Exploring the nature and origins of low luminosity type IIP supernovae
In this thesis I investigate the origins of a subset of core-collapse SNe (CCSNe); type IIP supernovae (SNe). Type II supernovae are the most common type of core collapse supernova (CCSN), and are defined by the presence of H in their envelope. Type IIP SNe are a subgroup that display a distinctive ’plateau’ feature on their light curve from which they get their name. Despite being the most common type of CCSN, the origin of the diversity observed in IIP SN light curves and spectra is still not fully understood. It is thought, however, that it is the result of differences in their progenitor stars. Of particular interest are low luminosity IIP SNe (LLSNe) as it is theorised that they arise from progenitor stars with just enough mass to undergo core collapse. They could therefore offer insight into the lower mass cutoff, the critical mass boundary that separates stars forming white dwarfs and core collapse forming neutron stars or black holes and a SN explosion. Progenitor stars of IIP SNe are being discovered at a slow, but steady rate. The sample of IIP progenitors is small with just over a dozen identified in the last 20 years. Given the difficulties in accurately determining the properties of stars in other galaxies, the luminosity and mass estimates are typically accompanied by large errors that make stying progenitor properties to SN properties challenging. This is made even more difficult as many of the progenitor detections are limited to a single band, which limits our ability to break the degeneracy between dust extinction, temperature and luminosity. If we were able to tie SN properties to those of the progenitor, it would allow us to put constraints on the progenitor of any IIP SN, even if the progenitor was not detected. First I investigated the low luminosity IIP SN 2014bi. This SN displayed many of the properties observed in other low luminosity SN such as a faint peak magnitude(MR=-14.84), low ejecta velocities (Sc II v = 2700km s−1at 25 d) and low 56Ni mass (<1×10−2M⊙ ). Yet both its light curve and spectra displayed unusual properties. The rise to the plateau was unusually long for SN 2014bi, approximately 16 days, which is more than double the average rise time for IIP SNe. This has only been observed in a few other IIP SNe, none of which were LL SNe. Its nebular phase spectra displayed significantly weaker [O I] emission than other CC SNe. Comparing the ratio of [Ca II]/[O I] (R),thought to be a tracer for core and progenitor mass, to other CC SNe show that SN 2014biis a clear outlier with a value significantly higher (R ∼9) than others (R <5). Taken at face value it would suggest that the progenitor was a red supergiant (RSG) star below the mass required to undergo core-collapse. As this is not possible, we briefly discuss fallback of inner O rich ejecta as a potential cause of these unusual traits, but conclude that this would be inconsistent with the light curve. Neither scenario can fully explain SN 2014bi. Despite occurring at a distance of only 12 Mpc, no progenitor could be identified in pre-explosion HST images probably due to the very high line of sight dust extinction AV ∼ 3 mag. Next, I studied the IIP SN 2018aoq. The SN was fainter than a normal IIP SNe, but brighter than the low luminosity group with MV = −15.7. Similarly I showed that its ejecta velocities were also between that of the low luminosity group and normal IIP SNe. SN 2018aoq therefore was an intermediate object. Aside from this, the SN did not display any remarkable or unexpected features. Given that it occurred at a distance of only 18 Mpc, I quickly identified a potential progenitor candidate in pre-explosion HST images in optical and near infrared (NIR) bands across multiple epochs. This is one of very few (4) progenitor candidates to have both multi-band and multi-epochphotometric data. I fit synthetic atmosphere models to the photometric data and found that the best fitting spectral energy distribution (SED) is that of a T = 3500 K RSG star with luminosity of Log(L/L⊙) ∼ 4.72. Comparing this temperature and luminosity value to stellar evolution models, we find that this is consistent with a MZAMS ∼10M⊙ star. While fitting SEDs to SN 2018aoq, I also decided to fit a number of previously identified progenitors to compare my own results with those from literature. In nearly all cases my results were in agreement with literature values, except for the progenitor of SN 2008bk. SN 2008bk is a noteworthy object for several reasons. It occurred at a distance of only 3.5 Mpc and was found to be a low luminosity IIP supernova. The progenitor was quickly identified in pre-explosion images in 8 different optical and NIR bands. This, then, should be one of the best constrained progenitors to date. Early studies of the progenitor estimated that the progenitor had a low temperature of ∼3500 K and a mass ∼8M⊙ . A later study using template subtracted photometry, estimated a temperature of 4330 K along with very high extinction (AV ∼2.4) and a mass of ∼13M⊙ .Given that the line of sight extinction has been shown to be low, the majority of this extinction would have to arise from CSM dust. These temperature and extinction values are similar to those observed in galactic RSGs. I decided to investigate this discrepancy further and found two equivalent fits for the photometry, matching previous results, a low temperature (∼3500 K) low extinction SED (AV <0.3) and a high temperature (4250 K - 4500 k), high extinction (AV ∼2.7)SED using different stellar atmospheric model sets. I found that previously unpublished mid-IR data is only consistent with the low temperature, low extinction SED and there-fore the high temperature, high extinction solution is the result of the degeneracy be-tween temperature and extinction. Using this SED I estimate the mass of the progenitor to be 8M⊙ , which is consistent with the earlier studies. Despite two decades of low luminosity IIP SNe and progenitor observations, we still have many unanswered questions. Even within the low luminosity IIP SN sub-group, there is still striking diversity observed in the light curves and spectra of these objects. SN 2014bi is a prime example of this diversity, sharing some properties of low luminosity IIP SNe, but wildly different in others. In order to understand the origin of this diversity we must look to the progenitors of these objects. However, this is problematic given the scarcity of progenitor detections. Not only is the progenitor sample size small, but the quality of the sample is not sufficient for us to draw any meaningful conclusions. Many of the detected progenitors have minimal wavelength coverage inhibiting our ability to place constraints on their temperature, luminosity and mass. Multi-band, multi-epoch detections like SN 2018aoq are few in number and are important additions to the sample. Studies on SN 2008bk show that observations in multiple optical bands is not enough to properly constrain the progenitor’s properties. It is vital for the mid-IR to have coverage to determine localised dust extinction. Looking to the future, the introduction of much deeper multi-band sky surveys as well as the continued work of instruments such as HST will greatly benefit our understanding of these SNe and their progenitors.
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