🔭 astrophysics

Two unseen massive white dwarf candidates in close binaries

This paper reports the discovery of two close binary systems containing slightly bloated G-type main-sequence stars and unseen massive white dwarf companions, identified through TESS photometry and Gaia radial velocity data, which are likely post-common envelope binaries with uncertain evolutionary futures ranging from cataclysmic variables to Type Ia supernovae.

Original authors: Yuta Shiraishi, Kenta Hotokezaka, Kento Masuda, Satoshi Honda, Ataru Tanikawa, Soetkin Janssens, Takato Tokuno, Takumi Shimasue, Ryoga Honjo, Bun'ei Sato, Masashi Omiya, Akito Tajitsu, Hideyuki Izumiu

Published 2026-02-17

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Yuta Shiraishi, Kenta Hotokezaka, Kento Masuda, Satoshi Honda, Ataru Tanikawa, Soetkin Janssens, Takato Tokuno, Takumi Shimasue, Ryoga Honjo, Bun'ei Sato, Masashi Omiya, Akito Tajitsu, Hideyuki Izumiura

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

The Big Picture: Hunting for Invisible Ghosts in the Night Sky

Imagine you are at a dance party. You see one person dancing wildly on the floor (a visible star). You notice they are being pulled in a circle, as if they are holding hands with an invisible partner. You can't see the partner, but you know they are there because of how the visible dancer is moving.

This is exactly what astronomers did in this paper. They found two binary star systems where one star is visible, and the other is a "ghost"—a massive, invisible object that we can't see directly.

How They Found the Ghosts

The team used two different "senses" to find these pairs, kind of like using both your eyes and your ears:

The "Eyes" (TESS Satellite): The TESS satellite watches stars for tiny dips in brightness. Usually, this happens when a planet passes in front of a star. But sometimes, a star gets stretched into a football shape by a nearby heavy partner, making it look brighter and dimmer as it spins. This is called ellipsoidal variation.
The "Ears" (Gaia Satellite): The Gaia satellite measures how stars wobble back and forth. If a star is dancing with a partner, it wobbles. Gaia measured the speed of this wobble (radial velocity).

By combining the "dance moves" (light curves) from TESS and the "wobble speed" from Gaia, the team filtered out thousands of false alarms (like stars that just have dark spots on them, like sunspots) and found two very promising candidates: J0144 and J2013.

The Investigation: What is the Invisible Partner?

Once they found the candidates, the team went to the "crime scene" with high-powered telescopes (the Seimei telescope in Japan) to take a close-up look at the visible stars. They analyzed the starlight like a detective analyzing fingerprints.

Here is what they discovered:

The Visible Dancers: Both visible stars are "G-type" stars (like our Sun, but slightly older and a bit puffed up, like a slightly bloated marshmallow). They are about the size of the Sun.
The Invisible Partners: By measuring how fast the visible stars were wobbly, they calculated the weight of the invisible partners.
- The invisible partner in J0144 weighs about 1.13 times the mass of our Sun.
- The invisible partner in J2013 weighs about 1.05 times the mass of our Sun.

The Big Question: What is heavy enough to weigh more than the Sun but invisible?

Is it a Black Hole? No. Black holes of that mass usually have different effects, and the math doesn't quite fit.
Is it a Normal Star? No. If it were a normal star that heavy, it would be glowing brightly. We would see it. Since we don't, it must be a "dead" star.
Is it a Neutron Star? Maybe, but those are usually even heavier and formed from violent explosions.
The Verdict: It is almost certainly a Massive White Dwarf.

The Analogy: Imagine a White Dwarf is like a diamond the size of the Earth, but with the weight of the Sun. It's incredibly dense. These two invisible partners are the "heavyweights" of the white dwarf world. They are so massive that they are on the edge of becoming something even more extreme (like a neutron star) if they gain a little more weight.

The Story of Their Past: The "Common Envelope"

How did these two get so close? The paper suggests they went through a dramatic phase called a Common Envelope.

Imagine a giant, fluffy star (the ancestor of the white dwarf) expanding like a balloon. It grew so big that it swallowed its smaller partner (the star we see today). Instead of crashing, the smaller star spiraled inside the giant's outer layers, acting like a brake. This friction slowed the giant down and squeezed the two stars closer together, eventually blowing off the giant's outer layers and leaving behind the dense, dead core (the white dwarf) and its partner, now dancing very close together.

