Pinwheels in the sky, with dust: 3D modeling of the Wolf-Rayet 98a environment

TL;DR

This paper presents detailed 3D hydrodynamic models of the Wolf-Rayet 98a system, revealing how dust forms and distributes in its rotating pinwheel nebula, and predicts observable features for current and future telescopes.

Contribution

The study introduces self-consistent 3D hydrodynamic simulations of WR 98a, incorporating dust creation, and explores the effects of radiative cooling on the nebula's structure and observability.

Findings

Effective wind mixing produces a spiral pattern conducive to dust formation.

Radiative cooling leads to thermal instabilities, creating dust clumps and filaments.

Predicted observable features include asymmetry in dust distribution and fine-structure detectable by ALMA and E-ELT.

Abstract

The Wolf-Rayet 98a (WR 98a) system is a prime target for interferometric surveys, since its identification as a "rotating pinwheel nebulae", where infrared images display a spiral dust lane revolving with a 1.4 year periodicity. WR 98a hosts a WC9+OB star, and the presence of dust is puzzling given the extreme luminosities of Wolf-Rayet stars. We present 3D hydrodynamic models for WR 98a, where dust creation and redistribution are self-consistently incorporated. Our grid-adaptive simulations resolve details in the wind collision region at scales below one percent of the orbital separation (~4 AU), while simulating up to 1300 AU. We cover several orbital periods under conditions where the gas component alone behaves adiabatic, or is subject to effective radiative cooling. In the adiabatic case, mixing between stellar winds is effective in a well-defined spiral pattern, where optimal conditions for dust creation are met. When radiative cooling is incorporated, the interaction gets dominated by thermal instabilities along the wind collision region, and dust concentrates in clumps and filaments in a volume-filling fashion, so WR 98a must obey close to adiabatic evolutions to demonstrate the rotating pinwheel structure. We mimic Keck, ALMA or future E-ELT observations and confront photometric long-term monitoring. We predict an asymmetry in the dust distribution between leading and trailing edge of the spiral, show that ALMA and E-ELT would be able to detect fine-structure in the spiral indicative of Kelvin-Helmholtz development, and confirm the variation in photometry due to the orientation. Historic Keck images are reproduced, but their resolution is insufficient to detect the details we predict.