Periods of magnetic field variations in the Ap star γ Equulei (HD 201601)

Abstract

This paper presents a series of 95 new measurements of the longitudinal (effective) magnetic field B e of the Ap star γ Equ (HD 201601). Observations were obtained at the coudé focus of the 1-m reflector at the Special Astrophysical Observatory (SAO RAS) in Russia over a time period of 4190 days (more than 11 years). We compiled a long record of B e points, adding our measurements to all published data. The time series of magnetic data consists of 395 B e points extending for 24488 days, i.e. over 67 years. Various methods of period determination were examined for the case in which the length of the observed time series is rather short and amounts only to 69 percent of the period. We argue that the fitting of a sine wave to the observed B e points by least squares yields the most reliable period in the case of γ Equ. Therefore, the best period for long-term magnetic variations of γ Equ, and hence the rotational period, is P rot = 35462.5 ± 1149 days = 97.16 ± 3.15 years.

Discovery of an extremely weak magnetic field in the white dwarf LTT 16093 = WD 2047+372

“…Note that the average of our Balmer line measurements of the field of γ Equ, about −520 G, is consistent with the observation by Bychkov et al (2016) that the values of B z measured for HD 201601 using Balmer lines tend to have smaller amplitude than those measured using metal lines, and in fact our measurement is in good agreement with the recent B z measurements of HD 201601 from dimaPol shown in Fig. 6 of Bychkov et al (2016).…”

“…excluding windows around each Balmer line from the analysis) is close to B z ≈ −1000 G. The ensemble of more than 60 yr of B z data available for the star, with many new measurements, has recently been studied by Bychkov et al (2006Bychkov et al ( , 2016, who fit a sine wave variation to all the available B z values, most of which are based on analysis of spectropolarimetry of metal line spectra. They derive a rotation period of 97.16 ± 3.15 yr, When their (corrected) new ephemeris and fitted sine wave parameters are used to compute the expected B z value for our measurement, their ephemeris predicts B z ≈ −740 G. This is (probably) significantly discrepant with our measurement of −1000 G. Since even the huge data set used by Bychkov et al (2016) does not yet cover one full rotation cycle, the period that they find is certainly affected by scatter due to different instrumental systems of measurement of B z (see Landstreet et al 2014), which means that the uncertainty in the period that they derive, ±3.15 yr, is probably underestimated. Our measurement suggests that the true rotation period may be a few years, perhaps even a decade, longer than the period of Bychkov et al (2016).…”

The long-term polarimetric variability of the strongly magnetic white dwarf Grw+70° 8247

“…Our period estimate may seem outrageously long, but there are main sequence stars known to have rotation periods near the lower limit, for example the star HD 201601 = γ Equ, with a period estimated to be about one century (Leroy et al 1994;Bychkov et al 2016). It is clear that physical processes exist that are capable of removing virtually all of a star's angular momentum, and that one of the most powerful of these processes acts through the stellar magnetic field.…”

Periodic Variability on Time-scales of Decades to Centuries in Magnetic Ap Stars: Challenges and Strategies

“…That will raise the value of the longest period that is known exactly to ∼ 30 y. Of the lower limits that have been set for the other stars, the highest one is 97 y (Bychkov et al 2016), for the star γ Equ. Obviously, the level to which upper limits can be set is defined by the time-span over which suitable observations are collected.…”