Unveiling the Kuiper belt from the JWST through stellar occultations

The stellar occultation technique is a very powerful tool to obtain the size and shape of Solar System bodies with high accuracy [6]. Size determination allows to obtain geometric albedos and, in the case of binary/multiple objects, even the mass density can be derived [3]. Satellites, atmospheres, and rings can also be detected and characterized [1,2,3]. The observation of stellar occultations produced by Kuiper Belt Objects (KBOs) and Centaurs with the James Webb Space Telescope (JWST) offers a unique possibility to extend our knowledge of these bodies [4] by providing key information on the body's ability to retain volatiles, surface thermal properties, roughness, porosity, etc.

We will present our Target of Opportunity (ToO) program [5] accepted within Heidi Hammel's JWST Guaranteed Time Observations (GTO), dedicated to observing stellar occultations by trans-Neptunian objects (TNO) and distant dwarf planets or particularly interesting centaurs (such as the ringed centaurs Chariklo [1] or Chiron [2]). Predictions of such events visible from JWST are challenging due to the chaotic motion of the space telescope around the Lagrange 2 (L2) point. Statistically, we expect there to be approximately a 50% chance of such an occultation of a star brighter than K=19 by a numbered TNO observable from JWST in Cycle 1. We will discuss the possible candidates for Cycle 1 occultations that we have identified so far. As JWST station-keeping maneuvers are executed, the list of possible occultations and their uncertainties will be revised. Very accurate relative astrometry will be performed using the latest releases of the Gaia catalog for particularly promising occultation events through established ground-based programs. Suppose a stellar occultation event is confirmed through such an astrometric revision to have a predicted impact parameter less than 3 times the estimated target radius and to have a 1 sigma uncertainty in prediction less than 2 times the target radius. In that case, the ToO observation will be triggered. JWST station-keeping and trajectory-prediction operations have been studied in the context of stellar occultations by solar system bodies [4]. The accuracy of the trajectory predictions is adequate to support this triggering mechanism up to roughly 30 days before an occultation event: the ToO response time is set to 14 days, the minimum value for a non-disruptive ToO.

The observations will be made with NIRCam and the F070W and F277W filters. These filters were chosen to maximize the flux from the star while minimizing the reflected flux from the TNO or Centaur. This filter combination could change based on the properties of the occulted star and the occulting TNO/Centaur. Other technical aspects and updates on this project will be provided during the presentation.

Acknowledgments. We acknowledge financial support from the Spanish grant AYA-RTI2018-098657-J-I00 “LEO-SBNAF” (MCIU/AEI/FEDER, UE) and from the State Agency for Research of the Spanish MCIU through the “Center of Excellence Severo Ochoa” award to the Instituto de Astrofísica de Andalucía (SEV-2017-0709). Funding from Spanish projects PID2020-112789GB-I00 from AEI and Proyecto de Excelencia de la Junta de Andalucía PY20-01309 is also acknowledged. Part of this work has received funding from the European Research Council under the European Community’s H2020 (2014-2020/ERC Grant Agreement no. 669416 “LUCKY STAR”). M.V-L. acknowledges funding from Spanish project AYA2017-89637-R (FEDER/MICINN).

References

[1] Braga-Ribas et al., Nature 508, 72 (2014)

[2] Ortiz et al., A&A 576, id.A18 (2015)

[3] Ortiz, Santos-Sanz et al., Nature 550, 219 (2017)

[4] Santos-Sanz et al., PASP 128, 959 (2016)

[5] Santos-Sanz, JWST Proposal. Cycle 1, ID. #1271

[6] Sicardy et al., Nature 439, 52 (2006)