Constraining the shaping mechanism of the Red Rectangle through spectro-polarimetry of its central star

TL;DR

High-sensitivity spectropolarimetric observations of the Red Rectangle's central star reveal polarization aligned with the nebula's outflow, providing constraints on its shaping mechanism and challenging some existing models.

Contribution

This study presents new spectropolarimetric data that constrains the nebula's shaping mechanism, ruling out uniform stellar winds and partially challenging precessing jet models.

Findings

Polarization aligned with the nebula's outflow spikes.

Balmer and Ca II K lines are polarized.

Uniform biconical stellar wind unlikely as the shaping mechanism.

Abstract

We carried out high-sensitivity spectropolarimetric observations of the central star of the Red Rectangle proto-planetary nebula with the aim of constraining the mechanism that gives its biconical shape. The stellar light of the central binary system is linearly polarised since it is scattered on the dust particles of the nebula. Surprisingly, the linear polarisation in the continuum is aligned with one of the spikes of the biconical outflow. Also, the observed Balmer lines as well as the Ca II K lines are polarised. These observational constraints are used to confirm or reject current theoretical models for the shaping mechanism of the Red Rectangle. We propose that the observed polarisation is very unlikely generated by a uniform biconical stellar wind. Also, the hypothesis of a precessing jet does not completely match the observations since it will require a jet aperture larger than that of the nebula.