Spectroscopy of candidate members of the η chamaeleontis and MBM 12 young associations

We present an analysis of candidate members of the η Cha and MBM 12A young associations. For an area of 0.7 deg² toward η Cha, we have performed a search for members of the association by combining JHK_s photometry from the Two Micron All Sky Survey and i photometry from DENIS with follow-up optical spectroscopy at Magellan Observatory. We report the discovery of three new members with spectral types of M5.25-M5.75, corresponding to masses of 0.13-0.08 M_⊙ by theoretical evolutionary models. Two and three of these members were found independently by Lyo and coworkers and Song and coworkers, respectively. Meanwhile, no brown dwarfs were detected in η Cha down to the completeness limit of 0.015 M_⊙. For MBM 12A, we have obtained spectra of three of the remaining candidate members that lacked spectroscopy at the end of the survey by Luhman, all of which are found to be field M dwarfs. Ogura and coworkers have recently presented four "probable" members of MBM 12A. However, two of these objects were previously classified as field dwarfs by the spectroscopy of Luhman. In this work, we find that the other two objects are field dwarfs as well.