| 1. |
- 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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| 2. |
- Dall` Olio, Daria, 1981, et al.
(författare)
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Methanol masers reveal the magnetic field of the high-mass protostar IRAS 18089-1732
- 2017
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 607
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Tidskriftsartikel (refereegranskat)abstract
- © ESO, 2017. Context. The importance of the magnetic field in high-mass-star formation is not yet fully clear and there are still many open questions concerning its role in the accretion processes and generation of jets and outflows. In the past few years, masers have been successfully used to probe the magnetic field morphology and strength at scales of a few au around massive protostars, by measuring linear polarisation angles and Zeeman splitting. The massive protostar IRAS 18089-1732 is a well studied high-mass-star forming region, showing a hot core chemistry and a disc-outflow system. Previous SMA observations of polarised dust revealed an ordered magnetic field oriented around the disc of IRAS 18089-1732. Aims. We want to determine the magnetic field in the dense region probed by 6.7 GHz methanol maser observations and compare it with observations in dust continuum polarisation, to investigate how the magnetic field in the compact maser region relates to the large-scale field around massive protostars. Methods. We reduced MERLIN observations at 6.7 GHz of IRAS 18089-1732 and we analysed the polarised emission by methanol masers. Results. Our MERLIN observations show that the magnetic field in the 6.7 GHz methanol maser region is consistent with the magnetic field constrained by the SMA dust polarisation observations. A tentative detection of circularly polarised line emission is also presented. Conclusions. We found that the magnetic field in the maser region has the same orientation as in the disk. Thus the large-scale field component, even at the au scale of the masers, dominates over any small-scale field fluctuations. We obtained, from the circular polarisation tentative detection, a field strength along the line of sight of 5.5 mG which appeared to be consistent with the previous estimates.
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| 3. |
- 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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| 4. |
- 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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| 5. |
- Decin, L., et al.
(författare)
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Study of the aluminium content in AGB winds using ALMA Indications for the presence of gas-phase (Al2O3)n clusters
- 2017
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 608, s. A55-
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Tidskriftsartikel (refereegranskat)abstract
- Context. The condensation of inorganic dust grains in the winds of evolved stars is poorly understood. As of today, it is not yet known which molecular clusters form the first dust grains in oxygen-rich (C/O 34) can be the potential agents of the broad 11 mu m feature in the SED and in the interferometric data and we propose potential formation mechanisms for these large clusters.
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| 6. |
- Fomalont, E. B., et al.
(författare)
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THE 2014 ALMA LONG BASELINE CAMPAIGN: AN OVERVIEW
- 2015
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Ingår i: Astrophysical Journal Letters. - : American Astronomical Society. - 2041-8213 .- 2041-8205. ; 808:1
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Tidskriftsartikel (refereegranskat)abstract
- A major goal of the Atacama Large Millimeter/submillimeter Array (ALMA) is to make accurate images with resolutions of tens of milliarcseconds, which at submillimeter (submm) wavelengths requires baselines up to similar to 15 km. To develop and test this capability, a Long Baseline Campaign (LBC) was carried out from 2014 September to late November, culminating in end-to-end observations, calibrations, and imaging of selected Science Verification (SV) targets. This paper presents an overview of the campaign and its main results, including an investigation of the short-term coherence properties and systematic phase errors over the long baselines at the ALMA site, a summary of the SV targets and observations, and recommendations for science observing strategies at long baselines. Deep ALMA images of the quasar 3C 138 at 97 and 241 GHz are also compared to VLA 43 GHz results, demonstrating an agreement at a level of a few percent. As a result of the extensive program of LBC testing, the highly successful SV imaging at long baselines achieved angular resolutions as fine as 19 mas at similar to 350 GHz. Observing with ALMA on baselines of up to 15 km is now possible, and opens up new parameter space for submm astronomy.
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| 7. |
- 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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| 8. |
- Humphreys, Roberta M., et al.
