XRISM presents a new theory about sulfur in the Milky Way, specifically in the stellar medium. NASA and JAXA are working together with X-rays to delve deeper into this new study. Lía Corrales is the principal investigator leading the project. Thus, troilitite, pyrrhotite, and pyrite are being closely studied to see how they relate to sulfur. This discovery was made possible by the GX 340+0 binary system, which has analyzed stars, molecular clouds, meteorites, and cosmic dust. Read on to learn more about this great galactic research.
Where is sulphur found?
Sulphur is everywhere. In our bodies, it forms part of proteins. On Earth, it participates in essential biogeochemical cycles. And beyond, in the universe, its history is much less clear. One might assume that, being a relatively abundant element, astronomers would already have a clear idea of where and how it is distributed among the stars. But no. For years, a significant portion of sulfur seemed to have “disappeared” from the interstellar medium, as if hiding from scientific instruments.
This mystery has just begun to be solved thanks to an extraordinary discovery: for the first time, interstellar sulfur has been detected in both gaseous and solid form, directly and in the same line of sight. The result was made possible by data obtained by the Japanese XRISM mission, which specializes in X-ray spectroscopy, and published in the Publications of the Astronomical Society of Japan by an international team led by Lía Corrales. This work not only helps to close one of the most persistent gaps in the chemistry of the cosmos, but also offers a new tool for understanding how stars, planets, and perhaps the basic ingredients of life are formed.
Sulfur: essential for life, elusive in space
Sulfur is a key element for life. It is involved in essential cellular processes and is present in amino acids, enzymes, and vitamins. However, in the interstellar medium—that vast environment of gas and dust that separates stars—its presence was not entirely clear. In low-density regions, it was detected as gas through ultraviolet observations. But in denser areas, such as the molecular clouds where stars are born, sulfur disappeared from the radar.
For decades, this disappearance has been attributed to a phenomenon of “depletion”: sulfur condenses and incorporates itself into dust grains, becoming invisible to many instruments. The problem is that, until now, this solid form had not been directly observed. As the study explains, “this is the first time that the detection of interstellar atomic sulfur in its SII Kβ absorption line has been demonstrated with a high signal.” In addition, absorption residues corresponding to solid sulfur compounds, such as troilite, pyrrhotite, and pyrite, have been identified.
An X-ray window to the other side of the galaxy
To achieve this breakthrough, the team used the Resolve spectrometer aboard XRISM, a joint mission between JAXA and NASA. The scientists observed the binary system GX 340+0, located about 11,000 parsecs away, at the other end of the Milky Way disk. They took advantage of the intense X-ray emission from this system as a background light source, which, as it passes through the interstellar medium, is “marked” with the chemical fingerprints of the elements present.
This technique allowed them not only to detect the Kβ line of ionized sulfur (SII) in gas, but also to record additional absorption that fits with the presence of sulfur in solid form, probably mixed with iron. As the paper details, “the absorption templates of three Fe-S compounds provide equally good fits to the residues” observed in the spectrum. This supports the hypothesis that some of the sulfur is trapped in minerals such as troilite, an iron sulfide commonly found in meteorites.
