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Thirty Circumbinary Disk Occultation Systems (KH 15D-like stars) from the OGLE Project

This paper presents a catalog of 30 candidate KH 15D-like circumbinary disk occultation systems identified through long-term OGLE monitoring, highlighting unique light curves and spectral data that reveal multi-year changes in eclipse characteristics.

Original authors: M. A. Urbanowicz, I. Soszyński, P. Pietrukowicz, A. Udalski, P. Mróz, M. Wrona, M. Ratajczak, M. K. Szymański, J. Skowron, D. M. Skowron, R. Poleski, S. Kozłowski, P. Iwanek, K. Ulaczyk, K. Rybicki, M

Published 2026-05-12

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CC BY 4.0

Original authors: M. A. Urbanowicz, I. Soszyński, P. Pietrukowicz, A. Udalski, P. Mróz, M. Wrona, M. Ratajczak, M. K. Szymański, J. Skowron, D. M. Skowron, R. Poleski, S. Kozłowski, P. Iwanek, K. Ulaczyk, K. Rybicki, M. Gromadzki, M. Mróz

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the night sky as a giant, cosmic stage. For decades, astronomers have been watching a very specific type of "act" performed by young stars. Usually, stars shine steadily, but some young stars put on a show where they seem to disappear and reappear, not because they are turning off, but because a giant, dusty curtain is swinging in front of them.

This paper is like a new, massive catalog of 30 actors on this stage who are performing this specific "curtain act." Here is the story of what the researchers found, explained simply:

The Star of the Show: KH 15D

To understand these 30 new stars, you first need to know the original star that started it all, called KH 15D.

The Setup: Imagine two young stars dancing around each other in a tight, wobbly circle.
The Curtain: Surrounding them is a giant, flat disk made of dust and gas (like a cosmic pizza dough, but made of space dust).
The Trick: As the stars dance, this dusty disk slowly tilts and spins (precesses). Sometimes, the disk swings in front of one star, hiding it completely. Later, it might hide both, or neither.
The Result: To us, watching from Earth, the star doesn't just twinkle; it goes through long periods of being bright, then suddenly gets dim for a long time, then bright again. It's like a lighthouse where the beam is being blocked by a slowly rotating wall.

The New Discovery: A Catalog of 30

The researchers used a powerful telescope in Chile (part of the OGLE project) to scan the sky for 25 years. They found 30 stars that behave just like KH 15D.

The Evidence: They didn't just take a snapshot; they watched these stars for decades. Some of the data goes back to 1997. This long-term view is crucial because it lets them see the "curtain" slowly changing its angle over time.
The Variety: While they all follow the same basic rule (dust blocking light), they are all unique. Some are blocked for a short time, others for most of their orbit. Some get very dim, others just a little bit.

What Makes These Stars Special?

The paper highlights a few "actors" that are doing something particularly interesting:

The Triple-Act Star (OGLE-CBO-018): Most of these stars have two main states: "Bright" and "Dim." But this one has three! It has a bright state, a medium state, and a dim state. It's like a lightbulb that has a "high," "medium," and "low" setting, and the dusty curtain is switching between them. The researchers think this star is currently in a transition phase and might eventually settle into the standard two-state pattern.
The Chameleon (OGLE-CBO-023): This star used to have shallow dips in brightness. Then, around 2011, it suddenly started having huge, deep dips (getting 3 times dimmer!). Recently, it has gone back to being less dramatic. It's like a performer who suddenly decided to wear a heavy black cloak, then took it off again.
The Shape-Shifter (OGLE-CBO-026): This star's "curtain" changes size dramatically. Sometimes the star is hidden for only 29% of the time, and other times for 89% of the time. It's as if the curtain is stretching and shrinking like a rubber band.

The Color Clues

The researchers also looked at the color of the light.

The Red Shift: When the dust blocks the star, the remaining light often looks redder. This is like looking at a sunset; the dust scatters the blue light away, leaving the red. This confirms that a thick, dusty disk is indeed the cause.
The Blue Surprise: Interestingly, a few stars actually look bluer when they are dim. The researchers suggest this might happen because the dust is scattering blue light from the star onto the disk, acting like a cosmic mirror that reflects blue light back to us just before the star gets fully hidden.

The Cast List

The paper provides a detailed list of these 30 stars, including their coordinates (where to find them in the sky) and how long their "curtain act" lasts (ranging from about 18 days to over 700 days). Many of these stars are found in young star clusters, which makes sense because they are all very young (less than 100 million years old)—essentially "teenage" stars that haven't finished growing up yet.

The Bottom Line

This paper is a "roll call" of 30 new cosmic mysteries. By watching them for so long, the astronomers can see the dusty disks slowly spinning and changing, giving us a front-row seat to the dynamic, messy, and beautiful process of how young binary stars and their surrounding disks evolve. It's a reminder that the universe isn't static; even the "curtains" in space are constantly moving.

