Sulfur Mystery in Planetary Nebulae Solved by Astrophysicists

An iconic collage showcasing 22 distinctive and famous Planetary Nebulae (PNe), artistically sorted into a spiral formation according to their physical sizes, has been created. This was a collaboration between ESA/Hubble, NASA, ESO, NOAO/AURA/NSF, and was initiated by the corresponding author and Ivan Bojičić, with additional inputs from David Frew and the author.

A 20-year conundrum about the lesser-than-anticipated quantity of sulfur in Planetary Nebulae (PNe) compared to other elements and various astrophysical objects, has been resolved by two astrophysicists from the Laboratory for Space Research (LSR) at The University of Hong Kong (HKU).

For quite a while, the expected amount of sulfur seemed “absent”. However, thanks to highly accurate and reliable data, the sulfur has now been accounted for. The results of their findings were recently published in the Astrophysical Journal Letters.

PNe, which are glowing gas shrouds ejected from dying stars, are transitory and their fascinating shapes have always been of interest to both professional and hobbyist astronomers. The lifespan of PNe, in contrast to their host stars, is quite brief - a few thousands of years in comparison to billions of years of the stars before transforming into “white dwarfs”.

A fleeting glimpse into the death throes of stars is provided by the PNe. They are an important scientific resource for studying the final stages of stellar evolution, given their detailed emission line spectra which help study their chemical makeup.

Optical spectra of previous PNe studies seemed to show an inconsistent deficit of sulfur – quite inexplicable, since sulfur, an “α element” should get produced along with other elements like oxygen, neon, argon, and chlorine in larger stars, making its cosmic abundance directly proportional.

An image captured by an ESO telescope based in Chile illustrates the Planetary Nebulae PN NGC 5189. Some believe it resembles a Chinese flying dragon floating in space.

Interestingly, though strong correlations between sulfur and oxygen quantities have been noticed in H II regions and compact blue galaxies, PNe that originate from low to intermediate-mass stars constantly demonstrate lower sulfur levels, leading to the mysterious “sulfur anomaly” that has puzzled astronomers for centuries.

Shuyu Tan, an HKU MPhil Physics graduate and research assistant at HKU LSR, and her supervisor Professor Quentin PARKER, the LSR Director, worked on a sample of nearly 130 PNe based in our Galaxy's center, with exceptional optical spectra. The dataset with minimum background noise helped in a detailed and clear inspection of the spectral features, aiding the team to solve the puzzling anomaly.

These PNe were studied with the European Southern Observatory (ESO) 8m Very Large Telescope in Chile. The sulfur emission anomaly in PNe spectra turned out to be a result of poor data quality. A better and stronger comparison with oxygen, sulfur and suggested as a more trustworthy indicator of metallicity, was found to be Argon.

The team noticed a strong correlation among sulfur in PNe when observation was done with a large number of PNe, spectroscopically and with a high Signal-to-noise ratio (S/N). The anomaly effectively disappeared with these observations.

Claims suggesting the sulfur anomaly in Planetary Nebulae was due to underestimated higher sulfur ionization stages or feeble sulfur line fluxes have been rejected by the study, stressing on the critical importance of high-quality data in cracking scientific quandaries.