Parallaxes, Proper Motions, and Near-Infrared Photometry for 173 L and T Dwarfs From The US Naval Observatory Infrared Astrometry Program

TL;DR

This work delivers high-precision near-infrared parallaxes and proper motions for 173 L- and T-type dwarfs observed with USNO/NOFS ASTROCAM across two long baselines, achieving median parallax uncertainty near 1.5 mas and proper-motion uncertainty near 1 mas yr$^{-1}$. It provides a uniform set of UKIRT/MKO $JHK$ photometry and constructs absolute magnitudes and color–magnitude diagrams that illuminate the L/T transition, subdwarfs, binaries, and spectral standards. An essential element is the Gaia DR3-based correction from relative to absolute quantities, allowing direct cross-checks with Gaia DR3 for 40 common objects and enabling an assessment of binarity effects on astrometry. The results refine the distance scale and kinematics of ultracool dwarfs, support population studies (young objects, wide companions, and subdwarfs), and establish a valuable benchmark set of brown-dwarf standards with well-determined distances and motions.

Abstract

We present near-infrared parallax and proper motion astrometry for 74 L-dwarfs and 99 T-dwarfs, as single objects or in binary systems, obtained with the ASTROCAM astrometric imager on the USNO, Flagstaff Station 1.55-m telescope over two observing periods. For all 173 objects the median number of observational epochs was 62 with a median time frame of 5.25 years, resulting in median uncertainties of $σ$($π_{abs}$) = 1.51 mas, $σ$($μ_{abs}$) = 1.02 mas yr$^{-1}$, and $σ$($V_{\rm tan}$) = 1.01 km s$^{-1}$. Our observations provide the first parallax/proper motion results for 16 objects and the highest precision parallaxes/proper motions for an additional 116 objects. A serendipitous overlap of 40 objects with Gaia DR3 astrometry allows direct comparison and confirmation of our results, along with an investigation on the effects of resolved binarity on astrometric results. We also provide a uniform set of $J$-, $H$-, $K_{S}$-band photometry in the UKIRT/MKO system, most of it being from new observations. We use these results to examine objects included in this study of special-interest populations, consisting of binaries, wide companions, young objects, subdwarfs, and brown dwarf spectral standards.

Figures (17)

• Figure 1: Distributions of $\pi_{abs}$ and $\sigma$($\pi_{abs}$) along with median values for the combined Series 1 and Series 2 results.
• Figure 2: Distributions of $\mu$$_{abs}$ and $\sigma$($\mu$$_{abs}$) along with median values for combined Series 1 and Series 2 results.
• Figure 3: Distributions of $V_{\rm tan}$ and $\sigma$($V_{\rm tan}$) along with median values. The red lines correspond to the $V_{\rm tan}$ velocities of the eight verified subdwarfs in this study (see text).
• Figure 4: Comparison of relative parallaxes determined in the $\alpha$ and $\delta$ directions independently. Blue points display Series 1 data while red points display Series 2 data.
• Figure 5: A plot of USNO versus Gaia $\pi_{abs}$ values for the 40 objects in common. Error bars for both sets of data are plotted along with the solid red line being the best fit to the data weighted by ($\sigma$($\pi_{abs}$ USNO$))^{-2}$, and a resulting slope of 1.006.