Diagnostic line ratios in the IC 1805 optical gas complex
  Large HII regions, with angular dimensions exceeding 10 pc, usually enclose
numerous massive O-stars. Stellar winds from such stars are expected to play a
sizeable role in the dynamical, morphological and chemical evolution of the
targeted nebula. Kinematically, stellar winds remain hardly observable i.e.,
the typical expansion velocities of wind-blown bubbles being often confused
with other dynamical processes also regularly found HII regions. However,
supersonic shock waves, developed by stellar winds, should favor shock
excitation and leave a well-defined spectral signature in the ionized nebular
content. In this work, the presence of stellar winds, observed through shock
excitation, is investigated in the brightest portions of the Galactic IC 1805
nebula, a giant HII region encompassing at least 10 O-stars from main-sequence
O9 to giant and supergiant O4. The use of the imaging Fourier transform
spectrometer SpIOMM enabled the simultaneous acquisition of the spectral
information associated to the Halpha6563A, [NII]6548, 6584A, and [SII]6716,
6731A ionic lines. Diagnostic diagrams, first introduced by Sabbadin and
collaborators, were used to circumscribe portions of the nebula likely subject
to shock excitation from other areas dominated by photoionization. The gas
compression, expected from supersonic shocks, is investigated by comparing the
pre- and post-shocked material's densities computed from the [SII]/[SII] line
ratio. The typical [NII]/[NII] line ratio slightly exceeds the theoretical
value of 3 expected in low-density regimes. To explain such behavior, a
scenario based on collisional de-excitations affecting the [NII]6548A line is
proposed.
