ALMA reveals a large structured disk and nested rotating outflows in DG Tau B (2001.09776v1)

Abstract: We present Atacama Large Millimeter Array (ALMA) Band 6 observations at 14-20 au spatial resolution of the disk and CO(2-1) outflow around the Class I protostar DG Tau B in Taurus. The disk is very large, both in dust continuum (R${\rm eff,95\%}$=174 au) and CO (R${CO}$=700 au). It shows Keplerian rotation around a 1.1$\pm$0.2 M${\odot}$ central star and two dust emission bumps at $r$ = 62 and 135 au. These results confirm that large structured disks can form at an early stage where residual infall is still ongoing. The redshifted CO outflow at high velocity shows a striking hollow cone morphology out to 3000 au with a shear-like velocity structure within the cone walls. These walls coincide with the scattered light cavity, and they appear to be rooted within $<$ 60 au in the disk. We confirm their global average rotation in the same sense as the disk, with a specific angular momentum $\simeq$ 65 au \kms. The mass-flux rate of 1.7-2.9 $\times$ 10$^{-7}$M${\odot}$ yr$^{-1}$ is 35$\pm$10 times that in the atomic jet. We also detect a wider and slower outflow component surrounding this inner conical flow, which also rotates in the same direction as the disk. Our ALMA observations therefore demonstrate that the inner cone walls, and the associated scattered light cavity, do not trace the interface with infalling material, which is shown to be confined to much wider angles ($> 70^{\circ}$). The properties of the conical walls are suggestive of the interaction between an episodic inner jet or wind with an outer disk wind, or of a massive disk wind originating from 2-5 au. However, further modeling is required to establish their origin. In either case, such massive outflow may significantly affect the disk structure and evolution.

• The paper demonstrates that DG Tau B hosts an unusually large circumstellar disk with distinct substructures, offering new insights into disk evolution.
• It reveals nested rotating outflows, including a high-velocity, conical disk wind that efficiently transports angular momentum.
• The findings support magnetohydrodynamic wind models and suggest early complex disk structures that may influence planet formation.

An Advanced Analysis of DG Tau B's Disk and Outflows Using ALMA Observations

The paper conducted on DG Tau B utilizes high-resolution observations from the Atacama Large Millimeter Array (ALMA) to dissect the structure and kinematics of a large circumstellar disk alongside its associated outflows. This research paper focuses on the Class I protostar DG Tau B, located in the Taurus star-forming region, and presents compelling insights into both the disk and the outflow dynamics. The results of this paper provide crucial data on the early stages of star formation, addressing important questions about disk formation and evolution as well as the mechanisms driving outflows.

Key Observations and Findings

DG Tau B's disk is found to be unusually large compared to other known Class II disks, with the disk size determined to be approximately 700 AU in the CO gas and 174 AU in dust continuum. The observations reveal a Keplerian disk with a strong rotational component extending to significant distances, indicating a central protostar mass of about 1.1 solar masses. A noteworthy discovery is the presence of two emission bumps within the disk, suggestive of substructures that could point to regions of enhanced dust growth or potential sites for planet formation. While the exact origins of these structures require further paper, they align with the theory that complex structures can form at early stages of disk evolution.

The paper presents striking ALMA maps of the molecular outflows emanating from DG Tau B, delineating two distinct components: an inner high-velocity, rotating conical outflow and a wider low-velocity component. The inner conical outflow is characterized by a well-defined hollow cone morphology with clear rotation in the same direction as the disk, rooted within approximately 60 AU of the disk. This structure may represent a disk wind, a common feature in star forming systems that channels mass and angular momentum away from the disk. The mass-flux ratio in the outflow reaches significant levels, comparable to the accretion rate itself, underscoring its potential impact on the surrounding disk environment.

Theoretical Implications

These findings have substantial implications for current theories of star and planet formation. The observation of a large and structured disk around a relatively young protostar like DG Tau B suggests the early development of complex disk structures, potentially driven by young planets or variations in disk density. Moreover, the detection of a rotating disk wind aligns with magnetohydrodynamic (MHD) models that propose such winds as mechanisms for angular momentum transport and disk evolution.

The nested, rotating outflow structures observed support notions of significant wind involvement in the efficient transfer of angular momentum, essential for the continued accretion of mass onto the central protostar. This provides further evidence for MHD wind scenarios where magnetic fields help drive outflows that subsequently shape disk morphology and characteristics.

Future Directions

The paper highlights areas for further research, such as constructing detailed models of DG Tau B's disk and outflows to refine our understanding of their origins and interactions. Future observations at even higher resolution may elucidate finer structural details such as potential gaps within the disk or variations in the outflow patterns which could indicate episodic accretion phases or the influence of forming planets.

Overall, the research on DG Tau B using ALMA serves as an exemplary case of how modern observational techniques can advance our comprehension of the intricate processes surrounding young stellar objects, providing a stepping stone for a deeper exploration of the physical conditions under which planet formation might commence.