Near-Ultraviolet and Optical Observations of the White Dwarf ZTF J0328$-$1219 Reveal Variable Periodicity in its Transiting Debris System

Joseph A. Guidry; J. J. Hermes; Zachary P. Vanderbosch; Boris Gaensicke; Simon Blouin; Laura K. Rogers; Jay Farihi; Stefan Arseneau; Lou Baya Ould Rouis; Siyi Xu; Bruce Gary; Thomas G. Kaye; Yarin-Meir Shani; Hannah Worters; Kaylee E. Grace; Steven Z. Savery; J. L. Provencal; Chun-Hai Bai; Keaton J. Bell; Barbara G. Castanheira; Paul Chote; Millie A. Dill; Inajara Endl; Torben Endl; Lester Fox Machado; Elizabeth M. Green; Kim Green; Easton J. Honaker; Nikoo Hosseininezhad; Xiao-Jun Jiang; Momin Y. Khan; Morgan A. Mitchell; Michael H. Montgomery; Rudy Morales; Tali Natan; Hui-Juan Wang; Jia-Qi Wang; Tu-Hong Zhong

AI summary · arXiv: 2611.13136v1 · astro-ph.SR

Near-Ultraviolet and Optical Observations of the White Dwarf ZTF J0328$-$1219 Reveal Variable Periodicity in its Transiting Debris System

Joseph A. Guidry, J. J. Hermes, Zachary P. Vanderbosch, Boris Gaensicke, Simon Blouin, Laura K. Rogers, Jay Farihi, Stefan Arseneau,

and 30 more authors

Lou Baya Ould Rouis, Siyi Xu, Bruce Gary, Thomas G. Kaye, Yarin-Meir Shani, Hannah Worters, Kaylee E. Grace, Steven Z. Savery, J. L. Provencal, Chun-Hai Bai, Keaton J. Bell, Barbara G. Castanheira, Paul Chote, Millie A. Dill, Inajara Endl, Torben Endl, Lester Fox Machado, Elizabeth M. Green, Kim Green, Easton J. Honaker, Nikoo Hosseininezhad, Xiao-Jun Jiang, Momin Y. Khan, Morgan A. Mitchell, Michael H. Montgomery, Rudy Morales, Tali Natan, Hui-Juan Wang, Jia-Qi Wang, Tu-Hong Zhong

Original paper Submitted to arXiv on 09 Nov. 2026 26 pages, 10 figures, accepted to The Astrophysical Journal License: CC BY 4.0 Also in: fr

How to read this summary? Illustrated guide

The essentials

ZTF J0328−1219’s transiting debris shifts between dominant orbital periods, while its transit colors are mostly consistent with wavelength-independent extinction and its accreted material appears chondritic.

Question

How do the transit periodicity and morphology of the debris system around ZTF J0328−1219 change over time, and what do ultraviolet and optical observations reveal about the debris color and accreted composition?

Method

The authors collected over 176 hours of ground-based time-series photometry across campaigns in November 2024 and autumn 2025, and obtained five consecutive HST/STIS near-ultraviolet spectra in October 2025. They searched the light curves with conditional-entropy and Lomb–Scargle periodograms, compared simultaneous multi-band transit depths, and modeled the NUV spectrum alongside a new optical spectrum to estimate atmospheric abundances.

Findings

Result The 2024 observations show the established 9.937-hour period, while 2025 observations shift from a predominant 11.17-hour period back toward the 9.937-hour period.
See the passage in the paper
The previously transient period $P_{\rm B}=11.17\,$hr began to dominate the line of sight in 2025 October; by 2025 November $P_{\rm A}$ was again predominant.
Result A candidate 7.64-hour transit period appears in observations from October 29 to November 1, but its stability and significance remain uncertain.
See the passage in the paper
It is unclear how stable this period is and its formal significance in the periodogram is less straightforward to assess.
Result The measured transit color is consistent with no significant NUV-to-optical color trend across HST orbits two through four.
See the passage in the paper
We detect no significant color term between the NUV and optical transits: $E(\mathrm{\textsc{nuv}}-\mathrm{\textsc{optical}})/A_{\mathrm{\textsc{optical}}}=-0.10\pm 0.12$.
Result A significant reddening trend is measured during the fifth HST orbit, though those observations are the most noise- and normalization-dominated.
See the passage in the paper
A significant reddening term emerges if we include the LAST photometry overlapping with the fifth HST orbit: $E(\mathrm{\textsc{nuv}}-\mathrm{\textsc{optical}})/A_{\mathrm{\textsc{optical}}}=0.59\pm 0.08$.
Authors’ interpretation The abundance analysis supports accretion of material with a chondritic-like composition.
See the passage in the paper
Our abundance analysis supports that the white dwarf is accreting debris consistent with originating from a chondritic-like parent body.

