Early and late time VLT spectroscopy of SN 2001el - progenitor constraints for a type Ia supernova

Abstract

We present early time high-resolution (VLT/UVES) and late time low-resolution (VLT/FORS) optical spectra of the normal type Ia supernova, SN 2001el. The high-resolution spectra were obtained at -9 and -2 days to allow the detection of narrow hydrogen and/or helium emission lines from the circumstellar medium of the SN. No such lines were detected, and we therefore use photoionisation models to derive upper limits of 9x10^-6 Msun/yr and 5x10^-5 Msun/yr for the mass loss rate from the progenitor system assuming velocities of 10 km/s and 50 km/s, respectively, for a wind extending to outside at least a few x 10^15 cm away from the SN explosion site. These limits exclude a symbiotic star in the upper mass loss rate regime from being the progenitor of SN 2001el. The low resolution spectrum was obtained in the nebular phase of the SN, 400 days after the maximum light, to search for any hydrogen rich gas originating from the SN progenitor system. However, we see no signs of Balmer lines in our spectrum. Therefore, we model the late time spectra to derive an upper limit of ~0.03 Msun for solar abundance material present at velocities lower than 1000 km/s within the SN explosion site. According to simulations of Marietta et al. (2000) this is less than the expected mass lost by a subgiant, red giant or main sequence secondary star at a small binary separation as a result of the SN explosion. Finally, we discuss the origin of high velocity Ca II lines. We see both the CaII IR triplet and the H&K lines in the -9 days spectrum at a very high velocity of up to 34000 km/s. The spectrum also shows a flat-bottomed Si II `6150 A' feature similar to the one previously observed in SN 1990N at -14 days. We compare these spectral features to those observed in SNe 1984A and 1990N at even higher velocities.