From the Laboratory to CW Leonis: A Hunt for a Metallic Molecule

This ultraviolet image of CW Leonis from NASA's Galaxy Evolution Explorer spacecraft shows the remarkable shell surrounding the star. Credit:NASA/JPL-Caltech

The sooty cloud surrounding the carbon star CW Leonis is known to contain more than 50 types of molecules, and the remaining unassigned spectral lines hint that many more molecules are present. Can a laboratory study of a metallic molecule help us identify some of these mystery spectral lines?

Another view of CW Leonis, this time from the Hubble Space Telescope. This image highlights the dusty layers shed by this evolved star. Credit: ESA/Hubble & NASA, T. Ueta, H. Kim; CC BY 4.0

Searching Space for Chemical Compounds

Astronomers have discovered more than 200 molecules in space since the first molecule was found in 1937. These discoveries confirmed something incredible — that in the cold, sparse space environment, individual atoms can link up to form complex molecules. Finding molecules in space represents both a challenge and an opportunity: how can we explain the presence of molecules in such an unforgiving environment, and how can we use the fact that they do exist to learn about the chemistry of interstellar and circumstellar space?

One of the best sites to study extraterrestrial molecules is in the dusty shroud and outflows of the star CW Leonis, also known as IRC+10216. CW Leonis is a carbon star: a supergiant star with a high abundance of carbon in its atmosphere. Among CW Leonis’s many molecules are metal-containing species like silicon dicarbide (SiC2), leading researchers to wonder if similar molecules might be responsible for any of the remaining unidentified lines in CW Leonis’s spectrum.

Summary of the known transitions and energy levels for magnesium dicarbide, as determined from laboratory and astrophysical observations. Credit: Changala et al. 2022

Making Magnesium Molecules

A team led by Bryan Changala‬ (Center for Astrophysics ∣ Harvard & Smithsonian) focused their search on magnesium dicarbide (MgC2). Changala and collaborators considered it likely that CW Leonis’s dusty shroud contains magnesium dicarbide because it’s chemically similar to the already-discovered silicon dicarbide, and many other magnesium-containing molecules have been found there.

How do you determine if a star’s spectral lines are due to a particular molecule, though? In order to be confident that we’ve discovered a molecule in space, we need to know its spectrum, which is best determined by studying the molecule in a lab. In the case of magnesium dicarbide, researchers have used quantum mechanical models to predict the molecule’s spectrum but had never confirmed it in a lab.

Changala‬ and coauthors combined magnesium atoms with acetylene molecules, which are made of carbon and hydrogen, hoping to synthesize magnesium dicarbide. The team successfully matched a spectral line from their sample to a line predicted by quantum mechanical models and performed additional tests to ensure that the molecule they created was actually magnesium dicarbide.

Example of a spectral line attributed to magnesium dicarbide. The fitted line profile is shown in red, and the blue “U” indicates unidentified lines. Adapted from Changala et al. 2022

Seeking a Spectral Match

Ultimately, Changala and collaborators used the spectrum of the newly synthesized molecule to assign 14 of CW Leonis’s unknown spectral lines to magnesium dicarbide and its isotopologues — molecules with the same chemical formula and structure in which one or more atoms has a different number of neutrons. What does the discovery of magnesium dicarbide in CW Leonis’s spectrum tell us? By comparing the abundance of magnesium dicarbide in the star’s surroundings with the abundances of other magnesium-containing molecules, researchers might be able to glean how these molecules are made. Additionally, these observations may help us understand how metals affect the chemistry of carbon-rich environments like the surroundings of carbon stars, helping to lift the veil on these dusty objects.

Citation

“Laboratory and Astronomical Discovery of Magnesium Dicarbide, MgC2,” P. B. Changala et al 2022 ApJL 940 L42. doi:10.3847/2041-8213/aca144

By Kerry Hensley

Source: American Astronomical Society - AAS Nova

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Hubble celebrates Halloween with a glowering, dying star

CW Leonis

Credit: Image: ESA/Hubble, NASA, Toshiya Ueta (University of Denver), Hyosun Kim (KASI)

Release images | Release videos

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A hypnotizing vortex? A peek into a witch's cauldron? A giant space-spider web?

