The earliest phases of high-mass star formation: a 3 square degree millimeter continuum mapping of Cygnus X
Aims:Our current knowledge of high-mass star formation is mainly based on follow-up studies of bright sources found by IRAS, and is thus biased against its earliest phases, inconspicuous at infrared wavelengths. We therefore started searching, in an unbiased way and in the closest high-mass star-forming complexes, for the high-mass analogs of low-mass pre-stellar cores and class 0 protostars.
Methods: We have made an extensive 1.2 mm continuum mosaicing study of the
Results: Our complete study of Cygnus X with ~0.09 pc resolution provides, for the first time, an unbiased census of massive young stellar objects. We discover 129 massive dense cores (FWHM size ~0.1 pc, M1.2~mm = 4-950 M_⊙, volume-averaged density ~105 cm-3), among which ~42 are probable precursors of high-mass stars. A large fraction of the Cygnus X dense cores (2/3 of the sample) remain undetected by the MSX satellite, regardless of the mass range considered. Among the most massive (≥40 M_⊙) cores, infrared-quiet objects are driving powerful outflows traced by SiO emission. Our study qualifies 17 cores as good candidates for hosting massive infrared-quiet protostars, while up to 25 cores potentially host high-luminosity infrared protostars. We fail to discover the high-mass analogs of pre-stellar dense cores (~0.1 pc, > 104 cm-3) in Cygnus X, but find several massive starless clumps (~ 0.8 pc, 7 × 103 cm-3) that might be gravitationally bound.
Conclusions: Since our sample is derived from a single molecular complex and covers every embedded phase of high-mass star formation, it gives the first statistical estimates of their lifetime. In contrast to what is found for low-mass class 0 and class I phases, the infrared-quiet protostellar phase of high-mass stars may last as long as their better-known high-luminosity infrared phase. The statistical lifetimes of high-mass protostars and pre-stellar cores (~ 3 × 104 yr and < 103 yr) in Cygnus X are one and two order(s) of magnitude smaller, respectively, than what is found in nearby, low-mass star-forming regions. We therefore propose that high-mass pre-stellar and protostellar cores are in a highly dynamic state, as expected in a molecular cloud where turbulent processes dominate.
Astronomy and Astrophysics
- Pub Date:
- December 2007
- H ii regions;
- ISM: structure;
- stars: formation;
- 32 pages, 62 figures to be published in Astronomy &