Optical polarization of the central and outer coma of the short-periodical comet 10P/Tempel 2
ATel #17862; Christian Woehler (Image Analysis Group, TU Dortmund University, Germany)
on 1 Jul 2026; 10:35 UT
Credential Certification: Christian Woehler (christian.woehler@tu-dortmund.de)
The comet 10P/Tempel 2 has a short orbital period of only 5.36 years (JPL Horizons, https://ssd.jpl.nasa.gov/horizons/app.html). In June 2026, we conducted optical polarimetric measurements of the central and outer coma of 10P/Tempel 2 at phase angles between 26.0 and 27.3 degrees. These observations complement earlier polarimetric data of 10P/Tempel 2 at ~45 degrees phase angle in the Johnson-Cousins V, R, and I bands, which are limited to the central coma (Myers et al., 1985, DOI:10.1016/0019-1035(85)90005-3). To the best of our knowledge, no further optical polarization data of this comet have been published so far.
For the observations, a portable 150/600 mm Newton reflector in combination with an electrically cooled QHY1253P polarization camera was used. Polarization filters with orientation angles of 0, 45, 90, and 135 degrees are attached to the sensor pixels of this camera, such that each 2x2 pixel subset yields one measurement of the unpolarized intensity, DoLP, and position angle of linear polarization (relative to the direction of the pixel rows), respectively. To increase the SNR, two-fold binning was performed, leading to an effective image resolution of 4.74 arcsec/pixel. Instrumental polarization effects were compensated based on observations of the polarization standard star HD183143 (Bailey and Hough, 1982, DOI:10.1086/131032) and the unpolarized standard star Gamma Bootis (Turnshek et al., 1990, DOI:10.1086/115413).
The observations were conducted at Erbéviller-sur-Amezule, France, on June 22 and 24, and near Wetter/Ruhr, Germany, on June 26. A SLOAN r filter was used in order to block green gas emissions and focus on the dust continuum. In each dataset, a circular region of 5,000 km radius around the photocenter (region 1) was defined to represent the central coma. A ring-shaped region around the photocenter with an inner and an outer radius of 10,000 and 15,000 km (region 2) was chosen to represent the outer coma (see also supplementary figures). For each polarizer orientation, the mean pixel intensities were computed for each region (after subtracting the dark current and the signal of the blank sky from each frame), and a sinusoidal function was fitted to derive the mean DoLP of each region. Due to the large-scale intensity averaging, the statistical measurement error of the DoLP is below 0.01 (Arnaut et al., 2026, DOI:10.1051/0004-6361/202555905).
Date: June_22.02 June_24.02 June_26.01
Phase angle [degrees]: 27.3 26.7 26.0
Distance from Earth [AU]: 0.561 0.549 0.536
Distance from Sun [AU]: 1.482 1.476 1.471
Exposure time [hours]: 01:41:15 02:12:45 01:29:15
DoLP (region 1): 0.027 0.024 0.011
DoLP (region 2): 0.026 0.015 0.032
Phase angles and distances are from JPL Horizons. The DoLP measurements are consistent with the polarimetric data of low-polarization comets in the red spectral domain (Kiselev et al., 2015, DOI:10.1017/CBO9781107358249.022). On June 22.00, the DoLP was also measured in the SLOAN g band (exposure time 21:30 minutes). The coma brightness was significantly higher than in the r band, hinting at a predominant C2 emission. The DoLP was 0.019 and 0.013 for region 1 and 2, respectively, which is consistent with the value of 0.015 predicted by the standard expression of emission band polarization (Le Borgne et al., 1987, A&A 187, 526).
The observation that the DoLP shows no clear dependence on the distance from the photocenter suggests that the particle size distribution (and other properties such as particle composition and morphology) is largely constant across the coma. This is in stark contrast to the radial increase in DoLP across the coma by a factor of ~2 observed for the dynamically new comet C/2023 A3 that was attributed to a changing size distribution favoring smaller particles with increasing distance from the photocenter (Arnaut et al., 2026, DOI:10.1051/0004-6361/202555905).
Supplementary figures
