiding the high tidal shear that can easily tear apart the nearby molecular clouds, preventing them from accumulating enough mass for fragmentation and core-collapse to proceed.

“The circumnuclear disk can be imagined as a factory of many doughs rotating around the supermassive black hole,” explains Pei-Ying Hsieh, principal investigator of this study and fellow astronomer at ALMA. “If the dough is too thin, it will be stretched like spaghetti by the black hole and so feed it; if the dough is dense enough, it has a chance to overcome the tidal shear and become ‘bread’, and so a star.”

The astronomers used ALMA to observe the carbon monosulfide molecule lines in the circumnuclear disk to achieve this image. Carbon monosulfide is a dense gas tracer that better samples the circumnuclear disk than carbon monoxide, a commonly used molecule to observe interstellar gas. This method provided a better way to constrain the gas densities and better understand what is going on in it.

The research team found that while a significant amount of gas is available to form stars, there is no clear evidence of star formation. The seemingly unstable clumps of molecular gas should then be marginally stabilized by other forces such as magnetic fields.

“Because the polarized signal generated by the magnetic field from dust emission is weak and difficult to measure, the magnetic field of the circumnuclear disk has not yet been probed at clump-scale (8000 AU),” explains Hsieh. “Thanks to the high resolution and sensitivity of ALMA, we have been granted the ALMA time to mosaic the magnetic field of the circumnuclear disk in future observations with ALMA. We will then continue to explore the role of magnetic fields in star formation in this region.”

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.


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Joint ALMA Observatory, Santiago - Chile
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Thursday, September 16, 2021

Untangling the Formation of Planetary Systems with Deuterium

ALMA images of the protoplanetary disks around young stars AS 209 and HD 163296. Different molecules have different distributions. Credit: ALMA (ESO/NAOJ/NRAO), Cataldi et al./Aikawa et al.

This image of ALMA data from the young star HD 163296 shows hydrogen cyanide emission. The MAPS project zoomed in on hydrogen cyanide and other organic and inorganic compounds in planet-forming disks to gain a better understanding of the compositions of young planets and how the compositions link to where planets form in a protoplanetary disk Credit: ALMA (ESO/NAOJ/NRAO)/D. Berry (NRAO), K. Öberg et al (MAPS)

An international research team using the Atacama Large Millimeter/submillimeter Array (ALMA) revealed the distribution of heavy hydrogen, or deuterium, in planet formation sites with the highest resolution ever achieved. This provides clues to understand the physical and chemical conditions during the formation of exoplanets and Solar System objects.

“The various bodies in our Solar System have a variety of chemical compositions,” says Yuri Aikawa, a professor at the University of Tokyo. “This variety could be due to differences in the chemical composition and physical state at their formation sites. Revealing the chemical variation within the planet-forming disks is thus fundamental to the study of planet formation.”

Protoplanetary disks around young stars contain a variety of molecules, each of which emits radio waves of specific wavelengths. In this study, researchers utilized the superb resolution and sensitivity of ALMA to understand the physical and chemical conditions in planet forming disks.

Gianni Cataldi, a postdoc at the University of Tokyo and the National Astronomical Observatory of Japan, and his team focused on deuterium, the heavy brother of hydrogen, in protoplanetary disks. Although there is only one deuterium atom for every 100,000 hydrogen atoms, it is known that the ratio is higher in certain molecules. This deuterium enrichment can be used as a footprint to infer where an object was formed in a disk.

The team analyzed ALMA data and measured the spatial distribution of the deuterium abundance ratio in protoplanetary disks. They found that the deuterium abundance ratios differed by a factor of about 100 among different locations within a single disk, with the abundance ratios becoming smaller closer to the central star.

“Two major reactions are thought to be responsible for the deuterium enrichment; one is active in very low-temperature regions and the other remain effective even in the relatively warm regions. Our observations show that both play an important role in disks,” says Cataldi.

Comparing the deuterium abundance ratios observed in protoplanetary disks with those of Solar System objects can provide information on the origin of the objects. For example, the deuterium abundance ratio in HCN molecules was measured for Comet Hale-Bopp, which approached the Sun around 1997 and could be seen brightly from Earth. The value for Comet Hale-Bopp was smaller than the one measured in the protoplanetary disks this time.

“This may suggest that Comet Hale-Bopp formed in the inner part of the disk, close to the young Sun (within 30 au),” says Yoshihide Yamato, a graduate student at the University of Tokyo and a co-author of the research paper. “Another possibility is that the HCN molecules in the comet originated from ices that condensed from the gas cloud at a much earlier stage of the formation of the disk, and were not affected by the deuterium enrichment in the disk.”

These observations are part of an ALMA Large Program, “Molecules with ALMA at Planet-forming Scales,” or MAPS, to detect radio waves emitted by molecules in protoplanetary disks with high spatial resolution. In this program researchers observed protoplanetary disks around five young stars, IM Lupi, GM Aurigae, AS 209, HD 163296, and MWC 480 with ALMA to infer the distribution of about 20 molecules, including deuterated molecules such as DCN and N2D+.

“With ALMA we were able to see how molecules are distributed where exoplanets are currently assembling,” said Karin Öberg, an astronomer at the Center for Astrophysics | Harvard & Smithsonian (CfA) and the Principal Investigator for MAPS. “One of the really exciting things we saw is that the planet-forming disks around these five young stars are factories of a special class of organic molecules, so-called nitriles, which are implicated in the origins of life here on Earth.”

Scientists also observed complex organic molecules like HC3N, CH3CN, and c-C3H2; notably these contain carbon, and therefore are most likely to act as the feedstock of larger, prebiotic molecules. Although these molecules have been detected in protoplanetary disks before, MAPS is the first systematic study across multiple disks at very high spatial resolution and sensitivity, and the first study to find the molecules in such significant quantities at small scales. “We found more of the large organic molecules than expected, a factor of 10 to 100 more, located in the inner disks on scales of the Solar System, and their chemistry appears similar to that of Solar System comets,” said John Ilee, an astronomer at the University of Leeds and the lead author of a MAPS paper. “The presence of these large organic molecules is significant because they are the stepping-stones between simpler carbon-based molecules such as carbon monoxide, which is found in abundance in space, and the more complex molecules that are required to create and sustain life.”

