We present a statistical study of infrared variability using the Near-Earth Object Wide-field Infrared Survey Explorer (NEOWISE) database for a sample consisting of 1085 high-mass young stellar objects (YSOs) related to 6.7 GHz methanol masers. A total of 383 maser sources were identified as NEOWISE variables and classified in two variability behavior classifications: 204 secular (linear, curved, and periodic) and 179 stochastic (burst, drop, and irregular) variables. Statistical analysis of the properties of these variables (e.g., the dust temperature, bolometric luminosity, hydrogen column density, W4 luminosity, and W1−W2 color) has revealed a potential evolutionary sequence among different light-curve types of variables. There is a possible general evolutionary (from less to more evolved) trend between the three variable types from secular to stochastic to nonvariable. For the specific classifications, the evolutionary trend for secular variables is linear to sin to sin+linear, and for stochastic variables it is burst to irregular to drop. These sequences may reflect the evolution of the envelope or accretion disk of high-mass YSOs, from large to small radii due to gravitational collapse. Although no significant variability correlation was established between the 6.7 GHz methanol maser and the W2-band emission based on the data collected so far, a number of candidates were found for further investigating the accretion burst events via future variability monitoring programs of both mid-infrared emission and masers.
We have conducted a systematic line survey, primarily focused on transitions of the methanol and ammonia molecules, and monitoring observations of masers toward the high-mass star-forming region NGC 6334I. These observations were undertaken between 2019 and 2022 in the C, K, Ka, and Q bands with the Tianma Radio Telescope. In total, 63 CH3OH (including 11 class I and nine class II maser or maser candidate), 18 13CH3OH, and 34 NH3 (including seven maser or maser candidate) transitions were detected. The emission is likely associated with the luminosity outburst source MM1. Rotation diagram analysis of multiple ammonia transitions shows that the gas temperature in the molecular core was a factor of 2 higher than that measured in previous observations in the pre-burst stage. This suggests that the molecular core has likely been heated by radiation originating from the luminosity outburst. Maser variability in the methanol and excited-state OH masers shows a general trend that the maser components associated with the luminosity outburst have decreased in their intensity since 2020. The decay in the maser luminosity indicates that the outburst is possibly declining, and as a result, the duration of the MM1 luminosity outburst may be shorter than the predicted 40 yr duration. Compared to the masers detected toward another luminosity outburst source, G358.93-0.03, abundant class I methanol masers and strong water maser flares were also detected toward NGC 633I, but masers from rare class II methanol transitions and new molecules were absent toward NGC 6334I. The large number of detections of maser transitions toward the two burst sources provided a database for further maser modeling to explore the physical environments associated with accretion burst events.
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