Can we find signposts of planet formation in disks, part 2?

Our picture of protoplanetary disks has undergone a radical revision thanks to new high-resolution imaging instruments, including near-infrared coronagraphy with adaptive optics, but especially with the advent of mm-wave interferometers, notably the Atacama Large Millimeter and Sub-Millimeter Array (ALMA), but also the NOEMA array in France, which the UM Astronomy Department has a share.  Spectral energy distributions are sensitive to only the largest holes and gaps in disks; the new imaging has shown a wealth of detail, indicating that planet formation occurs early, and giving us a window into the processes by which our own solar system formed.

Planets embedded in disks create perturbations due to their gravitational attraction.  These pertubations get sheared out due to the differential rotation in disks essentially in Keplerian rotation.  The planet will tend to pull back on inner gas which rotates faster- this causes the inner disk gas to lose angular momentum and thus move to smaller radii away from the planet.  The converse happens for the outer disk.  Therefore, a sufficiently massive planet opens a gap in the disk around its orbit (see page on disk structure).

As an example of recent observations, SAO 206462 has been studied extensively in near-infrared scattered light (left, above) and mm-wave thermal emission with ALMA (right, above).  The scattered light image from Garufi+13 clearly shows spiral arms, with a bright spot that may be a vortex.  Such vortices can be important because they are high-pressure systems which can concentrate dust for further solid body building.  The thermal emission from Perez+14 has less resolution, but clearly shows the same overall morphology in thermal emission.

In Bae+16 we conducted 2-dimensional simulations of the effect of an outer planet on the structure of the disk.  We found that a 10-15 Jupiter mass planet (seen at about 7 o’clock in the left image) could produce spiral arms quite similar to that of the observed ones.  In addition, a vortex develops (bright part of arm at about 4-5 o’clock) which creates a pressure maximum trapping drifting dust, producing stronger thermal emission as seen on the right panel, simulated with the same observational parameters as the ALMA image.

Most, if not all, of the star and planet formation community were stunned to see the ALMA submm image of the HL Tau disk (above left; ALMA Partnership+15).  The large number of nearly concentric rings and gaps suggested multiple planet formation; however, HL Tau is thought to be quite young, with infall still occuring to the disk (Ohashi+95).  The above right HST image shows light from the central source scattered off the surrounding envelope, with a cavity carved out by the action of a disk wind and jet (red narrow outflowing structure).  HL Tau is rapidly accreting, as indicated both by the jet (powered by accretion energy) as well as the strong accretion shock continuum emission which is much brighter than that of the stellar photosphere.

Left: surface density distribution of a disk with an embedded 30 Earth
mass planet, showing both a main gap and several inner gaps.  Right:
the contrast in density, showing more clearly where the inner spiral
arms propagate and shock to produce the inner gaps.  From Bae+17.

The youth and high accretion activity of HL Tau made it an unlikely object to have many massive planets formed already, as might have been indicated by the number of gaps in the disk, and so other mechanisms for producing the structure have been suggested, such as various snow lines where condensation of volatiles could produce larger dust at certain radii.  However, Dong+17 showed that a single superEarth mass planet could produce multiple gaps in a low-viscosity disk.  We had seen similar results in some of our previous calculations so we decided to explore the origin of these gaps.

In Bae+17 we used very high spatial resolution simulations to show that such a planet can excite two or three spiral arms.  The main spiral arm creates the large gap within which the planet orbits, while the secondary and tertiary arms propagate into the inner disk (see above figure). The inner gaps form at the locations where these propagating additional arms shock.  While the gaps may not be very deep in gas density, the tendency of dust to collect in higher-pressure regions can enhance the contrast in submm dust emission.  We further showed that several of the gaps and rings in HL Tau could be explained in this way (see also Dong+17).  This is an attractive explanation which indicates modest planet formation can occur early, but does not require large numbers of massive planets.

Research led by Jaehan Bae, who will be a Rubin Fellow at the Department of Terrestrial Magnetism of the Carnegie Institution, starting Sept. 2017.