Constraining properties of dust formed in Wolf-Rayet binary WR 112 using mid-infrared and millimeter observations

TL;DR

This study combines ALMA millimeter observations with JWST mid-infrared imaging to spatially resolve the dust around the WC binary WR 112, enabling detailed constraints on grain sizes. Through SED modeling across resolved dust arcs and stochastic heating with DustEM, the authors find that hydrogen-poor amorphous carbon grains dominate, with a robust bimodal size distribution comprising nanometer-sized grains and a secondary population near 0.1 µm. The results reconcile prior conflicting grain-size estimates and show the dust production rate is highly sensitive to the assumed size distribution, averaging around 2×10⁻⁷ M⊙ yr⁻¹ (about 0.7% of the wind mass loss). The work further discusses radiative torques and sublimation as potential mechanisms shaping the bimodality, highlighting the need for more comprehensive time-dependent modeling to confirm these processes in WR binaries.

Abstract

Binaries that host a carbon-rich Wolf-Rayet (WC) star and an OB-type companion can be copious dust producers. Yet the properties of dust, particularly the grain size distribution, in these systems remain uncertain. We present Band 6 observations of WR 112 by the Atacama Large Millimeter/submillimeter Array telescope (ALMA), which are the first millimeter observations of a WC binary system capable of resolving its dust emission. By combining ALMA observations with James Webb Space Telescope (JWST) images, we were able to analyze the spatially resolved spectral energy distribution (SED) of WR 112. We found that the SEDs are consistent with emissions from hydrogen-poor amorphous carbon grains. Notably, our results also suggest that the majority of grains in the system have radii below one micrometer, and the extended dust structures are dominated by nanometer-sized grains. Among four parameterizations of the grain radius distribution that we tested, a bimodal distribution, with abundant nanometer-sized grains and a secondary population of 0.1-micron grains, best reproduces the observed SED. This bimodal distribution helps to reconcile the previously conflicting grain size estimates reported for WR 112 and for other WC systems. We hypothesize that dust destruction mechanisms such as radiative torque disruption and radiative-driven sublimation are responsible for driving the system to the bimodal grain size distribution.

Figures (25)

• Figure 1: ALMA continuum images of WR 112 created using natural weighting and a $u-v$ taper of 2″ (left) and created using uniform weighting (right). Both panels share a common color scale. The white contours indicate the apertures used for the SED described in Section \ref{['sec:dust_sed_aperture']}, corresponding to the locations of the dust arcs seen in the JWST images. The light gray ellipse in the left bottom corner of each panel indicates the beam size.
• Figure 2: The stellar SED of WR 112 and its companion. The red and orange data points are from the spatially unresolved observations described in Section \ref{['sec:archival_obs']} and Table \ref{['tbl:other_obs']}. They are corrected for extinction with best-fit $R_V = 3.5$. While the red points are included in the fit, the orange points are excluded as they predominantly reflect dust rather than stellar emission. The green point at 1.3mm is the flux from the point source in ALMA continuum image. The black line indicates the median total stellar spectrum of WR 112 and its companion among the acceptable models. The gray region indicates the 16th and 84th percentile spectra. The black dotted line indicates $\lambda =$ 0.9µm.
• Figure 3: JWST 21µm image (right) with a zoomed-in view of the region outlined by the cyan box (left). White contours mark the apertures used for the SED described in Section \ref{['sec:dust_sed_aperture']}, 13 of which are in sector 1 and 10 are in sector 2.
• Figure 4: Probability density of grain radius $a$ from the MCMC joint fit of SEDs from the 23 apertures. The dark red dashed line indicates the median of the distribution, $\log [a/\textrm{meter}] = -7.86$.
• Figure 5: The temperature profile of dust in the two sectors, $216^{\circ} \leq \mathrm{PA} \leq 256^{\circ}$ (green) and $94^{\circ} \leq \mathrm{PA} \leq 134^{\circ}$ (orange), obtained from the MCMC joint fit. The bottom axis shows the deprojected distance from the star in arcsecond (see Section \ref{['sec:dust_sed_aperture']}), while the top axis shows the deprojected distance in au, assuming a distance of 3.39k to WR 112 Lau2020b. The blue line indicates the best-fit power law to the temperature profile, with power law index $\gamma=-0.384$. The narrow light blue region shows the credible interval of the fit.