Super-slowly rotating Ap (ssrAp) stars: New spectroscopic observations

¹ European Southern Observatory, Alonso de Cordova 3107, Vitacura, Santiago, Chile
² South African Astronomical Observatory, PO Box 9 Observatory, 7935 Cape Town, South Africa
³ School of Physics, Engineering and Technology, University of York, Heslington, York YO10 5DD, UK
⁴ INAF–Osservatorio Astrofisico di Catania, via S. Sofia 78, 95123 Catania, Italy
⁵ Centre for Space Research, North-West University, Mahikeng 2745, South Africa
⁶ Jeremiah Horrocks Institute, University of Central Lancashire, Preston PR1 2HE, UK
⁷ Dipartimento di Fisica e Astronomia, Sezione Astrofisica, Università di Catania, Via S. Sofia 78, I-95123 Catania, Italy

^⋆ Corresponding author: gmathys@eso.org

Received: 25 June 2025
Accepted: 4 September 2025

Abstract

Context. The rotation periods of Ap stars range over five to six orders of magnitude. The origin of their differentiation remains unknown.

Aims. We carry out a systematic study of the longest period Ap stars to gain insight into their properties.

Methods. We analysed newly obtained spectra of a sample of super-slowly rotating Ap (ssrAp) star candidates identified by a TESS photometric survey to confirm that their projected equatorial velocity, v sin i, is consistent with (very) long rotation periods; to obtain a first determination of their magnetic fields; and to test their binarity.

Results. The value of v sin i in 16 of the 18 studied stars is low enough for them to have moderately to extremely long rotation periods. All stars but one are definitely magnetic; for five of them, the magnetic field was detected for the first time. Another set of five new stars with resolved magnetically split lines were discovered. Five of the stars that were not previously known to be spectroscopic binaries were found to show radial velocity variations, and in one of them, lines from both components were observed.