New Photometry and Spectra of AB Doradus C: An Accurate Mass Determination of a Young Low‐Mass Object with Theoretical Evolutionary Tracks

We present new photometric and spectroscopic measurements for the unique,young, low-mass evolutionary track calibrator AB Dor C. While the new Ksphotometry is similar to that previously published in Close et al. (2005) thespectral type is found to be earlier. Based on new H & K IFS spectra of AB DorC (Thatte et al. 2007; paper 1) we adopt a spectral type of M5.5+/-1.0 for ABDor C. This is considerably earlier than the M8+/-1 estimated in Close et al.(2005) and Nielsen et al. (2005) yet is consistent with the M6+/-1independently derived by Luhman & Potter (2005). However, the spectrumpresented in paper 1 and analyzed here is a significant improvement over anyprevious spectrum of AB Dor C. We also present new astrometry for the systemwhich further supports a 0.090+/-0.005 Msun mass for the system. Once armedwith an accurate spectrum and Ks flux we find L=0.0021+/-0.0005 Lsun andTeff=2925{+170}{-145}K for AB Dor C. These values are consistent with a ~75 Myr0.090+/-0.005 Msun object like AB Dor C according to the DUSTY evolutionarytracks (Chabrier et al. 2000). Hence masses can be estimated from the HRdiagram with the DUSTY tracks for young low-mass objects like AB Dor C.However, we cautiously note that underestimates of the mass from the tracks canoccur if one lacks a proper (continuum preserved) spectra or is relying on NIRfluxes alone.Comment: To appear in the July 10, 2007 issue of the ApJ. 21 pages, 8 figure