7774$ [@Altavilla07; @Nugent11] are perhaps the cleanest way to detect HV material since there are no other strong lines just blueward of [Si$\,$]{} $\lambda 6355$ and [O$\,$]{} $\lambda 7774$. Other detections have been made by observing the Ca NIR triplet, often through spectropolarimetry [e.g., @Hatano99:94d; @Li01:00cx; @Kasen03; @Wang03; @Gerardy04; @Mazzali05:hvf], however there are several subtleties to this feature.

HV features must be caused by abundance and/or density enhancements in layers of the ejecta above the SN photosphere. Two distinct “layers” of material within a smooth density profile (i.e., an abundance enhancement) would necessarily be caused by the explosion, and observations of HV features could therefore restrict the possible explosion models. However, @Mazzali05:99ee suggested that abundance differences alone cannot reproduce the strength of the HV features, and therefore there must be a density enhancement. Density enhancements may be either caused by the explosion causing over-dense blobs or shells of material or by sweeping up circumbinary material [e.g., @Gerardy04; @Mazzali05:hvf; @Quimby06:05cg]. Spectropolarimetric observations have indicated that HV Ca NIR triplet features are probably caused from the explosion [e.g., @Kasen03; @Wang03; @Chornock08]. Because of its wavelength, it is difficult to obtain high-quality spectropolarimetric measurements of the Ca H&K feature. None the less, @Wang03 