| 1. |
- Matthews, L. D., et al.
(författare)
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The ALMA phasing system: A beamforming capability for ultra-high-resolution science at (sub)millimeter wavelengths
- 2018
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Ingår i: Publications of the Astronomical Society of the Pacific. - : IOP Publishing. - 0004-6280 .- 1538-3873. ; 130:983
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Tidskriftsartikel (refereegranskat)abstract
- The Atacama Millimeter/submillimeter Array (ALMA) Phasing Project (APP) has developed and deployed the hardware and software necessary to coherently sum the signals of individual ALMA antennas and record the aggregate sum in Very Long Baseline Interferometry (VLBI) Data Exchange Format. These beamforming capabilities allow the ALMA array to collectively function as the equivalent of a single large aperture and participate in global VLBI arrays. The inclusion of phased ALMA in current VLBI networks operating at (sub) millimeter wavelengths provides an order of magnitude improvement in sensitivity, as well as enhancements in u–v coverage and north–south angular resolution. The availability of a phased ALMA enables a wide range of new ultra-high angular resolution science applications, including the resolution of supermassive black holes on event horizon scales and studies of the launch and collimation of astrophysical jets. It also provides a high-sensitivity aperture that may be used for investigations such as pulsar searches at high frequencies. This paper provides an overview of the ALMA Phasing System design, implementation, and performance characteristics.
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| 2. |
- Vlemmings, Wouter, 1974, et al.
(författare)
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Magnetically aligned dust and SiO maser polarisation in the envelope of the red supergiant VY Canis Majoris
- 2017
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 603, s. A92-
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Tidskriftsartikel (refereegranskat)abstract
- Aims. Polarisation observations of circumstellar dust and molecular (thermal and maser) lines provide unique information about dust properties and magnetic fields in circumstellar envelopes of evolved stars. Methods. We use Atacama Large Millimeter/submillimeter Array (ALMA) Band 5 science verification observations of the red supergiant VY CMa to study the polarisation of SiO thermal /maser lines and dust continuum at similar to 1.7 mm wavelength. We analyse both linear and circular polarisation and derive the magnetic field strength and structure, assuming the polarisation of the lines originates from the Zeeman effect, and that of the dust originates from aligned dust grains. We also discuss other effects that could give rise to the observed polarisation. Results. We detect, for the first time, significant polarisation (similar to 3%) of the circumstellar dust emission at millimeter wavelengths. The polarisation is uniform with an electric vector position angle of similar to 8 degrees. Varying levels of linear polarisation are detected for the J = 4-3 (SiO)-Si-28 nu = 0; 1; 2; and (SiO)-Si-29 nu = 0; 1 lines, with the strongest polarisation fraction of similar to 30% found for the (SiO)-Si-29 nu = 1 maser. The linear polarisation vectors rotate with velocity, consistent with earlier observations. We also find significant (up to similar to 1%) circular polarisation in several lines, consistent with previous measurements. We conclude that the detection is robust against calibration and regular instrumental errors, although we cannot yet fully rule out non-standard instrumental effects. Conclusions. Emission from magnetically aligned grains is the most likely origin of the observed continuum polarisation. This implies that the dust is embedded in a magnetic field >13 mG. The maser line polarisation traces the magnetic field structure. The magnetic field in the gas and dust is consistent with an approximately toroidal field configuration, but only higher angular resolution observations will be able to reveal more detailed field structure. If the circular polarisation is due to Zeeman splitting, it indicates a magnetic field strength of similar to 1-3 Gauss, consistent with previous maser observations.
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| 3. |
- Richards, A.M.S., et al.
