S Orionis 70: Just a Foreground Field Brown Dwarf?
Abstract
We examine recent claims that the T-type brown dwarf S Ori 053810.1-203626 (S Ori 70) is a spectroscopically verified low-mass (3+5-1 MJup) member of the 1-8 Myr σ Orionis cluster. Comparative arguments by Martín & Zapatero Osorio asserting that S Ori 70 exhibits low surface gravity spectral features indicative of youth and low mass are invalidated by the fact that their comparison object was not the field T dwarf 2MASS 0559-1404, but rather a nearby background star. Instead, we find that the 1-2.5 μm spectra of S Ori 70 are well matched to older (age~few Gyr) field T6-T7 dwarfs. Moreover, we find that spectral model fits to late-type field T dwarf spectra tend to yield low surface gravities (logg=3.0-3.5), and thus young ages (<~5 Myr) and low masses (<~3 MJup), inconsistent with expected and/or empirical values. Finally, we show that the identification of one T dwarf in the field imaged by Zapatero Osorio et al. is statistically consistent with the expected foreground contamination. Based on the reexamined evidence, we conclude that S Ori 70 may simply be an old, massive (30-60 MJup) field brown dwarf lying in the foreground of the σ Orionis cluster. This interpretation should be considered before presuming the existence of so-called ``cluster planets.''
- Publication:
-
The Astrophysical Journal
- Pub Date:
- April 2004
- DOI:
- 10.1086/382129
- arXiv:
- arXiv:astro-ph/0312285
- Bibcode:
- 2004ApJ...604..827B
- Keywords:
-
- Galaxy: Open Clusters and Associations: Individual: Name: σ Orionis;
- Stars: Planetary Systems: Formation;
- Stars: Formation;
- Stars: Individual: Constellation Name: S Orionis 70;
- Stars: Low-Mass;
- Brown Dwarfs;
- Techniques: Spectroscopic;
- Astrophysics
- E-Print:
- 8 pages, 4 figures, accepted to ApJ