The extremely rapid rotational braking of the magnetic helium-strong star HD 37776
Authors:
Mikulasek, Z., Krticka, J., Henry, G. W., Zverko, J., Ziznovsky,, J. Bohlender, D., Romanyuk, I. I. et al.
Image & caption:
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Image caption::
The phase of the maxima of the light curves of HD 37776 as a function of time. The symbols represent different sources of data.
Description:
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). We analysed all available observations of the star obtained since 1976. These consist of 1707 photometric measurements obtained in uvby(β), (U)BV, V, BTVT, and Hp, including 550 of our recent observations, 53 spectrophotometric measurements of the He I λ 4026 Å line, 66 equivalent width measurements of He I spectral lines from 23 CFHT spectrograms, and 69 He I equivalent width measurements of spectral lines present in 35 SAO Zeeman spectrograms. All these individual observations obtained by various techniques were processed simultaneously by means of specially developed robust codes. We confirm the previously suspected gradual increase in the 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. 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.
Reference:
Astronomy and Astrophysics, Volume 485, Issue 2, 2008, pp.585-597
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