1. Steward Observatory, University of Arizona, Tucson, Arizona 85721, USA
2. Max-Plank-Institut fur Astronomie, Konigstuhl 17, D-69117 Heidelberg, Germany
3. Departament d'Astronomia i Astrofisica, Universitat de Valencia, E-46100 Burjassot, Valencia, Spain
4. European Southern Observatory, Alonso de Cordova 3107, Santiago 19, Chile

Correspondence to: Laird M. Close¹ Correspondence and requests for materials should be addressed to L.M.C. (Email: lclose@as.arizona.edu).

Abstract

Mass is the most fundamental parameter of a star, yet it is also one of the most difficult to measure directly. In general, astronomers estimate stellar masses by determining the luminosity and using the 'mass–luminosity' relationship^{1, 2}, but this relationship has never been accurately calibrated for young, low-mass stars and brown dwarfs³. Masses for these low-mass objects are therefore constrained only by theoretical models^{1, 2}. A new high-contrast adaptive optics camera^{4, 5, 6} enabled the discovery of a young (50 million years) companion only 0.156 arcseconds (2.3 au) from the more luminous (> 120 times brighter) star AB Doradus A. Here we report a dynamical determination of the mass of the newly resolved low-mass companion AB Dor C, whose mass is 0.090 0.005 solar masses. Given its measured 1–2-micrometre luminosity, we have found that the standard mass–luminosity relations^{1, 2} overestimate the near-infrared luminosity of such objects by about a factor of 2.5 at young ages. The young, cool objects hitherto thought to be substellar in mass are therefore about twice as massive, which means that the frequency of brown dwarfs and planetary mass objects in young stellar clusters has been overestimated.

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