AU Mic through new ALMA glasses

Title: Millimeter Emission Structure in the first ALMA Image of the AU Mic Debris Disk
Author: Meredith A. MacGregor, David J. Wilner, Katherine A. Rosenfeld, Sean M. Andrews, Brenda Matthews, A. Meredith Hughes, Mark Booth, Eugene Chiang, James R. Graham, Paul Kalas, Grant Kennedy, Bruce Sibthorpe
Institution: Harvard University, Cambridge, MA

I’ve got pretty bad eyesight. If I take off my glasses and look at the flowers on my window sill, they look like a fuzzy yellow blob. But with glasses, the petals and the patterns cast on them come into focus. This is how I felt when looking at the new ALMA observations of the debris disk around AU Mic. ALMA is Atacama Large Millimeter/submillimeter Array; see these astrobites on ALMA, including Adele’s astrobite introducing the array and Nick’s post on the first results from ALMA, which considered the dust ring around Fomalhault.

Debris disks such as AU Mic are comprised of continuously-supplied dust and grains resulting from the collision of planetesimals. Observations at long wavelengths (1.3mm is “long”) are sensitive to millimeter-sized grains. Grains this size trace the location and distribution of the parent population of eroding planetesimals.

I wrote about this debris disk, which last spring was the subject of a paper by Wilner et al. The previous work looked at AU Mic using the Submillimeter Array (SMA). You can check out the SMA data, model and the difference between the data and the model (the “residuals”) below. Contours of constant flux are plotted over the data and in the bottom left corner is shown the beam size. The beam size indicates what size of features are resolvable in the observations, so smaller is better.

Data, model and residuals for AU Mic, as observed with SMA by Wilner et al. (2012)

The paper I’ll present today takes a second look at AU Mic. Using ALMA, MacGregor et al. see the debris disk around AU Mic snap into focus with the new, higher resolution data. You can see this for yourself in the figure below (note that the y-axis scaling is actually smaller on this image than in the figure above!). MacGregor et al. can now see that the debris disk is composed of an outer disk and an inner emission source that is not resolved (all we know is that it’s smaller than the beam size, so less than 3 AU in radius).

Data, model and residuals for AU Mic, as observed with ALMA, by MacGregor et al. (2012). Notice that the range of the y axis is smaller.

The central emission source was not detected in the previous SMA observations or in observations at other wavelengths. The authors offer two possible explanations for the central source. First, it could be emission from the star, which is an M dwarf with frequent flares. However, they suggest that stellar activity is too variable and too weak to explain the central source, which is seen in all four of their individual ALMA observations. Second, it could be an analog to our Solar System’s asteroid belt (which is at about 2.7 AU). A cool, dusty belt at a radius of less than 3 AU with reasonable grain sizes and temperatures is consistent with both the ALMA observations and the lack of previous detections.

ALMA data for AU Mic again, in color. From MacGregor et al.

The new model for the outer debris disk is consistent with previous results, but ALMA’s superior resolution allows it to be studied in more detail. MacGregor et al. find that there’s radial structure in the disk: it gets brighter as you go farther away from the star, but then truncates rather abruptly at 40 AU. There’s no easy explanation for the gradient; however, our own Kuiper belt shows a similar truncation around 47 AU.

Putting on our ALMA glasses, the fuzzy debris disk around AU Mic is sharpening into something surprisingly consistent with our own Solar System. There’s a large, outer disk similar to the Kuiper Belt and perhaps a small, inner disk similar to the asteroid belt. The similarity extends farther than the location of the two belts. The authors estimate the mass ratio of the outer and inner belts in AU Mic to be about 100:1 – roughly the same ratio as for the Kuiper Belt and the asteroid belt. Moreover, the minimum mass of the inner belt is 1% of the Moon’s mass, comparable to the total mass of the asteroid belt (4% of the lunar mass). With further high resolution observations at a range of wavelengths, we will see whether this comparison continues to hold up.

About Elisabeth Newton

Elisabeth was a Harvard graduate student and an astrobites and ComSciCon co-founder and is now a professor at Dartmouth College.

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