# From Dust to Nanodust: Resolving Circumstellar Dust from the Colliding-Wind
Binary Wolf-Rayet (WR) 140

Ryan M. Lau  NSF’s NOIRLab, 950 N. Cherry Ave., Tucson, AZ 85719, USA
Institute of Space & Astronautical Science, Japan Aerospace Exploration
Agency, 3-1-1 Yoshinodai, Chuo-ku, Sagamihara, Kanagawa 252-5210, Japan  Jason
Wang  Center for Interdisciplinary Exploration and Research in Astrophysics
(CIERA), Northwestern University, 1800 Sherman Ave, Evanston, IL 60208, USA
Matthew J. Hankins  Department of Physical Sciences, Arkansas Tech University,
1701 N. Boulder Avenue, Russellville, AR 72801, USA  Thayne Currie  Subaru
Telescope, National Astronomical Observatory of Japan, 650 North A‘ohoku
Place, Hilo, HI 96720, USA  Vincent Deo  Subaru Telescope, National
Astronomical Observatory of Japan, 650 North A‘ohoku Place, Hilo, HI 96720,
USA  Izumi Endo  Department of Astronomy, School of Science, University of
Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan  Olivier Guyon  Subaru
Telescope, National Astronomical Observatory of Japan, 650 North A‘ohoku
Place, Hilo, HI 96720, USA  Steward Observatory, University of Arizona,
Tucson, AZ 85721, USA  Astrobiology Center of NINS, 2-21-1 Osawa, Mitaka,
Tokyo 181-8588, Japan  Yinuo Han  Institute of Astronomy, University of
Cambridge, Madingley Road, Cambridge CB3 0HA, UK  Anthony P. Jones  Institut
d’Astrophysique Spatiale, CNRS, Univ. Paris-Sud, Université Paris-Saclay, Bât.
121, 91405 Orsay Cedex, France  Nemanja Jovanovic  Department of Astronomy,
California Institute of Technology, Pasadena, CA 91125, USA  Julien Lozi
Subaru Telescope, National Astronomical Observatory of Japan, 650 North
A‘ohoku Place, Hilo, HI 96720, USA  Anthony F. J. Moffat  Département de
Physique, Université de Montréal, C.P. 6128, succ. centre-ville, Montréal (Qc)
H3C 3J7, Canada; and Centre de Recherche en Astrophysique du Québec, Canada
Takashi Onaka  Department of Physics, Faculty of Science and Engineering,
Meisei University, 2-1-1 Hodokubo, Hino, Tokyo 191-8506, Japan  Department of
Astronomy, School of Science, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku,
Tokyo 113-0033, Japan  Garreth Ruane  Department of Astronomy, California
Institute of Technology, Pasadena, CA 91125, USA  Andreas A. C. Sander
Zentrum für Astronomie der Universität Heidelberg, Astronomisches Rechen-
Institut,
Mönchhofstr. 12-14, 69120 Heidelberg, Germany  Samaporn Tinyanont  Department
of Astronomy and Astrophysics, University of California, 1156 High St., Santa
Cruz, CA 95064, USA  Peter G. Tuthill  Sydney Institute of Astronomy (SIfA),
School of Physics, The University of Sydney, NSW 2006, Australia  Gerd Weigelt
Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
Peredur M. Williams  Institute for Astronomy, University of Edinburgh, Royal
Observatory, Edinburgh EH9 3HJ, UK  Sebastien Vievard  Subaru Telescope,
National Astronomical Observatory of Japan, 650 North A‘ohoku Place, Hilo, HI
96720, USA  Astrobiology Center of NINS, 2-21-1, Osawa, Mitaka, Tokyo,
181-8588, Japan

(Accepted May 9, 2023)

###### Abstract

Wolf-Rayet (WR) 140 is the archetypal periodic dust-forming colliding-wind
binary that hosts a carbon-rich WR (WC) star and an O-star companion with an
orbital period of 7.93 years and an orbital eccentricity of 0.9. Throughout
the past several decades, multiple dust-formation episodes from WR 140 have
been observed that are linked to the binary orbit and occur near the time of
periastron passage. Given its predictable dust-formation episodes, WR 140
presents an ideal astrophysical laboratory for investigating the formation and
evolution of dust in the hostile environment around a massive binary system.
In this paper, we present near- and mid-infrared (IR) spectroscopic and
imaging observations of WR 140 with Subaru/SCExAO+CHARIS, Keck/NIRC2+PyWFS,
and Subaru/COMICS taken between 2020 June and Sept that resolve the
circumstellar dust emission linked to its most recent dust-formation episode
in 2016 Dec. Our spectral energy distribution (SED) analysis of WR 140’s
resolved circumstellar dust emission reveals the presence of a hot
($T_{\mathrm{d}}\sim 1000$ K) near-IR dust component that is co-spatial with
the previously known and cooler ($T_{\mathrm{d}}\sim 500$ K) mid-IR dust
component composed of $300\text{--}500$ Å-sized dust grains. We attribute the
hot near-IR dust emission to the presence of nano-sized (“nanodust”) grains
and suggest they were formed from grain-grain collisions or the rotational
disruption of the larger grain size  population by radiative torques in the
strong radiation field from the central binary. Lastly, we speculate on the
astrophysical implications of nanodust formation around colliding-wind WC
binaries, which may present an early source of carbonaceous nanodust in the
interstellar medium.

massive stars — Wolf-Rayet stars — circumstellar dust

††journal: AAS Journals††facilities: Subaru/SCExAO(CHARIS), Subaru(COMICS),
Keck II(NIRC2)††software: IRAF (Tody, 1986, 1993), COMICS Reduction Software,
pyKLIP (Wang et al., 2015), CHARIS pipeline (Brandt et al., 2017), DustEM
(Compiègne et al., 2011), Astropy (Astropy Collaboration et al., 2013, 2018)

##  1 Introduction

Dust grains are a ubiquitous component in the Universe and are present even in
high-redshift galaxies less than 1 Gyr after the Big Bang (Lesniewska and
Michalowski 2019; Hashimoto et al. 2019; Bakx et al. 2020). The infrared (IR)
emission from dust also presents a valuable tracer of star  and planet
formation and the chemical enrichment of the interstellar medium (ISM) of
galaxies across cosmic time. However, the origin(s) of substantial quantities
of dust measured in the ISM of both local and high-redshift galaxies remain(s)
uncertain (e.g., Dwek and Cherchneff 2011). Although the leading sources of
dust are thought to be core-collapse supernovae (SNe; e.g., Dwek 1998; Gall et
al. 2011; Lesniewska and Michalowski 2019) or Asymptotic Giant Branch (AGB;
e.g., Srinivasan et al. 2009; Valiante et al. 2009; Boyer et al. 2012) stars,
dust formation from Wolf-Rayet (WR) stars presents an early and potentially
significant source of dust (Marchenko & Moffat, 2017; Lau et al., 2020a).

Carbon-rich WR (WC) stars are capable of producing copious amounts of carbon-
rich dust ($\dot{M}_{d}\sim 10^{-10}-10^{-6}$ M⊙ yr-1; Zubko 1998; Lau et al.
2020a) and form on relatively short  evolutionary timescales ($\sim$Myr;
Crowther 2007). Recent results from Franco et al. (2021) have also highlighted
WR stars as potentially important sources of the chemical enrichment of
galaxies in the early Universe.  However, understanding dust formation around
WC stars has been challenging given their hostile circumstellar environment
from their fast winds and strong radiation field.  Notably, the condensation
of carbon grains requires much higher densities (greater than a factor of
$10^{3}$) than what is expected in the isotropic winds of a single WC star
(e.g., Hackwell et al. 1979; Cherchneff et al. 2000).

Observational and theoretical studies have demonstrated that dust formation in
such environments can be enabled by colliding winds between the WC star and a
massive companion such as an O-type star (Usov, 1991; Williams et al., 2009).
The  WC7pd +O5.5fc binary system WR 140 (= HD 193793), which briefly forms
dust around periastron passage, has provided one of the most important
astrophysical laboratories for investigating colliding-wind dust formation
given its well-defined $P_{\mathrm{orb}}=7.93$ yr highly eccentric ($e=0.90$)
orbit and the wealth of multi-wavelength observations taken over the past
several decades (Williams et al., 1978, 1990; Monnier et al., 2011; Sugawara
et al., 2015; Thomas et al., 2021; Williams et al., 2021; Pollock et al.,
2021). For this reason, WR 140 has been dubbed as the “Rosetta Stone” of
colliding-wind systems (Tuthill et al., 2003).

A comprehensive analysis on the orbitally modulated dust formation by WR 140
was conducted by Williams et al. (2009), who presented multi-year IR imaging
observations that captured the evolving morphology of the rapidly expanding
circumstellar dust after its 2001 periastron passage dust-formation episode.
With spatially resolved IR images of WR 140, Williams et al. (2009) presented
several possible models of the circumstellar dust morphology and conducted a
single-component isothermal dust SED analysis that indicated the presence of
$\sim 0.01$ $\mu$m-sized carbonaceous grains. Interestingly, they also
observed a decrease in the circumstellar dust mass by a factor of $\gtrsim 3$
over several years, which suggests dust destruction in WR 140’s circumstellar
environment.  Recent observations of WR 140 with the James Webb Space
Telescope (JWST; Gardner et al. 2006) by Lau et al. (2022) revealed over 17
nested circumstellar dust shells extending beyond $\sim 70,000$ au from the
central binary demonstrating the survival of the dust grains over at least 17
periods ($\gtrsim 130$ yr). Notably, JWST detected broad mid-IR emission
features at 6.4 and 7.7 $\mu$m from spatially resolved circumstellar dust
around WR 140 indicating a composition of carbon-rich aromatic compounds.
Given the revisited significance of WC binaries as prominent sources of dust
(Lau et al., 2020a) and the latest results from JWST (Lau et al., 2022), an
investigation of the evolution and physical properties of the circumstellar
dust around WR 140 is well motivated.

New significant technological developments in ground-based extreme adaptive
optics (AO) imaging tuned to obscured, red astrophysical sources have opened a
new window for investigations of dust-production from WC binaries like WR 140.
For example, the Keck Observatory has recently deployed an IR pyramid
wavefront sensor (PyWFS) on the Keck II AO system and the vortex coronagraph
on the NIRC2 instrument (Bond et al., 2020). The Subaru Coronagraphic Extreme
Adaptive Optics Project (SCExAO, Jovanovic et al. 2015a) on the Subaru
Telescope also presents a cutting-edge extreme AO platform with a Pyramid
wavefront sensor that operates at red optical wavelengths. The AO correction
combined with the high contrast achievable with the new technologies on Keck
and Subaru enable new studies of dust production and mass-loss from dust-
forming WC binaries.