In reality, it's a look at the red giant star CW Leonis as photographed by NASA's Hubble Space Telescope — just in time for celebrating Halloween with creepy celestial sights.

The orange-red "cobwebs" are dusty clouds of sooty carbon engulfing the dying star. They were created from the outer layers of CW Leonis being thrown out into the inky black void. The carbon, cooked up through nuclear fusion in the star's interior, gives it a carbon-rich atmosphere. Blasting the carbon back into space provides raw material for the formation of future stars and planets. All known life on Earth is built around the carbon atom. Complex biological molecules consist of carbon atoms bonded with other common elements in the universe.

At a distance of 400 light-years from Earth, CW Leonis is the closest carbon star. This gives astronomers the chance to understand the interplay between the star and its surrounding, turbulent envelope. The complex inner structure of shells and arcs may be shaped by the star’s magnetic field. Detailed Hubble observations of CW Leonis taken over the last two decades also show the expansion of threads of ejected material around the star.

The bright beams of light radiating outwards from CW Leonis are one of the star's most intriguing features. They've changed in brightness within a 15-year period — an incredibly short timespan in astronomical terms. Astronomers speculate that gaps in the dust shrouding CW Leonis may allow beams of starlight to pierce through and illuminate dust, like searchlight beacons through a cloudy sky. However, the exact cause of the dramatic changes in their brightness is as yet unexplained.

A star shines when the outward pressure from the fusion furnace at the core balances against the crush of gravity. When the star runs out of hydrogen fuel, the persistent pull of gravity causes the star to start collapsing. As the core shrinks, the shell of plasma surrounding the core becomes hot enough to begin fusing hydrogen, giving the star a second lease on life. It generates enough heat to dramatically expand the star's outer layers and swell up into a bloated red giant.

CW Leonis has an orange-reddish color due to its relatively low surface temperature of 2,300 degrees Fahrenheit. The green-tinted beams of light emanating from the star, however, glow at invisible mid-infrared wavelengths. In the absence of natural color, green has been added to the infrared image for better analysis through color-contrast.

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.

Credits: 

Release: ESA/Hubble, NASA, Toshiya Ueta (University of Denver), Hyosun Kim (KASI)

Media Contact:

Ray Villard
Space Telescope Science Institute, Baltimore, Maryland

Bethany Downer
ESA/Hubble.org

Science Contact:

Toshiya Ueta
University of Denver, Denver, Colorado

Hyosun Kim
Korea Astronomy and Space Science Institute, Daejeon, South Korea


Contact us:

Direct inquiries to the News Team.

Related links and documents:

• NASA's Hubble Portal
• ESA/Hubble's Release
• Goddard's "Hubble Spotted Something Scary" Video (on YouTube)

Source: HubbleSite/News

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Herschel probes the dusty history of a giant star

About 5 billion years from now, our Sun will expand into a red giant, swelling to such a size that it may swallow the Earth. It will then begin to shed huge amounts of dust, surrounding itself with an expanding circumstellar envelope (CSE) that ultimately will become a planetary nebula. New insights into this process have been revealed by ESA's Herschel Space Observatory, which is providing unprecedented images of the complex, outer structure of a nearby CSE.

A study of IRC+10216 has revealed a series of dust shells that have never been seen before. Credit: ESA/PACS/MESS Consortia. Hi-Res [jpg] 684.24 kb

As part of a long term programme to study aging stars, known as the Mass loss of Evolved StarS (MESS) survey, Herschel's Photodetector Array Camera and Spectrometer (PACS) instrument has been used to observe a nearby, carbon-rich star known as IRC+10216, or CW Leonis.

Classified as an Asymptotic Giant Branch (AGB) star, IRC+10216 has evolved into a red giant, several thousand times bigger than the Sun, and is now nearing the final stages of its life.

Nuclear reactions in its core have transformed most of its hydrogen into helium, and the star is now characterised by an inert carbon-oxygen core, surrounded by two separate layers where nuclear fusion is taking place - an inner layer of helium and an outer layer of hydrogen. These layers are surrounded by a strongly convective outer envelope of hydrogen.

