An analysis of star formation withHerschelin the Hi-GAL Survey

Abstract

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 340Newly 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 10 −3 M⊙ yr kpc 2 and 1.5 ± 0.3 10 −3 M⊙ yr kpc 2 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 10 −3 M⊙ yr kpc 2 and 0.8 ± 0.2 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.

Evolution of young protoclusters embedded in dense massive clumps. A new grid of population synthesis SED models and a new set of L/M evolutionary tracks

“…The 12 best fit models (in the χ 2 range selected) are all very similar in shape, and isolate a well defined area of parameters space. The models suggest that a cluster of young protostars is present, with a total stellar mass of ∼200M , corresponding to a present-day Star Formation Rate of 3.8 × 10 −4 M /yr, this time in relatively good agreement with the SFR value of ∼ 1.5×10 −4 M /yr computed with the method of Veneziani et al (2017). In this case our present tracks diagnose IRAS05137+3919 to be populated with ZAMS stars, representing a system that is closer (with respect to the previously examined source) to the terminal stage of its evolutionary path toward UCHii regions.…”

“…The SFR computed according to this method in IRAS19092+0841 is ∼ 6.3 × 10 −4 M yr −1 , that is about 200 times lower than the value given by the present tracks (0.13, see above). This is not surprising as in this work we are comparing clumps masses and luminosities with tracks that provide the present estimated embedded stellar mass and age, while Veneziani et al (2017) uses simulated embedded stellar masses and ages at the time the clump has reached luminosities typical of UCHii, therefore providing a terminal estimate of the SFR of the clump averaged over its early evolutionary path. Quite speculatively, the comparison would suggest that in IRAS19092+0841 we are probably looking at a very recent burst of star formation, where the future evolution will see an increase in the luminosity of the present protostellar objects rather than a further increase in their number.…”

“…The models suggest that a cluster of young protostars is present, with a current total stellar mass of ∼1800M , corresponding to a present-day Star Formation Rate of 0.13 M /yr. Recently Veneziani et al (2017) provided a method to estimate the Star Formation Rate of massive clumps from their mass. In short, upgrading an earlier prescription from Veneziani et al (2013), the clump mass is compared to our earlier set of evolutionary tracks Molinari et al (2008) where the clump, instead of a single forming massive star, hosts a cluster whose mass is simulated using Monte Carlo realizations of a star cluster following the IMF of Kroupa (2001) and producing the same bolometric luminosity typically observed for clumps of the same total mass but associated with UCHii regions.…”

“…While this is a good approximation for typical (massive) YSOs detected by GLIMPSE in the mid-infrared with ∼2 resolution (Morales & Robitaille 2017), it is unlikely to be the case for most compact sources revealed in the far-IR or submillimeter by single-dish facilities (e.g., Herschel) on the Galactic Plane (see above). In the case that multiple objects are indeed present in observed far-IR compact sources, assuming that sources are single objects would result in different star formation rates and efficiencies than would have been derived if higher-resolution data had been available and the multiple sources had been resolved (Veneziani et al 2017). This issue is particularly pressing for the systematic analysis of extensive compact source catalogues produced by the latest generation of Galactic Plane surveys in the infrared and submillimeter (Molinari et al 2016b;Elia et al 2017;Ginsburg et al 2013;Csengeri et al 2014), where the objects that are detected are in their vast majority clumps large enough to host multiple YSOs, and not cores.…”

The Hi-GAL compact source catalogue – I. The physical properties of the clumps in the inner Galaxy (−71$_{.}^{\circ}$0 < ℓ < 67$_{.}^{\circ}$0)

“…Hi-GAL observations of the fourth Galactic quadrant have been already used for studying InfraRed Dark Clouds (IRDCs, Egan et al 1998) in the 300 • 30 • , |b| ≤ 1 • range (Wilcock et al 2012a,b), highlighting the fundamental role of the Herschel FIR data for exploring the internal structure of these candidate sites for massive star formation. Furthermore, Veneziani et al (2017) used the catalogue presented here to study the compact source population in the far tip of the long Galactic bar. These data will be exploited, if needed, in this article as well, for instance for comparison between the fields studied in this paper and inner regions of the Galaxy.…”