Journal
ASTRONOMY & ASTROPHYSICS
Volume 377, Issue 2, Pages L9-L13Publisher
EDP SCIENCES S A
DOI: 10.1051/0004-6361:20011152
Keywords
giant planet formation; open clusters and associations : individual : sigma Orionis; stars : brown dwarfs
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We have obtained low resolution optical spectra of 15 isolated planetary mass objects (IPMOs) in the sigma Orionis cluster, and derived spectral types by comparison with nearby M and L dwarfs. The spectral types are in the range late M - mid L, in agreement with our expectations based on colors and magnitudes for bona fide members. Therefore, most of these objects have masses below the deuterium burning limit. About 2/3 show H alpha in emission at our spectral resolution. From our spectroscopic and photometric data, we infer that three IPMOs in this sample may be binaries with components of similar masses. These results confirm that the substellar mass function of the sigma Orionis cluster, in the form dN/dM, keeps rising in the planetary domain.
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