# HAT-P-67b: An Extremely Low Density Saturn Transiting an F-Subgiant Confirmed via Doppler Tomography

## TL;DR

HAT-P-67b is an extremely low-density Saturn-sized exoplanet orbiting a rapidly rotating F-subgiant, confirmed through Doppler tomography, with its properties challenging traditional mass measurement techniques due to stellar rotation.

## Contribution

This study reports the discovery and confirmation of HAT-P-67b using Doppler tomography, demonstrating a method to validate planets around rapidly rotating stars where radial velocities are limited.

## Key findings

- HAT-P-67b has a radius of approximately 2.09 Jupiter radii.
- The planet's mass is constrained to be less than 0.59 Jupiter masses.
- The planet's orbit is aligned within 12 degrees of the host star's spin.

## Abstract

We report the discovery of HAT-P-67b, a hot-Saturn transiting a rapidly rotating F-subgiant. HAT-P-67b has a radius of Rp = 2.085 -0.071/+0.096 RJ,, orbiting a M* = 1.642 -0.072/+0.155 Msun, R* = 2.546 -0.084/+0.099 Rsun host star in a ~4.81-day period orbit. We place an upper limit on the mass of the planet via radial velocity measurements to be Mp < 0.59 MJ, and lower limit of > 0.056 MJ by limitations on Roche lobe overflow. Despite being a subgiant, the host star still exhibits relatively rapid rotation, with a projected rotational velocity of v sin I* = 35.8 +/- 1.1 km/s, making it difficult to precisely determine the mass of the planet using radial velocities. We validated HAT-P-67b via two Doppler tomographic detections of the planetary transit, which eliminated potential eclipsing binary blend scenarios. The Doppler tomographic observations also confirmed that HAT-P-67b has an orbit that is aligned to within 12 degrees, in projection, with the spin of its host star. HAT-P-67b receives strong UV irradiation, and is amongst the one of the lowest density planets known, making it a good candidate for future UV transit observations to search for an extended hydrogen exosphere.