Diagnostic line ratios in the IC1805 optical gas complex

Abstract

Large H II regions, with angular dimensions exceeding 10pc, 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 favour 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 IC1805 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 with the Hαλ6563 Å, [N II]λλ6548, 6584 Å, and [S II]λλ6716, 6731 Å 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 line ratio. The typical line ratio slightly exceeds the theoretical value of 3 expected in low-density regimes. To explain such behaviour, a scenario based on collisional de-excitations affecting the [Nii]λ6548 Å line is proposed. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.