In the last years, a number of detailed observations have shown that molecular clouds are pervaded by dense structures called filaments. These structures seem to be ubiquitous and play an important role in the Galactic-scale star formation since they are probably the main routes to transport mass from the large scales of the clouds (few 10 pc) down to dense cores and disk (few 1000 AU). However, the physical processes leading to the formation of stars within filaments still remain to be well understood. The advent of ALMA allows now, for the first time and thanks to its unprecedent high resolution and sensitivity, to study the physical processes involved in filaments and permits statistical studies of sources in a variety of evolutionary stages and with different morphologies.
We have used ALMA to map the filamentary structure of three different star-forming complexes: the integral-shape filament Orion A, the filamentary hub MonR2 and the infrared-dark cloud G357.
We have studied the gas kinematics and mass accretion flow along filaments in MonR2, which harbors a network of filaments that converge into a massive and dense central hub where high-mass stars are forming. We determine velocity gradients of ~2.5 km/s/pc corresponding to mass accretion rates of ~10^(-3) Msun/yr. The filamentary structure extends towards the central hub, where we find spiraling filaments converging towards the central 0.05 pc where an already developed HII region is expanding. Interestingly, we find signs of fragmentation along the filaments, suggesting that stars can form along them before all the mass is gathered in the center.
The study of the fragmentation is conducted in more detail in two high line-mass filaments with very different levels of star formation: the dense integral-shape filament in Orion A, and the more quiescent infrared-dark cloud G357. The comparison of fragmentation in both filaments provide the first observational constraints for the evolutionary sequence of fragmentation in massive filaments. We identify fragmentation and clustering in different scales along the filament: cores appear strongly grouped below scales of 6000 AU, and appear in groups along the filament with separations of about 0.25 pc.
These studies constitute the first steps for a more comprehensive survey of a larger collection of star-forming filaments in our Galaxy.