In this artist’s conception, planets form from the gas and dust in the protoplanetary disk surrounding the young star. The gas is made up of many different molecules, including hydrogen cyanide and more complex nitriles—linked to the development of life on Earth—and other organic and inorganic compounds. From simple organic compounds to the more complex, the soup of molecules in a particular location in the disk shapes the future of the planet forming there, and determines whether or not that planet could support life as we know it. Credit: M.Weiss/Center for Astrophysics | Harvard & Smithsonian

Aikawa and the MAPS team also revealed the spatial distribution of ionized molecules in the disks. They found that ionized molecules are less abundant in the region inside the 100-au radius of disks. If ionized, the gas in the disk is more susceptible to magnetic fields, which can cause gas to start outflowing or, conversely, allow gas to flow into the central star, greatly affecting the growth of stars and planets. The observation also suggests that the ionization rate in the disk midplane might vary from object to object, which indicates that the physical conditions of planet-forming disks are quite complicated.

“I believe that we can approach the mystery of the formation process of our Solar System by combing the observations of protoplanetary disks using ALMA, observations and analysis of Solar System material, and predictions based on theoretical research,” summarizes Aikawa.

Paper Information

These observation results are presented as Gianni Cataldi et al. “Molecules with ALMA at Planet-forming Scales (MAPS) X: Studying deuteration at high angular resolution towards protoplanetary disks” and Yuri Aikawa et al. “Molecules with ALMA at Planet-forming Scales (MAPS) XIII: HCO+ and disk ionization structure” and other 18 papers in the MAPS special issue of the Astrophysical Journal Supplement Series.

This research was supported by:

JSPS KAKENHI (No. 18H05222, 20H05844, 20H05847, 18H05441, JP17K14244 and JP20K04017), NAOJ ALMA Joint Scientific Research Program (2019-13B, 2018-10B), World-leading Innovative Graduate Study Program (WINGS) of the University of Tokyo, NASA Hubble Fellowship grant (HST-HF2-51401.001, HST-HF2-51419.001, HST-HF2-51427.001-A, HST-HF2-51429.001-A, HST-HF2-51405.001-A, HST-HF2-51460.001-A), NASA Grant (No. 17-XRP17 2-0012), NSF AAG Grant (#1907653), FONDECYT Iniciación 11180904 and ANID project Basal AFB-170002, NSF Graduate Research Fellowship under Grant No. DGE1745303, Natural Science Foundation of China grant No. 11973090, David and Lucille Packard Foundation and Johnson & Johnson’s WiSTEM2D Program, Science and Technology Facilities Council of the United Kingdom (ST/T000287/1, ST/R000549/1, MR/T040726/1), CNES fellowship grant, ANR of France under contracts ANR-16-CE31-0013 and ANR-15-IDEX-02, Simons Foundation (SCOL #321183), Wisconsin Alumni Research Foundation, and Smithsonian Institution.

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.

Wednesday, September 15, 2021

Quasars as Cosmic Standard Candles

The quasar 3C 273 with its jet, as seen by the Chandra X-ray Observatory. Astronomers have found that the X-ray and ultraviolet luminosities of quasars are so tightly correlated, even for quasars at large cosmological distances, that quasars can be used as new "standard candles" to help determine cosmic distances and probe other fundamental cosmological parameters. Credit: 
Chandra X-ray Observatory

In 1929, Edwin Hubble published observations that galaxies' distances and velocities are correlated, with the distances determined using their Cepheid stars. Harvard astronomer Henrietta Swan Leavitt had discovered that a Cepheid star varies periodically with a period that is related to its intrinsic luminosity. She calibrated the effect, and when Hubble compared those calculated values with his observed luminosities he was able to determine their distances. But even today only Cepheid stars in relatively nearby galaxies can be studied in this way.In order to extend the distance scale back to earlier times in cosmic history, astronomers have used supernovae (SN) - the explosive deaths of massive stars – which can be seen to much greater distances. By comparing the observed brightness of a SN with its intrinsic brightness, based on its classification, astronomers are able to determine its distance; comparing that with the host galaxy's velocity (its redshift, measured spectroscopically) yields the "Hubble relation" relating the galaxy's velocity to its distance. The most reliable supernovae for this purpose, because of their cosmic uniformity, are so-called "Type Ia" supernovae, which are thought to be "standard candles," all having the same intrinsic brightness. However even SN become harder to study in this way as they lie farther away; to date the most distant Type Ia SN with a reliable velocity determination dates from an epoch about 3 billion years after the big bang.

CfA astronomers Susanna Bisogni, Francesca Civano, Martin Elvis and Pepi Fabbiano and their colleagues propose using quasars as a new standard candle. The most distant known quasars have been spotted from an era only about seven hundred million years after the big bang, dramatically extending the range of standard candle redshifts. Another advantage of quasars is that hundreds of thousands of them have been discovered in the past few years. Not least, the physical processes in quasars are different from those in SN, providing completely independent measures of cosmological parameters.

The new scheme proposed by the astronomers relies on their discovery that the X-ray and ultraviolet emission in quasars are tightly correlated. At the heart of a quasar is a supermassive black hole surrounded by a very hot disk of accreting material that emits in the ultraviolet. The disk in turn is surrounded by hot gas with electrons moving at speeds close to that of light, and when ultraviolet photons encounter these electrons their energy is boosted into the X-rays. The team, building on their previous methods, analyzed X-ray measurements of 2332 distant quasars in the new Chandra Source Catalog and compared them to ultraviolet results from the Sloan Digital Sky Survey. They found that the tight correlation already known to exist between the ultraviolet and X-ray luminosity of local quasars continues in distant quasars, back over 85% of the age of the Universe, becoming even tighter at earlier times. The implication is that these two quantities can determine the distance of each quasar, and those distances can then be used to test cosmological models. If the results are confirmed, they will provide astronomers with a dramatic new tool with which to measure the properties of the evolving universe.


The Chandra view of the relation between X-ray and UV emission in quasars,” S. Bisogni, E. Lusso, F. Civano, E. Nardini, G. Risaliti, M. Elvis, and G. Fabbiano,  Astronomy & Astrophysics, in press, 2021.


Tuesday, September 14, 2021

Rerun of Supernova Blast Is Expected to Appear in 2037

MACS J0138.0-2155
Credits: Lead Author: Steve A. Rodney (University of South Carolina), Gabriel Brammer (Cosmic Dawn Center/Niels Bohr Institute/University of Copenhagen). Iimage Processing: Joseph DePasquale (STScI). Release Images | Release Videos

It's challenging to make predictions, especially in astronomy. There are however, a few forecasts astronomers can depend on, such as the timing of upcoming lunar and solar eclipses and the clockwork return of some comets.

