| 1. |
- Glasbey, JC, et al.
(författare)
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- 2021
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swepub:Mat__t
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| 2. |
- 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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| 3. |
- Danilovich, Taissa, 1987, et al.
(författare)
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New observations and models of circumstellar CO line emission of AGB stars in the Herschel SUCCESS programme
- 2015
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 581
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Tidskriftsartikel (refereegranskat)abstract
- Asymptotic giant branch (AGB) stars are in one of the latest evolutionary stages of low to intermediate-mass stars. Their vigorous mass loss has a significant effect on the stellar evolution, and is a significant source of heavy elements and dust grains for the interstellar medium. The mass-loss rate can be well traced by carbon monoxide (CO) line emission. Aims. We present new Herschel/HIFI and IRAM 30 m telescope CO line data for a sample of 53 galactic AGB stars. The lines cover a fairly large range of excitation energy from the J = 1 → 0 line to the J = 9 → 8 line, and even the J = 14 → 13 line in a few cases. We perform radiative transfer modelling for 38 of these sources to estimate their mass-loss rates. Methods. We used a radiative transfer code based on the Monte Carlo method to model the CO line emission. We assume spherically symmetric circumstellar envelopes that are formed by a constant mass-loss rate through a smoothly accelerating wind. Results. We find models that are consistent across a broad range of CO lines for most of the stars in our sample, i.e., a large number of the circumstellar envelopes can be described with a constant mass-loss rate. We also find that an accelerating wind is required to fit, in particular, the higher-J lines and that a velocity law will have a significant effect on the model line intensities. The results cover a wide range of mass-loss rates (~10-8 to 2 × 10-5 M⊙ yr-1) and gas expansion velocities (2 to 21.5 km s-1), and include M-, S-, and C-type AGB stars. Our results generally agree with those of earlier studies, although we tend to find slightly lower mass-loss rates by about 40%, on average. We also present "bonus" lines detected during our CO observations.
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| 4. |
- Sanchez Contreras, C., et al.
(författare)
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A rotating fast bipolar wind and disk system around the B[e]-type star MWC 922
- 2019
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 629
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Tidskriftsartikel (refereegranskat)abstract
- We present interferometric observations with the Atacama Large Millimeter Array (ALMA) of the free-free continuum and recombination line emission at 1 and 3 mm of the Red Square Nebula surrounding the B[e]-type star MWC 922. The distance to the source, which is unknown, is usually taken to be d= 1.7-3 kpc. The unprecedented angular resolution (up to similar to 0 ''.02) and exquisite sensitivity of these data reveal for the first time the structure and kinematics of the nascent compact ionized region at its center. We imaged the line emission of H30 alpha and H39 alpha, previously detected with single-dish observations, and of H51 epsilon, H55 gamma, and H63 delta, detected for the first time in this work. The line emission is seen over a full velocity range of similar to 180 km s(-1) arising in a region of diameter <0 ''.14 (less than a few hundred au) in the maser line H30 alpha, which is the most intense transition reported here. We resolve the spatio-kinematic structure of a nearly edge-on disk rotating around a central mass of similar to 10 M-circle dot (d= 1.7 kpc) or similar to 18 M-circle dot (d = 3 kpc), assuming Keplerian rotation. Our data also reveal a fast (similar to 100 km s(-1)) bipolar ejection (possibly a jet) orthogonal to the disk. In addition, a slow (<15 km s(-1)) wind may be emanating from the disk. Both, the slow and the fast winds are found to be rotating in a similar manner to the ionized layers of the disk. This represents the first empirical proof of rotation in a bipolar wind expanding at high velocity (similar to 100 km s(-1)). The launching radius of the fast wind is found to be <30-51 au (i.e., smaller than the inner rim of the ionized disk probed by our observations). We believe that the fast wind is actively being launched, probably by a disk-mediated mechanism in a (possibly accretion) disk around a possible compact companion. We have modeled our observations using the radiative transfer code MORELI. This enables us to describe with unparalleled detail the physical conditions and kinematics in the inner layers of MWC 922, which has revealed itself as an ideal laboratory for studying the interplay of disk rotation and jet-launching. Although the nature of MWC 922 remains unclear, we believe it could be a similar to 15 M-circle dot post-main sequence star in a mass-exchanging binary system. If this is the case, a more realistic value of the distance may be d similar to 3 kpc.
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| 5. |
- Velilla Prieto, Luis, 1981, et al.
(författare)
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Circumstellar chemistry of Si-C bearing molecules in the C-rich AGB star IRC+10216
- 2018
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Ingår i: Proceedings of the International Astronomical Union. - 1743-9213 .- 1743-9221. ; 14, s. 535-537
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Konferensbidrag (refereegranskat)abstract
- Silicon carbide together with amorphous carbon are the main components of dust grains in the atmospheres of C-rich AGB stars. Small gaseous Si-C bearing molecules (such as SiC, SiCSi, and SiC2) are efficiently formed close to the stellar photosphere. They likely condense onto dust seeds owing to their highly refractory nature at the lower temperatures (i.e., below about 2500 K) in the dust growth zone which extends a few stellar radii from the photosphere. Beyond this region, the abundances of Si-C bearing molecules are expected to decrease until they are eventually reformed in the outer shells of the circumstellar envelope, owing to the interaction between the gas and the interstellar UV radiation field. Our goal is to understand the time-dependent chemical evolution of Si-C bond carriers probed by molecular spectral line emission in the circumstellar envelope of IRC+10216 at millimeter wavelengths.
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| 6. |
- Bujarrabal, V., et al.
