arXiv subjects

Anthony Moffat

Publications and source records attributed to Anthony Moffat.

7 recordsLinked to original sources

arXiv2025

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

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.

astro-ph.SR↗

arXiv2025

The Aromatic Infrared Bands around the Wolf-Rayet Binary WR140 Revealed by JWST

We have analyzed the aromatic infrared bands (AIBs) in the 6-11.2 $\mu$m range around the Wolf-Rayet binary WR140 (d=1.64 kpc) obtained with the James Webb Space Telescope (JWST) Mid-Infrared Instrument (MIRI) Medium-Resolution Spectrometer (MRS). In WR140's circumstellar environment, we have detected AIBs at 6 $\mu$m and 7.7 $\mu$m which are attributed to C-C stretching modes. These features have been detected in the innermost dust shell (Shell1; ~2100 au from WR140), the subsequent dust shell (Shell2; ~5200 au), and ``off-shell'' regions in the MRS coverage. The 11.2 $\mu$m AIB, which is associated with the C-H out-of-plane bending mode, has been tentatively detected in Shell2 and the surrounding off-shell positions around Shell2. We compared the AIB features from WR140 to spectra of established AIB feature classes A, B, C, and D. The detected features around WR140 do not agree with these established classes. The peak wavelengths and full width half maxima (FWHMs) of the 6 $\mu$m and 7.7 $\mu$m features are, however, consistent with those of R Coronae Borealis (RCB) stars with hydrogen-poor conditions. We discuss a possible structure of carbonaceous compounds and environments where they form around WR140. It is proposed that hydrogen-poor carbonaceous compounds initially originate from the carbon-rich WR wind, and the hydrogen-rich stellar wind from the companion O star may provide hydrogen to these carbonaceous compounds.

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arXiv2022

Constraints on Clumps in the Representative Wind of the WN8 Wolf-Rayet star HD96548 = WR40 with Simultaneous Broadband Light and Linear-Polarisation Variability

We report precision ground-based broadband optical intensity and linear-polarisation light-curves for the sky's brightest WN8 star, WR40. WN8 stars are notorious for their high level of variability, stemming from stochastic clumps in their strong winds that are slower and less hot than the winds of most other Wolf-Rayet (WR) stars. We confirm previous results that many WR stars display an amplitude of variability that is an order-of-magnitude higher in photometry than in polarimetry. For the first time, the unique nature of near simultaneity of our photometric and polarimetric observations of WR40 allows us to check whether the two types of variability show correlated behaviour, of which we find none. Assuming simple temporal functions for the brightness and polarisation of individual clumps, a model for simulated light curves is found to reproduce the properties of the observations, specifically the absence of correlation between photometric and polarimetric variations, the ratio of standard deviations for photometric and polarimetric variability, and the ratio of the average intrinsic polarisation relative to its standard deviation. Mapping the solution for time variability to a spatial coordinate suggests that the wind clumps of WR40 are free-free emitting in addition to light scattering.

astro-ph.SR↗

arXiv2022

NICER X-ray Observations of Eta Carinae During its Most Recent Periastron Passage

We report high-precision X-ray monitoring observations in the 0.4-10 keV band of the luminous, long-period colliding-wind binary Eta Carinae up to and through its most recent X-ray minimum/periastron passage in February 2020. Eta Carinae reached its observed maximum X-ray flux on 7 January 2020, at a flux level of $3.30 \times 10^{-10}$ ergs s$^{-1}$ cm$^{-2}$, followed by a rapid plunge to its observed minimum flux, $0.03 \times 10^{-10}$ ergs s$^{-1}$ cm$^{-2}$ near 17 February 2020. The NICER observations show an X-ray recovery from minimum of only $\sim$16 days, the shortest X-ray minimum observed so far. We provide new constraints of the "deep" and "shallow" minimum intervals. Variations in the characteristic X-ray temperature of the hottest observed X-ray emission indicate that the apex of the wind-wind "bow shock" enters the companion's wind acceleration zone about 81 days before the start of the X-ray minimum. There is a step-like increase in column density just before the X-ray minimum, probably associated with the presence of dense clumps near the shock apex. During recovery and after, the column density shows a smooth decline, which agrees with previous $N_{H}$ measurements made by SWIFT at the same orbital phase, indicating that changes in mass-loss rate are only a few percent over the two cycles. Finally, we use the variations in the X-ray flux of the outer ejecta seen by NICER to derive a kinetic X-ray luminosity of the ejecta of $\sim 10^{41}$ ergs s$^{-1}$ near the time of the "Great Eruption'.

astro-ph.HE↗

arXiv2020

CO, Water, and Possible Methanol in Eta Carinae Approaching Periastron

In circumstellar gas, the complex organic molecule methanol has been found almost exclusively around young stellar objects, and is thus regarded as a signpost of recent star formation. Here we report the first probable detection of methanol around an evolved high-mass star, in the complex circumstellar environment around the Luminous Blue Variable $η$ Carinae, while using ALMA to investigate molecular cloud conditions traced by CO (2-1) in an orbit phase of the massive binary preceding the 2020 periastron. Favoring methanol over a $^{13}$CS alternative, the emission originates from hot ($T_{\rm{gas}} \simeq$ 700 K) material, $\sim$2$''$ (0.02 pc) across, centered on the dust-obscured binary in contrast to the CO which traces inner layers of the extended massive equatorial torus, and is accompanied by prominent absorption in a cooler ($T_{\rm{gas}} \simeq$ 110 K) layer of gas. We also report detections of water in $Herschel$/HIFI observations at 557 GHz and 988 GHz. The methanol abundance is several to 50 times higher than observed towards several lower mass stars, while water abundances are similar to those observed in cool, dense molecular clouds. The very high methanol:water abundance ratio in the core of $η$ Carinae may suggest methanol formation processes similar to Fischer-Tropsch-type catalytic reactions on dust grains. These observations prove that complex molecule formation can occur in the chemically evolved environments around massive stars in the end stages of their evolution, given sufficient gas densities and shielding conditions as may occur in material around massive interacting companions and merger remnants.

astro-ph.SR↗

arXiv2020

QZ Carinae Orbit of the Two Binary Pairs

We present an updated O-C diagram of the light-time variations of the eclipsing binary (component B) in the system QZ Carinae as it moves in the long-period orbit around the non-eclipsing pair (component A). This includes new Variable Stars South members' measures from 2017 to 2019, BRITE satellite observations in 2017 and 2018, and 100 previously unpublished measures made at Auckland Observatory from 1974 to 1978. We conclude that QZ Carinae has not yet completed one orbit of the two pairs since discovery in 1971. The duration of totality of primary eclipses was measured to be 0.295 +/- 0.02 day (7.08 +/-0.48 hours), rather longer than earlier values from light curve models. Other observational findings include the shape of primary and secondary eclipses and small-scale short-term brightness changes.

astro-ph.SR↗

arXiv2017

Rotationally modulated photometric variations in B supergiants?

In this contribution, we present BRITE observations of the early-B supergiants $ε$ Ori and $κ$ Ori. We perform a preliminary analysis of the data acquired over the first two Orion observing runs. We evaluate whether they are compatible with co-rotating bright spots and discuss the challenges of such an approach.

astro-ph.SR↗