Stars form from the gravitational collapse of dense cores within molecular clouds. Observations of molecular lines in these cores reveal velocity gradients, which are likely due to the rotation of the cores. Assuming that the angular momentum is conserved during the gravitational collapse, cores should spin up as they collapse. Eventually, the centrifugal force will exceed the gravity, causing the gravitational collapse to cease. Clearly, the angular momentum must be redistributed somehow during the collapse, or stars would not form at all. But how the angular momentum is redistributed is still unclear.
In a recent paper, Mathilde Gaudel and co-workers from the CALYPSO team – including myself – study the angular momentum distribution in a sample of Class 0 prototars. Class 0 are the youngest protostars, so they are particularly interesting to study how the angular momentum it is redistributed during the earliest phases of star formation. For this study, we use line observations obtained with the NOEMA interferometer and the 30m single dish telescope. The advantage in using these two instruments lies in the fact that they are sensitive to different spatial scales. One the one hand, NOEMA has sharp resolution, but it is insensitive to large scale emission. The 30-m telescope, on the other hand, has a coarser resolution, but it does not suffer from large scale emission filtering. By combining observations from the two instruments, we get both spatial resolution and large scale sensitivity.
For this study, we also combine line observations from two different chemical species, C18O and N2H+. As explained in a previous post, these two species are present in different parts of the protostar envelope: C18O is present only in the innermost part of the envelope, within the so-called CO snowline. N2H+ is almost absent within the CO snowline, because it is destroyed by reactions with CO, but it is more abundant beyond the CO snowline, in the outermost part of the envelope. By looking at the profiles of lines from these two species, we can probe the kinematics in the entire envelope.
Figure 1: Apparent specific angular momentum as a function of the radius for several Class 0 protostars. The black line if a fit with a broken power-law. The change in the slope of the power-law at a characteristic radius of 1600 astronomical units is clearly seen. From Gaudel et al. (2020).
With the combination of the two instruments and the two lines, we can measure the rotational motions at scales between 50 and 1600 astronomical units (au). The results of this analysis are shown in Fig. 1. This figure shows the apparent specific angular momentum (that is the angular momentum per mass unit) obtained from the velocity profile. Each color point with an error bar is a measurement at one radius of a protostar. Each color correspond to a different protostar. When averaging the results for all protostars together, we identify two distinct regimes: at scales larger than 1,600 au, the apparent specific angular momentum scales as r-1.6, where r is the protostar radius. A scales smaller than 1,600 au, the specific angular momentum tends to become relatively constant, down to the smallest scales probed by our observations, i.e. about 50 au. This suggests that the specific angular momentum is conserved at scales smaller than 1,600 au, which probably results in the formation of a proto-planetary disk. Indeed, as explained in a a recent post, we detect Keplerian disks in two protostars of the CALYPSO sample. Disks are likely present in other protostars as well, but they might be too small to be detected with our observations.
The paper “Angular momentum profiles of Class 0 protostellar envelopes” by Gaudel et al. is available on arXiv.