The nature of the Na I D-lines in the Red Rectangle

TL;DR

This study analyzes the complex Na I D-line profiles in the Red Rectangle, revealing their origins from bipolar outflows, photospheric lines, and nebular components, using high-resolution spectra and empirical modeling.

Contribution

It provides a detailed interpretation of Na I D-line profiles in the Red Rectangle, including the origin of different spectral features and an empirical model to separate nebular and photospheric components.

Findings

Double-peaked emission from bipolar outflow

Red-shifted absorption from photosphere

Blue-shifted absorption from bipolar outflow

Abstract

In this paper we examine the profiles of the complex Na I D-lines in the Red Rectangle. The spectra were acquired with the ARCES \'{e}chelle spectrograph R = 38,000 on the 3.5-m telescope at the Apache Point Observatory. Additional spectra taken with STIS were acquired from the Hubble Legacy Archive (HLA) and were used to independently confirm the spatial origin of the spectral features. The profile of a single D-line consists of double-peaked emission, red-shifted absorption and blue-shifted absorption. We find that the double-peaked emission originates from the bipolar outflow, the red-shifted absorption feature is due to the photospheric line, and the blue-shifted absorption arises from the bipolar outflow as seen against the photosphere of the luminous post-AGB component in HD 44179. In order to better understand the Na I D-line profile, we examined the periodically variable asymmetric photospheric absorption lines. The asymmetric lines are interpreted as a signature of slow self-accretion following enhanced mass-loss around periastron. An empirical model was constructed to remove the photospheric component from the Na I D-line profile in order to study the nebular emission and absorption of sodium along the line-of-sight to the primary. This paper also discusses the different origins of the single-peaked emission, the double-peaked emission and the blue-shifted and red-shifted absorption components.