We summarize the first results from the Gould Belt Survey, obtained toward the Aquila rift and Polaris Flare regions during the science demonstration phase of Herschel. Our 70-500 μm images taken in parallel mode with the SPIRE and PACS cameras reveal a wealth of filamentary structure, as well as numerous dense cores embedded in the filaments. Between ∼350 and 500 prestellar cores and ∼45-60 Class 0 protostars can be identified in the Aquila field, while ∼300 unbound starless cores and no protostars are observed in the Polaris field. The prestellar core mass function (CMF) derived for the Aquila region bears a strong resemblance to the stellar initial mass function (IMF), already confirming the close connection between the CMF and the IMF with much better statistics than earlier studies. Comparing and contrasting our Herschel results in Aquila and Polaris, we propose an observationally-driven scenario for core formation according to which complex networks of long, thin filaments form first within molecular clouds, and then the densest filaments fragment into a number of prestellar cores via gravitational instability.

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.

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.

Abstract. Based on the ISO spectral catalogue of compact H ii regions by Peeters et al. (2002), we present a first analysis of the hydrogen recombination and atomic fine-structure lines originated in the ionized gas. The sample consists of 34 H ii regions located at galactocentric distances between R Gal = 0 and 15 kpc. The SWS H i recombination lines between 2 and 8 µm are used to estimate the extinction law at these wavelengths for 14 H ii regions. An extinction in the K band between 0 and ∼3 mag has been derived. The fine-structure lines of N, O, Ne, S and Ar are detected in most of the sources. Most of these elements are observed in two different ionization stages probing a range in ionization potential up to 41 eV. The ISO data, by itself or combined with radio data taken from the literature, is used to derive the elemental abundances relative to hydrogen. The present data thus allow us to describe for each source its elemental abundance, its state of ionization and to constrain the properties of the ionizing star(s). The main results of this study are as follows. The ionization ratios Ar ++ /Ar + , N ++ /N + , S +3 /S ++ and Ne ++ /Ne + , which measure the degree of ionization and to first order, the hardness of the stellar radiation, seem to increase with R Gal . These ionization ratios correlate well with each other, implying that the spectral hardening affects equally the full range of ionizing energies.