Star formation in the Milky Way


Stars are born in giant molecular clouds of dust and gas where the temperature and density are ideal for their formation. Because they are deeply embedded in the cloud, we have to rely on the infrared and millimeter part of the spectrum to have insights on the physical processes occurring during the early-stages of star formation.

High-mass stars emits UV photons which ionize the surrounding medium and lead to an HII region. This HII region further expands and sweeps-up the surrounding material to finally form a circular structure often called HII bubble, despite the fact that the geometry of such structure remains uncertain. When the layer of material enclosing the HII region reaches a certain density, it fragments to form the next generation of stars.
Despite the fact that 30% of Galactic high-mass sources are found at the edges of these HII regions, we are still not sure about the true effect of the pressure of the HII regions which is exterted on the layer of material, and therefore, on the star-formation occurring in it.

At NCBJ, we use the data provided by Herschel from The Herschel Imaging Survey of OB Stars (HOBYS) and the Herschel survey of the Galactic Plane (Hi-GAL) to understand the properties of HII regions from large to small scales. This is important as it give us crucial information on the mass fo the cores, the ionisation pressure of the HII region and the properties of the filaments inside the PhotoDissociation Region.
To understand the fragmentation process down to 0.01pc and below, the current Galactic plane surveys do not have enough spatial resolution. Therefore, we use high-resolution interferometric observations of the population of cores around HII regions. Such observations are performed with interferometers such as the Atacama Large Millimetre Array (ALMA). Thanks to them, we can observe and understand how the cores are fragmenting and what are the drivers of this fragmentation (gravity, turbulence, magnetic field, …)