Engineering topics

Andrews, Sean M.

Publications and source records attributed to Andrews, Sean M..

At least 19 records

DOE OSTI2019

Constraints on a Putative Planet Sculpting the V4046 Sagittarii Circumbinary Disk

We analyze the highest-resolution millimeter continuum and near-infrared (NIR) scattered-light images presented to date of the circumbinary disk orbiting V4046 Sgr, a ∼20-Myr-old actively accreting, close binary T Tauri star system located a mere 72.4 pc from Earth. We observed the disk with the Atacama Large Millimeter/submillimeter Array (ALMA) at 870 μm during Cycle 4, and we analyze these data in conjunction with archival NIR (H band) polarimetric images obtained with SPHERE/IRDIS on the ESO Very Large Telescope. At 0.″3 (20 au) resolution, the 870 μm image reveals a marginally resolved ring that peaks at ∼32 au and has an extension of ∼90 au. We infer a lower limit on a dust mass of ∼60.0 M {sub ⊕} within the 870 μm ring, and confirm that the ring is well aligned with the larger -scale gaseous disk. A second, inner dust ring is also tentatively detected in the ALMA observations; its position appears coincident with the inner (∼14 au radius) ring detected in scattered light. Using synthetic 870 μm and H-band images obtained from disk–planet interaction simulations, we attempt to constrain the mass of the putative planet orbiting at 20 au. Our trials suggest that a circumbinary Jovian-mass planet may be responsible for generating the dust ring and gap structures detected within the disk. We discuss the longevity of the gas-rich disk orbiting V4046 Sgr in the context of the binary nature of the system.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2018

Detecting Weak Spectral Lines in Interferometric Data through Matched Filtering

Modern radio interferometers enable observations of spectral lines with unprecedented spatial resolution and sensitivity. In spite of these technical advances, many lines of interest are still at best weakly detected and therefore necessitate detection and analysis techniques specialized for the low signal-to-noise ratio (S/N) regime. Matched filters can leverage knowledge of the source structure and kinematics to increase sensitivity of spectral line observations. Application of the filter in the native Fourier domain improves S/N while simultaneously avoiding the computational cost and ambiguities associated with imaging, making matched filtering a fast and robust method for weak spectral line detection. We demonstrate how an approximate matched filter can be constructed from a previously observed line or from a model of the source, and we show how this filter can be used to robustly infer a detection significance for weak spectral lines. When applied to ALMA Cycle 2 observations of CH{sub 3}OH in the protoplanetary disk around TW Hya, the technique yields a ≈53% S/N boost over aperture-based spectral extraction methods, and we show that an even higher boost will be achieved for observations at higher spatial resolution. A Python-based open-source implementation of this technique is available under the MIT license at http://github.com/AstroChem/VISIBLE.

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DOE OSTI2017

A millimeter Continuum Size–Luminosity Relationship for Protoplanetary Disks

We present a survey at subarcsecond resolution of the 340 GHz dust continuum emission from 50 nearby protoplanetary disks, based on new and archival observations with the Submillimeter Array. The observed visibility data were modeled with a simple prescription for the radial surface brightness profile. The results were used to extract intuitive, empirical estimates of the emission “size” for each disk, R{sub eff}, defined as the radius that encircles a fixed fraction of the total continuum luminosity, L{sub mm}. We find a significant correlation between the sizes and luminosities, such that R{sub eff}∝L{sub mm}{sup 0.5}, providing a confirmation and quantitative characterization of a putative trend that was noted previously. This correlation suggests that these disks have roughly the same average surface brightness interior to their given effective radius, ∼0.2 Jy arcsec{sup −2} (or 8 K in brightness temperature). The same trend remains, but the 0.2 dex of dispersion perpendicular to this relation essentially disappears, when we account for the irradiation environment of each disk with a crude approximation of the dust temperatures based on the stellar host luminosities. We consider two (not mutually exclusive) explanations for the origin of this size–luminosity relationship. Simple models of the growth and migration of disk solids can account for the observed trend for a reasonable range of initial conditions, but only on timescales that are much shorter than the nominal ages present in the sample. An alternative scenario invokes optically thick emission concentrated on unresolved scales, with filling factors of a few tens of percent, which is perhaps a manifestation of localized particle traps.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2017