(författare)
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High-resolution observations of the symbiotic system R Aqr Direct imaging of the gravitational effects of the secondary on the stellar wind
- 2018
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Ingår i: Astronomy and Astrophysics. - : EDP SCIENCES S A. - 0004-6361 .- 1432-0746. ; 616
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Tidskriftsartikel (refereegranskat)abstract
- We have observed the symbiotic stellar system R Aqr, aiming to describe the gravitational interaction between the white dwarf (WD) and the wind from the Mira star, the key phenomenon driving the symbiotic activity and the formation of nebulae in such systems. We present high-resolution ALMA maps of the (CO)-C-12 and (CO)-C-13 J = 3-2 lines, the 0.9 mm continuum distribution, and some high-excitation molecular lines in R Aqr. The maps, which have resolutions ranging between 40 mas and less than 20 mas probe the circumstellar regions at suborbital scales as the distance between the stars is similar to 40 mas. Our observations show the gravitational effects of the secondary on the stellar wind. The AGB star was identified in our maps from the continuum and molecular line data, and we estimated the probable position of the secondary from a new estimation of the orbital parameters. The (preliminary) comparison of our maps with theoretical predictions is surprisingly satisfactory and the main expected gravitational effects are directly mapped for the first time. We find a strong focusing in the equatorial plane of the resulting wind, which shows two plumes in opposite directions that have different velocities and very probably correspond to the expected double spiral due to the interaction. Our continuum maps show the very inner regions of the nascent bipolar jets, at scales of some AU. Continuum maps obtained with the highest resolution show the presence of a clump that very probably corresponds to the emission of the ionized surroundings of the WD and of a bridge of material joining both stars, which is likely material flowing from the AGB primary to the accretion disk around the WD secondary.
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| 7. |
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| 8. |
- Castro-Carrizo, A., et al.
(författare)
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Mapping the 12CO J = 1-0 and J = 2-1 emission in AGB and early post-AGB circumstellar envelopes. I. The COSAS program, first sample
- 2010
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 523:4, s. A59-
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Tidskriftsartikel (refereegranskat)abstract
- We present COSAS (CO Survey of late AGB Stars), a project to map and analyze the 12CO J = 1−0 and J = 2−1 line emission in a representative sample of circumstellar envelopes around AGB and post-AGB stars. The survey was undertaken with the aim of investigating small- and large-scale morphological and kinematical properties of the molecular environment surrounding stars in the late AGB and early post-AGB phases. For this, COSAS combines the high sensitivity and spatial resolving power of the IRAM Plateau de Bure interferometer with the better capability of the IRAM 30 m telescope to map extended emission. The global sample encompasses 45 stars selected to span a range in chemical type, variability type, evolutionary state, and initial mass. COSAS provides means to quantify variations in the mass-loss rates, assess morphological and kinematical features, and to investigate the appearance of fast aspherical winds in the early post-AGB phase. This paper, which is the first of a series of COSAS papers, presents the results from the analyses of a first sample of 16 selected sources. The envelopes around late AGB stars are found to be mostly spherical, often mingled with features such as concentric arcs (R Cas and TX Cam), a broken spiral density pattern (TX Cam), molecular patches testifying to aspherical mass-loss (WX Psc, IK Tau, V Cyg, and S Cep), and also with well-defined axisymmetric morphologies and kinematical patterns (X Her and RX Boo). The sources span a wide range of angular sizes, from relatively compact (CRL 2362, OH 104.9+2.4 and CRL 2477) to very large (χ Cyg and TX Cam) envelopes, sometimes partially obscured by self-absorption features, which particularly for IK Tau and χ Cyg testifies to the emergence of aspherical winds in the innermost circumstellar regions. Strong axial structures with more or less complex morphologies are detected in four early post-AGB stars (IRAS 20028+3910, IRAS 23321+6545, IRAS 19475+3119 and IRAS 21282+5050) of the sub-sample.
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| 9. |
- Velilla Prieto, Luis, 1981, et al.
(författare)
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IRC+10 degrees 216 mass loss properties through the study of lambda 3 mm emission
- 2019
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 629
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Tidskriftsartikel (refereegranskat)abstract
- Low-mass evolved stars are major contributors to interstellar medium enrichment as a consequence of the intense mass-loss process these stars experience at the end of their lives. The study of the gas in the envelopes surrounding asymptotic giant branch (AGB) stars through observations in the millimetre wavelength range provides information about the history and nature of these molecular factories. Here we present ALMA observations at subarsecond resolution, complemented with IRAM-30 m data, of several lines of SiO, SiS, and CS towards the best-studied AGB circumstellar envelope, IRC + 10 degrees 216. We aim to characterise their spatial distribution and determine their fractional abundances mainly through radiative transfer and chemical modelling. The three species display extended emission with several enhanced emission shells. CS displays the most extended distribution reaching distances up to approximately 20 ''. SiS and SiO emission have similar sizes of approximately 11 '', but SiS emission is slightly more compact. We have estimated fractional abundances relative to H-2, which on average are equal to f(SiO) similar to 10(-7), f(SiS) similar to 10(-6), and f(CS) similar to 10(-6) up to the photo-dissociation region. The observations and analysis presented here show evidence that the circumstellar material displays clear deviations from an homogeneous spherical wind, with clumps and low density shells that may allow UV photons from the interstellar medium (ISM) to penetrate deep into the envelope, shifting the photo-dissociation radius inwards. Our chemical model predicts photo-dissociation radii compatible with those derived from the observations, although it is unable to predict abundance variations from the starting radius of the calculations (similar to 10 R-*), which may reflect the simplicity of the model. We conclude that the spatial distribution of the gas proves the episodic and variable nature of the mass loss mechanism of IRC + 10 degrees 216, on timescales of hundreds of years.
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