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The ATLASGAL survey has characterised the properties of approximately 1000embedded HII regions and found an empirical relationship between the clump massand bolometric luminosity that covers 3-4 orders of magnitude. Comparing thisrelation with simulated clusters drawn from an initial mass function and usingdifferent star formation efficiencies we find that a single value is unable tofit the observed luminosity to mass ($L/M$) relation. We have used a MonteCarlo simulation to generate 200,000 clusters using the $L/M$-ratio as aconstraint to investigate how the star formation efficiency changes as afunction of clump mass. This has revealed that the star formation efficiencydecreases with increasing clump mass with a value of 0.2 for clumps with massesof a few hundred solar masses and dropping to 0.08 for clumps with masses of afew thousand solar masses. We find good agreement between our results and starformation efficiencies determined from counts of embedded objects in nearbymolecular clouds. Using the star formation efficiency relationship and theinfrared excess time for embedded star formation of $2\pm1$, Myr we estimatethe Galactic star formation rate to be approximately $0.9\pm0.45$ Msunyr$^{-1}$, which is in good agreement with previously reported values. Thismodel has the advantage of providing a direct means of determining the starformation rate and avoids the difficulties encountered in converting infraredluminosities to stellar mass that affect previous galactic and extragalacticstudies.

ATLASGAL -- Star forming efficiencies and the Galactic star formation rate