Low-Resolution Sodium D Absorption is a Bad Proxy for Extinction
  Dust extinction is generally the least tractable systematic uncertainty in
astronomy, and particularly in supernova science. Often in the past, studies
have used the equivalent width of Na I D absorption measured from
low-resolution spectra as proxies for extinction, based on tentative
correlations that were drawn from limited data sets. We show here, based on 443
low-resolution spectra of 172 Type Ia supernovae for which we have measured the
dust extinction as well as the equivalent width of Na I D, that the two barely
correlate. We briefly examine the causes for this large scatter that
effectively prevents one from inferring extinction using this method.
