2022ApJ...930....9M

We estimate the parameters of the donor of the accreting black hole binary MAXI J1820+070. The measured values of the binary period, rotational and radial velocities, and constraints on the orbital inclination imply the donor is a subgiant with the mass of {M}_{2}\approx {0.49}_{-0.10}^{+0.10}{M}_{\odot } and the radius of {R}_{2}\approx {1.19}_{-0.08}^{+0.08}{R}_{\odot }. We reanalyze the previously obtained optical spectrum from the Gran Telescopio Canarias and found it yields a strict lower limit on the effective temperature of T > 4200 K. We compile optical and infrared fluxes observed during the quiescence of this system. From the minima r- and i-band fluxes found in Pan-STARSS1 Data Release 2 prediscovery imaging and for a distance of D ≃ 3 kpc, reddening of E(B - V) = 0.23, and R₂ ≃ 1.11R_☉, we find T <= 4230 K, very close to the above lower limit. For a larger distance, the temperature can be higher, up to about 4500 K (corresponding to a K5 spectral type, preferred by previous studies) at D = 3.5 kpc, allowed by the Gaia parallax. We perform evolutionary calculations for the binary system and compare them to the observational constraints. Our model fitting the above temperature and radius constraints at D ≃ 3 kpc has a mass of 0.4M_☉, T ≃ 4200 K, and solar metallicity. Two alternative models require D >= 3.3-3.4 kpc at 0.4M_☉, T ≃ 4500 K and half-solar metallicity, and 0.5M_☉, T ≃ 4300 K, and solar metallicity. These models yield mass-transfer rates of ∼10^–10M_☉ yr^–1, compatible with those based on the estimated accreted mass of ≃2 x 10²⁵ g and the time between the 2018 discovery and the 1934 historical outburst.