Variable Sodium Absorption in a Low-extinction Type Ia Supernova

Recent observations have revealed that some Type Ia supernovae exhibit narrow, time-variable Na I D absorption features. The origin of the absorbing material is controversial, but it may suggest the presence of circumstellar gas in the progenitor system prior to the explosion, with significant implications for the nature of the supernova progenitors. We present the third detection of such variable absorption, based on six epochs of high-resolution spectroscopy of the Type Ia supernova SN 2007le from the Keck I telescope and the Hobby-Eberly Telescope. The data span a time frame of approximately three months, from 5 days before maximum light to 90 days after maximum. We find that one component of the Na I D absorption lines strengthened significantly with time, indicating a total column density increase of ∼ 2.5 × 10 cm. The data limit the typical timescale for the variability to be more than 2 days but less than 10 days. The changes appear to be most prominent after maximum light rather than at earlier times when the ultraviolet flux from the supernova peaks. As with SN 2006X, we detect no change in the Ca II H and K absorption lines over the same time period, rendering line-of-sight effects improbable and suggesting a circumstellar origin for the absorbing material. Unlike the previous two supernovae exhibiting variable absorption, SN 2007le is not highly reddened (EB−V = 0.27 mag), also pointing toward circumstellar rather than interstellar absorption. Photoionization calculations show that the data are consistent with a dense (10 cm) cloud or clouds of gas located ∼ 0.1 pc (3× 10 cm) from the explosion. These results broadly support the single-degenerate scenario previously proposed to explain the variable absorption, with mass loss from a nondegenerate companion star responsible for providing the circumstellar gas. We also present possible evidence for narrow Hα emission associated with the supernova, which will require deep imaging and spectroscopy at late times to confirm. Subject headings: circumstellar matter — supernovae: general — supernovae: individual (SN 1999cl, SN 2006X, SN 2007le) 1 Some of the data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and NASA. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. 2 Based in part on observations obtained with the Hobby-Eberly Telescope, which is a joint project of the University of Texas at Austin, the Pennsylvania State University, Stanford University, Ludwig-Maximilians-Universität München, and Georg-AugustUniversität Göttingen. 3 Observatories of the Carnegie Institution of Washington, 813 Santa Barbara St., Pasadena, CA 91101; [email protected] . 4 Benoziyo Center for Astrophysics, Faculty of Physics, Weizmann Institute of Science, 76100 Rehovot, Israel; [email protected] . 5 Theoretical Astrophysics, California Institute of Technology, Mail Code 130-33, 1200 E. California Blvd., Pasadena, CA 91125; [email protected] . 6 Chandra Fellow. 7 Department of Astronomy, California Institute of Technology, 1200 E. California Blvd., MS 105-24, Pasadena, CA 91125; [email protected] . 8 Department of Astronomy, University of California, Berkeley, CA 94720-3411; [email protected], [email protected], [email protected] [email protected], [email protected] . 9 European Southern Observatory, Karl Schwarzschild Str. 2, D-85748 Garching bei München, Germany; [email protected], [email protected] . 10 McDonald Observatory and Department of Astronomy, University of Texas, Austin, TX 71782; [email protected] . 11 Harvard-Smithsonian Center for Astrophysics, 60 Garden