Abstract DGP2026-109

Pre-perihelion imaging polarimetry of dust eruption events on comet C/2025 A6 (Lemmon)

Christian Wöhler (1), Mirza Arnaut (1), Prithish Halder (2), Shashikiran Ganesh (3), Goldy Ahuja (3), Megha Bhatt (3)

(1) Image Analysis Group, TU Dortmund University, Otto-Hahn-Str. 4, 44227 Dortmund, Germany (2) Department of Physics and Astronomy, University of Nebraska-Lincoln, Lincoln, NE, 68588-0299, USA (3) Physical Research Laboratory, Ahmedabad, 380009, India

Ground-based imaging polarimetry is a well-established method for analysing the morphological and compositional properties of cometary dust [1]. The polarisation of cometary tails has remained largely unexplored, though, with only few exceptions (e.g., [2, 3]). In this study, we present pre-perihelion polarimetric imaging observations of the coma and tail of comet C/2025 A6 (Lemmon) along with preliminary results of numerical dust modelling.

We conducted 14 observations of C/2025 A6 between Sep 28 and Nov 11, 2025, at phase angles between 54° and 103°, from Wetter (Ruhr), Germany (ten observations), and the Ballon d’Alsace mountain, France (four observations). A monochrome industrial-grade polarisation camera TheImagingSource DZK 33UX250 [4] was used in combination with a 150/600 mm Newton reflector and a 60/300 mm apochromatic refractor. The cometary gas emissions were blocked with a 570 nm long-pass filter. Our data processing technique is described in [3]. We fitted the empirical trigonometric DoLP model [5] to our coma and tail observations separately.

The trigonometric model provides a good fit to our coma observations and yields a maximum DoLP of 0.22 at 98° phase angle.

Before Oct 20, the comet only had a gas tail, and the diameter of the dust coma was less than 80,000 km. The trigonometric DoLP model fitted to six tail observations between Sep 28 and Oct 27 (excluding three on Oct 02, 03, and 20) covering phase angles of 54°–103° reveals a DoLP maximum of 0.83 at 82° phase angle. On the excluded dates, the DoLP was lower by 0.2–0.3 than the model prediction. These anomalies were accompanied by dust jets between Sep 28 and Oct 02/03 [6] and the development of a conspicuous dust tail between Oct 17 and 20. The tail DoLP of 0.67 observed on Oct 27 (103° phase angle, 0.60 AU solar distance) decreased to only 0.24 until Nov 07 (74°, 0.53 AU). Simultaneously, the dust coma diameter increased from 80,000 km to 170,000 km, and the dust tail brightened considerably.

Preliminary dust modelling based on MSTM5 [7] and DDSCAT [8] indicates that the predominant particle radii exceeded ~1 µm in the coma and were close to the Rayleigh limit (~0.1 µm) in the tail (similar to comet C/2023 A3 [3]) in the absence of dust ejection events. During the strong dust activity after Oct 27, the tail was dominated by particles with radii of several tenths of a micrometre, indicating an influx of fresh dust particles from the coma not having decayed yet into small particles characteristic of the “undisturbed” tail.

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[3] Arnaut et al. (2026) A&A, https://www.doi.org/10.1051/0004-6361/202555905
[4] https://www.theimagingsource.com/de-de/product/unique/33u-polarsens/dzk33ux250/
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