Astron. Astrophys. 332, 703-713 (1998)

Monte Carlo simulations at very high optical depth:
non-LTE transfer in H₂ O in the protostellar object B 335

Daniel Hartstein and René Liseau

Stockholm Observatory, S-133 36 Saltsjöbaden, Sweden (daniel@astro.su.se; rene@astro.su.se)

Received 12 March 1997 / Accepted 4 December 1997

Abstract

We present a formulation of the Monte Carlo method which should be capable of treating radiative transfer problems even at very high optical depths () and apply this tool to predict rotational H₂ O line spectra for the protostellar object B 335 for future observational tests with space borne facilities. The physical model of the source is based on published observations in lines of CS, for which we obtain model results which are in agreement with previous computations. We apply our model also to line profiles of CO isotopomers observed at high spatial resolution and derive the total CO cooling rate for B 335. From the comparison with the derived H₂ O cooling rates, we are led to conclude, quite generally, that H₂ O is probably not the major coolant of low-mass protostellar collapse.

Key words: ISM: clouds ISM: individual objects: B 335 ISM: molecules ISM: kinematics and dynamics stars: formation methods: numerical

Send offprint requests to: R. Liseau

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Contents

• 1. Introduction
• 2. The numerical method
  • 2.1. Monte Carlo simulations of radiative transfer
  • 2.2. MCM and very high optical depths: the core saturation method
  • 2.3. Continuum radiation from associated dust
  • 2.4. Performance and testings of the code
    • 2.4.1. Tests by comparison with other methods
    • 2.4.2. Choice of core parameter
    • 2.4.3. Numerical accuracy and computational speed
• 3. Results
  • 3.1. CS lines: determining the physical model
  • 3.2. Isotopic CO lines: infall profiles?
  • 3.3. H₂ O line spectra: the ortho-to-para ratio
  • 3.4. Prediction of H₂ O line spectra: ODIN observations
  • 3.5. Prediction of H₂ O line spectra: FIRST observations
• 4. Discussion
  • 4.1. CS line wings: infall or outflow?
  • 4.2. H₂ O rotational line emission
  • 4.3. CO and H₂ O cooling rates
  • 4.4. Limitations of the presented model and future work
• 5. Conclusions
• Acknowledgements
• Appendix
  • Appendix A: the Monte Carlo method of radiative transfer
  • Appendix B: Monte Carlo method with core saturation
    • B.1. Implementation of the core saturation method
    • B.2. Correcting the core saturation for lost photons
• References

© European Southern Observatory (ESO) 1998

Online publication: March 23, 1998
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