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

TL;DR

This study uses late-time radio observations of 16 supernovae to investigate their circumstellar environments and progenitor mass-loss histories over hundreds to thousands of years before explosion.

Contribution

It provides new insights into the diversity of mass-loss histories and circumstellar material in Type IIn and II-L supernovae through extensive radio data analysis.

Findings

Detected radio emission from four supernovae with a wide range of luminosities.

All detected sources show steep spectral indices consistent with optically-thin synchrotron emission.

Progenitor mass-loss rates are constrained to be between 10^-5 and 10^-3 Msun yr^-1/(1000 km s^-1).

Abstract

We present VLA 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 corresponding to mass-loss over hundreds to thousands of years before core collapse. We detect radio emission from four SNe (1998S, 2005ip, 2008fq, and PTF11iqb) with the remaining 12 yielding upper limits of nu L_nu < 10^35--10^36 erg s^-1 at 3--11 GHz. The detected sources span approximately two orders of magnitude in radio luminosity, reflecting a wide range of CSM densities and pre-explosion mass-loss histories. All detected sources exhibit steep spectral indices (~<-0.4) consistent with optically-thin synchrotron emission, and the spectral evolution supports internal free-free absorption as the dominant absorption mechanism at these late epochs. We infer progenitor mass-loss rates of \dot{M}/v_w ~< 10^-5--10^-3 Msun yr^-1/(1000 km s^-1), with the most radio-luminous objects requiring sustained mass-loss over hundreds to thousands of years. The detection of the intermediate SN IIn/SN II-L object PTF11iqb at luminosities between classical SNe IIn and SNe II-L supports a continuum between these subtypes in terms of 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. At epochs >5000 days, some SNe (e.g., 1979C and 1986J) maintain nearly constant radio luminosity while others decline rapidly, suggesting that the most radio-luminous SNe IIn arise from progenitors with sustained mass-loss extending >10^4 yr before explosion.