Benchmark cool companions: ages and abundances for the PZ Telescopii system

Abstract

We present new ages and abundance measurements for the pre-main-sequence star PZ Telescopii (more commonly known as PZ Tel). PZ Tel was recently found to host a young and low-mass companion. Such companions, whether they are brown dwarfs or planetary systems, can attain benchmark status by detailed study of the properties of the primary, and then evolutionary and bulk characteristics can be inferred for the companion. Using Fibre-fed Extended Range Optical Spectrograph spectra, we have measured atomic abundances (e.g. Fe and Li) and chromospheric activity for PZ Tel and used these to obtain the metallicity and age estimates for the companion. We have also determined the age independently using the latest evolutionary models. We find PZ Tel A to be a rapidly rotating (v sin i= 73 +/- 5 km s-1), approximately solar metallicity star [log N(Fe) =-4.37 +/- 0.06 dex or [Fe/H] = 0.05 +/- 0.20 dex]. We measure a non-local thermodynamic equilibrium lithium abundance of log N(Li) = 3.1 +/- 0.1 dex, which from depletion models gives rise to an age of 7 Myr for the system. Our measured chromospheric activity ( of -4.12) returns an age of 26 +/- 2 Myr, as does fitting pre-main-sequence evolutionary tracks (tevol= 22 +/- 3 Myr), both of these are in disagreement with the lithium age. We speculate on reasons for this difference and introduce new models for lithium depletion that incorporate both rotation and magnetic field effects. We also synthesize solar, metal-poor and metal-rich substellar evolutionary models to better determine the bulk properties of PZ Tel B, showing that PZ Tel B is probably more massive than previous estimates, meaning the companion is not a giant exoplanet, even though a planetary-like formation origin can go some way to describing the distribution of benchmark binaries currently known. We show how PZ Tel B compares to other currently known age and metallicity benchmark systems and try to empirically test the effects of dust opacity as a function of metallicity on the near-infrared colours of brown dwarfs. Current models suggest that in the near-infrared observations are more sensitive to low-mass companions orbiting more metal rich stars. We also look for trends between infrared photometry and metallicity amongst a growing population of substellar benchmark objects, and identify the need for more data in massagemetallicity parameter space.