Speakers
Description
The photometric observations of an asteroid can be used to derive the reduced magnitudes at different phase angles, V(α). A photometric function can be used to fit the magnitudes at different phase angles and to predict the behavior into the exact backscattering geometry at V(0), giving the absolute magnitude H of the object. In 2012 IAU adopted the H,G1,G2 photometric function which is an improvement to the H,G function, improving especially the behavior close to backscattering by allowing more variability in the phase-magnitude curve with small phase angles [1,2]. Since the absolute magnitude H can be used, with the asteroids’ geometric albedo p, to derive its size, the improved H estimates will produce improved size estimates.
The improved flexibility in the photometric function comes not for free, and estimating three parameters instead of two will require more observations to reach the same accuracy in the estimated parameters. For this reason, also a two-parameter version of the system was developed. In this H,G12 function the typical relation between the parameters G1 and G2 has been modeled and tight together in one parameter, the G12. In [2], this simplification was taken even further and average G1,G2 parameters were derived for several taxonomic classes of asteroids, simplifying the system even more so that the estimate for H is directly computed from V(α) observations without further parameter estimation. Both the G12 function and the method of fixing G1,G2 if the target taxonomy is known can be compared to previous practice of setting G=0.15 in the H,G function.
In this presentation, we will present the computational methods to estimate the absolute magnitude H for NEA observations using the H,G1,G2 function and its simplified forms.
References:
[1] Muinonen K, Belskaya IN, Cellino A, Delbò M, Levasseur-Regourd A-C, Penttilä A, and Tedesco EF (2010). A three-parameter magnitude phase function for asteroids. Icarus 209(2), 542–555. DOI: 10.1016/j.icarus.2010.04.003
[2] Penttilä A, Shevchenko VG, Wilkman O, and Muinonen K (2024). H,G1,G2 photometric phase function extended to low-accuracy data. Planetary and Space Science 123, 117–125. DOI: 10.1016/j.pss.2015.08.010i