(författare)
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Hot and cold running water: Understanding evolved star winds
- 2017
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Ingår i: Proceedings of the International Astronomical Union. - 1743-9213 .- 1743-9221. ; 13:S336, s. 347-350
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Tidskriftsartikel (refereegranskat)abstract
- Outstanding problems concerning mass-loss from evolved stars include initial wind acceleration and what determines the clumping scale. Reconstructing physical conditions from maser data has been highly uncertain due to the exponential amplification. ALMA and e-MERLIN now provide image cubes for five H2O maser transitions around VY CMa, at spatial resolutions comparable to the size of individual clouds or better, covering excitation states from 204 to 2360 K. We use the model of Gray et al. 2016, to constrain variations of number density and temperature on scales of a few au, an order of magnitude finer than is possible with thermal lines, comparable to individual cloud sizes or locally almost homogeneous regions. We compare results with the models of Decin et al. 2006 and Matsuura et al. 2014 for the circumstellar envelope of VY CMa; in later work this will be extended to other maser sources.
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| 4. |
- Baudry, A., et al.
(författare)
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Vibrationally excited water emission at 658 GHz from evolved stars
- 2018
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 609
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Tidskriftsartikel (refereegranskat)abstract
- Context. Several rotational transitions of ortho- and para-water have been identified toward evolved stars in the ground vibrational state as well as in the first excited state of the bending mode (v2 = 1 in (0, 1, 0) state). In the latter vibrational state of water, the 658 GHz J = 1 1,0 -1 0,1 rotational transition is often strong and seems to be widespread in late-type stars. Aims. Our main goals are to better characterize the nature of the 658 GHz emission, compare the velocity extent of the 658 GHz emission with SiO maser emission to help locate the water layers and, more generally, investigate the physical conditions prevailing in the excited water layers of evolved stars. Another goal is to identify new 658 GHz emission sources and contribute in showing that this emission is widespread in evolved stars. Methods. We have used the J = 1 1,0 -1 0,1 rotational transition of water in the (0, 1, 0) vibrational state nearly 2400 K above the ground-state to trace some of the physical conditions of evolved stars. Eleven evolved stars were extracted from our mini-catalog of existing and potential 658 GHz sources for observations with the Atacama Pathfinder EXperiment (APEX) telescope equipped with the SEPIA Band 9 receiver. The 13 CO J = 6-5 line at 661 GHz was placed in the same receiver sideband for simultaneous observation with the 658 GHz line of water. We have compared the ratio of these two lines to the same ratio derived from HIFI earlier observations to check for potential time variability in the 658 GHz line. We have compared the 658 GHz line properties with our H 2 O radiative transfer models in stars and we have compared the velocity ranges of the 658 GHz and SiO J = 2-1, v = 1 maser lines. Results. Eleven stars have been extracted from our catalog of known or potential 658 GHz evolved stars. All of them show 658 GHz emission with a peak flux density in the range ≈ 50-70 Jy (RU Hya and RT Eri) to ≈ 2000-3000 Jy (VY CMa and W Hya). Five Asymptotic Giant Branch (AGB) stars and one supergiant (AH Sco) are new detections. Three AGBs and one supergiant (VY CMa) exhibit relatively weak 13 CO J = 6-5 line emission while o Ceti shows stronger 13 CO emission. We have shown that the 658 GHz line is masing and we found that the 658 GHz velocity extent tends to be correlated with that of the SiO maser suggesting that both emission lines are excited in circumstellar layers close to the central star. Broad and stable line profiles are observed at 658 GHz. This could indicate maser saturation although we have tentatively provided first information on time variability at 658 GHz.
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| 5. |
- De Beck, Elvire, 1985, et al.
