Dust environment of long-period comet C/2023 A3 (Tsuchinshan-ATLAS)

We present a characterization of the dust environment of long-period comet C/2023 A3 (Tsuchinshan-ATLAS) by analysing an extensive data set, including dust tail images and photometric measurements, from 10 au pre-perihelion to 1.6 au post-perihelion, using our forward Monte Carlo code. For this analysis, we combine pre-perihelion images from the Zwicky Transient Facility with post-perihelion images from the Sierra Nevada Observatory (IAA-CSIC, Granada, Spain), along with both amateur and professional photometric measurements, including Afp and magnitude data. We find that the dust loss rate increases monotonically from the assumed start of activity at a heliocentric distance of approximately 15 au down to 4 au inbound, where the comet exhibits a notable decrease in dust production by about one order of magnitude. Following this period of reduced activity, the dust production rate rises again toward perihelion, reaching a peak production rate of 10⁵ kg s^-1 ⁠. The size distribution of the particles follows a power law, with its index decreasing towards perihelion, along with a reduction in the minimum particle radius, leading to both brightness and dust mass being dominated by small particles at perihelion. The particle speeds exhibit a dependence on heliocentric distance (⁠r_h⁠) that closely follows the classical r_h^-0.5 law near perihelion but deviates at larger heliocentric distances. We demonstrate that the dependence of particle speeds on the cosine of the solar zenith angle at the emission point plays a significant role in shaping the synthetic dust tails and in the formation of a dark stripe along the tail axis observed in high spatial resolution images near the comet’s perihelion.