Probing the Mass-loss Histories of Type IIn and Type II-L Supernovae with Late-time Radio Observations

We present Very Large Array observations of 16 Type IIn and Type II-L supernovae (SNe IIn and SNe II-L) at ≈1000–7000 days after explosion, probing circumstellar matter (CSM) at distances >10 16 cm from the progenitor. At these radii, the data directly constrain the density structure encountered by the shock. We detect radio emission from four SNe (1998S, 2005ip, 2008fq, and PTF11iqb) with the remaining 12 yielding upper limits of νL ν < 10 35 –10 36 erg s −1 at 3–11 GHz. The detected sources span approximately two orders of magnitude in radio luminosity, consistent with a broad distribution of CSM densities. All detected sources exhibit steep spectral indices ( α ≲ −0.4) consistent with optically thin synchrotron emission; their two-frequency spectral evolution is also consistent with, but does not uniquely require, models in which internal free–free absorption contributes at earlier epochs. Under simplified shock–CSM models, we obtain order-of-magnitude limits on M ̇ / v w of ≲10 −6 –10 −3 M ⊙ yr −1 /(100 km s −1 ), subject to substantial systematic uncertainties in microphysical parameters and absorption prescriptions. The detection of the intermediate SN IIn/SN II-L object PTF11iqb at luminosities between classical SNe IIn and SNe II-L is consistent with a continuum between these subtypes in CSM interaction strength. Our limits further suggest that SNe IIn and SNe II-L are not separated by long-term mass-loss rate at the radii probed here, but chiefly by the presence and strength of dense circumstellar material immediately before explosion.