University of Arizona researchers have observed, in unprecedented detail and spatial resolution, organic molecules in planetary nebulae, or the aftermath of dying stars. Their work sheds new light on how stars form and die.

Using the Atacama Large Millimeter Array, or ALMA, UArizona Regents Professor Lucy Ziurys and her collaborators observed 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.

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The researchers presented their findings during the virtual 238th Meeting of the American Astronomical Society on Tuesday. 

Planetary nebulae are bright objects produced when stars of a certain type reach the end of their evolution. Most stars in the galaxy, including the sun, are expected to end their lives this way.

As a dying star sheds large amounts of mass into space and becomes a white dwarf, it usually emits strong ultraviolet radiation. That radiation was long thought to break up any molecules hurled into the interstellar medium from the dying star and reduce them to atoms. However, detections of organic molecules in planetary nebulae in recent years have shown that this is not the case.

The new 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 thought to provide 90% of the material in the interstellar medium, with supernovae adding the remaining 10%.

"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," said Ziurys, a Regents Professor of Chemistry and Astronomy at UArizona. "But if the process starts with molecules instead, it could dramatically accelerate chemical evolution in nascent star systems."

The molecule emissions observed by Ziurys and her team outlined the shapes of 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 – equivalent to a dime viewed from 2.5 miles away – resulted in striking images of the nebulae, showing the complex geometries of the dense, ejected material with bars, lobes and arcs never clearly observed 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.

"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," Ziurys 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's possible that helium flashes, which originate in a hot, convective shell around the core of a dying star, could provide a source of explosive nuclear synthesis away from the star's center, 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," she said.

Many planetary nebulae are something of an enigma, Ziurys 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."

Organic Molecules Also Present in Outskirts of the Milky Way

In a second presentation at the AAS meeting, Lilia Koelemay, a doctoral student in Ziurys' research group, reported on the discovery of organic molecules in the outskirts of the Milky Way, more than twice as far from the galactic center than what is known as the Galactic Habitable Zone, or GHZ.

The Milky Way's GHZ region, which includes the solar system, is considered to have favorable 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 center.

Using the UArizona ARO 12-Meter Telescope on Kitt Peak near Tucson, Koelemay, Ziurys and their collaborators searched 20 molecular clouds in the Milky Way's Cygnus arms for signature emission spectra of methanol – a basic organic molecule. At 20 degrees Kelvin (approximately minus 423 degrees Fahrenheit), these clouds are extremely cold and far from the galactic center, at a distance of 13 to 23.5 kiloparsecs. The team detected methanol in all 20 clouds.

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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 center than the current established boundary.

"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," Koelemay said. "The widely held assumption was that in the outskirts of our galaxy, the chemistry necessary to form organics just doesn't occur."

That belief was partly based on the supposed dearth of organic molecules in the outer reaches of the galaxy, Koelemay said. The notion of the galactic habitable zone is based on the idea that for conditions to exist where life can evolve, a planetary system can't be too close to the galactic center with its extremely high density of stars and intense radiation. It also can't be too far out, because there would not be enough elements critical for life, such as oxygen, carbon and nitrogen.

Koelemay's observations were made possible by a new 2-millimeter wavelength receiver with unprecedented sensitivity. Developed in a collaboration with Ziurys, Steward Observatory engineer Gene Lauria and the National Radio Astronomy Observatory, the receiver allows for detection of molecular emission lines in a wavelength bandwidth radio astronomers in the U.S. couldn't access for years.

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"Without this new instrument, these observations would have taken hundreds of hours, which is not feasible," Ziurys said. "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." 

Koelemay has begun looking for other molecules besides methanol – such as 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 much interest, as many researchers deem them promising candidates for the emergence of life. When organic molecules are present in emerging planetary systems, they can condense 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, Koelemay's adviser and a co-author of the research. "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."