Polarimetry of rapid rotating Be stars

• Grant number: 17/08747-9
• Support Opportunities: Scholarships in Brazil - Scientific Initiation
• Start date: June 01, 2017
• End date: December 31, 2017
• Field of knowledge: Physical Sciences and Mathematics - Astronomy - Stellar Astrophysics
• Principal Investigator: Alex Cavaliéri Carciofi
• Grantee: Lucas Zillner Pinheiro
• Host Institution: Instituto de Astronomia, Geofísica e Ciências Atmosféricas (IAG). Universidade de São Paulo (USP). São Paulo , SP, Brazil

Abstract

Be stars are massive stars of spectral type B that have or have already had emission lines in their spectra. These lines are associated with the presence of a circumstellar decretion disk, formed from material ejected by the central star. Another feature that distinguishes them from other B stars is their extremely high rotation rate, often close to critical angular velocity. Be stars disks can also be studied through polarimetric observations. When the stellar light passes through the disk, the light becomes partially polarized by electron scattering. The canonical polarimetric signature of Be star's disk is a negative slope spectrum for denser disks, or near-zero inclination in the case of thin disks. However, Klement et al. (2015) observed a polarized spectrum with a positive slope for the beta CMi Be star. This surprising result is apparently linked to the star's high rotation, which causes the stellar flux to redirect to the pole, an effect known as gravity darkening. This project has the objective to initially investigate the cause of polarized positive spectrum in beta CMi using radiative transfer models and to establish if the real cause is the high stellar rotation. The models will also be used to study how positive polarized spectrum can be used to infer the stellar rotation rate. Finally, the BEACON polarimetric database will be searched for new examples of positive polarized Be stars, focusing on a future scientific paper about this subject. (AU)