Perihelion Activity of 3200 Phaethon Is Not Dusty

Perihelion Activity of 3200 Phaethon Is Not Dusty by Man-To Hui. on Tuesday 22 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