Exploring the Coldest Brown Dwarfs with Near-Infrared Colors
Brown dwarfs are cosmic objects that fall between stars and planets. They do not have enough mass to sustain nuclear fusion like stars, so they gradually cool over billions of years. The coldest known type, called Y dwarfs, has temperatures lower than 500 Kelvin (roughly room temperature to oven temperature!). In this paper, Leggett and collaborators investigate these frigid objects using new data from the James Webb Space Telescope (JWST), along with upcoming survey missions such as Euclid and Roman.
Looking at Y Dwarfs in the Near-Infrared
Most of the energy from Y dwarfs comes out at mid-infrared wavelengths, which are difficult to observe from Earth. However, ground-based telescopes and upcoming space missions like Euclid and Roman will focus on the near-infrared part of the spectrum. JWST has already provided the first detailed spectra of the faintest Y dwarfs in this range, allowing astronomers to measure their brightness through different filters and compare them across observing systems.
The authors synthesized new photometry, measurements of brightness through specific filters, for many T and Y dwarfs on the MKO, Euclid, and Roman systems. These synthetic values help astronomers compare observations across telescopes and prepare for future surveys.
A Closer Look at Five Y Dwarfs
From their expanded dataset, the team focused on five Y dwarfs with effective temperatures between 275 and 400 K. These objects span a range of near- to mid-infrared colors, making them good test cases. By comparing the JWST data with ATMO 2020++ models (a set of theoretical brown dwarf atmospheres), Leggett and colleagues found that Y dwarfs with bluer near-infrared colors tend to have lower gravity or more heavy elements (metal-rich) than redder dwarfs. These differences are seen most clearly in the 4–5-micron region, where molecules like carbon monoxide (CO) and carbon dioxide (CO₂) absorb light.
Temperature Trends and Model Comparisons
The study shows that the absolute brightness at 4.6 microns (a specific band called W2) is a strong indicator of a dwarf’s temperature. The authors even provide a polynomial relationship that astronomers can use to estimate temperature from brightness. However, the near-infrared colors show much more scatter, since they depend not only on temperature but also on metallicity and surface gravity. Models often underestimate certain parts of the spectrum, for example, the Y band near 1 micron, which suggests missing opacity sources in the models.
Euclid and Roman Perspectives
The paper also examines how Euclid and Roman will see Y dwarfs. Euclid’s filters are broad, so distinguishing cold dwarfs from distant galaxies and quasars will be tricky without additional information, like measuring how the objects move across the sky. Roman’s filter set is better matched to the bright parts of brown dwarf spectra, though some filters overlap with regions of strong molecular absorption. Both missions, however, will contribute valuable data on these elusive objects once they launch.
Conclusions: Challenges and Opportunities
Leggett and her team highlight that while mid-infrared brightness is a reliable thermometer for Y dwarfs, their near-infrared colors are influenced by a complex mix of chemistry, metallicity, and gravity. Models still fail to capture all the physics, especially for the coldest cases like the famous Y dwarf WISE 0855−07. Improving models of atmospheric chemistry and opacity at short wavelengths will be essential to fully unlock the secrets of these ultra-cool neighbors.
Source: Leggett