In this paper, we report the following near-contempor-aneous high-angular-
resolution IR observations of WR 140: spectroscopic imaging between 1.2  and
2.4 $\mu$m with the Coronagraphic High Angular Resolution Imaging Spectrograph
(CHARIS, Groff et al. 2016) behind the SCExAO system, $L^{\prime}$-band
imaging with Keck/NIRC2+PyWFS, and $N$-band imaging with the Cooled Mid-
Infrared Camera and Spectrograph (COMICS) on the Subaru Telescope. Our
observations resolve the circumstellar dust around WR 140 and present some of
the first extreme-AO observations of the circumstellar environment around
massive stars.

The paper is organized as follows. First, in Sec. 2 we present and describe
the WR 140 observations and data reduction with Subaru/SCExAO/CHARIS,
Keck/NIRC2+PyWFS, and Subaru/COMICS. In Sec. 3, we analyze the morphology of
the IR emission from circumstellar dust around WR 140 and conduct a dust SED
model analysis to investigate the dust properties. The dust SED analysis
reveals the presence of very small nano-sized (“nanodust”) grains around WR
140. Lastly, in Sec. 4, we discuss the possible mechanisms that could lead to
produce nanodust around WR 140 and the astrophysical implications of nanodust
formation around WC binaries.

Figure 1:  (Top Left) Subaru/SCExAO/CHARIS collapsed 1.2–2.4 $\mu$m image and
(Top Right) Keck/NIRC2 $L^{\prime}$-band image of WR 140 taken on 2020 Sept 2
and 2020 Aug 18, respectively. The central masks in the images correspond to
the $3\times$ the 113 mas inner working angle of the CHARIS observations and
the 80 mas inner working angle of the NIRC2 observations. The positions of
persistent dust features identified by Williams et al. (2009) as well as the
Western Arm are overlaid on each image. The bright and linear horizontal
feature at a vertical offset of +1$\farcs$0 is likely an artifact from the
data reduction.  The absence of this horizontal feature in the NIRC2
observations suggests it is indeed an artifact. (Bottom Row) CHARIS images of
WR 140 at individual slices corresponding to 1.28, 1.63 , and 2.21 $\mu$m with
a spectral resolution of $R\sim 19$. The rectangular aperture used to extract
the flux density of the ‘bar’ is overlaid on the CHARIS image slices.  Table
1: WR 140 Observation Summary

|  Subaru/CHARIS |  Keck/NIRC2+PyWFS |  Subaru/COMICS
---|---|---|---
Observing Mode |  Broadband+Lyot 113 mas |  $L^{\prime}$ Vortex Coronagraph |  Imaging
Observation Date (UTC) |  2020 Sept 2
(MJD 59094.2) |  2020 Aug 18
(MJD 59079.4) |  2020 June 27
(MJD 59027.5)
WR 140  Orbital Phase ($\varphi$) |  0.467 |  0.462 |  0.444
Wavelength ($\mu$m) |  1.15–2.39 |  3.8 |  11.7
Spec. Resolving Power ($\lambda/\Delta\lambda$) |  $\sim 19$ |  $\sim 5$ |  $\sim 12$
Angular Res. (FWHM) |  0.03–0.06″ |  0.074″ |  0.35″
Pixel Scale (mas) |  16.4 |  9.94 |  130
Field of View |  $\sim$2″$\times$ 2″ |  10″$\times$10″ |  42″$\times$ 32″

Note. — Summary of WR 140 observations with Subaru/SCExAO/CHARIS, Keck/NIRC2,
and Subaru/COMICS. The WR 140 orbital phase ($\varphi$) is derived from the
ephemeris derived by Monnier et al. (2011) with the Fahed et al. (2011) prior.

##  2 Observations and Data Reduction

###  2.1 Near-IR Integral Field Spectroscopy with Subaru SCExAO/CHARIS

We observed WR 140 (J2000 R.A. 20:20:27.98, Dec. +43:51:16.29; Lindegren et
al. 2021) on 2020 September 2 UT with SCExAO coupled to the CHARIS integral
field spectrograph operating in low-resolution/broadband mode, covering the
major $JHK$ passbands simultaneously ($\lambda_{\rm o}$ = 1.16–2.39 $\mu$m)
and using the $\sim$0$\farcs$113 radius Lyot coronagraphic mask. Seeing values
measured from the Mauna Kea Weather Center ranged between 0$\farcs$5 and
0$\farcs$8 but with high (20–25 mph) winds. The observations covered $-$2.3
to $-$0.94 in hour angle and were conducted in “pupil tracking”/angular
differential imaging mode (Marois et al., 2006) to allow the sky to rotate on
the CHARIS detector with time ($\Delta$PA = 30o). As WR 140 was slightly off
center from the CHARIS detector, we have full 360o coverage out to
$\sim$0$\farcs$96 instead of the more typical 1$\farcs$05\. For astrometric
calibration, we took four exposures with the artificial satellite spots
(Jovanovic et al., 2015b) turned on and obtained the rest of our observations
without satellite spots. We observed the K0-star HD 194479 afterwards as a PSF
reference star and for spectrophotometric calibration.

We used the standard CHARIS pipeline (Brandt et al., 2017) to extract CHARIS
data cubes from the raw data. For subsequent data reduction, we used the
CHARIS Data Processing Pipeline (Currie et al., 2020, 2022). For
spectrophotometric calibration drawn from  the HD 194479 data, we adopted a
Kurucz stellar atmosphere model appropriate for a K0V star. Simple inspection
showed that the ‘bar’  and C1 emission  features are bright (Fig. 1, Top
Left), especially at longer wavelengths, and detected with conservative PSF
subtraction methods. Thus, to subtract the PSF, we used a full-frame
implementation of reference star differential imaging (RDI) using the
Karhunen-Loève Image Projection (KLIP; Soummer et al., 2012) algorithm as in
Currie et al. (2019) retaining only the first 3 KL modes. Separately, we
performed a simple classical spectral differential imaging (SDI) reduction of
the post-RDI/KLIP residuals. While classical SDI produced some attenuation of
the bar at the shortest wavelengths, it fully preserved the overall shape and
morphology of the emission feature with reduced residual speckles. The bright
and linear horizontal emission feature at a vertical offset of +1$\farcs$0
near the edge of the field of view is an artifact from the data reduction
(Fig. 1, Top Left).

To estimate flux loss in RDI/KLIP-reduced data due to oversubtraction (Pueyo,
2016), we inserted into our RDI/KLIP sequence the wavelength-collapsed final
image produced using classical SDI on the post-RDI/KLIP residual images. We
then forward-modeled this image through our RDI/KLIP sequence to estimate
signal loss. Forward-modeling showed that the flux from the bar is well
preserved, with a throughput varying from 0.90 to 0.98 across the entire bar
morphology and over the full range of CHARIS channels. We therefore
conservatively adopt a $1\sigma$ photometric uncertainty of $10\%$ across the
CHARIS channels.

Flux densities of the C1 and the bar were extracted from the CHARIS data cubes
using a circular aperture with a 250 mas radius and a rectangular aperture
with a size of $1480\times 300$ mas, respectively. The rectangular aperture
for the bar was tilted by 1.7∘ south of west to match the angle of the bar
feature.

###  2.2 $L$’-band Imaging with the Keck/NIRC2 Vortex Coronagraph

Keck/NIRC2 L’-band ($\lambda_{c}=3.8$ $\mu$m) imaging observations of WR 140
were obtained on 2020 August 18 UT in the program C258 (PI - M. Hankins). The
vector vortex coronagraph (Vargas Catalán et al., 2016; Serabyn et al., 2017)
was used to suppress the diffraction pattern of the star with an inner working
angle of $\lambda/D\approx 80$ mas at L’-band. The infrared pyramid wavefront
sensor (Bond et al., 2020) was used to obtain good adaptive optics correction
as the star is much brighter in the infrared than at visible wavelengths due
to extinction. Observations were taken using the narrow camera with a
10″$\times$10″ field of view and a pixel scale of 9.94 mas. QACITS, the
standard focal plane wavefront sensing technique for the vector vortex
coronagraph, was used to obtain observations of the system while keeping the
star aligned behind the coronagraph (Huby et al., 2017). In total, we obtained
58 coronagraphic images, each consisting of 100 coadds of 0.18 s exposures. As
standard in the QACITS sequence, we also obtained sky background images and
unocculted images of the star for calibration. In addition to WR 140, we used
the same configuration to obtain 15 frames on two reference stars: the K5-star
HD 181681 immediately before, and the K0-star HD 194479 immediately
afterwards.

We used the vortex pre-processing pipeline (Xuan et al., 2018) to perform bad-
pixel correction, flat fielding, thermal background subtraction, and co-
registration of the images. The reference images from HD 181681 and HD 194479
were used to subtract off the stellar diffraction pattern of WR 140 using the
KLIP-RDI implementation in the open-source pyKLIP package (Wang et al., 2015).
Given the brightness of the dust in the data, we used 1 KL mode to model the
stellar diffraction pattern. The fully reduced image is shown in Figure 1 (Top
Right). To accurately measure the flux of the dust, we forward-modeled the
over-subtraction caused by KLIP-RDI by projecting the KL mode onto a model of
the dust, and measured a small (1–2%) flux loss due to KLIP-RDI that we
compensate for. We conservatively adopt a $1\sigma$ $L^{\prime}$-band
photometric uncertainty of $10\%$.

Flux densities of the C1 and the bar were extracted from the NIRC2 image using
a circular aperture with a 250 mas radius and a rectangular aperture with a
size of $1490\times 300$ mas, respectively. The rectangular aperture for the
bar was tilted by 1.7∘ south of west to match the angle of the bar feature.
The flux density of the C1 “Fossil” dust feature (see Sec. 3.3) was extracted
using an elliptical aperture with a semi-major axis and semi-minor axis of 600
and 150 mas, respectively. The elliptical C1 Fossil aperture was rotated by
11.6∘ north of west to match the orientation of the dust feature.

###  2.3 Mid-IR Imaging with Subaru/COMICS

Mid-IR imaging observations of WR 140 were obtained using the Cooled Mid-
Infrared Camera and Spectrometer (COMICS; Kataza et al. 2000; Okamoto et al.
2003) on the Cassegrain focus of the Subaru Telescope with the N11.7 filter
($\lambda_{c}=11.7$ $\mu$m, $\Delta\lambda=1.0$ $\mu$m) on 2020 June 27 UT
(Fig. 2). Individual 100 s exposures were performed using a perpendicular
chopping and nodding pattern with a chop throw and nod amplitude of
$10^{\prime\prime}$. The total integration time on WR 140 was 400 s. The pixel
scale of the detector is $0.13^{\prime\prime}$ pixel-1.

Observations of the K3II-star Gamma Aql were used as photometric and PSF
references based on the list of mid-IR standards in Cohen et al. (1999). The
measured full-width at half maximum (FWHM) in the N11.7 imaging observations
of Gamma Aql was $\sim 0.35^{\prime\prime}$, which is consistent with near
diffraction-limited performance at 11.7 $\mu$m ($0.29^{\prime\prime}$).

