Detecting stellar outbursts in the decade before massive stars explode as supernovae

• Applicant: Dr. Nora Linn Strotjohann
• Subject Area: Astrophysics and Astronomy
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 461903330

Luminous stellar outbursts have been detected months or years before massive stars explode as supernovae (SNe). This discovery was unexpected as most stellar evolution models do not predict such flares. I propose a systematic search for precursor eruptions prior to all SNe classes. The search is enabled by the rich dataset of the Zwicky Transient Facility (ZTF) survey which has collected thousands pre-explosion observations for positions in the Northern sky. Additional data from the Palomar Transient Factory (2009-2017) will extend the rate measurement to more than 10 years before the SN explosion.So far, I analyzed 100,000 ZTF pre-explosion images to search for outbursts prior to 196 interaction-powered SNe which are known for their luminous precursors. Many of the 19 detected eruptions happen immediately before the SN explosion and last for several months. With measured radiative energies of up to 1e49 erg, the associated mass loss is at least 1 solar mass, if the luminosity is powered through the collision with surrounding material. The precursor rate increases by a factor of 5 in the final 3 months before the SN explosion compared to earlier times (on average ~10 months before the SN). This implies that precursor eruptions are likely directly linked to the impending core collapse.I will improve the sensitivity of the existing analysis and use it to search for precursors prior to stripped-envelope SNe (1 million images available by June 2021) and to regular Type II SNe (5 million pre-explosion images by June 2022). An additional search for precursors prior to nearby, newly discovered SNe will allow me to trigger dedicated spectroscopic and multiwavelength follow-up observations to model these transients in detail.Characterizing the precursor properties will provide unprecedented constraints on the mechanism powering their luminosity and on the amount of ejected mass. Knowing in which environment the star explodes leads to improved, more robust modelling of SNe. Moreover, the detected precursors provide boundary conditions for hydrodynamical simulations of the stellar envelope and atmosphere. Rate measurements for different SN progenitor stars have the potential to reveal which stars have similar internal structures that allow the rapid energy transport from the core to the envelope. Finally, my research will pinpoint the physical mechanism that launches precursors. Current theoretical models attribute precursors to interaction with a binary partner, a reduction in gravity due to high neutrino fluxes, turbulent neon or oxygen core burning, or explosive oxygen shell burning. I will test these models by measuring the precursor energy, its timing with respect to the SN explosion, and how widespread these eruptions are among different SN progenitors. Since precursors are likely intimately linked to the core collapse, my work will provide critical information about the processes in the core that ultimately lead to the SN explosion.

• DFG Programme: WBP Fellowship
• International Connection: Israel
• Host: Professor Avishay Gal-Yam