A Clear View of a Cloudy Brown Dwarf Companion from High-resolution Spectroscopy

Abstract

Direct imaging studies have mainly used low-resolution spectroscopy (R ∼ 20–100) to study the atmospheres of giant exoplanets and brown dwarf companions, but the presence of clouds has often led to degeneracies in the retrieved atmospheric abundances (e.g., carbon-to-oxygen ratio, metallicity). This precludes clear insights into the formation mechanisms of these companions. The Keck Planet Imager and Characterizer (KPIC) uses adaptive optics and single-mode fibers to transport light into NIRSPEC (R ∼ 35,000 in the K band), and aims to address these challenges with high-resolution spectroscopy. Using an atmospheric retrieval framework based on petitRADTRANS, we analyze the KPIC high-resolution spectrum (2.29–2.49 μm) and the archival low-resolution spectrum (1–2.2 μm) of the benchmark brown dwarf HD 4747 B (m = 67.2 ± 1.8 M Jup, a = 10.0 ± 0.2 au, T eff ≈ 1400 K). We find that our measured C/O and metallicity for the companion from the KPIC high-resolution spectrum agree with those of its host star within 1σ–2σ. The retrieved parameters from the K-band high-resolution spectrum are also independent of our choice of cloud model. In contrast, the retrieved parameters from the low-resolution spectrum are highly sensitive to our chosen cloud model. Finally, we detect CO, H2O, and CH4 (volume-mixing ratio of log(CH4) = −4.82 ± 0.23) in this L/T transition companion with the KPIC data. The relative molecular abundances allow us to constrain the degree of chemical disequilibrium in the atmosphere of HD 4747 B, and infer a vertical diffusion coefficient that is at the upper limit predicted from mixing length theory.

Retrieving C and O Abundance of HR 8799 c by Combining High- and Low-resolution Data

“…This is not surprising given that the most constraining power comes from the lower-resolution data sets. We follow Xuan et al (2022) to quantify the relative constraining power between the high-resolution KPIC and the lower-resolution IFU data. The detection significance using KPIC high-resolution data is ∼10 (Wang et al 2021).…”

“…An alternative explanation is that high spectral resolution data are more effective in constraining chemical abundances and atmospheric properties (for HD 4747 B, Xuan et al 2022). However, in the case of HR 8799 c, we prefer results using DS III over DS II for the following reasons.…”

“…Second, our lowresolution spectral and photometric data covers a large wavelength from J to M band. The large wavelength coverage includes a spectral range from 2.2 to 2.5 μm and the L and M band, which are thought to be crucial in constraining chemical abundances (Xuan et al 2022) and cloud properties (Wang et al 2020). This is a key difference between the HD 4747 B data and the HR 8799 c data.…”

“…However, some of the issues in Wang et al (2020) had been addressed with, e.g., a more flexible temperature profile and a more realistic cloud treatment (Wang et al 2022a). Consequently, promising results had been shown when analyzing data from a benchmark brown dwarf: consistent chemical abundances and C/O ratios can be retrieved for benchmark brown dwarfs and their host stars (Wang et al 2022a;Xuan et al 2022). The promising results motivate a new retrieval analysis for the HR 8799 system.…”

“…With high spectral resolution data, they measured previously inaccessible properties such as planet projected rotational velocities. Furthermore, higher spectral resolution can better resolve molecular lines and provide key constraints on chemical compositions even in cloudy conditions (Xuan et al 2022).…”