High-precision atmospheric characterization of a Y dwarf with JWST NIRSpec G395H spectroscopy: isotopologue, C/O ratio, metallicity, and the abundances of six molecular species
arxiv(2024)
摘要
The launch of the James Webb Space Telescope (JWST) marks a pivotal moment
for precise atmospheric characterization of Y dwarfs, the coldest brown dwarf
spectral type. In this study, we leverage moderate spectral resolution
observations (R ∼ 2700) with the G395H grating of the Near-Infrared
Spectrograph (NIRSpec) onboard of JWST to characterize the nearby (9.9 pc) Y
dwarf WISEPA J182831.08+265037.8 (WISE 1828). With the NIRSpec G395H 2.88-5.12
μm spectrum, we measure the abundances of CO, CO_2, CH_4,
H_2S, NH_3, and H_2O, which are the major carbon, nitrogen, oxygen, and
sulfur bearing species in the atmosphere. Based on the retrieved volume mixing
ratios with the atmospheric retrieval framework CHIMERA, we report that the C/O
ratio is 0.45 ± 0.01, close to the solar C/O value of 0.55, and the
metallicity to be +0.30 ± 0.02 dex. Comparison between the retrieval
results with the forward modeling results suggests that the model bias for C/O
and metallicity could be as high as 0.03 and 0.97 dex respectively. We also
report a lower limit of the ^12CO/^13CO ratio of >40, being
consistent with the nominal solar value of 90. Our results highlight the
potential of JWST in measuring the C/O ratios down to percent-level precision
and characterizing isotopologues of cold planetary atmospheres similar to WISE
1828.
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