The data reduction was carried out using the COMICS Reduction
Software111https://subarutelescope.org/Observing/DataReduction/Cookbooks/COMICS_CookBook2p2E.pdf
and  the Image Reduction and Analysis Facility (IRAF; Tody 1986, 1993). The
chopping and nodding pair subtraction was employed to cancel out the
background radiation and to reduce a residual pattern from the chop
subtraction. Flat fielding was achieved using self-sky-flat made from each
image. A conversion factor for flux density calibration was calculated by the
reduced images of the standard star and its photometric data by Cohen et al.
(1999). We adopt a $1\sigma$ $N$-band photometric calibration uncertainty of
$\sim 10\%$.

Flux densities of the C1 and the bar were extracted from the COMICS image
using a circular aperture with a 520 mas radius and a rectangular aperture
with a size of $1820\times 650$ mas, respectively. The rectangular aperture
for the bar was tilted by 1.7∘ south of west to match the angle of the bar
feature. The flux density of the C1 Fossil dust feature was extracted using an
elliptical aperture with a semi-major axis and semi-minor axis of 1170 and 520
mas, respectively. Background “sky” annuli were used to measure and subtract
the mid-IR background emission. A circular annulus with inner and outer radii
of 780 mas and 1560 mas were used for the background annulus of C1. A
rectangular annulus with a 650 mas width and the same inner dimensions and
orientation as the rectangular aperture for the bar was used for the bar.
Lastly, the C1 Fossil background annulus had inner radii consistent with the
dimensions of the elliptical C1 Fossil aperture and width of 650 mas.

##  3 Results and Analysis

###  3.1 WR 140 Dust Morphology from December 2016 Periastron Passage

The Subaru/CHARIS 1.1–2.4 $\mu$m, Keck/NIRC2 3.8 $\mu$m $L^{\prime}$-band, and
11.7 $\mu$m $N$-band Subaru/COMICS observations of WR 140 resolve and detect
emission from dust formed during the periastron passage that occurred  in
December 2016 (Williams et al., 2009; Thomas et al., 2021). These features are
consistent with the “persistent dust features” identified by IR imaging of WR
140 in Williams et al. (2009) after its 2001 periastron passage. The most
prominent features are the two bright emission peaks located 0$\farcs$86 and
1$\farcs$15 southeast and southwest of the central binary, respectively (Fig.
1 & 2). These southeast and southwest emission peaks were labeled as ‘C0’ and
‘C1,’ respectively, by Williams et al. (2009). At wavelengths longer than
$\sim 1.6$ $\mu$m, we detect the aforementioned faint, linear ‘bar’ of
emission between C0 and C1. The near-IR CHARIS and 3.8 $\mu$m NIRC2
observations of the bar reveal a “clumpy” morphology that appear to bridge C0
and C1 (Fig. 1).

To the east of the bar, the NIRC2 and 11.7 $\mu$m COMICS images show a faint
arc of emission that curves to the north. Williams et al. (2009) refer to this
feature as the ‘arm,’ and, more recently, Han et al. (2022) label it as the
“Eastern Arm.” In this work, we refer to this feature as the Eastern Arm in
order to distinguish it from a fainter “Western Arm” detected in the NIRC2
image (Fig. 1; Top Right). The Eastern Arm exhibits an integrated flux density
$\sim 25$% of the bar, and in the COMICS image the Eastern Arm is $\sim 35$%
the intensity of the bar. The high spatial resolution of the NIRC2 image
reveals the clumpy morphology of the Eastern Arm similar to that of the bar.
The CHARIS observations did not detect significant emission from the Eastern
Arm; however, this feature is located near/slightly beyond the edge of the
CHARIS field of view.

Among the three observations, only the NIRC2 image reveals the Western Arm,
which extends from C0 through the central source and continues north/north-
west for $\sim 1$″(Fig. 1; Top Right). Northward of the central source, the
curvature of the Western Arm resembles that of the Eastern arm. The integrated
flux of the Western Arm in the NIRC2 image is a few percent of that of the
bar. Non-detection of the Western Arm in the CHARIS and COMICS observations is
likely due to its faintness as well as the lower SNR towards the central
source for the CHARIS observations.

Figure 2: Subaru/COMICS 11.7 $\mu$m imaging observations of WR 140 taken on
2020 June 27. The positions of persistent dust features are overlaid on the
image. The overlaid dashed circle indicates the approximate CHARIS field-of-
view.

Figure 3: (Left)  Orbital-plane and (Right)  on-sky projections of the
geometric wind-wind interface model where dust forms in the colliding winds in
the WR 140 binary around periastron passage. The positions of the dust
features are overlaid on the model, where the Eastern and Western Arms are on
the near side closest to the observer. The orbital parameters of the models
are indicated above, and the parameters in the observed view are adopted from
the Monnier et al. (2011) results with the Fahed et al. (2011) prior. The
half-opening angle of the colliding-wind interface is assumed to be
$\theta\approx 40^{\circ}$. The appearance of the model corresponds to the
timing of the CHARIS observations taken on 2020 Sept 2 ($\varphi=0.467$) and
assumes a constant dust expansion velocity of 2450 km s-1. The orbital
configuration of the WR and O stars and the direction of motion of the O star
(in the frame of reference of the WR star) is also shown in the models, where
the red region indicates the orbital range of active dust formation. The
adopted true anomaly range of dust formation is $-135^{\circ}$ to
$135^{\circ}$ (Han et al., 2022). The persistent circumstellar dust features
are identified in the observed view of the geometric wind-wind interface
model, where north is up and east is to the left.

With the exception of the Western Arm, all three observations capture the same
set of persistent dust features given their similar morphology and positions
relative to the central source. In the COMICS image, which was taken at
$\varphi=0.444$ in WR 140’s orbital phase, C0 and C1 were located $\sim
2.5\times$ and $\sim 3.0\times$ the 11.7 $\mu$m FWHM from the central source
which corresponds to $\sim$0$\farcs$8 and $\sim$1$\farcs$0, respectively. The
extent of the bar in the COMICS image is $\sim 1$$\farcs$0\. In both CHARIS
and NIRC2 observations, which were taken at $\varphi=0.467$ and
$\varphi=0.462$ in WR 140’s orbital phase, respectively, C0 and C1 were
located 0$\farcs$9 and 1$\farcs$1 from the central source, and the extent of
the bar was 1$\farcs$2\. The slightly increasing size and displacement of the
dust features measured over the $\sim 2$ months spanning the COMICS, CHARIS,
and NIRC2 observations are due to the high proper motion of the expanding dust
(240–320 mas yr-1; Williams et al. 2009)

###  3.2 Comparison to Predicted Dust Morphology

A geometric model of dust formed in the wind collision interface between the
WR 140 binary  components around periastron passage can be derived from its
well-defined orbital properties (Monnier et al., 2011; Thomas et al., 2021)
and the previously characterized half-opening angle, $\theta\approx
40^{\circ}$, of the colliding-wind shock from the central binary (Williams et
al., 2009; Fahed et al., 2011). Similar geometric models have successfully
reproduced the morphology of circumstellar dust formed in the colliding-wind
WC binaries WR 104 (Soulain et al., 2018), WR 112 (Lau et al., 2020b) , and
Apep (Callingham et al., 2019; Han et al., 2020). Figure 3 shows the  orbital-
plane and on-sky projections of the dust-forming wind-wind interface model of
WR 140’s dust shell assuming a constant dust expansion velocity of $2450$ km
s-1, which is consistent with the observationally derived expansion velocity
from Williams et al. (2009). The orbital parameters of WR 140 in the geometric
models are adopted from the Monnier et al. (2011) results with the Fahed et
al. (2011) prior: $e=0.9$, $\omega=46.8^{\circ}$, $\Omega=353.6^{\circ}$,
$i=119.6^{\circ}$. The orbital configuration of the central binary at an
orbital phase of $\varphi=0.47$ and the orbital range of dust formation is
also shown in Fig. 3. The adopted true anomaly range where dust formation
occurs is $-135^{\circ}$ to $135^{\circ}$ ; however, note that relatively
little dust is observed around anomaly ranges consistent with periastron
passage due to variable dust-formation rates(Han et al., 2022).

Figure 3 (Right) indicates the position of the C0 and C1 dust features as well
as the bar and the Western and Eastern Arms. These features likely correspond
to regions of the dust shell with larger integrated dust column densities
along the line of sight. The morphology of the Eastern Arm and the Western
Arm, the latter of which  captured only in the NIRC2 image, appear consistent
with the east and west edges of the near side of the geometric model. Based on
the geometric model, the southern edge of the near side of the dust shell
(i.e., the southern extent of the Arms) is coincident with C0 along the bar.
Given the lower angular resolution of the 11.7 $\mu$m COMICS observations
relative to  those of NIRC2 and CHARIS, the blended dust emission of those two
regions in the dust shell may contribute to C0 appearing brighter than C1 in
the COMICS image.

A notable  difference between the geometric wind-wind interface model of dust
formation and the observations of WR 140 is the absence of bright dust
emission along the northern edge of the dust shell, where the integrated
column density along the line of sight toward the observer should be
comparable to that of southern region associated with the bar. This is likely
due to WR 140’s highly variable dust-formation  rates throughout periastron
passage (Han et al., 2022; Eatson et al., 2022b). Further investigations
comparing colliding-wind simulations (e.g., Eatson et al. 2022a, b) with
observations are therefore important for testing our understanding of dust
formation in colliding-wind binaries; however, this is beyond the scope of
this paper.

Figure 4: Keck/NIRC2 and Subaru/COMICS images of WR 140 displayed with a flux
density range to highlight the detection of the C1 ‘Fossil’ that formed during
the 2009 periastron passage. The C0 Fossil and Eastern Arm Fossil are also
detected in the COMICS image.

###  3.3 WR 140 ‘Fossil’ Dust Morphology from  the January 2009 Periastron
Passage

The 3.8 $\mu$m NIRC2 and 11.7 $\mu$m COMICS images of WR 140 detect emission
from the C1 feature that formed during the January 2009 periastron passage,
one orbital period (7.93 yr) before the December 2016 periastron passage.
Figure 4 shows the NIRC2 and COMICS images and the location of this C1
‘Fossil’ that is located along the same position angle as C1. The COMICS image
also shows a faint detection of the C0 and Eastern Arm Fossils to the east of
the C1 Fossil. The COMICS image resembles previous Gemini South imaging
observations at 12.5 $\mu$m by Marchenko & Moffat (2007) and Williams et al.
(2009) as well as JWST imaging where  repeated dust features from multiple
past dust-formation episodes  were detected. The NIRC2 image presents the
first 3.8 $\mu$m detection of a previous dust-formation episode.

In order to verify that the C1 Fossil in the NIRC2 image is indeed from the
January 2009 dust-formation episode, the distance between C1 and the C1 Fossil
can be compared to the expected separation distance based on previously
measured proper motions of C1. The proper motion of C1 is 326 mas yr-1
(Williams et al., 2009); therefore, over a 7.93 yr orbital period the
separation distance between C1 and the C1 Fossil should be 2.6″, which is
almost exactly the measured separation distance in the NIRC2 image.

