HD 60431, the CP2 star with the shortest rotational period

Physical parameters and period analysis

¹ Faculty of Science, Masaryk University, Department of Theoretical Physics and Astrophysics, Kotlářská 2, 611 37, Brno, Czech Republic
² National Astronomical Research Institute of Thailand, 260 Moo 4, Donkaew, Maerim, Chiangmai 50180, Thailand
³ Bundesdeutsche Arbeitsgemeinschaft für Veränderliche Sterne e.V. (BAV), Munsterdamm 90, 12169 Berlin, Germany
e-mail: ernham@rz-online.de
⁴ American Association of Variable Star Observers (AAVSO), 49 Bay State Rd, Cambridge, MA 02138, USA
⁵ Institut de Physique, Laboratoire d’astrophysique, École Polytechnique Fédérale de Lausanne (EPFL), Observatoire de Sauverny, 1290 Versoix, Switzerland

Received: 23 March 2022
Accepted: 22 September 2022

Abstract

Context. CP2 stars show periodic photometric, spectroscopic, and magnetic variations with the rotational period. They are generally slow rotators, with rotational periods exceeding half a day, except for the late B-type star HD 60431, which has an unusually short rotational period of 0.4755 days. As slow rotation is deemed a necessary criterion for the establishment of chemical peculiarities, this characteristic renders HD 60431 a special object that might offer valuable insight into, and constraints on, the formation and evolution of CP2 stars.

Aims. The aims of our study are to analyse the light variability, derive atmospheric abundances, and determine the detailed physical parameters of HD 60431 to confirm its status as the CP2 star with the shortest known rotational period, with special emphasis on the rotational period evolution.

Methods. Photometric indices and high-resolution spectroscopy were employed to derive the physical parameters, evolutionary status, and atmospheric abundances of our target star. A light variability study was carried out using combined sets of photometric data from ground- and space-based facilities. A circularly polarised spectrum was employed to check the presence of a longitudinal magnetic field in the star.

Results. With an age of only 10 Myr, an effective temperature of T_eff = 13 000 ± 300 K, surface gravity log g = 4.10 ± 0.10, radius R = 1.97 ± 0.09 R_⊙, and mass M = 3.1 ± 0.1 M_⊙, HD 60431 is situated close to the zero age main sequence; it is a member of the open cluster NGC 2547 in the Vela OB2 complex. We confirm its status as a classical late B-type CP2 star showing strong overabundances of Mg (1.8 dex), Si (1.9 dex), Ca (1.6 dex), Ti (2.2 dex), and Fe (1.8 dex). No conclusive evidence for the presence of a strong magnetic field was found in the available spectroscopic data. The light curve of HD 60431 has remained constant over the last four decades. The available photometric time series data confirm the short rotational period and indicate a slight secular increase in the rotational period of Ṗ = 2.36(19) × 10⁻¹⁰ = 7.5(6) ms yr⁻¹. The following quadratic ephemeris has been derived: HJD_min(E) = 2 459 212.969 35 + 0. ^d475 516 64 E + 5. ^d62 × 10⁻¹¹E².

Conclusions. HD 60431 is indeed the CP2 star with the shortest known rotational period. Theory needs to explain the establishment and maintenance of chemical peculiarities in such a young and fast-rotating object. Our results furthermore raise the question of whether period variability on timescales significantly shorter than stellar evolution is inherent to all magnetic chemically peculiar stars.