Image of the orion nebula, the nearest high-mass star forming region.

From an astronomical perspective stars form quickly. Though forming a star typically takes a few million years, this is negligible compared to the total lifetime of a star. In the case of our Sun, her main sequence life will last for 12 billion years, about a thousand times longer than her formation time. Nevertheless, virtually all parameters of a star, including the existence of planetary systems, are set during its formation. Therefore, understanding star formation is one of the fundamental areas of research in modern astrophysics.

Regions of ongoing star formation are rather distant, at least 400 light-year (or 140 pc) away. They typically contain some hundreds of stars within a small region in the sky, e.g., the Orion Nebular Cloud (right figure). Star formation occurs in molecular clouds which are the coolest parts in our Galaxy (approx. 10 K). Eventually, the dense parts of those clouds start to collapse under their own gravity and the first stellar cores form. Initially, the star is embedded in its natal cloud and hidden from our view. This surrounding envelope disperses within approximately the first million years and the stars become visible in the optical. Those objects, called classical T Tauri stars (CTTS), are still surrounded by a gaseous disk from which they continue to accrete matter. Some fraction of the circumstellar disk might evolve into a planetary system. Finally, Hydrogen burning starts in the stellar interior - a new star is born.

Young stars strongly interact with their environment; they accrete matter and drive powerful outflows. Both processes determine the final appearance of a star on the main-sequence and are thus of major importance for understanding the star formation process. Our group uses major observatories like the Very Large Telescope (VLT) in Chile, the X-ray satellites XMM-Newton and Chandra as well as the Hubble Space Telescope (HST) to study accretion, outflows and the environment of young stars.

Chandra X-ray image of DG Tau (credit: M. Guedel)

Our group studies high-energy phenomena of young stellar objects. Among them is the inverstigation of the X-ray and ultraviolet emission generated in the accretion footpoints, i.e., where the accretion stream impacts the stellar surface. Using temperature and density sensitive line-ratios as well as hydrodynamic simulations, we demonstrated that this extra X-ray emission originates in the high-density environment of the accretion hotspot while being absent in more evolved stars. As an example, the figure at the bottom left shows the X-ray spectrum of the CTTS BP Tau with some of the important lines labeled. Studying these phenomena now includes also multi-wavelength studies, e.g., simultaneous observations with XMM-Newton and the VLT.

Additionally, outflows/jets of young stellar objects (YSOs) generate X-ray and UV emission. This emission from a high-temperature plasma shares many similarities with the hot plasma associated with the accretion process. Therefore, we also study the high-energy phenomena of jets (e.g., the X-ray jet of the star DG Tau; right figure) using XMM-Newton, Chandra and HST. This high-temperature plasma differs greatly from the usually studied cooler gas with YSO outflows and we try to understand the origin of this new plasma component to advance our still fragmentary knowledge of jet launching.

Similarly, we use optical/near-infrared data of YSOs to study those processes and their environment. On the lower right, the spectrum of the CTTS RU Lup is shown which contains emission lines tracing the accretion funnel and outflow properties. We study these features to reveal the interaction of young stars with their surrounding disks and enevelopes as well as the outflow properties.

bptau rulup
Emission lines in CTTS spectra. Left: XMM-Newton X-ray spectrum of BP Tau, right: VLT/X-Shooter optical/near-IR spectrum of RU Lup. Important diagnostic lines are labeled.