Angular momentum profiles of Class 0 protostellar envelopes

Abstract

Context. Understanding the initial properties of star forming material and how they affect the star formation process is a key question. The infalling gas must redistribute most of its initial angular momentum inherited from prestellar cores before reaching the central stellar embryo. Disk formation has been naturally considered as a possible solution to this "angular momentum problem". However, how the initial angular momentum of protostellar cores is distributed and evolves during the main accretion phase and the beginning of disk formation has largely remained unconstrained up to now. Aims. In the framework of the IRAM CALYPSO survey, we obtained observations of the dense gas kinematics that we used to quantify the amount and distribution of specific angular momentum at all scales in collapsing-rotating Class 0 protostellar envelopes. Methods. We used the high dynamic range C 18 O (2−1) and N 2 H + (1−0) datasets to produce centroid velocity maps and probe the rotational motions in the sample of 12 envelopes from scales ∼50 to ∼5000 au. Results. We identify differential rotation motions at scales 1600 au in 11 out of the 12 protostellar envelopes of our sample by measuring the velocity gradient along the equatorial axis, which we fit with a power-law model v ∝ r α . This suggests that coherent motions dominate the kinematics in the inner protostellar envelopes. The radial distributions of specific angular momentum in the CALYPSO sample suggest the following two distinct regimes within protostellar envelopes: the specific angular momentum decreases as j ∝ r 1.6±0.2 down to ∼1600 au and then tends to become relatively constant around ∼6 × 10 −4 km s −1 pc down to ∼50 au. Conclusions. The values of specific angular momentum measured in the inner Class 0 envelopes suggest that material directly involved in the star formation process (<1600 au) has a specific angular momentum on the same order of magnitude as what is inferred in small T-Tauri disks. Thus, disk formation appears to be a direct consequence of angular momentum conservation during the collapse. Our analysis reveals a dispersion of the directions of velocity gradients at envelope scales >1600 au, suggesting that these gradients may not be directly related to rotational motions of the envelopes. We conclude that the specific angular momentum observed at these scales could find its origin in other mechanisms, such as core-forming motions (infall, turbulence), or trace an imprint of the initial conditions for the formation of protostellar cores.

An observational correlation between magnetic field, angular momentum and fragmentation in the envelopes of Class 0 protostars?

“…Recent analysis have revealed a more complex interpretation, with sources showing shifts or even reversal of the gas velocity gradients within envelopes. In some sources, the observed gradients might actually originate from on-going infall, or linked with turbulence (Gaudel et al 2020). In all cases however, large velocity gradients are tracing more dynamical envelopes with larger kinetic energy.…”

“…For sources that are part of the CALYPSO sample (i.e. L1157, L1448C, L1448N-B, L1448-2A, IRAS4A, IRAS4B and SVS13-B), Gaudel et al (2020) recently presented observations of the dense gas kinematics using C 18 O and N 2 H + measurements and derive specific angular momentum estimates throughout their collapsing protostellar envelopes from 50 au to 10 000 au scales. In their analysis, velocity maps are produced from a combined Plateau de Bure (PdBI) + 30-m N 2 H + dataset.…”

“…Then velocity gradients are fitted over a 40 × 40 region around the sources and determined via the least-square minimization, with v grad = v 0 + a∆α + b∆β, with ∆α and ∆β the offsets with respect to the central source (Goodman et al 1993). Serp SMM18 is also part of the CALYPSO sample but is not included in the sample studied in Gaudel et al (2020). For this source, we determine the velocity gradient strength and position angle using the same 2D fitting technique.…”

“…L1157 is also part of their sample, with larger velocity gradient strength (from 3.5 to 10.5 km s −1 pc −1 ) than those derived in Gaudel et al (2020) (0.8 km s −1 pc −1 ) and different position angles depending on the use of PdBI, Combined Array for Research in Millimeter-wave Astronomy (CARMA) or Very Large Array (VLA) data. We keep the values from Gaudel et al (2020) in this paper, but remind the reader that the N 2 H + velocity field is extremely complex for this source, with a velocity gradient direction close to the outflow axis but whose symmetry is perturbed by a redshifted line emission southeast of the protostar, with potential contribution of the outflow cavity walls to the N 2 H + emission for this source (Tobin et al 2011;Gaudel et al 2020).…”

Formation and Evolution of Disks Around Young Stellar Objects

“…They found that RSDs also form because subsonic turbulence quickly develops and, angular momentum with respect to the moving star, is induced by the inertia forces. Therefore, the formation of initial disks can be a local process (∼10 3 au scale; see Gaudel et al 2020) and may not require organized rotation on the very large scale cloud (∼10 4 au).…”