A Three-dimensional View of Turbulence: Constraints on Turbulent Motions in the HD 163296 Protoplanetary Disk Using DCO{sup +}

Gas kinematics are an important part of the planet formation process. Turbulence influences planetesimal growth and migration from the scale of submicron dust grains through gas-giant planets. Radio observations of resolved molecular line emission can directly measure this non-thermal motion and, taking advantage of the layered chemical structure of disks, different molecular lines can be combined to map the turbulence throughout the vertical extent of a protoplanetary disk. Here we present ALMA observations of three molecules (DCO{sup +}(3-2), C{sup 18}O(2-1) and CO(2-1)) from the disk around HD 163296. We are able to place stringent upper limits (v {sub turb} < 0.06c {sub s}, <0.05c {sub s}, and <0.04c {sub s} for CO(2-1), C{sup 18}O(2-1), and DCO{sup +}(3-2) respectively), corresponding to α ≲ 3 × 10{sup −3}, similar to our prior limit derived from CO(3-2). This indicates that there is little turbulence throughout the vertical extent of the disk, contrary to theoretical predictions based on the magnetorotational instability and gravitoturbulence. In modeling the DCO{sup +} emission, we also find that it is confined to three concentric rings at 65.7 ± 0.9 au, 149.9{sub −0.7}{sup +0.5} au, and 259 ± 1 au, indicative of a complex chemical environment.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2017

Disentangling Time-series Spectra with Gaussian Processes: Applications to Radial Velocity Analysis

Measurements of radial velocity variations from the spectroscopic monitoring of stars and their companions are essential for a broad swath of astrophysics; these measurements provide access to the fundamental physical properties that dictate all phases of stellar evolution and facilitate the quantitative study of planetary systems. The conversion of those measurements into both constraints on the orbital architecture and individual component spectra can be a serious challenge, however, especially for extreme flux ratio systems and observations with relatively low sensitivity. Gaussian processes define sampling distributions of flexible, continuous functions that are well-motivated for modeling stellar spectra, enabling proficient searches for companion lines in time-series spectra. We introduce a new technique for spectral disentangling, where the posterior distributions of the orbital parameters and intrinsic, rest-frame stellar spectra are explored simultaneously without needing to invoke cross-correlation templates. To demonstrate its potential, this technique is deployed on red-optical time-series spectra of the mid-M-dwarf binary LP661-13. We report orbital parameters with improved precision compared to traditional radial velocity analysis and successfully reconstruct the primary and secondary spectra. We discuss potential applications for other stellar and exoplanet radial velocity techniques and extensions to time-variable spectra. The code used in this analysis is freely available as an open-source Python package.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2017

A Multi-ringed, Modestly Inclined Protoplanetary Disk around AA Tau

AA Tau is the archetype for a class of stars with a peculiar periodic photometric variability thought to be related to a warped inner disk structure with a nearly edge-on viewing geometry. We present high resolution (∼0.″2) ALMA observations of the 0.87 and 1.3 mm dust continuum emission from the disk around AA Tau. These data reveal an evenly spaced three-ringed emission structure, with distinct peaks at 0.″34, 0.″66, and 0.″99, all viewed at a modest inclination of 59.°1 ± 0.°3 (decidedly not edge-on). In addition to this ringed substructure, we find non-axisymmetric features, including a “bridge” of emission that connects opposite sides of the innermost ring. We speculate on the nature of this “bridge” in light of accompanying observations of HCO{sup +} and {sup 13}CO ( J = 3–2) line emission. The HCO{sup +} emission is bright interior to the innermost dust ring, with a projected velocity field that appears rotated with respect to the resolved disk geometry, indicating the presence of a warp or inward radial flow. We suggest that the continuum bridge and HCO{sup +} line kinematics could originate from gap-crossing accretion streams, which may be responsible for the long-duration dimming of optical light from AA Tau.

