Sulfur shows up around a dying supergiant, and its isotope ratio does not add up

A team led by Cristobal Bordiu of INAF's Catania Astrophysical Observatory has reported the first detection of sulfur monoxide and sulfur dioxide around a B[e] supergiant, using the Atacama Compact Array of ALMA for the first detailed millimeter-wavelength survey of HD 87643 ^[1][2]. The practical consequence is that the star's known molecular inventory went from one species to ten, so anyone modelling the material these stars shed has been working with a chemistry an order of magnitude thinner than what is actually there ^[3][4][5].

B[e] supergiants are rare, evolved massive stars that have finished core hydrogen burning, identifiable by their emission lines, including uncommon forbidden lines from the surrounding gas and dust, and by an infrared excess far stronger than an ordinary B-type star's ^[7]. As they approach the end, they dump large amounts of material into their surroundings: fast winds off the poles, and a slower, denser, dusty flow around the equator that behaves like a disk ^[8]. HD 87643 is an extreme example of the class ^[1].

The survey detected CO, 13CO, OCS, H2CO, SO, 33SO, SO2, HNCO, O13CS and 13CS ^[4]. Six of those ten carry sulfur ^[6]. That is not a marginal improvement on a well-studied problem; it is the opening of one. Sulfur is the tenth most abundant element in the universe and one of six elements essential to life as we know it ^[10], and sulfur-bearing molecules have been catalogued in the solar system, in planet-forming disks and in star-forming regions, but around hot, massive, dying stars the chemistry was almost entirely unexplored, with only a single sulfur-bearing molecule previously reported around a variable star ^[11]. The authors write that the detections "substantially expand the molecular inventory of HD 87643, previously limited to CO," and set "a new chemical benchmark for early-type supergiants" ^[9].

The interesting number is an isotope ratio. Two isotopologues of sulfur monoxide sit at a ratio of roughly 15, well below values measured elsewhere in the galaxy, implying the rarer heavy isotope is far more prevalent near this star than anywhere else in the Milky Way ^[12]. Standard nuclear physics does not account for that ^[13]. The team's proposed alternative is photochemical: the star's ultraviolet field breaks sulfur dioxide into sulfur monoxide, and the abundant light isotopologue is plentiful enough to shield itself, so only a small fraction is dissociated, while the rare heavy isotopologue cannot self-shield and is destroyed almost wholesale, converting a disproportionate share of it into sulfur monoxide ^[14]. That mechanism, mass-independent fractionation, has so far been documented only in ancient terrestrial rocks and in meteorites; if it holds here, HD 87643 is an extreme case of it ^[15].

Two cautions belong with the result. The interpretation of the isotope ratio is a proposal, not a measurement of the mechanism, and the team itself says higher-resolution ALMA observations are needed to complete the picture of this star's sulfur chemistry ^[14][16]. The compact-array data establish that the molecules are present; they are not the instrument for deciding which of them sit in the equatorial disk rather than the polar wind ^[1][16].

What to watch is whether follow-up imaging separates the sulfur-bearing gas by geometry, because a species confined to the disk would be a usable handle on how these stars lose mass. The broader payoff the authors point to is the same one that makes the chemistry worth resolving: how molecules form and survive in these conditions bears on the role massive stars play in distributing the elements that later go into planets ^[17].