Weak Turbulence in the HD 163296 Protoplanetary Disk Revealed by ALMA CO Observations (1510.01375v1)

Abstract: Turbulence can transport angular momentum in protoplanetary disks and influence the growth and evolution of planets. With spatially and spectrally resolved molecular emission line measurements provided by (sub)millimeter interferometric observations, it is possible to directly measure non-thermal motions in the disk gas that can be attributed to this turbulence. We report a new constraint on the turbulence in the disk around HD 163296, a nearby young A star, determined from ALMA Science Verification observations of four CO emission lines (the CO(3-2), CO(2-1), 13CO(2-1), and C18O(2-1) transitions). The different optical depths for these lines permit probes of non-thermal line-widths at a range of physical conditions (temperature and density) and depths into the disk interior. We derive stringent limits on the non-thermal motions in the upper layers of the outer disk such that any contribution to the line-widths from turbulence is <3% of the local sound speed. These limits are approximately an order of magnitude lower than theoretical predictions for full-blown MHD turbulence driven by the magneto-rotational instability, potentially suggesting that this mechanism is less efficient in the outer (R>30AU) disk than has been previously considered.

Analysis of Weak Turbulence in the HD 163296 Protoplanetary Disk

The paper conducted by Flaherty et al. presents a detailed analysis of turbulence within the protoplanetary disk of the young star HD 163296, utilizing high-resolution ALMA CO observations. The research focuses on quantifying the non-thermal motions in the disk gas that can be attributed to turbulence, a critical factor influencing angular momentum transport and the planet formation process.

The authors used a set of four CO isotopologue emission lines, specifically CO(3-2), CO(2-1), $^{13}$CO(2-1), and C$^{18}$O(2-1), to probe different depths and conditions within the disk. Importantly, these lines differ in optical depth, thus providing a comprehensive view of the turbulent environment under varied physical conditions. The findings show that the turbulence contributes less than 3% of the local sound speed in the upper layers of the outer disk, which is significantly lower than expectations from theoretical models predicting substantial magneto-hydrodynamic (MHD) turbulence driven by the magneto-rotational instability (MRI).

Key Results and Implications

• Turbulence Measurement: The paper provides an upper limit on velocity dispersion due to turbulence at under 3% of the local sound speed, equating to non-thermal velocities below 19 m/s in the disk's upper regions. This result challenges the standard expectations for turbulence strength deduced from MHD simulations, which predict more vigorous motion due to efficient angular momentum transport mechanisms, such as MRI.
• Accretion Discrepancy: The observed weak turbulence presents a conundrum when aligned with the relatively high accretion rates onto the star HD 163296, as calculated accretion-related viscosities do not match the observed turbulence limits. This discrepancy suggests the potential for a radial gradient in turbulence strength or other accretion processes, such as disk winds, that might operate without enhancing observable turbulence.
• CO Abundance: Analysis revealed a CO/H$_2$ depletion by a factor of approximately 4.9 compared to typical interstellar medium values. This finding could indicate complex chemical processes, such as grain surface reactions or selective CO photodissociation, modifying the expected CO abundance within the disk.
• Theoretical Models: The limits on turbulence derived from observations are substantially below those predicted by simulations assuming ideal conditions for MRI-driven turbulence, suggesting either suboptimal conditions for MRI, such as insufficient ionization or weak magnetic fields, or the need for alternative turbulence mechanisms. Moreover, potential non-turbulent processes might transport angular momentum effectively without contributing to measurable turbulence signatures in current observations.

Future Research Directions

This paper suggests several pathways for future inquiry:

1. Enhanced Spatial Resolution: Further detailed observations with even higher spatial resolution may elucidate finer structures in turbulence and help map variations more accurately across the disk.
2. Broader Scope: Expanding the sample to include disks around different stellar types and ages could provide comparative insights, allowing further examination of the role of stellar properties and evolutionary states on disk turbulence.
3. Advanced Modeling: Incorporating non-ideal MHD effects, improved chemistry models, and potential contributions from other instability-driven turbulent processes could refine theoretical predictions.
4. Alternative Angular Momentum Transport Mechanisms: Further studies should explore mechanisms such as disk winds in more detail to assess their role and efficiency in angular momentum transport without turbulence creation.

Flaherty et al.'s research offers critical insights into the turbulence dynamics within protoplanetary disks, challenges current theoretical frameworks, and sets the stage for a reassessment of turbulence's role in disk evolution and planet formation.