The Spectropolarimetric Evolution of V838 Monocerotis

I review photo-polarimetric and spectropolarimetric observations of V838 Mon, which revealed that it had an asymmetrical inner circumstellar envelope following its 2nd photometric outburst. Electron scattering, modified by preor post-scattering H absorption, is the polarizing mechanism in V838 Mon’s envelope. The simplest geometry implied by these observations is that of a spheroidal shell, flattened by at least 10% and having a projected position angle on the sky of ∼37◦. Analysis of V838 Mon’s polarized flux reveals that this electron scattering shell lies interior to the envelope region in which Hα and Ca II triplet emission originates. To date, none of the theoretical models proposed for V838 Mon have demonstrated that they can reproduce the evolution of V838 Mon’s inner circumstellar environment, as probed by spectropolarimetry. 1. Diagnostic Capabilities of Polarimetry Linear polarimetry can provide powerful diagnostic information regarding the geometry of unresolved astrophysical environments. Numerous literature resources (Nordsieck et al. 1992; Bjorkman 2000) eloquently discuss these diagnostic capabilities; for the purpose of this review I will simply summarize several fundamental principles. The observed intrinsic polarization of unresolved sources is simply the net integrated polarization of the system. The density, geometrical distribution, and scattering properties of scatterers in a system are several factors which will influence the strength of the observed intrinsic polarization; systems which either lack an extended envelope of material or are characterized by a symmetrical envelope will exhibit zero net intrinsic linear polarization. Non-uniform illumination of an extended envelope, by sources such as star-spots and/or binary companions, may also produce a net intrinsic polarization. Several factors may influence the wavelength dependence of observed intrinsic linear polarization, as discussed by Nordsieck et al. (1992) and Bjorkman (2000). These factors include: a) the scattering process (i.e. Thompson versus Mie scattering); b) the nature of the illuminating source; c) the dilution of polarized light by the presence of additional unpolarized (i.e. direct) light; and d) the preferential absorption of more scattered light than direct (unpolarized) light. One is typically is unable to directly measure the intrinsic polarization of astrophysical sources, as the actual observed polarization is comprised of interstellar (time independent) and intrinsic (possibly time dependent) components.