DIRECT DIAGNOSTICS OF FORMING MASSIVE STARS: STELLAR PULSATION AND PERIODIC VARIABILITY OF MASER SOURCES

Abstract

The 6.7 GHz methanol maser emission, a tracer of forming massive stars,\nsometimes shows enigmatic periodic flux variations over several 10-100 days. In\nthis Letter, we propose that this periodic variations could be explained by the\npulsation of massive protostars growing under rapid mass accretion with rates\nof Mdot > 10^-3 Msun/yr. Our stellar evolution calculations predict that the\nmassive protostars have very large radius exceeding 100 Rsun at maximum, and we\nhere study the pulsational stability of such the bloated protostars by way of\nthe linear stability analysis. We show that the protostar becomes pulsationally\nunstable with various periods of several 10-100 days, depending on different\naccretion rates. With the fact that the stellar luminosity when the star is\npulsationally unstable also depends on the accretion rate, we derive the\nperiod-luminosity relation log (L/Lsun) = 4.62 + 0.98log(P/100 day), which is\ntestable with future observations. Our models further show that the radius and\nmass of the pulsating massive protostar should also depend on the period. It\nwould be possible to infer such protostellar properties and the accretion rate\nwith the observed period. Measuring the maser periods enables a direct\ndiagnosis of the structure of accreting massive protostars, which are deeply\nembedded in dense gas and inaccessible with other observations.\n