Observations of comet 12P/Pons-Brooks after the July 19, 2023 outburst
ATel #16315; Alberto Silva Betzler (UFRB-CETENS, Brazil), Adam Popowicz (Silesian University of Technology, Gliwice, Poland), Orahcio Felicio de Sousa (UFRB-CFP, Brazil)
on 1 Nov 2023; 11:40 UT
Credential Certification: Alberto Betzler (betzler@ufrb.edu.br)
Subjects: Optical, Comet, Solar System Object
Referred to by ATel #: 16316
We observed comet 12P/Pons-Brooks at Otivar Observatory (Spain), a part of SUTO telescopes network, with a 0.30-m ASA Newtonian telescope equipped with a ZWO ASI1600MM-Cool CMOS camera and BVR filters on July 25.9385, 2023 UT, after the outburst that probably started on July 19.6+/-0.4, 2023 UT (ATEL #16194).
The reduction and photometric calibration procedures used in the study are described in ATEL #15812.
The coma had a mean full width at half maximum of 26+/5 arcsec in the x and y image directions through the R filter and a teardrop shape (Figure 1).
The Johnson-Kron-Cousin colors of the comet were B-R=0.9+/0.1, V-R=0.47+/-0.05, the magnitude V was equal to 14.9+/0.2, and the phase-corrected Af\rho by the Schleicher dust phase function was 6535+/349 cm in the R band, measured with a photometric aperture radius r of 4.1 arcsec. The comet colors are slightly bluer than the solar colors (Holmberg eta al. 2006, MNRAS, 367,449). There was a coma color gradient along radius r described approximately by B-V=(0.7+/-0.2)+(-0.2+/-0.2)*log10(r) for 4.1 =< r =< 9.6 arcsec (dashed red line, Figure 2). This suggests that the coma had a uniform color, bluer than that of the Sun, regardless of the aperture radius r, or that the coma became bluer with increasing distance from the optocenter. This B-V color behavior was previously observed with comets 21P/Giacobini-Zinner on October 23, 1959 UT and C/1954 O2 (Baade) on December 16, 1954 UT. The model for the B-V color of the coma proposed by Betzler et. al (2017,AdSpR,60, 612) assumes that the brightness profile of coma I is proportional to 1/r^m, with the exponent m for the steady state equal to 1, which is far from the observational reality with complex structures in the coma. The condition that the coma is outside the steady state can be indirectly demonstrated by analyzing the relationship between the magnitude V and the radius r, given by V=c0+2.5(m-2)*log10(r), where c0 is the magnitude of the coma's optocenter. Fitting the last equation with five measured V magnitudes with aperture radii between 4.1 and 9.6 arcsec gives V=(11.3+/-0.3)+(3.4+/-0.4)*log10(r) (solid red line, Figure 3). The exponent m >> 1 derived from the fitted curve cannot be explained by the effect of the solar wind deforming the coma (Jewitt & Meech, 1987, AJ, v.317, p.992), but by the probable presence of an extremely bright source at the optocenter, as suggested by Betzler (2023, MNRAS,523,3678) for centaur 29P/Schwassmann-Wachmann.
We suggest conducting multiband observations of possible future outbursts on this comet to verify whether the trends highlighted in this study
are relevant to other events.