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Magnetic Stars, 2011, pp. 458 ­ 461

Magnetic Variability of the Ap Star HD 9996
Bychkov V. D.1 , Bychkova L. V.1 , Madej J.2 , Shatilov A. V.1
1 2

Special Astrophysical Observatory, Nizhny Arkhyz, Russia Warsaw University Observatory, Poland

Abstract. The star HD 9996 (HR 465, GY And) belongs to the subclass of long­ period magnetic Ap stars. Its phase magnetic curve exhibits two components. Long-term variations of the longitudinal field are covered by faster oscillations with the period conterminous to the orbital period. We discuss a possible explanation of such a behaviour as a regular precession of the rotation axis of the main component.

1

Long­Perio d Magnetic Phase Curve

Compilation of early photometric measurements of HD 9996 allowed us to constrain the corresponding period P in the range of 7750 < Pph < 8550 days (Pyper & Adelman, 1986). The period obtained from magnetic measurements is Pmag = 7842 (Bychkov et al., 1997), and Pmag = 7692 (Bychkov et al., 2005). Since the magnetic phase curve was still poorly constrained, the monitoring pro ject for HD 9996 is active as now. As this ob ject is relatively bright, magnetic monitoring was performed in the coude-focus of the 1-m Zeiss-1000 reflector at the SAO RAS, equipped with the CEGS spectrometer and an analyzer of circular polarization (Bychkov, 2008). Finally, we collected 41 measurements of Be during the last 15 years. Figure 1 shows the ob ject's long-period magnetic phase curve. We plotted all the available data from 60 years of observations. The magnetic curve has a double wave with the period Pmag = 8019.24 days, and the parameters B0 = -429 G, B1 = 1330 G and B2 = 332 G. A large scatter of Be points suggests the existence of a possible short time variability.

2

Short­Term Magnetic Variability

We only selected the Be observations obtained by us in the search for a short magnetic period. We measured Ve sin I on all the spectra and determined that it is 8 km/s. The star has a radius of nearly 2.4 solar radii, therefore, the rotational period should be longer than 15.2 days. We have found the short magnetic period measuring the Be deviations from the mean magnetic curve. The short period coincided with the orbital period of Porb = 272.99 days (Scholz, 1978). Figure 2 shows magnetic variations plotted with that Julian date. Figure 3 as well shows the phase curve of the short-term variability with the parameters B0 = 4 G and B1 = 128 G about the long-term magnetic mean phase curve.

3

Discussion

We attempted to explain the magnetic behaviour by the precession about the rotational axis of the primary component in the double system with the period of 8020 days similarly as Lehmann (1987). the rotation period of the star is synchronized with the orbital period, and makes 272.99 days, angle


MAGNETIC VARIABILITY OF THE AP STAR HD 9996

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Figure 1: Long-period magnetic phase curve. The data from 60 years of observations are plotted.

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Figure 2: Magnetic variations plotted with the Julian date


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Figure 3: Pphase curve of the short­term variability with the parameters B0 = 4 G and B1 = 128 G about the long­term magnetic mean phase curve = 18 , Eulers angle = 43 . We assumed that the magnetic field of the main component is well approximated by a centered (not displaced) dipole described by the standard formalism of Preston (1967). This assumption is contradicted by the following data: 1. The amplitude of short-term variation should be maximal at at the phase when the averaged Be = 0 (the axis of rotation lies in the plane of the sky), and minimal when the averaged Be = max (the angle i is minimal). 2. Variability caused by the slow precession should be strictly harmonic, and does not exhibit a double wave.

4

Our Plans for the Future

We plan to continue regular magnetic monitoring of this ob ject. 1. We wish to study its magnetic behaviour in greater detail. 2. Modulation of the magnetic field with the orbital period is very interesting. Possibly it is not only a tidal effect. It may result from the interaction of magnetic fields if the secondary component is a magnetic white dwarf. 3. We wish to understand whether the rotation of the main component is synchronized with the orbital period or not.
Acknowledgements. We acknowledge the support from Polish Ministry of Science and Higher Education grant No. N N203 511638.

References
Bychkov V.D., Astrophysical Bulletin, 2008, 63, 83


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Bychkov V. D., Bychkova L. V., Madej J., 2005, A&A, 430, 1143 Bychkov V. D., Gerth E., Kroll R., Shtol V. G., 1997, in: Glagolevskij Yu. V., Romanyuk I I. (eds), Proc. Int. Conf., "Stellar Magnetic Fields", Nizhny Arkhyz, 204 Lehmann T., 1987, Astron. Nachr., 308, 333 Preston G. W., ApJ, 1967, 150, 547 Pyper D. M., Adelman S., 1986, IAPPP Communication, 25, 76 Scholz G., 1978, Astron. Nachr., 299, 81