Now, looking far beyond the solar system, astronomers have added a solid prediction of an event happening deep in intergalactic space: an image of an exploding star, dubbed Supernova Requiem, which will appear around the year 2037. Although this rebroadcast will not be visible to the naked eye, some future telescopes should be able to spot it.

It turns out that this future appearance will be the fourth-known view of the same supernova, magnified, brightened, and split into separate images by a massive foreground cluster of galaxies acting like a cosmic zoom lens. Three images of the supernova were first found from archival data taken in 2016 by NASA's Hubble Space Telescope.

The multiple images are produced by the monster galaxy cluster's powerful gravity, which distorts and magnifies the light from the supernova far behind it, an effect called gravitational lensing. First predicted by Albert Einstein, this effect is similar to a glass lens bending light to magnify the image of a distant object.

The three lensed supernova images, seen as tiny dots captured in a single Hubble snapshot, represent light from the explosive aftermath. The dots vary in brightness and color, which signify three different phases of the fading blast as it cooled over time.

"This new discovery is the third example of a multiply imaged supernova for which we can actually measure the delay in arrival times," explained lead researcher Steve Rodney of the University of South Carolina in Columbia. "It is the most distant of the three, and the predicted delay is extraordinarily long. We will be able to come back and see the final arrival, which we predict will be in 2037, plus or minus a couple of years." 

The light that Hubble captured from the cluster, MACS J0138.0-2155, took about 4 billion years to reach Earth. The light from Supernova Requiem needed an estimated 10 billion years for its journey, based on the distance of its host galaxy.

The team's prediction of the supernova's return appearance is based on computer models of the cluster, which describe the various paths the supernova light is taking through the maze of clumpy dark matter in the galactic grouping. Dark matter is an invisible material that comprises the bulk of the universe's matter and is the scaffolding upon which galaxies and galaxy clusters are built.

Each magnified image takes a different route through the cluster and arrives at Earth at a different time, due, in part, to differences in the length of the pathways the supernova light followed.

"Whenever some light passes near a very massive object, like a galaxy or galaxy cluster, the warping of space-time that Einstein's theory of general relativity tells us is present for any mass, delays the travel of light around that mass," Rodney said.

He compares the supernova's various light paths to several trains that leave a station at the same time, all traveling at the same speed and bound for the same location. Each train, however, takes a different route, and the distance for each route is not the same. Because the trains travel over different track lengths across different terrain, they do not arrive at their destination at the same time.

In addition, the lensed supernova image predicted to appear in 2037 lags behind the other images of the same supernova because its light travels directly through the middle of the cluster, where the densest amount of dark matter resides. The immense mass of the cluster bends the light, producing the longer time delay. "This is the last one to arrive because it's like the train that has to go deep down into a valley and climb back out again. That's the slowest kind of trip for light," Rodney explained.

The lensed supernova images were discovered in 2019 by Gabe Brammer, a study co-author at the Cosmic Dawn Center (DAWN) at the Niels Bohr Institute, University of Copenhagen, in Denmark. Brammer spotted the mirrored supernova images while analyzing distant galaxies magnified by massive foreground galaxy clusters as part of an ongoing Hubble program called REsolved QUIEscent Magnified Galaxies (REQUIEM).

He was comparing new REQUIEM data from 2019 with archival images taken in 2016 from a different Hubble science program. A tiny red object in the 2016 data caught his eye, which he initially thought was a far-flung galaxy. But it had disappeared in the 2019 images.

"But then, on further inspection of the 2016 data, I noticed there were actually three magnified objects, two red and a purple," he explained. "Each of the three objects was paired with a lensed image of a distant massive galaxy. Immediately it suggested to me that it was not a distant galaxy but actually a transient source in this system that had faded from view in the 2019 images like a light bulb that had been flicked off."

Brammer teamed up with Rodney to conduct a further analysis of the system. The lensed supernova images are arranged in an arc around the cluster's core. They appear as small dots near the smeared orange features that are thought to be the magnified snapshots of the supernova's host galaxy.

Study co-author Johan Richard of the University of Lyon in France produced a map of the amount of dark matter in the cluster, inferred from the lensing it produces. The map shows the predicted locations of lensed objects. This supernova is predicted to appear again in 2042, but it will be so faint that the research team thinks it will not be visible.

Catching the rerun of the explosive event will help astronomers measure the time delays between all four supernova images, which will offer clues to the type of warped-space terrain the exploded star's light had to cover. Armed with those measurements, researchers can fine-tune the models that map out the cluster's mass. Developing precise dark-matter maps of massive galaxy clusters is another way for astronomers to measure the universe's expansion rate and investigate the nature of dark energy, a mysterious form of energy that works against gravity and causes the cosmos to expand at a faster rate.

This time-delay method is valuable because it's a more direct way of measuring the universe's expansion rate, Rodney explained. "These long time delays are particularly valuable because you can get a good, precise measurement of that time delay if you are just patient and wait years, in this case more than a decade, for the final image to return," he said. "It is a completely independent path to calculate the universe's expansion rate. The real value in the future will be using a larger sample of these to improve the precision."

Spotting lensed images of supernovae will become increasingly common in the next 20 years with the launch of NASA's Nancy Grace Roman Space Telescope and the start of operations at the Vera C. Rubin Observatory. Both telescopes will observe large swaths of the sky, which will allow them to spot dozens more multiply imaged supernovae.

Future telescopes such as NASA's James Webb Space Telescope also could detect light from supernova Requiem at other epochs of the blast.

The team's results will appear on September 13 in the journal Nature Astronomy

The Hubble Space Telescope is a project of international cooperation between NASA and ESA (European Space Agency). NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Maryland, conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy in Washington, D.C.

