Perihelion Activity of 3200 Phaethon Is Not Dusty by Man-To Hui. on Wednesday 23 November

We present an analysis of asteroid (3200) Phaethon using coronagraphic

observations from 2008 to 2022 by the COR2 cameras onboard the twin Solar

TErrestrial RElations Observatory (STEREO) spacecraft. Although the asteroid

cannot be confidently detected in individual images, we managed to spot it in

image stacks combined from the same sets of perihelion observations, yet only

when observed at low phase angles ($\lesssim$30\deg) but not at large phase

angles ($\gtrsim$150\deg). The lack of a strong forward-scattering enhancement

that is expected for dust grains having sizes comparable to transmitted

wavelengths thereby implies that the perihelion activity of Phaethon is highly

unlikely to be relevant to the ejection of dust grains as suggested by earlier

studies based on STEREO's HI-1 observations. Assuming the observed activity of

Phaethon is caused by dust ejection will lead to an insurmountable discrepancy

in the inferred amount of dust no less than an order of magnitude between the

HI-1 and COR2 observations. Rather, we speculate that the perihelion activity

is caused by sodium and/or iron emissions, the former of which may have become

transmittable due to an ageing effect of the HI-1 cameras. The modelled

emission flux is qualitatively similar to the HI-1 observations in the case

where the peak of the atomic production rate is delayed by $\sim$1 day from

perihelion. We encourage future observations of Phaethon at small heliocentric

distances to verify our conjecture.

arXiv: http://arxiv.org/abs/http://arxiv.org/abs/2206.09704v2