Table 2: WR 140 Dust Feature Observed Flux Densities

$\lambda$ ($\mu$m) | $F_{\mathrm{bar}}$ | $F_{\mathrm{C1}}$ | $F_{\mathrm{C1,Fossil}}$ | Ext. Cor
---|---|---|---|---
1.241 | 3.62 | 2.26 | - | 2.26
1.284 | 7.54 | 3.5 | - | 2.17
1.329 | 8.04 | 4.21 | - | 2.09
1.375**Telluric-dominated channel | 9.09 | 5.1 | - | 2.02
1.422**Telluric-dominated channel | 11.83 | 5.43 | - | 1.95
1.471 | 13.79 | 6.46 | - | 1.89
1.522 | 15.34 | 7.19 | - | 1.83
1.575 | 15.75 | 7.79 | - | 1.77
1.63 | 18.83 | 9.5 | - | 1.72
1.686 | 19.26 | 10.76 | - | 1.68
1.744 | 22.93 | 12.39 | - | 1.64
1.805 | 26.78 | 15.06 | - | 1.6
1.867**Telluric-dominated channel | 30.1 | 16.75 | - | 1.56
1.932**Telluric-dominated channel | 38.58 | 19.78 | - | 1.53
1.999 | 42.78 | 21.47 | - | 1.5
2.068 | 48.43 | 25.2 | - | 1.47
2.139 | 59.82 | 29.02 | - | 1.44
2.213 | 65.86 | 31.74 | - | 1.41
2.29 | 69.44 | 32.67 | - | 1.39
2.369 | 99.2 | 46.23 | - | 1.37
3.8 | 385.26 | 169.47 | 10 | 1.17
11.7 | 439.04 | 270.98 | 23 | 1.13

Note. — Observed WR 140 flux densities (in mJy) of the bar, C1, and C1 Fossil.
The extinction correction (Ext. Cor) column indicates the multiplicative
derredening factor at each wavelength derived from the Gordon et al. (2021)
interstellar extinction curve with $A_{v}=2.21$. The $1\sigma$ photometric
uncertainties are $\sim 10\%$ (See Sec. 2).  The apertures used to measure the
flux densities are described in Sec. 2.

###  3.4 The IR SED of WR 140 Dust and the Dust Temperature Disparities

The resolved mid-IR emission from circumstellar dust around WR 140 has been
previously characterized and attributed to thermal emission heated radiatively
by the central binary (Williams et al., 2009). Monnier et al. (2002) had first
resolved near-IR emission around WR 140  with observations taken shortly after
periastron passage ($\varphi\approx 0.04\text{--}0.06$) and suggested it
originated from thermal emission from newly-formed dust. Based on $JHK$ light
curves of WR 140 and the decreasing dust temperatures as dust drifts further
from the central heating source, the near-IR emission from WR 140 was thought
to fade away by around $\varphi\sim 0.4-0.6$ (Williams et al., 2009). However,
the 0.04 mag uncertainties in the $JHK$ magnitudes from the (Williams et al.,
2009) light curve of WR 140 indicate that faint near-IR emission could still
persist at late times. Indeed, the CHARIS observations reveal the presence of
near-IR circumstellar emission around WR 140 at an orbital phase greater than
$\varphi\gtrsim 0.4$, which highlights the importance in understanding the
origin of the near-IR emission. The two most likely sources of circumstellar
near-IR emission at $\varphi\sim 0.45$, which is $\sim 3.6$ yr after the 2016
Dec dust-forming episode, are thermal dust emission and scattered light from
the central binary.

In order to investigate the properties of the spatially resolved near-IR
emission, the spectral energy distribution (SED) of the bar and C1 were
extracted from the CHARIS observations. The first two channels of the CHARIS
data cube, which correspond to 1.16 and 1.20 $\mu$m, were heavily affected by
data reduction artifacts and thus omitted from the SED analysis. The channels
corresponding to 1.37, 1.42, 1.87, and 1.93 $\mu$m are likely telluric-
dominated and were also omitted from the SED analysis. The extracted near-IR
flux densities  were corrected for interstellar extinction using the average
observed extinction curve of the Milky Way “G21_MWAvg” from the
“dust_extinction” Python package (Gordon et al., 2021)
222https://github.com/karllark/dust_extinction. A value of $A_{v}=2.21$333The
$v$ filter ($\lambda_{\mathrm{C}}=5160\,\AA$) is on the narrow-band system
designed by Smith (1968) where $A_{v}=1.1A_{\mathrm{V}}$ was adopted for the
interstellar extinction toward WR 140 (Rate & Crowther, 2020).

Figure 5 shows the near-IR SEDs of the bar and C1 from the CHARIS data cube
before and after extinction correction. The shape of the near-IR SEDs of both
the bar and C1 clearly demonstrates that the dust emission increases with
increasing wavelength.  Note that because the bar and C1 are spatially
resolved from the central stars, it is unlikely that there is any
contamination by emission from stellar winds in the SEDs. In order to
understand the source of the near-IR flux from the bar, the two most likely
scenarios were investigated: scattered light from the central binary or
thermal dust emission. For the scattered light model,
$F_{\nu}^{\mathrm{Sca}}(a)\propto Q_{\mathrm{Sca}}(a)\,F^{*}_{\nu}$, the
scattering efficiencies Q${}_{\mathrm{Sca}}(a)$ for aromatic-rich H-poor
amorphous carbon (a-C) models with radii $a=0.01$ $\mu$m and $a=1.0$ $\mu$m
were adopted (Jones et al., 2013). This  a-C dust model is also used  for SED
modeling in Sec. 3.5 & 3.6.

Figure 5: Observed (open circles) and  interstellar-extinction corrected
(filled circles) 1.2–2.4 $\mu$m CHARIS  SEDs of WR 140 extracted from the
(Left) bar and the (Right) C1 dust features. The  SEDs are overlaid with the
scattered light models,  which were normalized to the $H$-band flux density of
the SEDs, and  the modified black body emission model fit. The modified black
body thermal dust emission model (Eq. 1) presents the best fit to the shape of
the CHARIS  SEDs of both the bar and C1. Telluric dominated channels (See Tab.
2) are omitted from the plots.

The near-IR continuum emission from WR 140 is assumed to be dominated by the
free-free emission from the ionized stellar winds of the WC star, which can be
approximated as a power-law $F_{\nu}^{\mathrm{ff}}\propto\nu^{0.96}$ (Morris
et al., 1993). Both 0.01 and 1.0 $\mu$m grain size scattered light models,
which were normalized to the $H$-band flux density of the SEDs, fail to
reproduce the positive near-IR slope of SEDs and indicates that there is
little or no contribution from scattered light (Fig. 5).

A modified black-body with an emissivity power-law index of $\beta=1$ was
assumed for the thermal dust emission model,

$F_{\nu}^{\mathrm{MBB}}(T_{d})\propto B_{\nu}(T_{d})\,\nu^{\beta},$ (1)

where $T_{d}$ is the temperature of the emitting dust. The dust temperature
and a multiplicative scaling factor for $F_{\nu}^{\mathrm{MBB}}(T_{d})$ were
fit to the CHARIS SEDs of the bar and C1. The best-fit thermal dust emission
models with temperatures of $T_{d}\approx 960$ K and $T_{d}\approx 1020$ K for
the bar and C1, respectively, demonstrate a close agreement with the shape of
the near-IR SEDs. Even at late times ($\varphi\approx 0.45$), the near-IR
circumstellar emission around WR 140 is therefore most likely dominated by hot
thermal dust emission.

Based on previous resolved mid-IR imaging studies of WR 140, the circumstellar
3.8 and 11.7 $\mu$m emission observed by NIRC2 and COMICS likely arises from
thermal dust emission (Williams et al., 2009). The observed near-IR, 3.8
$\mu$m, and 11.7 $\mu$m flux densities extracted from the bar and C1 are
provided in Table 2 along with the extinction correction factors derived from
the Gordon et al. (2021) interstellar extinction curve with $A_{v}=2.21$.
Table 2 also provides the 3.8 and 11.7 $\mu$m flux densities measured from the
C1 Fossil.

Adopting the same modified black-body thermal dust emission model fit to the
near-IR emission (Eq. 1), the mid-IR 3.8 and 11.7 $\mu$m flux densities of the
bar and C1 indicate temperatures of $T_{d}\approx 560$ K and $T_{d}\approx
530$ K, respectively. These temperatures are notably consistent with the
circumstellar dust temperature derived from mid-IR observations by Williams et
al. (2009) at a similar phase, where $T_{d}\approx 560$ K. The dust
temperature of the C1 Fossil can also be estimated from its measured mid-IR
3.8 and 11.7 $\mu$m flux densities assuming they originate from the same dust
component. Eq. 1 provides a dust temperature of $T_{d}\approx 490$ K for the
C1 Fossil.

There are two notable disparities in the dust temperature calculations:

  * •

The near-IR thermal dust emission from the bar and C1 ($T_{d}\sim 1000$ K)
exhibits much hotter temperatures than those derived by the mid-IR 3.8 and
11.7 $\mu$m observations ($T_{d}\sim 550$ K).

  * •

The derived temperature of the C1 Fossil ($T_{d}\approx 490$ K)  from the 3.8
and 11.7 $\mu$m observations is a factor of $1.6$ hotter than predicted from
the decreasing radiative equilibrium temperature as a function of distance
from the heating source ($T_{d}\propto r^{-0.4}$).

Since the resolved circumstellar emission components at near- and mid-IR
wavelengths are largely co-spatial, it is highly unlikely that the first
disparity is due to spatial variations in the exposure to the central binary’s
radiation field that dominates the dust heating. This disparity between the
near- and mid-IR dust temperatures of the bar and C1 instead suggests that the
dust components probed by near- and mid-IR wavelengths possess different
physical properties such as grain size.

The second disparity arises since dust continuum emission in radiative thermal
equilibrium with a central heating source should decrease with distance (e.g.,
Williams et al. 2009; Lau et al. 2020a). The C1 Fossil is located over a
factor 3.8 further than C1 from the central binary, which implies the dust
temperature of the C1 Fossil should be $\sim 310$ K given the 530 K
temperature measured for C1 and adopting $T_{d}\propto r^{-0.4}$ for dust in
equilibrium heating. This is significantly lower than the estimated 490 K dust
temperature from the 3.8 and 11.7 $\mu$m flux measurements from Eq. 1. The
second disparity  can also be resolved if there are distinct dust components
probed by the 3.8 and 11.7 $\mu$m observations at the C1 Fossil. This scenario
at the C1 Fossil would invalidate the dust temperature estimate from Eq. 1,
which assumed the 3.8 and 11.7 $\mu$m emission originates from the same dust
component. These disparities are investigated in a detailed dust SED analysis
in the following sections.

