2019MNRAS.488.5536M

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 x 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.