Evolutionary inference and statistical constraints on Algols including SD2-type near contact binaries

Abstract

ABSTRACT Algol-type binaries (Algols) have aroused extensive interests due to the idiosyncratic evolutionary stages of both components. To better understand the evolutionary properties of such characteristic population, we collected mostly double-lined samples and performed systematic work concentrated on their physical constraints. It is found that orbital period cut-off for the OB and AF type Algols are P(OB) > 0.482 d, P(AF) >0.358 d; constraints on mean density and surface gravity of the secondary components are $\overline{\rho }_2$(OB) < 0.144 ρ⊙, $\overline{\rho }_2$(AF) < 0.26 ρ⊙, log g2(OB) < 4.719 cgs, and log g2(AF) < 4.517 cgs. Limitations of a(OB) $\gt 3.48\, {\rm R}_{\odot }$ and a(AF) $\gt 2.29\, {\rm R}_{\odot }$ have been deduced from the a–P relation. Moreover, statistical analysis of secular period changes reveal that Algols with higher f1 and relevant shorter period generally show weak period change. It hints that there is some connections between f1 and orbital evolution. Systems with lower f1 have the chance to evolve from classic Algols to SD2-type near contact binaries (NCBs); systems with higher f1 obey the evolutionary channel from SD2-type NCBs to CBs. However, it seems that SD2-type NCBs could not evolve towards classic Algols. As a natural lab, Algols are producing peculiar stellar experimental samples via complicated astrophysical processes, which will enhance our understanding on the physical properties and evolution of such binary populations.