Asteroid (3200) Phaethon: results of polarimetric, photometric, and spectral observations

We present results of polarimetric, photometric, and spectral observations of the near-Earth asteroid (3200) Phaethon carried out at the 6-m BTA telescope of the Special Astrophysical Observatory and the 2.6-m and 1.25-m telescopes of the Crimean Astrophysical Observatory over a wide range of phase angles during its close approach to the Earth at the end of 2017 (alpha = 19 degrees-135 degrees) and in 2020 at alpha = 52.2 degrees. Using our and other available in literature data, we found that the maximum degree of linear polarization of Phaethon in the V band is P-max = (45 +/- 1) per cent at the phase angle alpha(max) = 124.0 degrees +/- 0.4 degrees, whereas the inversion angle alpha(inv) = 21.4 degrees +/- 1.2 degrees and polarimetric slope is h = (0.326 +/- 0.027) per cent per degree. Using the dependence 'polarimetric slope - albedo,' we have found the geometric albedo of asteroid Phaethon to be p(v) = 0.060 +/- 0.005. This value falls into the lower range of albedo values for asteroids determined by different methods. The mean colour indices U-B = 0.207 (m) +/- 0.053 (m) and B-V = 0.639 (m) +/- 0.054 (m) of the asteroid are derived at heliocentric and geocentric distances 1.077 au and 0.102 au, respectively, and phase angle alpha = 23.78 degrees. The absolute magnitude of Phaethon is V(1,1,0) = 14.505 (m) +/- 0.059 (m). The effective diameter of Phaethon is estimated from obtained absolute magnitude and geometrical albedo, it is equal to 6.8 +/- 0.3 km. The best fit to the observed polarimetric data was obtained with the Sh-matrix model of conjugated random Gaussian particles composed of Mg-rich silicate (90 per cent) and amorphous carbon (10 per cent).