J/AJ/170/218 Radio light curve of WR112 from VLA & ATCA (Monnier+, 2025)
Revealing the accelerating wind in the inner region of colliding-wind binary
WR 112.
Monnier J.D., Han Y., Corcoran M.F., Bloot S., Callingham J.R., Danchi W.,
Edwards P.G., Greenhill L., Hamaguchi K., Hankins M.J., Lau R.,
Miller J.M., Moffat A.F.J., Ruane G., Russell C.M.P., Soulain A.,
Tinyanont S., Tuthill P., Wang J.J., Williams P.M.
<Astron. J., 170, 218 (2025)>
=2025AJ....170..218M 2025AJ....170..218M
ADC_Keywords: Radio sources; Stars, double and multiple; Photometry;
Stars, Wolf-Rayet; Interferometry
Keywords: Wolf-Rayet stars ; Stellar winds ; WC stars ; Circumstellar dust ;
High energy astrophysics ; Close binary stars
Abstract:
Colliding winds in massive binaries generate X-ray-bright shocks,
synchrotron radio emission, and sometimes even dusty "pinwheel"
spirals. We report the first X-ray detections of the dusty WC+O binary
system WR 112 from Chandra and Swift, alongside 27yr of Very Large
Array/Australia Telescope Compact Array radio monitoring and new
diffraction-limited Keck images. Because we view the nearly circular
orbit almost edge-on, the colliding-wind zone alternates between heavy
Wolf-Rayet wind self-absorption and near-transparent O-star wind
foreground each 20yr orbit, producing phase-locked radio and X-ray
variability. This scenario leads to a prediction that the radio
spectral index is flatter from a larger nonthermal contribution around
the radio intensity maximum, which indeed was observed. Existing
models that assume a single dust-expansion speed fail to reproduce the
combined infrared (IR) geometry and radio light curve. Instead, we
require an accelerating postshock flow that climbs from
near-stationary to ∼1350km/s in about one orbital cycle, naturally
matching the IR spiral from 5" down to within 0.1", while also fitting
the phase of the radio brightening. These kinematic constraints supply
critical boundary conditions for future hydrodynamic simulations,
which can link hot-plasma cooling, nonthermal radio emission, X-ray
spectra, and dust formation in a self-consistent framework. WR 112
thus joins WR 140, WR 104, and WR 70-16 (Apep) as a benchmark system
for testing colliding-wind physics under an increasingly diverse range
of orbital architectures and physical conditions.
Description:
Radio observations of WR 112 were obtained from 1995 to 2022 with the
Very Large Array (VLA) in New Meico, USA, and the Australia Telescope
Compact Array (ATCA), in Australia. These include 171 flux density
measurements not reported before by the principal investigators of
those programs, and 15 previously published observations. In all
observations the radio emission was unresolved, and here we report
only flux measurements in a variety of bands, though mostly at the
X-band (3.6cm, 8GHz). Almost all the radio epochs include an 8GHz
measurement.
We began a monitoring campaign which lasted for 15yr without return of
the original radio high state, and the VLA monitoring was terminated
in 2015. A renewed monitoring effort began in 2020 following an
estimate of the orbital period of 19.4yr from Lau+ (2020ApJ...900..190L 2020ApJ...900..190L)
based on the proper motion of mid-IR dust shells. Our new monitoring
began too late to catch the radio high state, as confirmed by one 2019
measurement by the VLA all-sky survey (VLASS; Gordon+2021, J/ApJS/255/30).
This long-term monitoring program began after Monnier+ (2002ApJ...566..399M 2002ApJ...566..399M)
highlighted the strong variability in WR 112's radio light curve based
on the earlier data of Leitherer+ (1997ApJ...481..898L 1997ApJ...481..898L) and
Chapman+ (1999ApJ...518..890C 1999ApJ...518..890C). The flux declined from 4mJy down to
<1mJy between 2000 February and 2000 October, accompanied by a
dramatic change in the spectral index.
