Post-outburst morphology and polarization behavior of the short-periodical comet 220P/McNaught

ATel #17984; Christian Woehler (TU Dortmund University, Germany)
on 18 Aug 2026; 13:07 UT
Credential Certification: Christian Woehler (christian.woehler@tu-dortmund.de)

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The short-periodical comet 220P/McNaught (5.5 years orbital period as per the JPL Horizons Ephemeris) has undergone two strong outbursts in early June (ATel #17829) and early August (ATel #17956), 2026. Between August 12 and 15, 2026, we conducted a series of optical imaging polarimetric observations at phase angles between 37.0 and 37.4 degrees in the SLOAN g, r, and i bands, using a portable 150/600 mm Newton reflector in combination with an electrically cooled QHY1253P polarization camera. The observing location was Wetter (Ruhr), Germany. After binning to increase the per-pixel SNR, an effective image resolution of 4.74 arcsec/pixel was obtained. The instrumental polarization was corrected based on an observation of the unpolarized standard star γ Boo (Turnshek et al., 1990, DOI:10.1086/115413).

The comet's structure is characterized by a greenish-blue outer coma, presumably indicating the emission of molecular carbon (C₂), a broad diffuse tail, and a narrow linear tail. The broad tail has a typical dust tail morphology, while the narrow tail resembles typical gas/ion tails of other comets. The narrow tail is about two times brighter (per pixel) than the broad tail in all three bands g, r, and i. This is somewhat unexpected since cometary gas typically shows fewer and weaker emission lines in the near-infrared than in the red and green domain.

The pixel-wise degree of linear polarization (DoLP) averaged over the central coma, the broad tail, and the narrow tail, respectively, was measured on August 14 in the r band and on August 15 in the i band, where the three different areas were selected manually in the image data. The given uncertainty is the standard error of the mean value over the respective area.

Central coma: 0.084±0.004 (r) 0.122±0.008 (i) Narrow tail: 0.114±0.003 (r) 0.164±0.003 (i) Broad tail: 0.117±0.001 (r) 0.163±0.001 (i)

The DoLP of the central coma in the r band is comparable to the values in the R domain observed at the same phase angle for high-P_max comets and is clearly above the range found for low-P_max (Jupiter-family) comets (Kiselev et al., 2015, DOI:10.1017/CBO9781107358249.022). The DoLP values of both tails are mutually similar and higher than the central coma DoLP by about one-third in both the r and the i band. In all three areas, the i-band DoLP is higher than the r-band DoLP by ~40%.

The standard expression for the DoLP of the emission of diatomic gas yields merely 0.027 at 37 degrees phase angle (Le Borgne et al., 1987, A&A 173(1), 180-182), suggesting that sunlight scattered by dust particles dominates all three areas of the comet. We furthermore conclude that on the average, the dust particles in the tails of 220P/McNaught are larger than in the dust tail of the dynamically new comet C/2023 A3 (Tsuchinshan-ATLAS), where the DoLP at 37 degrees phase angle was approximately twice as high (Arnaut et al., 2026, DOI:10.1051/0004-6361/202555905). The strong increase in the DoLP from the r to the i band indicates a predominance of sub-micrometer particle sizes close to the Rayleigh limit.

Prompt additional spectral and polarimetric observations of 220P/McNaught might shed further light on the comet's ongoing outburst-related processes. Acquisition of visible and near-infrared spectra of the broad vs. the narrow tail is particularly encouraged.

Supplementary figures illustrating the morphology of comet 220P/McNaught