2026MNRAS.545f1797J

We present phosphorus abundance measurements for a total of 102 giant stars, including 82 stars in 24 open clusters and 20 Cepheids, based on high-resolution near-infrared spectra obtained with GIANO-B. Evolution of phosphorus abundance, despite its astrophysical and biological significance, remains poorly understood due to a scarcity of observational data. By combining precise stellar parameters from the optical, a robust line selection and measurement method, we measure phosphorus abundances using available P I lines. Our analysis confirms a declining trend in $\mathrm{[P/Fe]}$ with increasing $\mathrm{[Fe/H]}$ around solar metallicity for clusters and Cepheids, consistent with previous studies. We also report a $\mathrm{[P/Fe]}$-age relation among open clusters older than 1 Gyr, indicating a time-dependent enrichment pattern. Such a pattern can be explained by the different stellar formation history of their parental gas, with more efficient stellar formation in the gas of older clusters (thus with higher phosphorus abundances). $\mathrm{[P/Fe]}$ shows a flat trend among Cepheids and clusters younger than 1 Gyr (along with three Cepheids inside open clusters), possibly hinting at the phosphorus contribution from the previous-generation low-mass stars. Such a trend suggests that the young clusters share a nearly common chemical history, with a mild increase in phosphorus production by low-mass stars.