Plot of the normalized Spitzer IRS spectra or Infrared Space Observatory Short Wavelength Spectrometer/Long Wavelength Spectrometer (LWS) spectra for the Galactic 21 μm sources, along with the carbon-rich PPN IRAS 01005+7910 which lacks the feature but which is included for comparison with the LMC object J051228. As in Figure 1 the peak λ F _λ values in units of 10 ⁻¹⁴ W m ⁻² are given in parentheses on the right within each panel. The spectra are generally from Hrivnak et al. (2000), Volk et al. (2002), and Hrivnak et al. (2009). The exceptions are the IRS spectrum of IRAS 01005+7910, from Cerrigone et al. (2009), plus the IRS spectra of IRAS 04296−3429 and 22223+4327 which were reduced from the Spitzer archive ( Spitzer program 00045, PI: T. Roellig). For IRAS 22574+6609 and IRAS 23304+6147, the ISO SWS/LWS spectra are shown for wavelengths not covered by the Spitzer IRS spectrum.