Technical Summary: Thirty Circumbinary Disk Occultation Systems (KH 15D-like stars) from the OGLE Project

Problem and Context
Since the discovery of KH 15D in 1998, a specific class of variable stars has been identified where brightness variations are caused by a circumbinary dusty disk periodically obscuring one or both stellar components in a binary system. These Circumbinary Disk Occultation (CBO) systems provide critical insights into the dynamics and evolution of circumbinary disks and planet formation in binary systems. Prior to this study, only 15 such objects were confirmed, most of which are young stellar objects (YSOs) associated with star-forming regions. The primary challenge in expanding this catalog is the necessity for long-term, high-cadence photometric monitoring to identify the characteristic light-curve morphologies—specifically broad, flat-bottomed minima that can persist for a significant fraction of the orbital period—and to track long-term evolutionary changes in eclipse width and amplitude caused by disk precession.

Methodology
This study utilizes data from the Optical Gravitational Lensing Experiment (OGLE) project, which has conducted systematic photometric observations of the Galactic bulge and disk since 1997. The authors analyzed light curves in the $I$ and $V$ bands, with the vast majority of data collected in the $I$ band. The time baselines for these objects range from approximately 10 years to nearly 30 years (1997–2025), allowing for the detection of secular changes in light-curve morphology.

The selection process involved a visual inspection of light curves from OGLE databases, originally searched for other variable star classes (e.g., long-period variables, eclipsing binaries). The authors identified candidates exhibiting unusual behavior: stars spending most of their time in one of two distinct brightness states with constant flux, separated by transitions on relatively short timescales. These light curves resemble eclipsing systems but feature broad, flat-bottomed minima persisting up to 80% of the orbital period. To increase catalog completeness, the sample was supplemented with five additional CBO systems previously identified by Hu et al. (2026) that were present in OGLE databases but not previously flagged as CBO candidates.

For three specific candidates (OGLE-CBO-018, OGLE-CBO-021, and OGLE-CBO-024), low-resolution spectroscopic observations were obtained using the ESO Faint Object Spectrograph and Camera 2 (EFOSC2) on the 3.58-m New Technology Telescope (NTT) in July 2021. These spectra were used to characterize the stellar types and identify emission or absorption features.

Key Contributions and Results
The paper presents a catalog of 30 CBO system candidates, increasing the known population of such objects by 25. The key findings include:

Catalog Compilation: The authors provide a comprehensive catalog (Tables 2 and 3) listing equatorial coordinates, alternative designations, orbital periods ( $P$ ), maximum brightness in $I$ and $V$ bands, $I$ -band amplitudes ( $A_I$ ), occultation widths (expressed as a percentage of the period), and occultation epochs. The periods range from ~18 days to ~759 days, with occultation widths varying from 0% to over 90%.
Long-Term Evolution: The extended OGLE time baselines reveal that many of these systems exhibit significant changes in light-curve morphology over years. Specifically, the authors document variations in eclipse width, amplitude, and shape. For instance, OGLE-CBO-010 shows a gradual decrease in both occultation width and amplitude until eclipses disappeared, while OGLE-CBO-023 transitioned from shallow, asymmetric eclipses to deep, symmetric broad eclipses and back to low-amplitude variability.
Spectroscopic Characterization:
- OGLE-CBO-018: Identified as a G-type star with metallic absorption features and no emission lines. Its light curve shows three distinct brightness levels (two minima and a bright maximum), a morphology similar to KH 15D during the 1995–1996 interval.
- OGLE-CBO-021: Exhibits an H $\alpha$ emission line, typical of young stellar objects.
- OGLE-CBO-024: Shows TiO bands characteristic of M-type stars and a mild H $\alpha$ emission line.
Color Variations: Analysis of $V-I$ color curves indicates that most systems become redder during eclipses, consistent with obscuration by a dusty disk. However, some systems (e.g., OGLE-CBO-012 and OGLE-CBO-027) become bluer, which the authors suggest may result from the disk obscuring the cooler component, allowing the hotter component to dominate, or from scattering effects.
Cluster Associations: Cross-matching with star cluster catalogs reveals that nine of the 30 systems are positionally and kinematically coincident with young star clusters (ages < 100 Myr), supporting the classification of these objects as young stellar systems.

Significance
The paper claims that this work significantly expands the known sample of KH 15D-like systems, providing a larger statistical basis for studying circumbinary disk dynamics. The primary value of the presented catalog lies in the unique, long-term OGLE light curves, which allow for the observation of evolutionary changes in eclipse morphology that are impossible to detect with shorter baselines. The authors emphasize that these observations enable a deeper understanding of disk precession and the physical processes occurring in young binary systems. While definitive confirmation of the binary nature and disk geometry for all candidates requires further spectroscopic follow-up, the photometric evidence strongly supports their classification as CBO systems. The study also highlights the diversity of light-curve behaviors within this class, ranging from stable two-state variability to complex, evolving morphologies.