Probing Late-type T dwarf J-H Color Outliers for Signs of Age

TL;DR

This study investigates late-type T dwarfs with unusual J-H colors using spectroscopy to identify signs of age, gravity, and metallicity, revealing some objects as potential old or young outliers and highlighting the need for improved atmospheric models.

Contribution

The paper provides new spectroscopic analysis of J-H color outliers among late-type T dwarfs, linking spectral features to age and metallicity, and discusses the limitations of current atmospheric models.

Findings

Some objects show spectral features indicating potential old age or youth.

Spectral morphologies are generally consistent with standards, despite color outliers.

Model fits vary significantly depending on the atmospheric model used.

Abstract

We present the results of a Keck/NIRSPEC follow-up survey of thirteen late-type T dwarfs (T6-T9), twelve of which have unusually red or blue J-H colors. Previous work suggests that J-H color outliers may represent the high-gravity, low-metallicity (old) and low-gravity, solar-metallicity (young) extremes of the late-type T dwarf population. We use medium-resolution Y- and H-band spectroscopy to probe regions of T dwarf atmospheres that are more sensitive to gravity and metallicity variations than the J band. We find that the spectral morphologies of our sample are largely homogeneous, with peak-normalized, Y- and H-band morphologies consistent with spectral standards. However, three objects stand out as potentially old, with overluminous Y-band spectra compared to their respective spectral standards, and a fourth object stands out as potentially young, with an underluminous Y band. Of these four objects, three have been previously identified as potential metallicity/gravity outliers, including the one object in our sample with a normal J-H color. We fit publicly available atmospheric model grids to our spectra and find that the best-fit physical parameters vary depending on the model used. As we continue to probe the characteristics of the late-T population, differences in synthetic spectra of ~10-20% in the blue wing of the Y band and ~45% at 1.65 microns, for the same physical parameters, must be reconciled. Further development and public availability of nonsolar metallicity models is also recommended. Future progress toward deciphering the impacts of gravity, metallicity, and variability in the late-type T dwarf population will also require high signal-to-noise, multiwavelength and multi-epoch photometry and spectroscopy.