Enhanced Momentum Feedback from Clustered Supernovae
  Young stars typically form in star clusters, so the supernovae (SNe) they
produce are clustered in space and time. This clustering of SNe may alter the
momentum per SN deposited in the interstellar medium (ISM) by affecting the
local ISM density, which in turn affects the cooling rate. We study the effect
of multiple SNe using idealized 1D hydrodynamic simulations which explore a
large parameter space of the number of SNe, and the background gas density and
metallicity. The results are provided as a table and an analytic fitting
formula. We find that for clusters with up to ~100 SNe the asymptotic momentum
scales super-linearly with the number of SNe, resulting in a momentum per SN
that can be an order of magnitude larger than for a single SN, with a maximum
efficiency for clusters with 10-100 SNe. We argue that additional physical
processes not included in our simulations -- self-gravity, breakout from a
galactic disk, and galactic shear -- can slightly reduce the momentum
enhancement from clustering, but the average momentum per SN still remains a
factor of 4 larger than the isolated SN value when averaged over a realistic
cluster mass function for a star-forming galaxy. We conclude with a discussion
of the possible role of mixing between hot and cold gas, induced by
multi-dimensional instabilities or preexisting density variations, as a
limiting factor in the buildup of momentum by clustered SNe, and suggest future
numerical experiments to explore these effects.
