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

TL;DR

This study tackles the detection and interpretation of secondary eclipses for two brown dwarfs in K2 fields, using a multi-dataset data-fusion approach to boost sensitivity. The authors employ a two-step method: single time-series modeling with Fourier-based stellar variability and systematics, followed by merging results across multiple photometric sources and campaigns via SES statistics. They detect a shallow eclipse for EPIC 219388192b with $\delta_{obs}=70\pm12\rm ext{ ppm}$ and a deeper eclipse for EPIC 211946007b with $\delta_{obs}=852\pm123\rm ext{ ppm}$; orbital phases are consistent with spectroscopic solutions. Atmospheric-model comparisons show EPIC 219388192b requires a high geometric albedo ($A_g\gtrsim0.6$) to reconcile the Kepler-band depth with Spitzer data, while EPIC 211946007b can be explained with a low albedo ($A_g\approx0.1$) given its young age and internal heat. Together, these results highlight how combining diverse photometric datasets can reveal faint occultations and constrain brown-dwarf atmospheres and evolution, guiding future multi-wavelength follow-ups.

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 comes mostly from the internal heat source and the absorbed stellar irradiation, the case of CWW 89Ab suggests very high albedo, because of the lack of sufficient thermal radiation in the Kepler waveband. Assuming 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 3 sigma less than the observed depth, the minimum geometric albedo becomes ~0.6.

Figures (11)

• Figure 1: Brief summary of the parameters and steps playing role in the analysis of the single datasets. See text for additional details.
• Figure 2: Two types of approach in searching for secondary eclipse by using multiple datasets (already filtered out from all other components -- see Sect. \ref{['sect:method1']} and Fig. \ref{['single_chart']}). Left: averaging SES statistics of the individual datasets by their SNR. Right: averaging individual LCs by their inverse variance, and then compute SES of the average LC. The sum of the weights are normalized to unity in both cases. See text for more.
• Figure 3: SES statistics for the non-reconstructed LCs of EPIC 219388192. From top to downward, plotted are SES statistics derived on the data sources of LUG, KEP, PET and VAN. The averaged SES is shown by black at the bottom. The phase scan is made from the transit center (phase zero) throughout the full orbital phase. All curves are plotted on the same (but arbitrary) scale and shifted properly for good visibility.
• Figure 4: As in Fig. \ref{['219388192_01_dip_stat']}, but for the reconstructed LCs.
• Figure 5: Comparison of the final SES statistics obtained from averaging the individual SES statistics (top) with the one obtained from the averaged LC (bottom). As for the earlier, similar plots, we employed arbitrary vertical shifts for better visibility. The results shown are based on reconstructed LCs.