Rotational deceleration and Rotational deceleration and accelerationacceleration of mof magnetic CP starsagnetic CP stars CU Vir and V901 CU Vir and V901 OriOri

Zdeněk Mikulášek,Zdeněk Mikulášek,Jiří Krtička, Jan Janík, Miloslav Zejda, Jiří Liška, DTPA MU Brno, Czech Republic Jiří Krtička, Jan Janík, Miloslav Zejda, Jiří Liška, DTPA MU Brno, Czech Republic

Juraj Zverko, Jozef Žižňovský, AI SAS Tatranska Lomnica, Slovak Republic Juraj Zverko, Jozef Žižňovský, AI SAS Tatranska Lomnica, Slovak Republic

Iosif Ivanovich Romanyuk, Dmitry Olegovich Kudryvtsev, SAO AV RAS, RussiaIosif Ivanovich Romanyuk, Dmitry Olegovich Kudryvtsev, SAO AV RAS, Russia ……

KOLOS 2011, Sninské Rybníky, SlovakiaKOLOS 2011, Sninské Rybníky, Slovakia

Rotation of upper main sequence starsRotation of upper main sequence stars The bestThe best instrument for instrument for testing of the rotational stability of testing of the rotational stability of upper upper MSMS

starsstars:: spotty magnetic CP stars with with strong strong global magnetic field global magnetic field

and stable surface structures.and stable surface structures.

Combining both present and archival photometric, spectroscopic and Combining both present and archival photometric, spectroscopic and

magnetic observations collected during many decades one can reconstruct magnetic observations collected during many decades one can reconstruct

the recent rotation history of outer parts of the star with a high accuracy.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 Careful period analyses were done for several dozens of mCP stars listed

in in On-line catalogue of photometric observations of mCP stars On-line catalogue of photometric observations of mCP stars (Mikulášek (Mikulášek

et al., et al., 2007, AN 328, 102007, AN 328, 10). They confirmed the expectations – the light ). They confirmed the expectations – the light

curves and rotational periods of the upper MS stars are quite steady.curves and rotational periods of the upper MS stars are quite steady.

There are some mCP stars showing light curves variations (56 Ari, V1039 There are some mCP stars showing light curves variations (56 Ari, V1039

Ori, 20 Eri, MW Vul, and 102Ori, 20 Eri, MW Vul, and 102  AqrAqr, Žižňovský et al, Adelman + somebody., Žižňovský et al, Adelman + somebody.), ),

obviously due to the obviously due to the precessionprecession caused by their magnetic distortion. caused by their magnetic distortion.

Nevertheless, there are also mCP stars displaying true Nevertheless, there are also mCP stars displaying true

period variations!period variations!

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 brakingrotational braking in the rate ṖṖ = 7.7 s/century = 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.

Si mCP star CU Virginis

Other type of period changes shows the famous fast-rotating Si

mCP star CU VirCU Vir = HD 124224 = HR 5313, the first stellar

radiopulsar.

Pyper et al. 1998, 2004 + our observations 2009-10: O-C diagram

documents two period jumpstwo 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 are non-variable – no

changes in the shape and amplitude have been found. This + the

extent of the observed period changes excluded precessionprecession as the

cause (detailed discussion in Mikulášek et al. 2008).

He-strong mCP star V901 Orionis

V901 OriV901 Ori = HD 37776 – a very young hot star (B2 IV) 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)

Interpretation of “O-C” diagram

The simplest expression:

JDmax= M0 + P0E + 1/2 P0 dP/dt E2 → constant lengthening of the

observed period of light and spectroscopic variations.

P0 = 1.5387121(10) d; dP/dt = 1.71(7) dP/dt = 1.71(7) ·· 10 10-8-8 (25 σ !)

Another term?

JDmax = M0 + P0E + 1/2 P0 dP/dt E2 + 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 to rotational braking?

He-strong mCP star V901 Orionis

V901 OriV901 Ori = HD 37776 – a very young hot star (B2 IV) 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 effectlight-time effect in a binary star, precessionprecession of rotational axis, evolutionary changesevolutionary changes as possible causes of the period change, we interpreted it as braking of star’s rotationbraking of star’s rotation (at least of surface layers) due to the AM lossAM loss through events in stellar magnetosphere.

Spin-down time paradox. Nature of mCP stars period variations

A quantitative expression of known mCPs with changing periods – spin-down timespin-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 cyclic nature

of mCP period variations.of mCP period variations. Does exist any accelerating mCP stars? Why do not we see them?Does exist any accelerating mCP stars? Why do not we see them?

V901 Ori + CU Vir once more (and better)V901 Ori + CU Vir once more (and better)

V 901 OriV 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 VirCU 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 The consequence of the dynamical interaction between

an outer ‘solid’ magnetically-confined envelope, braked by the stellar an outer ‘solid’ magnetically-confined envelope, braked by the stellar

wind, with an inner, faster rotating stellar body.wind, with an inner, faster rotating stellar body.