As the star evolves through the AGB phase, burning its nuclear fuel faster and faster, it is cooling and expanding, allowing dust to condense in its outer envelope. At the same time, IRC+10216 has begun to pulsate, causing a stellar wind of dust and gas to be expelled from its surface into the surrounding space. Measurements show that the dust is expanding outwards at a velocity of 14.5 km/s.

The presence of this dusty cocoon has been known for many years, but, until now, no instruments have been able to observe the structure of its cold outer regions, where the temperature plummets to –248 °C. Now, PACS infrared images taken at wavelengths of 70, 100 and 160 microns have revealed multiple dust shells in the circumstellar envelope of IRC+10216. The results are published this week in the journal Astronomy & Astrophysics.

Some new features in the circumstellar envelope of IRC+10216 (CW Leonis) are indicated in this annotated image. (A) The arc indicates the location of a bow shock, situated about 1 light year from the star; dust shells, corresponding to the ejection of material from the star at (B) 16 000 years, (C) 12 750 years, (D) 2500 years and (E) 1175 years ago are also indicated. Credit: ESA/PACS/MESS Consortia. Hi-Res [jpg] 1,267.26 kb

The extremely sensitive PACS instrument has unveiled at least a dozen dust shells (or arcs) that have never been seen before. While arcs which had been ejected within the last 4000 years were previously observed up to 80 arc seconds from the star, the new images show material which was ejected some 16 000 years ago and is now visible at a distance of 320 arc seconds.

Arcs which were shed much earlier than this are no longer visible. Although the mass-loss process started some 220 000 years ago, the earliest arcs have been destroyed by the violent interaction of the stellar wind with the interstellar medium at the bow shock interface, about one light year (almost 9.5 million million kilometers) from IRC+10216.

Surprisingly, the almost spherical shells are non-concentric, of variable thickness, and unevenly spaced. The arcs contain some 50 per cent more dust mass than the smooth envelope and local density variations are also visible within one of the arcs.

The complex internal structure of the nebula is a record of how the star has been losing mass during the recent past. A number of possible explanations for the asymmetric structure of the dust shells have been examined by the authors of the new paper.

"The shell separation distances indicate that they were ejected some 500 to 1700 years apart," said Leen Decin from the Instituut voor Sterrenkunde, Katholieke Universiteit Leuven, Belgium, lead author of the paper.

"The irregular spacing between the arcs suggests that the structure is not caused by the regular gravitational perturbations associated with an unseen binary companion in orbit around the star.

"A second hypothesis favours enhanced dust formation from magnetic cool spots on the star, rather like the coronal mass ejections which are associated with sunspots. However, the large size of the arcs suggests that there would have to be several sizeable starspots existing in close proximity at the same time. Furthermore, the spacing of the shells shows no evidence of the periodicity that would be expected if the star was experiencing a cycle of rising and falling magnetic activity, like our Sun.

"It seems more likely that the arcs are caused by slight variations in ejection velocity or in the time the ejection took place as the star pulsates and loses mass. Variations in the clumpiness of the dust, associated with temperature variations in the nebula, may also play a part."

Located some 500 light years from Earth, IRC+10216 is one of the best-known examples of the 150 or so evolved stars which are being studied in the MESS survey, one of the guaranteed time, key observational programmes being undertaken with the PACS and SPIRE instruments on board Herschel.

"The angular resolution of these instruments provides accurate maps of the far infrared emission of different types of evolved stars," said Groenewegen from the Observatory of Belgium in Brussels, Investigator of the MESS programme. "This helps us to infer detailed information on the mass, size and structure of the dust shells, and possible grain size/temperature gradients, significantly improving our knowledge of the mass-loss history of these giant stars."

