An Asymmetric Eclipse Seen Towards the Pre-Main Sequence Binary System V928 Tau

Abstract

K2 observations of the weak-lined T Tauri binary V928 Tau A+B show the detection of a single, asymmetric eclipse which may be due to a previously unknown substellar companion eclipsing one component of the binary with an orbital period $>$ 66 days. Over an interval of about 9 hours, one component of the binary dims by around 60%, returning to its normal brightness about 5 hours later. From modeling of the eclipse shape we find evidence that the eclipsing companion may be surrounded by a disk or a vast ring system. The modeled disk has a radius of $0.9923\pm 0.0005R_{\ast }$, with an inclination of $56.78\pm 0.03^{\circ }$, a tilt of $41.22\pm 0.05^{\circ }$, an impact parameter of $-0.2506\pm 0.0002R_{\ast }$ and an opacity of 1.00. The occulting disk must also move at a transverse velocity of $6.637\pm 0.002R_{\ast }{\mathrm{day}}^{-1}$, which depending on whether it orbits V928 Tau A or B, corresponds to approximately 73.53 or 69.26 ${\mathrm{kms}}^{-1}$. A search in ground based archival data reveals additional dimming events, some of which suggest periodicity, but no unambiguous period associated with the eclipse observed by K2. We present a new epoch of astrometry which is used to further refine the orbit of the binary, presenting a new lower bound of 67 years, and constraints on the possible orbital periods of the eclipsing companion. The binary is also separated by 18" ($\sim$2250 au) from the lower mass CFHT-BD-Tau 7, which is likely associated with V928 Tau A+B. We also present new high dispersion optical spectroscopy that we use to characterize the unresolved stellar binary.

30 pages, 9 tables, 10 figures, accepted for publication in ApJ, GitHub repository for scripts and figures on https://github.com/dmvandam/v928tau

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