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Gizmodo's Mega Comet Arriving From the Oort Cloud Is 85 Miles Wide says:

For the new study, Lellouch and his colleagues used the Atacama Large Millimeter Array (ALMA) in Chile to refine the comet’s size and reflectivity, or albedo. They did so on August 8, 2021, when Bernardinelli-Bernstein was 20 au from the Sun (1.86 billion miles). The team honed in on microwave radiation leaking out from the comet’s nucleus, while taking care to exclude radiation produced by the surrounding cloud of dust.

These thermal emissions pointed to the 85-mile (137 km) diameter, with a lower bound of 75 miles (120 km) and an upper bound of 96 miles (154 km). The large error bar is on account of uncertainties having to do with the object’s shape and reflectivity. Future observations should refine these estimates further.

Question: What exactly did ALMA observe at 20 AU from the "Mega Comet Arriving From the Oort Cloud" that will pass in 2031?

Did ALMA actually image the comet and spatially resolve the surrounding cloud of dust to separate it from the nucleus, or was all of that done without resolving the object?

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    $\begingroup$ The comet itself is spatially resolved (with synthesised beam 0.067”× 0.062, which is almost the best resolution you can get with ALMA). The observations are of the thermal continuum emission at 233 GHz. Image in the original paper (arxiv.org/pdf/2201.13188.pdf). The authors states the high resolution allows the dust contribution to be filtered out, although I'm not sure about the details of this. $\endgroup$
    – lucas
    Commented Feb 10, 2022 at 22:44
  • $\begingroup$ @lucas wow that was fast! It seems like you can post an answer already as well. $\endgroup$
    – uhoh
    Commented Feb 10, 2022 at 22:56
  • $\begingroup$ @lucas to that end I've just added a bounty $\endgroup$
    – uhoh
    Commented Mar 11, 2022 at 7:12

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ALMA measured the brightness of thermal emission from the comet (at 1287 µm or 233 GHz). The comet was not resolved directly, so Lellouch++2022 treated it as a point source. But spatial information in the ALMA data was used to rule out a significant dusty coma surrounding the nucleus.

Some details and methods:

  • The spatial resolution of the ALMA data was about 60 mas as @lucas said in the comments, but the Lellouch team's estimate for the diameter of the nucleus was 9.6 mas (their footnote 3). So they did not directly resolve the nucleus.

  • They got the size my making a bunch of assumptions in a model, combining the ALMA data to give the thermal radiation and other published measurements of the object's brightness at visible wavelengths. This is described in an Encyclopedia Britannica entry:

The most widely used technique for determining the sizes of asteroids (and other small bodies in the solar system) is that of thermal radiometry. That technique exploits the fact that the infrared radiation (heat) emitted by an asteroid must balance the solar radiation it absorbs. By using a so-called thermal model to balance the measured intensity of infrared radiation with that of radiation at visual wavelengths, investigators are able to derive the diameter of the asteroid.

  • They used a couple of methods to prove that there was no large contribution from a dust coma at the ALMA wavelength. A significant dust coma would have been resolved spatially if it were there.
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