Smear fitting: a new image-deconvolution method for interferometric data

Reid, Robert I.

doi:10.1111/j.1365-2966.2006.10084.x

Cited by 7 publications

(6 citation statements)

References 7 publications

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“…A final 2 http://www.vla.nrao.edu/astro/calib/manual/ examination of the calibrated visibilities was made prior to image construction. The visibility data for Cyg OB2 #5 were deconvolved via model fitting using the smerf patch (Reid 2006) to the difmap package (Shepherd et al 1995). This is a technique used widely in VLBI image reconstruction, rather than the well-known clean technique.…”

Section: Very Large Arraymentioning

confidence: 99%

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MODELING THE RADIO EMISSION FROM Cyg OB2 NO. 5: A QUADRUPLE SYSTEM?

Kennedy

Dougherty

Fink

et al. 2010

ApJ

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Fifty observations at frequencies between 1.4 GHz and 43 GHz of the 6.6-day O6.5-7+O5.5-6 binary Cyg OB2 #5 using the Very Large Array over 20 years are re-examined. The aim is to determine the location and character of the previously detected variable radio emission. The radio emission from the system consists of a primary component that is associated with the binary, and a non-thermal source (NE), 0.8 ′′ to the NE of the binary that has been ascribed to a wind-collision region (WCR) between the stellar winds of the binary and that of a B-type star (Star D) to the NE . Previous studies have not accounted for the potential contribution of NE to the total radio emission, most especially in observations where the primary and NE sources are not resolved as separate sources. NE shows no evidence of variation in 23 epochs where it is resolved separately from the primary radio component, demonstrating that the variable emission arises in the primary component. Since NE is non-variable, the radio flux from the primary can now be well determined for the first time, most especially in observations that do not resolve both the primary and NE components. The variable radio emission from the primary component has a period of 6.7 ± 0.3 years which is described by a simple model of a non-thermal source orbiting within the stellar wind envelope of the binary. Such a model implies the presence of a third, unresolved stellar companion (Star C) orbiting the 6.6-day binary with a period of 6.7 years and independent of Star D to the NE. The variable non-thermal emission arises from either a WCR between Star C and the binary system, or possibly from Star C directly. The model gives a mass-loss rate of 3.4 × 10 −5 M ⊙ yr −1 for Cyg OB2 #5, unusually high for an Of supergiant and comparable to that of WR stars, and consistent with an unusually strong He i 1.083-µm emission line, also redolent of WR stars. An examination of radial velocity observations available from the literature suggests reflex motion of the binary due to Star C, for which a mass of 23 +22 −14 M ⊙ is deduced. The natures of NE and Star D are also examined. If NE is a WCR, as suggested by other authors, then the required mass-loss rate is an order of magnitude higher than expected for an early B-type dwarf, and only just consistent with a supergiant. This raises the question of NE as a WCR, but its nonthermal luminosity is consistent with a WCR and a comparison of reddening between Cyg OB2 #5 and Star D do not rule out an association, implying Cyg OB2 #5 is a quadruple system. Pursuing alternative models for NE, such as an unassociated background source, would require very challenging observations.

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Section: Very Large Arraymentioning

confidence: 99%

“…The visibility data for Cyg OB2 #5 were deconvolved via model fitting using the smerf patch (Reid 2006) to the difmap package (Shepherd et al 1995). This is a technique used widely in VLBI image reconstruction, rather than the well-known clean technique.…”

Section: Very Large Arraymentioning

confidence: 99%

MODELING THE RADIO EMISSION FROM Cyg OB2 NO. 5: A QUADRUPLE SYSTEM?

