Tracing early evolutionary stages of high-mass star formation with molecular lines
SRON Netherlands Institute for Space Research, Landleven 12, 9747 AD
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2 Kapteyn Astronomical Institute, PO Box 800, 9700 AV Groningen, The Netherlands
3 Université Bordeaux 1, Laboratoire d’Astrophysique de Bordeaux, 2 rue de l’Observatoire, BP 89, 33271 Floirac Cedex, France
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4 CNRS/INSU, UMR 5804, BP 89, 33271 Floirac Cedex, France
Received: 27 October 2009
Accepted: 11 June 2000
Context. Despite its major role in the evolution of the interstellar medium, the formation of high-mass stars (M ≥ 10 M⊙) remains poorly understood. Two types of massive star cluster precursors, the so-called massive dense cores (MDCs), have been observed, which differ in terms of their mid-infrared brightness. The origin of this difference has not yet been established and may be the result of evolution, density, geometry differences, or a combination of these.
Aims. We compare several molecular tracers of physical conditions (hot cores, shocks) observed in a sample of mid-IR weakly emitting MDCs with previous results obtained in a sample of exclusively mid-IR bright MDCs. We attempt to understand the differences between these two types of object.
Methods. We present single-dish observations of HDO, HO, SO2, and CH3OH lines at λ = 1.3−3.5 mm. We study line profiles and estimate abundances of these molecules, and use a partial correlation method to search for trends in the results.
Results. The detection rates of thermal emission lines are found to be very similar for both mid-IR quiet and bright objects. The abundances of H2O, HDO (10-13 to 10-9 in the cold outer envelopes), SO2 and CH3OH differ from source to source but independently of their mid-IR flux. In contrast, the methanol class I maser emission, a tracer of outflow shocks, is found to be strongly anti-correlated with the 12 m source brightnesses.
Conclusions. The enhancement of the methanol maser emission in mid-IR quiet MDCs may be indicative of a more embedded nature. Since total masses are similar between the two samples, we suggest that the matter distribution is spherical around mid-IR quiet sources but flattened around mid-IR bright ones. In contrast, water emission is associated with objects containing a hot molecular core, irrespective of their mid-IR brightness. These results indicate that the mid-IR brightness of MDCs is an indicator of their evolutionary stage.
Key words: ISM: abundances / evolution / stars: formation / line: profiles
© ESO, 2010