Pair-production supernovae: Theory and observation

Nonrotating stars that end their lives with masses 140 M_⊙ ≤ M_* ≤ 260 M_⊙ should explode as pair-production supernovae (PPSNe). Here I review the physical properties of these objects, as well as the prospects for them to be observationally constrained. In very massive stars, much of the pressure support comes from the radiation field, meaning that they are loosely bound, and that (d lg p/d lg ρ)adiabatic near the center is close to the minimum value necessary for stability. Near the end of C/O burning, the central temperature increases to the point that photons begin to be converted into electron-positron pairs, softening the equation of state below this critical value. The result is a runaway collapse, followed by explosive burning that completely obliterates the loosely bound star. While these explosions can be up to 100 times more energetic than core collapse and Type Ia supernovae, their peak luminosities are only slightly greater. However, due both to copious Ni⁵⁶ production and hydrogen recombination, they are brighter much longer, and remain observable for ≈1 year. Since metal enrichment is a local process, PPSNe should occur in pockets of metal-free gas over a broad range of redshifts, greatly enhancing their detectability, and distributing their nucleosynthetic products about the Milky Way. This means that measurements of the abundances of metal-free stars should be thought of as directly constraining these objects. It also means that ongoing supernova searches, which limit the contribution of very massive stars to ≲ 1% of the total star-formation-rate density out to z ≈ 2, already provide weak constraints for PPSN models. A survey with the NIRCam instrument on JWST, on the other hand, would be able to extend these limits to z ≈ 10. Observing a 0.3 deg² patch of sky for ≈1 week per year for three consecutive years, such a program would either detect or rule out the existence of these remarkable objects.