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 ¹³CO line emission, and count the young stellar objects (YSOs) within these clouds using GLIMPSE and MIPSGAL 24 μm Spitzer databases. The masses for high-luminosity YSOs (L _bol > 10 L _⊙) are determined individually using pre-main-sequence evolutionary tracks and the evolutionary stages of the sources, whereas a mean mass of 0.5 M _⊙ was adopted to determine the masses in the low-luminosity YSO population. The star-formation rate surface density (Σ_SFR) corresponding to a gas surface density (Σ_gas) in each MC is obtained by counting the number of the YSOs within successive contours of ¹³CO line emission. We find a break in the relation between Σ_SFR and Σ_gas, with the relation being a power law (${\Sigma }_{SFR}\propto {{\Sigma }_{gas}}^N)$ with the index N varying between 1.4 and 3.6 above the break. The Σ_gas at the break is between 150–360 M _⊙ 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 1200 M _⊙ pc⁻² (∼0.25 g cm⁻²).