| 1. |
- 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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| 2. |
- Liseau, René, 1949, et al.
(författare)
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ALMA observations of α Centauri. First detection of main-sequence stars at 3 mm wavelength
- 2015
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 573, s. L4 (4 pages)-
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Forskningsöversikt (refereegranskat)abstract
- Context. The precise mechanisms that provide the non-radiative energy for heating the chromosphere and the corona of the Sun and those of other stars constitute an active field of research. By studying stellar chromospheres one aims at identifying the relevant physical processes. Defining the permittable extent of the parameter space can also serve as a template for the Sun-as-a-star. This feedback will probably also help identify stars that potentially host planetary systems that are reminiscent of our own.Aims: Earlier observations with Herschel and APEX have revealed the temperature minimum of α Cen, but these were unable to spatially resolve the binary into individual components. With the data reported in this Letter, we aim at remedying this shortcoming. Furthermore, these earlier data were limited to the wavelength region between 100 and 870 μm. In the present context, we intend to extend the spectral mapping (SED) to longer wavelengths, where the contrast between stellar photospheric and chromospheric emission becomes increasingly evident.Methods: The Atacama Large Millimeter/submillimeter Array (ALMA) is particularly suited to point sources, such as unresolved stars. ALMA provides the means to achieve our objectives with both its high sensitivity of the collecting area for the detection of weak signals and the high spatial resolving power of its adaptable interferometer for imaging close multiple stars.Results: This is the first detection of main-sequence stars at a wavelength of 3 mm. Furthermore, the individual components of the binary α Cen AB are clearly detected and spatially well resolved at all ALMA wavelengths. The high signal-to-noise ratios of these data permit accurate determination of their relative flux ratios, i.e., SyB / SyA> = 0.54 ± 0.04 at 440 μm, = 0.46 ± 0.01 at 870 μm, and = 0.47 ± 0.006 at 3.1 mm, respectively.Conclusions: The previously obtained flux ratio of 0.44±0.18, which was based on measurements in the optical and at 70 μm, is consistent with the present ALMA results, albeit with a large error bar. The observed 3.1 mm emission greatly exceeds what is predicted from the stellar photospheres, and undoubtedly arises predominantly as free-free emission in the ionized chromospheric plasmas of both stars. Given the distinct difference in their cyclic activity, the similarity of their submm SEDs appears surprising.
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| 3. |
- Sahai, R., et al.
(författare)
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ALMA Observations of the Water Fountain Pre-planetary Nebula IRAS 16342-3814: High-velocity Bipolar Jets and an Expanding Torus
- 2017
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Ingår i: Astrophysical Journal Letters. - 2041-8213 .- 2041-8205. ; 835:1
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Tidskriftsartikel (refereegranskat)abstract
- We have mapped 12CO J = 3-2 and other molecular lines from the "water fountain" bipolar pre-planetary nebula (PPN) IRAS 16342-3814 with ?0.?35 resolution using Atacama Large Millimeter/submillimeter Array. We find (i) two very high-speed knotty, jet-like molecular outflows; (ii) a central high-density ( cm-3), expanding torus of diameter 1300 au; and (iii) the circumstellar envelope of the progenitor AGB, generated by a sudden, very large increase in the mass-loss rate to M o yr-1 in the past ?455 years. Strong continuum emission at 0.89 mm from a central source (690 mJy), if due to thermally emitting dust, implies a substantial mass (0.017 M o) of very large (?millimeter-sized) grains. The measured expansion ages of the above structural components imply that the torus (age ?160 years) and the younger high-velocity outflow (age ?110 years) were formed soon after the sharp increase in the AGB mass-loss rate. Assuming a binary model for the jets in IRAS 16342, the high momentum rate for the dominant jet-outflow in IRAS 16342 implies a high minimum accretion rate, ruling out standard Bondi-Hoyle-Lyttleton wind accretion and wind Roche-lobe overflow (RLOF) models with white-dwarf or main-sequence companions. Most likely, enhanced RLOF from the primary or accretion modes operating within common-envelope evolution are needed.
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| 4. |
- Sahai, R., et al.
(författare)
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The Coldest Place in the Universe: Probing the Ultra-cold Outflow and Dusty Disk in the Boomerang Nebula
- 2017
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Ingår i: Astrophysical Journal. - : American Astronomical Society. - 1538-4357 .- 0004-637X. ; 841:2, s. Article Number: 110-
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Tidskriftsartikel (refereegranskat)abstract
- Our Cycle 0 ALMA observations confirmed that the Boomerang Nebula is the coldest known object in the universe, with a massive high-speed outflow that has cooled significantly below the cosmic background temperature. Our new CO 1-0 data reveal heretofore unseen distant regions of this ultra-cold outflow, out to greater than or similar to 120,000 au. We find that in the ultra-cold outflow, the mass-loss rate (M.) increases with radius, similar to its expansion velocity (V)-taking V alpha r, we find M alpha r(0.9-2.2). The mass in the ultra-cold outflow is greater than or similar to 3.3 M-circle dot, and the Boomerang's main-sequence progenitor mass is greater than or similar to 4 M-circle dot. Our high angular resolution (similar to 0.'' 3) CO J = 3-2 map shows the inner bipolar nebula's precise, highly collimated shape, and a dense central waist of size (FWHM) similar to 1740 au x 275 au. The molecular gas and the dust as seen in scattered light via optical Hubble Space Telescope imaging show a detailed correspondence. The waist shows a compact core in thermal dust emission at 0.87-3.3 mm, which harbors (4-7) x 10(-4) M-circle dot of very large (similar to millimeter-to-centimeter sized), cold (similar to 20-30 K) grains. The central waist (assuming its outer regions to be expanding) and fast bipolar outflow have expansion ages of less than or similar to 1925 years and
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