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DOE OSTI2017

Radial Surface Density Profiles of Gas and Dust in the Debris Disk around 49 Ceti

We present ∼0.″4 resolution images of CO(3–2) and associated continuum emission from the gas-bearing debris disk around the nearby A star 49 Ceti, observed with the Atacama Large Millimeter/Submillimeter Array (ALMA). We analyze the ALMA visibilities in tandem with the broadband spectral energy distribution to measure the radial surface density profiles of dust and gas emission from the system. The dust surface density decreases with radius between ∼100 and 310 au, with a marginally significant enhancement of surface density at a radius of ∼110 au. The SED requires an inner disk of small grains in addition to the outer disk of larger grains resolved by ALMA. The gas disk exhibits a surface density profile that increases with radius, contrary to most previous spatially resolved observations of circumstellar gas disks. While ∼80% of the CO flux is well described by an axisymmetric power-law disk in Keplerian rotation about the central star, residuals at ∼20% of the peak flux exhibit a departure from axisymmetry suggestive of spiral arms or a warp in the gas disk. The radial extent of the gas disk (∼220 au) is smaller than that of the dust disk (∼300 au), consistent with recent observations of other gas-bearing debris disks. While there are so far only three broad debris disks with well characterized radial dust profiles at millimeter wavelengths, 49 Ceti’s disk shows a markedly different structure from two radially resolved gas-poor debris disks, implying that the physical processes generating and sculpting the gas and dust are fundamentally different.

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NASA NTRS2017

Radial Surface Density Profiles of Gas and Dust in the Debris Disk Around 49 Ceti

We present approximately 0".4 resolution images of CO(3-2) and associated continuum emission from the gas-bearing debris disk around the nearby A star 49 Ceti, observed with the Atacama Large Millimeter/Submillimeter Array (ALMA). We analyze the ALMA visibilities in tandem with the broadband spectral energy distribution to measure the radial surface density profiles of dust and gas emission from the system. The dust surface density decreases with radius between approximately 100 and 310 au, with a marginally significant enhancement of surface density at a radius of approximately 110 au. The SED requires an inner disk of small grains in addition to the outer disk of larger grains resolved by ALMA. The gas disk exhibits a surface density profile that increases with radius, contrary to most previous spatially resolved observations of circumstellar gas disks. While approximately 80% of the CO flux is well described by an axisymmetric power-law disk in Keplerian rotation about the central star, residuals at approximately 20% of the peak flux exhibit a departure from axisymmetry suggestive of spiral arms or a warp in the gas disk. The radial extent of the gas disk (approx. 220 au) is smaller than that of the dust disk (approx. 300 au), consistent with recent observations of other gasbearing debris disks. While there are so far only three broad debris disks with well characterized radial dust profiles at millimeter wavelengths, 49 Ceti's disk shows a markedly different structure from two radially resolved gas-poor debris disks, implying that the physical processes generating and sculpting the gas and dust are fundamentally different.

Atacama Large Millimeter Submillimeter Array(ALMA)↗

DOE OSTI2017

H{sub 2}CO Distribution and Formation in the TW HYA Disk

H{sub 2}CO is one of the most readily detected organic molecules in protoplanetary disks. Yet its distribution and dominant formation pathway(s) remain largely unconstrained. To address these issues, we present ALMA observations of two H{sub 2}CO lines (3{sub 12}--2{sub 11} and 5{sub 15}--4{sub 14}) at 0.″5 (∼30 au) spatial resolution toward the disk around the nearby T Tauri star TW Hya. Emission from both lines is spatially resolved, showing a central depression, a peak at 0.″4 radius, and a radial decline at larger radii with a bump at ∼1″, near the millimeter continuum edge. We adopt a physical model for the disk and use toy models to explore the radial and vertical H{sub 2}CO abundance structure. We find that the observed emission implies the presence of at least two distinct H{sub 2}CO gas reservoirs: (1) a warm and unresolved inner component (<10 au), and (2) an outer component that extends from ∼15 au to beyond the millimeter continuum edge. The outer component is further constrained by the line ratio to arise in a more elevated disk layer at larger radii. The inferred H{sub 2}CO abundance structure agrees well with disk chemistry models, which predict efficient H{sub 2}CO gas-phase formation close to the star, and cold H{sub 2}CO grain surface formation, through H additions to condensed CO, followed by non-thermal desorption in the outer disk. The implied presence of active grain surface chemistry in the TW Hya disk is consistent with the recent detection of CH{sub 3}OH emission, and suggests that more complex organic molecules are formed in disks, as well.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2017

