Discovery of an M4 Spectroscopic Binary in Upper Scorpius: A Calibration Point for Young Low‐Mass Evolutionary Models

We report the discovery of a new low-mass spectroscopic (SB2) stellar binarysystem in the star-forming region of Upper Scorpius. This object, UScoCTIO5,was discovered by Ardila (2000), who assigned it a spectral class of M4. AKeckI HIRES spectrum revealed it to be double-lined, and we then carried out aprogram at several observatories to determine its orbit. The orbital period is34 days, and the eccentricity is nearly 0.3. The importance of such a discoveryis that it can be used to help calibrate evolutionary models at low masses andyoung ages. This is one of the outstanding problems in the study of formationmechanisms and initial mass functions at low masses. The orbit allows us toplace a lower limit of 0.64 +- 0.02 M_sol on the total system mass. Thecomponents appear to be of almost equal mass. We are able to show that thismass is significantly higher than predicted by evolutionary models for anobject of this luminosity and age, in agreement with other recent results. Moreprecise determination of the temperature and surface gravity of the componentswould be helpful in further solidifying this conclusion.Comment: 17 pages, 4 figures, accepted for publication in Ap