Apsidal motion in the short-period massive binary BD+60° 497

Abstract

A massive binary is a system of two massive stars orbiting around a common center of mass. A star is considered massive if its mass is greater than 8 M⊙. BD+60° 497 is a spectroscopic massive binary, a system with periodic variations in the wavelengths of its spectral lines. This binary has a short orbital period of a few days, and it can be classified as a close binary system. The short-period and the proximity between the two stars mean that important tidal interactions are at play in the system. This will cause dynamical deformations of the surfaces of the stars during their orbit, deformations that will change the expression of the gravitational potential of the system. This non-spherical potential will cause periodic variations of the orbital parameters of the binary. The most prominent consequence of these tidal interactions is the apsidal motion, that is the slow precession of the line connecting the periastron (closest point) and apastron (farthest point) of the orbit over a long period of hundreds of years. The rate of apsidal motion is directly linked to the internal structure of the stars inside the binary. Previous studies of BD+60° 497 have shown that the system undergoes a possible apsidal motion. Throughout this thesis, we will first discuss some concepts around massive stars, massive binaries, and their evolution. In the chapter 3, we will apply a disentangling method to the spectra of the binary to obtain the individual spectra of the stars. The disentangling of the spectra also comes with the computation of the radial velocities, which we will use to study the orbital solution, accounting for apsidal motion,and compare it with previous results. After that, we will properly introduce the apsidal motion, its mathematical expression, and the in- ternal structure constant linked with the apsidal motion rate. In the chapter that follows, we will apply an orbital fit code to two epochs of radial velocities data to show the ongoing apsidal motion in the system. A change in the value of the longitude of periastron ω over time will confirm the motion of the apsidal line. After that, we will compute the theoretical value of the rate of apsidal motion for different ages of the system. Using a global fitting method, we will determine the observational value of ˙ω, and then we will compare the two values. A section of this chapter will also talk about the photometry of the binary. Finally, we will conclude this thesis by reminding the reader of the results obtained in this work and the possible interpretations around them.