A new sample of super-slowly rotating Ap (ssrAp) stars from the Zwicky Transient Facility survey

¹ Bundesdeutsche Arbeitsgemeinschaft für Veränderliche Sterne e.V. (BAV), Munsterdamm 90, 12169 Berlin, Germany
² American Association of Variable Star Observers (AAVSO), 49 Bay State Rd, Cambridge, MA 02138, USA
³ Faculty of Science, Masaryk University, Department of Theoretical Physics and Astrophysics, Kotlářská 2, 611 37 Brno, Czech Republic

^★ Corresponding author; ernham@rz-online.de

Received: 31 August 2024
Accepted: 31 October 2024

Abstract

Context. The magnetic chemically peculiar Ap stars exhibit an extreme spread of rotational velocities, the cause of which is not clearly understood. Ap stars with rotation periods of 50 days or longer are know as super-slowly rotating Ap (ssrAp) stars. Photometrically variable Ap stars are commonly termed α² Canum Venaticorum (ACV) variables.

Aims. Our study aims to enlarge the sample of known ssrAp stars using data from the Zwicky Transient Facility (ZTF) survey to enable more robust and significant statistical studies of these objects.

Methods. Using selection criteria based on the known characteristics of ACV variables, candidate stars were gleaned from the ZTF catalogues of periodic and suspected variable stars and from ZTF raw data. ssrAp stars were identified from this list via their characteristic photometric properties, Δa photometry, and spectral classification.

Results. The final sample consists of 70 new ssrAp stars, which mostly exhibit rotation periods of between 50 and 200 days. The object with the longest period has a rotation period of 2551.7 days. We present astrophysical parameters and a Hertzsprung-Russell diagram for the complete sample of known ssrAp stars. With very few exceptions, the ssrAp stars are grouped in the middle of the main sequence with ages in excess of 150 Myr. ZTF J021309.72+582827.7 was identified as a possible binary star harbouring an Ap star and a cool component, possibly shrouded in dust.

Conclusions. With our study, we enlarge the sample of known ssrAp stars by about 150%, paving the way for more in-depth statistical studies.