Multi-epoch interferometric observations of the Be star ζ Tau

Abstract

Abstract. We present interferometric observations of the Be star ζ Tau obtained using the MIRC beam combiner at the CHARA Array during four epochs in [2007][2008][2009]. Fitting a geometric model to the data reveals a nearly edge-on disk with a FWHM of ∼ 1.8 milli-arcsec in the H-band. The non-zero closure phases indicate an asymmetry in the brightness distribution. Interestingly, when combining our results with previously published interferometric observations of ζ Tau, we find a correlation between the position angle of the disk and the spectroscopic V /R ratio, suggesting that the tilt of the disk might be precessing. This work is part of a multiyear monitoring campaign to investigate the development and outward motion of asymmetric structures in the disks of Be stars. Keywords. techniques: interferometric, circumstellar matter, stars: emission-line, BeThe bright Be star ζ Tau is an ideal target for optical/infrared interferometry. We used the MIRC beam combiner at the CHARA Array to measure interferometrically the size and orientation of the disk of ζ Tau in the H-band over four epochs in [2007][2008][2009]. The visibilities provide information on the size, orientation, and inclination of the disk. The non-zero closure phases indicate an asymmetry in the brightness distribution.We fit a two component geometric model to the MIRC data obtained for ζ Tau. The model is composed of a uniform disk with an angular diameter 0.40 mas (R = 5.5 R ) to fit the central star and an elliptical Gaussian surface brightness distribution to model the circumstellar disk. To account for the asymmetry, we modulated the elliptical Gaussian disk by a sinusoid as a function of azimuth. This creates a skewed disk model where the sinusoid causes the brightness distribution to peak on one side of the disk and places a depression in the brightness on the other side. The models are shown in Figure 1.Over the four epochs, the FWHM of the major axis of the disk ranges between 1.6 -2.1 mas in the H-band. This is similar to the K -band size of 1.8 mas computed by Gies et al. (2007) and smaller than the 3.1 -4.5 mas FWHM measured in Hα by Quirrenbach et al. (1997) and Tycner et al. (2004). We find that the star contributes on average about 55% of the light in the H-band. In comparison, the star contributes 41% of the flux in the K -band (Gies et al. 2007). These ratios are consistent with near-IR excess fluxes observed by Touhami et al. (2010).Combining our results with previous interferometric observations, we find that the position angle of the major axis of the ζ Tau disk varies as a function of the spectroscopic V /R phase (see Fig. 2). The disks of Be stars may develop a global, one-armed spiral instability. The oscillation mode forms a spiral density enhancement that precesses prograde with the disk rotation with a cycle time of a few years. We suspect that the 424 at https://www.cambridge.org/core/terms. https://doi