The RR Lyrae Delay-Time Distribution: A Novel Perspective on Models of
  Old Stellar Populations
  The delay-time distribution (DTD) is the occurrence rate of a class of
objects as a function of time after a hypothetical burst of star formation.
DTDs are mainly used as a statistical test of stellar evolution scenarios for
supernova progenitors, but they can be applied to many other classes of
astronomical objects. We calculate the first DTD for RR Lyrae variables using
29,810 RR Lyrae from the OGLE-IV survey and a map of the stellar-age
distribution (SAD) in the Large Magellanic Cloud (LMC). We find that $\sim
46\%$ of the OGLE-IV RR Lyrae are associated with delay-times older than 8 Gyr
(main-sequence progenitor masses less than 1 M$_{\odot}$), and consistent with
existing constraints on their ages, but surprisingly about $51\%$ of RR Lyrae
appear have delay times $1.2-8$ Gyr (main-sequence masses between $1 - 2$
M$_{\odot}$ at LMC metallicity). This intermediate-age signal also persists
outside the Bar-region where crowding is less of a concern, and we verified
that without this signal, the spatial distribution of the OGLE-IV RR Lyrae is
inconsistent with the SAD map of the LMC. Since an intermediate-age RR Lyrae
channel is in tension with the lack of RR Lyrae in intermediate-age clusters
(noting issues with small-number statistics), and the age-metallicity
constraints of LMC stars, our DTD result possibly indicates that systematic
uncertainties may still exist in SAD measurements of old-stellar populations,
perhaps stemming from the construction methodology or the stellar evolution
models used. We described tests to further investigate this issue.