Media Contact:

Donna Weaver
Space Telescope Science Institute, Baltimore, Maryland

Ray Villard
Space Telescope Science Institute, Baltimore, Maryland

Science Contact:

Steven A. Rodney
University of South Carolina, Columbia, South Carolina

Gabriel Brammer
Cosmic Dawn Center/Niels Bohr Institute/University of Copenhagen, Copenhagen, Denmark

Release: NASA, ESA, University of South Carolina, Cosmic Dawn Center/Niels Bohr Institute/University of Copenhagen

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Source: HubbleSite/News

Monday, September 13, 2021

A Closer Look at Hubble’s 31st Anniversary Snapshot

AG Carinae - Comparison
Credit: ESA/Hubble and NASA, A. Nota, C. Britt

A Closer Look at Hubble’s 31st Anniversary Snapshot

A Closer Look at Hubble’s 31st Anniversary Snapshot

This comparison view shows puffing dust bubbles and an erupting gas shell — the final acts of a monster star.You can explore the detail of the nebula surrounding the star AG Carinae by using the slider tool on the image above.

This comparison view shows puffing dust bubbles and an erupting gas shell — the final acts of a monster star.You can explore the detail of the nebula surrounding the star AG Carinae by using the slider tool on the image above.

This Picture of the Week showcases new views of the dual nature of the star AG Carinae, which was the target of the NASA/ESA Hubble Space Telescope’s 31st anniversary image in April 2021. This new perspective was developed thanks to Hubble’s observations of the star in 2020 and 2014, along with others captured by the telescope’s WFPC2 instrument in 1994.

The first image showcases the details of the ionised hydrogen and ionised nitrogen emissions from the nebula (seen here in red). In the second image, the blue demonstrates the contrasting appearance of the distribution of the dust that shines of reflected stellar light. Astronomers believe that the dust bubbles and filaments formed within and were shaped by powerful stellar wind .

This giant star is waging a tug-of-war between gravity and radiation to avoid self-destruction. The star is surrounded by an expanding shell of gas and dust — a nebula — that is shaped by the powerful winds emanating from the star. The nebula is about five light-years wide, equal to the distance from here to our nearest star, Alpha Centauri

AG Carinae is formally classified as a Luminous Blue Variable because it is hot (blue), very luminous, and variable. Such stars are quite rare because there are not many stars that are so massive. Luminous Blue Variable stars continuously lose mass in the final stages of their life, during which a significant amount of stellar material is ejected into the surrounding interstellar space, until enough mass has been lost that the star has reached a stable state.

AG Carinae is surrounded by a spectacular nebula, formed by material ejected by the star during several of its past outbursts. The nebula is approximately 10 000 years old, and the observed velocity of the gas is approximately 70 kilometres per second. While this nebula looks like a ring, it is in fact a hollow shell rich in gas and dust, the centre of which has been cleared by the powerful stellar wind travelling at roughly 200 kilometres per second. The gas (composed mostly of ionised hydrogen and nitrogen) is visible to us in these images as a thick bright red ring, which appears doubled in places — possibly the result of several outbursts colliding into each other. The dust, here visible in blue, has formed in clumps, bubbles and filaments that are shaped by the stellar wind.

Scientists who observed the star and its surrounding nebula note that the ring is not perfectly spherical; it appears to have a bipolar symmetry, indicating that the mechanism producing the outburst may have been caused by the presence of a disc in the centre, or that the star is not alone but might have a companion (known as a binary star). An alternative and simpler theory is that the star rotates very fast (as many massive stars have been found to do).

Friday, September 10, 2021

Astronomers nail down the origins of rare loner dwarf galaxies

Caption:In this image, the fall of a blue ultra-diffuse galaxy into a galaxy system and its subsequent ejection as a red ultra-diffuse galaxy, is depicted. Credits:Image: Vanina Rodriguez

By definition, dwarf galaxies are small and dim, with just a fraction of the stars found in the Milky Way and other galaxies. There are, however, giants among the dwarfs: Ultra-diffuse galaxies, or UDGs, are dwarf systems that contain relatively few stars but are scattered over vast regions. Because they are so diffuse, these systems are difficult to detect, though most have been found tucked within clusters of larger, brighter galaxies.

Now astronomers from MIT, the University of California at Riverside, and elsewhere have used detailed simulations to detect “quenched” UDGs — a rare type of dwarf galaxy that has stopped generating stars. They identified several such systems in their simulations and found the galaxies were not in clusters, but rather exiled in voids — quiet, nearly empty regions of the universe.

This isolation goes against astronomers’ predictions of how quenched UDGs should form. So, the team used the same simulations to rewind the dwarf systems’ evolution and see exactly how they came to be.

The researchers found that quenched UDGs likely coalesced within halos of dark matter with unusually high angular momentum. Like a cotton candy machine, this extreme environment may have spun out dwarf galaxies that were anomalously stretched out.

These UDGs then evolved within galaxy clusters, like most UDGs. But interactions within the cluster likely ejected the dwarfs into the void, giving them wide, boomerang-like trajectories known as “backsplash” orbits. In the process, the galaxies’ gas was stripped away, leaving the galaxies “quenched” and unable to produce new stars.

The simulations showed that such UDGs should be more common than what has been observed. The researchers say their results, published today in Nature Astronomy, provide a blueprint for astronomers to go looking for these dwarfish giants in the universe’s voids.

“We always strive to get a complete consensus of the galaxies that we have in the universe,” says Mark Vogelsberger, associate professor of physics at MIT. “This study is adding a new population of galaxies that the simulation actually predicts. And we now have to look for them in the real universe.”

Vogelsberger co-led the study with Laura Sales of UC Riverside and José A. Benavides of the Institute of Theoretical and Experimental Astronomy in Argentina.

Red v blue

The team’s search for quenched UDGs began with a simple survey for UDG satellites — ultra-diffuse systems that reside outside galaxy clusters. Astronomers predict that UDGs within clusters should be quenched, as they would be surrounded by other galaxies that would essentially rub out the UDG’s already-diffuse gas and shut off star production. Quenched UDGs in clusters should then consist mainly of old stars and appear red in color.

If UDGs exist outside clusters, in the void, they are expected to continue churning out stars, as there would be no competing gas from other galaxies to quench them. UDGs in the void, therefore, are predicted to be rich with new stars, and to appear blue.

When the team surveyed previous detections of UDG satellites, outside clusters, they found most were blue as expected — but a few were red.

“That’s what caught our attention,” Sales says. “And we thought, ‘What are they doing there? How did they form?’ There was no good explanation.”

