Jaan Laur defended his doctoral thesis "Variability survey of massive stars in Milky Way star clusters"

On 29 August 2017 Jaan Laur defended his doctoral thesis in the University of Tartu "Variability survey of massive stars in Milky Way star clusters".

Supervisors:
dr Indrek Kolka, Tartu Observatory
dr Laurits Leedjärv, Tartu Observatory

Oponents:
professor Andrzej Niedzelski, University of Torun, Poland
dr Michaela Kraus, Tartu Observatory

Summary

The formation and evolution of massive stars is still an open question as many physical phenomena inside massive stars are poorly understood. The field of asteroseismology helps us explore these stellar interiors by observing the surface brightness variability of stars. Although, space missions have made asteroseismic observations of hundreds of solar-like stars, our knowledge of massive stars is relatively poor due to the small number of them observed from space. Ground-based observations can not achieve the precision for proper asteroseismic analysis but, for massive stars with suitably large brightness amplitudes, they can be used to expand on the known asteroseismic constraints. The availability of small-scale telescopes helps to study massive stars with good temporal coverage that is unachievable from space missions.

In order to study massive stars in different evolutionary phases, we monitored 22 star clusters for three years and obtained light curves for over 3000 stars using a telescope in New Mexico, USA. We identified 354 variable stars of which 60% were first discoveries of variability. We used frequency analysis along with the pulsation HR diagram to classify our variable stars and determine multi-periodic stars. Analysing the probability density function of variable stars, we found the upper limit of cluster membership fraction to be higher than predicted from other works.

Mass loss in massive stars determines the evolution of their temperature, luminosity, etc. Mass-loss rates in binary stellar systems can be measured from the change in their periods, indicating either a spin-up or slow-down of the system. To study mass-loss rates in eclipsing massive binaries, we used a Bayesian approach to determine the period change of seven selected binary systems. For this, we used archival data together with our own observations. For three stars, we reported the period change for the first time. In addition, we determined the origin and values of the mass-loss rates for all seven binaries as well as modelled their orbital and stellar parameters.

URL: http://dspace.ut.ee/handle/10062/57044