ALMA detects sulfur molecules around rare B[e] supergiant star for first time

Astronomers have detected a chemically rich mixture of sulfur-bearing molecules around HD 87643, a rare B[e] supergiant star, using the Atacama Compact Array of the Atacama Large Millimeter/submillimeter Array (ALMA). The study, led by Cristobal Bordiu of the INAF Catania Astrophysical Observatory and published in The Astrophysical Journal Letters on July 1, marks the first detailed millimeter-wavelength survey of this extreme evolved massive star.

B[e] supergiants are evolved massive stars that have exhausted their core hydrogen and shed enormous amounts of material, creating complex environments of fast polar winds and a dense, dusty equatorial disk. Before this work, only carbon monoxide had been identified in the surroundings of HD 87643. The new observations revealed emissions from ten different molecules: CO, 13CO, OCS, H2CO, SO, 33SO, SO2, HNCO, O13CS, and 13CS.

Several of the newly detected molecules contain sulfur, making this the first detection of sulfur chemistry around any B[e] supergiant. Sulfur is the tenth most abundant element in the universe and one of six elements essential to life as we know it. While sulfur-bearing molecules have been found in many cosmic environments, including planet-forming disks and stellar nurseries, they had remained almost entirely unexplored around hot, massive, dying stars.

The researchers also uncovered a puzzling isotopic ratio in sulfur monoxide. The ratio between the two sulfur isotopes is approximately 15, far lower than values typically found elsewhere in the Milky Way, indicating the heavier isotope is present far more often near this star than expected. Standard nuclear physics cannot explain the imbalance.

The team proposes that the star's intense ultraviolet light breaks apart sulfur dioxide molecules into sulfur monoxide through a process called mass-independent fractionation. The common, lighter isotope of sulfur dioxide is abundant enough to shield itself from UV radiation, so only a small fraction dissociates. The rarer, heavier isotope lacks this self-shielding, so nearly all of it is exposed and destroyed, converting a much larger share into sulfur monoxide despite its lower initial abundance.

Mass-independent fractionation has previously been documented only in ancient Earth rock samples and meteorites. If confirmed, HD 87643 would represent an extreme astrophysical case of this phenomenon. The researchers note that future high-resolution ALMA observations will be needed to fully characterize the star's sulfur chemistry and its implications for how massive stars distribute chemical elements necessary for planet formation and life.