One element, two tracks: the unusual time evolution of phosphorus in the Milky Way

Using data acquired with GIARPS at the Telescopio Nazionale Galileo an international team (with researchers from Stockholm University, INAF, PMO Nanjing, Tokyo University, Malmö University and others) has produced the first age-resolved map of phosphorus of the Milky Way. By combining near-infrared spectroscopy with precise age tags from open clusters and classical Cepheids, the researchers uncovered a two track timeline for how this life-linked element accumulated in our Galaxy’s recent past. After the Big Bang produced mostly hydrogen, helium, and a trace of lithium, heavier elements were forged in stellar interiors, supernovae, kilonovae and so on, then recycled into new stars and planets.

The target in this study is phosphorus. It is part of the body’s “hardware”: it helps build bones and teeth together with calcium. Yet the origin of phosphorus is not well understood.

Fig. 1 The figure shows the evolution of phosphorus vs age using our sample of open clusters (OC) and Cepheids. Two regimes emerge: for older clusters (>1 Gyr) phosphorous rises with age, while for the younger populations (<1 Gyr) the trend is flatter.

This work uses weak phosphorus lines in the near-infrared to show that older clusters (>1 billion years) have increasing phosphorus with age; while younger populations, including Cepheids (young variable stars) show flat or only mildly changing phosphorus. These two tracks need to be explained by two different phosphorus formation scenarios. "Our phosphorus-age diagram acts like a chemical clock," says Dr. Mingjie Jian, the lead author from the Department of Astronomy, Stockholm University. "It tells us when the Galaxy stocked the shelves with a life-linked element and which stars did it."" Most of the previous studies on phosphorus lacked reliable ages for the stars. Without ages, it's hard to tell when different stellar sources dominated phosphorus production. Open clusters solve this: their stars share the same age and composition and ages can be determined with reliable methods, so phosphorus can be placed on a precise timeline.

The study observed stars of 24 open clusters together with 20 Cepheids using GIARPS at the TNG. The data is part of the Stellar Population Astrophysics (SPA) large program (PI L. Origlia) and in particular of SPA-Open clusters and SPA-variables (led by A. Bragaglia from INAF - OAS Bologna and G. Bono from Università di Roma Tor Vergata). "Seeing an early, massive star driven track alongside a later, low-mass powered track changes the timeline for life's ingredients in our corner of the Milky Way," adds Dr. Xiaoting Fu, the second author of this work from Purple Mountain Observatory. "It also gives modelers a sharper target for simulating how phosphorus evolve in our Milky Way ."

The results are published in a paper in press on MNRAS: "Stellar population astrophysics (SPA) with the TNG. The Phosphorus abundance on the young side of Milky Way", Jian M., Fu X., D'Orazi V., Bragaglia A., Bijavara Seshashayana S., Zhao H., Guo Z., et al., 2025, arXiv:2510.14791