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

We report new results in our continuing study of the unique compact (1 pc extent), nearby (d= 97 pc), young (t= 9 Myr) stellar cluster dominated by the B9 star η Chamaeleontis. An optical photometric survey spanning 1.3 × 1.3 pc adds two M5–M5.5 weak‐lined T Tauri (WTT) stars to the cluster inventory which is likely to be significantly complete for primaries with masses M > 0.15 M⊙. The cluster now consists of 17 primaries and ≃9 secondaries lying within 100 au of their primaries. The apparent distribution of 9 : 7 : 1 single : binary : triple systems shows 2–4 times higher multiplicity than in the field main‐sequence stars, and is comparable to that seen in other pre‐main‐sequence populations. The initial mass function (IMF) is consistent with that of rich young clusters and field stars. By extending the cluster IMF to lower masses, we predict 10–14 additional low‐mass stars with 0.08 < M < 0.15 M⊙ and 10–15 brown dwarfs with 0.025 < M < 0.08 M⊙ remain to be discovered. The η Cha cluster extends the established stellar density and richness relationship for young open clusters. The radial distribution of stars is consistent with an isothermal sphere, but mass segregation is present with >50 per cent of the stellar mass residing in the inner 6 arcmin (0.17 pc). Considering that the η Cha cluster is sparse, diffuse and young, the cluster may be an ideal laboratory for distinguishing between mass segregation that is primordial in nature, or arising from dynamical interaction processes.

High-resolution spectroscopic study of late-type members of the ≈9-Myr-old η Chamaeleontis star cluster shows that four stars, RECX 5, 9,, have broad Hα profiles indicative of ballistic accretion of material from circumstellar discs first identified by virtue of their infrared (IR) excess emission. Quantitative analysis of the profiles finds accretion in η Cha stars at rates comparable to that derived by Muzerolle et al. for members of the similarly aged TW Hydrae Association (TWA); rates 1-3 orders of magnitude lower than in younger classical T Tauri (CTT) stars. Together these studies indicate that the fraction of long-lived inner discs can be significantly higher than that inferred from study of younger premain sequence (PMS) populations, which suggest a disc lifetime of <6 Myr. The detection of long-lived discs may have implications for the formation of planetary systems. If slow accretion processes are the dominant formation mechanism for Jovian planets then long-lived discs may be ideal sites to search for evidence for protoplanets.

We present the first results from the B-fields In STar-forming Region Observations (BISTRO) survey, using the Sub-millimetre Common-User Bolometer Array2 camera, with its associated polarimeter (POL-2), on the James Clerk Maxwell Telescope in Hawaii. We discuss the survey's aims and objectives. We describe the rationale behind the survey, and the questions thatthe survey will aim to answer. The most important of these is the role of magnetic fields in the star formation process on the scale of individual filaments and cores in dense regions. We describe the data acquisition and reduction processes for POL-2, demonstrating both repeatability and consistency with previous data. We present a first-look analysis of the first results from the BISTRO survey in the OMC1 region. We see that the magnetic field lies approximately perpendicular to the famous "integral filament" in the densest regions of that filament. Furthermore, we see an "hourglass" magnetic field morphology extending beyond the densest region of the integral filament into the less-dense surrounding material, and discuss possible causes for this. We also discuss the more complex morphology seen along the Orion Bar region. We examine the morphology of the field along the lower-density northeastern filament. We find consistency with previous theoretical models that predict magnetic fields lying parallel to low-density, non-self-gravitating filaments, and perpendicular to higher-density, self-gravitating filaments.

We have analysed JHKL observations of the stellar population of the ≈9 Myr‐old η Chamaeleontis cluster. Using infrared (IR) colour–colour and colour‐excess diagrams, we find that the fraction of stellar systems with near‐IR excess emission is 0.60 ± 0.13 (2σ). This result implies considerably longer disc lifetimes than found in some recent studies of other young stellar clusters. For the classical T Tauri (CTT) and weak‐lined T Tauri (WTT) star population, we also find a strong correlation between the IR excess and Hα emission. The IR excesses of these stars indicate a wide range of star–disc activity: from a CTT star showing high levels of accretion to CTT–WTT transition objects with evidence for some on‐going accretion and WTT stars with weak or absent IR excesses. Of the 15 known cluster members, four stars with IR excesses Δ(K−L) > 0.4 mag are likely experiencing ongoing accretion owing to strong or variable optical emission. The resulting accretion fraction (0.27 ± 0.13; 2σ) shows that the accretion phase, in addition to the discs themselves, can endure for at least ∼10 Myr.

We present the 850 μm polarization observations toward the IC5146 filamentary cloud taken using the Submillimetre Common-User Bolometer Array 2 (SCUBA-2) and its associated polarimeter (POL-2), mounted on the James Clerk Maxwell Telescope, as part of the B-fields In STar forming Regions Observations. This work is aimed at revealing the magnetic field morphology within a core-scale (1.0 pc) hub-filament structure (HFS) located at the end of a parsec-scale filament. To investigate whether the observed polarization traces the magnetic field in the HFS, we analyze the dependence between the observed polarization fraction and total intensity using a Bayesian approach with the polarization fraction described by the Rice likelihood function, which can correctly describe the probability density function of the observed polarization fraction for low signal-to-noise ratio data. We find a power-law dependence between the polarization fraction and total intensity with an index of 0.56 in A V ∼20-300 mag regions, suggesting that the dust grains in these dense regions can still be aligned with magnetic fields in the IC5146 regions. Our polarization maps reveal a curved magnetic field, possibly dragged by the contraction along the parsec-scale filament. We further obtain a magnetic field strength of 0.5±0.2 mG toward the central hub using the Davis-Chandrasekhar-Fermi method, corresponding to a mass-to-flux criticality of ∼1.3±0.4 and an Alfvénic Mach number of <0.6. These results suggest that gravity and magnetic field are currently of comparable importance in the HFS and that turbulence is less important.