Table 3: DustEM SED Parameters

Property | Value
---|---
Grain Type | Aromatic-rich H-poor a-C
Bulk Density | 1.6 g cm-3
Size Distribution | log-normal
$a_{min}-a_{max}$ |  $9\times 10^{-4}\text{--}1.0$ $\mu$m
$r_{dust}$ | 1650 au
$r_{C1,Fossil}$ | 6200 au
$L_{*,WR}$ |  $9.3\times 10^{5}$ L⊙
$T_{2/3,WR}$ | 57,000 K
$R_{2/3,WR}$ | 9.79 R⊙
$L_{*,O}$ |  $5.0\times 10^{5}$ L⊙
$T_{2/3,O}$ | 36,000 K
$R_{2/3,O}$ | 18.2 R⊙
$d_{\mathrm{WR140}}$aaGaia-derived distance to WR 140 (Rate & Crowther, 2020) | 1.64 kpc
SED Grid Search Parameters
$a_{0}$ (Nanodust) | 9–13 Å ($\Delta a=0.5$ Å)
$\sigma$ (Nanodust) | 0.1
$a_{0}$ (Larger Grains) | 50–1100 Å ($\Delta a=100$ Å)
$\sigma$ (Larger Grains) | 1.0

Note. — The aromatic-rich H-poor amorphous carbon (a-C) models are defined in
Jones et al. (2013). The parameters $a_{\mathrm{min}}$, $a_{\mathrm{max}}$,
$r_{\mathrm{dust}}$, and $r_{\mathrm{C1,Fossil}}$ correspond to the minimum
and maximum grain sizes in the log-normal grain size distribution, the
distance between the circumstellar dust and the central heating source, and
the distance between the C1 Fossil and the central heating source,
respectively. The heating source parameters $L_{*}$, $T_{2/3}$, and $R_{2/3}$
correspond to the luminosity, temperature (at $\tau=2/3$), and radius (at
$\tau=2/3$) of the stellar heating sources, respectively. The SED grid search
parameters show the range of the centroid grain sizes $a_{0}$ and step
intervals $\Delta a$ for the nanodust and larger grain dust components in the
DustEM model fitting

Figure 6: Best-fit nanodust (orange dashed line) + larger grain (red dot-
dashed line) DustEM SED models fit to the extinction-corrected CHARIS, NIRC2,
and COMICS flux densities of circumstellar dust features (Left) C1 and (Right)
the bar. The solid black line shows the combined emission from the nanodust
and larger grain components. Telluric dominated channels from the CHARIS
observations (See Tab. 2) are omitted from the plots and SED fitting.

###  3.5 Dust SED Fitting of the 2016 Dust-Formation Episode: Revealing the
Presence of Nanodust

In order to investigate the properties of the near- and mid-IR dust components
of the circumstellar dust around WR 140, dust emission models were fit to the
1.2–11.7 $\mu$m SEDs of the bar and C1 using DustEM. DustEM is a numerical
tool that computes the emission from dust heated by an input radiation field
in the optically thin limit with no radiative transfer (Compiègne et al.,
2011). Using the formalism of Desert et al. (1986), DustEM also calculates
grain temperature distributions which is important for characterizing the
emission from very small ($a\lesssim 10$ Å) dust grains that are
stochastically heated by single photon absorption (e.g., Draine & Li 2001).
Unlike larger $a\gtrsim 100$ Å grains, these very small “nanodust” grains may
not be in local thermal equilibrium (LTE) with the radiation field given their
smaller absorption cross sections and lower heat capacities. Single photon
absorption by nanodust grains that are not in LTE result in transient
temperature spikes exceeding the temperatures of larger grains in LTE and can
therefore produce an excess in shorter wavelength IR emission relative to the
larger grains (Sellgren et al., 1983; Desert et al., 1986; Guhathakurta &
Draine, 1989). Notably, due to their lower heat capacities, smaller grains
exhibit higher temperatures than larger grains when exposed to the same
radiation field. Emissions from such nanodust grains are likely to be
important in explaining the hot temperatures of the near-IR component in WR
140.

The input parameters for our DustEM SED models were the radiation field of the
central binary, the radial distance of the dust components from the central
binary, the dust mass, and the grain size distribution properties. A log-
normal grain size distribution with $dn/d\mathrm{log}(a)\propto
e^{-(\mathrm{log}(a/a_{0})/\sigma)^{2}}$ was adopted for the DustEM models,
where the free parameters were the centroid grain size $a_{0}$ and the
distribution width $\sigma$. Minimum and maximum grain sizes in the models
were fixed at $9\times 10^{-4}$ $\mu$m and $1.0$ $\mu$m, respectively, where
the minimum grain size limit was defined by where the maximum dust
temperatures exceeded a sublimation temperature of $\sim 1800$ K when exposed
to the WR 140 radiation field.

To approximate the dust-heating radiation field from the WC7 and O5 stars in
WR 140, we used synthetic spectra from model atmospheres computed with the
PoWR code (Gräfener et al., 2002; Sander et al., 2012; Hainich et al., 2019).
The models were calculated to reproduce the spectral appearance of both stars
(including the light ratio) using a detailed binary analysis similar to those
described in Shenar et al. (2019). A consistent treatment of the hydrostatic
part is included in both the O and the WR model (Sander et al., 2015). A more
dedicated paper about the spectral modeling of WR 140 is currently in the
works (Shenar et al., in prep.). The derived effective temperatures at an
optical depth of $\tau=2/3$ are $T_{2/3,\text{WR}}=57,000$ K and
$T_{2/3,\text{O}}=36,000$ K, and the luminosities are $L_{*,\text{WR}}=1\times
10^{6}$ L⊙ and $L_{*,\text{O}}=5\times 10^{5}$ L⊙.

Table 4: WR 140 DustEM SED Properties of C1 and the bar

| C1 | bar
---|---|---
Property | Larger Grains | Nanodust Grains | Larger Grains | Nanodust Grains
Grain Size Dist. Centroid ($a_{0}$) |  $530^{+490}_{-210}$ Å |  $10.5^{+2.0}_{-0.5}$ Å |  $320^{+360}_{-200}$ Å |  $10.0^{+2.5}_{-1.0}$ Å*
Grain Size Dist. Width ($\sigma$) | $1.0$ | $0.1$ | $1.0$ | $0.1$
Dust Temp. ($T_{d}$) | 480 K | 930 K | 520 K | 930 K
IR Luminosity ($L_{\mathrm{IR}}$) |  $21.5^{+1.2}_{-3.3}$ L⊙ |  $4.4^{+1.2}_{-0.3}$ L⊙ |  $42.6^{+3.3}_{-7.0}$ L⊙ |  $8.0^{+2.8}_{-2.3}$ L⊙
Mass ($M_{d}$) |  $(2.4^{+1.6}_{-0.9})\times 10^{-9}$ M⊙ |  $(1.1^{+0.3}_{-0.1})\times 10^{-10}$ M⊙ |  $(3.2^{+2.0}_{-1.3})\times 10^{-9}$ M⊙ |  $(2.1^{+0.7}_{-0.6})\times 10^{-10}$ M⊙
$M_{\mathrm{d,Tot}}$ |  $(2.5^{+1.6}_{-0.9})\times 10^{-9}$ M⊙ |  $(3.4^{+2.0}_{-1.3})\times 10^{-9}$ M⊙
$M_{\mathrm{Nd}}$/$M_{\mathrm{d,Tot}}$ | $0.05^{+0.03}_{-0.02}$ |  $0.07^{+0.05}_{-0.03}$

Note. — Best-fit DustEM SED model parameters of WR 140’s circumstellar C1 and
bar dust features. The dust temperature $T_{\mathrm{d}}$ corresponds to the
average dust temperature “$T_{\mathrm{moy}}$” output by DustEM at the best-fit
grain size centroid $a_{0}$. $L_{\mathrm{IR}}$ is the integrated IR luminosity
of the larger grain or nanodust components. The individual dust masses
($M_{\mathrm{d}}$) of the larger and nanodust grains of the best-fit models
are provided as well as the total ($M_{\mathrm{d,Tot}}$) dust masses of both
components. $M_{\mathrm{Nd}}/M_{\mathrm{d,Tot}}$ corresponds to the dust mass
ratio of the nanodust component to the total dust mass.

*The lower uncertainty of $a_{0}$ for the nanodust grain  population of the bar hits the limit of the grid search.

Based on the H-poor environment around WR stars and the recent claim of C-rich
aromatic compounds in the circumstellar dust around WR 140 (Lau et al., 2022),
dust grains are assumed to be composed of aromatic-rich H-poor amorphous
carbon  for SED modeling.  The properties of these grains are consistent with
the aromatic-rich H-poor carbonaceous grains defined in Jones et al. (2013),
which have a bulk density of $\rho_{b}=1.6$ g cm-3. Notably, given its strong
UV-radiation field, the central WR star is capable of rapidly photo-processing
($\tau_{\mathrm{UV,pd}}\ll 1$ yr; Eq. 7 from Jones et al. 2014) newly formed
carbonaceous dust as large $a\lesssim 1000$ Å to a stable and fully evolved
state consistent with the aromatic-rich H-poor amorphous carbon grains
described by Jones et al. (2013). The distance to the circumstellar dust from
the central binary system can be approximated by the spatially resolved
imaging and the geometric model (Fig. 1 & 3). Adopting the Gaia-derived
distance to WR 140 of $d=1.64$ kpc (Rate & Crowther, 2020), the distance to
the bar and C1 is $r_{\mathrm{dust}}\approx 1650$ au and the distance to the
C1 Fossil is $r_{\mathrm{C1,Fossil}}\sim 6200$ au  at the time of the
observations.

DustEM SED model fits to the bar and C1 were initially attempted using a
single  population of $a\approx 100$ Å dust grains, which was motivated by the
$100$ Å grain size estimates from previous studies of circumstellar dust
around WR 140 (Williams et al., 2009; Lau et al., 2020a). However, the single
population fits failed to reproduce the emission at both near- and mid-IR
wavelengths. A second  population of  $\sim 10$ Å, nano-sized grains (i.e.,
“nanodust”) was then introduced in the SED fitting. The best-fit DustEM SED
models were obtained from a reduced $\chi^{2}$ analysis, where the amplitude
of the dust emission of the two components is modulated by their dust mass,
and a grid search through two parameters: the grain size centroid of the
nanodust and larger grain distributions. Due to degeneracies with the grain
size centroid, the nanodust and larger grain size distributions’ widths were
fixed at values of 0.1 and 1.0, respectively. The DustEM SED grid search
parameters and the other DustEM model parameters are provided in Table 3.

