Outflows from Evolved Stars: The Rapidly Changing Fingers of CRL618

TL;DR

This study investigates the finger-shaped outflows of CRL618, revealing their rapid evolution, complex interaction with surrounding material, and likely origin from episodic clump ejections rather than steady jets.

Contribution

It provides new proper motion measurements, surface brightness analysis, and 3D hydrodynamic models to understand the formation and evolution of CRL618's outflow fingers.

Findings

Fingers have an expansion age of about 100 years.

Significant brightness variations observed over a decade.

Evidence suggests fingers result from episodic clump ejections, not steady jets.

Abstract

Our ultimate goal is to probe the nature of the collimator of the outflows in the pre PN CRL618. CRL618 is uniquely suited for this purpose owing to its multiple, bright, and carefully studied finger-shaped outflows east and west of its nucleus. We compare new HST images to images in the same filters observed as much as 11 y previously to uncover large proper motions and surface brightness changes in its multiple finger-shaped outflows. The expansion age of the ensemble of fingers is close to 100y. We find strong brightness variations at the fingertips during the past decade. Deep IR images reveal a multiple ring- like structure of the surrounding medium into which the outflows propagate and interact. Tightly constrained three-dimensional ("3D") hydrodynamic models link the properties of the fingers to their possible formation histories. We incorporate previously published complementary information to discern whether each of the fingers of CRL618 are the results of steady, collimated outflows or a brief ejection event that launched a set of bullets about a century ago. Finally, we argue on various physical grounds that fingers of CRL618 are likely to be the result of a spray of clumps ejected at the nucleus of CRL618 since any mechanism that form a sustained set of unaligned jets is unprecedented.