The absolute integrated fluxes that we obtained (Paper I) for 34 H ii regions, with Galactocentric distances RG in the 6.6–17.7 kpc range, in the emission lines [O ii] λλ3726 and 3729, [O iii] λλ4363 and 5007, He iλ5876, Hα and Hβ, are analysed to derive the extinctions, the electron densities and temperatures, the ionic abundances O+/H+, O++/H+ and He+/H+ and the O/H abundances. The electron temperature has been derived from the [O iii] λ4363/λ5007 ratio in six H ii regions with RG between 6.6 and 14.8 kpc. These new measurements of Te, which are in good agreement with those from radio recombination lines, widen the RG range for which reliable Te measurements exist. Combining our new Te([O iii]) results with the radio values, we obtain the following temperature relationship: TeK]=(372±38)RG+4260±350. The O+/H+ and O++/H+ abundances have been obtained assuming a two‐temperature H ii region model. Our O/H relationship for 5 kpc

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. Young massive stars or clusters are often observed at the peripheries of H  regions. What triggers star formation at such locations? Among the scenarios that have been proposed, the "collect and collapse" process is particularly attractive because it permits the formation of massive objects via the fragmentation of the dense shocked layer of neutral gas surrounding the expanding ionized zone. However, until our recent article on Sh 104, it had not been convincingly demonstrated that this process actually takes place. In the present paper we present our selection of seventeen candidate regions for this process; all show high-luminosity near-IR clusters and/or mid-IR point sources at their peripheries. The reality of a "collect and collapse" origin of these presumably second-generation stars and clusters will be discussed in forthcoming papers, using new near-IR and millimetre observations.

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.

Abstract. Young massive stars or clusters are often observed at the peripheries of H  regions. What triggers star formation at such locations? Among the scenarios that have been proposed, the "collect and collapse" process is particularly attractive because it permits the formation of massive objects via the fragmentation of the dense shocked layer of neutral gas surrounding the expanding ionized zone. However, until our recent article on Sh 104, it had not been convincingly demonstrated that this process actually takes place. In the present paper we present our selection of seventeen candidate regions for this process; all show high-luminosity near-IR clusters and/or mid-IR point sources at their peripheries. The reality of a "collect and collapse" origin of these presumably second-generation stars and clusters will be discussed in forthcoming papers, using new near-IR and millimetre observations.