Interferometer phase calibration sources -- II. The region 0  &lt;=  B1950 &lt;= +20

Context. Cool, evolved stars undergo copious mass loss but the detailed mechanisms and the form in which the matter is returned to the ISM are still under debate. Aims. We investigated the structure and evolution of the wind at 5 to 50 stellar radii from asymptotic giant branch and red supergiant stars. Methods. 22-GHz water masers around seven evolved stars were imaged using MERLIN, at sub-AU resolution. Each source was observed at between 2 and 7 epochs, covering several stellar periods. We compared our results with long-term single dish monitoring provided by the Pushchino radio telescope. Results. The 22-GHz emission is located in approximately spherical, thick, unevenly filled shells. The outflow velocity increases twofold or more between the inner and outer shell limits. Water maser clumps could be matched at successive epochs separated by less than two years for AGB stars, or at least 5 years for RSG. This is much shorter than the decades taken for the wind to cross the maser shell, and comparison with spectral monitoring shows that some features fade and reappear. In five sources, most of the matched maser features brighten or dim in concert from one epoch to the next. A number of individual maser features show idiosyncratic behaviour, including one cloud in W Hya caught in the act of passing in front of a background cloud leading to 50-fold, transient amplification. The masing clouds are one or two orders of magnitude denser than the wind average and contain a substantial fraction of the mass loss in this region, with a filling factor <1%. The RSG clouds are about ten times bigger than those round the AGB stars. Conclusions. Proper motions are dominated by expansion, with no systematic rotation. The maser clouds presumably survive for decades (the shell crossing time) but the masers are not always beamed in our direction. Only radiative effects can explain changes in flux density throughout the maser shells on short timescales. The size of the clouds is proportional to that of the parent star, being of a similar radius to the star once the clumps reach the 22-GHz maser shell. Stellar properties such as convection cells must determine the clumping scale.

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“…9). The phase reference source J1635+1831 at 16:35:39.1588 +18:31:03.723, uncertainty (7, 12) milli-arcsec (Browne et al 1998) at 2.…”

Section: Phase Referencing Resultsmentioning

confidence: 99%

Evolved star water maser cloud size determined by star size

Richards

Etoka

Gray

et al. 2012

A&A

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“…The MASIV survey sources were selected to be compact and to have flat spectral indices (α ≥ −0.3, for S ∝ ν α ), based on non-coeval flux measurements from the 1.4 GHz NRAO VLA Sky Survey (Condon et al 1998), plus the Jodrell Bank VLA Astrometric Survey (Patnaik et al 1992;Browne et al 1998;Wilkinson et al 1998) and Cosmic Lens All-Sky Survey (Myers et al 1995) at 8.4 GHz. The MASIV source sample is flux-limited; about half the sources have flux densities of typically ∼ 100 mJy (classified as 'radioweak'), while the other half have flux densities of ∼ 1 Jy (classified as 'radio-strong').…”

Section: The Masiv Surveymentioning

confidence: 99%

The MASIV Survey – IV. Relationship between intra-day scintillation and intrinsic variability of radio AGNs

Koay

Macquart

Jauncey

et al. 2017

Monthly Notices of the Royal Astronomical Society

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We investigate the relationship between 5 GHz interstellar scintillation (ISS) and 15 GHz intrinsic variability of compact, radio-selected AGNs drawn from the Microarcsecond Scintillation-Induced Variability (MASIV) Survey and the Owens Valley Radio Observatory (OVRO) blazar monitoring program. We discover that the strongest scintillators at 5 GHz (modulation index, m 5 ≥ 0.02) all exhibit strong 15 GHz intrinsic variability (m 15 ≥ 0.1). This relationship can be attributed mainly to the mutual dependence of intrinsic variability and ISS amplitudes on radio core compactness at ∼ 100 µas scales, and to a lesser extent, on their mutual dependences on source flux density, arcsec-scale core dominance and redshift. However, not all sources displaying strong intrinsic variations show high amplitude scintillation, since ISS is also strongly dependent on Galactic line-of-sight scattering properties. This observed relationship between intrinsic variability and ISS highlights the importance of optimizing the observing frequency, cadence, timespan and sky coverage of future radio variability surveys, such that these two effects can be better distinguished to study the underlying physics. For the full MASIV sample, we find that Fermi-detected gamma-ray loud sources exhibit significantly higher 5 GHz ISS amplitudes than gamma-ray quiet sources. This relationship is weaker than the known correlation between gamma-ray loudness and the 15 GHz variability amplitudes, most likely due to jet opacity effects.

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“…JVAS contains the ∼2400 strongest flat spectrum sources in the northern sky (Patnaik et al 1992a;Patnaik 1993a;Browne et al 1998. All sources were observed with the VLA at 8.4 GHz giving a resolution of 200 mas.…”

Section: Introductionmentioning

confidence: 99%

The new gravitational lens system B1030 + 074

Xanthopoulos

Browne

King

et al. 1998

Monthly Notices RAS

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We report the discovery of a new double image gravitational lens system B1030+074 which was found during the Jodrell Bank -VLA Astrometric Survey (JVAS). We have collected extensive radio data on the system using the VLA, MERLIN, the EVN and the VLBA and optical observations using WFPC2 on the HST. The lensed images are separated by 1.56 arcseconds and their flux density ratio at centimetric wavelengths is approximately 14:1 although the ratio is slightly frequency dependent and the images appear to be time variable. The HST pictures show both the lensed images and the lensing galaxy close to the weaker image. The lensing galaxy has substructure which could be a spiral arm or an interacting galaxy.

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Interferometer phase calibration sources -- II. The region 0 <= B1950 <= +20

Cited by 139 publications

References 3 publications

Evolved star water maser cloud size determined by star size

Evolved star water maser cloud size determined by star size

The MASIV Survey – IV. Relationship between intra-day scintillation and intrinsic variability of radio AGNs

The new gravitational lens system B1030 + 074

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