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Perihelion Activity of (3200) Phaethon Is Not Dusty: Evidence from STEREO/COR2 Observations

Man-To Hui
Jun 2022
We present an analysis of asteroid (3200) Phaethon using coronagraphicobservations from 2008 to 2022 by the COR2 cameras onboard the twin SolarTErrestrial RElations Observatory (STEREO) spacecraft. Although undetected inindividual images, Phaethon was visible in stacks combined from the sameperihelion observations, yet only at small ($\lesssim$30\deg) but not large($\gtrsim$150\deg) phase angles. The observations are in line with thecontribution from a bare nucleus, thereby seriously contradicting theinterpretation based on HI-1 observations that attributes the perihelionactivity to the ejection of \micron-sized dust. We obtained an upper limit tothe effective cross-section of \micron-sized dust to be $\lesssim \! 10^{5}$m$^{2}$, at least three orders of magnitude smaller than earlier estimatesbased on HI-1 data. On the contrary, the COR2 observations cannot rule out theexistence of mm-sized or larger debris around Phaethon. However, the fact thatno postperihelion debris tail has ever been detected for Phaethon suggests theunimportance of such dust in the perihelion activity. We thus conclude that theperihelion activity of Phaethon is highly unlikely relevant to the ejection ofdust. Rather, we deduce that the activity is associated with gas emissions,possibly Fe I and/or Na D lines. To verify our conjecture and to fullyunderstand the perihelion activity of Phaethon, more observations at smallheliocentric distances are desired. We compile a list of observing windowsideal for the search of gas emissions of the asteroid from ground telescopes.The best opportunities will be during total solar eclipses.