Direct diagnostics of forming massive stars: stellar pulsation and periodic variability of maser sources

TL;DR

This paper proposes that periodic maser variability in massive star formation regions can be explained by pulsations of rapidly accreting, bloated protostars, providing a new diagnostic tool for their internal structure.

Contribution

It introduces a model linking protostellar pulsations to maser periodicity, deriving a period-luminosity relation and enabling direct probing of protostellar properties.

Findings

Protostars become pulsationally unstable with periods of 10-100 days.

Derived a period-luminosity relation: log(L/Lsun) = 4.62 + 0.98 log(P/100 days).

Protostellar radius and mass can be inferred from observed pulsation periods.

Abstract

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 this periodic variations could be explained by the pulsation of massive protostars growing under rapid mass accretion with rates of Mdot > 10^-3 Msun/yr. Our stellar evolution calculations predict that the massive protostars have very large radius exceeding 100 Rsun at maximum, and we here study the pulsational stability of such the 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/Lsun) = 4.62 + 0.98log(P/100 day), 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 inaccessible with other observations.