An ALMA Survey of DCN/H{sup 13}CN and DCO{sup +}/H{sup 13}CO{sup +} in Protoplanetary Disks

The deuterium enrichment of molecules is sensitive to their formation environment. Constraining patterns of deuterium chemistry in protoplanetary disks is therefore useful for probing how material is inherited or reprocessed throughout the stages of star and planet formation. We present ALMA observations at ∼0.″6 resolution of DCO{sup +}, H{sup 13}CO{sup +}, DCN, and H{sup 13}CN in the full disks around T Tauri stars AS 209 and IM Lup, in the transition disks around T Tauri stars V4046 Sgr and LkCa 15, and in the full disks around Herbig Ae stars MWC 480 and HD 163296. We also present ALMA observations of HCN in the IM Lup disk. DCN, DCO{sup +}, and H{sup 13}CO{sup +} are detected in all disks, and H{sup 13}CN in all but the IM Lup disk. We find efficient deuterium fractionation for the sample, with estimates of disk-averaged DCO{sup +}/HCO{sup +} and DCN/HCN abundance ratios ranging from ∼0.02–0.06 and ∼0.005–0.08, respectively, which is comparable to values reported for other interstellar environments. The relative distributions of DCN and DCO{sup +} vary between disks, suggesting that multiple formation pathways may be needed to explain the diverse emission morphologies. In addition, gaps and rings observed in both H{sup 13}CO{sup +} and DCO{sup +} emission provide new evidence that DCO{sup +} bears a complex relationship with the location of the midplane CO snowline.

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DOE OSTI2017

ALMA MEASUREMENTS OF CIRCUMSTELLAR MATERIAL IN THE GQ LUP SYSTEM

We present Atacama Large Millimeter/submillimeter Array observations of the GQ Lup system, a young Sun-like star with a substellar-mass companion in a wide-separation orbit. These observations of 870 μ m continuum and CO J = 3–2 line emission with beam size ∼0.″3 (∼45 au) resolve the disk of dust and gas surrounding the primary star, GQ Lup A, and provide deep limits on any circumplanetary disk surrounding the companion, GQ Lup b. The circumprimary dust disk is compact with an FWHM of 59 ± 12 au, while the gas has a larger extent with a characteristic radius of 46.5 ± 1.8 au. By forward-modeling the velocity field of the circumprimary disk based on the CO emission, we constrain the mass of GQ Lup A to be M {sub *} = (1.03 ± 0.05) ∗ ( d /156 pc) M {sub ⊙}, where d is a known distance, and determine that we view the disk at an inclination angle of 60.°5 ± 0.°5 and a position angle of 346° ± 1°. The 3 σ upper limit on the 870 μ m flux density of any circumplanetary disk associated with GQ Lup b of <0.15 mJy implies an upper limit on the dust disk mass of <0.04 M {sub ⊕} for standard assumptions about optically thin emission. We discuss proposed mechanisms for the formation of wide-separation substellar companions given the non-detection of circumplanetary disks around GQ Lup b and other similar systems.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2016

