Spectroscopic studies of three Cepheids with high positive pulsation period increments: SZ Cas, BY Cas, and RU Sct

Abstract

Three high-resolution spectra have been taken at different times with the 6-m SAO RAS telescope (LYNX and PFES spectrographs) for three Cepheids exhibiting high positive period increments: the small-amplitude (DCEPS) SZ Cas and BY Cas and the classical (DCEP) RU Sct. SZ Cas and RU Sct are members of the Galactic open clusters χ and h Per and Trump 35, respectively. Analysis of the spectra has shown that the interstellar Na I D1 and D2 lines in all objects are considerably stronger than the atmospheric ones and are redshifted in SZ Cas and BY Cas and blushifted in RU Sct. The core of the Hα absorption line in BY Cas has an asymmetric knifelike shape, while RU Sct exhibits an intense emission in the blue wing of this line. Such phenomena are observed in long-period Cepheids and bright hypergiants with an extended envelope. In this case, the strong Mg Ib 5183.62 A and Ba II 5853.67, 6141.713, and 6496.90 A lines with low χlow in SZ Cas and RU Sct also show characteristic knifelike profiles with an asymmetry in the red region, while the Ba II 4934.095 A line shows similar profiles in the blue one. The absorption lines of neutral atoms and singly ionized metals with different lowerlevel excitation potentials exhibit different degrees of asymmetry: from a pronounced one with secondary components in BY Cas (similar to those in the small-amplitude Cepheid BG Cru pulsating in the first overtone and having an envelope) to its insignificance or virtual absence in SZ Cas and RU Sct. Analysis of the secular changes in mean Teff determined from photometric color indices and spectra over the last 55 years for these stars has revealed periodic fluctuations of 200 K for SZ Cas and BY Cas and 500 K for RU Sct. For SZ Cas and RU Sct, Teff determined in some years from some color indices show much lower values, which together with the temperature fluctuations can be associated with mass loss and dust formation. Based on these facts, we hypothesize the existence of circumstellar envelopes around all three Cepheids. We have determined the atmospheric parameters and chemical composition of the program Cepheids. An appreciable carbon underabundance, a nitrogen overabundance (the result was obtained only for BY Cas), a nearly solar oxygen abundance, a sodium overabundance, and solar magnesium and aluminum abundances have been revealed in all stars, suggesting that these yellow supergiants has already passed the first dredge-up. The abundances of the Fe-peak elements, a-elements, and r- and s-process elements are nearly solar. [Fe/H] = −0.05 dex for SZ Cas and [Fe/H]= +0.05 dex for RU Sct can be used to estimate the metallicities of the open clusters χ and h Per and Trump 35, respectively.