Astron. Astrophys. 359, 113-130 (2000)

X-rays and regions of star formation: a combined ROSAT-HRI/near-to-mid IR study of the  Oph dark cloud ^*

N. Grosso ^1,2, T. Montmerle ¹, S. Bontemps ³, P. André ¹ and E.D. Feigelson <sup>4

1</sup> Service d'Astrophysique, CEA/DSM/DAPNIA, CEA-Saclay, 91191 Gif-sur-Yvette Cedex, France
² Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstrasse 1, 85740 Garching, Germany
³ Observatoire de Bordeaux, B.P. 89, 33270 Floirac, France
⁴ Departement of Astronomy and Astrophysics, Pennsylvania State University, University Park, PA 16802, USA

Received 21 July 1999 / Accepted 23 March 2000

Abstract

We have obtained two deep exposures of the Oph cloud core region with the ROSAT   High Resolution Imager . The improved position accuracy (1"-6") with respect to previous recent X-ray observations (ROSAT   PSPC , and ASCA ) allows us to remove positional ambiguities for the detected sources. We also cross-correlate the X-ray positions with IR sources found in the ISOCAM survey of the same region at 6.7 and 14.3 µm, in addition to sources (optical and IR) known from ground-based observations, which are young stars (T Tauri stars, with and without circumstellar disks, and protostars). We thus obtain the best-studied sample of X-ray emitting stars in a star-forming region (63 X-ray sources detected, and 55 identified).

We find that there is no statistically significant difference between the X-ray luminosity functions of HRI -detected Class II and Class III sources, i.e., T Tauri stars with and without disks, confirming that the contribution of these disks to X-ray emission (for instance by magnetic reconnection between the star and the disk), or to X-ray absorption, must be small.

X-ray variability of T Tauri stars can be studied by comparing the HRI data with the previously obtained PSPC data, but also using the fact that some HRI observations were done at different epochs. The resulting statistics show that most of the sources are variable, and that their variability is consistent with a solar-like (hence magnetic) flare origin.

We use the information given both by the ISOCAM survey and by our HRI deep exposure to study the T Tauri star population of the Oph dense cores. We confirm that essentially all Class II and Class III sources (embedded T Tauri stars) are X-ray emitters, and that a strong correlation exists between their X-ray luminosity, , and their stellar luminosity, , with . Most of the new ISOCAM Class II sources are not detected, however, which we explain by the fact that their X-ray luminosities "predicted" on the basis of this correlation are too faint to be detected by the HRI .

We predict that 40 unknown faint or embedded Class III sources remain to be discovered in X-rays in the HRI/ISOCAM overlapping area, down to a limit of erg s^-1. We show that the bulk of these unknown Class III sources should be made of low- to very low-mass stars (-0.6 ). Prospects for future detections with XMM-Newton and Chandra are discussed.

Key words: Galaxy: open clusters and associations: individual: ae Ophiuchi cluster stars: pre-main sequence stars: formation X-rays: stars infrared: stars

* Table A1, Fig. A1, Table B1 and Fig. B1 are only available in the on-line edition of the Journal (Table B1 is also available at the CDS).

Send offprint requests to: N. Grosso (ngrosso@xray.mpe.mpg.de)

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Contents

• 1. Introduction
• 2. The ROSAT HRI X-ray observations
  • 2.1. The ROSAT HRI images
  • 2.2. The HRI image analysis
  • 2.3. HRI X-ray source identifications
• 3. Nature and IR properties of the HRI sources
  • 3.1. New Class II and Class III source census after ISOCAM
  • 3.2. Cloud extinctions and stellar luminosities from near-IR photometry
  • 3.3. Nature of the HRI sources from IR data
• 4. X-ray luminosities of the HRI -detected T Tauri stars
  • 4.1. Derivation from source count rates
  • 4.2. Luminosity functions of HRI -detected Class II and Class III sources
  • 4.3. HRI source variability
• 5. X-ray detectability of the embedded T Tauri star population
  • 5.1. X-ray vs. stellar luminosities
  • 5.2. The X-ray undetected T Tauri star population
• 6. The unknown Class III source population
  • 6.1. Compared spatial distributions of the Class II and Class III sources
  • 6.2. Constraints on the nature of the unknown Class III sources
• 7. Summary and conclusions
  • 7.1. Main observational results
  • 7.2. What have we learned?
  • 7.3. The potential of XMM-Newton and Chandra
• Acknowledgements
• Appendices
  • A: HRI X-ray source detection
  • B: optical/IR counterparts of the HRI X-ray sources
  • C: comparison between HRI and PSPC observations
  • D: optical/IR counterpart without IR classification
• References

© European Southern Observatory (ESO) 2000

Online publication: June 30, 2000
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