# Angular Sizes and Effective Temperatures of O-type Stars from Optical Interferometry with the CHARA Array

## TL;DR

This study uses optical interferometry with the CHARA Array to measure the angular sizes and effective temperatures of six O-type stars, including the first direct measurement of rotational oblateness for a rapidly rotating star.

## Contribution

First direct measurement of rotational oblateness in an O-type star using optical interferometry, and validation of model-based angular diameters with observed data.

## Key findings

- Angular diameters range from 0.11 to 0.55 milliarcseconds.
- Rotational oblateness of $$ Oph measured for the first time.
- Model-based and observed angular diameters are in good agreement.

## Abstract

We present interferometric observations of six O-type stars that were made with the Precision Astronomical Visible Observations (PAVO) beam combiner at the Center for High Angular Resolution Astronomy (CHARA) Array. The observations include multiple brackets for three targets, $\lambda$~Ori~A, $\zeta$~Oph, and 10~Lac, but there are only preliminary, single observations of the other three stars, $\xi$~Per, $\alpha$~Cam, and $\zeta$~Ori~A. The stellar angular diameters range from 0.55 milliarcsec for $\zeta$~Ori~A down to 0.11 mas for 10~Lac, the smallest star yet resolved with the CHARA Array. The rotational oblateness of the rapidly rotating star $\zeta$ Oph is directly measured for the first time. We assembled ultraviolet to infrared flux measurements for these stars, and then derived angular diameters and reddening estimates using model atmospheres and an effective temperature set by published results from analysis of the line spectrum. The model-based angular diameters are in good agreement with observed angular diameters. We also present estimates for the effective temperatures of these stars derived by setting the interferometric angular size and fitting the spectrophotometry.