Young, cool, and on edge — an unstable protoplanetary disk

Title: Formation of dust clumps with sub-Jupiter mass and cold shadowed region in gravitationally unstable disk around class 0/I protostar in L1527 IRS

Authors: Satoshi Ohashi, Riouhei Nakatani, Hauyu Baobab Liu, Hiroshi Kobayashi, Yichen Zhang, Tomoyuki Hanawa, and Nami Sakai

First Author’s Institution: RIKEN Cluster for Pioneering Research, 2-1, Hirosawa, Wako-shi, Saitama 351-0198, Japan

Status: Published on ArXiv, 17 Jun 2022

When a cloud of gas in space has enough mass, the gravitational forces from all the gas overwhelm the gas pressure keeping the cloud puffed up, and it collapses under its own gravity to form a star. If the cloud is initially rotating, the contraction of the gas will magnify that rotation, due to the conservation of angular momentum – imagine spinning on a desk chair, and pulling your legs in towards your body. The rotation also drives material towards the equatorial plane ultimately resulting in a so-called protoplanetary disk — a flattened disk of leftover gas and dust orbiting the newly-formed star.

The protoplanetary disk that birthed the planets in our solar system is long gone, so we need to look to stars much younger than our Sun, to study these planetary nurseries. Today’s authors present a detailed analysis of a particular protoplanetary disk — one that is gravitationally unstable. 

Remember the gravitational instability that formed the star from a cloud? Well, the disk can be unstable to its own gravity, too, when the pressure and rotational forces are too small to prevent collapse. This can occur if the disk is very massive and also very cool. Gravitational instability in disks is one possible way of manufacturing giant planets. It causes the disk to fragment into many small blobs of gas, which then collapse into planets. Thus, understanding how gravitational instability begins is an important piece of the puzzle in understanding the formation of the diverse range of planetary systems discovered over the last twenty years.

Four observations of the protoplanetary disks in different bands.
Figure 1: ALMA images (Band 7,4,3) and JVLA image (Q band) of the L1527 protoplanetary disk viewed edge-on. The clumps detected in the Q band are marked with black crosses in the other panels. The white ellipse in the bottom left of each panel indicates the image resolution. Figure 2 in the paper.

Observing an edge-on protoplanetary disk

Two excellent tools for observing protoplanetary disks are the Atacama Large Millimeter Array (ALMA) and the Jansky Very Large Array (JVLA). Both use an array of dishes that look at the target in unison, acting as one massive telescope. Both can observe at different wavelengths, called bands, which can be combined to produce a more complete picture of the disk.

Sketch of a protoplanetary disk.
Figure 2: Sketch of L1527’s disk, as viewed from Earth. The hot regions (red) on the near side are obscured by the flared outer disk (blue), so the near side appears slightly hotter in temperature maps.  Figure 7 in the paper.

The disk observed by today’s authors is around the very young (less than hundred thousand years) star L1527 IRS in the Taurus molecular cloud at a distance of 137 parsecs. The disk is viewed nearly edge-on, and its host star is still accreting and the disk has not yet fully formed. Each band penetrates the disk to a different depth, so the observation will look very different depending on the filter. Figure 1 shows three ALMA images (bands 3,4,7) and one JVLA image (Q band). Viewed head-on, the center of the disk is expected to be symmetrical in temperature, so any temperature asymmetry in this region can be used to measure the disk inclination. The regions closest to the host star receive the most radiation, so they are the hottest. However, since the disk is flared (= it becomes thicker with distance to the star) , the inner regions on the near side should be mostly obscured, whereas the inner regions on the far side are better visible. This is sketched in the schematic in Fig. 2. Because of this asymmetry, the near side appears hotter in Fig. 1. The authors used this to put the disk inclination at around 5 degrees, with an additional warping potentially being present.

Assessing gravitational instability

A gravitationally unstable disk is characterized by a distinctive spiral structure. The problem is we can only view this disk edge-on, so we can’t see the spiral structure—much like how the Milky Way’s spiral structure isn’t visible from Earth. 

The author’s resolved this issue by assessing the stability of the L1527 disk using Toomre’s stability analysis with measured values for temperature and surface density. They find that the disk is expected to be gravitationally unstable. The left panel of Fig 3. shows the spiral structure typical to a gravitationally unstable disk in the face-on view. If we were to look at this model disk from an edge-on, 90 degree rotated view, we’d see two high density regions flanking the center of the disk (right panel of Fig. 3). This almost reproduces the shape of the Q-band observation (right most panel, Fig. 1) so the authors conclude that L1527’s disk is indeed likely to be gravitationally unstable.

A model of a gravitationally unstable protoplanetary disk. The face-on view shows the spiral structure. The projected edge-on view shows two clumps.
Figure 3: Model of a gravitationally unstable disk. If a massive disk cools enough such that its gas pressure cannot withstand the gas’ self-gravity, it starts to fragment and form a spiral structure (left panel, face-on view). The spiral structure projects two clumps on the edge-on view (right panel). Figure 17 in the paper.

One caveat of this assessment is that the surface density—which is a crucial quantity in Toomre’s stability analysis—has to be inferred indirectly by combining dust temperature measurements and opacity models that have some uncertainty attached to them (opacity is the ability of material to block photons). However, if truly unstable, the L1527 disk would be one of the youngest systems to be subject to gravitational instability, suggesting that this young star could have giant planets forming around it much sooner than expected. 

Astrobite edited by Sasha Warren.

About Konstantin Gerbig

I'm a PhD student in Astronomy at Yale University. I'm interested in the theory of (Exo)planets and protoplanetary disks and do hydro simulations thereof. I also like music, as well as dancing salsa and tango.

Discover more from astrobites

Subscribe to get the latest posts to your email.

Leave a Reply