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

TL;DR

This study significantly enlarges the sample of super-slowly rotating Ap stars using ZTF data, enabling better statistical understanding of their properties and evolution.

Contribution

The paper introduces 70 new ssrAp stars identified from ZTF data, increasing the known sample by about 150%, and provides detailed astrophysical parameters for these stars.

Findings

Most ssrAp stars have rotational periods between 50 and 200 days.

The longest observed period is 2551.7 days.

ssrAp stars are mainly found in the middle of the main sequence, older than 150 Myr.

Abstract

The magnetic chemically peculiar Ap stars exhibit an extreme spread of rotational velocities, the reason of which is not well understood. Ap stars with rotational periods of 50 days or longer are know as super-slowly rotating Ap (ssrAp) stars. Photometrically variable Ap stars are commonly termed alpha2 Canum Venaticorum (ACV) variables. Our study aims at enlarging 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. 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, Delta a photometry, and spectral classification. The final sample consists of 70 new ssrAp stars, which mostly exhibit rotational periods between 50 and 200 days. The object with the longest period has a rotational 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. With our study, we enlarge the sample of known ssrAp stars by about 150%, paving the way for more in-depth statistical studies.