Periodic 6.7 GHz $\mathrm{CH_3OH}$ maser emission in G353.273+0.641: First candidate for a pulsating high-mass protostar

TL;DR

The study presents a 13-year time-domain analysis of the 6.7 GHz CH$_3$OH maser in G353.273+0.641, revealing a robust ~309-day periodicity that is tightly correlated with mid-infrared variability, suggesting the maser responds to pulsations in protostellar luminosity. Through Lomb-Scargle and asymmetric power function analyses, the maser shows synchronized cycles across multiple velocity components and a characteristic dip-before-brightening profile, reminiscent of pulsating stars. Interpreting these findings within pulsation instability theory yields a cool, bloated protostar with a large radius ($R_*\,\sim\,5\times10^2$–$7\times10^2\,R_\odot$) and high accretion rate ($\dot{M}_*\sim$ a few $\times10^{-3}\,M_\odot$ yr$^{-1}$), though a periodic accretion scenario from an unresolved protobinary cannot be entirely ruled out. The results point toward protostellar pulsation as a plausible driver of maser variability, with future ngVLA observations poised to decisively test the pulsation versus binary accretion scenarios.

Abstract

We report on the periodic flux variations in the 6.7 GHz $\mathrm{CH_3OH}$ maser associated with the high-mass protostar G353.273+0.641, based on 13 yr of monitoring mainly by the Hitachi 32 m telescope. We identified a periodicity of 309 days based on a nearly complete light curve, with 833 epochs every few days. A strong correlation is found between the maser and the mid-infrared fluxes at 3.4 and 4.6 $μ$m observed by NEOWISE during these periods, suggesting that the maser emission responds to variations in the protostellar luminosity. The average profile of the maser light curve is asymmetric and shows a steep drop in intensity just before the brightening, resembling that of some pulsating variable stars. Assuming a protostellar pulsation as the origin of maser periodicity, the observed period implies a cool and highly bloated, red supergiant-like structure. Such a bloated structure is consistent with a theoretical model of protostellar evolution under high accretion rates. The inferred protostellar parameters are broadly consistent with the theoretical model of pulsational instability during the early phase of high-mass star formation. However, a periodic accretion scenario caused by an unresolved compact protobinary cannot be completely ruled out. Several irregular peaks that deviate from the periodicity may result from episodic accretion phenomena or jet-launching events independent of the protostellar pulsation. Extremely high-resolution imaging with next-generation interferometers such as the ngVLA will provide a conclusive test for both the protostellar pulsation and the binary accretion scenarios.

Figures (10)

• Figure 1: Averaged 6.7 GHz $\mathrm{CH_3OH}$ maser spectrum constructed only using channels $\geq\ 3\sigma$ (black). The spectra from the epochs of maximum (red) and minimum (blue) peak flux are also overlaid to show the range of variability.
• Figure 2: Intensity of the 6.7 GHz $\mathrm{CH_3OH}$ maser as a function of LSR velocity and modified Julian date of the observations. The plot covers the whole monitoring period. No emission was detected outside the displayed velocity range.
• Figure 3: Light curve of the 6.7 GHz $\mathrm{CH_3OH}$ maser at $V_\mathrm{LSR}=-4.5\ \mathrm{km\ s^{-1}}$ for G353. Gray, blue, and red points represent results from the Hitachi, Yamaguchi, and Tianma telescopes Song_2025ApJ...980..132S, respectively. Error bars indicate the 3-$\sigma$ noise level for each observation.
• Figure 4: The results of the LS analysis at $V_\mathrm{LSR}=-4.5\ \mathrm{km\ s^{-1}}$. The dashed lines represent the p-values of 0.01 and 0.0001 from the bottom.
• Figure 5: Light curves of the 6.7 GHz $\mathrm{CH_3OH}$ maser with the APF overlaid. From top to bottom, the light curve for the velocity component of $V_\mathrm{LSR}=-4.5, -3.7,$ and $-6.4\ \mathrm{km\ s^{-1}}$ are shown, respectively.