The Progenitors of Type Ia Supernovae: New Insights from Radio Observations

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Type Ia supernovae (SNe Ia) mark the complete disruption of a white dwarf star, or very old compact star, but we do not yet understand why the white dwarf explodes, nor in what kind of binary star system. Popular models alternatively hold that the white dwarf accretes from a normal companion star until it reaches the maximum mass of a stable white dwarf, known as the Chandrasekhar limit, or that it may be driven to explode by merging with another white dwarf. Indeed, SNe Ia may have a diversity of progenitors, with a mix that varies with stellar age. The progenitor system and the mass of the material ejected in the explosion determine how the luminosity of the SN evolves with time, affecting the use of SNe Ia as standard candles in cosmology. Therefore, there is an urgent need to determine the progenitors of SNe Ia, beyond just solving a longstanding astrophysical mystery. A research group at Michigan State University (MSU) intends to constrain the progenitors of SNe Ia by mapping out their environments with radio telescope observations. The program will survey SNe Ia for signatures of the binary progenitor by using the Karl G. Jansky Very Large Array (VLA) and the new MeerKAT telescope in South Africa to search for emission from the interaction of the SN blast wave with circumbinary material. The principal investigator will engage school-age youth from rural backgrounds through Michigan 4-H, leading programs at the residential Exploration Days and activity nights at the MSU Observatory. The MSU Observatory and SkyNet network of telescopes will be used to offer hands-on observing experience and projects to these students.In the context of popular SN Ia models, non-degenerate companion stars should pollute their environments with stellar winds and nova shells, while white dwarf mergers will explode in relatively clean environments---but be surrounded by unusually dense, discrete shells expelled in the merger or during the common envelope phase. This program will constrain the progenitors of SNe Ia by: (1) Searching for a low-density wind expelled by the binary companion by observing SNe Ia at radio wavelengths at very early times, just ~1-2 days after explosion to identify the cause of early "bumps" in SN optical light curves. (2) Determining if SNe Ia are surrounded by recently ejected common envelopes, as might be expected if they are produced by the merger of two white dwarfs, using radio observations of SNe Ia taken 1-60 years after the explosion. (3) Mapping the circumbinary environments of SNe Ia-CSM. Observations will determine whether the circumstellar material (CSM) is more consistent with a stellar wind (evidence of a non-degenerate companion) or of a common envelope (as expected for a white dwarf merger). (4): Observing Galactic novae with red giant companions. These systems serve as scaled-down testbeds for the expected radio emission resulting from the interaction between ejecta in an explosion and CSM.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.