Colloquium on Dec. 5, 2019

Evidence of dynamical processing of the field binary stars

Speaker: Chao Liu (NAOC)

Venue: Room 2111, SWIFAR Building

Time: 3:00 PM, Thursday, December 5, 2019

Abstract: 

Binary stars are very common phenomena in the universe.  In recent years, astronomers are realizing that binaries play critical roles from the evolution of the whole cosmos to the planetary systems. For instance, the massive binaries can be the progenitors of black hole binaries, which are the important sources of gravitational waves. However, it is still not clear how the binaries form and evolve and how they affect the stellar evolution tracks. I would like to talk about the recent studies about the binaries based on the LAMOST and Gaia stars in the solar neighborhood. I made statistics about the fractions of binary system and the mass-ratio distributions for stellar populations with different metallicity and stellar masses. For the first time, the binary fraction is found tightly anti-correlated with the shape of mass-ratio distribution. In other word, the stellar populations with smaller binary fraction contains more binaries with larger companions, and the populations with larger binary fractions have more binaries with smaller companions. This substantial anti-correlation strongly implies that binary stars have experienced significant dynamical processing in the early times when they were still in the embedded star clusters. The dynamical processing tends to destroy binaries with smaller primary mass, smaller mass-ratio, and wider separation. Consequently, the populations containing smaller stellar mass is more effectively influenced and hence contains less binaries, many of which have larger mass-ratio and shorter period. However, the populations with larger stellar mass are less affected by the dynamical processing. These are evident that the present-day binary properties of field main-sequence stars do not necessarily reflect the original status when the binaries form.