What Happens Next? The Future is Uncertain

These two systems are currently "dormant." The invisible white dwarf isn't eating the visible star yet. But in the future, the visible star will evolve and expand.

Scenario A (The Feast): If the visible star expands and starts feeding the white dwarf, the white dwarf might get so heavy it explodes in a Type Ia Supernova (a cosmic explosion that lights up the whole galaxy).
Scenario B (The Collapse): If the white dwarf is made of specific heavy elements (Oxygen/Neon), it might collapse into a neutron star instead of exploding.
Scenario C (The Merge): If the orbit gets too tight, the two stars might crash into each other, creating a chaotic mess or a new type of exotic star.

Why Does This Matter?

Finding these two systems is like finding two missing puzzle pieces. For a long time, we knew massive white dwarfs existed, but we didn't know how many were hiding in close pairs with normal stars.

By finding J0144 and J2013, the astronomers proved that their "hunting method" (combining TESS and Gaia data) works. This means there are likely many more of these "invisible heavyweight" pairs out there waiting to be discovered. Understanding them helps us predict how stars die, how supernovas happen, and how the universe recycles its matter.

In short: They found two cosmic couples where the invisible partner is a super-dense, massive dead star, and they are waiting to see if these couples will end in a beautiful explosion or a dramatic collapse.

1. Problem Statement

The identification of "dormant" binary systems—those containing a compact object (white dwarf, neutron star, or black hole) that is not currently accreting mass and thus emits no X-rays—is a significant challenge in stellar astrophysics.

Detection Difficulties: Short-period dormant binaries ( $\lesssim 100$ $≲ 100$ days) are difficult to find because:
- Stellar companions can be hidden by bright giant stars.
- Rapidly rotating stars have broadened spectral lines, making radial velocity (RV) measurements difficult.
- Photometric variability caused by stellar spots often mimics or obscures the ellipsoidal modulation expected from close binaries, leading to false positives (e.g., "imposters" like the Unicorn system).
Scientific Gap: While large surveys like Gaia have identified long-period binaries ( $\sim 1$ year), characterizing short-period systems with massive white dwarf (WD) companions is crucial for understanding stellar evolution pathways, particularly those leading to Type Ia supernovae (SNe Ia) and accretion-induced collapse (AIC).

2. Methodology

The authors employed a multi-messenger approach combining photometric and astrometric data to filter candidates, followed by high-resolution spectroscopy for confirmation.

A. Sample Selection Strategy

Data Sources:
- TESS: High-cadence photometric light curves (Quick-Look Pipeline) to identify periodic variations.
- Gaia DR3: Astrometric data providing radial velocity amplitudes ( $rv\_amplitude\_robust$ ).
Selection Criteria:
- Pseudo-Mass Function ( $\tilde{f}$ ): A proxy for the spectroscopic mass function was constructed using the photometric period ( $P_{LC}$ ) and the robust RV amplitude ( $K \approx rv\_amplitude\_robust / 2$ ). The formula used was:
  $\tilde{f} = \frac{2 P_{LC}^2}{2\pi G} \left( \frac{rv\_amplitude\_robust}{2} \right)^3$
  Candidates were selected if $\tilde{f} > 0.1 M_{\odot}$ (assuming the visible star is a main-sequence star).
- Stellar Constraints: Targets were restricted to FGKM-type main-sequence (MS) stars ( $M < 1.5 M_{\odot}$ ) to ensure the unseen companion, if massive, would be the dominant mass component, reducing the likelihood of the companion being a hidden giant.
- Period Constraint: $P_{LC} < 10$ days.
Result: This yielded $\sim 600$ candidates in the northern hemisphere. Two systems, J0144 and J2013, were selected for follow-up due to their high pseudo-mass functions.

