A speckle-imaging search for close and very faint companions to the nearest and brightest Wolf–Rayet stars

Shara, Michael M.; Howell, Steve B.; Furlan, Elise; Gnilka, Crystal L.; Moffat, A. F. J.; Scott, Nicholas J.; Zurek, David

doi:10.1093/mnras/stab2666

Monthly Notices of the Royal Astronomical Society

2021

DOI: 10.1093/mnras/stab2666

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A speckle-imaging search for close and very faint companions to the nearest and brightest Wolf–Rayet stars

Michael M. Shara

Steve B. Howell

Elise Furlan

et al.

Abstract: Gravitationally bound companions to stars enable determinations of their masses, and offer clues to their formation, evolution and dynamical histories. So motivated, we have carried out a speckle imaging survey of eight of the nearest and brightest Wolf-Rayet (WR) stars to directly measure the frequency of their resolvable companions, and to search for much fainter companions than hitherto possible. We found one new, close companion to each of WR 113, WR 115 and WR 120 in the separation range ∼ 0.2” - 1.2”. Ou… Show more

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Cited by 4 publications

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Insight into the occurrence of particle acceleration through the investigation of Wolf–Rayet stars using uGMRT observations

Blanco,

De Becker,

Saha

et al. 2024

A&A

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Massive stars produce strong stellar winds that consist of continuous outflows of material at speeds of thousands of $ $. These winds convey large amounts of kinetic power, especially in the case of Wolf--Rayet (WR) stars. When these winds interact with nearby material, they will likely produce shocks. Among other processes, particle acceleration is expected to occur. This is particularly well established in the case of massive binary systems, where the stellar winds collide, allowing these systems to be identified thanks to the detection of synchrotron radio emission, produced by a population of relativistic particles accelerated in the shocks. Our goal is to investigate the occurrence of particle acceleration among massive stars in their pre-supernova evolution phases. We observed a subset of five WR stars in the radio domain using the upgraded Giant Metrewave Radio Telescope (uGMRT), located in India. The observations were carried out in bands 4 (550--950 MHz) and 5 (1050--1450 MHz) for all the targets. We detected radio emission for only WR 110 in bands 4 and 5. Its thermal spectrum displays a consistent index of +0.74 down to uGMRT bands. The four other targets were not detected and we derived 3sigma upper limits. Our upper limits in Band 4 are the first provided for these targets below 1 GHz. None of the targets was identified as a synchrotron radio emitter in these radio bands. If some synchrotron emission is produced in these systems, the non-detection with uGMRT can be most likely attributed to strong free-free absorption (FFA). This is especially relevant for WR98a which is catalogued as a particle accelerator based on previous measurements at higher radio frequencies. The prominence of FFA constitutes a severe obstacle to identifying particle accelerators in the radio domain.

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Insight into the occurrence of particle acceleration through the investigation of Wolf–Rayet stars using uGMRT observations

Blanco,

De Becker,

Saha

et al. 2024

A&A

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Investigating 39 Galactic Wolf-Rayet stars with VLTI/GRAVITY

Deshmukh,

Sana,

Mérand

et al. 2024

A&A

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Wolf-Rayet stars (WRs) represent one of the final evolutionary stages of massive stars and are thought to be the immediate progenitors of stellar-mass black holes. Their multiplicity characteristics form an important anchor point in single and binary population models for predicting gravitational-wave progenitors. Recent spectroscopic campaigns have suggested incompatible multiplicity fractions and period distributions for N- and C-rich Galactic WRs (WNs and WCs) at both short and long orbital periods, in contradiction with evolutionary model predictions. In this work, we employed long-baseline infrared interferometry to investigate the multiplicity of WRs at long periods and explored the nature of their companions. We present a magnitude-limited ($K<9$; $V<14$) survey of 39 Galactic WRs, including 11 WN, 15 WC, and 13 H-rich WN (WNh) stars. We used the $K$-band instrument GRAVITY at the Very Large Telescope Interferometer (VLTI) in Chile. The sensitivity of GRAVITY at spatial scales of sim 1--200 milliarcseconds and flux contrast of $1<!PCT!>$ allowed an exploration of periods in the range $10^ $ d and companions down to sim 5$\,M_ We carried out a companion search for all our targets, with the aim of either finding wide companions or calculating detection limits. We also explored the rich GRAVITY dataset beyond a multiplicity search to look for other interesting properties of the WR sample. We detected wide companions with VLTI/GRAVITY for only four stars in our sample: WR 48, WR 89, WR 93, and WR 115. Combining our results with spectroscopic studies, we arrived at observed multiplicity fractions of $f^ WN obs WC obs and WNh obs The multiplicity fractions and period distributions of WNs and WCs are consistent in our sample. For single WRs, we placed upper limits on the mass of potential companions down to sim 5$\,M_ for WNs and WCs, and sim 7$\,M_ for WNh stars. In addition, we also found other features in the GRAVITY dataset, such as (i) a diffuse extended component contributing significantly to the $K$-band flux in over half the WR sample; (ii) five known spectroscopic binaries resolved in differential phase data, which constitutes an alternative detection method for close binaries; and (iii) spatially resolved winds in four stars: WR 16, WR 31a, WR 78, and WR 110. Our survey reveals a lack of intermediate- (a few hundred days) and long- (a few years to decades) period WR systems. The 200d peak in the period distributions of WR+OB and BH+OB binaries predicted by Case B mass-transfer binary evolution models is not seen in our data. The rich companionship of their O-type progenitors in this separation range suggests that the WR progenitor stars expand and interact with their companions, most likely through unstable mass transfer, resulting in either a short-period system or a merger.

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Speckle imaging of γ2 Velorum: the inner wind possibly resolved

Shara,

Howell,

Furlan

et al. 2023

Monthly Notices of the Royal Astronomical Society

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Accurately quantifying the rates dM/dt at which massive stars lose mass is essential to any understanding of their evolution. All dM/dt estimates to date assume wind clumping factors; not allowing for clumping leads to overestimates of dM/dt and underestimates of lifetimes and masses when these stars explode as supernovae. Mid-IR spectroscopy suggested that the wind of the nearest Wolf-Rayet star, γ2 Vel, is resolved with a Full Width at 10 per cent intensity of 0.5 arcsec, or 171 AU at the 342 pc distance of the star. As the Zorro speckle imager on Gemini-South is capable of ∼ 0.02 arcsec resolution, we have used it to image γ2 Vel at two orbital phases (0.30 and 0.44) with two narrowband and two intermediate-band filters in an attempt to resolve its wind. Our observations demonstrate that γ2 Vel’s wind may be resolved as a ∼ 0.07 arcsec westward elongation through an 832 nm filter at orbital phase 0.3. If confirmed, this is the smallest scale (∼24 AU) at which a WR star wind asymmetry has been directly imaged. Similar imaging at multiple phases is needed to determine if the asymmetry is due to stochastic wind clumping, co-rotating interaction regions or colliding-wind, cone-shaped shocks.

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A speckle-imaging search for close and very faint companions to the nearest and brightest Wolf–Rayet stars

Cited by 4 publications

References 76 publications

Insight into the occurrence of particle acceleration through the investigation of Wolf–Rayet stars using uGMRT observations

Insight into the occurrence of particle acceleration through the investigation of Wolf–Rayet stars using uGMRT observations

Investigating 39 Galactic Wolf-Rayet stars with VLTI/GRAVITY

Speckle imaging of γ2 Velorum: the inner wind possibly resolved

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