Galactic cube

To find one, the researchers looked to TNG50, a detailed cosmological simulation of galaxy formation developed by Vogelsberger and others at MIT and elsewhere. The simulation runs on some of the most powerful supercomputers in the world and is designed to evolve a large volume of the universe, from conditions resembling those shortly after the Big Bang to the present day. The simulation is based on fundamental principles of physics and the complex interactions between matter and gas, and its results have been shown in many scenarios to agree with what astronomers have observed in the actual universe. TNG50 has therefore been used as an accurate model for how and where many types of galaxies evolve through time. In their new study, Vogelsberger, Sales, and Benavides used TNG50 to first see if they could spot quenched UDGs outside galaxy clusters. They started with a cube of the early universe measuring about 150 million light years wide, and ran the simulation forward, up through the present day. Then they searched the simulation specifically for UDGs in voids, and found most of the ones they detected were blue, as expected. But a surprising number — about 25 percent — were red, or quenched.

They zeroed in on these red satellite dwarfs and used the same simulation, this time as a sort of time machine to see how, when, and where these galaxies originated. They found that the systems were initially part of clusters but were somehow thrown out into the void, on a more elliptical, “backsplash” orbit.

“These orbits are almost like those of comets in our solar system,” Sales says. “Some go out and orbit back around, and others may come in once and then never again. For quenched UDGs, because their orbits are so elliptical, they haven’t had time to come back, even over the entire age of the universe. They are still out there in the field.”

The simulations also showed that the quenched UDGs’ red color arose from their ejection — a violent process that stripped away the galaxies’ star-forming gas, leaving it quenched and red. Running the simulations further back in time, the team observed that the tiny systems, like all galaxies, originated in halos of dark matter, where gas coalesces into galactic disks. But for quenched UDGs, the halos appeared to spin faster than normal, generating stretched out, ultra-diffuse galaxies.

Now that the researchers have a better understanding of where and how quenched UDGs arose, they hope astronomers can use their results to tune telescopes, to identify more such isolated red dwarfs — which the simulations suggest must be lurking in larger numbers than what astronomers have so far detected.

“It’s quite surprising that the simulations can really produce all these very small objects,” Vogelsberger says. “We predict there should be more of this kind of galaxy out there. This makes our work quite exciting.”

Jennifer Chu | MIT News Office

Source: MIT/News

Thursday, September 09, 2021

ESO captures best images yet of peculiar “dog-bone” asteroid

Asteroid Kleopatra from different angles
Asteroid Kleopatra from different angles (annotated)
Size comparison of asteroid Kleopatra with northern Italy
Size comparison of asteroid Kleopatra with Chile
Processed SPHERE image showing the moons of Kleopatra


Location of Kleopatra in the Solar System
Location of Kleopatra in the Solar System

Using the European Southern Observatory’s Very Large Telescope (ESO’s VLT), a team of astronomers have obtained the sharpest and most detailed images yet of the asteroid Kleopatra. The observations have allowed the team to constrain the 3D shape and mass of this peculiar asteroid, which resembles a dog bone, to a higher accuracy than ever before. Their research provides clues as to how this asteroid and the two moons that orbit it formed.

Kleopatra is truly a unique body in our Solar System,” says Franck Marchis, an astronomer at the SETI Institute in Mountain View, USA and at the Laboratoire d'Astrophysique de Marseille, France, who led a study on the asteroid — which has moons and an unusual shape — published today in Astronomy & Astrophysics. “Science makes a lot of progress thanks to the study of weird outliers. I think Kleopatra is one of those and understanding this complex, multiple asteroid system can help us learn more about our Solar System.

Kleopatra orbits the Sun in the Asteroid Belt between Mars and Jupiter. Astronomers have called it a “dog-bone asteroid” ever since radar observations around 20 years ago revealed it has two lobes connected by a thick “neck”. In 2008, Marchis and his colleagues discovered that Kleopatra is orbited by two moons, named AlexHelios and CleoSelene, after the Egyptian queen’s children.

To find out more about Kleopatra, Marchis and his team used snapshots of the asteroid taken at different times between 2017 and 2019 with the Spectro-Polarimetric High-contrast Exoplanet REsearch (SPHERE) instrument on ESO’s VLT. As the asteroid was rotating, they were able to view it from different angles and to create the most accurate 3D models of its shape to date. They constrained the asteroid’s dog-bone shape and its volume, finding one of the lobes to be larger than the other, and determined the length of the asteroid to be about 270 kilometres or about half the length of the English Channel.

In a second study, also published in Astronomy & Astrophysics and led by Miroslav Brož of Charles University in Prague, Czech Republic, the team reported how they used the SPHERE observations to find the correct orbits of Kleopatra’s two moons. Previous studies had estimated the orbits, but the new observations with ESO’s VLT showed that the moons were not where the older data predicted them to be.

This had to be resolved,” says Brož. “Because if the moons’ orbits were wrong, everything was wrong, including the mass of Kleopatra.” Thanks to the new observations and sophisticated modelling, the team managed to precisely describe how Kleopatra’s gravity influences the moons’ movements and to determine the complex orbits of AlexHelios and CleoSelene. This allowed them to calculate the asteroid’s mass, finding it to be 35% lower than previous estimates.

Combining the new estimates for volume and mass, astronomers were able to calculate a new value for the density of the asteroid, which, at less than half the density of iron, turned out to be lower than previously thought [1]. The low density of Kleopatra, which is believed to have a metallic composition, suggests that it has a porous structure and could be little more than a “pile of rubble”. This means it likely formed when material reaccumulated following a giant impact.

Kleopatra’s rubble-pile structure and the way it rotates also give indications as to how its two moons could have formed. The asteroid rotates almost at a critical speed, the speed above which it would start to fall apart, and even small impacts may lift pebbles off its surface. Marchis and his team believe that those pebbles could subsequently have formed AlexHelios and CleoSelene, meaning that Kleopatra has truly birthed its own moons.

The new images of Kleopatra and the insights they provide are only possible thanks to one of the advanced adaptive optics systems in use on ESO’s VLT, which is located in the Atacama Desert in Chile. Adaptive optics help to correct for distortions caused by the Earth’s atmosphere which cause objects to appear blurred — the same effect that causes stars viewed from Earth to twinkle. Thanks to such corrections, SPHERE was able to image Kleopatra — located 200 million kilometres away from Earth at its closest — even though its apparent size on the sky is equivalent to that of a golf ball about 40 kilometres away.

ESO’s upcoming Extremely Large Telescope (ELT), with its advanced adaptive optics systems, will be ideal for imaging distant asteroids such as Kleopatra. “I can’t wait to point the ELT at Kleopatra, to see if there are more moons and refine their orbits to detect small changes,” adds Marchis.



