The May 26, 2020 multi-chord stellar occultation by the trans-Neptunian object (119951) 2002 KX14

Abstract

Centaurs and trans-Neptunian objects (TNOs) are considered to be the most pristine members of our solar system, beside Oort cloud objects.

The observation of stellar occultations by solar system objects is a powerful technique to directly measure their size and profile shapes with kilometer accuracy [e.g. 1, and references therein], to probe the environment of them with the possibility to reveal satellites and / or rings [e.g. 2, 3] and to detect or to constrain an atmosphere down to the nanobar level [e.g. 4]. Finally it provides a high-accuracy astrometric measurement, which can for example be used for improving the prediction of subsequent occultation events within short or mid-term time spans.

Here we report the observation of an occultation event of the star Gaia DR2 4111560308371475840 (G = 14.6 mag) by the TNO (119951) 2002 KX₁₄ on May 26, 2020. The shadow was predicted to cross eastern Europe (Fig. 1) and the event was observed successfully by ten stations supplemented by another good dozen of stations which had a miss (no event detected).

2002 KX₁₄ is a low-inclination (i ~ 0.4°), low-eccentricity (e ~ 0.04) cold classical TNO, orbiting the Sun at an average distance of a ~ 39 au. The radiometric diameter is given as 455 ± 27 km [5]. On April 26, 2012, an occultation by this object was observed with the 4.2-m William Herschel Telescope on La Palma (Spain) at high cadence. From this single-chord observation (with a chord length of 415 ± 1 km), combined with accurate astrometry at the time of occultation, an area-equivalent diameter of at least 365 (+30, -21) km was estimated [6]. The rotational period is yet unknown. The lightcurve amplitude is reported as Δm < 0.05 mag (from 3 nights, [7]).

The occultation was predicted within the Lucky Star project. 121 astrometric observations filed at the MPC plus 56 observations derived at observatories used by the LS collaboration, covering the time span 1984-2018, were used to derive the NIMAv7 [8] orbit solution, which was used for the occultation event search. As the event was suitable to be observed by smaller telescopes, the amateur community was informed via mail lists etc. Details of the prediction are given on the LS project website [9].

Ten stations recorded the occultation with apertures between 20 cm and 80 cm. Five of them used analogue video technique for the recording, and five  applied CCD cameras. 14 stations reported a non-detection of the occultation event within their observing windows (Fig. 1).

By applying the method described in [10], we fitted the extremities of the positive chords to an ellipse to derive the instantaneous limb of 2002 KX₁₄. From the five fit parameters (center of the ellipse, semi-major and semi-minor axis and the position angle of the semi-minor axis) we derived the size and shape of the 2D limb as well as a high-accurate astrometric position for the time of occultation. Photometric observations to derive the light curve amplitude and the rotational period, with the aim to put constraints on the albedo as well as on the 3D size and the shape of 2002 KX₁₄ are planned.

Acknowledgements

We 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) and the financial support by the Spanish grant AYA-2017-84637-R. M.V-L. acknowledges funding from Spanish project AYA2017-89637-R (FEDER/MICINN). P.S-S. acknowledges financial support by the Spanish grant AYA-RTI2018-098657-J-I00 "LEO-SBNAF" (MCIU/AEI/FEDER, UE). This campaign was carried out within the “Lucky Star" umbrella that agglomerates the efforts of the Paris, Granada and Rio teams. It is funded by the European Research Council under the European Community’s H2020 programme (2014-2020/ERC Grant Agreement No. 669416). This work has made use of data from the European Space Agency (ESA) mission Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC, https://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement.

References

[1] Souami, D., et al. 2020, A&A, 643, A125

[2] Braga-Ribas, F., et al. 2014, Nature, 508, 72

[3] Ortiz, J. L., et al. 2017, Nature, 550, 219

[4] Sicardy, B., et al. 2011, Nature, 478, 493

[5] Vilenius, E., et al. 2012, A&A 541, A94

[6] Alvarez-Candal, A., et al. 2014, A&A 571, A48

[7] Benecchi, S. D., et al. 2013, AJ 145, 124

[8] Desmars, J., et al. 2015, A&A, 584, A96

[9] https://lesia.obspm.fr/lucky-star/occ.php?p=39276

[10] Szpak, Z. L., et al. 2015, Journal of Mathematical Imaging and Vision 52, 173

 

Figure 1 – The map shows the nominal Lucky Star prediction (NIMAv7) together with the location of the observation sites. The centre line of the shadow is given in red, the northern and southern shadow limits are given by the blue lines, based on an assumed mean diameter of 455 km. The southern 1-σ limit is marked by the grey line. The observation sites are represented by green markers (with positive occultation observation) and by red markers, were no occultation was detected.