Differential Proper-Motion Measurements of The Cygnus Egg Nebula: The Presence of Equatorial Outflow

TL;DR

This study uses Hubble data to analyze the Egg Nebula's proper motions, revealing bipolar outflows and radially expanding concentric arcs, supporting models of binary-influenced mass-loss modulation.

Contribution

It provides the first differential proper-motion measurements of the Egg Nebula, confirming colatitudinal velocity trends and supporting binary modulation theories.

Findings

Lobes have been expanding linearly for about 400 years.

Concentric arcs are moving radially at approximately 10 km/s for 5,500 years.

The velocity field shows a colatitudinal increase, consistent with binary interaction models.

Abstract

We present the results of differential proper-motion analyses of the Egg Nebula (RAFGL 2688, V1610 Cyg) based on the archived two-epoch optical data taken with the Hubble Space Telescope. First, we determined that the polarization characteristics of the Egg Nebula is influenced by the higher optical depth of the central regions of the nebula (i.e., the "dustsphere" of about 1000 AU radius), causing the nebula illuminated in two steps -- the direct starlight is first channeled into bipolar cavities and then scattered off to the rest of the nebula. We then measured the amount of motion of local structures and the signature concentric arcs by determining their relative shifts over the 7.25 yr interval. Based on our analysis, which does not rely on the single-scattering assumption, we concluded that the lobes have been excavated by a linear expansion along the bipolar axis for the past 400 yr, while the concentric arcs have been generated continuously and moving out radially at about 10 km/s for the past 5,500 yr, and there appears to be a colatitudinally-increasing trend in the radial expansion velocity field of the concentric arcs. There exist numerical investigations into the mass-loss modulation by the central binary system, which predict such a colatitudinally-increasing expansion velocity field in the spiral-shock trails of the mass-loss ejecta. Therefore, the Egg Nebula may represent a rare edge-on case of the binary-modulated circumstellar environs, corroborating the previous theoretical predictions.