Atmospheric retrieval of Subaru/IRD high-resolution spectrum of the archetype T-type brown dwarf Gl 229 B

TL;DR

This study demonstrates high-resolution atmospheric retrieval of the archetype T-type brown dwarf Gl 229 B using Subaru/IRD spectra, revealing its binary nature, consistent C/O ratio with its host star, and validating molecular line lists, advancing brown dwarf atmospheric characterization.

Contribution

First high-resolution atmospheric retrieval of Gl 229 B using ExoJAX, confirming its binary status and validating molecular line lists, with implications for brown dwarf and exoplanet studies.

Findings

Gl 229 B is a binary object.

C/O ratio matches host star, supporting formation theories.

Validated molecular line lists with high-res spectrum.

Abstract

Brown dwarfs provide a unique opportunity to study atmospheres and their physical and chemical processes with high precision, especially in temperature ranges relevant to exoplanets. In this study, we performed high-resolution ($R \sim 70,000$) spectroscopy using Subaru/IRD ($Y$, $J$, $H$-bands) of the T7.0p-type object Gl 229 B, the first discovered T-type brown dwarf, which orbits an M1V host star at a separation of 33 au. We conducted atmospheric retrieval on the reduced $H$-band spectrum using the high-resolution spectrum model compatible with automatic differentiation and GPU, ExoJAX. In contrast to previous retrieval studies on medium-resolution spectra, we obtained a C/O ratio consistent with that of the host star, aligning with the expected formation process for such a massive brown dwarf. Additionally, based on the strong constraint on temperature from the high-resolution spectrum and previously measured photometric magnitude, our analysis indicates that Gl 229 B is a binary, which was also proposed by Brandt et al. (2021) and recently confirmed by Xuan et al. (2024). Finally, we validated current molecular line lists by leveraging the obtained high-resolution, high signal-to-noise ratio spectrum of this warm ($\sim 900$ K) atmosphere. This study highlights the importance of observing companion brown dwarfs as benchmark objects for establishing characterization techniques for low-mass objects and enhancing our understanding of their atmospheres, given the wealth of available information and the relative ease of observation.