Figure 7:  C1 Fossil DustEM SED model of nanodust (orange dashed line) +
larger grain (red dot-dashed line)  emission assuming the C1 Fossil dust is
located a distance of $\sim 6200$ au from the central binary with the grain
size distribution centroids ($a_{0}$) of the nanodust and larger grains from
the C1 SED model (Tab. 4). Similar to Fig. 6, the NIRC2 and COMICS flux
densities of the C1 Fossil have been corrected for interstellar extinction.
The solid black line shows the combined emission from the nanodust and larger
grain components.

The best-fit DustEM SED models for the emission from the bar and C1 are shown
in Figure 6. Both of the SED model fits demonstrate that the near-IR flux
observed by CHARIS is dominated by thermal emission from $\sim 10$ Å nanodust.
Emission from the nanodust component between 3–14 $\mu$m also exhibits IR
emission bands that are consistent with “unidentified infrared (UIR)” features
found in various astrophysical environments (Leger & Puget, 1984; Allamandola
et al., 1985; Tielens, 2008). Although the UIR features from dust around WR
140 were not covered by the ground-based observations presented in this work,
they have been identified from mid-IR integral-field unit spectroscopy with
JWST (Lau et al., 2022).

The mid-IR flux observed by NIRC2 and COMICS is dominated by thermal emission
from larger $\sim 300\text{--}500$ Å grains  in the best-fit SED models.
Although consistent within uncertainties, it is interesting to note that the
grain sizes of this larger  population lie in between the $\sim 100$ Å grains
inferred from the analysis in Williams et al. (2009); Lau et al. (2020a) and
the $\sim 700$ Å grains determined by Marchenko et al. (2003) from optical
photometric  absorption around periastron passage. The dust SED properties of
the best-fit models are provided in Table 4.

The dust temperature444$T_{d}$ corresponds the average dust temperature output
by DustEM, “$T_{\mathrm{moy}}$”., $T_{d}$, of the nanodust and larger grain
size  populations are $930$ K and $\sim 500$ K, respectively, and are
consistent with the modified blackbody temperatures estimated in Sec. 3.4. The
total integrated IR luminosity of C1 and the bar are a very small fraction of
the luminosity of the central binary ($L_{\mathrm{IR}}/L_{*}<0.01\%$).

The total dust mass derived for the bar is $\sim 3\times 10^{-9}$ M⊙. As
expected, this value is slightly lower than the total WR 140 circumstellar
dust mass ($6.4^{+3.8}_{-3.3}\times 10^{-9}$ M⊙) from similar SED modeling
results from Lau et al. (2020a), which was fitted to archival data taken at
similar orbital phase ($\varphi\approx 0.43$). In both C1 and the bar, the
ratio of the nanodust mass to the total dust mass is $\sim 5\%$. The general
agreement between the dust temperatures and nanodust mass ratios of C1 and the
bar suggests that C1 does not  possess significantly different dust properties
from the other regions in the bar.

###  3.6 Dust SED of the 2009 Dust-Formation Episode: Evidence of Continued
Nanodust Production

In order to estimate changes in mass from dust formed in the 2016 and 2009
dust-production episodes, the DustEM SED model  fit to the C1 feature was
applied to the 3.8 and 11.7 $\mu$m detections of the C1 Fossil (Fig. 4).
Several free parameters  were therefore fixed due to the limited wavelength
coverage of the C1 Fossil: the grain size distribution centroids ($a_{0}$) of
the nanodust and larger grains was adopted from the derived C1 values (Tab.
4). The same heating source properties as the C1 and bar models were used. The
separation distance between the central binary and the C1 Fossil was estimated
from the NIRC2 observations to be $\sim 6200$ au, which resulted in cooler
dust temperatures for the larger and nanodust grains in the C1 Fossil. The
only free parameters were therefore the dust masses of the nanodust and larger
grain size  populations, which were derived by scaling the nanodust and larger
grain models to the 3.8 and 11.7 $\mu$m flux measurements.

The  scaled SED model of the C1 Fossil shown in Fig. 7 and the dust properties
are provided in Table 5. The dust SED fit suggests that the 3.8 $\mu$m
emission is dominated by nanodust while the 11.7 $\mu$m emission is dominated
by larger grains. Note that in the C1 and bar SED models (Fig. 6), the
emission at both 3.8 and 11.7 $\mu$m emission was dominated by the larger
grains. The different dust  population probed by the 3.8 $\mu$m emission in
the C1 Fossil is  likely due to the cooler dust temperatures as well as an
enhancement in the abundance of nanodust. The cooler dust temperatures of 300
K and 590 K exhibited by the larger grains and nanodust grains, respectively,
in the C1 Fossil is expected due to $r^{-2}$ decrease in the radiative flux
from the central binary heating source.  A potentially intriguing result is
the increase in the C1 Fossil nanodust mass relative to the C1 dust mass by a
factor of $\sim 3$ and the decrease of the C1 larger grain dust mass by a
factor $\sim 0.4$. The ratio of the nanodust mass to the total dust mass in
the C1 Fossil, $\sim 40\%$, presents a substantial increase from the nanodust
mass ratio in C1 of $\sim 5\%$.

The total dust mass in the C1 Fossil, $\sim 1.3\times 10^{-9}$ M⊙, is
approximately half that of the total dust mass in C1, which  suggests that
circumstellar dust around WR 140 is destroyed as it propagates away from the
central binary. This is consistent with the interpretation and observations of
the decreasing circumstellar dust mass by Williams et al. (2009) from their
multi-epoch mid-IR imaging of WR 140. The $\sim 4$ $\mu$m imaging observations
presented by Williams et al. (2009), however, did not capture the fossil dust
emission, which was likely due to the limiting sensitivity of the
observations. The NIRC2 observations of WR 140 therefore uniquely present
evidence of an increasing abundance of nanodust. If the nanodust population is
enhanced by the disruption of the larger grain population, then $\sim 20\%$ of
the total mass removed from the larger grain population between C1 and the C1
Fossil must have been transferred to new nanodust grains.  It is important to
note that these interpretations are derived from only two flux density
measurements and should therefore be treated with caution until verified by
observations at higher sensitivity and with broader wavelength coverage.
However, UIR features that are typically attributed to very small carbonaceous
grains and/or complex hydrocarbons were observed from the C1 Fossil with JWST
(Lau et al., 2022), which supports the presence of nanodust grains derived
from our SED models. The observation of such features in addition to the
detection of the C1 feature from the past $\sim 17$ dust-formation episodes at
7.7 $\mu$m (Lau et al., 2022) suggest that nanodust grains are indeed present
and survive in the extended circumstellar environment around WR 140.

Table 5: WR 140 DustEM SED Properties of the C1 Fossil

Property | Larger Grains | Nanodust Grains
---|---|---
$a_{0}$ |  $530$ Å |  $10.5$ Å
$\sigma$ | $1.0$ | $0.1$
$T_{d}$ | 300 K | 590 K
$L_{\mathrm{IR}}$ |  $1.0$ L⊙ ($0.05\times$) |  $0.6$ L⊙ ($0.1\times$)
$M_{d}$ |  $\sim 9.4\times 10^{-10}$ M⊙ ($0.4\times$) |  $\sim 3.6\times 10^{-10}$ M⊙ ($3\times$)
$M_{\mathrm{d,Tot}}$ |  $\sim 1.3\times 10^{-9}$ M⊙ ($0.5\times$)
$M_{\mathrm{Nd}}$/$M_{\mathrm{d,Tot}}$ |  $\sim 0.4$ ($8\times$)

Note. — DustEM SED model parameters of the C1 Fossil dust feature. The grain
size centroid for the larger grain and nanodust grain models ($a_{0}$) were
adopted from the best-fit values of the C1 SED model. The dust temperatures,
Td, of the C1 Fossil are lower than that of the C1 feature (Tab. 4) due to the
larger separation distance of the C1 Fossil from the central binary (Tab. 3).
The factors in the parentheses following $L_{\mathrm{IR}}$, $M_{\mathrm{d}}$,
$M_{\mathrm{d,Tot}}$, and $M_{\mathrm{Nd}}/M_{\mathrm{d,Tot}}$ indicate the
multiplicative difference from the values derived for the C1 dust feature
(e.g., the best-fit dust mass of the nanodust grains for the C1 Fossil is a
factor of $\sim 3$ greater than that of C1).

##  4 Discussion

###  4.1 Nanodust Formation around WR 140

In this section, we consider the possible mechanisms responsible for the
apparent enhancement of nanodust and destruction of larger $\sim 300-500$ Å
grains in the circumstellar environment of WR 140. This process must occur
over the 7.93-yr timescale (i.e., the WR 140 orbital period) and the 1650 –
6200 au distance scale that characterize how dust propagates from C1 to the
location of the C1 Fossil. Based on the increasing nanodust abundance, we can
rule out several dust destruction mechanisms that would produce the opposite
result and destroy nanodust grains more efficiently than larger dust grains.
Thermal sublimation, which is caused by heating grains  to, and beyond, their
sublimation temperature, would preferentially  tend to destroy small,
stochastically-heated grains since they are heated to higher temperatures than
larger grains when exposed to an identical heating source, this is due to
their lower heat capacities

Thermal and non-thermal sputtering of dust grains by collisions with gas in
the circumstellar environment of WR 140 would destroy grains on timescales
proportional to the grain size (e.g., Tielens et al. 1994), which implies that
$\sim 10$ Å nanodust would be destroyed via sputtering $\sim 50$ times faster
than $\sim 500$ Å grains. Both thermal sublimation and (non-)thermal
sputtering can therefore be ruled out as dominant mechanisms for enhancing
nanodust abundance.

We now consider the two most likely dust processing mechanisms that can
produce smaller dust grains from larger grains: shattering via grain-grain
collisions and rotational disruption from radiative torques (Hirashita, 2010;
Asano et al., 2013; Hoang et al., 2019). The shattering of larger grains via
grain-grain collisions is believed to be an important mechanism for
significantly altering grain size distributions and enhancing the population
of small grains (e.g., Jones et al. 1994, 1996; Hirashita 2010; Asano et al.
2013). Another mechanism that can increase the abundance of small grains
relative to large grains is dust disruption from the centrifugal stress within
fast-rotating grains spun up by radiative torques. Hoang et al. (2019) claim
that this radiative torque disruption (RATD) process should be particularly
relevant in environments with strong radiation fields from sources such as
massive stars and supernovae. The viability of grain-grain collisions compared
to disruption from radiative torques in the circumstellar environment of WR
140 can be investigated by comparing the relevant timescales of each
mechanism. We specifically focus on the C1 dust concentration and its derived
properties for estimating and comparing the timescales.

