Disentangling Blended K2 Photometry: Determining the Planetary Host Star

Abstract

The presence of companion stars, whether bound or unbound, make correct identification of the planetary hosting star difficult when a planet has been detected through a photometrically blended transiting event. We present an approach that uses a combination of light curve analysis and stellar modeling to disentangle 8 K2 photometrically blended binary systems that have either a confirmed or suspected planet to identify the probable host star. The key to our approach is the use of the mean stellar density, calculated using the transit geometry, as a discriminator. The approach is strongly dependent on the difference in magnitude between the stars and the observed transit depth, which is corrected by the flux ratio between the two stars. While our approach does not lead to a definitive result for all systems, we were able to determine for the 8 systems in this case study: two systems where the primary was highly likely to be the planet hosting star (> 90% likelihood); three systems where the primary was likely to be the hosting star (> 55% likelihood); one system where the secondary was likely to be the planet hosting star (> 55% likelihood); and two systems where we were uncertain which star was the planet hosting star (≈ 50% likelihood to be either the primary or the secondary). Two systems, denoted by their EPIC number, EPIC 201546283 and EPIC 201862715, had confirmed planets, K2-27b and K2-94b respectively, with radii of 4.0 ± 0.1 and 16.4 ± 2.9 R ⊕ respectively.

Determining Which Binary Component Hosts the TESS Transiting Planet

“…Figure 7 shows the primary host star probability as a function of the the binary magnitude difference. While we do not see the strong trend found by Payne et al (2018), where the host component in equal brightness pairs were uncertain due to the similarity in temperature and density, the equal brightness systems in our sample do have slightly lower primary host probabilities that unequal brightness pairs. Furthermore, the systems with faint secondaries (Δm 4 mag) are even more likely to have a primary star host (89%), as seen in Figure 7.…”

“…We calculated the mean stellar density for each component (ρ 1 , ρ 2 , ρ 3 ) following Equations (8)-( 11) in Payne et al (2018), which correct the observed transit depth for the flux contamination from the other components. For example, if the planet orbits the primary star in a binary system, the primary's stellar density is…”

“…We pulled the TIC parameters for each TOI, kept only stars with uncertainties in radius, temperature, and mass less than 20%, and calculated the mean density for each system. Following the method of Payne et al (2018), we then created a model density-T eff curve by fitting a 6th-order polynomial to these temperatures and densities. Figures 5 and 6 show the stellar density versus effective temperature plots for all of the TOI's in our sample.…”

“…Seager & Mallén-Ornelas (2003) derived a method to determine the stellar density of a planet host star using the transit geometry, which can be extended to planet hosts in multistar systems if the transit depth is corrected for the flux contribution of each component. Payne et al (2018) used this method to identify the planet host star in 8 K2 binary systems and found that the brighter primary star was more likely to host the planet in five out of eight systems. They also found that systems with fainter companions were more likely to have primary host stars.…”

Determining Which Binary Component Hosts the TESS Transiting Planet

“…Seager & Mallén-Ornelas (2003) derived a method to determine the stellar density of a planet host star using the transit geometry, which can be extended to planet hosts in multistar systems if the transit depth is corrected for the flux contribution of each component. Payne et al (2018) used this method to identify the planet host star in 8 K2 binary systems and found that the brighter primary star was more likely to host the planet in 5 out of 8 systems. They also found that systems with fainter companions were more likely to have primary host stars.…”