The 2018 eruption and long-term evolution of the new high-mass Herbig Ae/Be object Gaia-18azl = VES 263

We have been monitoring, at high cadence, the photometric and spectroscopic evolution of VES 263 following the discovery in 2018 of a brightening labelled as event Gaia-18azl. VES 263 is so far a neglected emission-line object discovered in the 1960s on objective prism plates, tentatively classified as a semiregular AGB cool giant by automated analysis of ASASSN light curves. We have discovered that VES 263 is a bona fide massive pre-main-sequence object (∼12 M_⊙), of the Herbig AeBe type. It is located at 1.68 ± 0.07 kpc distance, within the Cyg OB2 star-forming region, and it is highly reddened [E(B - V) = 1.80 ± 0.05] by interstellar extinction. In quiescence, the spectral energy distribution is dominated by the ∼20 000 K photospheric emission from the central B1II star, and at λ ≥ 6 μm by emission from circumstellar warm dust (T ≤ 400 K). The 2018-19 eruption was caused by a marked brightening of the accretion disc around the B1II star as traced by the evolution with time of the integrated flux and the double-peaked profile of emission lines. At the peak of the eruption, the disc has a bulk temperature of ∼7500 K and a luminosity L ≥ 860 L_⊙, corresponding to a mass accretion rate \dot{M} ≥ 1.1 × 10^-5 M_⊙ yr^-1. Spectroscopic signatures of possible bipolar jets (at -700 and +700 km s^-1) of variable intensity are found. We have reconstructed from Harvard, Moscow, and Sonneberg photographic plates the photometric history of VES 263 from 1896 to 1995.