Momentum Injection by Clustered Supernovae: Testing Subgrid Feedback
  Prescriptions
  Using a 1D Lagrangian code specifically designed to assess the impact of
multiple, time-resolved supernovae (SNe) from a single star cluster on the
surrounding medium, we test three commonly used feedback recipes: delayed
cooling (e.g., used in the GASOLINE-2 code), momentum-energy injection (a
resolution-dependent transition between momentum-dominated feedback and
energy-dominated feedback used, e.g., in the FIRE-2 code), and simultaneous
energy injection (e.g., used in the EAGLE simulations). Our work provides an
intermediary test for these recipes: we analyse a setting that is more complex
than the simplified scenarios for which many were designed, but one more
controlled than a full galactic simulation. In particular, we test how well
these models reproduce the enhanced momentum efficiency seen for an 11 SN
cluster simulated at high resolution (0.6 pc; a factor of 12 enhancement
relative to the isolated SN case) when these subgrid recipes are implemented in
low resolution (20 pc) runs. We find that: 1) the delayed cooling model
performs well -- resulting in 9 times the momentum efficiency of the fiducial
isolated SN value -- when SNe are clustered and $10^{51}$ erg are injected per
SN, while clearly over-predicting the momentum efficiency in the single SN test
case; 2) the momentum-energy model always achieves good results, with a factor
of 5 boost in momentum efficiency; and 3) injecting the energy from all SNe
simultaneously does little to prevent over-cooling and greatly under-produces
the momentum deposited by clustered SNe, resulting in a factor of 3 decrease in
momentum efficiency on the average.
