Photometry of variable dust-enshrouded stars

Supernovae Ia

Max-Planck-Institut für Radiastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
e-mail: endrik.krugel@googlemail.com

Received: 26 August 2014
Accepted: 17 October 2014

Abstract

We study the time evolution of the spectral energy distribution (SED) of a transient star surrounded by a spatially unresolved spherical dust envelope. We perform radiative transfer calculations following the Monte Carlo method of Bjorkman & Wood (2001, ApJ, 554, 615) which we extend to include time variability. In a preparatory step, we compute the SED of sources whose intrinsic light curve is a step function. Although the stellar spectrum is constant while the star is switched on, the SED, the extinction curve and the colors are variable. In a second step, we model dust-embedded, distant and therefore spatially unresoved supernovae Ia because of the profound cosmological consequences of their photometric data. As before, the colors, the extinction curve and the SED develop in time. The peak bolometric luminosity is much reduced compared to an unreddend SN. The maximum brightness at wavelengths where grains do not radiate (λ< 2 μm) is also substantially reduced relative to a non-variable star of the same luminosity. The light curves in optical bands are broadened and the time of maximum emission delayed, the extinction curves vary with time. All this is caused by the stochastic travel times of interacting photons. The dust temperature in the dust shell is remarkably constant. In the SED, the emission changes from mainly optical to predominantly mid IR. Changing the scattering properties of dust, without changing its extinction coefficient can lead to an underestimate of the supernova peak brightness.