Stellar surface imaging

Long-baseline optical interferometry has provided a method with which stars are not only resolved, but directly imaged. Resolving features on stellar surfaces is becoming routine for observers using the Center for High Angular Resolution Astronomy (CHARA) Array or the Very Large Telescope Interferometer (VLTI), in particular. The CHARA Array presently has six 1-m telescopes at permanent locations with baselines ranging from 34-331 m.1 Currently for imaging, the six CHARA Array telescopes are combined with the Michigan InfraRed Combiner-eXeter (MIRC-X; formerly the Michigan Infrared Combiner, MIRC).2 Working in H- and J -bands, MIRC-X provides an angular resolution of θH ∼ 0.5 mas and θJ ∼ 0.4 mas, respectively. Four telescopes at a time can be combined at VLTI, either the 8.2-m Unit Telescopes (UTs) or the 1.8-m Auxiliary Telescopes (ATs). The ATs can be moved to provide multiple configurations to increase the possible baselines (up to 130 m). The Precision Integrated-Optics Near-Infrared Imaging ExpeRiment (PIONIER),3 which operates in H-band (θH ∼ 1.3 mas), is the current beam combiner frequently used for imaging. In the following sections, the types of stars imaged by long-baseline optical interferometry will be discussed with significant astrophysical results highlighted.