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Issue A&A
Volume 377, Number 2, October II 2001
Page(s) 538 - 556
Section Formation, structure and evolution of stars
DOI http://dx.doi.org/10.1051/0004-6361:20011093



A&A 377, 538-556 (2001)
DOI: 10.1051/0004-6361:20011093

X-ray emission from young stars in Taurus-Auriga-Perseus: Luminosity functions and the rotation -activity -age -relation

B. Stelzer and R. Neuhäuser

Max-Planck-Institut für extraterrestrische Physik, Postfach 1312, 85741 Garching, Germany

(Received 4 September 2000 / Accepted 26 July 2001)

Abstract
We report on a systematic search for X-ray emission from pre-main sequence and young main sequence stars in the Taurus-Auriga-Perseus region. Our stellar sample consists of all T Tauri stars from the Taurus-Auriga region, and all late-type stars from the Pleiades and Hyades clusters which have been observed by the ROSAT PSPC in pointed observations. We present the X-ray parameters for all observed stars in tables. Next to the basic results of the data analysis (such as count rates, exposure time, and off-axis angle) we give X-ray luminosities and hardness ratios for all detected stars. Upper limits are given for non-detections. Detection rates for different spectral types are compiled. We use these results to study the connection between coronal X-ray activity and stellar parameters for different subgroups of our sample. In particular we compile X-ray luminosity functions (XLF), and discuss the relations between X-ray emission and spectral type, age, and rotation, which have been disputed extensively in the past. Here, we study these questions with the largest sample so far. The XLF for classical and weak-line T Tauri stars are different, with weak-lines being the stronger X-ray emitters. Proceeding towards the main-sequence (Pleiades, Hyades) the X-ray luminosity declines for all spectral types examined (G, K, and M stars). Within an age group $L_{\rm x}$ decreases towards later spectral types, while $L_{\rm x}/L_{\rm bol}$ remains constant or even increases, reflecting the opposed influence of stellar radius, i.e. emitting area, and convection zone depth. For a given spectral type the fastest rotators show the highest X-ray luminosity. Rotation rate and X-ray emission are clearly correlated for all groups of stars with power law indices for ${\log{(L_{\rm x}/L_{\rm bol})}}$ versus $\log{P_{\rm rot}}$ of ~-0.7 to -1.5. The study of XLF for binary stars shows that the known unresolved secondaries likely contribute a significant amount to the X-ray emission.


Key words: X-rays: stars -- stars: late-type, pre-main sequence, coronae, activity

Offprint request: B. Stelzer, stelzer@xray.mpe.mpg.de

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