Astron. Astrophys. 359, 1147-1161 (2000)

Spectroscopy of molecular hydrogen in outflows from young stars ^*

J. Eislöffel ¹, M.D. Smith ² and C.J. Davis <sup>3,4

1</sup> Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
² Armagh Observatory, College Hill, Armagh BT61 9DG, Ireland
³ School of Cosmic Physics, Dublin Institute of Advanced Studies, 5 Merrion Square, Dublin 2, Ireland
⁴ Joint Astronomy Centre, 660 N. A'ohoku Place, University Park, Hilo, HI 96720, USA

Received 20 March 2000 / Accepted 23 May 2000

Abstract

We present new medium-resolution longslit spectra in the near-infrared and examine the excitation of molecular hydrogen across several outflows from young stars. In contrast to previous studies, in which the brightest patches in the flows were selected, we find evidence for variations in excitation with position within several sources.

We present and comment on a number of H₂ excitation mechanisms of outflows from the literature, and compare them with our observations. H₂ position-velocity diagrams and line ratios support a picture in which the excitation variations are caused by the large-scale shock geometry. The variations are best interpreted by magnetohydrodynamic C-shocks, although some locations which demonstrate hydrodynamic J-type excitation are found. In particular, VLA 1623 is accurately modelled by a C-type bow, whereas a planar J-type model is favoured for HH 57. HH 1/2 consists of a collection of molecular bow and planar shocks. The H₂ emission from the leading edge of the HH 1 bow structure is actually from a low-excitation oblique wing from one of many mini bow shocks rather than from fluorescence or a magnetic precursor.

In two of our objects, the jets themselves are also traced in molecular hydrogen emission. Radial velocities in the infrared jet in HH 47C demonstrate acceleration consistent with the proper motions, and hint at ballistic motions originating in an eruptive event about 280 years ago. In this jet, as well as in the HH 1 infrared jet, we find the excitation to decline with distance from the source.

Key words: shock waves molecular processes stars: mass-loss ISM: jets and outflows ISM: kinematics and dynamics ISM: molecules

* Based on observations collected at the European Southern Observatory, La Silla, Chile.

Send offprint requests to: J. Eislöffel

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Contents

• 1. Introduction
• 2. Observations and data reduction
• 3. Diagnostic background
  • 3.1. Excitation of H₂ and the Column Density Ratios (CDR)
  • 3.2. Excitation models for the H₂ gas
• 4. Results
  • 4.1. HH 1/2
    • 4.1.1. Radial velocities and H₂ line fluxes in HH 1/2
    • 4.1.2. Excitation of H₂ in HH 1/2
  • 4.2. HH 46/47
    • 4.2.1. Radial velocities and H₂ line fluxes in HH 46/47
    • 4.2.2. Excitation of H₂ in HH 46/47
  • 4.3. VLA 1623
    • 4.3.1. Radial velocities and H₂ line fluxes in VLA 1623
    • 4.3.2. Excitation of H₂ in VLA 1623
  • 4.4. NGC 6334I
    • 4.4.1. Radial velocities and H₂ line fluxes in NGC 6334I
    • 4.4.2. Excitation of H₂ in NGC 6334I
  • 4.5. HH 57
    • 4.5.1. Radial velocities and H₂ line fluxes in HH 57
    • 4.5.2. Excitation of H₂ in HH 57
• 5. Discussion
  • 5.1. Excitation models
  • 5.2. The leading H₂ knots in the HH 1 and HH 47C bows
  • 5.3. Kinematics of the southwestern HH 47 counterjet
• 6. Conclusions
• Acknowledgements
• References

© European Southern Observatory (ESO) 2000

Online publication: July 13, 2000
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