Interferometric observations of hot stars

Accurate determination of fundamental stellar properties is of vital importance in understanding and testing models of stellar structure and evolution. To date, most stars with independent determinations of stellar properties based on high-resolution observations by optical interferometers belong to stars of type F5 or later, which are characterized by the existence of a convective envelope. Early-type stars represent a relatively modest fraction in those studies, mainly for two reasons: angular resolution at a given brightness and stellar rotation. Because their radiation peaks in the visible part of the spectrum, they are usually both faint and unresolved for an infrared beam combiner. Their almost ubiquitous high rotation rates require accurate diameter estimates at a range of position angles. The recent compilation of diameters and inferred stellar properties in Boyajian et al. (2013) shows no star hotter than Teff= 10000 K (spectral type A0), and shows a much less dense coverage in the A0-F5 spectral range. Similar measurements for earlier types come almost exclusively from the seminal (but now half-century old! ) observations with the Narrabri Stellar Intensity Interferometer (NSII, Hanbury Brown et al. 1974a; Code et al. 1976). This thesis adds 10 new stars with spectral types earlier than F6 for which we have obtained fundamental properties. Comparison of the results with existing color-temperature relations show less robust results when stars with radiative envelopes are included in the sample, mostly due to the lack of observations, and the orientation-dependent scatter in effective temperature and luminosity induced by rotation (Maestro et al. 2013). The role of rotation in standard stellar models has been generally assumed to be negligible for the last decades, in order to make their computation tractable. High angular resolution studies of rapid rotators using optical interferometry in the nearinfrared (e.g. McAlister et al. 2005; Monnier et al. 2007) revealing the fingerprint of rotation in distorted photospheres have re-invigorated this field, based on onedimensional Roche-von Zeipel models (point mass plus rigid rotation) that incorporate rotation rate and gravity darkening as independent parameters. The recent development of two-dimensional stellar models including a self-consistent rotation description (incorporating differential rotation) in stars with radiative envelopes (Espinosa Lara and Rieutord 2011, 2013) has provided a method to describe the effects of rotation that links them to fundamental properties, based on a small set of parameters determining the stellar structure (mass, composition, rotation). In this thesis, we have incorporated these 2-D models into a code capable of generating a model synthesizing all observables into a robust and self-consistent framework. Using PAVO interferometric measurements, Hipparcos parallaxes, photometry and spectroscopy in the ultraviolet, visible and infrared we have derived fundamental stellar parameters describing four A-type stars exhibiting projected velocities in excess of 200 km s−1. The luminosities and effective temperatures obtained, together with rotation dependent evolutionary tracks are used to determine mass and age of the stars, corrected for the bias induced by the stellar rotation axis orientation.