The paper is highly cited because it analyzes the longest, most sensitive, and most comprehensive observation ever acquired on the X-ray emission of T Tauri stars (TTS), i.e., very young stars with less than about two solar masses.

“The paper is highly cited because it analyses the longest, most sensitive, and most comprehensive observation ever acquired on the X-ray emission of T Tauri stars (i.e. very young stars with less than about 2 solar masses).”

Stars like the Sun emit a small fraction of their light in the X-ray band due to the presence of hot, magnetically confined plasma at their surface. It has been known for more than 25 years that young solar-like stars are hundreds or thousands of times more X-ray luminous than the Sun today.

But these early studies observed only small samples of young stars with short exposures, and left many questions unanswered. In the "Chandra Orion Ultradeep Project" (COUP), a collaboration of 37 scientists from eight countries observed the nearest rich cluster of very young stars in the Orion Nebula with the Chandra X-ray observatory for a period of 13 days.

With its very long observing time and the resulting detection of more than 1,600 X-ray sources, the COUP removed many limitations of earlier data sets and allowed a detailed look at the relations between the X-ray emission and stellar parameters of the T Tauri stars.

I was selected as one of the science group leaders by the principal investigator of COUP, Professor Eric Feigelson of the Department of Astronomy and Astrophysics at Penn State. The work of the COUP team was very efficient and successful; the initial set of 13 scientific papers filled a whole volume of the Astrophysical Journal Supplements.

The detailed knowledge of the X-ray properties of young stars obtained from the COUP provides essential templates for studies of more distant young clusters and for studying star formation on a galactic scale.

Not directly, but the objects in the sky and the question about the abundance of life in the universe have always been of great interest to humans. An important aspect in that respect is that many T Tauri stars are surrounded by circumstellar disks of dust and gas, out of which planetary systems can form. The strong X-ray emission of the young stars has direct implications for the physical processes involved in the formation of planets and the early evolution of protoplanetary atmospheres.