EDP Sciences

Vol. 601
In section 6. Interstellar and circumstellar matter

Medium-resolution échelle spectroscopy of the Red Square Nebula, MWC 922

by N. Wehres, B. Ochsendorf, A.G.G.M. Tielens, N.L.J. Cox, L. Kaper, J. Bally, and T.P. Snow A&A 601, A69


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The authors present a study of a remarkable B[e] star, MWC 922 (a.k.a. IRAS 18184-1302). This is a star of ambiguous evolutionary status embedded within the Red Square, that the authors have investigated using long slit spectroscopy in sequences separated by about two years. The nebula shares many morophological properties with the Red Rectangle, a protoplanetary nebula. Using a compare and contrast method, the authors show that the two are quite different, meaning that MWC 922 is not necessarily an evolved star. Choosing their orientations relative to the circumstellar medium they can separate the stellar photosphere (relatively hot, likely a B star), fast (orthogonal, atomic) outflow, high density disk (atomic and molecular), and the more extensive, low density circumstellar (atomic and molecular) gas. Moleclar tracers of colder, denser gas are particularly emphaized and interesting. The CO overtone bands between 1 and 2.5 microns are present, indicating temperatures around 3000K, and ISO observations show PAH emission bands and the SiO. No other molecules are found to be present, despite an intensive search. The cool material comes from the disk, while the forbidden line zoo arises from the nebula whose velocity field is partially resolved. They also detect a fast wind outflow in the permitted lines and shocks indicated by the nebular tracers. The evolutionary state of the central star is still ambiguous, but the authors argue that it is likely to be a binary system that is responsible for the driving and shaping of the nebula. Readers will also find the extensive line list in the appendix very useful for future studies.