2009A&A...498..115M

We present the results of high-resolution optical spectroscopy, low-resolution near-IR spectroscopy and near-infrared speckle interferometry of the massive young stellar object candidate V645 Cyg, acquired to refine its fundamental parameters and the properties of its circumstellar envelope. Speckle interferometry in the H- and K-bands and an optical spectrum in the range 5200-6680Å with a spectral resolving power of R=60000 were obtained at the 6m telescope of the Russian Academy of Sciences. Another optical spectrum in the range 4300-10500Å with R=79000 was obtained at the 3.6m CFHT. Low-resolution spectra in the ranges 0.46-1.4µm and 1.4-2.5µm with R∼800 and ∼700, respectively, were obtained at the 3m Shane telescope of the Lick Observatory. Using a novel kinematical method based on the non-linear modeling of the neutral hydrogen density profile in the direction toward the object, we propose a distance of D=4.2±0.2kpc. We also suggest a revised estimate of the star's effective temperature, T_eff∼25000K. We resolved the object in both H- and K-bands. Using a two-component ring fit, we derived a compact component size of 14mas and 12mas in the H- and K-band, respectively, which correspond to 29 and 26AU at the revised distance. Analysis of our own and previously published data indicates a ∼2mag decrease in the near-infrared brightness of V645 Cyg at the beginning of the 1980's. At the same time, the cometary nebular condensation N1 appears to fade in this wavelength range with respect to the N0 object, representing the star with a nearly pole-on optically-thick disk and an optically-thin envelope. We conclude that V645 Cyg is a young, massive, main-sequence star, which recently emerged from its cocoon and has already experienced its protostellar accretion stage. The presence of accretion is not necessary to account for the high observed luminosity of (2-6)x10⁴M_☉/yr. The receding part of a strong, mostly uniform outflow with a terminal velocity of ∼800km/s is only blocked from view far from the star, where forbidden lines form. The near-infrared size of the source is consistent with the dust sublimation distance close to this hot and luminous star and is the largest among all young stellar objects observed interferometrically to-date.