Context. The temperature and density structure of molecular cloud cores are the most important physical quantities that determine the course of the protostellar collapse and the properties of the stars they form. Nevertheless, density profiles often rely either on the simplifying assumption of isothermality or on observationally poorly constrained model temperature profiles. The instruments of the Herschel satellite provide us for the first time with both the spectral coverage and the spatial resolution that is needed to directly measure the dust temperature structure of nearby molecular cloud cores. Aims. With the aim of better constraining the initial physical conditions in molecular cloud cores at the onset of protostellar collapse, in particular of measuring their temperature structure, we initiated the guaranteed time key project (GTKP) "The Earliest Phases of Star Formation" (EPoS) with the Herschel satellite. This paper gives an overview of the low-mass sources in the EPoS project, the Herschel and complementary ground-based observations, our analysis method, and the initial results of the survey. Methods. We study the thermal dust emission of 12 previously well-characterized, isolated, nearby globules using FIR and submm continuum maps at up to eight wavelengths between 100 μm and 1.2 mm. Our sample contains both globules with starless cores and embedded protostars at different early evolutionary stages. The dust emission maps are used to extract spatially resolved SEDs, which are then fit independently with modified blackbody curves to obtain line-of-sight-averaged dust temperature and column density maps. Results. We find that the thermal structure of all globules (mean mass 7 M ) is dominated by external heating from the interstellar radiation field and moderate shielding by thin extended halos. All globules have warm outer envelopes (14-20 K) and colder dense interiors (8-12 K) with column densities of a few 10 22 cm −2 . The protostars embedded in some of the globules raise the local temperature of the dense cores only within radii out to about 5000 AU, but do not significantly affect the overall thermal balance of the globules. Five out of the six starless cores in the sample are gravitationally bound and approximately thermally stabilized. The starless core in CB 244 is found to be supercritical and is speculated to be on the verge of collapse. For the first time, we can now also include externally heated starless cores in the L smm /L bol vs. T bol diagram and find that T bol < 25 K seems to be a robust criterion to distinguish starless from protostellar cores, including those that only have an embedded very low-luminosity object.

The dense cores which conceive and cradle young stars can be explored through continuum emission from associated dust grains. We have performed a wide field survey for dust sources at 1.2 millimetres in the ρ Ophiuchi molecular cloud, covering more than 1 square degree in an unbiased fashion. We detect a number of previously unknown sources, ranging from extended cores over compact, starless cores to envelopes surrounding young stellar objects of Class 0, Class I, and Class II type. We analyse the mass distribution, spatial distribution and the potential equilibrium of the cores. For the inner regions, the survey results are consistent with the findings of previous narrower surveys. The core mass function resembles the stellar initial mass function, with the core mass function shifted by a factor of two to higher masses (for the chosen opacity and temperature). In addition, we find no statistical variation in the core mass function between the crowded inner regions and those in more isolated fields except for the absence of the most massive cores in the extended cloud. The inner region contains compacter cores. This is interpreted as due to a medium of higher mean pressure although strong pressure variations are evident in each region. The cores display a hierarchical spatial distribution with no preferred separation scale length. However, the frequency distribution of nearest neighbours displays two peaks, one of which at 5000 AU can be the result of core fragmentation. The orientations of the major axes of cores are consistent with an isotropic distribution. In contrast, the relative orientations of core pairs are preferentially in the NW-SE direction on all separation scales. These results are consistent with core production and evolution in a turbulent environment. Finally, we report the discovery of a new, low-mass Class 0 object candidate and its CO outflow.

