Optical polarization of the long-periodical comet C/2026 A2 (Bok)
ATel #18139; Christian Woehler (TU Dortmund University)
on 3 Nov 2026; 14:34 UT
Credential Certification: Christian Woehler (christian.woehler@tu-dortmund.de)
The long-periodical comet C/2026 A2 (Bok) has an orbital period of more than 23,000 years, and by the end of October 2026 its distance from the Sun and the Earth was 2.04 and 1.64 AU, respectively, at a phase angle of 28.7 deg, less than two degrees below the maximum phase angle of 30.4 deg that will be reached by mid-December 2026 (JPL Horizons Ephemeris). This comet provides a unique opportunity of acquiring pre-perihelion polarization data of a relatively far-out long-periodical comet.
In the night of October 30/31, 2026, optical imaging polarimetric observations were conducted in the SLOAN r and i bands, using a portable 150/600 mm Newtonian reflector in combination with an electrically cooled QHY1253P polarization camera. The observing location was Wetter (Ruhr), Germany. Polarization filters with orientation angles of 0, 45, 90, and 135 deg are attached to each 2x2 pixel block of the camera sensor, so that one frame contains four simultaneously acquired sub-frames corresponding to the four polarizer orientations. Two sets of 531 frames in the r band and 166 frames in the i band were taken on October 30.97 and 31.11, respectively, with a per-frame exposure time of 20 s. The resolution of the sub-frames is 2.4 arcsec/pixel. For each polarizer orientation, the corresponding sub-frames were centered on the comet and stacked, and the dark signal and the brightness of the blank sky were subtracted from each of the four stacked images. A circular aperture of 15,000 km radius around the photocenter was used to represent the inner coma and a concentric ring-shaped region of 15,000 - 30,000 km radius to represent the outer coma (see supplementary figures). The mean pixel intensities were then computed as a function of the polarizer orientation for the inner and outer coma region, respectively, and a sinusoidal function was fitted to derive the corresponding DoLP values. The instrumental polarization was taken into account based on an observation of the unpolarized standard star γ Bootis (Turnshek et al., 1990, DOI:10.1086/115413). The error analysis of Arnaut et al. (2026, DOI:10.1051/0004-6361/202555905) was used to estimate the DoLP measurement errors (given here as one standard deviation). In the i band data, "contamination" of the outer coma by a star trail has been masked out manually.
The coma appears to be highly condensed, and in the r band a diffuse tail of about 80,000 km length is apparent. The following DoLP values were measured for the central and outer coma:
- Central coma: 0.022±0.009 (r) 0.049±0.030 (i)
- Outer coma: 0.028±0.009 (r) 0.049±0.030 (i)
Despite the long-periodical nature of C/2026 A2, the DoLP of its central coma in the r band is similar to the values in the red continuum typically observed at the same phase angle for short-periodical, low-Pmax Jupiter-family comets (Kiselev et al., 2015, DOI:10.1017/CBO9781107358249.022). Both in the inner and outer coma, the DoLP is higher in the i band than in the r band (with large measurement errors, though, in the i band). This behavior suggests a predominance of sub-micrometer particle sizes close to the Rayleigh limit. Furthermore, under the assumption of similar dust composition and morphology, it can be concluded that the dust particles in the central coma of C/2026 A2 are larger than in the coma of the dynamically new comet C/2023 A3 (Tsuchinshan-ATLAS), whose DoLP was at least 50% higher at the same phase angle (Arnaut et al., 2026, DOI:10.1051/0004-6361/202555905).
Supplementary figures
