Stellar population astrophysics (SPA) with the TNG. The Phosphorus abundance on the young side of MilkyWay

TL;DR

This work addresses the poorly constrained Galactic phosphorus evolution by measuring P abundances in 82 cluster giants and 20 Cepheids using high-resolution near-infrared spectra from GIANO-B, complemented by optical data. It employs a robust line-selection and LTE synthesis approach with PySME, leveraging VALD3 Pi lines and MARCS atmospheres to derive $A( ext{P})$ and $[P/Fe]$ across a wide age range. The results show a modest decrease of $[P/Fe]$ with increasing $[Fe/H]$ near solar metallicity, and reveal a V-shaped $A( ext{P})$–age relation: older clusters reflect diverse, historically richer P enrichment, while younger clusters and Cepheids indicate local, quiescent enrichment likely sourced from low-mass stars. The findings support a two-regime Galactic chemical evolution for phosphorus and highlight the role of star-formation history and AGB contributions in shaping P abundances, with no strong spatial gradient detected across the sampled region.

Abstract

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 [P/Fe] with increasing [Fe/H] around solar metallicity for clusters and Cepheids, consistent with previous studies. We also report a [P/Fe]-age relation among open clusters older than 1 Gyr, indicating a time-dependent enrichment pattern. Such 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). [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 trend suggests that the young clusters share a nearly common chemical history, with a mild increase in phosphorus production by low-mass stars.

Figures (11)

• Figure 1: Stellar total yield of phosphorus in three different initial metallicities ($\mathrm{ini}Z$). The yield has been weighted by Kroupa IMF, and the stellar population has been normalized to 1 $\rm M_{\odot}$.
• Figure 2: CMD of the target clusters. The grey dots represent stars with membership probabilities greater than 0.5 from Cantat-Gaudin2020, while the blue circles highlight our selected cluster giant stars.
• Figure 3: The Kiel diagram of our sample stars. The underlying contours show the EW (mÅ) of Pi line at 10529.524 Å at solar metallicity.
• Figure 4: Phosphorus abundance determination from the Pi $10529.524\,$Å line in the solar spectrum. Top panel: Synthetic spectra after varying the abundances of P by $\pm0.1$,dex (blue shaded profiles) together with spectra that contains only the Pi transition (solid blue), and other species (gray). The orange dotted vertical line marks the line centre, while the orange shaded strip marks the wavelength range used in the fit. Bottom panel: Observed spectrum (blue points) compared with the best–fitting synthetic spectrum (solid orange) and models corresponding to $\pm1\sigma$ uncertainty in $A(\mathrm{P})$ (orange dashed). The light-grey dashed "stair-step" curve (right-hand $y$-axis) shows the pixel mask: cont (continuum windows), line (pixels included in the fit), and bad (pixels excluded). The orange shaded region is the same fitting window as in the top panel. Similar plots for the other analysed P lines are available in the Supporting Information.
• Figure 5: Similar as Figure \ref{['fig:P-line-fit-exmaple-sun']}, but for the line at $10581.577\,$Å of Alessi 1-2. Here several CN lines appear around the target P line, and their synthetic spectrum is also plotted in the upper panel. Similar plots for other lines and other stars are available in the Supporting Information.