We present the results of a comprehensive infrared, submillimetre, and millimetre continuum emission study of isolated low-mass star-forming cores in 32 Bok globules, with the aim to investigate the process of star formation in these regions. The submillimetre and millimetre dust continuum emission maps together with the spectral energy distributions are used to model and derive the physical properties of the star-forming cores, such as luminosities, sizes, masses, densities, etc. Comparisons with ground-based near-infrared and space-based mid and far-infrared images from Spitzer are used to reveal the stellar content of the Bok globules, association of embedded young stellar objects with the submm dust cores, and the evolutionary stages of the individual sources. Submm dust continuum emission was detected in 26 out of the 32 globule cores observed. For 18 globules with detected (sub)mm cores we derive evolutionary stages and physical parameters of the embedded sources. We identify nine starless cores, most of which are presumably prestellar, nine Class 0 protostars, and twelve Class I YSOs. Specific source properties like bolometric temperature, core size, and central densities are discussed as function of evolutionary stage. We find that at least two thirds (16 out of 24) of the star-forming globules studied here show evidence of forming multiple stars on scales between 1,000 and 50,000 AU. However, we also find that most of these small prototstar and star groups are comprised of sources with different evolutionary stages, suggesting a picture of slow and sequential star formation in isolated globules.

Aims. A census of molecular hydrogen flows across the entire Orion A giant molecular cloud is sought. With this paper we aim to associate each flow with its progenitor and associated molecular core, so that the characteristics of the outflows and outflow sources can be established. Methods. We present wide-field near-infrared images of Orion A, obtained with the Wide Field Camera, WFCAM, on the United Kingdom Infrared Telescope. Broad-band K and narrow-band H 2 1-0S(1) images of a contiguous ∼8 square degree region are compared to mid-IR photometry from the Spitzer Space Telescope and (sub)millimetre dust-continuum maps obtained with the MAMBO and SCUBA bolometer arrays. Using previously-published H 2 images, we also measured proper motions for H 2 features in 33 outflows, and use these data to help associate flows with existing sources and/or dust cores. Results. Together these data give a detailed picture of dynamical star formation across this extensive region. We increase the number of known H 2 outflows to 116. A total of 111 H 2 flows were observed with Spitzer; outflow sources are identified for 72 of them (12 more H 2 flows have tentative progenitors). The MAMBO 1200 μm maps cover 97 H 2 flows; 57 of them (59%) are associated with Spitzer sources and either dust cores or extended 1200 μm emission. The H 2 jets are widely distributed and randomly orientated. The jets do not appear to be orthogonal to large-scale filaments or even to the small-scale cores associated with the outflow sources (at least when traced with the 11 resolution of the 1200 μm MAMBO observations). Moreover, H 2 jet lengths (L) and opening angles (θ) are not obviously correlated with indicators of outflow source age -source spectral index, α (measured from mid-IR photometry), or (sub)millimetre core flux. It seems clear that excitation requirements limit the usefulness of H 2 as a tracer of L and θ (though jet position angles are well defined). Conclusions. We demonstrate that H 2 jet sources are predominantly protostellar sources with flat or positive mid-IR spectral indices, rather than disc-excess (or T Tauri) stars. Most protostars associated with molecular cores drive H 2 outflows; however, not all molecular cores are associated with protostars or H 2 jets. On statistical grounds, the H 2 jet phase may be marginally shorter than the protostellar phase, though it must be considerably (by an order of magnitude) shorter than the prestellar phase. In terms of range and mean value of α, H 2 jet sources are indistinguishable from protostars. The spread in α observed for both protostars and H 2 outflow sources is probably a function of inclination angle as much as source age. The few true protostars without H 2 jets are almost certainly more evolved than their H 2 -jet-driving counterparts, although these later stages of protostellar evolution (as the source transitions to being a "disc-excess" source) must be very brief, since a large fraction of protostars do drive H 2 flows. We also find that the protostars that power molecul...

We present a catalogue of Molecular Hydrogen emission-line Objects (MHOs) in outflows from young stars, most of which are deeply embedded. All objects are identified in the near-infrared lines of molecular hydrogen, all reside in the Milky Way, and all are associated with jets or molecular outflows. Objects in both low and high-mass star forming regions are included. This catalogue complements the existing database of Herbig-Haro objects; indeed, for completeness, HH objects that are detected in H 2 emission are included in the MHO catalogue.