Tracing the early lives of stars

The crucial process of star formation must have occurred countless times in our Universe in the past. The earliest phases in the lives of stars determine their future fate. For example, the production of chemical elements, which are used to trace the history of Galactic evolution, depends on the initial mass and metallicity. Also, the angular momentum obtained during the birth process is vital for the further stellar life. In the dusty, circumstellar disks that appear at star formation planetary systems like our own solar system are formed, thus connecting the origin of planets closely to the early evolution of the central stellar object. Therefore, understanding the physical processes that occur in these early stages of stars is essential. Although we have a general concept of how stars are formed and evolve, our current knowledge of early stellar evolution is limited and contains a lot of unsolved questions such as the determination of pre-main sequence (pre-MS) lifetimes and ages, the speed of early stellar evolution and the mass dependent evolution of angular momentum and chemical composition. Young intermediate-mass stars with ~1 to 6 solar masses have similar atmospheric properties than their evolved counterparts in the (post-)main sequence phase. Hence, it is not possible to constrain the evolutionary stage of a given star by its atmospheric properties (i.e., effective temperature, surface gravity and luminosity) alone. Asteroseismology is the only technique to investigate the interiors of pulsating stars through the analysis of their pulsation modes, similar to the study of earthquakes, which allows us to gain knowledge on the interior of the Earth. Each pulsation mode carries different information on the inner structure of the pulsating star. As young stars differ from their more evolved analogues of same atmospheric properties mostly in their interiors, asteroseismology provides an independent method to constrain the evolutionary stage of a field star. It also allows investigating the relevance of various physical processes to early stellar evolution. My recent study (Zwintz et al. 2014) revealed a first connection between the oscillation properties of pre-MS stars and their relative stage in the pre-MS evolution illustrating the enormous potential for asteroseismic methods in the early evolutionary phases. With this project I will conduct for the first time a homogeneous seismic description of pulsating pre- MS stars using dedicated ground- and space based observations and model calculations. I aim to find empirical scaling relations that will allow the direct identification of the pre-MS ages. I will combine the results from asteroseismology with a spectroscopic study of the rotational and chemical properties for selected intermediate mass pre-MS stars. With this work I will contribute to solve some of the open questions of early stellar evolution.

The earliest phases in the lives of stars their childhood and youth belong to the least understood stages of stellar evolution because young stars are still deeply embedded in the gas and dust from which they were born. Hence, they first remain hidden from our view in the optical light until they have evolved enough to become visible, and with it, observable. Some of the open questions of early stellar evolution include the speed of early stellar evolution, the interior structures of young stars, and when and how planets are formed around the early stars. We are also aiming to find the young Sun the Sun as it was at an age of a few million years to test our concept how the Sun and solar system were formed. In the course of the Elise Richter project, I was investigating the above-mentioned topics using a method called asteroseismology which is the study of stellar oscillations. Similar to seismology on the Earth where we use earthquakes to probe the interior structure of our planet, we can analyze star quakes to describe the inner regions of stars. Although a wealth of different types of pulsating stars is known among the older, more evolved, adult stars, the research area of asteroseismology of young stars is relatively young and only exists since about 20 years. In my project Tracing the early lives of stars, I could for the first time measure the speed of early stellar evolution through measurements of the evolutionary changes in the pulsation periods of young stars, and, hence, test whether the theoretical models and the observations agree or disagree. For three stars the measured period changes of 0.001 to 0.005 seconds per year agree to the theoretical predictions, while for the fourth star we measure a period change a factor of 2 larger than predicted. This brings up new questions: why does this star seem to evolve twice as fast as the others and what physical processes cause this effect? Another highlight of my research focused around the young star Pictoris, which hosts a Jupiter-like, giant gas planet, Pictoris b, in its huge circumstellar gas and dust disk. The star itself has a surface temperature of 8200K and, hence, is significantly hotter than our Sun, and shows oscillations with periods ranging from 20 to 45 minutes. The planet, Pictoris b, needs close to 18 years for one revolution around its host star where the closest approach of both objects occurred in the year 2017. Using the most recent observations conducted with the BRITE-Constellation satellite mission I could describe the pulsational properties of Pictoris more accurately and investigate its interior structure. To be able to search for additional planets, comets and moons around the star, a precise description of the pulsations is crucial: even if the pulsation amplitudes are only at maximum a few thousands of the stellar brightness, a signal from a transiting additional planet is even smaller with only a few millionths of the stellar magnitude. In order to find these low signals, the pulsational variability has to be subtracted first based on its most precise analysis.