A. M. S. Richards

University of Manchester

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M. D. Gray

National Astronomical Research Institute of Thailand

S. Etoka

University of Manchester

L. Decin

Institute of Astronomy

T. W. B. Muxlow

University of Manchester

A. Baudry

Université de Bordeaux

J. A. Yates

University College London

P. Kervella

Qualissima (France)

W. H. T. Vlemmings

Chalmers University of Technology

Iain McDonald

University of Manchester

W. Homan

KU Leuven

Cooperative Institutions

Centre National de la Recherche Scientifique

194

Max Planck Society

178

University of Manchester

160

Max Planck Institute for Radio Astronomy

154

SKA Observatory

116

Center for Astrophysics Harvard & Smithsonian

Commissariat à l'Énergie Atomique et aux Énergies Alternatives

CEA Paris-Saclay

European Southern Observatory

Netherlands Institute for Radio Astronomy

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An unusual face-on spiral in the wind of the M-type AGB star EP Aquarii

Astronomy and Astrophysics (2018)

W. Homan A. M. S. Richards L. Decin A. de Koter P. Kervella

High-resolution interferometric observations of the circumstellar environments of AGB stars show a variety of morphologies. Guided by the unusual carbon monoxide line profile of the AGB star EP Aquarii, we have observed its circumstellar environment with ALMA band 6 in cycle 4. We describe the morphological complexity of the CO, SiO and SO2 molecular emission. The CO emission exhibits the characteristics of a bi-conical wind with a bright nearly face-on spiral feature around the systemic velocity. This is the first convincing detection of a spiral morphology in an O-rich wind. Based on the offsets of the centers of the two bi-conical wind hemispheres, we deduce the position angle of the inclination axis to be ~150 deg measured counterclockwise from north. Based on the velocity width of the spiral signature we estimate the inclination angle of the system to be between 4 deg and 18 deg. The central emission zone exhibits a morphology that resembles simulations modelling the spiral-inducing wind-Roche-lobe-overflow mechanism. Though the spiral may be a (companion-induced) density enhancement in the stellar outflow, the extremely narrow width of the spiral signature in velocity space suggests that it may be a hydrodynamical perturbation in a face-on differentially rotating disk. The SiO emission does not show the spiral, but exhibits a local emission void approximately 0.5'' west of the continuum brightness peak. We hypothesise that this may be a local environment caused by the presence of a (stellar) companion with a mass of at most 0.1 Msol, based on its non-detection in the continuum...

Asymptotic giant branch

Outflow

10.1051/0004-6361/201832834

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Citations (36)

MERLIN and eMERLIN OH maser observations toward the star forming region complex W49 A

K. Asanok S. Etoka M. D. Gray A. M. S. Richards B. Hutawarakorn Kramer

Merlin (protein)

Star (game theory)

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AGN and starbursts at high redshift: High resolution EVN radio observations of the Hubble Deep Field

arXiv (Cornell University) (2000)

M. Garrett T. W. B. Muxlow S. T. Garrington W. Alef A. Alberdi

We present deep, wide-field European VLBI Network (EVN) 1.6 GHz observations of the Hubble Deep Field (HDF) region with a resolution of 0.025 arcseconds. Above the 210 microJy/beam (5sigma) detection level, the EVN clearly detects two radio sources in a field that encompasses the HDF and part of the Hubble Flanking Fields (HFF). The sources detected are: VLA J123644+621133 (a z=1.013, low-luminosity FR-I radio source located within the HDF itself) and VLA J123642+621331 (a dust enshrouded, optically faint, z=4.424 starburst system). A third radio source, J123646+621404, is detected at the 4sigma level. The VLBI detections of all three sources suggest that most of the radio emission of these particular sources (including the dusty starburst) is generated by an embedded AGN.

Very-long-baseline interferometry

Hubble Ultra-Deep Field

Source counts

10.48550/arxiv.astro-ph/0012293

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High Resolution Radio Imaging of the Hubble Deep and Flanking Fields

T. W. B. Muxlow S. T. Garrington A. M. S. Richards E. A. Richards M. Garrett

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The structure of the Proto-Planetary Nebula OH 231.8+4.2 as revealed by VLTI and MERLIN interferometric observations

S. Etoka E. Lagadec A. A. Zijlstra O. Chesneau A. M. S. Richards

Merlin (protein)

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OH MASERS TOWARDS THE W49A STAR-FORMING REGION WITH MERLIN AND e-MERLN OBSERVATIONS

Publications of The Korean Astronomical Society (2015)

K. Asanok S. Etoka M. D. Gray A. M. S. Richards B. Hutawarakorn Kramer

We present preliminary results from OH ground state phase referenced observations carried out with the Multi Element Radio Linked Interferometer Network (MERLIN) and e-MERLIN towards the massive star forming region W49A. There are three active SFRs within this complex: W49 North (W49 N), W49 South (W49 S) and W49 South West (W49 SW). The first epoch of observations was obtained in 2005 with MERLIN while the second epoch was obtained in 2013 with the e-MERLIN upgraded system. In this paper, we present 1665 and 1720 MHz maser emission towards W49 S and W49 SW. Overall, both epochs show good agreement with the previous observations of Argon et al. (2000) carried out with the Very Large Array (VLA). The better sensitivity and wider velocity coverage of the MERLIN/e-MERLIN observations allowed us to discover a new 1720 MHz OH maser site in W49 S.

