The Kinematic Age of the Coolest T Dwarfs

TL;DR

This study uses kinematic data of 75 T dwarfs to estimate their ages, finding they are generally around 2 billion years old, which challenges some atmospheric model predictions about their youth.

Contribution

It provides the first comprehensive kinematic analysis of a large sample of mid to late T dwarfs to independently estimate their ages.

Findings

Mid and late T dwarf populations have similar velocity distributions.

Estimated mean age of T dwarf population is about 2 Gyr.

Kinematic ages suggest models may overestimate the youth of these objects.

Abstract

Surprisingly, current atmospheric models suggest that the coolest T dwarfs (T8.5 to T10) are young and very low mass (0.06-2Gyr, 5-20Mjup, Leggett et al.2009, 2010, 2012). Studies of population kinematics offer an independent constraint on the age of the population. We present kinematic data of a sample of 75 mid to late T dwarfs drawn from a variety of sources. We define our samples, T5.5 to T8 and T8.5 to T10, as mid and late T respectively. UKIDSS LAS kinematics were derived from our automated LAS proper motion pipeline and distance estimates derived from spectral types and photometry for the minority of sources that lack parallaxes. Our results show that the mid and late T populations do not have distinctly separate tangential velocity distributions to 95% probability. They also give an approximate mean kinematic age equal to that of a population with B-V colour 0.51-0.54, and a spectral type late F, which corresponds to an age of about 2 Gyr. However the median and modal ages are greater. This indicates that while model atmospheres correctly predict some trends in colour with gravity and age, reliable ages cannot yet be inferred from them. More benchmark objects are needed to anchor the models.