Secondary eclipses of two brown dwarfs in the K2 fields: Detection by multiple dataset merging

¹ Konkoly Observatory, Research Center for Astronomy and Earth Sciences of HUN-REN, MTA Center of Excellence, Budapest, 1121 Konkoly Thege ut. 15-17, Hungary
² Department of Physics & Astronomy, College of Charleston, Rita Hollings Science Center, 58 Coming Street, Charleston, SC 29421, USA
³ Escuela Superior de Física y Matemáticas, Instituto Politécnico Nacional (IPN), Luis Enrique Erro S/N, San Pedro Zacatenco, 07738 Ciudad de México, Mexico

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Received: 9 September 2025
Accepted: 15 November 2025

Abstract

By using various data sources for the stellar fluxes in overlapping campaign fields and employing full time-series modeling, we report the detection of the secondary eclipses of two brown dwarfs (CWW 89Ab = EPIC 219388192b and HSHJ 430b = EPIC 211946007b). The detections yielded timings in agreement with the orbital elements derived from the earlier radial velocity measurements and eclipse depths of 70 ± 12 ppm (CWW 89Ab) and 852 ± 123 ppm (HSHJ 430b). While the high depth in the Kepler waveband for HSHJ 430b is in agreement with the assumption that the emitted flux mostly comes from the internal heat source and the absorbed stellar irradiation, the case of CWW 89Ab suggests a very high albedo because of the lack of sufficient thermal radiation in the Kepler waveband. Assuming a completely reflective dayside hemisphere, without circulation, the maximum value of the eclipse depth due to the reflection of the stellar light is 56 ppm. By making the extreme assumption that the true eclipse depth is 3σ less than the observed depth, the minimum geometric albedo becomes ~0.6.