1:45 PM - 2:00 PM
[PPS03-18] Ground-based observations of (3200) Phaethon in 2021
Keywords:Asteroid, (3200)Phaethon, Photometric observation, Polarimetric observation, Occultation observation
The DESTINY+ science team has been carrying out various ground-based observations to learn more about Phaethon's physical properties in advance of its scheduled flyby in 2028. When Phaethon made its closest approach to Earth in December 2019, The DESTINY+ science team launched a Phaethon observing campaign to encourage researchers from around the world to make intensive, multi-faceted observations. As a result, the true properties of Phaethon have gradually become clearer. However, the reflectivity of Phaethon is still highly uncertain, with values varying from 0.041 (McAdams et al. 2018) to 0.22 (Harris et al. 1998) depending on the observations or the thermal models. This uncertainty affects the exposure time for the camera observations during the flyby.
In order to reduce the uncertainty in Phaethon's reflectivity, we conducted photometric, polarimetric and occultation observations of Phaethon in 2021.
The inaccuracy of Phaethon's reflectance is due to errors in the estimation of Phaethon's diameter. The diameter of an asteroid is usually estimated from its absolute magnitude, but in the case of Phaethon it is difficult to obtain an accurate absolute magnitude. This is because the geometrical relationship between the Sun and the orbits of Earth and Phaethon prevents observations at a solar phase angle of 0 degrees, which is necessary to obtain an absolute magnitude. However, in November 2021 we had the opportunity to observe Phaethon at a solar phase angle of less than 10 degrees. Since it is possible to observe at a relatively small solar phase angle, it would be possible to obtain an absolute magnitude with a small estimation error. Therefore, Photometric observations were carried out to determine the absolute magnitude of Phaethon with the Seimei telescope in late Oct. and Nov. 2021.
Polarization observations of Phaethon were carried out at Nishi-Harima Observatory and Higashi-Hiroshima Observatory. The minimum polarization of asteroids and the slope of the polarization phase angle curve when it changes from negative to positive polarization are used to estimate the albedo of asteroids. By observing Phaethon for about 2 months (from Oct. to Dec. 2021), we obtained a polarization phase angle curve to estimate Phaethon's albedo. It makes us to obtain Polarimetric albedo of Phaethon.
There is a way to measure the diameter of Phaethon directly. It is the occultation observation. This method does not rely on thermal models or empirical methods and allows us to determine the cross-section of Phaethon during the occultation with a relatively small error. The occultation team observed Phaethon's stellar occultation of 12th magnitude star UCAC4 646-021974 = GSC 2894-00131 on October 3, 2021 and measured the size of Phaethon's shadow on the ground. The cross-section of the Phaethon during this occultation was found to be approximated by an ellipse with a major diameter of 6.13 ± 0.05 km and a minor diameter of 4.40 ± 0.06 km (Yoshida et al. in preparation).
Photometric and polarization data are currently being analyzed. Based on these observations, we present the best currently possible estimate of the albedo of Phaethon.
References
McAdams et al. 2018, American Astronomical Society, DPS meeting #50, id.312.01
Harris 1998, Icarus 131, 291-301
In order to reduce the uncertainty in Phaethon's reflectivity, we conducted photometric, polarimetric and occultation observations of Phaethon in 2021.
The inaccuracy of Phaethon's reflectance is due to errors in the estimation of Phaethon's diameter. The diameter of an asteroid is usually estimated from its absolute magnitude, but in the case of Phaethon it is difficult to obtain an accurate absolute magnitude. This is because the geometrical relationship between the Sun and the orbits of Earth and Phaethon prevents observations at a solar phase angle of 0 degrees, which is necessary to obtain an absolute magnitude. However, in November 2021 we had the opportunity to observe Phaethon at a solar phase angle of less than 10 degrees. Since it is possible to observe at a relatively small solar phase angle, it would be possible to obtain an absolute magnitude with a small estimation error. Therefore, Photometric observations were carried out to determine the absolute magnitude of Phaethon with the Seimei telescope in late Oct. and Nov. 2021.
Polarization observations of Phaethon were carried out at Nishi-Harima Observatory and Higashi-Hiroshima Observatory. The minimum polarization of asteroids and the slope of the polarization phase angle curve when it changes from negative to positive polarization are used to estimate the albedo of asteroids. By observing Phaethon for about 2 months (from Oct. to Dec. 2021), we obtained a polarization phase angle curve to estimate Phaethon's albedo. It makes us to obtain Polarimetric albedo of Phaethon.
There is a way to measure the diameter of Phaethon directly. It is the occultation observation. This method does not rely on thermal models or empirical methods and allows us to determine the cross-section of Phaethon during the occultation with a relatively small error. The occultation team observed Phaethon's stellar occultation of 12th magnitude star UCAC4 646-021974 = GSC 2894-00131 on October 3, 2021 and measured the size of Phaethon's shadow on the ground. The cross-section of the Phaethon during this occultation was found to be approximated by an ellipse with a major diameter of 6.13 ± 0.05 km and a minor diameter of 4.40 ± 0.06 km (Yoshida et al. in preparation).
Photometric and polarization data are currently being analyzed. Based on these observations, we present the best currently possible estimate of the albedo of Phaethon.
References
McAdams et al. 2018, American Astronomical Society, DPS meeting #50, id.312.01
Harris 1998, Icarus 131, 291-301