Thin flux tube models for star spots

The possibility of understanding stellar activity as an up‐scaled version of the activity of our Sun is investigated. A theoretical model to explain properties of sunspots is used for explaining observed latitudes of star spots. The model is based on thin‐flux‐tube simulations that study the path of magnetic flux tubes from their origin in a stellar overshoot layer to photospheric layers. A direct comparison of the simulation results with individual stars is given. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Photometric monitoring of the young star Par 1724 in Orion

We report new photometric observations of the ∼200 000 year old naked weak‐line run‐away T Tauri star Par 1724, located north of the Trapezium cluster in Orion. We observed in the broad band filters B, V, R, and I using the 90 cm Dutch telescope on La Silla, the 80 cm Wendelstein telescope, and a 25 cm telescope of the University Observatory Jena in Großschwabhausen near Jena. The photometric data in V and R are consistent with a ∼5.7 day rotation period due to spots, as observed before between 1960ies and 2000. Also, for the first time, we present evidence for a long‐term 9 or 17.5 year cycle in photometric data (V band) of such a young star, a cycle similar to that to of the Sun and other active stars (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

A ten-year-long peak of the X-ray flux from the burster 4U 1724-307 in the globular cluster Terzan 2: Evolution of the donor star or the influence of a third star?

We analyze observations of the burster 4U 1724-307 (1E 1724-3045) in the globular cluster Terzan 2 from 1971 until 2001. Uhuru, OSO-8, Einstein, and EXOSAT observations are used. In addition, we analyze data from the TTM telescope onboard the Mir/Kvant observatory and from the ASM and PCA instruments onboard the RXTE observatory. Based on PCA/RXTE scanning observations, we have mapped the sky in the Galactic-center region. The derived light curve of the burster 4U 1724-307 shows the source to be variable on a time scale of 30 years: over the first 20 years of observations (1970–1990), the source flux was approximately constant, while in the 1990s, it rose over ～5 years and declined over approximately the same period, approaching its original value. We discuss several scenarios to explain the behavior of the light curve, including the evolution of the mass outflow rate from the donor-star surface, the episodic appearance of a second source in the globular cluster, and gravitational microlensing. As one of the scenarios, we also consider the possibility that the passage of a third star affects the accretion rate in the binary 4U 1724-307. We show that if this event is responsible for the 10-year-long rise in the luminosity of the burster 4U 1724-307, then the size and period of this binary can be estimated.