The extremely rapid rotational braking of the magnetic helium-strong star HD 37776

Abstract

Context. Light and spectrum variations of the magnetic chemically peculiar (mCP) stars are explained by the oblique rigid rotator model with a rotation period usually assumed to be stable on a long time scale. A few exceptions, such as CU Vir or 56 Ari, have been reported as displaying an increase in their rotation period. A possible increase in the period of light and spectrum variations has also been suggested from observations of the helium-strong mCP star HD 37776 (V901 Ori). Aims. In this paper we attempt to confirm the possible period change of HD 37776 and discuss a possible origin of this change as a consequence of i) duplicity; ii) precession; iii) evolutionary changes; and iv) continuous/discrete/transient angular momentum loss. Results. We confirm the previously suspected gradual increase in the 1. d 5387 period of HD 37776 and find that it has lengthened by a remarkable 17.7 ± 0.7 s over the past 31 years. We also note that a decrease in the rate of the period change is not excluded by the data. The shapes of light curves in all colours were found to be invariable. Conclusions. After ruling out light-time effects in a binary star, precession of the rotational axis, and evolutionary changes as possible causes for the period change, we interpret this ongoing period increase as a braking of the star's rotation, at least in its surface layers, due to the momentum loss through events or processes in the extended stellar magnetosphere.

Phenomenological modelling of eclipsing system light curves

“…Bevington & Robinson 2003;. It is advisable to orthogonalize the models at least in the ephemeris parameters, in accordance with what we did in Mikulášek (2007) and Mikulášek et al (2008).…”

“…The period itself and its development over time cannot be observed directly, but both can be derived through analysing time series of light changes or extremum timings. For that purpose, it was useful to introduce a monotonically rising phase function ϑ(t), as a sum of the epoch E(t) and the phase in the common usage ϕ and its inversion function t(ϑ) (Mikulášek et al 2008).…”

“…Functions ϑ(t) are tied with the instantaneous observed period P(t), P(ϑ) by the following differential equations (see Kalimeris et al 1994;Mikulášek et al 2008Mikulášek et al , 2012b:…”

Magnetic Doppler imaging of early-type stars

“…It is also one of the few spotted stars for which the rotational spin-down has been directly measured (Mikulášek et al 2008) and the light variations were explained with a physically realistic star-spot model (Krtička et al 2007). Uniquely for an early-type chemically peculiar star, HD 37776 shows a complex double-wave longitudinal magnetic field variation (Thompson & Landstreet 1985), suggesting that its magnetic field topology significantly departs from an oblique dipolar field common for other Bp stars.…”

Modeling the winds and magnetospheres of active OB stars

“…For instance, it is not clear how to extend the parametrization above to handle oblique-dipole fields or higher-order fields. With the recent measurement of braking in other active OB stars -in particular, the quadrupole-field HD 37776 (Mikulášek et al 2008) -the ball is clearly in the theoreticians' court.…”