Mathilde Gaudel

Stars born in the molecular clouds which are huge reservoirs of dust and gas. These clouds are organized by filaments. Inside filaments, a succession of compressions and fragmentations leads to the creation of gas nuclei called pre-stellar cores. These dense cores collapse and one or several stellar embryos form in the center of an envelope composed of the matter of the parent pre-stellar core. This is the protostellar phase. The embryo will then grow by progressively accreting a large part of the matter which surrounds it. It is during this accretion phase that the star will obtain its adult stellar mass. When the internal temperature is high enough to trigger the nuclear fusion reactions, the stellar object obtain the official status of a star like our Sun.

A rotating core has a specific angular momentum, noted j, defined as the rotational velocity times the radius. In the classical scenario of star formation, which is described above, stellar embryo inherits its rotational motions from the parent pre-stellar core in which it form. As a consequence of angular momentum conservation, if the angular momentum of the pre-stellar core is totally transferred to the stellar embryo during the collapse and the accretion phase, the embryo would rotate too quickly and would fragment before becoming a star like our Sun. To better understand the link between rotational motions at different scales, we studied in detail the velocity field within protostellar envelopes, from the stellar embryo (1 au) to the outer edges of the envelope (5 000 au). Young protostars, which are characterized by an envelope mass much larger than the mass of the stellar embryo, are key objects to contrain mecanisms of stellar formation. They are at the beginning of the accretion phase and they keep imprint of the initial conditions from pre-stellar cores.

To do this, we used high-resolution observations of kinematic tracers from IRAM Plateau de Bure Interferometer (PdBI) and 30-meter telescope (30m) obtained in the framework of the CALYPSO program (Continuum and Lines in Young Protostellar Objects, PI: Ph. André) for a sample of 12 young protostars with a distance between 140 and 400 pc.

The kinematic analysis allows us to build, for the first time in a large sample, radial distributions of angular momentum from the embryo scale to the outer edges of the envelope (Gaudel et al. 2020). Two distinct regimes are revealed: a relatively constant profile in the inner part of the envelope and an increase in angular momentum in the outer part. In the outer part of the envelopes, the orientation of the mean velocity gradients measured to build the angular momentum distributions becomes random and disorganized (Gaudel et al. 2020). This suggests that the origin of the velocity field is not the same as in the inner envelope where rotational motions are identified. Thus, the motions in the outer part are not due to pure envelope rotation but seem to be dominated by other mechanisms. The increase in angular momentum observed in the outer part of the protostellar envelopes is consistent with the angular momentum observed in pre-stellar cores and molecular clouds. Historically, the velocity gradients measured in the pre-stellar cores were interpreted as rotational motions (Goodman et al. 1993, Caselli et al. 2002) but a recent study (Tatematsu et al. 2016) interpreted them as turbulence or gravitationally-driven turbulence.

These new results question the origin of star rotation: the rotational motions of the stellar embryos would not be inherited from the pre-stellar phase, as postulated in the classical scenario of star formation, but would be a consequence of the collapse and/or turbulence during star formation.