Abstract
The rapidly rotating Be star phi Persei was spun up by mass and angular momentum transfer from a now stripped-down, hot subdwarf companion. Here we present the first high angular resolution images of phi Persei made possible by new capabilities in longbaseline interferometry at near-IR and visible wavelengths. We observed phi Persei with the MIRC and VEGA instruments of the CHARA Array. Additional MIRC-only observations were performed to track the orbital motion of the companion, and these were fit together with new and existing radial velocity measurements of both stars to derive the complete orbital elements and distance. The hot subdwarf companion is clearly detected in the near-IR data at each epoch of observation with a flux contribution of 1.5% in the H band, and restricted fits indicate that its flux contribution rises to 3.3% in the visible. A new binary orbital solution is determined by combining the astrometric and radial velocity measurements. The derived stellar masses are 9.6+-0.3Msol and 1.2+-0.2Msol for the Be primary and subdwarf secondary, respectively. The inferred distance (186 +- 3 pc), kinematical properties, and evolutionary state are consistent with membership of phi Persei in the alpha Per cluster. From the cluster age we deduce significant constraints on the initial masses and evolutionary mass transfer processes that transformed the phi Persei binary system. The interferometric data place strong constraints on the Be disk elongation, orientation, and kinematics, and the disk angular momentum vector is coaligned with and has the same sense of rotation as the orbital angular momentum vector. The VEGA visible continuum data indicate an elongated shape for the Be star itself, due to the combined effects of rapid rotation, partial obscuration of the photosphere by the circumstellar disk, and flux from the bright inner disk.
Highlights
For about 30 years, Be stars have been favorite targets of optical interferometers (Thom et al 1986; Mourard et al 1989), andBased on observations with MIRC-6T and VEGA-4T instruments on the CHARA Array.Table 2 and Appendix A are available in electronic form at http://www.aanda.org they are important objects to study the role of rotation in the link between a star and its close environment
We know of two other Be plus sdO binaries, FY CMa and 59 Cygni, that share many of the properties of φ Persei (Peters et al 2013), and it is probable that other hot companions of Be stars await discovery (Koubský et al 2012, 2014)
Thanks to advanced visible and infrared interferometric instrumentation at the CHARA Array, we have been able to obtain the first high angular resolution images of the Be binary φ Persei, including the first spectrally resolved images in the visible. These images show the highly inclined circumstellar disk surrounding the Be star, and the helium star remnant of the mass donor star is detected for the first time through MIRC observations of its orbital motion
Summary
For about 30 years, Be stars have been favorite targets of optical interferometers (Thom et al 1986; Mourard et al 1989), and. In a recent series of two papers on γ Cas, Smith et al (2012) and Stee et al (2012) have reviewed the most remarkable features of this star: X-ray activity, magnetic field, critical rotation, Keplerian rotation, disk elongation, and binarity These papers have shown once more that a multiwavelength and multitechnique approach is key to making progress in understanding the physics of these complex systems. A second possibility is that Be stars were spun up by mass and angular momentum transfer in a binary (Pols et al 1991; de Mink et al 2013) In this case, the companion would lose most of its envelope and would appear as a hot, stripped-down He-star remnant.
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