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 C18O (2−1) and N2H+ (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 ∝ rα. 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 ∝ r1.6±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.
Highlights
Stars form via the gravitational collapse of 0.1 pc dense cores, which are embedded within molecular clouds (André et al 2000; Ward-Thompson et al 2007; di Francesco et al 2007)
This paper presents an analysis of envelope kinematics, for the 12 sources from the CALYPSO sample located at d ≤ 350 pc and discuss our results on the properties of the angular momentum in Class 0 protostellar envelopes
When velocity gradients with a blue- and a red-shifted velocity components observed on each side of the protostellar embryo are continuous from inner to outer envelope scales but shifted from the equatorial axis (∆Θ ≥ 60◦), we only report upper limits on rotational velocities in the PVrot diagrams
Summary
Stars form via the gravitational collapse of 0.1 pc dense cores, which are embedded within molecular clouds (André et al 2000; Ward-Thompson et al 2007; di Francesco et al 2007). Thanks to observations of the dense molecular gas emission, rotational motions where characterized in seven Class 0 or I protostellar envelopes at scales between 3500 and 10000 au (Ohashi et al 1997a; Belloche et al 2002; Chen et al 2007) These envelopes exhibit an average angular momentum of ∼10−3 km s−1 pc at scales of r < 5000 au, consistent with the j measured in prestellar cores by Caselli et al (2002) (∼10−3 km s−1 pc). One of the main goals of this large observing program is to understand how the circumstellar envelope is accreted onto the central protostellar object during the Class 0 phase, and tackle the angular momentum problem of star formation. We describe the dataset properties exploited to characterize the kinematics of the envelopes at radii between r ∼50 and 5000 au from the central object
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