2008A&A...489..157L

We aim to characterize the properties of the prestellar and protostellar condensations to understand the star formation processes at work in a young HII region We have obtained maps of the 1.25 mm thermal dust emission and the molecular gas emission over a region of 20'x10 ' around the Trifid Nebula (M20), with the IRAM 30 m and the CSO telescopes as well as in the mid-infrared wavelength with ISO and SPITZER. Our survey is sensitive to features down to N(H₂) ∼10²²cm^–2 in column density. The cloud material is distributed in fragmented dense gas filaments (n(H₂) of a few 10³cm^–3) with sizes ranging from 1 to 10pc. A massive filament, WF, with properties typical of Infra Red Dark Clouds, connects M20 to the W28 supernova remnant. We find that these filaments pre-exist the formation of the Trifid and were originally self-gravitating. The fragments produced are very massive (typically 100M_☉ or more) and are the progenitors of the cometary globules observed at the border of the HII region. We could identify 33 cores, 16 of which are currently forming stars. Most of the starless cores have typical H₂ densities of a few 10⁴cm^–3. They are usually gravitationally unbound and have low masses of a few M_☉. The densest starless cores (several 10⁵cm^–3) are located in condensation TC0, currently hit by the ionization front, and may be the site for the next generation of stars. The physical gas and dust properties of the cometary globules have been studied in detail and have been found very similar. They all are forming stars. Several intermediate-mass protostars have been detected in the cometary globules and in the deeply embedded cores. Evidence of clustering has been found in the shocked massive cores TC3-TC4-TC5. M20 is a good example of massive-star forming region in a turbulent, filamentary molecular cloud. Photoionization appears to play a minor role in the formation of the cores. The observed fragmentation is well explained by MHD-driven instabilities and is usually not related to M20. We propose that the nearby supernova remnant W28 could have triggered the formation of protostellar clusters in nearby dense cores of the Trifid.