JWST NIR Nucleus Surface Reflectance Spectrum of Centaur 29P/Schwassmann-Wachmann 1 Provides Link to its Progenitor TNO Population
Authors: Schambeau Charles , Fernandez Yanga , McKay Adam , Faggi Sara , de Pra Mario , Pinilla-Alonso Noemi , Harrington Pinto Olga , Villanueva Geronimo , Kelley Michael , Bockelee-Morvan Dominique , Womack Maria , Feaga Lori , DiSanti Michael , Bauer James , Licandro Javier , Wierzchos Kacper 
Published: July 2024

The active Centaur 29P/Schwassmann-Wachmann 1 has been an enigma since its discovery almost a century ago due to the combination of its orbital properties and outburst prone cometary behaviors. Its nearly circular trans-Jovian orbit (perihelion distance q ~ 5.8 au) provides a relatively stable thermal environment, yet 29P displays a moderately variable persistent dust and gas production sprinkled with frequent short-lived major outbursts when its visual magnitude brightens by ~ 1 to 6 magnitudes. This type of dust production behavior has been well documented since its discovery and it is unique. It is natural to then question whether 29P nucleus&#8217; compositional nature is also unique, or whether its behaviors are explained by a fortuitous combination of orbital and nuclear properties (i.e., size, spin state, shape, etc.) such that 29P represents an extreme case of cometary activity that other objects would similarly display provided the right circumstances.We present the first NIR nucleus surface reflectance spectrum of 29P with the goal of using it to better explain its activity behaviors. Acquiring nucleus surface information for 29P is challenging due to the persistent presence of a dust coma which confuses attempts to disentangle nucleus vs. coma flux. The new spectrum we present was enabled by the combination of the wavelength coverage, sensitivity, and stable point spread function (PSF) of the JWST NIRSpec utilized in the integral field unit (IFU) mode. The spectral data were acquired with the PRISM disperser, covering a wavelength range of 0.6 &#8211; 5.3 microns, on UT 2023 February 20 as part of the Cycle 1 GO Program 2416 [1]. The IFU&#8217;s 3&#8221; x 3&#8221; field of view enabled application coma modeling and removal [2] to isolate the nucleus&#8217; flux contributions over each of the datacube&#8217;s wavelengths to produce a nucleus-only reflectance spectrum.The NIR spectrum of 29P presents a shape retaining distinct characteristics of the &#8220;Bowl&#8221;-type trans-Neptunian Objects (TNOs) spectral classification established by the JWST Cycle 1 Program 2418 [2; DiSCo-TNOs]. TNOs from this population are hypothesized to have abundant water ice and to have formed interior to the CO2 ice line [4, 5]. The prospects of 29P&#8217;s progenitor nucleus forming in a region too warm for the condensation of CO and CO2, yet the gas comae environment observed with an abundance of both [6, 7] provides potential evidence of its nucleus having retained abundant amorphous water ice (AWI) trapping the two gas species. &#160;However, future studies are necessary to better understand the surface evolution experienced by Centaurs as they become thermally activated, potentially causing objects to change between spectral classification types, and trace the possible evolution of active surface areas. In our presentation we will show the surface reflectance spectrum and preliminary compositional modeling of the surface and thermophysical modeling of the nucleus&#8217;s bulk interior.References: [1] McKay, A., et al., JWST Proposal Cycle 1, ID. #2416. [2] Fern&#225;ndez, Y, R., et al., 2013, Icarus, 226, 1138-1170. [3] Pinilla-Alonso, N., et al., JWST Proposal. Cycle 1, ID. #2418. [4] Pinilla-Alonso, N., et al., Nature Astronomy, in review. [5] De Pr&#225;, M., et al., 2024, Nature Astronomy, in press. [6] Bockel&#233;e-Morvan, D., et al., 2022, A&amp;A, 664, id.A95. [7] Faggi, S., et al., Nature Astronomy, in review.