B. Observations and Analysis

Spectroscopy: High-resolution spectra ( $R \approx 65,000$ ) were obtained using the GAOES-RV spectrograph on the 3.8m Seimei telescope.
Spectral Analysis:
- Cross-Correlation Function (CCF): Confirmed both systems are single-lined spectroscopic binaries (SB1) with no detectable spectral features from the companion.
- Atmospheric Parameters: Derived using jaxspec (Bayesian spectral fitting) to determine $T_{eff}$ , $\log g$ , metallicity, and projected rotational velocity ( $v_{rot} \sin i$ ).
Stellar Parameters: Derived using jaxstar (isochrone fitting with MIST models) combining spectroscopic data with Gaia parallax and 2MASS photometry.
Orbital Dynamics:
- RV curves were fitted with circular orbit models using Bayesian inference (MultiNest).
- Inclination Constraint: Since ellipsoidal modulation was contaminated by stellar spots, the orbital inclination ( $i$ ) was constrained by assuming tidal synchronization between the stellar rotation and the orbit:
  $\sin i \approx \frac{P_{orb} (v_{rot} \sin i)_{spec}}{R_{MS}}$

3. Key Results

System J0144 + 5106

Visible Star: G-type MS star, $M \approx 0.89 M_{\odot}$ , slightly bloated ( $R \approx 1.14 R_{\odot}$ ).
Orbit: Period $P_{orb} = 1.37$ days, circular.
Companion Mass:
- Minimum mass ( $M_{c,min}$ ): $1.078^{+0.058}_{-0.060} M_{\odot}$ .
- Estimated mass (assuming synchronization): $1.13^{+0.11}_{-0.08} M_{\odot}$ .
Nature: The companion is too massive to be a typical WD but lacks spectral features of an MS star. It is identified as a massive O/Ne or C/O white dwarf.

System J2013 + 1734

Visible Star: G-type MS star, $M \approx 0.97 M_{\odot}$ , slightly bloated ( $R \approx 1.27 R_{\odot}$ ).
Orbit: Period $P_{orb} = 2.76$ days, circular.
Companion Mass:
- Minimum mass ( $M_{c,min}$ ): $0.943^{+0.048}_{-0.049} M_{\odot}$ .
- Estimated mass (assuming synchronization): $1.05^{+0.15}_{-0.10} M_{\odot}$ .
Nature: Similar to J0144, the companion is a massive white dwarf.

Photometric Behavior

Both systems showed significant contamination from stellar spots.
J0144: Light curve dominated by rotational modulation (spots), obscuring ellipsoidal variation.
J2013: Light curve showed a mix of ellipsoidal variation and spot modulation.
Spot Coverage: Estimated at 6–13% for J0144 and 1–2% for J2013, consistent with solar-type stars.

4. Significance and Evolutionary Implications

Post-Common Envelope Binaries (PCEBs): The short orbital periods and the masses of the components suggest these systems are PCEBs. The progenitor of the massive WD likely had a Zero-Age Main Sequence (ZAMS) mass of $6–8 M_{\odot}$ and initiated a common envelope phase while on the Asymptotic Giant Branch (AGB).
Future Evolution: The fate of these systems depends on the stability of the next mass transfer event as the visible MS star evolves:
- Stable Mass Transfer: Could lead to Cataclysmic Variables (CVs), Type Ia Supernovae (if the WD is C/O and accretion rates are high), or Accretion-Induced Collapse (if the WD is O/Ne).
- Unstable Mass Transfer: Could lead to a second common envelope phase, resulting in an ultra-compact binary (AM CVn system) or a merger.
Methodological Validation: The study validates the strategy of combining Gaia RV amplitudes with TESS photometry to identify dormant massive WDs, effectively filtering out false positives caused by stellar activity.
Population Statistics: The discovery of these two systems suggests that massive WDs in close binaries with MS stars are more common than previously thought, providing a new sample for testing binary evolution models and the progenitors of SNe Ia.

5. Conclusion

Shiraishi et al. successfully identified two dormant binary systems containing massive white dwarfs ( $\sim 1.1 M_{\odot}$ ) orbiting slightly bloated G-type main-sequence stars. By overcoming the challenges of stellar spot contamination through a combination of high-resolution spectroscopy and Bayesian modeling, the authors confirmed these systems as post-common envelope binaries. These findings offer critical constraints on the formation channels of massive white dwarfs and the potential progenitors of thermonuclear supernovae.