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.

We present new continuum and line observations, along with modelling, of the faint (6−8) Myr old T Tauri star ET Cha belonging to the η Chamaeleontis cluster. We have acquired Herschel/PACS photometric fluxes at 70 μm and 160 μm, as well as a detection of the [OI] 63 μm fine-structure line in emission, and derived upper limits for some other far-IR OI, CII, CO and o-H 2 O lines. These observations were carried out in the frame of the open time key programme GASPS, where ET Cha was selected as one of the science demonstration phase targets. The Herschel data is complemented by new simultaneous ANDICAM B−K photometry, new HST/COS and HST/STIS UV-observations, a non-detection of CO J = 3 → 2 with APEX, re-analysis of a UCLES high-resolution optical spectrum showing forbidden emission lines like [OI] 6300 Å, [SII] 6731 Å and 6716 Å, and [NII] 6583 Å, and a compilation of existing broad-band photometric data. We used the thermo-chemical disk code ProDiMo and the Monte-Carlo radiative transfer code MCFOST to model the protoplanetary disk around ET Cha. The paper also introduces a number of physical improvements to the ProDiMo disk modelling code concerning the treatment of PAH ionisation balance and heating, the heating by exothermic chemical reactions, and several non-thermal pumping mechanisms for selected gas emission lines. By applying an evolutionary strategy to minimise the deviations between model predictions and observations, we find a variety of united gas and dust models that simultaneously fit all observed line and continuum fluxes about equally well. Based on these models we can determine the disk dust mass with confidence, M dust ≈ (2−5) × 10 −8 M whereas the total disk gas mass is found to be only little constrained, M gas ≈ (5 × 10 −5 −3 × 10 −3 ) M . Both mass estimates are substantially lower than previously reported. In the models, the disk extends from 0.022 AU (just outside of the co-rotation radius) to only about 10 AU, remarkably small for single stars, whereas larger disks are found to be inconsistent with the CO J = 3 → 2 non-detection. The low velocity component of the [OI] 6300 Å emission line is centred on the stellar systematic velocity, and is consistent with being emitted from the inner disk. The model is also consistent with the line flux of H 2 v = 1 → 0 S(1) at 2.122 μm and with the [OI] 63 μm line as seen with Herschel/PACS. An additional high-velocity component of the [OI] 6300 Å emission line, however, points to the existence of an additional jet/outflow of low velocity 40−65 km s −1 with mass loss rate ≈10 −9 M /yr. In relation to our low estimations of the disk mass, such a mass loss rate suggests a disk lifetime of only ∼0.05−3 Myr, substantially shorter than the cluster age. If a generic gas/dust ratio of 100 was assumed, the disk lifetime would be even shorter, only ∼3000 yrs. The evolutionary state of this unusual protoplanetary disk is discussed.

We present a study of calibrated low-resolution spectra of the 18 known primaries of the ≈9-Myr-old η Chamaeleontis (η Cha) pre-main-sequence (PMS) star cluster. Using synthetic broad-band colours and narrow-band continuum-sensitive, temperature-sensitive and gravitysensitive indices derived from the spectra, we compare the η Cha stars to standard dwarfs. We find that the VRI colours of the PMS stars are indistinguishable from those of mainsequence stars, but that a B-band excess attaining ≈0.2 mag for late-M cluster stars is present, which might be an indicator of gravity, metallicity and/or activity differences between the two samples of stars. The narrow-band spectral indices for the η Cha stars possibly indicate higher metallicity and strongly indicate lower surface gravity than the dwarf indices, consistent with the elevated location of the cluster in the Hertzsprung-Russell evolutionary diagram. Using the derived synthetic colours and indices, we adopt spectral types for the late-type η Cha stars. We then produce a table of absolute optical magnitudes and colours representing the cluster isochrone for comparison with PMS evolutionary models. From our results we also conclude that the η Cha stars are unreddened, consistent with the group being a sample of older PMS stars distant from obscuring molecular clouds, except for the A1 member HD 75505 for which we confirm A V = 0.4 mag likely due to the presence of circumstellar material. the gravity-sensitive K I and Na I lines are weaker in young Pleiades brown dwarfs than in field objects of similar spectral type, while Mohanty et al. (2004) have demonstrated the gravity sensitivity of the TiO molecular bands, which increase in strength with decreasing effective gravity, in samples of PMS stars in Upper Scorpius and Taurus star-forming regions. The differences in these features are relevant to all young PMS populations, with ages several Gyr less than old disc dwarfs and elevated in the Hertzsprung-Russell (HR) diagram by several magnitudes above the main sequence.Nearby young clusters have distinct advantages for the study of fundamental properties of PMS stars. They provide a sample of stars across a wide range of spectral types at essentially uniform age, distance and metallicity, and are sufficiently bright for precise spectroscopic characterization. Our laboratory is the ≈9-Myr-old η Chamaeleontis (η Cha) cluster (Mamajek, Lawson & Feigelson 1999). The cluster is an ideal PMS group for the study of 'intermediate-aged' PMS stars owing to its well-defined distance from Hipparcos measurements (d = 97 ± 3 pc), compactness (extent ∼1 pc), the apparent high degree of coevality of the stellar C 2004 RAS