Rotational properties of the retrograde object (468861) 2013 LU28

Trans-Neptunian Objects (TNOs) are thought to be among the least evolved Solar System objects, which retain information on the origin and evolution of the outer parts of it. They are located at far distances of the Sun, where the influence of our star is less dramatic than in the closer regions. Thus, these icy objects are extremely interesting bodies that hide plenty of information on the physical and dynamical processes that
shaped our Solar System.
We only know a few retrograde TNOs so far (e.g. 2008 KV42 [1], 2011 KT19 [2], 2004 XR190). One of the few known retrograde objects listed in the MPC database as a scattered disk object is 2013 LU28, which has a high orbital eccentricity (e = 0.95), a large semimajor axis (a= 181 AU) and a very high inclination (i = 125.4º). This exotic object is also classified as an “extended centaur”, because its perihelion at 8.7 AU moves it into the centaur region.
The physical properties of 2013 LU28, such as its rotational period and light curve amplitude, are unknown but can be revealed through photometry. With this aim, we observed this object during three observing runs on 2021 January and March using two telescopes, the 1.23 m telescope at Calar Alto Observatory in Almería, Spain and the 1.5 m telescope at Sierra Nevada Observatory in Granada, Spain. From these observations we derived the first determination of the rotational light curve of 2013LU28 from which we derived its rotational period and its peak-to-peak light curve amplitude. The obtained amplitude turned out to be higher than the average amplitude of most TNOs, which points toward an elongated or a binary object. Other magnitudes, such as its absolute magnitude (H) were also derived. We will present and discuss preliminary results on all the above.

Acknowledgements
The authors acknowledge financial support 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). P.S-S. acknowledges financial support by the Spanish grant AYA-    RTI2018-098657-J-I00 "LEO-SBNAF" (MCIU/AEI/FEDER, UE). We are grateful to the CAHA and OSN staffs. This research is partially based on observations collected at the Centro Astronómico Hispano Alemán (CAHA) at Calar Alto, operated jointly by Junta de Andalucı́a and Consejo Superior de Investigaciones Cientı́ficas (IAA-CSIC). This research was also partially based on observation carried out at the Observatorio de Sierra Nevada  (OSN) operated by Instituto de Astrofı́sica de Andalucı́a (CSIC).

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