Astron. Astrophys. 326, 801-810 (1997)

The formation of C-shocks: structure and signatures

Michael D. Smith ¹ and Mordecai-Mark Mac Low <sup>2

1</sup> Astronomisches Institut der Universität Würzburg, Am Hubland, D-97074 Würzburg, Germany
² Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg, Germany
(smith@astro.uni-wuerzburg.de,mordecai@mpia-hd.mpg.de)

Received 28 March 1997 / Accepted 30 April 1997

Abstract

Shock waves in molecular clouds should evolve into continuous or C-type structures due to the magnetic field and ion-neutral friction. We here determine whether and how this is achieved through plane-parallel numerical simulations using an extended version of ZEUS. We first describe and test the adapted code against analytical results, laying the necessary foundations for subsequent works on supersonic ambipolar diffusion, including C-type jets and shock instability.

The evolution away from jump shocks toward the numerous steady C-shock sub-types is then investigated. The evolution passes through four stages, which possess distinctive observational properties. The time scales and length scales cover broad ranges. Specific results are included for shock types including switch, absorber, neutralised, oblique, transverse and intermediate. Only intermediate Type II shocks and `slow shocks', including switch-off shocks, remain as J-type under the low ion levels assumed. Other shocks transform via a steadily growing neutral precursor to a diminishing jump. For neutralised shocks, this takes the form of an extended long-lived ramp.

Molecular hydrogen emission signatures are presented. After the jump speed has dropped to under 25 km s^-1, a non-dissociative jump section can dominate the spectra for a long period. This produces a high-excitation spectrum. Once the jump has further weakened, to 8 km s^-1, the fully developed ion front is responsible for brisk progress towards a constant C-type excitation. The time scale for emission-line variations is ) yr, where n_i is the pre-shock ion number density.

Key words: shock waves MHD ISM: kinematics and dynamics ISM: clouds ISM: jets and outflows ISM: lines and bands

Send offprint requests to: M.D. Smith

Contents

• 1. Introduction
• 2. Framework
  • 2.1. Numerical methods
  • 2.2. Analytical background and parameter definitions
  • 2.3. Scaling
  • 2.4. Test of accuracy
• 3. The evolution from J-shock to C-shock
  • 3.1. The four stages for transverse shocks
  • 3.2. Low Alfvén number shocks
  • 3.3. Oblique field
  • 3.4. Near-parallel field
  • 3.5. Neutralisation in transverse shocks
• 4. Observables
• 5. Conclusions
• Acknowledgements
• References

© European Southern Observatory (ESO) 1997

Online publication: November 3, 1997
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