Multicolor Photometry and Peculiarities of the Spectrum for the post-AGB Candidate AU Vulpeculae (IRAS 20160+2734)

Abstract

We present the results of a UBVRcIc and JHKLM photometric monitoring campaign conducted in 2016–2020, low-resolution spectroscopy, and an analysis of the ASAS-3 and ASAS-SN photometric data for the semiregular variable and post-AGB candidate AU Vul (IRAS 20160+2734). The star is shown to experience quasi-periodic brightness oscillations with a variable amplitude and periods of 67–75 and 145–150 days in different time intervals. Its spectrum at maximum light is classified as that of an early G supergiant. In this spectrum H$\alpha$, H$\beta$, and higher Balmer lines are weakened, possibly, because of emission components, while the absorptions of s-process elements (Ba II, Sr II, and Y II) are slightly strengthened. An H$\alpha$ emission has been detected in the spectrum on the ascending branch of the light curve. TiO absorption bands are observed in the spectra at different brightness levels with maximum intensity at minimum light. The spectral energy distribution in the range 0.44 ($B$)–2.2 ($K$) $\mu$m corresponds to spectral types from G2I at maximum to G8I at minimum light. An emission excess has been detected in the $L$ and $M$ bands. The spectral energy distribution in the range 0.44–90 $\mu$m constructed from our own observations and WISE, MSX, IRAS, and AKARI data is satisfactorily described by the sum of three components: a star and dust envelopes with $T_{\textrm{hot}}=1000$ K and $T_{\textrm{cold}}=150$ K. Using the distance based on the Gaia EDR3 parallax, $D\approx 2300$ pc, we have estimated the absolute magnitude, $M_{V}\approx-4\overset{m}{.}45$, and the luminosity, $L\approx 5450L_{\odot}$. A comparison with evolutionary models has shown that AU Vul may be at the very beginning of the post-AGB stage of evolution and have a mass ${\sim}0.55M_{\odot}$. We note that the star has a number of properties that distinguish it from typical post-AGB objects and make it akin to RV Tau stars.