# The accelerating rotation of the magnetic He-weak star HD 142990

## TL;DR

This study re-evaluates the rotation period of the magnetic He-weak star HD 142990, revealing a spin-up over 30 years that challenges existing models and suggests complex rotational behavior.

## Contribution

The paper provides a 40-year combined dataset analysis showing that HD 142990's rotation period has decreased, indicating a possible irregular or cyclic spin-up not explained by current models.

## Key findings

- HD 142990's rotation period has decreased over 30 years.
- The star's spin-up rate is approximately -0.6 s/yr.
- Possible slowing of spin-up suggests irregular rotational evolution.

## Abstract

HD 142990 (V 913 Sco; B5 V) is a He-weak star with a strong surface magnetic field and a short rotation period ($P_{\rm rot} \sim 1$ d). While it is clearly a rapid rotator, recent determinations of $P_{\rm rot}$ are in formal disagreement. In this paper we collect magnetic and photometric data with a combined 40-year baseline in order to re-evaluate $P_{\rm rot}$ and examine its stability. Both period analysis of individual datasets and $O-C$ analysis of the photometric data demonstrate that $P_{\rm rot}$ has decreased over the past 30 years, violating expectations from magnetospheric braking models, but consistent with behaviour reported for 2 other hot, rapidly rotating magnetic stars, CU Vir and HD 37776. The available magnetic and photometric time series for HD 142990 can be coherently phased assuming a spin-up rate $\dot{P}$ of approximately $-0.6$ s/yr, although there is some indication that $\dot{P}$ may have slowed in recent years, possibly indicating an irregular or cyclic rotational evolution.