[Paper Review] Dust Formation in Massive WR+O Binaries: Recent Results

С. В. Марченко, A. F. J. Moffat|arXiv (Cornell University)|Oct 17, 2006

Astrophysics and Star Formation Studies3 citations

TL;DR

This paper investigates dust formation in massive Wolf-Rayet + O binary systems using high-resolution mid-infrared imaging, revealing that colliding winds in eccentric binaries drive episodic dust production. Key findings include the detection of expanding dust arcs linked to periastron passages, confirmation of large amorphous carbon grains (~0.1–0.5 µm), and dust survival for up to ~100 years in hostile environments.

ABSTRACT

The massive, luminous Population I Wolf-Rayet stars can be considered as stars with the highest known sustained mass loss rates. Around 10% of WR stars may form carbon-rich dust in their dense and inhomogeneous winds. Though we are yet to find how dust is formed in such an extremely hostile environment, we have made substantial progress over the past decade. Here we discuss the results of recent high-resolution mid-infrared imaging of a sample of the most prodigious WR 'dustars'. This allows one to map rapidly changing dust-forming regions and derive some basic properties of the freshly formed dust.

Motivation & Objective

• To map rapidly evolving dust-forming regions in massive WR+O binaries using high-resolution mid-infrared imaging.
• To determine the physical properties of freshly formed dust, including temperature, size, and composition.
• To distinguish between single-star and binary-driven dust formation mechanisms in WC-type Wolf-Rayet stars.
• To estimate absolute dust formation rates and assess dust survival timescales in extreme stellar wind environments.
• To identify new dust-producing WR stars using 2MASS photometric data and infrared color diagnostics.

Proposed method

• High-resolution mid-infrared imaging at 12.3–12.5 µm using TReCS/Gemini-South and Michelle/Gemini-North to spatially resolve dust envelopes.
• Time-series imaging of WR112, WR140, and WR137 to track dust expansion and correlate with orbital phases and periastron passages.
• Spectral energy distribution (SED) modeling of mid-IR fluxes to derive dust temperature profiles and thermal equilibrium conditions.
• Analysis of dust grain size distribution using characteristic grain size fitting to mid-IR flux profiles and angular extent.
• Use of 2MASS J, H, K-band photometry to de-redden sources and identify dust-producing WR stars via J-H and H-K color criteria.
• Comparison of dust morphology with wind-wind collision models to infer shock compression and dust formation zones.

Experimental results

Research questions

• RQ1 What is the spatial morphology and expansion rate of dust clouds in WR+O binaries like WR112, WR137, and WR140, and how do they relate to orbital phases?
• RQ2 What are the characteristic sizes, composition, and temperature profiles of dust grains formed in the colliding winds of massive binaries?
• RQ3 How long can dust survive in the harsh environment of a WR wind, and what factors influence its longevity?
• RQ4 Can mid-infrared imaging distinguish between single-star and binary-driven dust formation channels in WC-type WR stars?
• RQ5 What is the incidence of dust-producing WR stars in the Galaxy, and how do their infrared colors compare between binary and single systems?

Key findings

• Concentric dust arcs in WR140’s mid-IR images (12.5 µm) are unambiguously linked to dust formation episodes in 1993 and 2001, confirming orbital modulation.
• The dust in WR140 exhibits a characteristic grain size of ~0.1 µm, indicating the formation of unusually large dust particles.
• Dust in WR112 expands at a rate consistent with a radial velocity of ~1200 km/s, yielding a distance estimate of 2.0⁺¹.⁷₋₀.⁸ kpc.
• Mid-IR imaging of WR48a reveals a spectacular dust envelope with bright knots, possibly linked to secondary outbursts in 1990 and 1994.
• Dust survival in WR winds is confirmed to last at least ~100 years, as shown by the slow expansion of the broken spiral in WR112.
• The 2MASS survey identified WR102e as a new dust-producing WR star, with no significant color difference between binary and single dustars, though binaries are slightly bluer due to hot companions.