Star Formation in the Trifid Nebula

40 pages, 16 figures, 5 tables, 2 appendix.-- Pre-print archive.
Aims: 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.
Methods: We have obtained maps of the 1.25 mm thermal dust emission and the molecular gas emission over a region of 20' x 10' around the Trifid Nebula (M 20), 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) ∼1022 cm-2 in column density.
Results: The cloud material is distributed in fragmented dense gas filaments (n(H2) of a few 103 cm-3) with sizes ranging from 1 to 10 pc. A massive filament, WF, with properties typical of Infra Red Dark Clouds, connects M 20 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 100 M⊙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 104 cm-3. They are usually gravitationally unbound and have low masses of a few M⊙. The densest starless cores (several 105 cm-3) are located in condensation TCO, 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.
Conclusions: M 20 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 M 20. We propose that the nearby supernova remnant W28 could have triggered the formation of protostellar clusters in nearby dense cores of the Trifid.
J.R. Pardo and J. Cernicharo acknowledge Spanish Ministerio de Educacion y Ciencia for supporting this research under grants AYA2003-2785, AYA2006-14876, ESP2004-00665, ESP2007-65812- C02-01, Communidad de Madrid for research grant ASTROCAM S- 0505ESP-0237, and the Molecular Universe FP6Marie Curie Research Training Network.