Luminosity Outburst of a High-mass Young Stellar Object Triggered by the Surrounding Radiation Field

Abstract

We present observations of the 6.7 GHz methanol and 4.8 GHz formaldehyde masers toward the high-mass young stellar object G24.33+0.14 (hereafter G24). Our observations were conducted from 2019 to 2021 using the Shanghai Tianma 65 m Radio Telescope and the Very Large Array in response to the luminosity outburst event traced by these two species masers in 2019. Our results indicate that the provenance of the maser flares is unlikely to be ascribed to the protostar of G24 itself. Through analyzing NEOWISE infrared monitoring data, we identified two light curves of G24 with long-term (3083 days, ∼8.5 yr) and short-term (424 days) periods. Intriguingly, 11 periodic variable sources located in the same bubble as G24 exhibiting periods comparable to the short-term period of G24 were also detected. The analysis of the spectral energy distributions of these periodic variables revealed a possible correlation between their temperature fluctuations and the surrounding radiation field that possibly emanates from the driving source of the bubble. This source could be an individual supergiant protostar of a few hundred solar masses with periodic pulsation potentially accounting for the observed short-term period in the G24 region.

Sample and Statistical Analysis on NEOWISE Variability of ATLASGAL Sources

“…The flux variability of the 6.7 GHz methanol maser and the WISE/NEOWISE infrared emission at 4.6 μm in G24.33+0.14 show a clear correlation (Hirota et al 2022;Liu et al 2023). During the accretion burst, the brightness of G24.33+0.14 in the W2 band increased by approximately 1.3 mag over a period of 3 yr, reaching its maximum value.…”

“…The remaining three physical parameters (volume density, radius, and mass) cannot be distinguished, suggesting that the cause of variability in this stage may differ from that in the early phase. A UV radiation field can trigger a luminosity outburst, and the fluctuation of UV radiation from supermassive YSOs can effectively induce a change in temperature in the surrounding environment (Liu et al 2023). One plausible explanation is that the variability observed in the mid-infrared during the H II region stage can be primarily attributed to the propagation of the UV radiation field.…”