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A Candidate Low-mass Disk-eclipsing Binary in the ~316 Myr Open Cluster UPK 13

This paper presents multi-band photometric evidence reclassifying the object UPK 13-c2 from a white dwarf-main sequence binary to a late-K/early-M binary with a misaligned circumbinary disk, potentially making it the oldest known main-sequence disk-eclipsing binary if its membership in the ~316 Myr UPK 13 cluster is confirmed.

Original authors: Jiamao Lin, Yongkang Sun, Chengyuan Li

Published 2026-06-18

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CC BY 4.0

Original authors: Jiamao Lin, Yongkang Sun, Chengyuan Li

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 a cosmic dance floor where two stars are locked in a tight embrace, orbiting each other. Now, imagine a giant, invisible curtain sweeping back and forth across their path, periodically hiding one of the dancers from our view. That is essentially what astronomers have found in a star system called UPK 13-c2.

For a long time, scientists thought this system was a mismatched pair: a tiny, dead star (a white dwarf) and a normal, living star. But this new paper argues that the "dead star" idea is wrong. Instead, it's likely a pair of two normal, cool stars (like our Sun's smaller, redder cousins) being hidden by a massive, tilted ring of dust.

Here is the breakdown of the discovery using simple analogies:

1. The Mystery of the "Square" Eclipse

Usually, when one star passes in front of another, the light dims gradually, creating a smooth, U-shaped curve (like a gentle hill). But UPK 13-c2 is different. Its light curve looks like a flat-bottomed box or a square wave.

The Analogy: Imagine shining a flashlight at a wall. If you slowly move a small pebble in front of the light, the shadow grows and shrinks smoothly. But if you hold a large, flat piece of cardboard directly in front of the light, the shadow appears instantly, stays perfectly dark for a while, and then disappears just as suddenly.
The Paper's Claim: The "flat bottom" of the eclipse means the object blocking the light (a disk of dust) is completely covering one of the stars for about 10 days. The edges of this "cardboard" are so sharp that it takes about 2.5 days just to slide the star in and out of the shadow.

2. Why It Can't Be a "Dead Star" (White Dwarf)

The previous theory suggested a tiny, dead white dwarf was the one getting hidden. The paper uses two main arguments to say, "No way."

Argument A: The Size Mismatch
- The Analogy: A white dwarf is the size of Earth, while a normal star is the size of the Sun. If a white dwarf were the one getting hidden by that sharp-edged dust disk, it would zip across the edge in less than 2 hours. It would be a tiny, needle-thin dip in the light.
- The Reality: We see a dip that lasts for 2.5 days to enter and 10 days to stay hidden. This only makes sense if the object being hidden is a full-sized star, not a tiny dead one.
Argument B: The Color Mismatch
- The Analogy: Imagine trying to identify a person in a crowd by the color of their shirt. A white dwarf is very hot and blue; a normal star is cooler and redder. If you subtract the light of the visible star from the total light, you are left with the light of the hidden star.
- The Reality: When the scientists did the math, the "hidden light" looked exactly like a cool, reddish star (a late-K or early-M dwarf). If it were a white dwarf, the colors wouldn't match the data at all. It's like trying to fit a square peg into a round hole.

3. The "Tilted Curtain" (The Disk)

So, what is hiding the star? It's a circumbinary disk—a giant ring of gas and dust orbiting both stars.

The Analogy: Think of a hula hoop spinning around two dancers. If the hoop is tilted, it won't block the dancers all the time. But as the dancers move in their elliptical (oval) orbit, they eventually pass behind the edge of the hoop.
The Paper's Claim: The disk is tilted and the stars are moving in a stretched-out oval orbit. This geometry creates the perfect conditions for the disk to completely block one star for a long time, creating that "square" eclipse.

4. Why This Discovery is Special

Most of these "disk-eclipsing" systems we know about are babies—very young stars only a few million years old. Their dust disks haven't had time to disappear yet.

The Analogy: Finding a system like this is like finding a house with a brand-new, pristine garden in a neighborhood where all the other houses are 300 years old. Usually, gardens disappear or get overgrown after a few decades.
The Paper's Claim: If this star system is part of the UPK 13 cluster (which is about 316 million years old), it would be the oldest known example of this phenomenon. It suggests that these massive dust rings can survive for hundreds of millions of years, which challenges our current understanding of how long dust disks can last around mature stars.

Summary

The paper concludes that UPK 13-c2 is likely two normal, cool stars dancing in an oval orbit, with a giant, tilted ring of dust periodically sweeping across one of them. This creates a long, flat eclipse that proves the hidden object is a full-sized star, not a tiny dead one. If confirmed, it's a rare, ancient example of a cosmic "curtain" that has survived much longer than scientists expected.

