Aleksandar Cikota
Spectropolarimetry is a powerful technique that enables us to study interstellar and circumstellar dust along lines of sight to Type Ia Supernovae (by measuring the continuum polarization), and the supernova Ia (SNe Ia) ejecta asymmetry (by measuring polarization of absorption lines). This provides insights on global and local asymmetries of SN explosions. These aspects are fundamental for our understanding of the phenomenon and are hardly approachable by any other observational technique.
Recent polarimetric studies of highly reddened type Ia supernovae (SNe Ia) with low RV values have revealed peculiar linear polarization curves raising toward blue wavelengths, with peak polarizations at short wavelengths (lambda_max ≲ 0.4 /mum, Patat et al., 2015, A&A, 577, A53). These profiles clearly deviate for what is observed in the Milky Way (where sightlines to normal stars displays polarization curves with lambda_max ~ 0.55 /mum), for reasons that are not well understood and may be related to the environment in which these explosions occur (in terms of peculiar dust properties) and/or to local effects (e.g. scattering by circumstellar dust).
Furthermore, the explosion geometry problem was tackled with a statistical approach, using archival data of 35 SNe Ia, observed with VLT-FORS at 128 epochs in total. In particular, we examined the polarization of the Si II line, which displays an evolution in time with a variety of peak polarization degrees, at different epochs relative to peak brightness (Cikota et al., 2019, MNRAS, 490, 578C).
In this poster I will explain the different polarization mechanisms and discuss the possible implications of the observed polarization measurements on the SN Ia progenitor system.
Slide 2 briefly explains relevant continuum and line polarization mechanisms.
1. There are three relevant continuum polarization mechanisms:
(i) Polarization produced due to linear dichroism in non-spherical grains: When light passes through the interstellar medium, or a cloud of non-spherical supramagnetic dust grains, which are aligned with the galactic magnetic field, the electric vector of the light wave parallel to the major axis of the dust grains will experience higher extinction than light waves parallel to the minor axis of the dust grain, and thus we will observe a net polarization curve, with a polarization peak, depending on the dust grain size distribution, of ~0.55 microns.
(ii) Polarization by scattering from nearby material: Single scattering from nearby dust clouds or sheets produces polarized light perpendicular to the scattering plane. The polarization curve produced by scattering will be increasing towards the blue with a power law.
(iii) Polarization induced by electron scattering in globally aspherical photospheres: In case of spherical photospheres, the intensity of scattered (linearly polarized) light from free electrons will be equal in orthogonal directions, and thus, we do not observe a net polarization. In the case of an aspherical photosphere, on the other hand, the net intensity of scattered light perpendicular to the major axis of the projected photosphere will be larger than the intensity of the scattered light perpendicular to the minor axis, and therefore, we will observe a net polarization.
2. Line polarization: If there is an inhomogeneous distribution of material in front of the spherical photosphere, it will obscure part of the polarized light, and we will observe polarization across spectral lines. Depending on the geometry and velocity distribution of the blocking material, different properties in the Stokes q–u plane will be observed. If the distribution of the absorbers (or clumps) is not spherically symmetric and does not have a common symmetry axis, and if there is a velocity-dependent range of position angles, we will observe a smooth change in the polarization angle, i.e. we will observe loops in the q–u plane as a function of wavelength.
Slide 3: Summarized are results from continuum polarization and line polarization studies.
Recent polarimetric studies of highly reddened type Ia supernovae (SNe Ia) with low RV values have revealed peculiar linear polarization curves raising toward blue wavelengths, with peak polarizations at short wavelengths (lambda_max ≲ 0.4 \mum, Patat et al., 2015, A&A, 577, A53). These profiles clearly deviate for what is observed in the Milky Way (where sightlines to normal stars displays polarization curves with lambda_max ~ 0.55 \mum), for reasons that are not well understood and may be related to the environment in which these explosions occur (in terms of peculiar dust properties) and/or to local effects (e.g. scattering by circumstellar dust).
Furthermore, the explosion geometry problem was tackled with a statistical approach, using archival data of 35 SNe Ia, observed with VLT-FORS at 128 epochs in total. In particular, we examined the polarization of the Si II line, which displays an evolution in time with a variety of peak polarization degrees, at different epochs relative to peak brightness (Cikota et al., 2019, MNRAS, 490, 578C).