A Survey for Low‐Mass Stars and Brown Dwarfs in the η Chamaeleontis and ε Chamaeleontis Young Associations

I present the results of a search for new low-mass stars and brown dwarfs inthe Eta Cha and Eps Cha young associations. Within radii of 1.5 and 0.5 degsurrounding Eta Cha and Eps Cha, respectively, I have constructedcolor-magnitude diagrams from DENIS and 2MASS photometry and have obtainedspectra of the candidate low-mass members therein. The five candidates in EtaCha are classified as four field M dwarfs and one carbon star. No new membersare found in this survey, which is complete for M_sun=0.015-0.15 according tothe evolutionary models of Chabrier and Baraffe. Thus, an extended populationof low-mass members is not present in Eta Cha out to four times the radius ofthe known membership. Meanwhile, the three candidate members of Eps Cha areclassified as young stars, and thus likely members of the association, based onLi absorption and gravity-sensitive absorption lines. These new sources havespectral types of M2.25, M3.75, and M5.75, corresponding to masses of 0.45,0.25, and 0.09 M_sun by the models of Chabrier and Baraffe. For one of thesestars, intense H(alpha) emission, forbidden line emission, and strong K-bandexcess emission suggest the presence of accretion, an outflow, and a disk,respectively. This young star is also much fainter than expected for anassociation member at its spectral type, which could indicate that it is seenin scattered light. No brown dwarfs are detected in Eps Cha down to thecompleteness limit of 0.015 M_sun. The absence of brown dwarfs in theseassociations is statistically consistent with the mass functions measured instar-forming regions, which exhibit only ~2 and ~1 brown dwarfs for stellarsamples at the sizes of the Eta Cha and Eps Cha associations.Comment: 19 pages, The Astrophysical Journal, 2004, v616 (December 1