[Paper Review] POLARIZED LINE PROFILES AS DIAGNOSTICS OF CIRCUMSTELLAR GEOMETRY IN TYPE IIN SUPERNOVAE

Jennifer L. Homan|arXiv (Cornell University)|Jan 1, 2007

Gamma-ray bursts and supernovae1 references4 citations

TL;DR

This paper develops a Monte Carlo radiative transfer code to simulate polarized Hα line profiles in Type IIn supernovae, accounting for scattering, absorption, thermal emission, and complex circumstellar geometries. It finds that a shock region between the photosphere and circumstellar shell is essential to reproduce narrow polarized emission features at Hα rest wavelength, offering new diagnostics for circumstellar material structure and kinematics.

ABSTRACT

Supernovae of Type IIn possess spectral signatures that indicate an intense interaction between the supernova ejecta and surrounding dense circumstellar material cast o by the star in pre-explosion mass-loss episodes. Studying this interaction can yield clues to the nature of Type IIn progenitors and their mass loss history. In particular, polarization spectra of Type IIn’s show complex line polarization and position angle features that arise from a combination of geometrical and optical eects. I have constructed a Monte Carlo code that simulates the transfer of the H line through circumstellar shells with various geometrical congurations and optical characteristics. The superposition of broad and narrow line components produced in dieren t regions of the circumstellar environment and modied by electron and line scattering, hydrogen absorption, thermal emission, and geometrical and viewing angle eects gives rise to a variety of polarized line shapes in the model spectra. Comparison of these results with recent high-quality spectropolarimetric observations of Type IIn supernovae suggests that a model \shock region between the supernova photosphere and the circumstellar shell is necessary to produce the narrow polarized emission features at the rest wavelength of H seen in some IIn’s. Further model results point toward other features in the polarized line prole that can be used to constrain the characteristics of the circumstellar material in these intriguing objects. The code’s usefulness will be extended by the treatment of Doppler eects due to expansion of the circumstellar scattering region, such as those that characterize the polarized H proles of the Type IIn SN 1997eg.

Motivation & Objective

• To understand the origin of complex polarized line profiles in Type IIn supernovae, which arise from interaction between ejecta and dense circumstellar material.
• To address the challenge of interpreting spectropolarimetric observations that reveal asymmetric scattering and viewing-angle-dependent polarization features.
• To develop a physically motivated model that links observed polarized line shapes to the geometry and optical properties of circumstellar shells.
• To identify diagnostic features in polarized line profiles that constrain the structure and kinematics of circumstellar material in Type IIn progenitors.

Proposed method

• A Monte Carlo radiative transfer code is developed to simulate the propagation of Hα photons through circumstellar shells with varied geometries and optical properties.
• The model includes electron scattering, line scattering, hydrogen absorption, thermal emission, and geometrical effects such as viewing angle and asymmetries.
• The superposition of broad and narrow line components from different regions of the circumstellar environment is computed to reproduce observed line profiles.
• The code incorporates Doppler shifts due to expansion of the scattering region, particularly relevant for SN 1997eg.
• Model spectra are compared with high-quality spectropolarimetric observations of Type IIn supernovae to test consistency and constrain parameters.
• The model is iteratively adjusted to reproduce observed polarization features, especially narrow polarized components at Hα rest wavelength.

Experimental results

Research questions

• RQ1 What circumstellar geometry is required to reproduce the narrow polarized Hα emission features observed in some Type IIn supernovae?
• RQ2 How do scattering processes (electron and line) and absorption affect the polarization of Hα lines in asymmetric circumstellar environments?
• RQ3 To what extent do viewing angle and shell geometry influence the observed line profile and polarization position angle features?
• RQ4 Can the presence of a shock region between the photosphere and circumstellar shell explain the observed narrow polarized emission in Hα?
• RQ5 What specific features in the polarized line profile can be used to constrain the kinematics and density structure of circumstellar material?

Key findings

• A shock region located between the supernova photosphere and the circumstellar shell is necessary to reproduce the narrow polarized emission features observed at the Hα rest wavelength in some Type IIn supernovae.
• The model successfully reproduces complex polarized line profiles through the combined effects of scattering, absorption, thermal emission, and geometrical asymmetries.
• Polarization features in the line profile are sensitive to viewing angle and the spatial distribution of scattering material, providing diagnostic power for circumstellar geometry.
• The model predicts that Doppler shifts from expanding circumstellar shells significantly affect the shape and polarization of Hα profiles, especially in objects like SN 1997eg.
• Specific features in the polarized line profile—such as the depth and width of polarization dips—can be used to constrain the density and kinematics of circumstellar material.
• The code’s extension to include Doppler effects improves its ability to match high-resolution spectropolarimetric data, enhancing its utility for future observational comparisons.