Limitations

The candidate 7.64-hour period is not established as stable or formally significant.
See the passage in the paper
It is unclear how stable this period is and its formal significance in the periodogram is less straightforward to assess.
The apparent fifth-orbit reddening comes from the shallowest, most noise- and normalization-dominated transits.
See the passage in the paper
At the same time, we raise the caveat that both the NUV and optical transits during the fifth HST orbit are the shallowest and normalization dominated, and thus the most noise dominated.
The proposed collision-driven explanation for the changing periodicity is an interpretation, not a directly established cause.
See the passage in the paper
We postulate that transits associated with $P_{\rm B}$ could have become the predominant pattern along the line of sight if a collisional cascade occurred within this orbit, boosting its occulting fraction enough to overtake that of the debris transiting on the $P_{\rm A}$ orbit, either through elevating the optical depth or increasing the ring’s scale height.

Why it matters

The observations document a change in the dominant transit period and morphology on short timescales, which the authors argue offers a way to study dynamical processes and dust evolution in white-dwarf debris systems in real time. They emphasize that continued long-term monitoring and broad-wavelength observations are important for understanding these systems.

Written by AI from the paper. Passages are checked word for word against the text we analysed; the rest may contain errors.

AI-generated keywords: polluted white dwarf transiting debris circumstellar dust ultraviolet spectroscopy atmospheric abundances

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Comprehensive Summary
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Original abstract

The paper investigates how transiting debris around the polluted white dwarf ZTF J0328−1219 changes over time. Earlier observations established a recurring transit period of about 9.937 hours, designated period A, and identified a second, temporary period near 11.2 hours, designated period B. The authors examine whether these patterns persist, how the transit shapes and colors vary, and what the white dwarf’s atmospheric abundances reveal about the material it is accreting. The study combines more than 176 hours of ground-based time-series photometry from campaigns in November 2024 and autumn 2025 with five consecutive Hubble Space Telescope orbits of near-ultraviolet spectroscopy and time-resolved photometry in October 2025. The 2024 observations used ULTRACAM to observe the target simultaneously in the u, g, and r bands. The 2025 campaign used multiple telescopes and filters, including optical observations taken simultaneously or nearly simultaneously with Hubble’s ultraviolet measurements. The authors searched for periodic signals using conditional-entropy and Lomb–Scargle periodograms and compared transit depths across wavelengths to test for color-dependent extinction. They also combined the Hubble spectrum with a new optical spectrum to model the white dwarf’s atmosphere and measure elemental abundances. In November 2024, the transits were dominated by period A, consistent with the previously established 9.937-hour recurrence. In autumn 2025, the pattern changed: period B, measured at about 11.17 hours, initially became dominant while period A weakened. Over the roughly two-week campaign, the transit shapes changed rapidly, and period A regained prominence toward the end. The authors also identify a possible third recurrence, period C, at 7.64 hours in observations from October 29 to November 1. Its significance and persistence remain uncertain, and nearby periodogram signals may include aliases. Overall, period A appears stable across observing epochs, but the debris pattern dominating the line of sight can vary. Transit-color measurements generally show no convincing wavelength dependence. The simultaneous 2024 u, g, and r observations are consistent with colorless transits; small apparent trends are not statistically distinguishable from a model with no color term. For Hubble orbits two through four, the near-ultraviolet-to-optical color term is also consistent with no change in color as transit depth varies, with a measured slope of −0.10 ± 0.12. However, ultraviolet flux was, on average, more strongly extinguished than optical flux by about 0.18 magnitudes. A significant reddening slope of 0.59 ± 0.08 appears when data from the fifth Hubble orbit are included. Because those observations contain the shallowest transits and are especially affected by normalization and noise, this result is tentative. The spectral analysis yields an effective temperature of 7,310 ± 100 K and a white-dwarf mass of 0.662 ± 0.020 solar masses. The measured abundances, including carbon, indicate that the accreted material is broadly consistent with a chondritic parent body, although silicon is an exception to the reported close agreement with chondritic ratios. The authors interpret the changing transit activity as evidence of an evolving debris system, with collisions and a resulting dust cascade as one plausible explanation. They also consider orbital interactions and a cyclical viewing effect as alternatives. The observations do not establish which mechanism is responsible, whether period C persists, or whether the color change during the fifth Hubble orbit is real. Longer-term monitoring and infrared observations are needed to test these possibilities.

