Diagnostic line ratios in the IC 1805 optical gas complex

Abstract

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.

Origins and interpretation of the tridimensional kinematical disorder in H ii regions

“…None the less, an important result of this work resides in the positive value obtained for ∇ turb . This will be largely used in our ongoing work on turbulent motions in NGC 595 (Lagrois & Joncas 2011).…”

“…This substantially complicates the extraction of pure turbulent motions from velocity profiles of nebular gases. The appropriate modus operandi of such an operation will be addressed in a subsequent paper (Lagrois & Joncas 2011). However, unless viscous forces became surprisingly dominant, the reader should consider all objects investigated in this work as having developed turbulent motions (see below).…”

“…By high‐ and low‐amplitude turbulence, this paper refers to turbulent motions, in velocities, leading, respectively, to relatively strong and weak line‐broadening effects. This will be largely discussed in Lagrois & Joncas (2011) by investigating turbulence associated with the ionized content of NGC 595.…”

“…In fact, many reasons can be proposed suggesting that ∇ obs ≃∇ sim ≃∇ turb . For example, non‐thermal motions in H ii regions might not perfectly result from the quadratic summation of the velocity‐gradient and turbulence individual contributions (see Lagrois & Joncas 2011). Moreover, our data sample could be easily biased towards certain ages and/or particular geometries.…”

Testing diagnostics of triggered star formation

“…The conditions of H ii regions and planetary nebulae have long been studied using ratios of forbidden lines that are sensitive to the electron density or temperature (e.g. Osterbrock ; Spitzer ; Caplan et al ; Deharveng et al ; Lagrois, Joncas & Drissen , and references therein). Single forbidden lines have been used in studies to identify pre‐main‐sequence (PMS) stars in and around BRCs as an indicator of triggered star formation (e.g.…”