Periodic Reporting for period 2 - StarDestroyers (Realizing the Potential of the Transients Boom: A Consolidated Study of Stellar Demise)
Reporting period: 2022-01-01 to 2023-06-30
The goals of this project are to gain a better understanding of the physics behind three types of high-energy phenomena in the Universe: Core-collapse supernovae, tidal disruption events and neutron-star mergers. Core-collapse supernovae are the explosive deaths of massive stars, which collapse and then explode violently after exhausting their nuclear fuel. These explosions involve some of the highest densities, temperatures and energies known, they are responsible for creating the heavy elements crucial for life, they can affect the formation of new stars and the evolution of galaxies, and they give birth to some of the most intriguing objects in the Universe: neutron stars and black holes. Tidal disruption events are the destruction of stars by supermassive black holes in galaxy centers. These extremely rare events reveal the presence and properties of otherwise dormant black holes, the dynamics of matter in strong gravitational fields, and the dynamics of stars in galaxy centers. Neutron star mergers are sources of gravitational waves, they produce the heaviest elements in the Universe, involve physics on the smallest scales and strongest gravity (where new physics is waiting to be discovered), and can be used to measure the expansion rate of the Universe as a whole.
These seemingly unrelated types of events present unique opportunities to study the laws of nature in ways that are not accessible by any other means. These events present many puzzles which are not currently explained with physical models, meaning that we are missing something, or somethings, of great essence. Recently, astronomical surveys have started to scan the sky to greater distances and more often than ever before, and gravitational wave detectors have come online with unprecedented sensitivities. The result is an explosion of discoveries of more and more supernovae, tidal disruption events, and soon, neutron-star mergers. However, finding these events is not enough. Real time rapid follow-up observations are the key to making the most of these discoveries. That is how this project is making progress. By consolidating discoveries from various surveys, running large follow-up observing programs with innovative strategies, and obtaining the most constraining observations that can be compared to state-of-the-art theoretical models.
These seemingly unrelated types of events present unique opportunities to study the laws of nature in ways that are not accessible by any other means. These events present many puzzles which are not currently explained with physical models, meaning that we are missing something, or somethings, of great essence. Recently, astronomical surveys have started to scan the sky to greater distances and more often than ever before, and gravitational wave detectors have come online with unprecedented sensitivities. The result is an explosion of discoveries of more and more supernovae, tidal disruption events, and soon, neutron-star mergers. However, finding these events is not enough. Real time rapid follow-up observations are the key to making the most of these discoveries. That is how this project is making progress. By consolidating discoveries from various surveys, running large follow-up observing programs with innovative strategies, and obtaining the most constraining observations that can be compared to state-of-the-art theoretical models.
Core collapse supernovae:
We have discovered, followed and studied a number of rare supernovae. Rare events provide unique clues regarding possible missing physical insights in our models. We studied an event from a rare class of explosions (called Type Ibn) which occur from stars that lost all of their hydrogen prior to explosion, and were likely in the process of losing their helium. Using our observations, we were able to confirm that such explosions require recent mass loss to have occurred, and to constrain the parameters and type of mass loss. We also studied an event from the most common type of stellar explosions (Type IIP), and showed that it, surprisingly, suffered no mass loss prior to explosion. These discoveries provide stringent constraints for models of massive star evolution, especially during their final, and currently most uncertain, stages. In parallel we have modeled the evolutionary paths of actual supernova progenitors (seen before the explosion), that lost mass before the explosion, in order to better understand the causes and conditions leading to this mass loss. In addition, we have developed a new method to study the oscillations of massive stars, which hold vital clues as to their still uncertain internal structure.
Tidal Disruption Events:
We discovered several new events, of this very rare class of astronomical transients, in their early phases, before reaching maximum brightness, as well as one event with a dust echo formed closer to the black hole than ever observed before. These observations have revealed new phenomena in the way the light from these events evolve, providing clues regarding the still debated emission physics of stellar accretion and disruption by black holes. We have also quantified the relative rates of such events in various types of galaxies, and using various types of search filters in transient alert streams. This informs, not only models of the physics leading to these events, but also strategies on how to find more of them in existing and future astronomical surveys.
Neutron-Star Mergers:
Given the lack of new gravitational-wave events, and delays in re-starting gravitational-wave detectors, we have focused on analyzing our, as well as the community’s follow-up strategies during previous gravitational-wave detector observing runs, and on building tools to improve these strategies. In parallel, we searched for neutron-star mergers without gravitational-waves, and while we didn’t find any, we did find other intriguing events, and were able to set limits on the rates of neutron-star mergers.
We have discovered, followed and studied a number of rare supernovae. Rare events provide unique clues regarding possible missing physical insights in our models. We studied an event from a rare class of explosions (called Type Ibn) which occur from stars that lost all of their hydrogen prior to explosion, and were likely in the process of losing their helium. Using our observations, we were able to confirm that such explosions require recent mass loss to have occurred, and to constrain the parameters and type of mass loss. We also studied an event from the most common type of stellar explosions (Type IIP), and showed that it, surprisingly, suffered no mass loss prior to explosion. These discoveries provide stringent constraints for models of massive star evolution, especially during their final, and currently most uncertain, stages. In parallel we have modeled the evolutionary paths of actual supernova progenitors (seen before the explosion), that lost mass before the explosion, in order to better understand the causes and conditions leading to this mass loss. In addition, we have developed a new method to study the oscillations of massive stars, which hold vital clues as to their still uncertain internal structure.
Tidal Disruption Events:
We discovered several new events, of this very rare class of astronomical transients, in their early phases, before reaching maximum brightness, as well as one event with a dust echo formed closer to the black hole than ever observed before. These observations have revealed new phenomena in the way the light from these events evolve, providing clues regarding the still debated emission physics of stellar accretion and disruption by black holes. We have also quantified the relative rates of such events in various types of galaxies, and using various types of search filters in transient alert streams. This informs, not only models of the physics leading to these events, but also strategies on how to find more of them in existing and future astronomical surveys.
Neutron-Star Mergers:
Given the lack of new gravitational-wave events, and delays in re-starting gravitational-wave detectors, we have focused on analyzing our, as well as the community’s follow-up strategies during previous gravitational-wave detector observing runs, and on building tools to improve these strategies. In parallel, we searched for neutron-star mergers without gravitational-waves, and while we didn’t find any, we did find other intriguing events, and were able to set limits on the rates of neutron-star mergers.
We are eagerly looking forward to several new developments that will advance our project in the coming year, namely the next gravitational-wave detector observing run, which began in May 2023, the launch of the new Black-GEM transient survey (currently in commissioning) and the LS4 transient survey (expected early 2024), the start of operations of the new SoXS spectrograph and the LSST survey, set for late 2024, and the launch of ULTRASAT, a first of its kind wide field ultraviolet survey, planned for mid 2026, but which is guiding our research more and more already pre-launch. These new and improved facilities will allow us to find even more rare supernovae, tidal disruption events, and neutron star mergers, all of which will continue to provide new discoveries and follow-up capabilities that are directly in line with the goals of our project.