Constraints on Circumstellar Material around the Type Ia Supernova 2007af

Patat et al. recently inferred the existence of circumstellar material around a normal Type Ia supernova (SN Ia) for the first time, finding time-variable Na I D absorption lines in the spectrum of SN 2006X. We present high-resolution spectroscopy of the bright SN Ia 2007af at three epochs and search for variability in any of the Na D absorption components. Over the time range from 4 days before to 24 days after maximum light, we find that the host-galaxy Na D lines appear to be of interstellar rather than circumstellar origin and do not vary down to the level of 18 mÅ (column density of 2 × 10 11 cm -2 ). We limit any circumstellar absorption lines to be weaker than ~10 mÅ (6 × 10 10 cm -2 ). For the case of material distributed in spherically symmetric shells of radius ~10 16 cm surrounding the progenitor system, we place an upper limit on the shell mass of ~(3 × 10 -8 )/ X M ☉ , where X is the Na ionization fraction. We also show that SN 2007af is a photometrically and spectroscopically normal SN Ia. Assuming that the variable Na D lines in SN 2006X came from circumstellar matter, we therefore conclude that either there is a preferred geometry for the detection of variable absorption components in SNe Ia, or SN 2007af and SN 2006X had different types of progenitor systems.