Spectral Ornaments: The Chemical Fabric of NGC 2264’s Christmas Tree

Abstract

This study presents a detailed spectroscopic and photometric analysis of the early B-type stars HD 47777, HD 47887, and HD 261810 in the Christmas Tree Cluster. Using fully self-consistent non-LTE TLUSTY208 / SYNSPEC54 modelling of high-resolution ESPaDOnS spectra, combined with SEDs and TESS photometry, we derive significantly improved stellar parameters. Unlike previous He-weak classifications, HD 47777 shows a solar helium abundance, whereas HD 47887 and HD 261810 exhibit mild helium enrichment ([He/H] = 0.22 dex and 0.16 dex). We also report a considerably revised effective temperature of 27,000 K for HD 47887 based on Si iv λ4116 and O ii λ4189 lines. TESS data reveal rotational modulation for HD 47777 (P = 2.6319 d) and HD 47887 (P = 1.9464 d). For HD 261810, alongside confirming its SPB nature, we detect an extraordinary 17-peak high-frequency harmonic ladder (Δf ≈ 0.541775 d−1). Supported by SED-derived infrared excess and an asymmetric dual-dip phase-folded light curve, we suggest that this modulation may be driven by magnetospheric accretion, where gas from a truncated inner disk (~4 R*) is channelled into two discrete magnetic funnels. Overall, these findings highlight the complex interplay of rotation, localized magnetic fields, and active accretion environments shaping the early B-type population within the Christmas Tree Cluster.