What Shaped Elias 2-27’s Disk?

The young star Elias 2-27 is surrounded by a massive disk with spectacular spiral arms. A team of scientists from University of Cambridge’s Institute of Astronomy has now examined what might cause this disk’s appearance.

Top: ALMA 1.3-mm observations of Elias 2-27’s spiral arms, processed with an unsharp masking filter. Two symmetric spiral arms, a bright inner ellipse, and two dark crescents are clearly visible. Bottom: a deprojection of the top image (i.e., what the system would look like face-on). [Meru et al. 2017]

Top: ALMA 1.3-mm observations of Elias 2-27’s spiral arms, processed with an unsharp masking filter. Two symmetric spiral arms, a bright inner ellipse, and two dark crescents are clearly visible. Bottom: a deprojection of the top image (i.e., what the system would look like face-on). [Meru et al. 2017]

ALMA-Imaged Spiral Arms

With the dawn of new telescopes such as the Atacama Large Millimeter/submillimeter Array, we’re now able to study the birth of young stars and their newly forming planetary systems in more detail than ever before. But these new images require new models and interpretations!

Case in point: Elias 2-27 is a low-mass star that’s only a million years old and is surrounded by an unusually massive disk of gas and dust. Recent spatially-resolved ALMA observations of Elias 2-27 have revealed the stunning structure of the star’s disk: it contains two enormous, symmetric spiral arms, as well as additional features interior to the spirals.

What caused the disk to develop this structure? Led by Farzana Meru, a group of  Institute of Astronomy researchers has run a series of simulations that explore different ways that Elias 2-27’s disk might have evolved into the shape we see today.

Modeling a Disk

Meru and collaborators performed a total of 72 three-dimensional smoothed particle hydrodynamics simulations tracking 250,000 gas particles in a model disk around a star like Elias 2-27. They then modeled the transfer of energy through these simulated disks and produced synthetic ALMA observations based on the outcomes.

Left: Synthetic ALMA observations of disks shaped by an internal companion (top), an external companion (middle), and gravitational instability within the disk (bottom). Right: Deprojections of the images on the left. Scale is the same as in the actual observations above. The external companion and the gravitational instability scenarios match the actual ALMA observations of Elias 2-27 well. [Adapted from Meru et al. 2017]

Left: Synthetic ALMA observations of disks shaped by an internal companion (top), an external companion (middle), and gravitational instability within the disk (bottom). Right: Deprojections of the images on the left. Scales are the same as in the actual observations above. The external companion and the gravitational instability scenarios match the actual ALMA observations of Elias 2-27 well. [Adapted from Meru et al. 2017]

By comparing these synthetic observations to the true ALMA observations of Elias 2-27, the authors hoped to determine which of three possible scenarios could produce the disk shape we see: 1) a companion (a planet or star) internal to the spiral arms, 2) a companion external to the spirals, or 3) gravitational instabilities operating within the disk.

Gravity or a Companion?

Meru and collaborators find that two scenarios produce observations that are very similar to what ALMA imaged. In the first, the disk is so massive that it becomes gravitationally unstable. Self-gravity of the disk then forms the spiral structures. In the second scenario, the arms are formed by a planetary companion of up to ~10–13 Jupiter masses orbiting Elias 2-27 outside of the spiral arms, at a large distance roughly in the range of 300–700 AU.

Though the possible companion inside the spiral arms is ruled out, the scenarios of a gravitational instability or an external companion remain plausible. If the former is true, then Elias 2-27 would be one of the first examples of an observed self-gravitating disk. If the latter is true, then Elias 2-27’s disk likely fragmented recently, forming the giant planet that shapes the disk. This would be the first evidence for a disk that has fragmented into planetary-mass objects.

Future deep near-infrared imaging may offer the chance to distinguish between these scenarios by allowing us to search for the heat from the possible companion.

Citation

F. Meru et al 2017 ApJL 839 L24. doi:10.3847/2041-8213/aa6837

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