On absorption by circumstellar dust, with the progenitor of SN 2012aw as a case study

Abstract

We use the progenitor of SN 2012aw 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) mischaracterize the optical/UV absorption by assuming that scattered photons are lost to the observer. The primary consequences for the progenitor of SN 2012aw are that both the luminosity and the absorption are significantly overestimated. In particular, the stellar luminosity is most likely in the range 104.8 < 105.0 and the star was not extremely massive for a Type IIP progenitor, with M* < 15 M⊙. Given the properties of the circumstellar dust and the early X-ray/radio detections of SN 2012aw, the star was probably obscured by an ongoing wind with Ṁ ∼ 10 -5.5 to 10-5.0 M⊙ yr-1 at the time of the explosion, roughly consistent with the expected mass-loss rates for a star of its temperature (T* ≃ 3600+300-200K) and luminosity. In the spirit of Galactic extinction laws, we supply simple interpolation formulae for circumstellar extinction by dusty graphitic and silicate shells as a function of wavelength (λ ≥ 0.3 μm) and total (absorption plus scattering) V-band optical depth (τV ≤ 20). These do not include the contributions of dust emission, but provide a simple, physical alternative to incorrectly using interstellar extinction laws. © 2012 The American Astronomical Society. All rights reserved.