Astron. Astrophys. 349, 475-484 (1999)

Radio counterparts to extreme X-ray YSO's

Kester Smith ^1,2, Manuel Güdel ^2,1 and A.O. Benz <sup>1

1</sup> Institut für Astronomie, ETH-Zentrum, CH-8092 Zürich, Switzerland (kester, guedel, benz@astro.phys.ethz.ch)
² Paul Scherrer Institut, Würenlingen und Villigen, CH-5232 Villigen PSI, Switzerland

Received 26 February 1999 / Accepted 29 June 1999

Abstract

We search for radio counterparts to two recently-detected strong X-ray sources associated with highly embedded young stellar objects (SVS4/EC 95 and SVS16). We detect a radio source (S68-2) consistent with the position of EC95. We fail to detect a counterpart for SVS16, and place upper limits on its quiescent radio brightness. For S68-2, we show that the radio source has a falling spectrum, suggestive of a gyrosynchrotron emission mechanism, and that it is variable on a timescale of years. We search for, but do not detect, evidence for flaring activity on timescales of minutes to hours. We also search for, but do not detect, circular polarisation. We derive the radio luminosity and compare the object to an empirical X-ray - radio luminosity relationship established for dMe stars. We find that the object is consistent with the dwarfs relation, but is unusually X-ray rich compared to other high-luminosity coronal sources. By comparing the objects to a sample of active galactic nuclei in the - diagram, we rule out the possibility that either object is a background AGN. We discuss the ways in which a normal stellar coronal model might be modified to explain the strong, X-ray rich characteristics of the source, which appears to be the most extreme stellar corona yet found.

Key words: stars: activity stars: coronae stars: magnetic fields stars: pre-main sequence radio continuum: stars X-rays: stars

Send offprint requests to: K. Smith

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Contents

• 1. Introduction
• 2. VLA archive data
• 3. The EC95/Ser-X3 region
  • 3.1. Properties of the radio source associated with EC95
• 4. Search for a radio counterpart to SVS16/X11
• 5. X-ray properties of Ser-X3 and X11
  • 5.1. Flaring versus quiescent emission
• 6. Discussion
  • 6.1. L_X - L_R relation for coronal sources
  • L_X/L_Bol and the problem of saturation
  • 6.2. The nature of SVS16 - colliding winds?
  • 6.3. The nature of EC95
    • HAeBe coronae
    • Companion source
  • 6.4. Background object
• 7. Summary
• Acknowledgements
• References

© European Southern Observatory (ESO) 1999

Online publication: September 2, 1999
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