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 ∼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 ∼350 GHz. Observing with ALMA on baselines of up to 15 km is now possible, and opens up new parameter space for submm astronomy.

Context. On the asymptotic giant branch (AGB) low-and intermediate-mass stars eject a large fraction of their envelope, but the mechanism driving these outflows is still poorly understood. For oxygen-rich AGB stars, the wind is thought to be driven by radiation pressure caused by scattering of radiation off dust grains. Aims. We study the photosphere, the warm molecular layer, and the inner wind of the close-by oxygen-rich AGB star R Doradus. We focus on investigating the spatial distribution of the dust grains that scatter light and whether these grains can be responsible for driving the outflow of this star. Methods. We use high-angular-resolution images obtained with SPHERE/ZIMPOL to study R Dor and its inner envelope in a novel way. We present observations in filters V, cntHα, and cnt820 and investigate the surface brightness distribution of the star and of the polarised light produced in the inner envelope. Thanks to second-epoch observations in cntHα, we are able to see variability on the stellar photosphere. We study the polarised-light data using a continuum-radiative-transfer code that accounts for direction-dependent scattering of photons off dust grains. Results. We find that in the first epoch the surface brightness of R Dor is asymmetric in V and cntHα, the filters where molecular opacity is stronger, while in cnt820 the surface brightness is closer to being axisymmetric. The second-epoch observations in cntHα show that the morphology of R Dor has changed completely in a timespan of 48 days to a more axisymmetric and compact configuration. This variable morphology is probably linked to changes in the opacity provided by TiO molecules in the extended atmosphere. The observations show polarised light coming from a region around the central star. The inner radius of the region from where polarised light is seen varies only by a small amount with azimuth. The value of the polarised intensity, however, varies by between a factor of 2.3 and 3.7 with azimuth for the different images. We fit the radial profile of the polarised intensity using a spherically symmetric model and a parametric description of the dust density profile, ρ(r) = ρ • r −n . On average, we find exponents of −4.5 ± 0.5 that correspond to a much steeper density profile than that of a wind expanding at constant velocity. The dust densities we derive imply an upper limit for the dust-to-gas ratio of ∼2 × 10 −4 at 5.0 R . Considering all the uncertainties in observations and models, this value is consistent with the minimum values required by wind-driving models for the onset of a wind, of ∼3.3 × 10 −4 . However, if the steep density profile we find extends to larger distances from the star, the dust-to-gas ratio will quickly become too small for the wind of R Dor to be driven by the grains that produce the scattered light.

Our current understanding of the chemistry and mass-loss processes in solar-like stars at the end of their evolution depends critically on the description of convection, pulsations and shocks in the extended stellar atmosphere (1). Threedimensional hydrodynamical stellar atmosphere models provide observational predictions (2), but so far the resolution to constrain the complex temperature and velocity structures seen in the models has been lacking. Here we present submillimeter continuum and line observations that resolve the atmosphere of the asymptotic giant branch star W Hya. We show that hot gas with chromospheric characteristics exists around the star. Its filling factor is shown to be small. The existence of such gas requires shocks with a cooling time larger than commonly assumed. A shocked hot layer will be an important ingredient in the models of stellar convection, pulsation and chemistry that underlie our current understanding of the late stages of stellar evolution.Asymptotic giant branch (AGB) stars are among the most important sources of enrichment of the Galactic interstellar medium (ISM). Molecules and dust formed in the warm extended atmospheres and the cool and dense circumstellar envelopes (CSEs) around AGB stars are injected into the ISM by a stellar wind that has overcome stellar gravity (1). It is generally assumed that the stellar wind is driven by radiation pressure on dust that forms at a few stellar radii, where the temperature in the CSE has dropped so that dust condensation can occur (3). In order for the gas in the extended stellar atmosphere to reach the dust formation region, the most recent AGB mass-loss models typically invoke stellar pulsations and convective motions (2,(4)(5)(6).Both convective motions and pulsations induce outward moving shocks that critically affect the upper layers of the AGB atmosphere where the stellar mass loss is determined. The propagation of shocks also strongly affects the chemistry in the stellar atmosphere (7-9). In early AGB atmosphere models, the outward propagation of strong shocks is responsible for the creation of a chromosphere (6, 10), from which ultraviolet line and continuum emissions originate. Such emissions have been observed from AGB stars (11,12). However, the observations of molecules and dust close to the star are not consistent with the extended chromosphere produced by the models. Observations have so far not been able to resolve this ambiguity. High angular resolution images of the stellar disks of AGB stars have revealed asymmetries of which the source is not yet clear, but convective motions are believed to play a role (13)(14)(15)(16). Since at most wavelengths, the observations are probing distinct molecular opacity sources (16), or averages over the stellar disk (17), the dynamics and temperature structures in the atmosphere closest to the stellar photosphere have not yet been observed in detail.We present observations of the AGB star W Hya that reveal evidence for the presence of shocks and map the distribution of molecular g...