THE COUPLED PHYSICAL STRUCTURE OF GAS AND DUST IN THE IM Lup PROTOPLANETARY DISK

The spatial distribution of gas and solids in protoplanetary disks determines the composition and formation efficiency of planetary systems. A number of disks show starkly different distributions for the gas and small grains compared to millimeter–centimeter-sized dust. We present new Atacama Large Millimeter/Submillimeter Array observations of the dust continuum, CO, {sup 13}CO, and C{sup 18}O in the IM Lup protoplanetary disk, one of the first systems where this dust–gas dichotomy was clearly seen. The {sup 12}CO is detected out to a radius of 970 au, while the millimeter continuum emission is truncated at just 313 au. Based upon these data, we have built a comprehensive physical and chemical model for the disk structure, which takes into account the complex, coupled nature of the gas and dust and the interplay between the local and external environment. We constrain the distributions of gas and dust, the gas temperatures, the CO abundances, the CO optical depths, and the incident external radiation field. We find that the reduction/removal of dust from the outer disk exposes this region to higher stellar and external radiation and decreases the rate of freeze-out, allowing CO to remain in the gas out to large radial distances. We estimate a gas-phase CO abundance of 5% of the interstellar medium value and a low external radiation field ( G {sub 0} ≲ 4). The latter is consistent with that expected from the local stellar population. We additionally find tentative evidence for ring-like continuum substructure, suggestions of isotope-selective photodissociation, and a diffuse gas halo.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2016

Constraints on planetesimal collision models in debris disks

Observations of debris disks offer a window into the physical and dynamical properties of planetesimals in extrasolar systems through the size distribution of dust grains. In particular, the millimeter spectral index of thermal dust emission encodes information on the grain size distribution. We have made new VLA observations of a sample of seven nearby debris disks at 9 mm, with 3{sup ′′} resolution and ∼5 μJy beam{sup −1}rms. We combine these with archival ATCA observations of eight additional debris disks observed at 7 mm, together with up-to-date observations of all disks at (sub)millimeter wavelengths from the literature, to place tight constraints on the millimeter spectral indices and thus grain size distributions. The analysis gives a weighted mean for the slope of the power-law grain size distribution, n(a)∝a{sup −q}, of 〈q〉=3.36±0.02, with a possible trend of decreasing q for later spectral type stars. We compare our results to a range of theoretical models of collisional cascades, from the standard self-similar, steady-state size distribution (q = 3.5) to solutions that incorporate more realistic physics such as alternative velocity distributions and material strengths, the possibility of a cutoff at small dust sizes from radiation pressure, and results from detailed dynamical calculations of specific disks. Such effects can lead to size distributions consistent with the data, and plausibly the observed scatter in spectral indices. For the AU Mic system, the VLA observations show clear evidence of a highly variable stellar emission component; this stellar activity obviates the need to invoke the presence of an asteroid belt to explain the previously reported compact millimeter source in this system.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2016

RINGED SUBSTRUCTURE AND A GAP AT 1 au IN THE NEAREST PROTOPLANETARY DISK

We present long baseline Atacama Large Millimeter/submillimeter Array (ALMA) observations of the 870 μm continuum emission from the nearest gas-rich protoplanetary disk, around TW Hya, that trace millimeter-sized particles down to spatial scales as small as 1 au (20 mas). These data reveal a series of concentric ring-shaped substructures in the form of bright zones and narrow dark annuli (1–6 au) with modest contrasts (5%–30%). We associate these features with concentrations of solids that have had their inward radial drift slowed or stopped, presumably at local gas pressure maxima. No significant non-axisymmetric structures are detected. Some of the observed features occur near temperatures that may be associated with the condensation fronts of major volatile species, but the relatively small brightness contrasts may also be a consequence of magnetized disk evolution (the so-called zonal flows). Other features, particularly a narrow dark annulus located only 1 au from the star, could indicate interactions between the disk and young planets. These data signal that ordered substructures on ∼au scales can be common, fundamental factors in disk evolution and that high-resolution microwave imaging can help characterize them during the epoch of planet formation.