[1] The newly calculated density is 3.4 grams per cubic centimetre, while previously Kleopatra was believed to have a mean density of about 4.5 grams per cubic centimetre.

More Information

This research, based on observations with SPHERE on ESO’s VLT (Principal Investigator: Pierre Vernazza), was presented in two papers to appear in Astronomy & Astrophysics.

The team of the paper entitled “(216) Kleopatra, a low density critically rotating M-type asteroid” ( is composed of F. Marchis (SETI Institute, Carl Sagan Center, Mountain View, USA and Aix Marseille University, CNRS, Laboratoire d’Astrophysique de Marseille, France [LAM]), L. Jorda (LAM), P. Vernazza (LAM), M. Brož (Institute of Astronomy, Faculty of Mathematics and Physics, Charles University, Prague, Czech Republic [CU]), J. Hanuš (CU), M. Ferrais (LAM), F. Vachier (Institut de mécanique céleste et de calcul des éphémérides, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, UPMC University Paris 06 and Université de Lille, France [IMCCE]), N. Rambaux (IMCCE), M. Marsset (Department of Earth, Atmospheric and Planetary Sciences, MIT, Cambridge, USA [MIT]), M. Viikinkoski (Mathematics & Statistics, Tampere University, Finland [TAU]), E. Jehin (Space sciences, Technologies and Astrophysics Research Institute, Université de Liège, Belgium [STAR]), S. Benseguane (LAM), E. Podlewska-Gaca (Faculty of Physics, Astronomical Observatory Institute, Adam Mickiewicz University, Poznan, Poland [UAM]), B. Carry (Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange, France [OCA]), A. Drouard (LAM), S. Fauvaud (Observatoire du Bois de Bardon, Taponnat, France [OBB]), M. Birlan (IMCCE and Astronomical Institute of Romanian Academy, Bucharest, Romania [AIRA]), J. Berthier (IMCCE), P. Bartczak (UAM), C. Dumas (Thirty Meter Telescope, Pasadena, USA [TMT]), G. Dudziński (UAM), J. Ďurech (CU), J. Castillo-Rogez (Jet Propulsion Laboratory, California Institute of Technology, Pasadena,USA [JPL]), F. Cipriani (European Space Agency, ESTEC - Scientific Support Office, Noordwijk, The Netherlands [ESTEC]​​), F. Colas (IMCCE), R. Fetick (LAM), T. Fusco (LAM and The French Aerospace Lab BP72, Chatillon Cedex, France [ONERA]​​), J. Grice (OCA and School of Physical Sciences, The Open University, Milton Keynes, UK [OU]), A. Kryszczynska (UAM), P. Lamy (Laboratoire Atmosphères, Milieux et Observations Spatiales, CNRS [CRNS] and Université de Versailles Saint-Quentin-en-Yvelines, Guyancourt, France [UVSQ]), A. Marciniak (UAM), T. Michalowski (UAM), P. Michel (OCA), M. Pajuelo (IMCCE and Sección Física, Departamento de Ciencias, Pontificia Universidad Católica del Perú, Lima, Perú [PUCP]), T. Santana-Ros (Departamento de Física, Ingeniería de Sistemas y Teoría de la Señal, Universidad de Alicante, Spain [UA] and Institut de Ciéncies del Cosmos, Universitat de Barcelona, Spain [UB]), P. Tanga (OCA), A. Vigan (LAM), O. Witasse (ESTEC), and B. Yang (European Southern Observatory, Santiago, Chile [ESO]).

The team of the paper entitled “An advanced multipole model for (216) Kleopatra triple system” ( is composed of M. Brož (CU), F. Marchis (SETI and LAM), L. Jorda (LAM), J. Hanuš (CU), P. Vernazza (LAM), M. Ferrais (LAM), F. Vachier (IMCCE), N. Rambaux (IMCCE), M. Marsset (MIT), M. Viikinkoski (TAU), E. Jehin (STAR), S. Benseguane (LAM), E. Podlewska-Gaca (UAM), B. Carry (OCA), A. Drouard (LAM), S. Fauvaud (OBB), M. Birlan (IMCCE and AIRA), J. Berthier (IMCCE), P. Bartczak (UAM), C. Dumas (TMT), G. Dudziński (UAM), J. Ďurech (CU), J. Castillo-Rogez (JPL), F. Cipriani (ESTEC​​), F. Colas (IMCCE), R. Fetick (LAM), T. Fusco (LAM and ONERA), J. Grice (OCA and OU), A. Kryszczynska (UAM), P. Lamy (CNRS and UVSQ), A. Marciniak (UAM), T. Michalowski (UAM), P. Michel (OCA), M. Pajuelo (IMCCE and PUCP), T. Santana-Ros (UA and UB), P. Tanga (OCA), A. Vigan (LAM), O. Witasse (ESTEC), and B. Yang (ESO).

ESO is the foremost intergovernmental astronomy organisation in Europe and the world’s most productive ground-based astronomical observatory by far. It has 16 Member States: Austria, Belgium, the Czech Republic, Denmark, France, Finland, Germany, Ireland, Italy, the Netherlands, Poland, Portugal, Spain, Sweden, Switzerland and the United Kingdom, along with the host state of Chile and with Australia as a Strategic Partner. ESO carries out an ambitious programme focused on the design, construction and operation of powerful ground-based observing facilities enabling astronomers to make important scientific discoveries. ESO also plays a leading role in promoting and organising cooperation in astronomical research. ESO operates three unique world-class observing sites in Chile: La Silla, Paranal and Chajnantor. At Paranal, ESO operates the Very Large Telescope and its world-leading Very Large Telescope Interferometer as well as two survey telescopes, VISTA working in the infrared and the visible-light VLT Survey Telescope. Also at Paranal ESO will host and operate the Cherenkov Telescope Array South, the world’s largest and most sensitive gamma-ray observatory. ESO is also a major partner in two facilities on Chajnantor, APEX and ALMA, the largest astronomical project in existence. And on Cerro Armazones, close to Paranal, ESO is building the 39-metre Extremely Large Telescope, the ELT, which will become “the world’s biggest eye on the sky”.



