Astron. Astrophys. 358, 521-534 (2000)

The radial distribution of OB star formation in the Galaxy ^*

L. Bronfman ¹, S. Casassus ^1,2, J. May ¹ and L.-Å. Nyman <sup>3,4

1</sup> Departamento de Astronomía, Universidad de Chile, Casilla 36-D, Santiago, Chile
² Department of Physics, University of Oxford, Keble Road, Oxford OX1 3RH, UK
³ SEST, ESO-La Silla, Casilla 19001, Santiago 19, Chile
⁴ Onsala Space Observatory, 43992 Sweden

Received 6 January 1999 / Accepted 28 February 2000

Abstract

We present the azimuthally averaged radial distribution of 748 regions of OB star formation in the whole galactic disk, based on our previous CS(2-1) survey of UC H II regions. Embedded massive stars produce a total FIR luminosity of within the range in galactocentric radius. We find 492 massive star forming regions within the solar circle, producing of the total FIR luminosity. Separate analyses of the 349 sources in the I and II quadrant (north), and of the 399 sources in the III and IV quadrant (south), yield FIR luminosities (extrapolated to the complete galactic disk) of and of , respectively. Massive star formation is distributed in a layer with its centroid following that of molecular gas for all galactocentric radii, both north and south. Its thickness for is pc (FWHM), the thickness of the molecular gas disk. The FIR luminosity produced by massive stars has a well defined maximum at , with a gaussian FWHM of - compared with for the H₂ surface density distribution. Toward the outer Galaxy, down from the maximum, the face-on FIR surface luminosity decays exponentially with a scale length of , compared with for the H₂ surface density. Massive star formation per unit H₂ mass is maximum for in the southern Galaxy, with a FIR surface luminosity to H₂ surface density ratio of , compared with at the same radius in the north, and with an average of for the whole galactic disk within the solar circle.

Key words: Galaxy: structure infrared: ISM: continuum ISM: clouds ISM: H ii regions ISM: molecules infrared: ISM: continuum

* Based partly on results collected at the European Southern Observatory, La Silla, Chile

Send offprint requests to: L. Bronfman

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Contents

• 1. Introduction
• 2. Observational results
• 3. Mean radial distribution of embedded OB stars
  • 3.1. Centroid and thickness of the OB star formation layer
  • 3.2. Azimuthally averaged FIR surface luminosity produced by embedded OB stars
  • 3.3. Errors derivation
  • 3.4. Spiral structure and the axisymmetric analysis
  • 3.5. Completeness of the sample and contamination by low luminosity sources in the solar neighborhood
• 4. The galactic disk traced by embedded OB stars and by molecular clouds
  • 4.1. Centroid and thickness of the disk
  • 4.2. Embedded OB stars, FIR surface luminosity, and H₂ surface density
  • 4.3. Large scale asymmetries within the solar circle
  • 4.4. The contribution of embedded OB stars to the FIR luminosity of GMCs in the Galaxy
• 5. Conclusions
• Acknowledgements
• References

© European Southern Observatory (ESO) 2000

Online publication: June 8, 2000
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