Herschel image of IRC+20216 (CW Leonis). The bow shock is clearly visible to the left of the star. Credit: ESA/PACS/SPIRE/MESS Consortia. Hi-Res [jpg] 714.34 kb

IRC+10216 was also the subject of a recent paper, published in the journal Nature, in which Leen Decin described the detection of warm water vapour in the sooty envelope of the carbon star. One way to create water in this carbon-rich environment is by means of photochemistry, induced by the penetration of highly energetic interstellar photons of ultraviolet light into a non-homogeneous envelope.

The new PACS detection of arcs in the outer envelope confirms that the nebula surrounding IRC+10216 is variable in structure, and that photochemistry is an important process in creating warm water vapour.

"By virtue of its large telescope enabling us to see these structures in such fine detail, Herschel is adding to our understanding of this iconic star," commented Göran Pilbratt, ESA's Herschel Project Scientist.

Reference publication

L. Decin, et al., "Discovery of multiple dust shells beyond 1 arcmin in the circumstellar envelope of IRC+10216 using Herschel/PACS". Published online in Astronomy & Astrophysics on 20 September 2011.

Contacts

Leen Decin
Katholieke Universiteit Leuven
Department Natuurkunde en Sterrenkunde
Belgium
Email: Leen.Decin@ster.kuleuven.be
Phone: +32 16 327041

Martin Groenewegen
Royal Observatory of Belgium
Ringlaan 3
B-1180 Brussels
Belgium
Email: marting@oma.be
Phone: +32 2 3730203

Göran Pilbratt
Herschel Project Scientist
Research and Scientific Support Department
Science and Robotic Exploration Directorate
ESA, The Netherlands
Email: gpilbratt@rssd.esa.int
Phone: +31 71 565 3621

CW Leonis

CW Leonis

Image Credit: ESA / SPIRE / PACS / MESS

The red giant carbon star CW Leonis is too bright to be seen well by the SPIRE and PACS cameras, but it is releasing material in a violent stellar wind. Some of that material is seen as a “bow shock” to the left of the star in this image, glowing as it piles up against the interstellar medium. Observations with the PACS and SPIRE spectrometers have shown that water vapour is being formed deep down near the surface of the star; a place where it was previously thought to be impossible.

When astronomers discovered an unexpected cloud of water vapour around the old star CW Leonis in 2001, using the Submillimeter Wave Astronomy Satellite (SWAS), they immediately began searching for the source. Water is known to be present around several types of stars, but CW Leonis is a “carbon star” and therefore thought not to produce water. Initially they suspected the star’s heat must be evaporating comets or even dwarf planets to produce the water.

CW Leonis is a red giant star which is only a few times the mass of the Sun but has expanded to hundreds of times its size. Nuclear fusion reactions deep inside the star are converting helium into carbon, much of which has ended up in the outer layers of the star’s atmosphere. It is this abundance of Carbon in the atmospheres of these types of stars that has previously led scientists to believe that water could not exist; with so much carbon all of the oxygen should be locked up in carbon monoxide (CO).

Now, the SPIRE and PACS spectrometers ave shown that the water vapour is being formed much closer to the star itself than preivously thought, at temperatures approaching 1000oC. These results, published in Nature, suggests that the water is being created by a previously unsuspected chemical process where ultraviolet radiation from interstellar space is breaking up the carbon monoxide and releasing oxygen atoms that can then react with hydrogen to form water molecules.

Ultraviolet starlight would normally be blocked by the material flowing from the star as its outer layers billow out in a stellar wind. It was already known that the stellar wind is “clumpy” but the Herschel results have shown that some regions around the star must have no wind. These empty regions allow the ultraviolet light to reach the deepest layers of the star’s atmosphere and initiate the chemical reactions that produce the water.

About this image: CW Leonis as seen by SPIRE and PACS, with the emission lines due to water also shown. Image credit: ESA / SPIRE / PACS / MESS Consortia

The water is detected by the fact that it preferentially emits light with specific wavelengths, with the relative brightnesses of the "emission lines" indicating the conditions in which the water is forming.

The wavelength range of the SPIRE and PACS spectrometers, between them covering 60--670 micrometers (frequencies of 0.45--5 THz), allows astronomers to detect elements and molecules previously impossible. The wide range now also makes it possible to determine the temperature od the environment in which the water is forming, which wasn't possible before.