The Star-formation Law in Galactic High-mass Star-forming Molecular Clouds

We study the star-formation (SF) law in 12 Galactic molecular clouds with ongoing high-mass star-formation (HMSF) activity, as traced by the presence of a bright IRAS source and other HMSF tracers. We define the molecular cloud (MC) associated with. each IRAS source using (CO)-C-13 line emission, and count the young stellar objects (YSOs) within these clouds using GLIMPSE and MIPSGAL 24 mu m Spitzer databases. The masses for high-luminosity YSOs (L-bol >10 L-circle dot) are determined individually using pre-main-sequence evolutionary tracks and the evolutionary stages of the sources, whereas a mean mass of 0.5M(circle dot) was adopted to determine the masses in the low-luminosity YSO population. The star-formation. rate surface density (Sigma(SFR)) corresponding to a gas surface density (Sigma(gas)) in each MC is obtained by counting the number of the YSOs within successive contours of (CO)-C-13 line emission. We find a break in the relation between Sigma(SFR) and Sigma(gas), with the relation being a. power. law (Sigma(SFR) proportional to Sigma(N)(gas)) with the index N varying between 1.4 and 3.6 above the break. The Sigma(gas) at the break is between 150-360M(circle dot) pc(-2) for the sample clouds, which compares well with the threshold gas density found in recent studies of Galactic star-forming. regions. Our clouds treated as a whole lie between the Kennicutt relation and the linear relation for Galactic and extra-galactic dense star-forming. regions. We find a tendency for the high-mass YSOs to be found preferentially in dense regions at densities higher than 1200M(circle dot) pc(-2) (similar to 0.25 g cm(-2)).