Bow shocks, bow waves, and dust waves. IV. Shell shape statistics

TL;DR

This study introduces a new two-dimensional classification scheme for stellar bow shock shapes and applies it to diverse observational datasets, revealing shape diversity and potential causes like instabilities, wind variability, and environmental factors.

Contribution

The paper develops and applies a novel shape classification method to different bow shock observations, highlighting shape diversity and proposing possible physical explanations.

Findings

OB star bow shocks mostly match thin-shell models but show greater shape diversity.

Cool star bow shocks have more closed wings, possibly due to emission from shocked stellar wind.

Orion Nebula arcs are more open and flatter, with multiple potential influencing factors.

Abstract

Stellar bow shocks result from relative motions between stars and their environment. The interaction of the stellar wind and radiation with gas and dust in the interstellar medium produces curved arcs of emission at optical, infrared, and radio wavelengths. We recently proposed a new two-dimensional classification scheme for the shape of such bow shocks, which we here apply to three very different observational datasets: mid-infrared arcs around hot OB stars; far-infrared arcs around luminous cool stars; and H alpha emission-line arcs around proplyds and other young stars in the Orion Nebula. For OB stars, the average shape is consistent with simple thin-shell models for the interaction of a spherical wind with a parallel stream, but the diversity of observed shapes is many times larger than such models predict. We propose that this may be caused by time-dependent oscillations in the bow shocks, due to either instabilities or wind variability. Cool star bow shocks have markedly more closed wings than hot star bow shocks, which may be due to the dust emission arising in the shocked stellar wind instead of the shocked interstellar medium. The Orion Nebula arcs, on the other hand, have both significantly more open wings and significantly flatter apexes than the hot star bow shocks. We test several possible explanations for this difference (divergent ambient stream, low Mach number, observational biases, and influence of collimated jets), but the evidence for each is inconclusive.