Analysis of Actinium Abundances in the Atmosphere of Cepheid HIP13962

Abstract

Actinium is a radioactive element that has an isotope ²²⁷Ac with the longest half-life of 21.772(3) years. It is the third element in the actinoid group, in addition to thorium and uranium, the abundance of which can be studied in the atmospheres of stars. Its presence in the atmosphere of a particular star primarily indicates some mechanism of its production. The first studies of the actinium absorption lines in the spectra of certain stars showed that the appearance of actinium in their spectrum is associated with the presence of deformation of strong lines, such as hydrogen lines and sodium doublet lines. In some cases, profiles of strong lines contain emission components. In the search for actinium absorption lines in the stellar spectra, attention was focused on such class of stars as Cepheids, which are characterized by deformation of strong lines due to pulsations. The absorption lines of actinium were studied in the spectral interval of 378.0–887.7 nm for the runaway star and Cepheid HIP13962 using the spectra obtained in 2014 with a 1.8-m telescope at Bohyunsan Optical Astronomical Observatory (BOAO, South Korea) with a spectral resolution greater than R = 80 000. The archived HIP13962 spectrum for 1995 in the wavelength range of 400.0–680.0 nm with a spectral resolution R = 42 000, which was obtained with the 1.93-m telescope of the Haute Provence Observatory (France), was also used. The modeling showed that the actinium abundance in the atmosphere of HIP13962 log N(Ac) = –1.2 on the hydrogen scale log N(H) = 12.0, with the model atmosphere T_eff = 5930 K, log g = 1.0, V_micro = 6 km s^–1. This value turns out to be 0.2 more at an increase in the effective temperature T_eff = 6250 K: logN(Ac) = –1.0 on the hydrogen scale logN(H) = 12.0.