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Detection of a Very Slow Magnetic Rotator HD 965

Russian version

    The chemically peculiar star HD965 belongs to a rare type of very slow rotators. As far back as in the 90s, G.Mathys (ESO, Chile) supposed its extremely slow rotation as observing split Zeeman components in the spectrum, from which he measured the magnetic field on its surface. Field variation proved to be very slow, and G.Mathys determined that the variability period should be longer than two years.
    In 1999, at G.Wade's suggestion, HD965 was included in the joint program of magnetic field observations with the SAO RAS 6-m telescope (BTA). First results of the observations were published in 2005 (Elkin et al., 2005, MNRAS, 358, 1100). Magnetic monitoring of HD 965 had been carried out on the BTA in the period from 2000 to 2015 with the Main Stellar Spectrograph.
    Our 15-year observations showed that the positive extremum of the star's longitudinal magnetic field (+600 Gs) was observed in 2005, and the negative one (-1300 Gs) in summer 2015. Thus, our observations have not covered the entire rotation period of the star, although there are strong reasons to consider it to be equal to about 20 years. Up to now, only two stars with periods longer than 20 years are known. Those are γ Equ with a period of about 100 years, and HD9996 - 21-22 years. HD965 so far ranks third in the period length among all (about 300) magnetic CP stars with determined or estimated rotation periods.
    Detection of another magnetic star with a rotation period of more than 20 years is indicative of the fact that ultra slow rotation is very rare but not unique. We obtained one more argument for the fossil origin of the magnetic field in CP stars. As already three very slow rotators have been found, we can draw a conclusion that magnetic fields greater than 1 kGs can also form in such objects, which in turn proves that the field formation mechanism is not associated with rotation. Different dynamo mechanisms fail to effectively generate strong global magnetic fields of chemically peculiar stars during their Main Sequence (MS) lifetime. Global magnetic fields form with a star itself, before it enters the MS stage. There is no magnetic field generation but only a very slow, gradual fading (at timescale of billions of years) and simplification of the magnetic field structure during the MS lifetime of the star.
I.I.Romanyuk, D.O.Kudryavtsev, E.A.Semenko, and I.A.Yakunin, "Magnetic Field Monitoring of the Very Slowly Rotating CP Star HD 965", 2015, Astrophysical Bulletin, 70, 456

Contact - I.I.Romanyuk

Fig.1. Longitudinal magnetic field curve of the CP star HD965 in the time interval from 2000 to 2015. The blue squares are the measurements from the shift of oppositely circularly polarized Zeeman components