UArizona Find Organic Molecules That Offer Clues About Dying Stars

UArizona researchers have observed in never before seen detail and spatial resolution, organic molecules in planetary nebulae, or the aftermath of dying stars, this work gives new insight to the star life cycle.

Using the Atacama Large Millimeter Array (ALMA), UArizona Regents Professor of Chemistry Lucy Ziurys and her collaborators examined radio emissions from hydrogen cyanide, formyl ion, and carbon monoxide in five planetary nebulae, M2-48, M1-7, M3-28, K3-45, and K3-58. These findings were presented at the 238th meeting of The American Astronomical Society (AAS).

UArizona’s Team’s Findings

These planetary nebulae are bright objects created when certain stars read the end of their evolution, most stars, like our Sun, will be expected to end their life cycle this way. Dying stars shed large amounts of mass into space and becomes a white dwarf and usually gives off ultraviolet radiation,

It was believed that this radiation eradicated any molecules released into space and reduce them to atoms, however, detections of organic molecules in planetary nebulae in recent years have shown this is not true. Observations by Ziurys and her team further support the idea that planetary nebulae instead seed the interstellar medium with molecules that serve as the raw ingredients for the formation of new stars and planets.

Planetary nebulae are believed to provide 90% of the material in the interstellar medium,  with supernovae adding the remaining 10%. Ziurys said “It was thought that molecular clouds, which would give rise to new stellar systems, would have to start from scratch and form these molecules from atoms”, she also said, “But if the process starts with molecules instead, it could dramatically accelerate chemical evolution in nascent star systems.”

These molecule emissions observed by Ziurys and her team outlined the shapes o the planetary nebulae, which previously had only been observed in visible light. In some cases, molecular signatures revealed previously unseen features.

A high resolution of one arcsecond, which is like looking at a coin from 2.5 miles away, resulted in striking images of the nebulae, showing the complex geometries of the dense material with bars, lobes, and arcs that are ejected, these have never been seen before. Ziurys and her team believe the shapeshifting behavior in the nebulae geometry may be driven by certain processes involved in nucleosynthesis or the forging of new elements inside a star.

Ziurys said, “It tells us that in a dying star, which is spherical until its final phase, some very interesting dynamics occur once it goes through the planetary nebula stage, which changes that spherical shape”. She also said, “These stars just lose their mass, and so there’s really no mechanism for them to all of a sudden become bipolar or even quadrupolar.”

It is possible that helium flashes, which are created in a hot, convective shell around the hearth of the dying star, could provide a source of explosive nuclear synthesis away from the star’s centre, resulting in the complex shapes seen in some nebulae. Ziurys said, “This could probably distort the spherical shape because a helium flash can explode through the poles of a star, where it will be directed by magnetic fields, and that will have an effect on the shape of the nebula that will form around it”.

Ziurys noted many planetary nebulae are still very much an enigma, she said “It’s been a puzzle to astronomers as to how you go from a spherical geometry into these multipolar geometries,” she said. “The molecules we observed trace the polar geometries beautifully, and so we’re hoping that this is going to give us some insight into the shaping of planetary nebulae.”

Lilia Koelemay, a doctoral student in Ziurys’ research group, in the second presentation at the AAS meeting, reported on the discovery of organic molecules in the outskirts of the Milky Way, more than twice as far from the galactic centre than what is known as the Galactic Habitable Zone (GHZ). The Milky Way’s GHZ region, including the solar system, is considered to have more favourable conditions for the formation of life.

It is thought to extend to only up to 10 kiloparsecs, or about 32,600 light-years, from the galactic centre. Using UArizona’s ARO 12-Metre Telescope on Kitt Peak near Tucson, Ziurys, Koelemay and their collaborators searched the 20 molecular clouds in the Milky Way’s Cygnus arms for signature emission spectra of methanol, a basic organic module.

At 20 degrees Kelvin, around 423 degrees Fahrenheit, these cloud are extremely cold and far from the galactic centre at a distance of 13 to 23.5 kiloparsecs, the team detected methanol in all 20 clouds.  According to Koelemay, the detection of these organic molecules at the galactic edge may imply that organic chemistry is still prevalent at the outer reaches of the galaxy, and the GHZ may extend much further from the galactic centre than the currently established boundary.

Speaking on this Koelemay said, “Scientists have wondered about the extent of organic chemistry in our galaxy for a long time, and it was always thought that not too far beyond our sun, we’re not going to see a lot of organic molecules”. She also said, “The widely held assumption was that in the outskirts of our galaxy, the chemistry necessary to form organics just doesn’t occur.”

The belief was partly centred around the supposed lack of organic molecules in the outer corners of the galaxy. The notion of the galactic habitable zone is based on the idea that for conditions to exist where life can evolve, a planetary system cant be too close to the galactic centre with its extremely high density of stars and intense radiation.

It also cannot be too far out as there would not be enough elements to make a planet habitable for life, like oxygen, carbon and nitrogen. Koelemay’s observations were possible thanks to a new 2-millimetre wavelength receiver with unprecedented sensitivity.

Developed in a collaboration with UArizona’s Ziurys, Steward Observatory engineer Gene Lauria and the National Radio Astronomy Observatory, the receiver allows for the detection of molecular emission lines in a wavelength bandwidth radio astronomers in the U.S. couldn’t access for years.

Speaking on this, Ziurys said, “Without this new instrument, these observations would have taken hundreds of hours, which is not feasible”. She said also “With this new capability, we expect to dramatically open our observation window and detect molecules in other regions of our galaxy previously thought to be devoid of such chemistry.”

UArizona’s Koelemay has begun looking for other molecules besides methanol like methyl cyanide, organic molecules with ring structures, and others that contain functional groups known to be crucial building blocks for biomolecules. Discoveries of those molecules in the interstellar medium have attracted a lot of interest, many researchers deem in emerging planetary systems, they can be condensed onto the surfaces of asteroids, which then deliver them to nascent planets, where they could potentially jumpstart the evolution of life.

“We’re finding these species way on the outskirts of the galaxy, and the abundance doesn’t even drop off 10 kiloparsecs from the solar system, where the chemistry necessary for building the molecules necessary for life just wasn’t believed to occur,” said Ziurys. She also said, “The fact that they’re there expands the prospects of habitable planets forming far beyond what has been considered the habitable zone, and it is extremely exciting.”