Measurement of the photosphere oblateness of $\gamma$ Cassiopeiae via Stellar Intensity Interferometry with the VERITAS Observatory

TL;DR

This study measures the oblateness of the rapidly rotating star $ ext{γ}$ Cassiopeiae using stellar intensity interferometry, providing the first such measurement of an oblate photosphere with this technique.

Contribution

It demonstrates the first measurement of an oblate stellar photosphere using intensity interferometry, expanding the application of this method to stellar shape analysis.

Findings

Measured the star's oblateness and orientation with high precision.

Fitted the data with a stellar rotation model including limb and gravity darkening.

Results are consistent with previous spectroscopic and interferometric observations.

Abstract

We use the stellar intensity interferometry system implemented with the Very Energetic Radiation Imaging Telescope Array System (VERITAS) at Fred Lawrence Whipple Observatory (FLWO) as a light collector to obtain measurements of the rapid rotator star $\gamma$ Cassiopeiae, at a wavelength of 416 nm. Using data from baselines sampling different position angles, we extract the size, oblateness, and projected orientation of the photosphere. Fitting the data with a uniform ellipse model yields a minor-axis angular diameter of $0.43\pm0.02$ mas, a major-to-minor-radius ratio of $1.28\pm0.04$, and a position angle of $116^\circ\pm5^\circ$ for the axis of rotation. A rapidly-rotating stellar atmosphere model that includes limb and gravity darkening describes the data well with a fitted angular diameter of $0.604^{+0.041}_{-0.034}$ mas corresponding to an equatorial radius of 10.9$^{+0.8}_{-0.6}~R_\odot$, a rotational velocity with a $1~\sigma$ lower limit at $97.7\%$ that of breakup velocity, and a position angle of $114.7^{+6.4}_{-5.7}$ degrees. These parameters are consistent with H$\alpha$ line spectroscopy and infrared-wavelength Michelson interferometric measurements of the star's decretion disk. This is the first measurement of an oblate photosphere using intensity interferometry.