  Ionized regions around early-type stars are believed to be well-known
objects, but until recently, our knowledge of the relation between the
free-free radio emission and the IR emission has been observationally hindered
by the limited angular resolution in the far-IR. The advent of Herschel has now
made it possible to obtain a more precise comparison between the two regimes,
and it has been found that about a third of the young HII regions emit more
Lyman continuum photons than expected, thus presenting a Lyman excess. With the
present study we wish to distinguish between two scenarios that have been
proposed to explain the existence of the Lyman excess: (i) underestimation of
the bolometric luminosity, or (ii) additional emission of Lyman-continuum
photons from an accretion shock. We observed an outflow (SiO) and an infall
(HCO+) tracer toward a complete sample of 200 HII regions, 67 of which present
the Lyman excess. Our goal was to search for any systematic difference between
sources with Lyman excess and those without. While the outflow tracer does not
reveal any significant difference between the two subsamples of HII regions,
the infall tracer indicates that the Lyman-excess sources are more associated
with infall signposts than the other objects. Our findings indicate that the
most plausible explanation for the Lyman excess is that in addition to the
Lyman continuum emission from the early-type star, UV photons are emitted from
accretion shocks in the stellar neighbourhood. This result suggests that
high-mass stars and/or stellar clusters containing young massive stars may
continue to accrete for a long time, even after the development of a compact
HII region.