Kennedy

Dougherty

Fink

et al. 2010

ApJ

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“…This was done using an automated model-fitting routine, the modcons macro within the smerf patch (Reid 2006) to the Difmap package (Shepherd 1997). The major advantage of the "smear fitting" method implemented by smerf is that it yields higher resolution for significantly detected features than estimated by the oft-used Rayleigh criteria (Reid 2006), represented by the FWHM of the synthesized beam. A model was established for the 8.6-GHz visibilities consisting of point and Gaussian source models, using the image data to guide the modelling process.…”

Section: Observationsmentioning

confidence: 99%

Radio emission from the massive stars in the galactic super star cluster Westerlund 1

Dougherty

Clark

Negueruela

et al. 2010

A&A

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Aims. Current mass-loss rate estimates imply that main sequence line-driven winds are not sufficient to strip away the H-rich envelope to yield Wolf-Rayet (WR) stars. The rich transitional population of the young massive cluster Westerlund 1 (Wd 1) provides an ideal laboratory to observe and constrain mass-loss processes throughout the transitional phase of stellar evolution. Methods. We present an analysis of deep radio continuum observations of Wd 1 obtained with the Australia Telescope Compact Array at four frequency bands that permit investigation of the intrinsic characteristics of the radio emission. Results. We detect 18 cluster members, a sample dominated by the cool hypergiants, with additional detections amongst the hotter OB supergiants and WR stars. The radio properties of the sample are diverse, with thermal, non-thermal and composite thermal/nonthermal sources present. Mass-loss rates determined for stars with partially optically thick stellar winds are ∼10 −5 M yr −1 across all spectral types, insufficient to enable the formation of WRs during a massive star lifetime, and the stars must undergo a period of greatly enhanced mass loss. The sgB[e] star W9, the brightest radio source in Wd 1, may provide an example, with a current mass-loss rate an order of magnitude higher than the other cluster members, and an extended nebula interpreted as a wind from an earlier epoch with a density ∼3× the current wind. Such an envelope structure in W9 is reminiscent of luminous blue variables, and one that shows evidence of two eras of high, possibly eruptive mass loss. Surprisingly, three of the OB supergiants are detected, implying unusually dense winds, though they are embedded in more extended emission regions that may influence the derived parameters. They also may have composite spectra, suggesting binarity, which can lead to a higher flux than expected from a stellar wind. Spatially resolved nebulae are associated with three of the four RSGs and three of the six YHGs in the cluster, which are due to quiescent mass loss rather than outbursts. The extended nebulae of W20 and W26 have a cometary morphology, implying significant interaction with either the intracluster medium or cluster wind. For some of the cool star winds, the ionizing source may be a companion star though the cluster radiation density is sufficiently high to provide the necessary ionizing radiation. Five WR stars are detected with composite spectra, interpreted as arising in colliding-wind binaries.

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“…MEM techniques have been also extended to take into account source structure through the use of multiresolution and wavelet based techniques [9], global non-negative least squares was proposed by Briggs [10], matrix based parametric imaging such as the Least Squares Minimum Variance Imaging (LS-MVI), maximum likelihood based techniques in [11], [12], [13] and sparse L1 reconstruction in [14] and [15]. Source modeling is an important issue and various techniques to improve modeling over simple point source models by using shapelets, wavelets and Gaussians [16] have been implemented. A more extensive overview of classical techniques and implementation issues is given in [17] or [18].…”

Section: Introductionmentioning

confidence: 99%

Computationally efficient radio astronomical image formation using constrained least squares and and the MVDR beamformer

Sardarabadi

Leshem

Veen

2015

2015 IEEE International Conference on Acoustics, Speech and Signal Processing (ICASSP)

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Linear image deconvolution for radio-astronomy isan ill-posed problem. For this reason a-priori knowledge is crucial for improving the performance of the deconvolution. In this paper we show that combining non-negativity constraints with an upper bound on the magnitude of each pixel in the image can significantly improve the image formation algorithm. We also show that the minimum variance distortionless response (MVDR) dirty image provides the tightest upper bound among all beamformers. We then show how LS-MVI image formation a;gorithm can be reformulated as a preconditioned weighted least squares algorithm. The resulting algorithm can be efficiently solved using the active-set method. The performance of the algorithm is demonstrated in simulation and compared with constrained least squares based on the classical dirty image.

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Smear fitting: a new image-deconvolution method for interferometric data

Cited by 7 publications

References 7 publications

MODELING THE RADIO EMISSION FROM Cyg OB2 NO. 5: A QUADRUPLE SYSTEM?

MODELING THE RADIO EMISSION FROM Cyg OB2 NO. 5: A QUADRUPLE SYSTEM?

Radio emission from the massive stars in the galactic super star cluster Westerlund 1

Computationally efficient radio astronomical image formation using constrained least squares and and the MVDR beamformer

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