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DOE OSTI2016

RESOLVED CO GAS INTERIOR TO THE DUST RINGS OF THE HD 141569 DISK

The disk around HD 141569 is one of a handful of systems whose weak infrared emission is consistent with a debris disk, but still has a significant reservoir of gas. Here we report spatially resolved millimeter observations of the CO(3-2) and CO(1-0) emission as seen with the Submillimeter Array and CARMA. We find that the excitation temperature for CO is lower than expected from cospatial blackbody grains, similar to previous observations of analogous systems, and derive a gas mass that lies between that of gas-rich primordial disks and gas-poor debris disks. The data also indicate a large inner hole in the CO gas distribution and an outer radius that lies interior to the outer scattered light rings. This spatial distribution, with the dust rings just outside the gaseous disk, is consistent with the expected interactions between gas and dust in an optically thin disk. This indicates that gas can have a significant effect on the location of the dust within debris disks.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2016

RESOLVED MILLIMETER-WAVELENGTH OBSERVATIONS OF DEBRIS DISKS AROUND SOLAR-TYPE STARS

The presence of debris disks around young main-sequence stars hints at the existence and structure of planetary systems. Millimeter-wavelength observations probe large grains that trace the location of planetesimal belts. The Formation and Evolution of Planetary Systems Spitzer Legacy survey of nearby young solar analogues yielded a sample of five debris disk-hosting stars with millimeter flux suitable for interferometric follow-up. We present observations with the Submillimeter Array (SMA) and the Combined Array for Research in Millimeter-wave Astronomy at ∼2″ resolution that spatially resolve the debris disks around these nearby (d ∼ 50 pc) stars. Two of the five disks (HD 377, HD 8907) are spatially resolved for the first time and one (HD 104860) is resolved at millimeter wavelengths for the first time. We combine our new observations with archival SMA and Atacama Large Millimeter/Submillimeter Array data to enable a uniform analysis of the full five-object sample. We simultaneously model the broadband photometric data and resolved millimeter visibilities to constrain the dust temperatures and disk morphologies, and perform a Markov Chain Monte Carlo analysis to fit for basic structural parameters. We find that the radii and widths of the cold outer belts exhibit properties consistent with scaled-up versions of the Solar System's Kuiper Belt. All the disks exhibit characteristic grain sizes comparable to the blowout size, and all the resolved observations of emission from large dust grains are consistent with an axisymmetric dust distribution to within the uncertainties. These results are consistent with comparable studies carried out at infrared wavelengths.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2015

CHEMICAL IMAGING OF THE CO SNOW LINE IN THE HD 163296 DISK

The condensation fronts (snow lines) of H{sub 2}O, CO, and other abundant volatiles in the midplane of a protoplanetary disk affect several aspects of planet formation. Locating the CO snow line, where the CO gas column density is expected to drop substantially, based solely on CO emission profiles, is challenging. This has prompted an exploration of chemical signatures of CO freeze-out. We present ALMA Cycle 1 observations of the N{sub 2}H{sup +} J = 3−2 and DCO{sup +} J = 4−3 emission lines toward the disk around the Herbig Ae star HD 163296 at ∼0.″5 (60 AU) resolution, and evaluate their utility as tracers of the CO snow line location. The N{sub 2}H{sup +} emission is distributed in a ring with an inner radius at 90 AU, corresponding to a midplane temperature of 25 K. This result is consistent with a new analysis of optically thin C{sup 18}O data, which implies a sharp drop in CO abundance at 90 AU. Thus N{sub 2}H{sup +} appears to be a robust tracer of the midplane CO snow line. The DCO{sup +} emission also has a ring morphology, but neither the inner nor the outer radius coincide with the CO snow line location of 90 AU, indicative of a complex relationship between DCO{sup +} emission and CO freeze-out in the disk midplane. Compared to TW Hya, CO freezes out at a higher temperature in the disk around HD 163296 (25 versus 17 K in the TW Hya disk), perhaps due to different ice compositions. This highlights the importance of actually measuring the CO snow line location, rather than assuming a constant CO freeze-out temperature for all disks.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2015

WEAK TURBULENCE IN THE HD 163296 PROTOPLANETARY DISK REVEALED BY ALMA CO OBSERVATIONS

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 Atacama Large Millimeter/submillimeter Array Science Verification observations of four CO emission lines (the CO(3-2), CO(2-1), {sup 13}CO(2-1), and C{sup 18}O(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 magnetohydrodynamic turbulence driven by the magnetorotational instability, potentially suggesting that this mechanism is less efficient in the outer (R ≳ 30 AU) disk than has been previously considered.

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