(författare)
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ALMA observations of TiO2 around VY Canis Majoris
- 2015
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 580
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Tidskriftsartikel (refereegranskat)abstract
- Context. Titanium dioxide, TiO2, is a refractory species that could play a crucial role in the dust-condensation sequence around oxygen-rich evolved stars. To date, gas phase TiO2 has been detected only in the complex environment of the red supergiant VY CMa. Aims. We aim to constrain the distribution and excitation of TiO2 around VY CMa in order to clarify its role in dust formation. Methods. We analyse spectra and channel maps for TiO2 extracted from ALMA science verification data. Results. We detect 15 transitions of TiO2, and spatially resolve the emission for the first time. The maps demonstrate a highly clumpy, anisotropic outflow in which the TiO2 emission likely traces gas exposed to the stellar radiation field. An accelerating bipolar-like structure is found, oriented roughly east-west, of which the blue component runs into and breaks up around a solid continuum component. A distinct tail to the south-west is seen for some transitions, consistent with features seen in the optical and near-infrared. Conclusions. We find that a significant fraction of TiO2 remains in the gas phase outside the dust-formation zone and suggest that this species might play only a minor role in the dust-condensation process around extreme oxygen-rich evolved stars like VY CMa.
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| 6. |
- De Beck, Elvire, 1985, et al.
(författare)
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ALMA observations of TiO2 around VY CMa
- 2016
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Ingår i: Journal of Physics: Conference Series. - : IOP Publishing. - 1742-6588 .- 1742-6596. ; 728:2, s. Art. no. UNSP 022009-
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Konferensbidrag (refereegranskat)abstract
- Titanium dioxide, TiO2, is a refractory species that could play a crucial role in the dust-condensation sequence around oxygen-rich evolved stars. We present and discuss the detections of 15 emission lines of TiO2 with ALMA in the complex environment of the red supergiant VY CMa. The observations reveal a highly clumpy, anisotropic outflow in which the TiO2 emission likely traces gas exposed to the stellar radiation field. We find evidence for a roughly east-west oriented, accelerating bipolar-like structure, of which the blue component runs into and breaks up around a solid continuum component. We see a distinct tail to the south-west for some transitions, consistent with features seen in the optical and near-infrared. We find that a significant fraction of TiO2 remains in the gas phase outside the dust-formation zone and suggest that this species might play only a minor role in the dust-condensation process around extreme oxygen-rich evolved stars like VY CMa.
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| 7. |
- Decin, L., et al.
(författare)
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ALMA-resolved salt emission traces the chemical footprint and inner wind morphology of VY Canis Majoris
- 2016
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 592:A76
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Tidskriftsartikel (refereegranskat)abstract
- Context. At the end of their lives, most stars lose a significant amount of mass through a stellar wind. The specific physical and chemical circumstances that lead to the onset of the stellar wind for cool luminous stars are not yet understood. Complex geometrical morphologies in the circumstellar envelopes prove that various dynamical and chemical processes are interlocked and that their relative contributions are not easy to disentangle. Aims. We aim to study the inner-wind structure (R
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| 8. |
- Humphreys, E. M., et al.
(författare)
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Simultaneous 183 GHz H2O maser and SiO observations towards evolved stars using APEX SEPIA Band 5
- 2017
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 603, s. A77-
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Tidskriftsartikel (refereegranskat)abstract
- Aims. The aim is to investigate the use of 183 GHz H2O masers for characterization of the physical conditions and mass loss process in the circumstellar envelopes of evolved stars. Methods. We used APEX SEPIA Band 5 (an ALMA Band 5 receiver on the APEX telescope) to observe the 183 GHz H2O line towards two red supergiant (RSG) and three asymptotic giant branch (AGB) stars. Simultaneously, we observed the J = 4-3 line for (SiO)-Si-28 nu = 0, 1, 2 and 3, and for (SiO)-Si-29 nu = 0 and 1. We compared the results with simulations and radiative transfer models for H2O and SiO, and examined data for the individual linear orthogonal polarizations. Results. We detected the 183 GHz H2O line towards all the stars with peak flux densities >100 Jy, including a new detection from VYCMa. Towards all five targets, the water line had indications of being caused by maser emission and had higher peak flux densities than for the SiO lines. The SiO lines appear to originate from both thermal and maser processes. Comparison with simulations and models indicate that 183 GHz maser emission is likely to extend to greater radii in the circumstellar envelopes than SiO maser emission and to similar or greater radii than water masers at 22, 321 and 325 GHz. We speculate that a prominent blue-shifted feature in the WHya 183 GHz spectrum is amplifying the stellar continuum, and is located at a similar distance from the star as mainline OH maser emission. We note that the coupling of an SiO maser model to a hydrodynamical pulsating model of an AGB star yields qualitatively similar simulated results to the observations. From a comparison of the individual polarizations, we find that the SiO maser linear polarization fraction of several features exceeds the maximum fraction allowed under standard maser assumptions and requires strong anisotropic pumping of the maser transition and strongly saturated maser emission. The low polarization fraction of the H2O maser however, fits with the expectation for a non-saturated maser. Conclusions. 183 GHz H2O masers can provide strong probes of the mass loss process of evolved stars. Higher angular resolution observations of this line using ALMA Band 5 will enable detailed investigation of the emission location in circumstellar envelopes and can also provide information on magnetic field strength and structure.
