The Surprising Evolution of the Shadow on the TW Hya Disk*

We report new total-intensity visible-light high-contrast imaging of the TW Hya disk taken with the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope. This represents the first published images of the disk with STIS since 2016, when a moving shadow on the disk surface was reported. We continue to see the shadow moving in a counterclockwise fashion, but in these new images the shadow has evolved into two separate shadows, implying a change in behavior for the occulting structure. Based on radiative-transfer models of optically thick disk structures casting shadows, we infer that a plausible explanation for the change is that there are now two misaligned components of the inner disk. The first of these disks is located between 5 and 6 au with an inclination of 5.5° and position angle (PA) of 170°, and the second between 6 and 7 au with an inclination of 7° and PA of 50°. Finally, we speculate on the implications of the new shadow structure and determine that additional observations are needed to disentangle the nature of TW Hya's inner-disk architecture.</p

Recurring Planetary Debris Transits and Circumstellar Gas around White Dwarf ZTF J0328-1219

We present follow-up photometry and spectroscopy of ZTF J0328-1219, strengthening its status as a white dwarf exhibiting transiting planetary debris. Using TESS and Zwicky Transient Facility photometry, along with follow-up high-speed photometry from various observatories, we find evidence for two significant periods of variability at 9.937 and 11.2 hr. We interpret these as most likely the orbital periods of different debris clumps. Changes in the detailed dip structures within the light curves are observed on nightly, weekly, and monthly timescales, reminiscent of the dynamic behavior observed in the first white dwarf discovered to harbor a disintegrating asteroid, WD 1145+017. We fit previously published spectroscopy along with broadband photometry to obtain new atmospheric parameters for the white dwarf, with M∗ = 0.731 ± 0.023 M⊙, Teff = 7630 ± 140 K, and [Ca/He] = -9.55 ± 0.12. With new high-resolution spectroscopy, we detect prominent and narrow Na D absorption features likely of circumstellar origin, with velocities 21.4 ± 1.0 km s-1 blueshifted relative to atmospheric lines. We attribute the periodically modulated photometric signal to dusty effluents from small orbiting bodies such as asteroids or comets, but we are unable to identify the most likely material that is being sublimated, or otherwise ejected, as the environmental temperatures range from roughly 400 to 700 K