Remembering the original confusion over galactic distances that arose because it wasn't realized that there were two classes of Cepheid variables, which were and are used as 'standard candles', and that SN Ia are today used as standard candles measuring cosmological distances, might the possible existence of two classes of SNe Ia lead to a similar confusion over cosmological expansion rates?We present a summary that suggests there are at least two distinct populations of normal, cosmologically useful SNe Ia.
(Emphasis mine)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.
An SN 1a progenitor class refers to a specific type of supernova explosion that occurs in binary star systems. It involves a white dwarf star accreting material from a companion star until it reaches a critical mass, causing a runaway nuclear fusion reaction and resulting in a bright and uniform explosion.
SN 1a progenitor classes are identified through statistical analysis of observed supernova events, such as their light curves and spectral properties. This analysis helps determine the type of progenitor star involved in the explosion.
There are two main types of SN 1a progenitor classes: single degenerate and double degenerate. In single degenerate systems, the white dwarf accretes material from a non-degenerate companion star, while in double degenerate systems, the white dwarf accretes material from another white dwarf.
Studying SN 1a progenitor classes can help us better understand the evolution of binary star systems and the mechanisms behind supernova explosions. It also plays a crucial role in using supernovae as standard candles for cosmological distance measurements.
The statistical analysis of SN 1a progenitor classes allows us to estimate the frequency of these events in different types of galaxies, which can provide insights into the history and evolution of galaxies. It also helps refine models of stellar and supernova evolution, contributing to our overall understanding of the universe.