Spectral changes in AC Herculis during its rise to a maximum in 2014 July

AC Herculis

AC Her was first reported as a variable by Dorothy Applegate in a Harvard College Observatory Circular in 1921.  B. P. Gerasimovic, in a similar publication in 1929, listed AC Her as one of ‘twelve undoubted RV Tauri variables’.

RV Tauri stars are luminous pulsating variables, located between the Cepheids and long period variables on the Hertzsprung-Russell diagram, which show two unequal minima per cycle. According to the General Catalogue of Variable Stars (GCVS), AC Her is a RVA variable with a period between successive primary minima of 75.5 days and V magnitude between 6.85 and 9.0. RVA variables are RV Tauri stars that do not vary in mean magnitude. The quoted period contains two pulsation cycles, sometimes of similar amplitude and sometimes of significantly different amplitudes. Alternate minima which are on average the deepest are referred to as primary minima and the following maxima are primary maxima.

Historical spectroscopic observations of AC Her
W. F. Waterfield, in a Harvard College Observatory Bulletin in 1927, reported that the spectral type of AC Her had been found to be variable by Margaret Walton and subsequently observed by Annie Cannon to be F8 at maximum, passing through G0 and G5 as it faded to K5 at minimum. A series of studies of AC Her over the next 65 years reported the spectral type as varying from anywhere between F1 and F4 at maximum to between K0 and K5 at minimum. In some cases classification of the spectrum depended on which part of the spectrum was used and as a result the spectrum was described as Fp (p= peculiar).

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