Speaker
Description
We present Keck/KPIC high-resolution ($R\sim35,000$) $K$-band thermal emission spectroscopy of the ultra-hot Jupiter WASP-33b. The use of KPIC's single-mode fiber greatly improves both blaze and line-spread stabilities relative to slit spectrographs, enhancing the cross-correlation detection strength. We perform separate retrievals for the dayside and nightside emission spectra with a cross-correlation-to-logL nested sampling pipeline which fits for orbital parameters, the atmospheric pressure-temperature profile, and molecular abundances. While the dayside shows a thermal inversion which is absent on the nightside, we measure consistent CO and H$_2$O abundances for both hemispheres (nightside $\log\rm H_2O_{MMR} = -3.3^{+0.9}_{-0.6}$, $\log\rm CO_{MMR} = -1.5^{+0.9}_{-0.6}$ versus dayside $\log\rm H_2O_{MMR} = -4^{+1}_{-1}$, $\log\rm CO_{MMR} = -2^{+1}_{-1}$), suggesting limited dayside H$_2$O dissociation at the pressures probed. The retrieved abundances suggest a carbon- and possibly metal-enriched atmosphere, with a gas-phase C/O ratio of $1.0\pm0.1$, consistent with the accretion of high-metallicity gas near the CO$_2$ snow line and post-disk migration. The dayside hemisphere shows a systemic redshift compared to the nightside in excess of WASP-33b's rotation speed, suggesting day-to-night winds may be contributing to the offset. We also find tentative evidence for $\rm ^{13}CO/^{12}CO \sim 40$, consistent with values expected in protoplanetary disks. These observations demonstrate KPIC's ability to characterize close-in planets and the utility of KPIC's improved instrumental stability for cross-correlation techniques.
