Superluminous supernovae
Superluminous supernovae (SLSNe) are a rare class of stellar death, multiple magnitudes more luminous than their normal counterparts — and too luminous to be powered by ordinary mechanisms. SLSNe, like normal SNe, are divided into type I (H-poor) and II (H-rich). Most SLSNe I can be powered by the birth of an extremely magnetic, fast-rotating neutron star (a magnetar) that deposits its rotational energy into the SN ejecta through magnetic braking, but in some cases the light curves aren’t consistent with this scenario alone. The shape of the light curve in some cases points toward interaction with the circumstellar medium (CSM), which can efficiently release the kinetic energy of the SN as radiation. SLSNe I are often divided into fast and slow subclasses, but some intermediate events also exist, pointing at a continuum of properties. Most H-rich SLSNe, on the other hand, show narrow emission lines, clear signs of CSM interaction, and belong to the “IIn” type. However, roughly a fifth of H-rich SLSNe lack narrow lines, and their power source is less obvious. These objects are extremely rare, but I studied the largest sample of these SLSNe II to date as of 2023 and uncovered other signs of CSM interaction such as an excess of ultraviolet emission. However, interaction alone may not be enough to power the most powerful SLSNe II, whose tremendous radiated energy may also require a central engine such as a magnetar.
Gamma-ray burst afterglows
Gamma-ray bursts (GRBs) are some of the most powerful transient events in the universe. Their power sources, too, are in question: either a magnetar central engine or accretion onto a black hole can launch the jets with a huge Lorentz factor that cause a GRB. I have studied some especially energetic long GRBs to determine whether a magnetar can provide enough energy, and it seems unlikely assuming realistic magnetar and jet parameters. However, the radio afterglow of these GRBs deviates from the standard theory, calling into question the validity of the resulting model parameters. I also studied a larger sample of radio afterglows and determined that this deviation is widespread among GRBs in general. More work is needed before we can trust the energetics and rates of GRBs.
Environments and progenitors of supernovae
It is important to try and connect SNe with their progenitor stars in order to shed light on the little-known last stages of the evolution of these massive stars. The locations of SNe in their host galaxies can, for example, be examined and their correlation with nearby star formation quantified. A stronger correlation with star formation implies a younger and therefore more massive progenitor star. While this method alone can produce a relative mass sequence between SN types, more quantitative results can be obtained by carefully comparing the locations of SNe to those of evolved massive stars. So-called stripped-envelope SNe, belonging to type I, tend to have more massive progenitors than those of type II SNe, but mostly not massive enough (>25 Solar masses) to have lost their envelopes through winds — they are instead likely dominated by less massive stars that lose mass to a close companion in a binary system.
SN 1987A
SN 1987A is the closest SN observed in centuries, and the best-observed SN ever by a long shot. It provides a unique opportunity to study the evolution of a SN into a SN remnant nebula in the best possible detail. Information on the mechanisms active in the ejecta, abundances of elements in the progenitor star and from the explosive nucleosynthesis, the mass loss of the progenitor and more can be obtained by examining the evolution of the SN through decades of observations from the ultraviolet to the infrared. These observations can constrain, for example, clumpy outflows of matter between the progenitor and the famous rings surrounding it; power sources acting to produce different lines in the ejecta; the mass of dust in the SN remnant; and mechanisms of emission from the CSM interaction.
