Lectura de la tesi doctoral d’Antonia Morales Garoffolo: “Observation and Interpretation of Type IIb Supernova Explosions”

Among the various types, there is a group of relatively infrequent CC-SNe termed type IIb, which appear to be hybrids between normal type II SNe (those characterised by H emission) and type Ib (those that lack H features in their spectra but exhibit prominent HeI lines).

The nature of the stellar progenitors leading to type IIb SNe is currently unknown, although two channels are contemplated: single massive stars that have lost part of their outer envelope as a consequence of stellar winds, and massive stars that she mass by Roche-Lobe overflow to a companion. The latter is in fact the favoured scenario for most of the objects observed up to now. In the majority of cases, when there are no direct progenitor detections, some hints about type IIb SN progenitors (e.g., initial mass) can be derived indirectly from the objects' light curves (LCs) and spectra.

Motivated by the relatively few well-sampled observational datasets that exist up to date for type IIb SNe and the unknowns on their progenitors, we carried out extensive observations (mainly in the optical domain) for the young type IIb SNe 2011fu and 2013df. Both these SNe are particularly interesting because they show a first LC peak caused by shock breakout, followed by a secondary ⁵⁶Ni-decay-powered maximum. The analysis of the data for SNe 2011fu and 2013df points to precursors that seem to have been stars with large radii (of the order of 100 Rsun), with low mass hydrogen envelopes (tenths of Msun), and relatively low initial masses (12-18 Msun), which could have formed part of interacting binary systems.

The nature of a third SN IIb candidate, OGLE-2013-SN-100, proved to be enigmatic. OGLE-2013-SN-100, shows a first peak in the LC, and other characteristics somewhat similar to those of type IIb SNe. However, after a deeper analysis, we conclude OGLE-2013-SN-100 is likely not a SN of type IIb. We provide an alternative possible explanation for this object, which implies a combination of a SN explosion and interaction of its ejecta with circumstellar-material.

SNe 2011fu and 2013df were included in a larger sample of type IIb SNe to carry out a comparative study of their observables and environment. Regarding the host galaxies, 90 % of the objects are located in giant (r<-18 mag) hosts.

In addition, the SNe are about equally split in low star formation and high star formation rate spiral galaxies. Concerning the SN ultra-violet (UV), optical, and near-infrared (NIR) LCs, we find a dispersion in both shape and brightness. Particularly, a few objects show a sharp declining early phase in the UV and double-peaked optical-NIR LCs. However, the absence of a first LC peak, in some of the cases, may be due to lack of early observations.

In addition, we found dispersion in the evolution of the colour indices of the SNe, making the colour comparison method not suitable to estimate extinction toward a type IIb SN. In the optical domain, the study of the (secondary) peak brightness in the R band shows that low luminosity events could be uncommon and the average brightness of the sample is ~-17.5 mag. As for the spectral properties, the SNe that show an early spike in their LCs exhibit blue, shallow-lined early-time spectra and arise from extended progenitors (R~100 Rsun).

Additionally, while there is an overall resemblance of the measured ejecta velocities, there is also dispersion of equivalent widths, nebular line luminosities and ratios among all the objects that could indicate differences in the ionisation state of the ejecta and mixing. All in all, we find heterogeineity in the studied observables of the sample of type IIb SNe, which reflects the variety of their explosion parameters and progenitor properties Core-collapse supernovae (CC-SNe) explosions represent the final demise of massive stars.

Among the various types, there is a group of relatively infrequent CC-SNe termed type IIb, which appear to be hybrids between normal type II SNe (those characterised by H emission) and type Ib (those that lack H features in their spectra but exhibit prominent HeI lines). The nature of the stellar progenitors leading to type IIb SNe is currently unknown, although two channels are contemplated: single massive stars that have lost part of their outer envelope as a consequence of stellar winds, and massive stars that shed mass by Roche-Lobe overflow to a companion.

The latter is in fact the favoured scenario for most of the objects observed up to now. In the majority of cases, when there are no direct progenitor detections, some hints about type IIb SN progenitors (e.g., initial mass) can be derived indirectly from the objects' light curves (LCs) and spectra. Motivated by the relatively few well-sampled observational datasets that exist up to date for type IIb SNe and the unknowns on their progenitors, we carried out extensive observations (mainly in the optical domain) for the young type IIb SNe 2011fu and 2013df.

Both these SNe are particularly interesting because they show a first LC peak caused by shock breakout, followed by a secondary ⁵⁶Ni-decay-powered maximum. The analysis of the data for SNe 2011fu and 2013df points to precursors that seem to have been stars with large radii (of the order of 100 Rsun), with low mass hydrogen envelopes (tenths of Msun), and relatively low initial masses (12-18 Msun), which could have formed part of interacting binary systems.

The nature of a third SN IIb candidate, OGLE-2013-SN-100, proved to be enigmatic. OGLE-2013-SN-100, shows a first peak in the LC, and other characteristics somewhat similar to those of type IIb SNe. However, after a deeper analysis, we conclude OGLE-2013-SN-100 is likely not a SN of type IIb.

We provide an alternative possible explanation for this object, which implies a combination of a SN explosion and interaction of its ejecta with circumstellar-material.

SNe 2011fu and 2013df were included in a larger sample of type IIb SNe to carry out a comparative study of their observables and environment. Regarding the host galaxies, 90 % of the objects are located in giant (r<-18 mag) hosts. In addition, the SNe are about equally split in low star formation and high star formation rate spiral galaxies.

Concerning the SN ultra-violet (UV), optical, and near-infrared (NIR) LCs, we find a dispersion in both shape and brightness. Particularly, a few objects show a sharp declining early phase in the UV and double-peaked optical-NIR LCs. However, the absence of a first LC peak, in some of the cases, may be due to lack of early observations.