The candidates of long-periodic variable sources in 6.7 GHz methanol masers associated with four high-mass star-forming regions.

This article presents results from a decade-long monitoring of 6.7 GHz Class II methanol masers—tracers of high-mass star-forming regions (HMSFRs)—using the Hitachi 32 m radio telescope. Periodic flux variability was detected in four HMSFRs (G06.795−0.257, G10.472+0.027, G12.209−0.102, and G13.657−0.599) with tentative periods ranging from 968 to 1624 days. The symmetric, continuous flux variations and consistency with a protostellar period–luminosity relation in G10.472+0.027 and G12.209−0.102 suggest protostellar pulsation instability as the likely mechanism, indicating very high-mass protostars (~40 solar masses) with high accretion rates (~2 × 10⁻² solar masses per year). In contrast, G06.795−0.257 and G13.657−0.599 exhibit intermittent variation patterns and lower luminosities inconsistent with this relation, implying that their periodicity may arise from binary system interactions, with periodic accretion models favored. The study highlights the need for high-resolution Very Long Baseline Interferometry (VLBI) monitoring to further elucidate the spatial and physical origins of these maser periodicities.