Proposal #493

Dear AAVSONet Telescope Allocation Committee,

As part of the PLATO benchmark eclipsing binaries initiative and also member of AAVSO, I would like to propose the observation of eclipses for the detached eclipsing binary EPIC 212822491, using the Optical Craftsman 0.61m Telescope (OC61), located at the Mount John University Observatory (MJUO), via the AAVSONet.
The PLATO (PLAnetary Transits and Oscillations of stars) is the third medium-class mission in ESA's Cosmic Vision program. PLATO was adopted by ESA Science Programme Committee in June 2017 and is due to be launched in 2026. Its objective is to find and study a large number of extrasolar planetary systems, with emphasis on the properties of terrestrial planets in the habitable zone around Sun like stars. PLATO has also been designed to investigate seismic activity in stars, enabling the precise characterization of the planet host stars, including its age.

The PLATO benchmark eclipsing binaries initiative, conducted by the PLATO Benchmark stars work package (WP 125 500) of the Determination of Stellar Parameters work package (WP 125 000), as part of the Stellar Science work package (WP12) (https://warwick.ac.uk/fac/sci/physics/research/astro/plato-science/resources/psmdocuments/plato-uwa-psm-wbs-0013_i4.0_wp120000_stellar_science_structure.pdf), has as one of its main objetives the coordination of benchmark observations for testing stellar models used by the PLATO mission, and for validation of the stellar parameters that will be provided in the PLATO mission data products (Gent et al. 2022).
The 20 detached eclipsing binary stars (DEBS) selected as targets for the benchmark show a total secondary eclipse with depths around 1 per cent and narrow eclipses (Maxted et al. 2023).
Observations in multiple filters of the secondary eclipse are needed to measure the flux ratio, in order to help improve the the accuracy and reliability of the effective temperature measurements for the two stars in the binary system. However, observations of the primary eclipse should also be useful for the analysis.

The observations of each eclipse must be conducted in high cadence using, using a par of photometric filters B and V (bright targets) or R and I (faint targets), in order to cover a complete ingress and/or egress of an eclipse plus some data before and after (~20 minutes) to measure the out-of-eclipse level and eclipse depth.

In addition, the 24" OC61 telescope is particularly required for these observations due to the following factors:
- The 24" OC61 telescope is well suited for this target in order to achieve the photometric precision required for this 21.05 ppt depth of the secondary eclipse and 11.8 V-mag star;
- The observability from the southern celestial hemisphere at the Mount John University Observatory is favorable for the target at -4.83° in declination, and its elevation at 1029 meters and Class 2 Bortle scale sky should be also helpful;
- The secondary eclipse duration of 3.64 hours, plus the required baseline observation of 20 minutes pre-ingress or post-egress, leads to a duration of at least 2.15 hours for the observing session of an ingress or egress event. According to the Dr Karen Pollard (Director UC Mt John Observatory), this should provide a good match for the climate variations at the New Zealand site, and should also help minimize the conflict with other observation activities currently in place.

Finally, the observations of EPIC 212822491 would require a high cadence alternating between the most suitable pair of filters, and having the integration time adjusted accordingly in order to achieve a SNR~500 (or ~2 ppt RMS), not to exceed an interval of 2 minutes between exposures and also having the minimum dead time between them as low as possible, in order not to impact the precision of the eclipse model fit to the data.
The following list of predicted eclipse observations are suggested for the next 120 days:
- 2024-04-12 08:38 (Session start time in UTC): secondary eclipse with 17% Moon illumination at 133˚ distance;
- 2024-04-19 09:42 (Session start time in UTC): primary eclipse with 83% Moon illumination at 45˚ distance;
- 2024-06-01 08:49 (Session star time in UTC): primary eclipse with 29% Moon illumination at 160˚ distance;
- 2024-07-07 06:55 (Session star time in UTC): secondary eclipse with 2% Moon illumination at 84˚ distance;

Cordially,
Andre Kovacs