Determining Empirical Stellar Masses and Radii Using Transits and Gaia Parallaxes as Illustrated by Spitzer Observations of KELT-11b

Abstract

Using the Spitzer Space Telescope, we observed a transit at 3.6um of KELT-11b (Pepper et al. 2017). We also observed three partial transits from the ground. We simultaneously fit these observations, ground-based photometry from Pepper et al. (2017), radial velocity data from Pepper et al. (2017), and an SED model utilizing catalog magnitudes and the Hipparcos parallax to the system. The only significant difference between our results and Pepper et al. (2017) is that we find the orbital period is shorter by 37 seconds, $4.73610\pm 0.00003$ vs. $4.73653\pm 0.00006$ days, and we measure a transit center time of BJD_TDB $2457483.4310\pm 0.0007$, which is 42 minutes earlier than predicted. Using our new photometry, we measure the density of the star KELT-11 to 4%. By combining the parallax and catalog magnitudes of the system, we are able to measure KELT-11b's radius essentially empirically. Coupled with the stellar density, this gives a parallactic mass and radius of $1.8{\mathrm{M}}_{\odot }$ and $2.9{\mathrm{R}}_{\odot }$, which are each approximately $1\sigma$ higher than the adopted model-estimated mass and radius. If we conduct the same fit using the expected parallax uncertainty from the final Gaia data release, this difference increases to $4\sigma$. This demonstrates the role that precise Gaia parallaxes, coupled with simultaneous photometric, RV, and SED fitting, can play in determining stellar and planetary parameters. With high precision photometry of transiting planets and high precision Gaia parallaxes, the parallactic mass and radius uncertainties of stars become 1% and 3%, respectively. TESS is expected to discover 60 to 80 systems where these measurements will be possible. These parallactic mass and radius measurements have uncertainties small enough that they may provide observational input into the stellar models themselves.

15 pages, 8 figures, 5 tables. Submitted to AAS journals. Updated based on post-submission and referee comments

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