Merlin (protein)

Epoch (astronomy)

Star (game theory)

Very-long-baseline interferometry

10.5303/pkas.2015.30.2.125

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Tracing the inner regions of circumstellar envelopes via high-excitation water transitions

Proceedings of the International Astronomical Union (2020)

S. Etoka A. Baudry A. M. S. Richards M. D. Gray L. Decin

Abstract Water is a ubiquitous molecule in circumstellar envelopes (CSEs). Its emission has been detected at a wide range of distances from the central oxygen-rich evolved star. In particular, the water maser transition at 22 GHz, typically extending from about 5–20 stellar radii to as far as several hundred stellar radii from the star, has been commonly used to probe the structure and dynamics of the intermediate regions of the CSE where dust is condensing and the inner wind is being accelerated. The advent of ALMA has opened the door to high-angular resolution mapping of much higher excitation transitions of water, probing the inner regions of the CSEs, some of which are anticipated to exhibit maser action. The ALMA ATOMIUM large program observed many such transitions towards a sample of AGB stars & red supergiants. The preliminary results show that while some transitions depart only slightly from LTE, others clearly show signs of maser action. The Gaussian fitting of the non-diffuse/compact part of some of the (quasi) thermal & maser transitions reveal interesting velocity gradients, signatures of outflowing and infalling motions hence providing important constraints for stellar wind models.

10.1017/s1743921322000680

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The H₂O maser proper motions of RT Vir and VX Sgr

Symposium - International Astronomical Union (2002)

J. Yates A. M. S. Richards M. D. Gray

We have measured significant proper motions using the water maser clouds in the outflows from RT Vir (6 epochs) and VX Sgr (2 epochs). In both cases proper motions are measured in the bright emission found perpendicular to the OH emission. This strongly suggests latitude dependent mass-loss with the water masers tracing a dense equatorial outflow and the OH emission tracing a less dense polar outflow. In both cases the rotational velocities are < 1 km s −1 thus ruling out a strongly rotating circumstellar envelope. This suggests the outflow is not shaped by an orbiting companion. In the case of VX Sgr the proper motions are contained in a wedge perpendicular to the measured magnetic field axis, thus strongly suggesting that the magnetic field is helping to shape the mass outflow. The proper motion results have thrown up a puzzle. For VX Sgr the material is being radially accelerated and material enters the water maser zone at 4 km s −1 . This is consistent with the mass loss model of Bowen (1988). However the RT Vir proper motion results show no such radial acceleration. The masers have already received their acceleration before they reach the water maser zone at 3-AU (our numerical models seem to support the small radius). This was probably done in the pulsation zone. The acceleration mechanism is unclear for this source.

Outflow

Proper motion

10.1017/s0074180900222584

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ATOMIUM: Probing the inner wind of evolved O-rich stars with new, highly excited H₂O and OH lines

Astronomy and Astrophysics (2023)

A. Baudry K. T. Wong S. Etoka A. M. S. Richards H. S. P. Müller

Water and the hydroxyl radical are major constituents of the envelope of O-rich late-type stars. Transitions involving energy levels that are highly excited have been observed in both H$_2$O and OH. These and more recently discovered transitions can now be observed at a high sensitivity and angular resolution with the ALMA Array. Spectra and maps of H$_2$O and OH observed with an angular resolution of 20 to $\sim$200 mas were obtained at two epochs with the ALMA array. Observations with the Compact Array were also used to check for time variability of water transitions. Radiative transfer models of water were revisited to characterize masing conditions and up-to-date chemical models were used for comparison with our observations. Ten rotational transitions of H$_2$O with energies up to 9000 K were observed in various vibrational states. All but one are new detections in space, and from these we have derived accurate rest frequencies. Hyperfine split $\Lambda$-doubling transitions in v = 0, J = 27/2 and 29/2 levels of the $^2\Pi_{3/2}$ state and, $J = 33/2$ and 35/2 of the $^2\Pi_{1/2}$ state of OH with excitation energies up to 8900 K were also observed. Four of these transitions are new detections in space. Combining our measurements with earlier observations of OH, the v = 0 and v = 1 $\Lambda$-doubling frequencies have been improved. Our H$_2$O maps show compact emission and extensions up to twelve stellar radii or more. The 268.149 GHz emission line of water in the v$_2$ = 2 state is time variable, tends to be masing with dominant radiative pumping, and is widely excited. The widespread but weaker 262.898 GHz water line in v$_2$ = 1 also shows signs of maser emission. Emission and absorption of both H$_2$O and OH reveal an infall of matter and complex kinematics influenced by binarity. From our observed column densities, we derived OH/H$_2$O abundance ratios in a few stars.

Submillimeter Array

Asymptotic giant branch

Photosphere

10.1051/0004-6361/202245193

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Citations (13)

Maser Astrometry with VLBI and the SKA

Jimi Green H. J. van Langevelde A. Brunthaler S. P. Ellingsen Hiroshi Imai

We discuss the unique opportunities for maser astrometry with the inclusion of the Square Kilometre Array (SKA) in Very Long Baseline Interferometry (VLBI) networks.The first phase of the SKA will enable observations of hydroxyl and methanol masers, positioning the latter to an accuracy of 5 microarcseconds, and the second phase may allow water maser observations.These observations will provide trigonometric distances with errors as small as 1%.The unrivalled sensitivity of the SKA will enable large-scale surveys and, through joint operations, will turn any VLBI network into a fast astrometry device.Both evolved stars and high mass star formation regions will be accessible throughout the (Southern) Milky Way, completing our understanding of the content, dynamics and history of our Galaxy.Maser velocities and proper motions will be measurable in the Local Group of galaxies and beyond, providing new insights into their kinematics and evolution.

Very-long-baseline interferometry

Astrometry

10.22323/1.215.0119

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Citations (12)