Technical Summary: A Candidate Low-Mass Disk-Eclipsing Binary in the $\sim$ 316 Myr Open Cluster UPK 13

Problem Statement
The paper addresses the classification of UPK 13-c2, a candidate member of the $\sim$ 316 Myr open cluster UPK 13. Previously identified by Grondin et al. (2024) as a white dwarf + main-sequence (WD+MS) binary with 99.44% confidence, the system exhibits periodic dimming. The authors investigate whether this system is indeed a WD+MS binary or, alternatively, a late-K/early-M main-sequence binary (MS+MS) occulted by a misaligned circumbinary disk. Resolving this is critical because known disk-eclipsing binaries (e.g., KH 15D, Bernhard-1/2) are exclusively young ( $\lesssim$ 20 Myr). Confirming a similar system at $\sim$ 300 Myr would challenge current models of disk longevity and binary-disk evolution.

Methodology
The authors utilize multi-band photometry spanning the optical to mid-infrared, including data from ZTF (g, r), NEOWISE-R (W1, W2), AllWISE (W3, W4), 2MASS (JHKs), Pan-STARRS1 (i, z, y), and SPHEREx spectrophotometry (0.75–5.0 $\mu$ m).

Data Processing: Astrometric parameters were corrected using Gaia DR3 parallax zero-point recipes (Maíz Apellániz 2022), yielding a distance of $d \approx 951$ pc. Extinction was fixed to $A_V = 0.85$ mag based on the Bayestar2019 dust map.
Light Curve Analysis: Period analysis (BLS and Lomb–Scargle) established an orbital period of $P = 36.71 \pm 0.01$ days. The light curve exhibits a flat-bottomed, trapezoidal eclipse with a duration of $\sim$ 10 days and ingress/egress times of $\sim$ 2.5 days.
SED Decomposition: The authors employed a "decoupled" three-step fitting procedure to compare two hypotheses:
- Hypothesis A (WD+MS+Disk): Fits the difference spectrum ( $F_{high} - F_{low}$ ) using Koester DA white dwarf atmospheres.
- Hypothesis B (MS+MS+Disk): Fits the difference spectrum using reddened late-K/early-M stellar templates.
- In both cases, the low-state flux ( $F_{low}$ ) and high-state flux ( $F_{high}$ ) were modeled independently before checking for global consistency.
Geometric Modeling: A forward modeling approach using MCMC (emcee) was applied to a sharp-edge disk occultation model to constrain orbital eccentricity ( $e$ ), argument of periastron ( $\omega$ ), and the disk azimuthal arc ( $\alpha$ ).

Key Results

Eclipse Morphology: The system displays a stable, flat-bottomed eclipse with an approximately achromatic flux decrement of $\sim$ 40% from optical to W1. The W2 depth is reduced ( $\sim$ 28%) due to thermal dilution by the disk, while W3/W4 show a prominent mid-infrared excess.
Rejection of WD+MS Hypothesis:
- Morphological: The flat-bottomed eclipse with a 2.5-day ingress requires the complete occultation of an extended stellar component ( $R \sim 0.7 R_\odot$ ). A white dwarf ( $R \sim 0.013 R_\odot$ ) would cross the disk edge in $\lesssim$ 2 hours, producing a needle-narrow dip inconsistent with the observed trapezoid.
- Spectral: The difference spectrum ( $F_{diff}$ ) is well-fit by a single $\sim$ 4000 K thermal SED ( $\chi^2_{diff} \approx 0.9$ for a K9V template). A WD model yields a poor fit ( $\chi^2_{diff} \approx 45$ ) because the Rayleigh–Jeans tail of a WD cannot simultaneously match the optical eclipse depth and the significant mid-infrared decrement.
Adoption of MS+MS+Disk Model:
- The system is best described as a late-K/early-M binary (M1V + K9V) with a total mass $M_{tot} \approx 1.4 M_\odot$ (subject to $\sim$ 20% systematic uncertainty).
- The occulting structure is a misaligned circumbinary disk with an inner edge at $R_{in} \approx 0.48–0.72$ AU, consistent with tidal truncation and the observed dust temperature ( $T_{disk} \approx 220$ K).
- Forward modeling suggests an eccentric orbit ( $e \approx 0.71$ ) with the eclipse occurring near apoastron, and a localized disk arc ( $\alpha \approx 13^\circ$ ).
Cluster Membership: UPK 13-c2 satisfies kinematic and parallax criteria for membership in UPK 13, with proper motion within $2.5\sigma$ of the cluster mean and a parallax consistent with the cluster distance.

Significance and Claims
The paper claims that if cluster membership is confirmed, UPK 13-c2 represents the oldest known candidate for a main-sequence disk-eclipsing binary, with an age of $\sim$ 316 Myr compared to the $\lesssim$ 20 Myr of previously confirmed systems (e.g., Bernhard-2).

The authors posit that this system raises a fundamental question regarding disk evolution: whether such strongly occulting structures can persist into the mature main-sequence phase. If confirmed, the disk could represent either an exceptionally long-lived primordial reservoir preserved by binary tidal truncation or a massive second-generation debris disk entering an extreme collisional phase. The paper emphasizes that while the photometric evidence strongly favors the MS+MS+disk interpretation over the WD+MS classification, spectroscopic confirmation of the radial velocity and dynamical masses is required to definitively confirm the system's architecture and cluster membership.

1. The Mystery of the "Square" Eclipse

2. Why It Can't Be a "Dead Star" (White Dwarf)

3. The "Tilted Curtain" (The Disk)

4. Why This Discovery is Special

Summary

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