| 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. |
- Goicoechea, J. R., et al.
(författare)
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Spatially resolved images of reactive ions in the Orion Bar
- 2017
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 601, s. Article Number: L9, pp. 1-9
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Tidskriftsartikel (refereegranskat)abstract
- We report high angular resolution (4.9" X 3.0") images of reactive ions SH+, HOC+, and SO+ toward the Orion Bar photodissociation region (PDR). We used ALMA-ACA to map several rotational lines at 0.8 mm, complemented with multi-line observations obtained with the IRAM 30m telescope. The SH+ and HOC+ emission is restricted to a narrow layer of 2" - to 10" -width (approximate to 800 to 4000AU depending on the assumed PDR geometry) that follows the vibrationally excited H-2(*) emission. Both ions efficiently form very close to the H/H-2 transition zone, at a depth of AV less than or similar to 1 mag into the neutral cloud, where abundant C+, S+, and H-2(*) coexist. SO+ peaks slightly deeper into the cloud. The observed ions have low rotational temperatures (T-rot approximate to 10-30K
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| 3. |
- Goicoechea, Javier R., et al.
(författare)
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The ALMA view of UV-irradiated cloud edges: unexpected structures and processes
- 2018
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Ingår i: Proceedings of the International Astronomical Union. - 1743-9213 .- 1743-9221. ; S332, s. 210-217
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Konferensbidrag (refereegranskat)abstract
- Far-UV photons (FUV, E < 13.6 eV) from hot massive stars regulate, or at least influence, the heating, ionization, and chemistry of most of the neutral interstellar medium (H i and H2 clouds). Investigating the interaction between FUV radiation and interstellar matter (molecules, atoms and grains) thus plays an important role in astrochemistry. The Orion Bar, an interface region between the Orion A molecular cloud and the H ii region around the Trapezium cluster, is a textbook example of a strongly illuminated dense PDR (photodissociation region). The Bar is illuminated by a FUV field of a few 104 times the mean interstellar radiation field. Because of its proximity and nearly edge-on orientation, it provides a very good template to investigate the chemical content, structure, and dynamics of a strongly irradiated molecular cloud edge. We have used ALMA to mosaic a small field of the Bar where the critical transition from atomic to molecular gas takes place. These observations provide an unprecedented sharp view of this transition layer (≲ 1″ resolution or ≲ 414 AU). The resulting images (so far in the rotational emission of CO, HCO+, H13CO+, SO+, SO, and reactive ions SH+ and HOC+) show the small-scale structure in gas density and temperature, and the steep abundance gradients. The images reveal a pattern of high-density substructures, photo-ablative gas flows and instabilities at the edge of the molecular cloud. These first ALMA images thus show a more complex morphology than the classical clump/interclump static model of a PDR. In order to quantify the chemical content in strongly FUV-irradiated gas, we have also used the IRAM-30 m telescope to carry out a complete line-survey of the illuminated edge of the Bar in the millimeter domain. Our observations reveal the presence of complex organic molecules (and precursors) that were not expected in such a harsh environment. In particular, we have reported the first detection of the unstable cis conformer of formic acid (HCOOH) in the ISM. The energy barrier to internal rotation (the conversion from trans to cis) is approximately 4827 cm−1 (≈7000 K). Hence, this detection is surprising. The low inferred trans-to-cis abundance ratio of 2.8±1.0 supports a photoswitching mechanism: a given conformer absorbs a FUV stellar photon that radiatively excites the molecule to electronic states above the interconversion barrier. Subsequent fluorescent decay leaves the molecule in a different conformer form. This mechanism, which we have specifically studied with ab initio quantum calculations, was not considered so far in astrochemistry although it can affect the structure of a variety of molecules in PDRs.
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| 4. |
- Csengeri, T., et al.
(författare)
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Search for high-mass protostars with ALMA revealed up to kilo-parsec scales (SPARKS): I. Indication for a centrifugal barrier in the environment of a single high-mass envelope
- 2018
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 617
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Tidskriftsartikel (refereegranskat)abstract
- The conditions leading to the formation of the most massive O-type stars are still an enigma in modern astrophysics. To assess the physical conditions of high-mass protostars in their main accretion phase, here we present a case study of a young massive clump selected from the ATLASGAL survey, G328.2551-0.5321. The source exhibits a bolometric luminosity of 1.3 × 104L·, which allows us to estimate that its current protostellar mass lies between ∼11 and 16 M·. We show high angular resolution observations with ALMA that reach a physical scale of ∼400 au. To reveal the structure of this high-mass protostellar envelope in detail at a ∼0.17′′ resolution, we used the thermal dust continuum emission and spectroscopic information, amongst others from the CO (J = 3-2) line, which is sensitive to the high-velocity molecular outflow of the source. We also used the SiO (J = 8-7) and SO2(J = 82,6-71,7) lines, which trace shocks along the outflow, as well as several CH3OH and HC3N lines that probe the gas of the inner envelope in the closest vicinity of the protostar. Our observations of the dust continuum emission reveal a single high-mass protostellar envelope, down to our resolution limit. We find evidence for a compact, marginally resolved continuum source that is surrounded by azimuthal elongations that could be consistent with a spiral pattern. We also report on the detection of a rotational line of CH3OH within its vt= 1 torsionally excited state. This shows two bright emission peaks that are spatially offset from the dust continuum peak and exhibit a distinct velocity component ±4.5 km s-1offset from the systemic velocity of the source. Rotational diagram analysis and models based on local thermodynamic equilibrium assumption require high CH3OH column densities that reach N(CH3OH) = 1.2-2 × 1019cm-2, and kinetic temperatures of the order of 160-200 K at the position of these peaks. A comparison of their morphology and kinematics with those of the outflow component of the CO line and the SO2line suggests that the high-excitation CH3OH spots are associated with the innermost regions of the envelope. While the HC3N v7= 0 (J = 37-36) line is also detected in the outflow, the HC3N v7= 1e (J = 38-37) rotational transition within the first vibrationally excited state of the molecule shows a compact morphology. We find that the velocity shifts at the position of the observed high-excitation CH3OH spots correspond well to the expected Keplerian velocity around a central object with 15 M·consistent with the mass estimate based on the bolometric luminosity of the source. We propose a picture where the CH3OH emission peaks trace the accretion shocks around the centrifugal barrier, pinpointing the interaction region between the collapsing envelope and an accretion disc. The physical properties of the accretion disc inferred from these observations suggest a specific angular momentum several times higher than typically observed towards low-mass protostars. This is consistent with a scenario of global collapse setting on at larger scales that could carry a more significant amount of kinetic energy compared to the core-collapse models of low-mass star formation. Furthermore, our results suggest that vibrationally excited HC3N emission could be a new tracer for compact accretion discs around high-mass protostars.
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