Abstract
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 (SN) 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 NaID absorption lines strengthened significantly with time, indicating a total column density increase of ~2.5 × 10^(12) cm^(–2). 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 SN 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 (E_(B – V) = 0.27 mag), also pointing toward circumstellar rather than interstellar absorption. Photoionization calculations show that the data are consistent with a dense (10^7 cm^(–3)) cloud or clouds of gas located ~0.1 pc (3 × 10^(17) 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 SN, which will require deep imaging and spectroscopy at late times to confirm.
Highlights
The origin of Type Ia supernovae (SNe Ia) has important implications for understanding the physics of these dramatic explosions, the evolution of binary stars, and the expansion of the universe
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
We have presented a series of high-resolution spectra of the nearby SN Ia SN 2007le, a normal SN Ia in the subgroup that exhibits broad lines and high velocity gradients, similar to SN 2002bo and SN 2006X
Summary
The origin of Type Ia supernovae (SNe Ia) has important implications for understanding the physics of these dramatic explosions, the evolution of binary stars, and the expansion of the universe. Recent interest in this last issue is high because SNe Ia are the most effective distance indicators known on cosmological scales. These objects are the focus of many current and future dark energy experiments (e.g., Astier et al 2006; Riess et al 2007; Wood-Vasey et al 2007; Aldering 2005). It is widely thought that SNe Ia occur in close binary systems
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
