Pixel Centroid of the HN Tau Jet

Position centroids of HN Tau for the 2000 epoch, measured using a 21-pixel aperture centered on the stellar position (this is not an absorption line spectrum). We use this plot to identify emission lines in the jet, which will have lower centroids (smaller y-values) than stellar lines or continuum. The spikes on either side of Halpha are caused by a bright halo that surrounds this line, which is saturated in our images. The centroids gradually increase with wavelength because the spectrum tilts slightly on the CCD. Individual emission lines are discussed in the text; the feature marked as [O II] is a blend of four [O II] lines and a [Ca II] line.