Alice Townsend
The aim of this project was to analyse the light curves of several Type IIn supernovae and calculate a volumetric rate. Type II supernovae are the explosions of massive stars at the end of their lifetime. For Type IIn supernovae (a sub-type of Type II supernovae), the light we see from the explosion is due to the interaction of the supernova with the material surrounding the star (mainly consisting of hydrogen). Therefore, the light curves (a plot of the brightness of the explosion over time) can tell us information about the exploding star and the material surrounding it. Our supernova sample consisted of nearby explosions (within approximately 100 Mpc), which meant that fainter supernovae were more likely to be detected. The volumetric rate (the number of supernovae occurring per year per volume of space) of Type IIn supernovae was calculated. Our volumetric rate was smaller than a value determined by another survey, which suggests that Type IIn supernovae are rarer than previously thought.
The subject of this section is light curves. Light curves are a plot of the brightness (also known as the absolute magnitude) of the supernova over time. In our light curves, two wavelengths of light were detected (orange and cyan). Two examples of a light curve plot are shown. For both, the y-axis is absolute magnitude, measured in magnitudes, and the x-axis is the modified Julian date, measured in days. On the first light curve, the data points are declining linearly. Then, there is a significant drop in brightness, and the curve subsequently declines with a steeper gradient. This could be due to non-uniformity in the CSM; for example, if there was an outer, less dense layer. The second light curve shows a quick rise and then a slight decline before a plateau of roughly 20 days. This could belong to a sub-type of Type IIn supernovae known as IIn-P, where the ’P’ indicates the presence of a plateau.
The subject of this section is the volumetric rate. The volumetric rate of Type IIn supernovae is the number of supernovae occurring per year per volume of space. A lot of previous supernova research has focused on brightness-limited surveys, meaning that fainter objects were not being detected. As a result, the current supernova volumetric rates could be inaccurate. Our survey, the ATLAS (Asteroid Terrestrial-impact Last Alert System) survey, collected a supernova sample across 2.5 years and within the local universe. This is a volume-limited survey. Looking at the local universe is the best way to detect fainter supernova, as closer objects appear brighter. The volumetric rate was calculated from this sample. There is a figure which illustrates the volume of space used to calculate the volumetric rate, which was approximated as a sphere.
The final section is a discussion and conclusion. Firstly, the Type IIn supernovae in this sample displayed a wide variety of light curves. In the future, with a larger sample size, it could be possible to group all the Type IIn supernovae into sub-types (such as IIn-P) based on their properties. Secondly, our Type IIn supernova volumetric rate is approximately half the value determined by the Lick Observatory Supernova Search (LOSS) survey. This suggests that Type IIn supernovae are rarer than previously thought. Going forward, the ATLAS group aims to calculate volumetric rates for different types of supernova using the sample from the local universe.
To conclude, I would like to thank my supervisor, Michael Fulton, and the rest of the ATLAS team at Queen’s University Belfast who gave me the opportunity to take part in this summer studentship and supported me along the way.