Direct Diagnostics of Forming Massive Stars: Stellar Pulsation and Periodic Variability of Maser Sources

The 6.7 GHz methanol maser emission, a tracer of forming massive stars, sometimes shows enigmatic periodic flux variations over several 10-100 days. In this Letter, we propose that these periodic variations could be explained by the pulsation of massive protostars growing under rapid mass accretion with rates of \,\dot{M}_{\ast }\gtrsim 10^{-3} \,{M}_{\odot }\, yr^{-1}. Our stellar evolution calculations predict that the massive protostars have very large radii exceeding 100 R _⊙ at maximum, and here we study the pulsational stability of such bloated protostars by way of the linear stability analysis. We show that the protostar becomes pulsationally unstable with various periods of several 10-100 days depending on different accretion rates. With the fact that the stellar luminosity when the star is pulsationally unstable also depends on the accretion rate, we derive the period-luminosity relation log (L/ L _⊙) = 4.62 + 0.98log (P/100 days), which is testable with future observations. Our models further show that the radius and mass of the pulsating massive protostar should also depend on the period. It would be possible to infer such protostellar properties and the accretion rate with the observed period. Measuring the maser periods enables a direct diagnosis of the structure of accreting massive protostars, which are deeply embedded in dense gas and are inaccessible with other observations.