(författare)
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The Unexpected Spectrum of the Innermost Ejecta of the Red Hypergiant VY CMa
- 2019
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Ingår i: Astrophysical Journal Letters. - : American Astronomical Society. - 2041-8213 .- 2041-8205. ; 874:2
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Tidskriftsartikel (refereegranskat)abstract
- Hubble Space Telescope/STIS spectra of the small clumps and filaments closest to the central star in VY CMa reveal that the very strong K I emission and TiO and VO molecular emission, long thought to form in a dusty circumstellar shell, actually originate in a few small clumps hundreds of au from the star. The K I lines are 10-20 times stronger in these nearest ejecta than on the star. The observations also confirm VO as a circumstellar molecule. In this Letter we discuss the spectra of the features, their motions and ages, and the identification of the molecular emission. The strength of the atomic and molecular features in the small clumps present an astrophysical problem for the excitation process. We show that the clumps must have a nearly clear line of sight to the star's radiation.
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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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| 11. |
- O Gorman, Eamon, 1984, et al.
(författare)
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Temporal evolution of the size and temperature of Betelgeuse's extended atmosphere
- 2015
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 580
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Tidskriftsartikel (refereegranskat)abstract
- Spatially resolved multi-wavelength centimeter continuum observations of cool evolved stars can not only constrain the morphology of the radio emitting regions, but can also directly probe the mean gas temperature at various depths of the star's extended atmosphere. Here, we use the Very Large Array (VLA) in the A configuration with the Pie Town (PT) Very Long Baseline Array (VLBA) antenna to spatially resolve the extended atmosphere of Betelgeuse over multiple epochs at 0.7, 1.3, 2.0, 3.5, and 6.1 cm. The extended atmosphere deviates from circular symmetry at all wavelengths while at some epochs we find possible evidence for small pockets of gas significantly cooler than the mean global temperature. We find no evidence for the recently reported e-MERLIN radio hotspots in any of our multi-epoch VLA/PT data, despite having sufficient spatial resolution and sensitivity at short wavelengths, and conclude that these radio hotspots are most likely interferometric artefacts. The mean gas temperature of the extended atmosphere has a typical value of 3000 K at 2 R∗ and decreases to 1800 K at 6 R∗, in broad agreement with the findings of the single epoch study from Lim et al. (1998, Nature, 392, 575). The overall temperature profile of the extended atmosphere between 2 R∗ ≲ r ≲ 6 R∗ can be described by a power law of the form Tgas(r) ∝ r-0.6, with temporal variability of a few 100 K evident at some epochs. Finally, we present over 12 yr of V band photometry, part of which overlaps our multi-epoch radio data. We find a correlation between the fractional flux density variability at V band with most radio wavelengths. This correlation is likely due to shock waves induced by stellar pulsations, which heat the inner atmosphere and ionize the more extended atmosphere through radiative means. Stellar pulsations may play an important role in exciting Betelgeuse's extended atmosphere.
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| 12. |
- Richards, A. M. S., et al.
(författare)
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ALMA Reveals VY CMa's Sub-mm H2O Maser and Dust Distribution
- 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. 337-338
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Konferensbidrag (refereegranskat)abstract
- Cool, evolved stars have copious, enriched winds but observation constrains do not yet fully constrain models for need the dynamics of mass loss from the pulsating stellar surface to 10 stellar radii, where radiation pressure on dust is fully effective. We retrieved ALMA Science Verification data providing images of sub-mm line and continuum emission from VY CMa. This enables us to locate water masers with milli-arcsec precision and resolve the dusty continuum. The 658-, 321- and 325-GHz masers lie in irregular, thick shells at increasing distances from the centre of expansion. For the first time this is confirmed as the stellar position. Their distribution is broadly consistent with excitation models but the conditions and kinematics are complicated by wind collisions, clumping and asymmetries.
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| 13. |
- 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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| 14. |
- 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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