Rotational deceleration and acceleration of m agnetic CP stars CU Vir and V901 Ori

1. Rotational deceleration and accelerationof magnetic CP stars CU Vir and V901Ori Zdeněk Mikulášek, Jiří Krtička, Jan Janík, Miloslav Zejda, Jiří Liška, DTPA MU Brno, Czech Republic Juraj Zverko, Jozef Žižňovský, AI SAS Tatranska Lomnica, Slovak Republic Iosif Ivanovich Romanyuk, Dmitry Olegovich Kudryvtsev, SAO AV RAS, Russia… KOLOS 2011, Sninské Rybníky, Slovakia

2. Rotation of upper main sequence stars • The bestinstrument for testing of the rotational stability of upper MSstars:spotty magnetic CP stars with strong global magnetic field and stable surface structures. • Combining both present and archival photometric, spectroscopic and magnetic observations collected during many decades one can reconstruct the recent rotation history of outer parts of the star with a high accuracy. • Careful period analyses were done for several dozens of mCP stars listed in On-line catalogue of photometric observations of mCP stars (Mikulášek et al., 2007, AN 328, 10). They confirmed the expectations – the light curves and rotational periods of the upper MS stars are quite steady. • There are some mCP stars showing light curves variations (56 Ari, V1039 Ori, 20 Eri, MW Vul, and 102Aqr, Žižňovský et al, Adelman + somebody.), obviously due to the precession caused by their magnetic distortion. • Nevertheless, there are also mCP stars displaying true period variations!

3. He-strong mCP star σ Orionis E • σ Ori E = HD 37479 – a hybrid of a classical He-strong CP star and a Be one with strong stellar wind. • The light curves in optical domain are unusual – deep minima namely in u (U) cannot by explained only with spots on the surface – we need ‘eclipses’ with an accretion disc. • Townsend et al. (2010) have quite recently discovered a smooth rotational braking in the rate Ṗ = 7.7 s/century from their own U ( 2004-9) and Hessers u observations (1977). • They explained it by magnetic braking through strong stellar wind.

4. Si mCP star CU Virginis • Other type of period changes shows the famous fast-rotating Si mCP star CU Vir = HD 124224 = HR 5313, the first stellar radiopulsar. • Pyper et al. 1998, 2004 + our observations 2009-10: O-C diagram documents two period jumps in 1984 and 1998 – the period suddenly arose by several seconds. • The amplitude of the light variations is relatively large – the behaviour of the star is reliably documented. • The light curves of HD 37776 and CU Vir arenon-variable – no changes in the shape and amplitude have been found. This + the extent of the observed period changes excluded precession as the cause (detailed discussion in Mikulášek et al. 2008).

5. He-strong mCP star V901 Orionis • V901 Ori= HD 37776– a very young hot star (B2IV) residing in the emission nebula IC 432, with a complex (quadruple) global magnetic field (Thompson & Landstreet,1985). • It can be ranked among the He strong mCPs, however the light variations are due to spots of overabundant Si (Krtička et al., 2007)

6. Interpretation of “O-C” diagram The simplest expression: JDmax= M0 + P0E + 1/2 P0 dP/dtE2→ constant lengthening of the observed periodof light and spectroscopic variations. • P0 = 1.5387121(10) d; dP/dt = 1.71(7) · 10-8(25 σ!) Another term? JDmax= M0 + P0E + 1/2 P0 dP/dtE2 + 1/6 P02 d2P/dt2 E3 • d2P/dt2 = 3.2(2.2) · 10-12 d-1 – questionable, based mainly on one, uncertain point (Hipparcos). Two intersecting straight lines? • The same uncertainty + problems with physics. Is the lengthening of the period due torotational braking?

7. He-strong mCP star V901 Orionis • V901 Ori= HD 37776– a very young hot star (B2IV) residing in the emission nebula IC 432, with a complex (quadruple) global magnetic field (Thompson & Landstreet,1985). • It can be ranked among the He strong mCPs, however the light variations are due to spots of overabundant Si (Krtička et al., 2007) • Using photometry and spectroscopy, Mikulášek et al. (2008, 2010) found gradual changes of the observed period that can be very well fit by a parabola (O-C diagram – cubic parabola). • After ruling out the light-time effect in a binary star, precession of rotational axis, evolutionary changes as possible causes of the period change, we interpreted it as braking of star’s rotation (at least of surface layers) due to the AM loss through events in stellar magnetosphere.

8. Spin-down time paradox. Nature of mCP stars period variations • A quantitative expression of known mCPs with changing periods – spin-down time(SDT): τ = P/Ṗ • All τ > 0– braking of rotation → the process is irreversible. Assuming that SDT is constant, we estimate the maximum duration of the process. • Except for the very young σ Ori SDT are much shorter than the ages of the stars. Does it mean that the process of the rotational braking sets on later than the star arrives at MS? Why? Why we do not see a larger % of CP stars with extremely long periods? Is it possible to brake the whole star so drastically? • Are the abrupt changes of period of CU Vir astrophysically permitted (the most dramatic case)? Stępień (1998): “We have to abandon the assumption of the necessity of a rigid rotation and to admit that the outer layers controlled by magnetic field and denser inner parts can rotate differently.” • Mikulášek et al. 2010, Magnetic stars: This allows us to speculate about a cyclic nature of mCP period variations. • Does exist any accelerating mCP stars? Why do not we see them?

9. V901 Ori + CU Vir once more (and better) • V 901 Ori - We analyzed period variations using all available data (1976-2011, 2611 individual measurements) by own sophisticated method. • We found that since 2004 the rotation is accelerating – it might be an indication of the altering nature of rotation variations – the duration of a cycle > 90 years. • CU Vir – Period analysis on the basis of all available data of all kind (8965 individual measurements, 1949-2011). • We found well defined harmonic-like oscillation in the rotational period – an amplitude 3.7 s, in the time-scale of 70 years. • How to interpret this astonishing and unexpected phenomenon? • Our proposal: The consequence of the dynamical interaction between an outer ‘solid’ magnetically-confined envelope, braked by the stellar wind, with an inner, faster rotating stellar body.