The fundamental parameters of the roAp star alpha Circini

TL;DR

This study provides the first direct measurement of the radius and temperature of the roAp star alpha Cir using interferometry and spectra, revealing its physical properties and surface composition.

Contribution

It presents the first interferometric measurement of an roAp star's radius and temperature, offering new observational constraints for stellar models.

Findings

Radius of alpha Cir is 1.967±0.066 Rs.

Effective temperature is 7420±170 K, lower than previous estimates.

Spectral analysis shows nearly solar abundances of some elements and overabundance of others.

Abstract

We have used the Sydney University Stellar Interferometer (SUSI) to measure the angular diameter of alpha Cir. This is the first detailed interferometric study of a rapidly oscillating A (roAp) star, alpha Cir being the brightest member of its class. We used the new and more accurate Hipparcos parallax to determine the radius to be 1.967+-0.066 Rs. We have constrained the bolometric flux from calibrated spectra to determine an effective temperature of 7420+-170 K. This is the first direct determination of the temperature of an roAp star. Our temperature is at the low end of previous estimates, which span over 1000 K and were based on either photometric indices or spectroscopic methods. In addition, we have analysed two high-quality spectra of alpha Cir, obtained at different rotational phases and we find evidence for the presence of spots. In both spectra we find nearly solar abundances of C, O, Si, Ca and Fe, high abundance of Cr and Mn, while Co, Y, Nd and Eu are overabundant by about 1 dex. The results reported here provide important observational constraints for future studies of the atmospheric structure and pulsation of alpha Cir.