Probing Unique Regimes of Exoplanet Science with Long Baseline Interferometry

Abstract

Thousands of exoplanets have been discovered to date. Each exoplanet detection method is sensitive to specific regimes, leaving many gaps in exoplanet parameter space. In this thesis, we exploit the unique capabilities of long baseline interferometry to fill two gaps in exoplanet parameter space: 1) the detection of new planets around stars more massive than the Sun (Project ARMADA), and 2) the characterization of known planets that are extremely close to their host star (Project PRIME).

Most detection methods have focused on finding planets around solar-type and lower mass stars. More massive stars are not well-probed for planets. We developed a method to search for planets around such stars with Project ARMADA. Given that a star is part of a binary system, we can use interferometry to measure the position of one star relative to the other down to a precision of 10 micro-arcseconds. This allows us to detect the ``wobble" motion from previously unseen companions in the system. We first demonstrate the power of interferometry for studying binary stars with orbits, masses, and evolutionary state of A-type binaries alp Del and del Del. For del Del, we find that our astrometry allows us to probe for planets down to 2 MJ at 1 au. We then introduce our observations and wavelength calibration schemes for Project ARMADA, which is a long term monitoring effort with MIRC-X/CHARA and GRAVITY/VLTI to search for companions orbiting individual components of sub-arcsecond binaries. We present 15 detections of new stellar mass companions, with median precision levels often at the 20-50 micro-arcsecond level. We also detect two candidate brown dwarfs around A-type binaries HD6456 and HD82446. For ARMADA binaries without detections, we compute non-detection limits and show that we are probing down to the substellar mass regime.

Project PRIME uses interferometry to characterize known close-in exoplanets. Single dish telescopes cannot see planets within the diffraction limit of the telescope, which is at the 100 milli-arcsecond level. Long baseline interferometry can be used to directly detect planets down to a few milli-arcseconds from their host star, if the high contrast can be demonstrated. We use the H-band MIRC-X instrument at the CHARA array to observe the non-transiting hot Jupiter planet Ups And b. We developed a self-calibration routine and observing setup to achieve the precision closure phases necessary for such a detection. In September 2019, we achieved a promising tentative detection of the planet at a contrast level of 2.2e-4. We seemingly confirmed the detection in an independent MIRC-X follow-up run in 2021 October. However, we do not see the planet in our 2021 October K-band MYSTIC data. We compute updated model spectra for Ups And b, and confirm that the K-band data is expected to have more favorable contrast than in H-band. By injecting simulated planets at the expected location from the MIRC-X detection, we show that we should have recovered the planet down to a contrast level of 2e-4 in K-band. We are near the level of being able to directly detect close-in hot Jupiter planets. This will be a promising technique in the era of Gaia, when many close-in high contrast systems will need interferometric follow-up.