A Deep Search for Emission from "Rock Comet" (3200) Phaethon at 1 au

We present a deep imaging and spectroscopic search for emission from (3200) Phaethon, a large near-Earth asteroid that appears to be the parent of the strong Geminid meteoroid stream, using the 4.3 m Lowell Discovery Telescope. Observations were conducted on 2017 December 14-18 when Phaethon passed only 0.07 au from the Earth. We determine the 3σ upper level of dust and CN production rates to be 0.007-0.2 kg s^-1 and 2.3 × 10²² molecules s^-1 through narrowband imaging. A search in broadband images taken through the SDSS r' filter shows no 100 m class fragments in Phaethon's vicinity. A deeper but star-contaminated search also shows no sign of fragments down to 15 m. Optical spectroscopy of Phaethon and comet C/2017 O1 (ASASSN) as a comparison confirms the absence of cometary emission lines from Phaethon and yields 3σ upper levels of CN, C₂, and C₃ of ∼10²⁴-10²⁵ molecules s^-1, 2 orders of magnitude higher than the CN constraint placed by narrowband imaging, due to the much narrower on-sky aperture of the spectrographic slit. We show that narrowband imaging could provide an efficient way to look for weak gas emission from near-extinct bodies near the Earth, though these observations require careful interpretation. Assuming Phaethon's behavior is unchanged, our analysis shows that the DESTINY⁺ mission, currently planning to explore Phaethon in 2026, may not be able to directly detect a gas coma.