The Luminosities of the Coldest Brown Dwarfs

TL;DR

This study provides new distance measurements for the coldest brown dwarfs, confirming they form a continuation of the T dwarf sequence and revealing diverse atmospheric properties, including cloud presence and variability potential.

Contribution

It offers the first comprehensive set of distances for Y dwarfs, advancing understanding of their luminosities, temperatures, and atmospheric cloud characteristics compared to prior models.

Findings

Y dwarfs extend the T dwarf sequence to lower luminosities and temperatures.

The coolest brown dwarfs show a wide range of absolute magnitudes at similar colors.

Atmospheric models generally agree with observed magnitudes, but some objects suggest binarity or cloud effects.

Abstract

In recent years brown dwarfs have been extended to a new Y-dwarf class with effective temperatures colder than 500K and masses in the range 5-30 Jupiter masses. They fill a crucial gap in observable atmospheric properties between the much colder gas-giant planets of our own Solar System (at around 130K) and both hotter T-type brown dwarfs and the hotter planets that can be imaged orbiting young nearby stars (both with effective temperatures of in the range 1500-1000K). Distance measurements for these objects deliver absolute magnitudes that make critical tests of our understanding of very cool atmospheres. Here we report new distances for nine Y dwarfs and seven very-late T dwarfs. These reveal that Y dwarfs do indeed represent a continuation of the T dwarf sequence to both fainter luminosities and cooler temperatures. They also show that the coolest objects display a large range in absolute magnitude for a given photometric colour. The latest atmospheric models show good agreement with the majority of these Y dwarf absolute magnitudes. This is also the case for WISE0855-0714 the coldest and closest brown dwarf to the Sun, which shows evidence for water ice clouds. However, there are also some outstanding exceptions, which suggest either binarity or the presence of condensate clouds. The former is readily testable with current adaptive optics facilities. The latter would mean that the range of cloudiness in Y dwarfs is substantial with most hosting almost no clouds -- while others have dense clouds making them prime targets for future variability observations to study cloud dynamics.