2001/10/01 by Gerald van Belle, David R. Ciardi, R. R. Thompson +2 · 11 citations
Physics and Astronomy · #Stellar, planetary, and galactic studies #Adaptive optics and wavefront sensing #Astrophysics and Star Formation Studies
paper · doi:10.1086/322340
openalex publication_date 2001/10/01 · openalex created_date 2025/10/10 · openalex updated_date 2026/07/31
We present infrared interferometric angular size measurements for the A7IV-V star Altair that indicate a noncircular projected disk brightness distribution. Given the known rapid rotation of this star, we model the data as arising from an elongated rigid atmosphere. To first order, an ellipse may be fitted to our interferometric diameter measurements, with major and minor axes of 2 a = 3.461 ± 0.038 mas and 2 b = 3.037 ± 0.069 mas, respectively, for a difference of 424 ± 79 μas between 2 a and 2 b , and with an axial ratio of a / b = 1.140 ± 0.029. Assuming that the apparent oblateness of the photosphere is due to the star's rapid rotation, a more rigorous evaluation of the observation data in the context of a rigidly rotating Roche model shows that an estimate of v sin i = 210 ± 13 km s -1 can be derived that is independent of spectroscopic techniques. Also derived are values for the mean effective temperature and the mean linear radius, and an observational constraint upon the relationship between rotation velocity and stellar inclination is established. Altair is the first main-sequence star for which direct observations of an oblate photosphere have been reported and the first star for which v sin i has been established from observations of the star's photospheric geometry.