We present the first results from the science demonstration phase for the Hi-GAL survey, the Herschel key program that will map the inner Galactic plane of the Milky Way in 5 bands. We outline our data reduction strategy and present some science highlights on the two observed 2 • × 2 • tiles approximately centered at l = 30 • and l = 59 • . The two regions are extremely rich in intense and highly structured extended emission which shows a widespread organization in filaments. Source SEDs can be built for hundreds of objects in the two fields, and physical parameters can be extracted, for a good fraction of them where the distance could be estimated. The compact sources (which we will call cores' in the following) are found for the most part to be associated with the filaments, and the relationship to the local beam-averaged column density of the filament itself shows that a core seems to appear when a threshold around A V ∼ 1 is exceeded for the regions in the l = 59 • field; a A V value between 5 and 10 is found for the l = 30 • field, likely due to the relatively higher distances of the sources. This outlines an exciting scenario where diffuse clouds first collapse into filaments, which later fragment to cores where the column density has reached a critical level. In spite of core L/M ratios being well in excess of a few for many sources, we find core surface densities between 0.03 and 0.5 g cm −2 . Our results are in good agreement with recent MHD numerical simulations of filaments forming from large-scale converging flows.

Context. To investigate the process of star formation triggered by the expansion of an H ii region, we present a multi-wavelength analysis of the Galactic H ii region RCW 120 and its surroundings. The collect and collapse model predicts that the layer of gas and dust accumulated between the ionization and shock fronts during the expansion of the H ii region collapses and forms dense fragments, giving rise to potential sites of massive-star formation. Aims. The aim of our study is to look for such massive fragments and massive young stars on the borders of RCW 120. Methods. We mapped the RCW 120 region in the cold dust continuum emission at 1.2 mm to search for these fragments. We supplemented this study with the available near-(2MASS) and mid-IR (GLIMPSE) data to locate the IR sources observed towards this region and to analyse their properties. We then compared the observational results with the predictions of Hosokawa & Inutsuka's model (2005, ApJ, 623, 917; 2006, ApJ, 646, 240). Results. At 1.2 mm we detected eight fragments towards this region, five located on its borders. The largest fragment has a mass of about 370 M . Class I and Class II young stellar objects are detected all over the region, with some observed far from the ionization front. This result emphasises the possible importance of distant interactions between the radiation, escaping from the ionized region, and the surrounding medium.

ABSTRACT. Hi-GAL, the Herschel infrared Galactic Plane Survey, is an Open Time Key Project of the Herschel Space Observatory. It will make an unbiased photometric survey of the inner Galactic plane by mapping a 2°wide strip in the longitude range |l| < 60°in five wavebands between 70 μm and 500 μm. The aim of Hi-GAL is to detect the earliest phases of the formation of molecular clouds and high-mass stars and to use the optimum combination of Herschel wavelength coverage, sensitivity, mapping strategy, and speed to deliver a homogeneous census of starforming regions and cold structures in the interstellar medium. The resulting representative samples will yield the variation of source temperature, luminosity, mass and age in a wide range of Galactic environments at all scales from massive YSOs in protoclusters to entire spiral arms, providing an evolutionary sequence for the formation of intermediate and high-mass stars. This information is essential to the formulation of a predictive global model of the role of environment and feedback in regulating the star-formation process. Such a model is vital to understanding star formation on galactic scales and in the early universe. Hi-GAL will also provide a science legacy for decades to come with incalculable potential for systematic and serendipitous science in a wide range of astronomical fields, enabling the optimum use of future major facilities such as JWST and ALMA.

Context. This study deals with the star formation triggered by H ii regions.Aims. We wish to take advantage of the very simple morphology of RCW 120 -a perfect bubble -to understand the mechanisms triggering star formation around an H ii region and to establish what kind of stars are formed there.Methods. We present 870 μm observations of RCW 120, obtained with the APEX-LABOCA camera. These show the distribution of cold dust, and thus of neutral material. We use Spitzer-MIPS observations at 24 μm and 70 μm to detect the young stellar objects present in this region and to estimate their evolutionary stages.Results. A layer of dense neutral material surrounds the entire H ii region, having been swept up during the region's expansion.This layer has a mass greater than 2000 M and is fragmented, with massive fragments elongated along the ionization front (IF). We measured the 24 μm flux of 138 sources. Of these, 39 are Class I or flat-spectrum young stellar objects (YSOs) observed in the direction of the collected layer. We show that several triggering mechanisms are acting simultaneously in the swept-up shell, where they form a second generation of stars. No massive YSOs are detected. However, a massive, compact 870 μm core lies adjacent to the IF. A 70 μm source with no 24 μm counterpart is detected at the same position. This source is a likely candidate for a Class 0 YSO. Also at 24 μm, we detect a chain of about ten regularly spaced Class I or flat spectrum sources, parallel to the IF, in the direction of the most massive fragment. We suggest that the formation of these YSOs is the result of Jeans gravitational instabilities in the collected layer. Finally, the 870 μm emission, the 24 μm emission, and the Hα emission show the existence of an extended and partially ionized photodissociation region around RCW 120. This demonstrates the long-distance influence of the H ii region upon its surrounding medium.

Aims. We study three Galactic H ii regions -RCW 79, RCW 82, and RCW 120 -where triggered star formation is taking place. Two stellar populations are observed: the ionizing stars of each H ii region and young stellar objects on their borders. Our goal is to show that they represent two distinct populations, as expected from successive star-forming events. Methods. We use near-infrared integral field spectroscopy obtained with SINFONI on the VLT to make a spectral classification. We derived the stellar and wind properties of the ionizing stars using atmosphere models computed with the code CMFGEN. The young stellar objects were classified according to their K-band spectra. In combination with published near and mid infrared photometry, we constrained their nature. Linemaps were constructed to study the geometry of their close environment. Results. We identify the ionizing stars of each region. RCW 79 is dominated by a cluster of a dozen O stars, identified for the first time by our observations. RCW 82 and RCW 120 are ionized by two and one O star, respectively. All ionizing stars are early-to-late O stars, close to the main sequence. The cluster ionizing RCW 79 formed 2.3 ± 0.5 Myr ago. Similar ages are estimated, albeit with a larger uncertainty, for the ionizing stars of the other two regions. The total mass-loss rate and ionizing flux is derived for each region. In RCW 79, where the richest cluster of ionizing stars is found, the mechanical wind luminosity represents only 0.1% of the ionizing luminosity, questioning the influence of stellar winds on the dynamics of these three H ii regions. The young stellar objects show four main types of spectral features: H 2 emission, Brγ emission, CO bandheads emission, and CO bandheads absorption. These features are typical of young stellar objects surrounded by disks and/or envelopes, confirming that star formation is taking place on the borders of the three H ii regions. The radial velocities of most YSOs are consistent with that of the ionized gas, firmly establishing their association with the H ii regions. Exceptions are found in RCW 120 where differences up to 50 km s −1 are observed. Outflows are detected in a few YSOs. All YSOs have moderate-to-strong near-IR excess. In the [24] versus K-[24] diagram, the majority of the sources dominated by H 2 emission lines stand out as redder and brighter than the rest of the YSOs. The quantitative analysis of their spectra indicates that, for most of them, the H 2 emission is essentially thermal and likely produced by shocks. We tentatively propose that they represent an earlier phase of evolution compared to sources dominated by Brγ and CO bandheads. We suggest that they still possess a dense envelope in which jets or winds create shocks. The other YSOs have partly lost their envelopes and show signatures of accretion disks. Overall, the YSOs show distinct spectroscopic signatures compared to the ionizing sources, confirming the presence of two stellar populations.