Discovery of an optical cocoon tail behind the runaway HD 185806

TL;DR

This study reports the discovery and detailed analysis of an optical cocoon tail behind the star HD 185806, revealing its structure, velocity, and evolutionary status through multi-wavelength observations and modeling.

Contribution

It presents the first detailed characterization of an optical cocoon tail around HD 185806, combining spectroscopy, SED fitting, and morpho-kinematic modeling.

Findings

The cocoon tail exhibits velocities from -100 to 40 km/s.

HD 185806 is classified as a 1.3 M star between RGB and early AGB.

An ellipsoidal structure with 19-degree inclination explains the tail morphology.

Abstract

Studies on the circumstellar structures around evolved stars provide vital information on the evolution of the parent star and the properties of the local interstellar medium. In this work, we present the discovery and characterization of an optical cocoon tail behind the star HD 185806. The cocoon apex emission is puzzling, as it is detected in the infrared but shows no signal in the optical wavelength. The H-alpha and [OIII] fluxes of the nebular structure vary from 2.7 to 8.5x10^{-12} erg s^{-1} cm^ {-2} and from 0.9 to 7.0x10^{-13} erg s^{-1} cm^{-2}, respectively. Through high-resolution spectroscopy, we derive the spectral type of the star, construct the position-velocity diagrams of the cocoon tail for the H-alpha, [OIII] and [NII] emission lines, and determine its velocity in the range of -100 to 40 km s ^{-1} . Furthermore, we use SED fitting and MESA evolutionary models adopting a distance of 900 pc, and classify HD 185806 as a 1.3 M star, in the transition phase between the RGB and early AGB stages. Finally, we study the morpho-kinematic structure of the cocoon tail using the astronomical software SHAPE. An ellipsoidal structure, with an inclination of 19 degrees with respect to the plane of sky is found to better reproduce the observed cocoon tail of HD 185806.