|
Description
Medium-resolution echelle spectra of the Red Square Nebula surrounding the star MWC 922 are presented. The spectra have been obtained in 2010 and 2012 using the XSHOOTER spectrograph mounted on the Very Large Telescope (VLT) in Paranal, Chile. The spectrum covers a wavelength range between 300nm-2.5um and shows that the nebula is rich in emission lines. We aim to identify the emission lines and use them as a tool to determine the physical and chemical characteristics of the nebula. The emission lines are also used to put constraints on the structure of the nebula and on the nature of the central stars. We analyzed and identified emission lines that indicated that the Red Square Nebula consists of a low density bipolar outflow, eminent in the broad emission component seen in [FeII], as well as in P Cygni line profiles indicative of fast outflowing material. The narrow component in the [FeII] lines is most likely formed in the photosphere of a surrounding disk. Some of the emission lines show a pronounced double peaked profile, such as CaII, indicating an accretion disk in Keplerian rotation around the central star. [OI] emission lines are formed in the neutral atomic zone separating the ionized disk photosphere from the molecular gas in the interior of the disk, which is prominent in molecular CO emission in the near-IR. [NII] and [SII] emission clearly originates in a low density but fairly hot (7000-10000K) nebular environment. HI recombination lines trace the extended nebula as well as the photosphere of the disk.
|
