Astron. Astrophys. 331, 193-210 (1998)

X-rays and star formation: ROSAT observations of the Monoceros and Rosette molecular clouds

J. Gregorio-Hetem ^{1, 2}, T. Montmerle ¹, S. Casanova ¹ and E.D. Feigelson <sup>3

1</sup> Service d'Astrophysique, CEA/DAPNIA/SAp, Centre d'Études de Saclay, F-91191 Gif-sur-Yvette Cedex, France
² Universidade de São Paulo - IAG/USP, Depto Astronomia, Caixa Postal 3386 - 01060-970 - São Paulo/SP, Brazil
³ Department of Astronomy & Astrophysics, Pennsylvania State University, University Park, PA 16802, USA

Received 8 March 1997 / Accepted 24 October 1997

Abstract

We report ROSAT observations of two molecular clouds previously unstudied in the X-ray band, examining the possibility that X-rays can serve as tracers of star formation at distances 1 kpc. The targets are the molecular clouds associated with the Monoceros R2 reflection nebula (d = 0.8 kpc) and with the Rosette giant HII region (d = 1.5 kpc). In both cases, the PSPC images show point sources and extended emission. Most of the 41 distinct ROSAT sources in the Monoceros cloud and 21 sources in the Rosette cloud have optical counterparts on digitized plates, but a significant minority (20 - 30%) do not and are probably embedded in the clouds. Comparison with recent near-infrared surveys of the Monoceros and Rosette clouds supports the broad correspondence, previously found in nearby clouds, between X-ray and near-infrared sources associated with Young Stellar Objects. X-ray luminosities are erg s^-1. The X-ray hardness ratios and ratios of the Monoceros and Rosette X-ray sources are comparable to those found for low- and intermediate-mass pre-main sequence stars in nearby star forming clouds. We conclude that most of the point-like ROSAT sources are T Tauri and Herbig Ae/Be stars.

Extended X-ray "hot spots" are seen in both clouds, which likely result from the partially resolved emission of embedded young star clusters, similar to the Oph cluster. The detection of individual stars in the high-luminosity tail of the low-mass stellar population, and of embedded young star clusters in star-forming molecular cores, confirm that X-rays can efficiently trace low- and intermediate-mass star formation at significant distances across the Galaxy.

Key words: stars: ISM: Monoceros clouds; Rosette nebula; pre-main sequence; formation X-rays: stars

Send offprint requests to: J. Gregorio-Hetem

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Contents

• 1. Introduction
• 2. The target clouds
  • 2.1. The Monoceros cloud
  • 2.2. The Rosette cloud
• 3. The ROSAT images
  • 3.1. Image analysis
  • 3.2. Source detection and extended features
• 4. X-ray point source identifications
  • 4.1. Optical counterparts
    • 4.1.1. Method
    • 4.1.2. Visible counterparts
    • 4.1.3. Blank fields
  • 4.2. Infrared counterparts
• 5. Properties of the X-ray sources
  • 5.1. Individual fluxes and luminosities
  • 5.2. X-ray hardness ratios
  • 5.3. ratios
  • 5.4. Summary: nature of the X-ray source population
• 6. X-ray "hot spots" and extended emission
  • 6.1. The Monoceros X-ray hot spot and molecular core
  • 6.2. The Rosette X-ray hot spot and CO observations
  • 6.3. Diffuse emission associated with the Rosette HII region
• 7. Summary and conclusions
  • 7.1. ROSAT observations of the Monoceros and Rosette clouds
  • 7.2. Relevance for star formation studies
• Acknowledgements
• References

© European Southern Observatory (ESO) 1998

Online publication: February 4, 1998
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