Paper Title: The κ Andromedae System: New Constraints on the Companion Mass, System Age & Further Multiplicity
Authors: Sasha Hinkley, Laurent Pueyo, Jacqueline K. Faherty, Ben R. Oppenheimer, and 28 others
First Author’s Institution: Caltech, Pasadena, California
So What’s the Age?If the star is not a member of the Columba association, then we can’t assume it’s the same age as the cluster stars, and its age must be determined by other methods. As we’ve mentioned previously, measuring the ages of individual stars on the main sequence is a difficult task. In this case, Hinkley et al. find the star has a higher luminosity and a lower surface gravity than would be expected for a main sequence star at a temperature of 11,400 K, as is measured for this star. Both of these measurements suggest the star has begun to evolve off the main sequence, on its path to becoming a red giant. Therefore, its age can be estimated.We must rely on stellar models to determine the age of such stars, and these models are poorly constrained for stars significantly different from our Sun. One example is shown in the figure to the right. The data point shows the temperature and luminosity of the star (both of which are observed directly), while the dotted and dashed lines correspond to theoretical stellar masses and ages. By determining which theoretical model matches the observed quantities, the star’s fundamental parameters can be estimated: this model suggests an age near 2.85 solar masses and an age of 140 million years. By testing many different stellar evolutionary models, the authors use the distribution of estimated ages to determine an measurement and the uncertainty of the age (and mass) of this star. By combining several models, they find the star has an age of 220±100 million years and a mass of 2.8+0.1-0.2 solar masses.
What about the Companion?Since the estimate of the age of the primary star increases, the “Super-Jupiter” companion mass must be higher than previously estimated, as it has had more time to cool. By using the same evolutionary models as the discovery paper, Hinkley et al. determine the object to have a mass of 50+16-13 times the mass of Jupiter. This mass places the object firmly in the brown dwarf, not planetary, regime.The authors also obtained a low-resolution spectrum using the “Project 1640” integral field spectrograph, which can be combined with an adaptive optics system to collect spectra of objects that might not be resolved with a traditional optics system. Their data is shown in the figure to the left. They find the brown dwarf is spectral type L1±1, consistent with what would be expected for a brown dwarf of this mass and age.With all of this information together, it is very likely the companion is a brown dwarf of ~50 Jupiter masses, orbiting a star with an age of ~220 million years and a mass of ~2.8 solar masses. The best way to definitively determine this in the future without of the use of stellar models will be by measuring the position of the companion in time as it orbits its host star to measure a dynamical mass. Since the separation between the two objects is small, this is feasible in the next few years. Stay tuned!