Emission at the wavelength of [Ar II], [Ne II], and [Ne III] from HK Tau A (top panel, yellow line) and B (bottom panel, pink line), from left to right. For [Ar II] and [Ne III], the top panel includes the fitted water spectrum for HK Tau A, as the lines fall in a water-rich wavelength region. We also show the resulting water-subtracted spectra for HK Tau A (dashed orange line). The rest-frame wavelength is highlighted in green.