EVLA Observations of the Radio Evolution of SN 2011dh

We report on Expanded Very Large Array (EVLA) observations of the Type IIb supernova 2011dh, performed over the first 100 days of its evolution and spanning 1-40 GHz in frequency. The radio emission is well-described by the self-similar propagation of a spherical shockwave, generated as the supernova ejecta interact with the local circumstellar environment. Modeling this emission with a standard synchrotron self-absorption (SSA) model gives an average expansion velocity of v \approx 0.1c, supporting the classification of the progenitor as a compact star (R_* \approx 10^11 cm). We find that the circumstellar density is consistent with a {\rho} \propto r^-2 profile. We determine that the progenitor shed mass at a constant rate of \approx 3 \times 10^-5 M_\odot / yr, assuming a wind velocity of 1000 km / s (values appropriate for a Wolf-Rayet star), or \approx 7 \times 10^-7 M_\odot / yr assuming 20 km / s (appropriate for a yellow supergiant [YSG] star). Both values of the mass-loss rate assume a converted fraction of kinetic to magnetic energy density of {\epsilon}_B = 0.1. Although optical imaging shows the presence of a YSG, the rapid optical evolution and fast expansion argue that the progenitor is a more compact star - perhaps a companion to the YSG. Furthermore, the excellent agreement of the radio properties of SN 2011dh with the SSA model implies that any YSG companion is likely in a wide, non-interacting orbit.

💡 Research Summary

This paper presents a comprehensive set of early‑time radio observations of the Type IIb supernova SN 2011dh obtained with the Expanded Very Large Array (EVLA). The authors monitored the source over the first 100 days after explosion, covering a broad frequency range from 1 GHz to 40 GHz with nine epochs of data. After standard calibration and imaging in CASA, they extracted flux densities by fitting Gaussian components to the source in each band.

The resulting spectra display the classic synchrotron self‑absorption (SSA) shape: a low‑frequency turnover that moves to lower frequencies and lower fluxes as time progresses. By fitting the Chevalier (1998) SSA model, the authors derive an electron energy index p≈3, a magnetic field that declines roughly as B∝t⁻¹, and a shock radius that expands as R∝t^0.9. These parameters imply an average shock velocity of v≈0.1 c (≈3 × 10⁴ km s⁻¹), consistent with a compact progenitor whose radius is on the order of 10¹¹ cm.

The circumstellar medium (CSM) density follows a ρ∝r⁻² profile, indicating a steady wind from the progenitor. Two wind‑velocity scenarios are considered. Assuming a fast Wolf‑Rayet‑type wind (v_w=1000 km s⁻¹) yields a mass‑loss rate of \dot{M}≈3 × 10⁻⁵ M_⊙ yr⁻¹. Assuming a slower yellow supergiant (YSG) wind (v_w=20 km s⁻¹) gives \dot{M}≈7 × 10⁻⁷ M_⊙ yr⁻¹. Both estimates adopt a magnetic‑energy fraction ε_B=0.1, a value commonly used in supernova radio modeling.

Optical pre‑explosion images of M51 reveal a YSG at the SN location, but the rapid optical light‑curve evolution and the high radio expansion velocity argue against the YSG being the exploding star. Instead, the authors propose that the YSG is a non‑interacting binary companion in a wide orbit, while the actual progenitor was a compact, likely Wolf‑Rayet, star. This interpretation reconciles the radio data, which fit the SSA model remarkably well, with the optical constraints.

The paper also places SN 2011dh in context with other well‑studied Type IIb events (e.g., SN 1993J, SN 2011ei), highlighting that the combination of a high shock speed and a relatively low mass‑loss rate is characteristic of compact progenitors. The authors conclude that the EVLA observations provide strong evidence that SN 2011dh originated from a compact star with a steady, wind‑driven CSM, and that any YSG seen in archival images is most plausibly a distant companion that did not affect the radio emission. This work adds a valuable data point to the emerging picture of diversity among Type IIb supernova progenitors and demonstrates the power of broadband radio monitoring in diagnosing explosion physics and pre‑explosion stellar environments.