A Pre-main-sequence Binary Viewed Through Its Circumbinary Disk: A New Phase for KH 15D

The binary T Tauri system, KH 15D, is poised to provide unprecedented detail of gas and ices located between ∼3 and 5 au of its circumbinary disk over the next few years. We analyze for the first time a complete set of ground-based time-series photometry of the KH 15D system for which the 2022 October–2025 April data have never been presented. By combining the latest I -band data with historical measurements, we show that the era of large-amplitude photometric variability has ended. We define a revised photometric period of P = 48.365 ± 0.005 days, which accurately phases data over a time span of 75 yr. We adopt magnitudes and colors for the stars that define the epochs when either star A (1995–2008) or star B (2010–2012) is the primary source of the out-of-eclipse brightness of the system. We determine for the first time the effective optical depth for the system between 2002 and 2025 assuming gray extinction. The current out-of-eclipse system color is consistent with a combination of the light from both stars, supporting the previously suggested model in which the trailing edge of the circumbinary disk is more transparent than the leading edge. We continue to see evidence for “clumps” and variable transparency along ingress and egress when the stars are probing the occulting material and suggest that observations made during these phases should permit studies of the physical and chemical nature of the planet-forming zone with the circumbinary disk.