Constraining the Presence of Helium in Type Ia Supernovae
Citation:Skillen, Kelly Laura, Constraining the Presence of Helium in Type Ia Supernovae, Trinity College Dublin.School of Physics, 2021
Trinity_Masters_Thesis.pdf (PDF) 5.732Mb
The aim of this thesis is to investigate the possibility of visible helium features in the near-infrared region of the spectra of Type Ia supernovae, in particular a single feature thought to appear around 10250 ̊A, caused by the He I 10830 ̊Aline. It is theorised that helium features will appear around this region if the supernova was caused by a helium shell detonation (Boyle et al. 2017). However, there have been very few detections of these types of events in nature. The purpose of this thesis was to create a large sample of Type Ia supernovae, spanning a range of their different spectral classes to try and find a handful of objects which may have been possible helium shell detonation events. The spectral data of 33 Type Ia supernovae were taken using the X-Shooter echelle spectograph on the Very Large Telescope (VLT). Common spectral features (Si II 6355 ̊A, Ca near-infrared (NIR) triplet and O I 7773 ̊A) were then fit using a new velocity fitting code for all supernovae in the sample. These are three features which appear in nearly all Type Ia spectra and can be used to build an idea of the line forming regions of the ejecta. This is important in cases when investigating uncertain spectral features,such as the helium features in the NIR region, especially as this feature can be blended in with the C I 10693 ̊AandMgII10927 ̊Alines.These velocity fits can help to break the degeneracy between different blends.The next step after measuring velocity ranges for different features across the sample was to visually inspect the NIR region of all of the spectra to see which contained features around 10250 ̊A. Out of the 33 supernovae in this sample, 9 objects had features which were in the correct spectral region to be possible helium detections. Again, the velocity fitting code was used to simulate these features and estimate velocities for their constituents. The initial velocities from the Si, Ca and O fits were then used to determine which of these elements was the most likely cause of the NIR feature.Out of the 9 objects with a feature, 7 were found to most likely be made entirely of Mg II. 2 of the objects, SN 2017gah and SN 2017ejb, were found to be a likely blend of the Mg II 10927 ̊A and He I 10830 ̊Alines.To confirm or deny the possibility of a helium feature in these two events, the radiative transfer code TARDIS was used to create model spectra of the objects to test whether synthetically, the feature could be made with a blend of helium and magnesium. In these models, a subluminous density and abundance profile, based on the work of Heringer et al. 2017, were used to model the objects as they were both deemed to be subluminous events.It was found that both events were reasonably well modelled with helium shell detonations. SN 2017gah was best fit by a helium shell model spanning a velocity range of11836–27014 km s–1with a post-burning helium shell mass of 0.0236-0.0591 M. This corresponds to a helium velocity of 13735kms–1. Again, this is in good agreement with the initial velocity fit (13922 km s–1). Both of these velocities are lower than those predicted by the models of Boyle et al.2017 (⇠16000–19000 km s–1). It is thought that the difference between the two sets of models is due to the density profiles used in each - Boyle et al. 2017 used steeper density profiles, similar to the common w 7 model used for normal luminosity events. The ones in this thesis were shallower and it is likely that this allowed for the difference between the recovered velocities of features, though further work would need to be carried out to prove this hypothesis.Out of a total of 33 type Ia supernovae, 1-2 events containing possible helium features corresponds to a minimum of⇠3–6% of the sample which could have been potential helium shell detonations. The true number could, in fact, be higher as a non-detection of helium does not necessarily preclude the possibility of a helium shell detonation,as found in this thesis. Both of these objects were subluminous events, though it is thought that helium features may also be visible in normal luminosity events if they were caused by a helium shell detonation. If this rate is indicative of subluminous events, then it would be expected that a handful of helium shell detonations would be detected each year. If normal luminosity events can also be included in this figure,then the number of expected detections should increase even more. The fact that there have been so few detections of these kinds of objects suggests that perhaps some of these events are being missed and a new observing strategy focusing on near-infrared detections such as those used in this thesis may help to improve the sample size in the future.
Author: Skillen, Kelly Laura
Publisher:Trinity College Dublin. School of Physics. Discipline of Physics
Type of material:Thesis
Availability:Full text available