Astron. Astrophys. 324, 249-262 (1997)

Cometary globules

III. Triggered star formation in IC 1848

Bertrand Lefloch ^{1, 2}, Bernard Lazareff ³ and Alain Castets <sup>2

1</sup> IRAM, Avda. Divina Pastora 7,N.C., E-18012 Granada, Spain
² Laboratoire d'Astrophysique, Observatoire de Grenoble, Université Joseph Fourier, B.P. 53X, F-38041 Grenoble, France
³ IRAM, 300 Rue de la Piscine, F-38406 St. Martin d'Hères Cedex, France

Received 20 November 1996 / Accepted 31 January 1997

Abstract

We present a multiwavelength study from molecular and continuum observations of the cometary globule CG5 in the HII region IC1848. CG5 is associated with the luminous IRAS point source 02252+6120 (L= ) and shows evidence for an energetic outflow and ongoing star formation (a cluster of low-mass stars has formed inside the cloud). CG5 is presently undergoing a second episode of photo-ionisation from a cluster of O stars in the nebula, characterised by a shock at the surface of the cloud; the shock was detected in several millimeter lines (CO, CS) and in the thermal continuum dust emission. Photo-ionisation conditions at the surface of the globule were estimated from measurements of the VLA free-free continuum emission. A virial analysis suggests that CG5 is probably permeated by a static magnetic field of strength and is subject to an outer pressure 4 times as large as its inner pressure. It is in a globally magnetically subcritical state whereas its core is close to virial equilibrium. Photo-ionisation is found to play a key role in the evolution of CG5 because a) it confines the molecular gas and contains the gas dispersal favored by the energy injection from the outflow while evaporating the superficial layers at the same time , b) the overpressure of the compressed gas ahead of the ionisation front is sufficient to trigger the collapse of initially (sub)critical condensations inside the cloud and initiate star formation. We find that the outflow contains enough momentum to have contributed to this star-formation process. In addition, the energy of the outflow is sufficient to sustain turbulence in the cloud and maintain it in a magnetically subcritical state, i.e. it tends to stabilize the cloud against collapse and regulate star formation.

Key words: ISM: general ISM: clouds ISM: kinematics and dynamics ISM: star formation ISM: molecules

Send offprint requests to: B. Lefloch

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Contents

• 1. Introduction
• 2. Morphology and location in Sharpless 190
• 3. Ionising conditions at the boundary of CG5
  • 3.1. Free-free emission of a photo-ionised globule
  • 3.2. Observations
  • 3.3. Quantitative analysis
• 4. Structure of molecular gas
  • 4.1. The CO(2-1) data
  • 4.2. The CO data
  • 4.3. The C O data
  • 4.4. The CS and C S data
• 5. Bipolar outflow
  • 5.1. The CO outflow
    • 5.1.1. Morphology
  • 5.2. Physical parameters of the outflow
  • 5.3. The CS emission
• 6. Thermal dust emission
• 7. Discussion
  • 7.1. Dynamical interaction of CG5 with the UV flux
  • 7.2. Pressures and magnetic field
  • 7.3. Gravitational stability and star formation
  • 7.4. Multiple star formation in CG5
• 8. Conclusion
• References

© European Southern Observatory (ESO) 1997

Online publication: May 26, 1998

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