Sulfur turns up around a dying supergiant, and takes a mass-independent oddity with it

A team led by Cristobal Bordiu of INAF's Catania Astrophysical Observatory has reported sulfur monoxide and sulfur dioxide in the material around HD 87643, the first detection of sulfur-bearing species toward a B[e] supergiant ^[1][2][8]. The paper, published in The Astrophysical Journal Letters on July 1, matters less for the novelty than for the arithmetic: an object whose molecular inventory was one species now has ten ^[3][4][5][7].

B[e] supergiants are rare evolved massive stars that have left the main hydrogen-burning phase, identified by emission lines that include rare forbidden lines and by an infrared excess far stronger than an ordinary B-type star's ^[9]. Late in life they shed large amounts of material into an awkward two-part geometry: fast winds off the poles, and a slower, denser, dusty flow around the equator ^[10]. That is the environment the ALMA Atacama Compact Array was pointed at, in what the authors describe as the first detailed millimeter-wavelength survey of HD 87643, an extreme member of the class ^[8].

The detected list is CO, 13CO, OCS, H2CO, SO, 33SO, SO2, HNCO, O13CS and 13CS ^[5]. Six of the ten carry sulfur ^[6]. Prior to this, the surroundings of HD 87643 had yielded carbon monoxide and nothing else ^[4], and the wider field was barely better off: sulfur-bearing molecules are known from the solar system to protoplanetary disks to star-forming regions, but around hot, massive, dying stars only a single sulfur-bearing molecule had ever been reported, around a variable star ^[12]. This is not an exotic element being chased for its own sake. Sulfur is the tenth most abundant element in the universe and one of six considered essential to life as we know it ^[11]. The team writes that the detections "substantially expand the molecular inventory of HD 87643, previously limited to CO," and establish "a new chemical benchmark for early-type supergiants" ^[13].

The awkward result is an isotope ratio. The two isotopic variants of sulfur monoxide come out at roughly 15, low enough that the heavier, rarer form appears more abundant here than anywhere else measured in the Milky Way ^[14]. Standard nuclear physics does not produce that ^[15]. The authors' proposed alternative is photochemical: the star's ultraviolet field dissociates sulfur dioxide into sulfur monoxide, and the common lighter isotopologue is abundant enough to shield itself, so only a small fraction is destroyed, while the rare heavier one cannot self-shield and is almost entirely processed ^[16]. The net effect is that a disproportionate share of the heavy isotope ends up as sulfur monoxide ^[16]. That mechanism, mass-independent fractionation, has so far been documented in ancient terrestrial rocks and in meteorites, not in circumstellar gas; if it holds up, HD 87643 is an extreme case ^[17].

What to watch is confirmation and geometry. The authors say higher-resolution ALMA observations are needed to pin down the full sulfur inventory ^[18], and the self-shielding story is testable: it predicts where in the UV-exposed material the fractionation should be strongest. The broader payoff claimed is bookkeeping on how massive stars distribute the elements that later go into planets ^[19]. That case is only as good as the number of sgB[e] stars surveyed, which currently stands at one ^[4][8].