Grain-grain collisions. For grain shattering, the mean timescale for
collisions between two grains of size $a$ can be estimated as follows:

$\tau_{\mathrm{gg}}=\frac{1}{\pi a^{2}n_{\mathrm{gr}}v_{\mathrm{gr}}},$ (2)

where $n_{\mathrm{gr}}$ is the number density of dust grains and
$v_{\mathrm{gr}}$ is the relative velocity of the grains. Given the complexity
of estimating the relative velocity of circumstellar dust formed via
colliding-winds, we adopt an upper limit of $v_{\mathrm{gr}}$ as the drift
velocity of dust grains through gas, $v_{\mathrm{drift}}$ (i.e.,
$v_{\mathrm{gr}}<v_{\mathrm{drift}}$).  The drift velocity can be estimated
from the observationally derived radial expansion velocity of the
circumstellar dust around WR 140, $v_{\mathrm{dust}}\sim 2600$ km s-1 (Lau et
al., 2022), and the terminal velocity of the WC star, $v_{\infty}\approx 2900$
km s-1 (Williams & Eenens, 1989). Therefore,
$v_{\mathrm{drift}}=v_{\infty}-v_{\mathrm{dust}}\sim 300$ km s-1.

${\color[rgb]{0,0,0}v_{\mathrm{drift}}=v_{\infty}-v_{\mathrm{dust}}\sim
300\,\mathrm{km}\,\mathrm{s}^{-1}}$ (3)

The number density of dust grain in the circumstellar dust shell is difficult
to measure given its complex 3-D morphology (Fig. 3). However, we can estimate
an upper limit on the dust grain number density for the partially spatially
resolved C1 dust feature assuming that its derived dust mass (Tab. 4) is
concentrated in a sphere with a radius equivalent to half the FWHM of the
angular resolution in the Keck/NIRC2 imaging observations at the 1.64 kpc
distance to WR 140, $\Delta r_{\mathrm{C1}}\approx 60$ au. An upper limit of
the dust grain number density in the C1 feature can be expressed as

$n_{\mathrm{gr,C1}}\lesssim\frac{M_{\mathrm{d,C1}}}{m_{\mathrm{gr}}\frac{4}{3}\pi{\Delta
r_{\mathrm{C1}}}^{3}}\approx\frac{M_{\mathrm{d,C1}}}{(\frac{4}{3}\pi
a^{3}\rho_{\mathrm{bulk}})\,\frac{4}{3}\pi{\Delta r_{\mathrm{C1}}}^{3}},$ (4)

where $M_{\mathrm{d,C1}}$ is the dust mass of the larger grains in C1,
$m_{\mathrm{gr}}$ is the mass of an individual larger grain, and
$\rho_{\mathrm{bulk}}$ is the bulk density of the dust grain. Based on the
adopted and derived SED model dust properties of C1, we derive the following
value as an upper limit

$n_{\mathrm{gr,C1}}\lesssim 1.8\times
10^{-6}\left(\frac{M_{\mathrm{d,C1}}}{2.4\times
10^{-9}\,\mathrm{M}_{\odot}}\right)\left(\frac{\Delta
r_{\mathrm{C1}}}{60\,\mathrm{au}}\right)^{-3}\\\
\left(\frac{\rho_{\mathrm{bulk}}}{1.6\,\mathrm{g}\,{\mathrm{cm}^{-3}}}\right)^{-1}\left(\frac{a}{500\,\mathrm{\AA}}\right)^{-3}\,\mathrm{cm}^{-3}.$
(5)

A lower limit on the grain-grain collision timescale of
$\tau_{\mathrm{gg,C1}}\gtrsim 7.5$ yr can then be derived by combining Eqs. 3
& 5 with Eq. 2:

$\tau_{\mathrm{gg,C1}}\gtrsim{\color[rgb]{0,0,0}7.5}\left(\frac{n_{\mathrm{gr,C1}}}{1.8\times
10^{-6}\,\mathrm{cm}^{-3}}\right)^{-1}\\\
\left(\frac{v_{\mathrm{drift}}}{{\color[rgb]{0,0,0}300}\,\mathrm{km}\,\mathrm{s}^{-1}}\right)^{-1}\left(\frac{a}{500\,\mathrm{\AA}}\right)^{-2}\,\mathrm{yr}.$
(6)

Given that grain-grain collision timescales are less than the 7.93-yr orbital
period of WR 140, it is  possible that grain-grain collisions may be causing
the redistribution of mass from larger to smaller dust grains.

Radiative torque disruption (RATD). In strong radiation fields ($U\gg 1$) with
mean wavelength $\bar{\lambda}$, the RATD mechanism can disrupt dust grains
with radii larger than a critical grain size $a_{\mathrm{disr}}$ (Hoang et
al., 2019). Utilizing the derivation of $a_{\mathrm{disr}}$ from Eq. 26 in
Hoang et al. (2019) for environments where
$a_{\mathrm{disr}}\lesssim\bar{\lambda}/1.8$, we can estimate
$a_{\mathrm{disr}}$ for circumstellar dust around WR 140 as follows:

$a_{\mathrm{disr}}\simeq
84\left[\left(\frac{\gamma}{1.0}\right)^{-1}\left(\frac{L_{\mathrm{*,WR+O}}}{1.43\times
10^{6}\,\mathrm{L}_{\odot}}\right)^{-1/3}\right.\\\
\left.\left(\frac{r_{\mathrm{dust}}}{1650\,\mathrm{au}}\right)^{2/3}\left(\frac{\bar{\lambda}_{\mathrm{WR+O}}}{0.11\,\mu\mathrm{m}}\right)^{1.7}\left(\frac{S_{\mathrm{max}}}{10^{9}\,\mathrm{erg}\,\mathrm{cm}^{-3}}\right)^{1/2}\right]^{\frac{1}{2.7}}\,\mathrm{\AA},$
(7)

where $\gamma$ corresponds to the anisotropy of the radiation field (i.e.,
$\gamma=1$ for unidirectional radiation sources), $L_{\mathrm{*,WR+O}}$ is the
combined luminosity of the WR and O stars in WR 140, $r_{\mathrm{dust}}$ is
the distance between the circumstellar dust and the central binary,
$\bar{\lambda}_{\mathrm{WR+O}}$ is the mean wavelength of the radiation field
from the central WR+O binary, and $S_{\mathrm{max}}$ is the maximum tensile
strength of the grain material.

In Eq. 7, we have adopted $S_{\mathrm{max}}=10^{9}$ erg cm-3, which is a
typical value for compact grains and is also consistent with polycrystaline
graphite (Hoang et al., 2019); however, we note that strong materials such as
monocrystaline graphite and diamond can exhibit tensile strengths of
$S_{\mathrm{max}}\gtrsim 10^{11}$ erg cm-3 (Hoang et al., 2019). The
calculation of $a_{\mathrm{disr}}$ in Eq. 7 demonstrates that the larger grain
size  population in our SED models of the bar and C1 ($a\sim 300\text{--}500$
Å; Tab. 4) satisfies the conditions for which RATD should occur.

The RATD disruption time for grains of size $a$ can be defined as the time
required to spin-up a grain to its rotational disruption velocity. Hoang et
al. (2019) provide the expression for the RATD disruption time in their Eq.
27, which we can apply to the circumstellar environment of WR 140 for dust
located at $r_{\mathrm{dust}}=1650$ au (e.g., the C1 feature) from the central
binary:

$\tau_{\mathrm{RATD}}\simeq
1.6\left(\frac{\bar{\lambda}_{\mathrm{WR+O}}}{0.11\,\mu\mathrm{m}}\right)^{1.7}\left(\frac{L_{\mathrm{*,WR+O}}}{1.43\times
10^{6}\,\mathrm{L}_{\odot}}\right)^{-1}\\\
\left(\frac{a}{500\,\mathrm{\AA}}\right)^{-0.7}\left(\frac{r_{\mathrm{dust}}}{1650\,\mathrm{au}}\right)^{2}\left(\frac{S_{\mathrm{max}}}{10^{9}\,\mathrm{erg}\,\mathrm{cm}^{-3}}\right)^{1/2}\,\mathrm{days}.$
(8)

A comparison of the grain-grain collision and RATD timescales (Eq. 6 & 8)
indicates that $\tau_{\mathrm{RATD}}\ll\tau_{\mathrm{gg}}$ for circumstellar
dust around WR 140 at C1 located $r_{\mathrm{dust}}=1650$ au from the central
binary. Dust disruption from radiative torques by the strong radiation field
of the central binary may therefore be the dominant grain disruption
mechanism; however, such short RATD timescales suggest the larger grain
population should be entirely disrupted by the time C1 reaches the position of
the C1 Fossil, which disagrees with the presence of larger grains in the C1
Fossil (Tab. 5, Fig. 7). It is possible that the maximum tensile strength of
the dust grains ($S_{\mathrm{max}}$) around WR 140 is underestimated by
factors of $\sim 10\text{--}100$, which would result in increased RATD
timescales by factors of $\sim 3\text{--}10$ (Eq. 8). In the RATD
calculations, we have also disregarded the possible effects of grain
rotational damping (e.g., by gas collisions), which may mitigate the grain
disruption from radiative torques (Hoang et al., 2019). We also note that
there are significant uncertainties in the grain density estimates for the
grain-grain collision timescale calculation (Eq. 4). Ultimately, given the
strong and hard radiation field around WR 140, we slightly favor RATD over
grain-grain collisions as the dominant mechanism that increases the abundance
of nanodust grains relative to the larger grains in WR 140.

Figure 8: The critical size ($a_{\mathrm{disr}}$) for radiative torque
disruption of dust around WR 140 as a function of distance from the central
binary based on the analytical RATD expressions from Hoang et al. (2019).
Grains larger than $a_{\mathrm{disr}}$ will be disrupted by the RATD
mechanism. The grain size centroids, $a_{0}$, of the nanodust and larger grain
components fit to the circumstellar C1 and C1 Fossil dust features are
overlaid as orange dot-dashed and red dashed lines, respectively. The
separation distances from the central binary to the C1 and C1 Fossil features
are also overlaid on the plot.

We can investigate the impact of RATD in the circumstellar environment of WR
140 starting from the point of dust formation at $r_{\mathrm{dust}}\sim 125$
au (Williams et al., 2009). Figure 8 shows the critical grain size
$a_{\mathrm{disr}}$ as a function of $r_{\mathrm{dust}}$ from WR 140 based on
Eq. 7. The $\sim 10$ Å nanodust grains notably fall below $a_{\mathrm{disr}}$,
which implies that RATD should not destroy newly formed nanodust created from
disrupted larger grains. Interestingly, Fig. 8 indicates that larger grains
($a\gtrsim 100$ Å) around WR 140 should continue to be disrupted by RATD at
separation distances as large as the position of the C1 Fossil
($r_{\mathrm{dust}}\approx 6200$ au). The nanodust to total dust mass ratio
should therefore increase beyond the $\sim 40\%$ measured from our SED
analysis of the C1 Fossil observations (Tab. 5).

