JWST Spectroscopy of a Blue Binary Cold Classical Kuiper Belt Object

Abstract

We present observations of two binary systems within the cold classical region of the Kuiper belt$-$2001 XR254 and 2016 BP81$-$obtained with the JWST Near-Infrared Spectrograph. The measured reflectance spectrum of 2001 XR254 is characteristic of the red cold classicals, with strong features due to carbon dioxide, carbon monoxide, and methanol ices. In contrast, 2016 BP81 is a blue binary, with a water-ice-rich surface composition. The two components of the 2016 BP81 binary display identical spectral profiles, consistent with coeval formation from gravitational collapse. Through qualitative and quantitative comparisons of water-ice-rich Kuiper belt objects observed with JWST, we identify a small subclass, including 2016 BP81, that appears to differ in systematic ways from the rest of the population. The relatively deep carbon dioxide ice absorption bands and enhanced signatures of aliphatic organics suggest that objects within this subclass may have originated in a distinct formation environment from the other water-ice-rich Kuiper belt objects. The implications of our findings are discussed within the context of recent models of Kuiper belt formation and evolution.

Figures (6)

• Figure 1: Median-averaged IFU image slices from the first dithered exposures of 2001 XR254 and 2016 BP81. A logarithmic stretch has been applied to the flux scaling to accentuate the PSF shapes and background variations. The black points and circles indicate the measured source centroid positions and spectral extraction apertures. The primary and secondary components of 2016 BP81 are well separated and labeled.
• Figure 2: Comparison plot of spectra of the solar analog star SNAP-2 obtained from two JWST observations that utilized different dither patterns (blue and red curves). The CALSPEC model spectrum of SNAP-2 is overplotted in the solid black line. The difference between the two JWST spectra of SNAP-2 at short wavelengths is attributed to variations in sensitivity across the NIRSpec IFU detector that are not corrected by the current calibration pipeline. The solar standard spectrum from the PSG is also included (black dashed line), which shows a discrepant spectral slope at short wavelengths from the SNAP-2 model spectrum. All spectra have been renormalized to match the blue curve at 2.0 $\mu$m.
• Figure 3: Reflectance spectra of 2001 XR254 and 2016 BP81, normalized to unity at 2.5 $\mu$m and offset for clarity. The individually extracted spectra of the primary and secondary components of 2016 BP81 are overlaid on top of one another and show identical spectral profiles.
• Figure 4: Spectral comparison plot of 2001 XR254 and 2016 BP81 with the KBOs presented in pinillaalonso2025. The latter are divided into panels based on their compositional class. All spectra are normalized to unity at 2.5 $\mu$m. The spectrum of 2016 BP81, shown in the black curve in each panel, has the greatest affinity to the H$_{2}$O-type KBOs. Meanwhile, 2001 XR254 (purple line in the bottom panel) has a surface composition that is consistent with organics-type objects.
• Figure 5: Grid plot of two-parameter distributions for the measured 2 $\mu$m H$_{2}$O, 3 $\mu$m H$_{2}$O, and 4.27 $\mu$m CO$_{2}$ absorption bands among objects in the H$_{2}$O-type KBO compositional class. Different colors and shapes denote the corresponding dynamical classes of the objects. Dashed black lines connect the blue binary 2016 BP81 with the Neptune Trojan 2011 SO277 and the ETNO 2016 QV89. The three objects' spectra show comparable spectral properties, which appear to differ systematically from the rest of the H$_{2}$O-type KBOs in the sample.