Franck Marchis
SETI Institute and Laboratoire d’Astrophysique de Marseille
Mountain View and Marseille, France and USA
Cell: +1-510-599-0604

Miroslav Brož
Charles University
Prague, Czech Republic

Pierre Vernazza
Laboratoire d’Astrophysique de Marseille
Marseille, France
Tel: +33 4 91 05 59 11

Bárbara Ferreira
ESO Media Manager
Garching bei München, Germany
Tel: +49 89 3200 6670
Cell: +49 151 241 664 00

 Source: ESO/News

Wednesday, September 08, 2021

Stellar Collision Triggers Supernova Explosion

The Sequence of Events -- Clockwise, from top left: (1.) A neutron star or black hole orbits a "normal" companion star (light blue), growing closer over thousands of years. (2.) The neutron star or black hole enters its companion's atmosphere, throwing gas outward in an expanding spiral. (3.) When the intruder reaches the companion's core, material briefly forms a disk that propels a superfast jet outward, poking its way out of the star. The nuclear fusion that held the companion's core up against its own gravity is disrupted, triggering a collapse and subsequent supernova explosion. (4.) The material blasted out by the supernova explosion catches up to the material thrown out by the earlier interaction, causing strong shock waves that produce the radio waves observed with the VLA. Credit: Bill Saxton, NRAO/AUI/NSF.Hi-res File

Fast-moving debris from a supernova explosion triggered by a stellar collision crashes into gas thrown out earlier, and the shocks cause bright radio emission seen by the VLA. Credit: Bill Saxton, NRAO/AUI/NSF.Hi-res File

Astronomers have found dramatic evidence that a black hole or neutron star spiraled its way into the core of a companion star and caused that companion to explode as a supernova. The astronomers were tipped off by data from the Very Large Array Sky Survey (VLASS), a multi-year project using the National Science Foundation’s Karl G. Jansky Very Large Array (VLA).

“Theorists had predicted that this could happen, but this is the first time we’ve actually seen such an event,” said Dillon Dong, a graduate student at Caltech and lead author on a paper reporting the discovery in the journal Science.

The first clue came when the scientists examined images from VLASS, which began observations in 2017, and found an object brightly emitting radio waves but which had not appeared in an earlier VLA sky survey, called Faint Images of the Radio Sky at Twenty centimeters (FIRST). They made subsequent observations of the object, designated VT 1210+4956, using the VLA and the Keck telescope in Hawaii. They determined that the bright radio emission was coming from the outskirts of a dwarf, star-forming galaxy some 480 million light-years from Earth. They later found that an instrument aboard the International Space Station had detected a burst of X-rays coming from the object in 2014.

The data from all these observations allowed the astronomers to piece together the fascinating history of a centuries-long death dance between two massive stars. Like most stars that are much more massive than our Sun, these two were born as a binary pair, closely orbiting each other. One of them was more massive than the other and evolved through its normal, nuclear fusion-powered lifetime more quickly and exploded as a supernova, leaving behind either a black hole or a superdense neutron star.

The black hole or neutron star’s orbit grew steadily closer to its companion, and about 300 years ago it entered the companion’s atmosphere, starting the death dance. At this point, the interaction began spraying gas away from the companion into space. The ejected gas, spiraling outward, formed an expanding, donut-shaped ring, called a torus, around the pair.

Eventually, the black hole or neutron star made its way inward to the companion star’s core, disrupting the nuclear fusion producing the energy that kept the core from collapsing of its own gravity. As the core collapsed, it briefly formed a disk of material closely orbiting the intruder and propelled a jet of material outward from the disk at speeds approaching that of light, drilling its way through the star.

“That jet is what produced the X-rays seen by the MAXI instrument aboard the International Space Station, and this confirms the date of this event in 2014,” Dong said.

The collapse of the star’s core caused it to explode as a supernova, following its sibling’s earlier explosion.

“The companion star was going to explode eventually, but this merger accelerated the process,” Dong said.

The material ejected by the 2014 supernova explosion moved much faster than the material thrown off earlier from the companion star, and by the time VLASS observed the object, the supernova blast was colliding with that material, causing powerful shocks that produced the bright radio emission seen by the VLA.

“All the pieces of this puzzle fit together to tell this amazing story,” said Gregg Hallinan of Caltech. “The remnant of a star that exploded a long time ago plunged into its companion, causing it, too, to explode,” he added.

The key to the discovery, Hallinan said, was VLASS, which is imaging the entire sky visible at the VLA’s latitude — about 80 percent of the sky — three times over seven years. One of the objectives of doing VLASS that way is to discover transient objects, such as supernova explosions, that emit brightly at radio wavelengths. This supernova, caused by a stellar merger, however, was a surprise.

“Of all the things we thought we would discover with VLASS, this was not one of them,” Hallinan said.

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

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(505) 241-9210

Link to Scientific Paper


Source: National Radio Astronomy Observatory (NRAO)/News

Tuesday, September 07, 2021

Galaxies Pump Out Contaminated Exhausts

Galaxies pump out contaminated exhausts

Credit: James Josephides, Swinburne Astronomical Productions

AUSTIN — Galaxies pollute the environment they exist in, researchers have found.

A team of astronomers including The University of Texas at Austin’s Danielle Berg and John Chisholm used a new imaging system at the WM Keck Observatory in Hawaii to confirm that what flows into a galaxy is a lot cleaner than what flows out. The research is published today in The Astrophysical Journal.

“Enormous clouds of gas are pulled into galaxies and used in the process of making stars,” said co-lead author Deanne Fisher, associate professor at the Centre for Astrophysics and Supercomputing at Swinburne University in Australia.

“On its way in it is made of hydrogen and helium. By using a new piece of equipment called the Keck Cosmic Web Imager, we were able to confirm that stars made from this fresh gas eventually drive a huge amount of material back out of the system, mainly through supernovas.

“But this stuff is no longer nice and clean — it contains lots of other elements, including oxygen, carbon, and iron.”

The process of atoms flooding into galaxies — known as ‘accretion’ – and their eventual expulsion — known as ‘outflows’ — is an important mechanism governing the growth, mass and size of galaxies.

Until now, however, the composition of the inward and outward flows could only be guessed at.

"These are some of the first observations to reveal the long-sought cycle of gas in and out of galaxies," UT Austin's Chisholm said. "This work uses the chemical composition of the gas to tag whether the gas is freshly coming into the galaxy or is being ejected out of the galaxy. This cycle of gas regulates the buildup stars and the evolution of galaxies in general. These observations provide concrete benchmarks to test our theory of galaxy evolution."

