| Interferometry has the advantage of overcoming the diffraction limit of individual telescopes and achieving higher angular resolution. Modern long baseline optical interferometers can reach sub-milliarcsecond angular resolution, allowing us to resolve nearby stellar systems to reveal the detailed structures of stellar surfaces as well as circumstellar disks. This thesis work includes a scientific study of two rapidly rotating stars, a Be star observed in H band by an optical interferometer (CHARA/MIRC), and instrumental developments on the interferometer.;A solid body rotation model fitting the interferometric data of the two rapid rotators Beta Cas and Alpha Leo reveals close-to-breakup rotations in both cases, which result in oblate geometry of the photospheres. Consequently, the equatorial temperatures are much lower than the polar temperatures due to the gravity darkening effect. Model-independent photospheric images are constructed, confirming the geometry and temperature distribution from the model fitting. A rotational correction is pro- posed to more accurately estimate stellar ages and masses of rapid rotators on the traditional HR diagram. The correction takes into account the non-uniform temperature distribution and the oblate geometry, which can only be obtained through optical interferometry. The preferred non-standard gravity darkening coefficients of rapid rotators from this work agrees with previous studies, suggesting a breakdown of von Zeipel's law. One possible explanation is that the temperature and pressure difference across latitudes caused by the rotation induces meridional flow, which violates the radiative envelope assumption.;Spectroscopic and photometric observations of the high-eccentricity Be binary system Delta Sco during the periastron in 2000 indicated that the secondary passage triggered the gaseous disk formation around the primary star. Our 7 nights of interferometric observations of the system right after its periastron in 2011 resolved the binary as well as the circumstellar disk around the primary. The modeling and imaging results showed a mainly symmetric disk with stable and consistent H band flux contributions from the primary disk over the 7 nights. This result suggests no significant material outflow from the primary star due to the gravitational interference from the secondary star, contrary to the results from the periastron in 2000.;In order to improve the scientific results from MIRC and the sensitivity of the CHARA array, I have participated in three instrumental projects. In the first project, I have developed the subsystem Photometric Channels for MIRC to directly measure the fluxes of the individual beams in real time to improve the calibration of the interferometric data. The Photometric Channels have not only reduced the uncertainty of the visibility measurements from 10% to 3%, but also increased the observational efficiency. In the second project, I have upgraded MIRC from a 4-beam combiner to a 6-beam combiner to exploit the full usage of the CHARA 6 telescopes. The upgrade obtains 2.5 times more of visibility measurements and recovers ∼ 3 times more of phase information in a single snapshot, allowing imaging and modeling of more complex stellar systems such as circumstellar disks and spotted stars. In the third project, I have developed the Wavefront Sensor (WFS) for the CHARA Adaptive Optics upgrade. The WFS commissioning run in January 2014 has shown an improvement of sensitivity of 4 magnitudes in R band, allowing 5 times more Young Stellar Objects (YSOs) in Taurus to be observable with the CHARA array, as well as a few brightest Active Galactic Nuclei (AGN) and microquasars. |
