From forced collapse to H ii region expansion in Mon R2: Envelope
  density structure and age determination with Herschel
  The surroundings of HII regions can have a profound influence on their
development, morphology, and evolution. This paper explores the effect of the
environment on H II regions in the MonR2 molecular cloud. We aim to investigate
the density structure of envelopes surrounding HII regions and to determine
their collapse and ionisation expansion ages. The Mon R2 molecular cloud is an
ideal target since it hosts an H II region association. Column density and
temperature images derived from Herschel data were used together to model the
structure of HII bubbles and their surrounding envelopes. The resulting
observational constraints were used to follow the development of the Mon R2
ionised regions with analytical calculations and numerical simulations. The
four hot bubbles associated with H II regions are surrounded by dense, cold,
and neutral gas envelopes. The radial density profiles are reminiscent of those
of low-mass protostellar envelopes. The inner parts of envelopes of all four
HII regions could be free-falling because they display shallow density
profiles. As for their outer parts, the two compact HII regions show a density
profile, which is typical of the equilibrium structure of an isothermal sphere.
In contrast, the central UCHii region shows a steeper outer profile, that could
be interpreted as material being forced to collapse. The size of the heated
bubbles, the spectral type of the irradiating stars, and the mean initial
neutral gas density are used to estimate the ionisation expansion time, texp,
0.1Myr,for the dense UCHII and compact HII regions and 0.35 Myr for the
extended HII region. The envelope transition radii between the shallow and
steeper density profiles are used to estimate the time elapsed since the
formation of the first proto stellar embryo, Tinf : 1Myr, for the
ultra-compact, 1.5 / 3Myr for the compact, and greater than 6Myr for the
extended HII regions.
