Photometric and spectroscopic study of the supergiant with an infrared excess V1027 Cygni

Abstract

We present the results of our U BV and JHKLM -photometry for the semiregular pulsating variable V1027 Cyg, a supergiant with an infrared excess, over the period from 1997 to 2015 (U BV ) and in . Together with the new data, we analyze the photometric observations of V1027 Cyg that we have obtained and published previously. Our search for a periodicity in the U BV brightness variations has led to several periods from P = 212 10. An ambiguous correlation of the B − V and U − B colors with the brightness has been revealed. For example, a noticeable bluing of the star was observed during the deep 1992, 2008, and 2011 minima, while the variations with smaller amplitudes show an increase in B − V at the photometric minima. The spectral energy distribution for V1027 Cyg from our photometry in the range 0.36 (U )-5.0 (M ) µm corresponds to spectral types from G8I to K3I at different phases of the pulsation cycle. Low-resolution spectra of V1027 Cyg in the range λ4400-9200 A were taken during 16 nights over the period . At the 1995 and 2011 photometric minima the star's spectrum exhibited molecular TiO bands whose intensity corresponded to spectral types M0-M1, while the photometric data point to a considerably earlier spectral type. We hypothesize that the TiO bands are formed in the upper layers of the extended stellar atmosphere. We have measured the equivalent widths of the strongest absorption lines, in particular, the infrared Ca II triplet in the spectrum of V1027 Cyg. The calcium triplet (Ca T) with W λ (Ca T) = 20.3 ± 1.8Å as a luminosity indicator for supergiants places V1027 Cyg in the region of the brightest G-K supergiants. V1027 Cyg has been identified with the infrared source IRAS 20004+2955 and is currently believed to be a candidate for post-AGB stars. The evolutionary status of the star and its difference from other post-AGB objects are discussed.