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 9 and 2 days before (B-band) maximum light in order to detect narrow hydrogen and/or helium emission lines from the SN CSM. No such lines were detected in our data. We therefore use photoionisation models to derive upper limits of 1x10^-5 and 6x10^-5 Msol/yr, assuming wind velocities of 10 and 50 km/s, respectively, for the mass loss rate from the progenitor system of SN 2001el. This excludes 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 supernova, \\~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 Msol for solar abundance material present at velocities lower than 1000 km/s within the SN explosion site. This is less than the expected mass stripped by the SN explosion (Marietta et al. 2000) from a subgiant, red giant or a main-sequence secondary star at a small binary separation. Finally, we discuss the origin of high velocity Ca II lines previously observed in a few type Ia SNe before the maximum light. We see both the Ca II IR triplet and the H&K lines in our earliest (-9 days) spectrum at a very high velocity of up to ~34 000 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 in SN 2001el to those observed in SNe 1984A and 1990N at even higher velocities.