  Massive protostars grow and evolve under the effect of rapid accretion of
circumstellar gas and dust, falling at high rates ($\ge 10^{-4}$-$10^{-3}\, \rm
M_{\odot}\, \rm yr^{-1}$). This mass infall has been shown, both numerically
and observationally, to be episodically interspersed by accretion of dense
gaseous clumps migrating through the circumstellar disc to the protostellar
surface, causing sudden accretion and luminous bursts. Using numerical
gravito-radiation-hydrodynamics and stellar evolution calculations, we
demonstrate that, in addition to the known bloating of massive protostars,
variable episodic accretion further influences their evolutionary tracks of
massive young stellar objects (MYSOs). For each accretion-driven flare, they
experience rapid excursions toward more luminous, but colder regions of the
Hertzsprung-Russell diagram. During these excursions, which can occur up to the
end of the pre-main-sequence evolution, the photosphere of massive protostars
can episodically release much less energetic photons and MYSOs surreptitiously
adopt the same spectral type as evolved massive (supergiants) stars. Each of
these evolutionary loop brings the young high-mass stars close to the forbidden
Hayashi region and might make their surrounding H II regions occasionally
fainter, before they recover their quiescent, pre-burst surface properties. We
interpret such cold, intermittent pre-main-sequence stellar evolutionary
excursions and the dipping variability of HII regions as the signature of the
presence of a fragmenting circumstellar accretion disc surrounding the MYSOs.
We conjecture that this mechanism might equivalently affect young stars in the
intermediate-mass regime.

Solar and Stellar Astrophysics