A SURVEY FOR LOW-MASS STARS AND BROWN DWARFS IN THE η CHAMAELEONTIS AND ε CHAMAELEONTIS YOUNG ASSOCIATIONS

摘要

I present the results of a search for new low-mass stars and brown dwarfs in the η Cha and ε Cha young associations. Within radii of 1.5° and 0.5° surrounding η Cha and ε Cha, respectively, I have constructed color-magnitude diagrams from Deep Near Infrared Survey (DENIS) and Two Micron All Sky Survey (2MASS) photometry and have obtained spectra of the candidate low-mass members therein. The five candidates in η Cha are classified as four field M dwarfs and one carbon star. No new members are found in this survey, which is complete for M/solar mass = 0.015-0.15, according to the evolutionary models of Chabrier and Baraffe. Thus, an extended population of low-mass members is not present in η Cha out to 4 times the radius of the known membership. Meanwhile, the three candidate members of ε Cha are classified as young stars, and thus likely members of the association, based on Li absorption and gravity-sensitive absorption lines. These new sources have spectral types of M2.25, M3.75, and M5.75, corresponding to masses of 0.45, 0.25, and 0.09 solar mass by the models of Chabrier and Baraffe. For one of these stars, intense Hα emission, forbidden line emission, and strong K-band excess emission suggest the presence of accretion, an outflow, and a disk, respectively. This young star is also much fainter than expected for an association member at its spectral type, which could indicate that it is seen in scattered light. No brown dwarfs are detected in ε Cha down to the completeness limit of 0.015 solar mass. The absence of brown dwarfs in these associations is statistically consistent with the mass functions measured in star-forming regions, which exhibit only ～2 and ～1 brown dwarfs for stellar samples at the sizes of the η Cha and ε Cha associations.