A Changing View of Debris Around a White Dwarf

Some white dwarfs are surrounded by the remains of planetary systems. When fragments of those remains pass in front of the star, they create dips in its brightness. A new study of ZTF J0328−1219 finds that the timing and shape of these transits can shift noticeably over just a few weeks, offering a rare look at a changing debris system.

In “Near-Ultraviolet and Optical Observations of the White Dwarf ZTF J0328−1219 Reveal Variable Periodicity in its Transiting Debris System,” Joseph A. Guidry, J. J. Hermes, Zachary P. Vanderbosch, Boris Gaensicke, Simon Blouin, Laura K. Rogers et al. combine extensive ground-based observations with ultraviolet measurements from the Hubble Space Telescope. Their results show that one recurring transit pattern remains recognizable, even as another pattern temporarily takes over.

Watching the Transits Change

Earlier observations had established a recurring period of about 9.937 hours, called period A. They also revealed a second, temporary pattern near 11.2 hours, called period B. To track how these signals evolved, the researchers collected more than 176 hours of ground-based time-series photometry during campaigns in November 2024 and autumn 2025.

The 2024 campaign used ULTRACAM to observe the star simultaneously in three optical bands: u, g, and r. The 2025 effort drew on multiple telescopes and filters. It also included five consecutive Hubble orbits of near-ultraviolet spectroscopy and time-resolved photometry in October 2025, alongside optical observations taken at the same time or nearly so.

The team searched the light curves for repeating signals using two methods, conditional-entropy and Lomb–Scargle periodograms. They also compared transit depths at different wavelengths to test whether the debris blocked some colors more strongly than others.

One Period Fades as Another Takes Over

In November 2024, period A dominated the observations, matching the previously known 9.937-hour recurrence. The picture looked different in autumn 2025. Period B, measured at about 11.17 hours, initially became the stronger signal while period A weakened. Across the roughly two-week campaign, transit shapes changed rapidly, and period A became more prominent again toward the end.

The researchers also report a possible third recurrence, period C, at 7.64 hours in observations from October 29 to November 1. Its status is uncertain: it may not persist, and nearby signals in the periodograms could be aliases rather than separate rhythms. The clearest conclusion is that period A remains stable across observing epochs, but the pattern of debris crossing the line of sight can change.

Do the Transits Change Color?

Comparing wavelengths can help reveal whether the obscuring material has wavelength-dependent effects. In the simultaneous 2024 u, g, and r observations, the transit measurements are consistent with no color dependence. Small apparent trends are not statistically distinct from a model with no color term.

For Hubble orbits two through four, the near-ultraviolet-to-optical color slope was −0.10 ± 0.12, also consistent with no change in color as transit depth varied. However, ultraviolet light was, on average, more strongly extinguished than optical light by about 0.18 magnitudes.

A different result appears when data from Hubble’s fifth orbit are included: the measured reddening slope is 0.59 ± 0.08. The authors caution that this finding is tentative. The fifth-orbit observations contain the shallowest transits and are particularly sensitive to normalization and noise, so further measurements are needed to determine whether the color change is real.

Clues in the White Dwarf’s Atmosphere

The team combined the Hubble spectrum with a new optical spectrum to model the white dwarf’s atmosphere and measure the elements it is accreting. Their analysis gives an effective temperature of 7,310 ± 100 K and a mass of 0.662 ± 0.020 times the mass of the Sun.

The measured abundances, including carbon, broadly match a chondritic parent body, a type of rocky material with proportions similar to primitive meteorites. Silicon is an exception to the reported close agreement with chondritic ratios. These atmospheric measurements provide another way to investigate the composition of material falling onto the star.

What Could Be Driving the Changes?

The shifting transit patterns point to an evolving debris system, but the observations do not identify a single cause. Collisions among fragments could produce a cascade of dust and alter what passes in front of the white dwarf. The authors also consider orbital interactions and a cyclical viewing effect as possible explanations.

Important questions remain: Does period C persist? Is the fifth-orbit color trend genuine? Which mechanism changes the debris pattern along our line of sight? Longer-term monitoring and infrared observations could help answer them. For now, ZTF J0328−1219 offers a striking example of how quickly the observable behavior of debris around a white dwarf can change.

Created on 11 Nov. 2026

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Near-Ultraviolet and Optical Observations of the White Dwarf ZTF J0328$-$1219 Reveal Variable Periodicity in its Transiting Debris System

The essentials

Question

Method

Findings

Limitations

Why it matters

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Definitions

A Changing View of Debris Around a White Dwarf

Watching the Transits Change

One Period Fades as Another Takes Over

Do the Transits Change Color?

Clues in the White Dwarf’s Atmosphere

What Could Be Driving the Changes?

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