Forum der Vereinigung der Sternfreunde

There's been quite a bit of discussion on this comet here, especially on recent disintegration claims that seem to be drawing increasing outside attention. Others have already commented on and refuted the arguments on phase effect and astrometry, but have skipped over several of the other arguments variously made. As it turns out, those other arguments are also flawed, and a closer examination shows that not only that is no evidence the comet has disintegrated, but that much of the support data indicate that the comet has, in fact, *not* disintegrated:


1. The tail of C/2023 A3 is optically dominated by submillimeter-sized, β ~ 0.01 dust grains instead of micron-sized, β ~ 1 grains. This is completely normal for comets at r > ~2 au, and in fact, comets showing otherwise at this distance are quite rare by comparison. Some other "normal" comets exhibiting similar tail dust include C/2013 A1, C/2013 US10, and C/2021 A1. The micron-sized grains tend to appear at r < ~1 au, possibly as some of the bigger grains fragment under increased sunlight.

2. A slow brightening with a flat/slightly decreasing Afρ from 4 to 2 au is *normal* for a distantly active dynamically new comet of all sizes regardless of whether they disintegrate or not. For just a few examples with published data, see C/2012 S1, C/2013 A1, and C/2017 K2, where the first later disintegrated, the second may or may not have partially disrupted later in an outburst, and the third was much brighter and never did anything of note, yet all showed exactly the same behavior. This also does *not* mean dust production is flat or decreasing; Afρ may be related to dust production, but does not directly quantify dust production, instead only providing a measure of the dust cross section in the coma. All else equal, a comet with flat Afρ but r^-2 outgassing has a dust production rate roughly following r^-1 due to the faster terminal velocity of dust grains entrained in the denser gas outflow. However, even that is a simplification that does not account for changes in dust composition, grain fragmentation, etc. that are known to vary with r, and further decouple Afρ from the dust production rate. Either way, the fact that C/2023 A3's Afρ has remained fairly steady in recent months just like normal dynamically new comets supports that the comet hasn't done anything unusual.

3. Dynamically new comets are often ~100% active. That was the case for C/2012 S1 based on normally gas production rates and its estimated ~1 km diameter, as well as for C/2013 A1 which even had a directly observed ~1 km nucleus. Actually, "active fraction" can easily be above 100% because distantly active comets are known to eject water ice grains that must necessarily sublimate as the comet/the grains approach the Sun, which can elevate water production rate above a sublimating nucleus alone. But ignoring that extra contribution now that the comet is well within the water ice line, the latest published water production rate measurement of ~2e28 molecules/s at r=1.8 au is about the rate expected for a ~100% active nucleus of ~2 km diameter. That's a perfectly reasonable size for a comet nucleus, and substantially bigger than the more common ~1 km nuclei prone to disintegration at similar perihelion distances.

4. A field of active boulders *cannot* physically produce the observed coma. The dust coma has its characteristic parabolic shape because dust grains ejected from the nucleus with nonzero speed are repelled tailward by sunlight. Sunward ejected grains are stopped and turned tailward only after radiation pressure fully negates the initial ejection velocity, which means the dust coma size is set by the dust ejection speed and radiation pressure acceleration (set by β). My latest data from July 14 show the dust coma extending ~25 arcsec sunward, corresponding a dust turnaround distance of ~18000 km (approximating the coma as a paraboloid directed away from the Sun). Since the dust is β ~ 0.01, that means the dust is ejected from the nucleus with a terminal speed of ~28 m/s.

This speed is attained as a consequence of gas drag on the ejected dust grains, and is closely related to the nucleus size. Gas density is roughly equal near the surface of every ~100% active nucleus, but falls off as (distance from nucleus)^-2, so drops to near zero (thus becoming incapable of accelerating the dust grains any further) over a much shorter distance if the activity were on a bunch of hectometer-class (or smaller) boulders than if it were from a single multi-kilometer nucleus. There are a few ways of calculating terminal speed as a function of nucleus size that generally give similar results; here's a detailed step-by-step example, where substituting in the appropriate thermodynamic properties of water finds the observed ~28 m/s terminal velocity of β ~ 0.01 => ~0.1 mm radius dust would be produced by a ~2 km diameter nucleus, which is consistent with the previous point.

A field of still-active boulders would produce a much smaller coma to give a comet-within-a-comet appearance like C/2014 Y4 (or just an elongated pseudonucleus in the early stages, which is just a tail extending from a tiny, unresolved coma still in the process of forming), if the original high-velocity coma produced by the pre-fragmented nucleus hadn't dissipated yet. The narrow stripe within the main coma of C/2011 W3 following that comet's disintegration is a similar feature. Given that nothing has been seen of the sort, that confirms the observed activity is overwhelming coming from a single, big nucleus, and not a bunch of small boulders.