Is U Aquarii a Thorne‐Żytkow Object?

Abstract

The R Coronae Borealis (RCB) star U Aqr was known to show an unusual abundance pattern with respect to the post-iron peak elements. The abundances of elements in the light s-process peak—Rb, Sr, Y, and Zr—are greatly enhanced, whereas elements representative of the heavy s-process peak, such as Ba, do not exhibit large abundance enhancements. Recent calculations of the abundances of elements formed by rapid proton capture in Thorne-Żytkow objects indicate that qualitatively the abundance pattern of post-iron peak elements in Thorne-Żytkow objects appeared to match that observed in U Aqr. New high-resolution spectra of U Aqr and two other stars are used to explore the possibility that U Aqr is a Thorne-Żytkow object. Our spectra show that on the basis of the light s-process elements, in particular the absence of an enhancement of Mo, Cu, and Zn, it does not appear that U Aqr is a good candidate for a Thorne-Żytkow object. However, the light s-process abundance pattern resembles that of Sakurai's star that has recently begun to show RCB characteristics. We suggest that these stars represent a possible source for the weak component of the solar system s-process distribution of heavy elements.