Photometric results of two PHAs: (52768) 1998 OR2 and (99942) Apophis

We report an analysis of photometric observations for two potentially hazardous asteroids – (52768) 1998 OR2 and (99942) Apophis. Data for 1998 OR2 was obtained in March – April 2020 and for Apophis in February – March 2021, when both objects experienced their close approaches to Earth. The aim of the observations was mainly to refine the physical characteristics (rotation periods, absolute magnitudes H, slope parameter G, to find out the approximate taxonomy, or even to estimate the heliocentric position of the north pole).

Observations and data reduction

The observations of both PHAs were carried out with the 0.61-m f/4.3 Newton reflector at the Skalnaté Pleso Observatory (Slovakia, IAU code 056) and CCD camera SBIG ST-10XME. We obtained photometric data using broad-band Johnson-Cousins B, V, and R filters, 2×2 binning, and a resolution of 1.07 arcsec/px. The calibration with dark and flatfield frames was applied in the standard way. Next, we used a differential aperture photometry technique to obtain lightcurves.

Results

Asteroid 1998 OR2 was observed for 9 nights. From each night we obtained 3 – 4 hr series in BVR filters, from which we determined the exact synodic rotation periods. Figure 1 shows the composite light curves with periods of 4.11 hours. We divided them into three groups to emphasize the time- and phase-angle-dependent changes in the lightcurve shape.

Data from the V filter we used to determine the absolute magnitude H, which has a value of 15.72 ± 0.02 mag (Fig. 2). We kept the slope parameter G at 0.15 because of the narrow range of phase angles. Next, we calculated the approximate effective diameters of the asteroid using an albedo value of 0.2. These varied only minimally between 2.1 and 2.3 km on each observing night, with a mean value of 2.14 ± 0.06 km.

Color photometry also allowed us to determine an approximate taxonomic class in the Tholen classification. Figure 3 shows the positions of the color indices on each night along with the mean value of the B-V and V-R indices. With high probability, the asteroid surface is dominated by a metallic material, which characterizes the X-class. Our finding is confirmed by Hromakina et al. (2021) [2].

Fig. 1 Three composite lightcurves of asteroid 1998 OR2 obtained from March 24 to April 22 through the R filter. We divided them into three groups to show the changing of some features in brightness in the range of rotational phase from 0 to 0.2. All lightcurves have the same scale of relative magnitudes (y-axis) for better comparison. The typical rms of all composite lightcurves is less than 0.010 mag.

 

Fig. 2 Plot of magnitude-phase relation of 1998 OR2. In this case, the slope parameter G is fixed at 0.15.

 

Fig. 3 Color plots of V-R vs. B-V for 1998 OR2 within various Tholen taxonomic classes [1].

 

In the spring of 2021, perhaps the most famous asteroid, Apophis, approached the Earth. We used this time to obtain photometric material to refine the physical characteristics known so far. The aim was not to make long-term observations to determine the rotational period, as this is known, a fact complicated by the excited rotational state (tumbling). We mainly focused on finding the B-V and V-R color indices, the absolute magnitude H, and the diameter. Already after the first observations, it was clear that either the shape of the object is highly elongated or the rotation is not performed along one axis. The values of the effective diameter varied significantly from 0.31 to 0.40 km each night. We observed a similar effect when determining the absolute magnitude, which we calculated to be 19.08 ± 0.06 mag (with fixed G = 0.24).

Similarly to the previous asteroid, we used color photometry. It shows almost unequivocally that Apophis is classified as an S-type asteroid (Fig. 4). A similar finding was obtained by Hromakina et al. (2021) [2] or Lin et al. (2018) [3].

 

Fig. 4 Color plots of V-R vs. B-V for Apophis within various Tholen taxonomic classes [1].

References

[1] Dandy, C. L., Fitzsimmons, A., and Collander-Brown, S. J., “Optical colors of 56 near-Earth objects: trends with size and orbit”, Icarus, vol. 163, no. 2, pp. 363–373, 2003. doi:10.1016/S0019-1035(03)00087-3.

[2] Hromakina, T., “Photometric survey of 55 near-earth asteroids”, Astronomy and Astrophysics, vol. 656, 2021. doi:10.1051/0004-6361/202141600.

[3] Lin, C.-H., Ip, W.-H., Lin, Z.-Y., Cheng, Y.-C., Lin, H.-W., and Chang, C.-K., “Photometric survey and taxonomic identifications of 92 near-Earth asteroids”, Planetary and Space Science, vol. 152, pp. 116–135, 2018. doi:10.1016/j.pss.2017.12.019.