A Census of Chemically Peculiar Stars in Stellar Associations

TL;DR

This study conducts a comprehensive search for chemically peculiar (CP) stars within young open clusters and stellar associations to constrain the early evolution of CP phenomena. By combining HDBSCAN‑based membership, synthetic $\Delta a$ photometry, MKCLASS spectral classifications, and TESS light‑curve analyses, the authors identify 971 CP stars and candidates across 217 clusters, with ~10% displaying pre‑main‑sequence (PMS) characteristics. SED fitting via VOSA reveals infrared excess in 92 objects and H$\alpha$ emission in 12, underscoring a sizeable PMS CP population and their potential connection to magnetic fields and diffusion processes. The results substantially expand the catalog of candidate PMS CP stars and offer a valuable resource for probing the origin of CP phenomena during early stellar evolution, while also highlighting the need for higher‑quality spectra and broader coverage to refine membership and classifications.

Abstract

The pre-main-sequence evolution of the chemically peculiar (CP) stars on the upper main sequence is still a vast mystery and not well understood. Our analysis of young associations and open clusters aims to find (very) young CP stars to try to put a lower boundary on the age of such objects. Using three catalogues of open clusters and associations, we determined membership probabilities using HDBSCAN. The hot stars from this selection were submitted to synthetic $Δa$ photometry, spectral, and light curve classification to determine which ones are CP stars and candidates. Subsequently, we used spectral energy distribution fitting and emission line analysis to check for possible PMS CP stars. The results were compared to the literature. We detected 971 CP stars and candidates {in 217 clusters and associations}. A relatively large fraction, $\sim$10\% of those, show characteristics of PMS CP stars. This significantly expands the known list of candidate PMS CP stars, bringing us closer to solving the mystery of their origin.

Figures (16)

• Figure S2: Result of the clustering on the Monocerotis south-west region, called as such by 2021ApJ...917...23K. The stars are colour-coded by membership probability. Three of the sub-plots show astrometric properties ($XYZ$-distances and transversal velocities). The fourth one on the upper left shows a colour--magnitude diagram (CMD) of the region as a visual check if the clustering gives a reasonable result.
• Figure S3: Extinction corrected Colour--Absolute Magnitude Diagram of the clustered stars in the StarHorse21 catalogue. The red stars denote the stars hotter than $T_{eff}>6500$ K.
• Figure S4: Filter curves used for $\Delta a$ photometry.
• Figure S5: Result of the synthetic $\Delta a$ photometry. See Section \ref{['syn_phot']} for details.
• Figure S6: Process of extracting the lightcurves for a circular mask (Section \ref{['methods']}). Left: Target pixel file with aperture. Right: Raw (black) and corrected (red) normalised flux. The corrected flux values have been shifted down by 1.5 per cent for clarity.