APEX, the Atacama Pathfinder EXperiment, has been successfully commissioned and is operational. This novel submillimeter telescope is located at 5107 m altitude on Llano de Chajnantor in the Chilean High Andes, on what is considered one of the world's outstanding sites for submillimeter astronomy. The primary reflector with 12 m diameter has been carefully adjusted by means of holography. Its surface smoothness of only 17-18 µm makes APEX suitable for observations up to 200 µm, through all atmospheric submm windows accessible from the ground. First scientific results will be presented in the accompanying papers of this special issue.

The Large APEX BOlometer CAmera, LABOCA, has been commissioned for operation as a new facility instrument at the Atacama Pathfinder Experiment 12 m submillimeter telescope. This new 295-bolometer total power camera, operating in the 870 μm atmospheric window, combined with the high efficiency of APEX and the excellent atmospheric transmission at the site, offers unprecedented capability in mapping submillimeter continuum emission for a wide range of astronomical purposes.

Aims. The composition of planetary solids and gases is largely rooted in the processing of volatile elements in protoplanetary disks. To shed light on the key processes, we carry out a comparative analysis of the gas-phase carbon abundance in two systems with a similar age and disk mass, but different central stars: HD 100546 and TW Hya. Methods. We combine our recent detections of C 0 in these disks with observations of other carbon reservoirs (CO, C + , C 2 H) and gas-mass and warm-gas tracers (HD, O 0 ), as well as spatially resolved ALMA observations and the spectral energy distribution. The disks are modelled with the DALI 2D physical-chemical code. Stellar abundances for HD 100546 are derived from archival spectra. Results. Upper limits on HD emission from HD 100546 place an upper limit on the total disk mass of ≤0.1 M . The gas-phase carbon abundance in the atmosphere of this warm Herbig disk is, at most, moderately depleted compared to the interstellar medium, with [C]/[H] gas = (0.1−1.5) × 10 −4 . HD 100546 itself is a λ Boötis star, with solar abundances of C and O but a strong depletion of rockforming elements. In the gas of the T Tauri disk TW Hya, both C and O are strongly underabundant, with [C]/[H] gas = (0.2−5.0)×10 −6 and C/O > 1. We discuss evidence that the gas-phase C and O abundances are high in the warm inner regions of both disks. Our analytical model, including vertical mixing and a grain size distribution, reproduces the observed [C]/[H] gas in the outer disk of TW Hya and allows to make predictions for other systems.

Context. The NGC 1333 IRAS 4A and IRAS 4B sources are among the most well-studied Stage 0 low-mass protostars, which drive prominent bipolar outflows. Spectrally resolved molecular emission lines provide crucial information about the physical and chemical structure of the circumstellar material as well as the dynamics of the different components. Most studies have so far concentrated on the colder parts (T ≤ 30 K) of these regions. Aims. The aim is to characterize the warmer parts of the protostellar envelope using the new generation of submillimeter instruments. This will allow us to quantify the feedback of the protostars on their surroundings in terms of shocks, ultraviolet (UV) heating, photodissociation, and outflow dispersal. Methods. The dual frequency 2 × 7 pixel 650/850 GHz array receiver CHAMP + mounted on APEX was used to obtain a fully sampled, large-scale ∼4 × 4 map at 9 resolution of the IRAS 4A/4B region in the 12 CO J = 6-5 line. Smaller maps were observed in the 13 CO 6-5 and [C i] J = 2-1 lines. In addition, a fully sampled 12 CO J = 3-2 map made with HARP-B on the JCMT is presented and deep isotopolog observations are obtained at selected outflow positions to constrain the optical depth. Complementary Herschel-HIFI and ground-based lines of CO and its isotopologs, from J = 1-0 up to 10-9 (E u /k ≈ 300 K), are collected at the source positions and used to construct velocity-resolved CO ladders and rotational diagrams. Radiative-transfer models of the dust and lines are used to determine the temperatures and masses of the outflowing and photon-heated gas and infer the CO abundance structure. Results. Broad CO emission-line profiles trace entrained shocked gas along the outflow walls, which have an average temperature of ∼100 K. At other positions surrounding the outflow and the protostar, the 6-5 line profiles are narrow indicating UV excitation. The narrow 13 CO 6-5 data directly reveal the UV heated gas distribution for the first time. The amount of UV-photon-heated gas and outflowing gas are quantified from the combined 12 CO and 13 CO 6-5 maps and found to be comparable within a 20 radius around IRAS 4A, which implies that UV photons can affect the gas as much as the outflows. Weak [C I] emission throughout the region indicates that there is a lack of CO dissociating photons. Our modeling of the C 18 O lines demonstrates the necessity of a "drop" abundance profile throughout the envelopes where the CO freezes out and is reloaded back into the gas phase through grain heating, thus providing quantitative evidence of the CO ice evaporation zone around the protostars. The inner abundances are less than the canonical value of CO/H 2 = 2.7 × 10 −4 , however, implying that there is some processing of CO into other species on the grains. The implications of our results for the analysis of spectrally unresolved Herschel data are discussed.

Context. In the low-mass regime, molecular cores have spatially resolved temperature and density profiles allowing a detailed study of their chemical properties. It is found that the gas-phase abundances of C-bearing molecules in cold starless cores rapidly decrease with increasing density. Here the molecules tend to stick to the grains, forming ice mantles. Aims. We study CO depletion in a large sample of massive clumps, and investigate its correlation with evolutionary stage and with the physical parameters of the sources. Moreover, we study the gradients in Methods. From the ATLASGAL 870 µm survey we selected 102 clumps, which have masses in the range ∼10 2 −3 × 10 4 M , sampling different evolutionary stages. We use low-J emission lines of CO isotopologues and the dust continuum emission to infer the depletion factor f D . RATRAN one-dimensional models were also used to determine f D and to investigate the presence of depletion above a density threshold. The isotopic ratios and optical depth were derived with a Bayesian approach. Results. We find a significant number of clumps with a high degree of CO depletion, up to ∼20. Larger values are found for colder clumps, thus for earlier evolutionary phases. For massive clumps in the earliest stages of evolution we estimate the radius of the region where CO depletion is important to be a few tenths of a pc. The value of the [ 12 C]/[ 13 C] ratio is found to increase with distance from the Galactic centre, with a value of ∼66 ± 12 for the solar neighbourhood. The [ 18 O]/[ 17 O] ratio is approximately constant (∼4) across the inner Galaxy between 2 kpc and 8 kpc, albeit with a large range (∼2−6). Clumps are found with total masses derived from dust continuum emission up to ∼20 times higher than M vir , especially among the less evolved sources. These large values may in part be explained by the presence of depletion: if the CO emission comes mainly from the low-density outer layers, the molecules may be subthermally excited, leading to an overestimate of the dust masses. Conclusions. CO depletion in high-mass clumps seems to behave as in the low-mass regime, with less evolved clumps showing larger values for the depletion than their more evolved counterparts, and increasing for denser sources.