###  4.2 Astrophysical Implications: An Early Source of Carbonaceous Nanodust?

Owing to their rapid evolutionary formation timescales ($\sim$Myr) and high
dust production rates ($\dot{M_{d}}\sim 10^{-10}-10^{-6}$ M⊙ yr-1; Lau et al.
2020a), dust-forming WC binaries have been revisited in recent studies as
early and important sources of interstellar dust (Marchenko & Moffat, 2017;
Lau et al., 2020a, 2021, 2022). The  confirmation of nanodust formation around
a colliding-wind WC binary system like WR 140 therefore presents potentially
significant implications on the origin of very small carbonaceous grains in
the ISM of galaxies in the local and early Universe.  Here, we consider
whether or not all dust-forming WC binaries are capable of nanodust production
like WR 140  based on the RATD mechanism.

Newly formed dust in the circumstellar environment around all WC binaries
should be exposed to a strong radiation field powered by the high luminosities
($\sim 10^{5}-10^{6}$ L⊙) and hot photospheric temperatures ($\gtrsim 40,000$
K) of the central WC star as well as that of the  OB-star companion. Based on
Eq. 7 & 8, even the lowest luminosity WC9 stars ($L_{*}\sim 10^{5}$ L⊙) should
be capable of disrupting grains via RATD that are larger than
$a_{\mathrm{disr}}\sim 100$ Å at a distance of $r_{\mathrm{dust}}=1650$ au on
timescales as short as $\tau_{\mathrm{RATD}}\sim 20$ days. Interestingly, in
their dust SED model fitting of Galactic dust-forming WC systems, Lau et al.
(2020a) find that the episodic dust-forming WC systems such as WR 140, WR 125,
WR 137, and WR 19 are better fit by smaller grain size models
($a=0.01\text{--}0.1$ $\mu$m) whereas the persistent dust-forming systems such
as WR 95, WR 98a, WR 104, WR 106, and WR 118 are better fit by larger grain
size models ($a=0.1\text{--}1.0$ $\mu$m). The dust production rates are higher
in the persistent dust-forming systems compared to the episodic systems, which
suggests that self shielding and larger optical depths could play a role in
enhancing grain growth and mitigating the RATD mechanism in persistent dust
makers.

The Lau et al. (2020a) dust SED model fits of the persistent dust-forming WC
systems that favor larger grains, however, do not preclude the presence of
nanodust in these systems given the challenges of detecting nanodust emission.
Indeed, previous mid-IR spectroscopic studies of dust-forming WC systems that
included persistent dust-formers like WR 104 show that they exhibit UIR
features and therefore indicate the presence of very small carbonaceous grains
and/or complex hydrocarbons (Chiar et al., 2002; Marchenko & Moffat, 2017;
Endo et al., 2022). Our observations of WR 140 notably demonstrate that high
angular resolution multi-wavelength IR imaging observations are crucial for
distinguishing nanodust from the longer wavelength IR emission by cooler and
larger dust grains as well as the near-IR free-free emission from the ionized
winds of the central WR star. We therefore speculate that all dust-forming WC
binaries should exhibit nanodust production due to the RATD mechanism on
larger grains. The nanodust production efficiency  may differ based on the
dust-formation conditions around the WC binary.

Given that the evolutionary timescales to form WR stars ($\sim$Myr) is much
shorter than that of AGB stars ($\gtrsim$ 100 Myr), which have been considered
as leading sources of dust in the local Universe (e.g., Gall et al. 2011),  we
argue that WC binaries present an early source of C-rich nanodust.  We
speculate that nanodust formed by WC binaries  are the C-rich aromatic
carriers of the UIR features detected from dusty WC binaries (Endo et al.,
2022; Lau et al., 2022).  Nanodust from WC binaries also present a potential
candidate as a carrier of the 2175 Å UV extinction bump, which is thought to
arise from very small carbonaceous grains (Stecher & Donn, 1965; Duley &
Seahra, 1998; Draine, 2009).  Recent observations with JWST of galaxies from
the first billion years of cosmic time notably present evidence of the 2175 Å
absorption feature, which highlights the possible presence of carbonaceous
dust in the early Universe (Witstok et al., 2023). Continued  ground-based,
high contrast, and high angular resolution near/mid-IR observations  in
combination with sensitive spaced-based observations with JWST will be
essential for investigating the impact of nanodust formation in WC binaries.

##  5 Conclusions

In this work, we presented high contrast and high spatial resolution IR
observations of the dust-forming colliding-wind binary WR 140 with
Subaru/SCExAO + CHARIS, Subaru/COMICS, and Keck/NIRC2+PyWFS (Fig. 1 & 2). All
observations were taken within $\sim 2$ months and enabled a $1\text{--}12$
$\mu$m morphological analysis of the circumstellar dust formed in the most
recent periastron passage and dust-production episode from WR 140 in December
2016. The persistent circumstellar dust features referred to as C0, C1, and
the bar were present in all observations, but the dust feature referred to as
the Eastern Arm was only detected at longer IR wavelengths (3.8 and 11.7
$\mu$m) by NIRC2 and COMICS. The imaging observations were compared against a
geometric model of the dust-forming regions of WR 140’s wind-wind interface at
a consistent orbital phase ($\varphi=0.47$; Fig. 3). The observed morphology
of the circumstellar dust emission appeared consistent with regions in the
geometric model that should exhibit the largest integrated column density
along our line of sight. However, other regions of enhanced column density
predicted by the geometric model were not detected in the observations , which
is likely due to variable dust production around periastron passage (Han et
al., 2022). The 3.8 $\mu$m NIRC2 and 11.7 $\mu$m COMICS images detected faint
‘Fossil’ dust emission linked to the WR 140’s dust-formation event during the
January 2009 periastron passage.

An SED analysis of the IR circumstellar dust emission revealed that the
spectral shape in the near-IR was consistent with thermal dust emission at a
temperature of $\sim 1000$ K as opposed to dust-scattered light from the
central binary (Fig. 5). However, the $\sim 1000$ K dust temperature derived
from the near-IR emission was inconsistent with the $\sim 550$ K dust
temperatures determined from the 3.8 and 11.7 $\mu$m observations. Another
circumstellar dust temperature disparity was identified: the $490$ K dust
temperature of the C1 Fossil derived from the 3.8 and 11.7 $\mu$m images was a
factor of 1.6 hotter than expected based on decreasing radiative equilibrium
temperatures as a function of distance from the heating source.

In order to investigate the dust properties and resolve the temperature
disparities, we conducted a detailed dust SED analysis of the bar, C1, and the
C1 Fossil with DustEM (Fig. 6 & 7). The best-fit DustEM SED models indicated
the presence of a hot dust component composed of very small ($a\sim 10$ Å)
nanodust grains. The presence of this hot dust component in addition to the
previously known larger ($a\sim 300\text{--}500$ Å) and cooler dust component
resolved the dust temperature disparities. Interestingly, the DustEM SED model
fit to the C1 Fossil  indicated a larger abundance of nanodust and a
decreasing abundance of the larger grains in comparison to the more recently
formed C1 feature (Tab. 4 & 5). Our observations and analysis  present
evidence of nanodust formation in the circumstellar environment around a dust-
forming WC binary.

We discussed the possible mechanisms that could reduce the abundance of larger
grains while enhancing the abundance of nanodust in the circumstellar
environment of WR 140. The two most likely processes were shattering via
grain-grain collisions and rotational disruption from radiative torques. An
analysis of the grain-grain collision and RATD timescales (Eq. 6 & 8) suggests
that both mechanisms are plausible in the circumstellar environment around WR
140. However, we slightly favor the RATD mechanism over grain-grain collisions
as the dominant grain disruption mechanism due to the strong and hard
radiation field from the WR 140’s WR and O stars.

Following analytical formulas derived by Hoang et al. (2019) on the RATD
mechanism, we showed that the strong radiation field of the central WC binary
in WR 140 should be capable of disrupting grains larger than $a\gtrsim 100$ Å
at a distance of $\sim 6000$ au from the central binary (Fig. 8). One of the
major issues presented by the RATD mechanism, however, is that the short RATD
disruption timescale of $\sim$days (Eq. 8) at the position of C1 suggests that
the larger grain population should be rapidly and entirely disrupted, which
disagrees with the presence of larger grains in the C1 Fossil. Further
investigation of RATD and grain-grain collisions in the circumstellar
environment of dust-forming WC binaries will be important for addressing the
dominant dust-disruption / nanodust-production mechanism(s) from these Carbon-
rich dust factories.

Lastly, we introduced the astrophysical implications of nanodust formation
around dust-forming colliding-wind WC binaries like WR 140. We speculate that
all dust-forming WC binaries should exhibit some degree of nanodust formation
from the RATD mechanism given the strong radiation field emitted by the
central WC star. Given the rapid evolutionary timescales to form WR stars
($\sim$Myr), WC binaries may therefore provide an early source of C-rich
nanodust in the ISM of galaxies in the local and early Universe. As
demonstrated in this work, high contrast and high spatial resolution IR
observations will be crucial for investigating the impact of nanodust
formation from WC binaries  especially in the era of JWST.

RML thanks Thiem Hoang for the insightful discussions on grain disruption and
the RATD mechanism. RML also thanks T. Fujiyoshi, the Subaru Observatory
staff, and the Keck Observatory staff for supporting our observations of WR
140.  We also thank the anonymous referee for their valuable feedback on our
analysis an interpretation. The work of RML is supported by NOIRLab, which is
managed by the Association of Universities for Research in Astronomy (AURA)
under a cooperative agreement with the National Science Foundation. RML
acknowledges the Japan Aerospace Exploration Agency’s International Top Young
Fellowship (ITYF). AACS is funded by the Deutsche Forschungsgemeinschaft (DFG,
German Research Foundation) in the form of an Emmy Noether Research Group
(grant number SA4064/1-1, PI Sander) and acknowledges additional support by
the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) –
Project-ID 138713538 – SFB 881 (“The Milky Way System”, subproject P04).  AFJM
is grateful for financial aid from NSERC (Canada). IE acknowledges support
from Grant-in-Aid for JSPS Fellows (grant No. 21J13200) This research is based
on data collected at Subaru Telescope, which is operated by the National
Astronomical Observatory of Japan. The development of SCExAO was supported by
the Japan Society for the Promotion of Science (Grant-in-Aid for Research
#23340051, #26220704, #23103002, #19H00703 & #19H00695), the Astrobiology
Center of the National Institutes of Natural Sciences, Japan, the Mt Cuba
Foundation and the director’s contingency fund at Subaru Telescope.  Some of
the data presented herein were obtained at the W. M. Keck Observatory, which
is operated as a scientific partnership among the California Institute of
Technology, the University of California and the National Aeronautics and
Space Administration. The Observatory was made possible by the generous
financial support of the W. M. Keck Foundation. The authors wish to recognize
and acknowledge the very significant cultural role and reverence that the
summit of Maunakea has always had within the indigenous Hawaiian community,
and are most fortunate to have the opportunity to conduct observations from
this mountain. CHARIS was built at Princeton University under a Grant-in-Aid
for Scientific Research on Innovative Areas from MEXT of the Japanese
government (#23103002) This research made use of
Astropy,555http://www.astropy.org a community-developed core Python package
for Astronomy (Astropy Collaboration et al., 2013, 2018).

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