An analysis of star formation with Herschel in the Hi-GAL Survey: II. the tips of the Galactic bar

Context. We present the physical and evolutionary properties of prestellar and protostellar clumps in the Herschel Infrared GALactic plane survey (Hi-GAL) in two large areas centered in the Galactic plane and covering the tips of the long Galactic bar at the intersection with the spiral arms. The areas fall in the longitude ranges 19° <l < 33° and 340° < l < 350°, while latitude is −1° < b < 1°. Newly formed high mass stars and prestellar objects are identified and their properties derived and compared. A study is also presented on five giant molecular complexes at the further edge of the bar, identified through ancillary ^(12)CO(1–0) data from the NANTEN observatory. Aims. One of the goals of this analysis is assessing the role of spiral arms in the star-formation processes in the Milky Way. It is, in fact, still a matter of debate if the particular configuration of the Galactic rotation and potential at the tips of the bar can trigger star formation. Methods. The star-formation rate was estimated from the quantity of proto-stars expected to form during the collapse of massive turbulent clumps into star clusters. The expected quantity of proto-stars was estimated by the possible final cluster configurations of a given initial turbulent clump. This new method was developed by applying a Monte Carlo procedure to an evolutionary model of turbulent cores and takes into account the wide multiplicity of sources produced during the collapse. Results. The star-formation rate density values at the tips are 1.2 ± 0.3 x 10^(-3) M_☉/�yr kpc^2 and 1.5 ± 0:3 x 10^(-3) M_☉/yr kpc^2 and in the first and fourth quadrant, respectively. The same values estimated on the entire field of view, that is including the tips of the bar and background and foreground regions, are 0.9 ± 0.2 x 10^(-3) M_☉/yr kpc^2 and 0.8 ± 0:2 x 10^(-3) M_☉/yr kpc^2. The conversion efficiency indicates the percentage amount of material converted into stars and is approximately 0.8% in the first quadrant and 0.5% in the fourth quadrant, and does not show a significant difference in proximity of the bar. The star forming regions identified through CO contours at the further edge of the bar show star-formation rate and star-formation rate densities larger than the surrounding regions but their conversion efficiencies are comparable. Conclusions. The tips of the bar show an enhanced star-formation rate with respect to background and foreground regions. However, the conversion efficiency shows little change across the observed fields suggesting that the star-formation activity at the bar is due to a large amount of dust and molecular material rather than being due to a triggering process.