Aims. We present the size, shape, and flux densities at millimeter continuum wavelengths, based on ALMA science verification observations in Band 3 (∼94.6 GHz) and Band 6 (∼228.7 GHz), from the binary Mira A (o Ceti) and Mira B. Methods. The Mira AB system was observed with ALMA at a spatial resolution down to ∼25 mas. The extended atmosphere of Mira A and the wind around Mira B sources were resolved, and we derived the sizes of Mira A and of the ionized region around Mira B. The spectral indices within Band 3 (between 89-100 GHz) and between Bands 3 and 6 were also derived. Results. The spectral index of Mira A is found to change from 1.71 ± 0.05 within Band 3 to 1.54 ± 0.04 between Bands 3 and 6. The spectral index of Mira B is 1.3 ± 0.2 in Band 3, in good agreement with measurements at longer wavelengths; however, it rises to 1.72 ± 0.11 between the bands. For the first time, the extended atmosphere of a star is resolved at these frequencies, and for Mira A the diameter is ∼3.8 × 3.2 AU in Band 3 (with brightness temperature T b ∼ 5300 K) and ∼4.0 × 3.6 AU in Band 6 (T b ∼ 2500 K). Additionally, a bright hotspot ∼0.4 AU, with T b ∼ 10 000 K, is found on the stellar disk of Mira A. The size of the ionized region around the accretion disk of Mira B is found to be ∼2.4 AU. Conclusions. The emission around Mira B is consistent with emission from a partially ionized wind of gravitationally bound material from Mira A close to the accretion disk of Mira B. The Mira A atmosphere does not fully match predictions with brightness temperatures in Band 3 significantly higher than expected, potentially owing to shock heating. The hotspot is very likely due to magnetic activity and could be related to the previously observed X-ray flare of Mira A.

We observed Betelgeuse using ALMA's extended configuration in band 7 ( f ≈ 340 GHz, λ ≈ 0.88 mm), resulting in a very high angular resolution of 18 mas. Using a solid body rotation model of the 28 SiO( = 2, J = 8−7) line emission, we show that the supergiant is rotating with a projected equatorial velocity of eq sin i = 5.47 ± 0.25 km s −1 at the equivalent continuum angular radius R star = 29.50 ± 0.14 mas. This corresponds to an angular rotation velocity of ω sin i = (5.6 ± 1.3) × 10 −9 rad s −1 . The position angle of its north pole is PA = 48.0 ± 3.5• . The rotation period of Betelgeuse is estimated to P/sin i = 36 ± 8 years. The combination of our velocity measurement with previous observations in the ultraviolet shows that the chromosphere is co-rotating with the star up to a radius of ≈10 au (45 mas or 1.5× the ALMA continuum radius). The coincidence of the position angle of the polar axis of Betelgeuse with that of the major ALMA continuum hot spot, a molecular plume, and a partial dust shell (from previous observations) suggests that focused mass loss is currently taking place in the polar region of the star. We propose that this hot spot corresponds to the location of a particularly strong "rogue" convection cell, which emits a focused molecular plume that subsequently condenses into dust at a few stellar radii. Rogue convection cells therefore appear to be an important factor shaping the anisotropic mass loss of red supergiants.