Investigating the nature of the Fried Egg nebula: CO mm-line and optical spectroscopy of IRAS 17163-3907

TL;DR

This study uses CO mm-line and optical spectroscopy to analyze the Fried Egg nebula IRAS 17163-3907, revealing bipolar outflows and confirming its classification as a yellow hypergiant.

Contribution

It provides new spectroscopic evidence of bipolar outflows and detailed circumstellar properties, strengthening the classification of IRAS 17163-3907 as a yellow hypergiant.

Findings

Detection of asymmetrical CO line profiles indicating bipolar outflows.

Optical spectrum similarity to IRC+10420 supports hypergiant classification.

Circumstellar material within 20" of the star identified.

Abstract

Through CO mm-line and optical spectroscopy, we investigate the properties of the Fried Egg nebula IRAS 17163-3907, which has recently been proposed to be one of the rare members of the yellow hypergiant class. The CO J=2-1 and J=3-2 emission arises from a region within 20" of the star and is clearly associated with the circumstellar material. The CO lines show a multi-component asymmetrical profile, and an unexpected velocity gradient is resolved in the east-west direction, suggesting a bipolar outflow. This is in contrast with the apparent symmetry of the dust envelope as observed in the infrared. The optical spectrum of IRAS 17163-3907 between 5100 and 9000 {\AA} was compared with that of the archetypal yellow hypergiant IRC+10420 and was found to be very similar. These results build on previous evidence that IRAS 17163-3907 is a yellow hypergiant.