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| 9. |
- O Gorman, Eamon, 1984, et al.
(författare)
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ALMA Observations of Anisotropic Dust Mass Loss around VY CMa
- 2015
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Ingår i: 4th ALMA Science Conference on Revolution in Astronomy with ALMA: The Third Year, Tokyo, Japan, 8-11 December. - 9781583818831 ; 499, s. 335-336
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Konferensbidrag (refereegranskat)abstract
- We present high resolution ALMA Science Verification data of the continuum emission around the highly evolved oxygen-rich red supergiant VY CMa. These data enable us to study the dust in its inner circumstellar environment at a spatial resolution of 129 mas at 321 GHz and 59 mas at 658 GHz, thus allowing us to trace dust on spatial scales down to 11 R-star (71 AU). Two prominent dust components are detected and resolved. We find that at least 17% of the dust mass around VY CMa is located in clumps ejected within a more quiescent roughly spherical stellar wind, with a quiescent dust mass loss rate of 5 x 10(-6) M-circle dot yr(-1). The anisotropic morphology of the dust indicates a continuous, directed mass loss over a few decades, suggesting that this mass loss cannot be driven by large convection cells alone.
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| 10. |
- O Gorman, Eamon, 1984, et al.
(författare)
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ALMA observations of anisotropic dust mass loss in the inner circumstellar environment of the red supergiant VY Canis Majoris
- 2015
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 573, s. Article no. L1-
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Tidskriftsartikel (refereegranskat)abstract
- The processes leading to dust formation and the subsequent role it plays in driving mass loss in cool evolved stars is an area of intense study. Here we present high resolution ALMA Science Verification data of the continuum emission around the highly evolved oxygen-rich red supergiant VY CMa. These data enable us to study the dust in its inner circumstellar environment at a spatial resolution of 129 mas at 321 GHz and 59 mas at 658 GHz, thus allowing us to trace dust on spatial scales down to 11 R-star (71 AU). Two prominent dust components are detected and resolved. The brightest dust component. C, is located 334 mas (61 R-star) southeast of the star and has a dust mass of at least 2.5 x 10(-4) M-circle dot. It has a dust emissivity spectral index of beta = -0.1 at its peak, implying that it is optically thick at these frequencies with a cool core of T-d less than or similar to 100 K. Interestingly, not a single molecule in the ALMA data has emission close to the peak of this massive dust clump. The other main dust component. VY, is located at the position of the star and contains a total dust mass of 4.0 x 10(-)5 M-circle dot. It also contains a weaker dust feature extending over 60 R-star to the north with the total component having a typical dust emissivity spectral index of beta = 0.7. We find that at least 17% of the dust mass around VY CMa is located in clumps ejected within a more quiescent roughly spherical stellar wind, with a quiescent dust mass loss rate of 5 x 10(-6) M-circle dot yr(-1). The anisotropic morphology of the dust indicates a continuous, directed mass loss over a few decades, suggesting that this mass loss cannot be driven by large convection cells alone.
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