Multicolor Photometry and Spectroscopy of the Yellow Supergiant with Dust Envelope HD 179821 = V1427 Aquilae

Abstract

We present the results of multicolor (UBVJHKLM) photometry (2009-2017) and low-resolution spectroscopy (2016-2017) of the semi-regular variable V1427 Aql=HD 179821, a yellow supergiant with gas-dust envelope. The star displays low-amplitude ({\Delta}V<0.2 mag) semi-periodic brightness variation superimposed on a long-term trend. The light curve shape and timescale change from cycle to cycle. There are temperature variations characteristic for pulsations, and brightness oscillations with no significant change of color are also observed. The UBV data for the 2009-2011 interval are well reproduced by a superposition of two periodic components with P=170 d and P=141 d (or P=217 d - the one year alias of P=141 d). The variation became less regular after 2011, the timescale increased and exceeded 250 days. In 2009-2016 the annual average brightness monotonically increased in V, J, K, whereas it decreased in U and B. The annual average U-B, B-V, and J-K colors grew, the star was getting redder. The cooling and expanding of the star photosphere along with the increasing of luminosity may explain the long-term trend in brightness and colors. Based on our photometric data we suppose that the photosphere temperature decreased by ~400 K in the 2008-2016 interval, the radius increased by ~24 %, and the luminosity grew by ~19 %. Low-resolution spectra in the {\lambda}4000-9000 {\AA} wavelength range obtained in 2016-2017 indicate significant changes in the spectrum of V1427 Aql as compared with the 1994-2008 interval, i.e. the BaII and near-infrared CaII triplet absorptions have turned stronger while the OI {\lambda}7771-4 triplet blend has weakened that points out the decrease of temperature in the region where the absorptions are formed. The evolutionary stage of the star is discussed. We also compare V1427 Aql with post-AGB stars and yellow hypergiants.