This research is the first time the full cycle has been confirmed in a galaxy other than the Milky Way.

To make their findings, the researchers focused on a galaxy called Mrk 1486, which lies about 500 light years from the Sun and is going through a period of very rapid star formation.

“We found there is a very clear structure to how the gases enter and exit,” explained study co-leader Alex Cameron of the UK’s University of Oxford.

“Imagine the galaxy is a spinning frisbee. The gas enters relatively unpolluted from the cosmos outside, around the perimeter, and then condenses to form new stars. When those stars later explode, they push out other gas – now containing these other elements – through the top and bottom.”

The elements — comprising more than half the Periodic Table — are forged deep inside the cores of the stars through nuclear fusion. When the stars collapse or go nova the results are catapulted into the universe – where they form part of the matrix from which newer stars, planets, asteroids and, in at least one instance, life emerges.

Mrk 1486 was the perfect candidate for observation because it lies “edge-on” to Earth, meaning that the outflowing gas could be easily viewed, and its composition measured. Most galaxies sit at awkward angles for this type of research.

“This work is important for astronomers because for the first time we’ve been able to put limits on the forces that strongly influence how galaxies make stars,” added Professor Fisher. “It takes us one step closer to understanding how and why galaxies look the way they do — and how long they will last.”

Other scientists contributing to the work are based at the University of Maryland at College Park, the University of California at San Diego, and the Universidad de Concepcion in Chile.

Media Contact:

Rebecca Johnson, Communications Mgr.
McDonald Observatory
The University of Texas at Austin

Science Contact:

Dr. John Chisholm, Asst. Professor
Department of Astronomy
The University of Texas at Austin

Source:  McDonald Observatory/News

Monday, September 06, 2021

Hubble discovers hydrogen-burning White Dwarfs enjoying slow aging-

To investigate the physics underpinning white dwarf evolution, astronomers compared cooling white dwarfs in two massive collections of stars: the globular clusters M13 and M3. These two clusters share many physical properties such as age and metallicity, but the populations of stars which will eventually give rise to white dwarfs are different. This makes M13 and M3 together a perfect natural laboratory in which to test how different populations of white dwarfs cool. Credits: Science: ESA, NASA, Giampaolo Piotto.
Release Images

Could dying stars hold the secret to looking younger? New evidence from NASA's Hubble Space Telescope suggests that white dwarf stars could continue to burn hydrogen in the final stages of their lives, causing them to appear more youthful than they actually are. This discovery could have consequences for how astronomers measure the ages of star clusters, which contain the oldest known stars in the universe.

These results challenge the prevalent view of white dwarfs as inert, slowly cooling burned-out stars where nuclear fusion has stopped. Now, an international group of astronomers has discovered the first evidence that white dwarfs can slow down their rate of aging by burning hydrogen on their surfaces.

"We have found the first observational evidence that white dwarfs can still undergo stable thermonuclear activity," explained Jianxing Chen of the Alma Mater Studiorum Università di Bologna and the Italian National Institute for Astrophysics, who led this research. "This was quite a surprise, as it is at odds with what is commonly believed."

White dwarfs have cast off their outer layers during the last stages of their lives. They are common objects in the cosmos; roughly 98% of all the stars in the universe will ultimately end up as white dwarfs, including our own Sun. Studying these cooling stages helps astronomers understand not only white dwarfs, but also their earlier stages as well.

To investigate the physics underpinning white dwarf evolution, astronomers compared cooling white dwarfs in two massive collections of stars: the globular clusters M3 and M13. These two clusters share many physical properties such as age and metallicity (the abundance of heavier elements), but the populations of stars which will eventually give rise to white dwarfs are different. This makes M3 and M13 together a perfect natural laboratory in which to test how different populations of white dwarfs cool.

"The superb quality of our Hubble observations provided us with a full view of the stellar populations of the two globular clusters," continued Chen. "This allowed us to really contrast how stars evolve in M3 and M13."

Using Hubble's Wide Field Camera 3 the team observed M3 and M13 at near-ultraviolet wavelengths, allowing them to compare more than 700 white dwarfs in the two clusters. They found that M3 contains standard white dwarfs, which are simply cooling stellar cores. M13, on the other hand, contains two populations of white dwarfs: standard white dwarfs and those which have managed to hold on to an outer envelope of hydrogen, allowing them to burn for longer and hence cool more slowly.

Comparing their results with computer simulations of stellar evolution in M13, the researchers were able to show that roughly 70% of the white dwarfs in M13 are burning hydrogen on their surfaces, slowing down the rate at which they are cooling.

This discovery could have consequences for how astronomers measure the ages of stars in the Milky Way galaxy. The evolution of white dwarfs has previously been modeled as a predictable cooling process. This relatively straightforward relationship between age and temperature has led astronomers to use the white dwarf cooling rate as a natural clock to determine the ages of star clusters, particularly globular and open clusters. However, white dwarfs burning hydrogen could cause these age estimates to be inaccurate by as much as 1 billion years.

"Our discovery challenges the definition of white dwarfs as we consider a new perspective on the way in which stars get old," added Francesco Ferraro of the Alma Mater Studiorum Università di Bologna and the Italian National Institute for Astrophysics, who coordinated the study. "We are now investigating other clusters similar to M13 to further constrain the conditions which drive stars to maintain the thin hydrogen envelope which allows them to age slowly."

The Hubble Space Telescope is a project of international cooperation between NASA and ESA (European Space Agency). NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Maryland, conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy in Washington, D.C.

Media Contact:

Bethany Downer


Ray Villard
Space Telescope Science Institute, Baltimore, Maryland

Science Contact:

Jianxing Chen
University of Bologna, Bologna, Italy

Francesco R. Ferraro
University of Bologna, Bologna, Italy

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Saturday, September 04, 2021

Young star HD 142527

Observations made with the Atacama Large Millimeter/submillimeter Array (ALMA) telescope of the disc of gas and cosmic dust around the young star HD 142527 show vast streams of gas flowing across the gap in the disc. These are created by giant planets guzzling gas as they grow. The dust in the outer disc is shown in red. Dense gas in the streams flowing across the gap, as well as in the outer disc, is shown in green. Diffuse gas in the central gap is shown in blue. The gas filaments can be seen at the three o’clock and ten o’clock positions, flowing from the outer disc towards the center. Credit: ALMA (ESO/NAOJ/NRAO), S. Casassus et al. Hi-res image