Astron. Astrophys. 354, 621-635 (2000)

T Tauri stars: The UV/X-ray connection

V.M. Costa ^1,2, M.T.V.T. Lago ^1,3, L. Norci ⁴ and E.J.A. Meurs <sup>4

1</sup> Centro de Astrofisica da Universidade do Porto, Rua das Estrelas, 4150 Porto, Portugal
² Departamento de Matemática, Instituto Superior de Engenharia do Porto, Portugal
³ Departamento de Matemática Aplicada, Faculdade de Ciências da Universidade do Porto, Portugal
⁴ Dunsink Observatory, Castleknock, Dublin 15, Ireland

Received 26 March 1999 / Accepted 22 November 1999

Abstract

We have analysed newly calibrated IUE and ROSAT data for three T Tauri stars from different subclasses: TW Hya, V410 Tau and CS Cha, a Classical T Tauri star (CTTS), a Weak T Tauri star (WTTS) and a T Tauri star intermediate between CTTS and WTTS, respectively. In the ultraviolet the continuum seems to be well explained by the sum of the stellar black body emission plus a hydrogenic component with temperature in the range 1.4 to 510⁴ K. TW Hya requires additionally a third component at 7900 K covering approximately 5% of the stellar surface. Using UV line fluxes, we have also analysed the energy emitted by these stars up to temperatures of 10⁵ K. All three stars show emission strongly enhanced relatively to the Sun and peaking at temperatures characteristic of the transition region. In this respect the behaviour of the three stars seems to extend in a harmonic way from the ultraviolet into the X-ray band. The X-ray spectral analyses are consistent with emission from a two-temperature, optically thin plasma in collisional equilibrium. The lowest temperature plasma is at about 10⁶ K while the highest reaches 10⁷ K. We have also checked for and found no evidence for flaring activity in our sources at the time of the observations. Nevertheless, some level of variability is present for the stars in the sample. The ROSAT datasets for V410 Tau show a variation in terms of count rate on timescales of months to years.

Key words: stars: evolution stars: pre-main sequence ultraviolet: stars X-rays: stars

Send offprint requests to: vcosta@astro.up.pt

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Contents

• 1. Introduction
• 2. The sample of stars
  • 2.1. TW Hya
  • 2.2. V410 Tau
  • 2.3. CS Cha
• 3. The UV observations and analysis
  • 3.1. The line identification
  • 3.2. The continuum
  • 4.1. Source detection and identification
  • 4.2. The spectral analysis
  • 4.3. Variability
    • 4.3.1. Short-term variability
    • 4.3.2. Long-term variability
• 6. Conclusions
• Acknowledgements
• References

© European Southern Observatory (ESO) 2000

Online publication: February 9, 2000
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