Astron. Astrophys. 318, 416-428 (1997)

Rotation of the outer disc from classical cepheids ^*

F. Pont , D. Queloz , P. Bratschi and M. Mayor

Geneva Observatory, CH-1290 Sauverny, Switzerland

Received 19 April 1996 / Accepted 13 June 1996

Abstract

Radial velocities and distances have been measured for a sample of 48 remote classical cepheids located in the outer disc of the Galaxy (). The distances are determined from BVI photometry, with semi-empirical metallicity corrections calibrated on the Magellanic Clouds. Using these cepheids as tracers, the rotation curve of the disc is determined between and . The result is a flat rotation curve about 30 km s^-1 lower than , km s^-1 for kpc and km s^-1 assumed, or km s^-1 for kpc and km s^-1.
The possible presence of non-axisymmetric components in the rotation of the outer disc is considered. We find a very small or vanishing value for any radial motion of the LSR or expansion/contraction motion. In particular, the data exclude large radial motions such as have been proposed in some models to account for features observed in the kinematics of the gas.
The rotation curve indicated by cepheids is markedly lower than that currently derived using HII regions. Possible explanations are examined. A real kinematical difference is possible, caused for instance by radial motions in the gas induced by spiral arms, but an important distance scale difference is not excluded. A systematic error added to a high dispersion on HII region distances could explain the mismatch.

Key words: stars: distance kinematics cepheids Galaxy: kinematics and dynamics

* Based on observations made at the Observatoire de Haute-Provence, France and at the European Southern Observatory, La Silla, Chile.

Send offprint requests to: F. Pont

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Contents

• 1. Introduction
• 2. Observations
  • 2.1. Radial velocities
  • 2.2. Photometry
  • 2.3. Mean values
• 3. Distance determination
  • 3.1. Distances using (B-V) et V
  • 3.2. Distances using (V-I) and I
  • 3.3. Choice of distance scale
  • 3.4. Metallicity gradient
  • 3.5. Overtone pulsators
  • 3.6. Type II cepheids
  • 3.7. Position in the Galaxy
• 4. Galactic rotation
  • 4.1. Axisymmetric solution
  • 4.2. Non-axisymmetric components
• 5. Discussion
  • 5.1. Kinematical difference
  • 5.2. Distance scale difference
  • 5.3. Effect of changes in the assumptions
  • 5.4. Non-axisymmetric models of the Galaxy
• 6. Conclusion
• Acknowledgements
• Appendix A
• References

© European Southern Observatory (ESO) 1997

Online publication: July 8, 1998
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