An In Situ Study of Turbulence Near Stellar Bow Shocks

TL;DR

This study combines Hubble and Voyager observations to analyze turbulence near stellar bow shocks, revealing enhanced density fluctuations likely due to shock interactions with the surrounding medium.

Contribution

It provides the first detailed measurements of turbulence spectra near stellar bow shocks using combined in situ and remote sensing data.

Findings

Turbulence amplitude near the Guitar Nebula is constrained to log10Cn^2 = -0.8±0.2 m^-20/3.

Voyager data shows turbulence amplitude in the local ISM as log10Cn^2 = -1.57±0.02 m^-20/3.

Density fluctuations near bow shocks are often amplified compared to typical diffuse ionized medium values.

Abstract

Stellar bow shocks are observed in a variety of interstellar environments and are shaped by the conditions of gas in the interstellar medium (ISM). In situ measurements of turbulent density fluctuations near stellar bow shocks are only achievable with a few observational probes, including H$\alpha$ emitting bow shocks and the Voyager Interstellar Mission (VIM). In this paper, we examine density variations around the Guitar Nebula, an H$\alpha$ bow shock associated with PSR B2224$+$65, in tandem with density variations probed by VIM near the boundary of the solar wind and ISM. High-resolution Hubble Space Telescope observations of the Guitar Nebula taken between 1994 and 2006 trace density variations over scales from 100s to 1000s of au, while VIM density measurements made with the Voyager 1 Plasma Wave System constrain variations from 1000s of meters to 10s of au. The power spectrum of density fluctuations constrains the amplitude of the turbulence wavenumber spectrum near the Guitar Nebula to ${\rm log}_{10}C_{\rm n}^2 = -0.8\pm0.2$ m$^{-20/3}$ and for the very local ISM probed by Voyager ${\rm log}_{10}C_{\rm n}^2 = -1.57\pm0.02$ m$^{-20/3}$. Spectral amplitudes obtained from multi-epoch observations of four other H$\alpha$ bow shocks also show significant enhancements in $C_{\rm n}^2$ from values that are considered typical for the diffuse, warm ionized medium, suggesting that density fluctuations near these bow shocks may be amplified by shock interactions with the surrounding medium, or by selection effects that favor H$\alpha$ emission from bow shocks embedded in denser media.