Journal Article

Diagnostic line ratios in the IC 1805 optical gas complex

Dominic Lagrois, Gilles Joncas and Laurent Drissen

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 420, issue 3, pages 2280-2299
Published in print March 2012 | ISSN: 0035-8711
Published online February 2012 | e-ISSN: 1365-2966 | DOI:
Diagnostic line ratios in the IC 1805 optical gas complex

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Large H ii 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 H ii 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 IC 1805 nebula, a giant H ii 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 Hα λ6563 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 [N ii] λ6548 Å line is proposed.

Keywords: techniques: spectroscopic; H ii regions; ISM: individual objects: IC 1805; ISM: lines and bands

Journal Article.  11276 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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