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

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5. Conclusions

Optical studies of massive stars are restricted to the solar neighborhood due to extinction by intervening dust. The FIR spectral signature of embedded OB stars provides a unique opportunity to extend the study of their distribution to the whole galactic disk. We have analyzed here the first complete galactic survey of dense molecular cloud cores associated with OB star formation. Our main conclusions are:

  1. We have derived the mean radial distribution for 748 regions of massive star formation in the whole galactic disk. These regions produce a total FIR luminosity of [FORMULA] within the range [FORMULA]. We find 492 regions, representing [FORMULA] of the sample, within the solar circle; they produce a FIR luminosity of [FORMULA], [FORMULA] of the galactic total.

  2. Separate analyses of the 349 sources in the northern Galaxy, and of the 399 sources in the southern Galaxy, yield total FIR luminosities (extrapolated to the complete galactic disk) of [FORMULA] and of [FORMULA], respectively. The fraction of the total FIR luminosity produced within the solar circle ([FORMULA]) is similar north and south.

  3. Regions of massive star formation in the galactic disk are distributed in a layer with its centroid [FORMULA] following that of molecular gas for all galactocentric radii, both north and south. Within the solar circle the mean thickness of the OB star formation layer is [FORMULA] pc (FWHM), a factor of 0.62 thinner than the H2 layer ([FORMULA] pc FWHM).

  4. The FIR luminosity produced by massive stars, azimuthally averaged over the whole galactic disk within the solar circle, has a well defined maximum at [FORMULA], like the H2 surface density, but with a gaussian FWHM of [FORMULA], a factor of 0.54 narrower than for the H2 ([FORMULA] FWHM).

  5. Toward the outer Galaxy, down from the maximum, the face-on FIR surface luminosity, azimuthally averaged over the whole galactic disk, decays with an exponential scale length of [FORMULA], steeper than the H2 surface density exponential decay which has a scale length of [FORMULA].

  6. Massive star formation per unit H2 mass is higher than average in the region of the molecular annulus. While the mean FIR luminosity to H2 mass ratio within the solar circle is [FORMULA], for [FORMULA] the ratio grows to [FORMULA]; and for [FORMULA] it reaches [FORMULA]

  7. Massive star formation per unit H2 mass is maximum in the southern Galaxy at [FORMULA]. The average FIR luminosity to H2 mass ratio there is [FORMULA], compared with [FORMULA] for its northern counterpart.

There is a fair amount of further work that we hope will follow from the present results. The availability of a fairly complete and homogeneous sample of OB star forming regions in the Galaxy allows statistical studies of the general physical process of massive star formation, and in particular of the high-mass end of the IMF. Bearing in mind that embedded massive stars should be the best tracers of spiral arm structure in our Galaxy, we foresee a concerted effort to resolve in a case-by-case basis their two-fold distance ambiguity within the solar circle.

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© European Southern Observatory (ESO) 2000

Online publication: June 8, 2000
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