On Absorption by Circumstellar Dust, With the Progenitor of SN2012aw as a Case Study

TL;DR

This paper demonstrates that using Galactic extinction laws to model circumstellar dust leads to overestimations of progenitor luminosity and mass, and provides improved formulas for more accurate extinction correction.

Contribution

It introduces simple interpolation formulas for circumstellar dust extinction that account for dust composition and optical depth, improving upon standard interstellar extinction laws.

Findings

Progenitor luminosity is likely between 10^4.8 and 10^5.0 Lsun.

The star's mass was probably less than 15 Msun.

The circumstellar dust caused significant absorption and obscuration.

Abstract

We use the progenitor of SN2012aw to illustrate the consequences of modeling circumstellar dust using Galactic (interstellar) extinction laws that (1) ignore dust emission in the near-IR and beyond; (2) average over dust compositions, and (3) mis-characterize the optical/UV absorption by assuming that scattered photons are lost to the observer. The primary consequences for the progenitor of SN2012aw are that both the luminosity and the absorption are significantly over-estimated. In particular, the stellar luminosity is most likely in the range 10^4.8 < L/Lsun < 10^5.0 and the star was not extremely massive for a Type IIP progenitor, with M < 15Msun. Given the properties of the circumstellar dust and the early X-ray/radio detections of SN2012aw, the star was probably obscured by an on-going wind with Mdot ~ 10^-5.5 to 10^-5.0 Msun/year at the time of the explosion, roughly consistent with the expected mass loss rates for a star of its temperature (T_* ~ 3600K) and luminosity. In the spirit of Galactic extinction laws, we supply simple interpolation formulas for circumstellar extinction by dusty graphitic and silicate shells as a function of wavelength (>0.3 micron) and total (absorption plus scattering) V-band optical depth (tau < 20). These do not include the contributions of dust emission, but provide a simple, physical alternative to incorrectly using interstellar extinction laws.