Few X-ray observations of WR 112 exist. X-ray emission from a source
about 8.1" from the known position of WR 112 was made in a short
(853s) exposure with the Neil Gehrels Swift X-ray Telescope (XRT) in
photon-counting mode as part of a Director's Discretionary Time
observation on the space telescope. This short exposure only detected
15cts over the full XRT energy band (approximately 0.2-10keV). In
2022, the field centered on WR 112 was observed four additional times
with the Swift/XRT in photon-counting mode, confirming the detection
of this source. In order to confirm that the Swift source was in fact
WR 112, we observed the region around WR 112 on 2023 July 3-4 with
Chandra. The exposure time of the first observation segment, ObsID
25128, was 17.8ks, while the exposure time of the second observation
segment, ObsID 27939, was 16.7ks.
WR 112 was observed 11 times between 1998 and 2004 at the Keck
Observatory in Hawaii, USA, using the annulus aperture mask with the
Near-infrared Camera (NIRC) instrument. Most of the images were taken
with an effective wavelength of ∼2.2um. L'-band (3.426-4.126um)
observations of WR 112 were carried out with Keck/NIRC2 on UT 2020 Aug
18 using the vortex coronagraph. We obtained 87 frames of the target
in total, each consisting of 200 coadds of 0.053s, with the central
star being obscured by the vortex coronagraph. This resulted in a
total integration time of 922s on source.
Objects:
--------------------------------------------------------------------
RA (2000) DE Designation(s)
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18 16 33.49 -18 58 42.3 WR 112 = Gaia DR3 4094789697991379712
--------------------------------------------------------------------
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table6.dat 57 186 Radio observations of WR112
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See also:
VIII/111 : VCCS Epoch 1 Bright Catalog Release (Peters+, 2021)
J/ApJS/255/30 : VLASS QL Ep.1 Catalog, CIRADA version (Gordon+, 2021)
J/ApJ/923/191 : RXTE, Swift & NICER obs. of WR 140 (Pollock+, 2021)
J/MNRAS/515/5159 : Radio and winds in BALQSOs (Petley+, 2022)
J/AJ/168/288 : VLASS stellar radio sources Gaia counterparts (Ayanabha+, 2024)
J/A+A/684/A19 : The XXL Survey. LII. (Slaus+, 2024)
Byte-by-byte Description of file: table6.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 10 A10 "Y/M/D" Date UT date of the observation
12- 16 A5 --- Inst Facility identifier (1)
18- 21 F4.1 cm Wave [0.7/20.0] Wavelength of the observation
23- 26 F4.1 GHz Freq [1.5/42.9] Frequency of the observation
28- 28 A1 --- l_Flux The 2σ limit flag on Flux
29- 32 F4.2 mJy Flux [0.13/4.4] Flux density at Freq
34- 37 F4.2 mJy e_Flux [0.01/0.56]? Uncertainty in Flux
39- 57 A19 --- Bib Data reference bibcode (2)
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Note (1): Facility as follows:
ATCA = Australia Telescope Compact Array (ATCA), Australia; 20 occurrences
VLA = Very Large Array (VLA), New Mexico, USA; 165 occurrences
VLASS = Very Large Array Sky Survey (VLASS), Gordon+2021 (J/ApJS/255/30);
1 occurrence
Note (2): Reference as follows:
This work = 171 occurrences
2021ApJS..255...30G 2021ApJS..255...30G = Gordon+2021 (J/ApJS/255/30); 1 occurrence
1997ApJ...481..898L 1997ApJ...481..898L = Leitherer+1997; 2 occurrences
1999ApJ...518..890C 1999ApJ...518..890C = Chapman+1999; 2 occurrences
2002ApJ...566..399M 2002ApJ...566..399M = Monnier+2002; 10 occurrences
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History:
From electronic version of the journal
License: CC-BY-4.0
(End) Prepared by [AAS], Robin Leichtnam [CDS] 17-Jul-2026