Irradiated Interfaces in the Ara OB1, Carina, Eagle Nebula, and Cyg OB2
  Massive Star Formation Regions
  Regions of massive star formation offer some of the best and most
easily-observed examples of radiation hydrodynamics. Boundaries where
fully-ionized H II regions transition to neutral/molecular photodissociation
regions (PDRs) are of particular interest because marked temperature and
density contrasts across the boundaries lead to evaporative flows and fluid
dynamical instabilities that can evolve into spectacular pillar-like
structures. When detached from their parent clouds, pillars become ionized
globules that often harbor one or more young stars. H2 molecules at the
interface between a PDR and an H II region absorb ultraviolet light from
massive stars, and the resulting fluoresced infrared emission lines are an
ideal way to trace this boundary independent of obscuring dust. This paper
presents H2 images of four regions of massive star formation that illustrate
different types of PDR boundaries. The Ara OB1 star formation region contains a
striking long wall that has several wavy structures which are present in H2,
but the emission is not particularly bright because the ambient UV fluxes are
relatively low. In contrast, the Carina star formation region shows strong H2
fluorescence both along curved walls and at the edges of spectacular pillars
that in some cases have become detached from their parent clouds. The
less-spectacular but more well-known Eagle Nebula has two regions that have
strong fluorescence in addition to its pillars. While somewhat older than the
other regions, Cyg OB2 has the highest number of massive stars of the regions
surveyed and contains many isolated, fluoresced globules that have head-tail
morphologies which point towards the sources of ionizing radiation. These
images provide a collection of potential astrophysical analogs that may relate
to ablated interfaces observed in laser experiments of radiation hydrodynamics.
