Astron. Astrophys. 347, L15-L18 (1999)

4. Multiplicity of the massive Trapezium stars

Several recent studies (P98; Simon et al. 1999) have concordantly found that the binary frequency of the low-mass stars in the Orion nebula cluster (ONC) is comparable to that of solar type field stars, which is about 60% with a median number of companions per primary of about 0.5 (c.f. Duquennoy & Mayor 1991; Fischer & Marcy 1992). The binary frequency of O-type and early B-type field stars seems to be similar (cf. Abt 1983; Mason et al. 1998).

Our detection of C₂ increases the number of known companions to the four Trapezium stars to 7. The average number of at least 1.75 companions per primary among the high-mass Trapezium stars is clearly higher than the corresponding number for the low-mass stars in the ONC. A similar trend has been found by Abt et al. (1991) and Morrell & Levato (1991): most of the spectroscopic binaries in the ONC are among the O- and early B-type stars, and much less frequent among the later B- and A-type stars. This finding suggests different formation mechanisms for the high-mass and low-mass multiple systems. This is consistent with the recent results of Bonnell et al. (1998) who assumed that high-mass stars form through accretion-induced collisions of protostars. Their theory predicts that close binary systems should be very common among the massive stars. This is supported by our results.

© European Southern Observatory (ESO) 1999

Online publication: June 30, 1999
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