5 CONCLUSIONS
In this paper, we have presented a sample of 17 low-redshift SNe Ia observed with the XShooter intermediate resolution spectrograph on the VLT. We conducted a search for narrow Na i D absorption profiles in these spectra and, where present, have measured its blueshift (or non-blueshift) relative to the systemic velocity of the SN in its host galaxy. We combined these new data with events from the literature to form a single sample of 32 SNe Ia with intermediate-high resolution spectra and light curve data. We also measured the strength of the narrow Na i D absorption features through pEW measurements and investigated the connection to SN observables. Our main conclusions are:
(i) Combining our new data with the S11 sample, we find an excess of SNe Ia with blueshifted Na i D absorption features over those with no-blueshifted Na i D, with ∼20 per cent of SNe Ia having an additional blueshifted Na i D absorption feature.
(ii) SNe Ia with Na i D absorption features in their spectra have, on average, broader light curves (or higher stretches) and are more luminous events than SNe Ia without Na i D absorption features.
(iii) SNe Ia with blueshifted Na i D absorption features are most likely to be found in late-type galaxies containing younger stellar populations. No SNe Ia in our sample with blueshifted Na i D were found in an E/S0 galaxy.
(iv) SNe Ia with blueshifted Na i D absorption features show stronger Na i D pEWs than those without blueshifted features, suggestive of an additional contribution to the Na i D absorption from CSM.
(v) Within the sample of SNe Ia with blueshifted Na i D absorption, we find that the strength of the ‘blueshifted’ a i D absorption features correlates with SN B − V colour at maximum, strongly suggesting this material is associated with the progenitor system.
(vi) We find no statistically significant preference for SNe Ia with blueshifted Na i D features to have higher Si ii 6355 Å velocities than SNe Ia without blueshifted Na i D features.
The simplest explanation for the presence of additional blueshifted Na i D absorption features in SN Ia spectra is that it arises due to CSM from the progenitor system of the SN. This suggests a progenitor channel where one would expect outflowing shell-like structures - the most obvious being the SD scenario. A SD origin for the CSMis supported by clear observational evidence with recurrent nova systems being observed to show time-varying Na i D features very similar to those in some SNe Ia. However, some recent DD models may now also produce similar narrow absorption features, but not currently at the rate necessary to explain
our results.
Table 5 summarises the observational evidence for two distinct families of ‘normal’ SNe Ia with different light curve, spectral, and host galaxy properties. The rates of the different channels are consistent with one population with high luminosity, short delay-timesand evidence for outflowing material, with the other population displaying no Na i D absorption features, low luminosity and long delay-times indicative of an older population. Whether these ‘families’ correspond to separate progenitor channels (SD and DD) or
can be explained within the framework of one channel (i.e. different types of companion stars in the SD channel) is still very much a topic under debate.
