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Two Be or Not Two Be: A New Companion detection for HD 52244 using HST/FGS

Using the Hubble Space Telescope's Fine Guidance Sensors during a 2021 operational survey of Be stars, researchers detected a new companion to HD 52244 with a separation of approximately 74 AU, establishing a foundation for future investigations into wide or third companions of Be stars.

Original authors: Keefe J. Kamp, Saida M. Caballero-Nieves, Edmund P. Nelan, Nancy Remage Evans, Douglas R. Gies, Noel D. Richardson

Published 2026-03-30

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CC BY 4.0

Original authors: Keefe J. Kamp, Saida M. Caballero-Nieves, Edmund P. Nelan, Nancy Remage Evans, Douglas R. Gies, Noel D. Richardson

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 you are looking at a crowded dance floor from a very high balcony. Most of the dancers are spinning wildly on their own, but some are actually dancing in pairs, holding hands and spinning together. In the world of astronomy, these "dancers" are stars, and the "pairs" are binary systems.

This paper is about a team of astronomers who used a special tool to find a hidden dance partner for a star named HD 52244.

Here is the story of how they did it, explained simply:

1. The "Be" Stars: The Spin Doctors

First, let's talk about the stars they were looking for. They focused on a specific group called Be stars.

The Analogy: Imagine a figure skater spinning so fast that they start to fling their arms out, creating a ring of ice around them. Be stars are like that. They spin so fast that they fling off a disk of gas and dust around them.
The Mystery: Scientists have long wondered: What makes them spin so fast? One leading theory is that they got a "push" from a binary companion (a partner star) stealing their energy. But to prove this, we need to find these partners. The problem is, these stars are often dim, variable (they flicker), and their gas disks make them look blurry, making it hard to spot a partner hiding nearby.

2. The Accidental Opportunity: The "Side-B" Mode

In late 2021, the Hubble Space Telescope (HST) had a bit of a glitch. A crucial computer module that usually controls all the fancy cameras broke.

The Analogy: Think of Hubble as a high-end smartphone. Suddenly, the main camera app stopped working. But, the phone still had a basic "selfie camera" (the Fine Guidance Sensor, or FGS) that was still functional.
The Pivot: Instead of letting the telescope sit idle, the team decided to use this "selfie camera" to take a quick survey of six Be stars. The FGS is usually used just to keep the telescope pointed steadily at a target, but it's actually a super-precise instrument capable of seeing tiny details.

3. The Detective Work: The "S-Curve"

How do you find a twin star when they are so close together that they look like a single dot of light?

The Analogy: Imagine you are looking at a lighthouse through a foggy window. If there is only one lighthouse, the light pattern is smooth. But if there are two lighthouses very close together, the light waves crash into each other, creating a wavy, interference pattern.
The Tool: The FGS creates a graph that looks like a sideways "S" (called an S-curve).
- If the star is alone, the "S" is smooth and predictable.
- If there is a companion star, the "S" gets distorted, like a smooth road that suddenly has a bump in it.

4. The Big Discovery: HD 52244

The team scanned six stars. Five of them looked like solo dancers; their "S-curves" were smooth.

The Breakthrough: One star, HD 52244, had a bumpy "S-curve." It was screaming, "I'm not alone!"
The Evidence: By running complex math (comparing the bumpy curve to a smooth one), they confirmed there was a second star hiding just 42.7 milliarcseconds away.
- To visualize that distance: If you held a human hair at arm's length, that separation is roughly the width of that hair seen from that distance.
The Partner: The companion is about half as bright as the main star and is located about 74 Astronomical Units away (that's 74 times the distance from the Earth to the Sun). It would take this pair about 110 years to orbit each other once.

5. Why This Matters

This discovery is a big deal for a few reasons:

It's New: No one knew HD 52244 had a partner before.
It Proves the Theory: Finding a companion supports the idea that these "spin doctors" (Be stars) might get their speed from a partner.
The Method Works: It shows that even when a telescope's main cameras are broken, its "guidance sensors" can still do serious science. It's like finding a diamond in a broken watch.

The Bottom Line

The astronomers used a backup tool on the Hubble Space Telescope to catch a Be star in the act of dancing with a partner. They found that HD 52244 isn't a solo act after all; it has a companion star orbiting it far away. This helps us understand how massive stars are born, how they spin, and how they interact with their cosmic neighbors.

In short: They fixed a broken telescope by using its "backup camera," found a hidden star partner, and proved that even the fastest-spinning stars might have a dance partner they never told us about.

1. Problem Statement

Classical Be stars are B-type stars characterized by hydrogen emission lines (primarily H $\alpha$ ) caused by circumstellar decretion disks, often resulting from near-critical rotation. While binary interactions are a leading theory for spinning up these stars, the multiplicity fraction of Be stars remains poorly constrained compared to other stellar populations.

The Gap: Existing studies on Be star binarity have been limited by scope and methodology. Spectroscopic surveys are often biased against wide-orbit companions, and previous interferometric or speckle studies have found low binary fractions (e.g., 8–10%), which contradicts trends seen in O and G stars where multiplicity is higher.
The Challenge: Be stars exhibit intrinsic variability and are often dimmer than O stars, making it difficult to disentangle binary signatures in light curves and spectra. Furthermore, the presence of circumstellar disks complicates astrometric solutions (e.g., inflating Gaia RUWE values), making it difficult to distinguish between disk-induced anomalies and actual binary companions.
Objective: To probe the multiplicity of Be stars at high angular resolution to detect wide or third companions that spectroscopic methods miss, specifically filling the gap in period distribution for massive stars.

2. Methodology

The study utilized a unique observational opportunity during Hubble Space Telescope (HST) Cycle 29. Due to a failure in the Science Instrument Command and Data Handling (SI C&DH) module, the telescope's science instruments were temporarily unavailable, but the Fine Guidance Sensors (FGS) remained operational.

Instrument & Mode: The team used FGS2 in Transfer (TRANS) Mode. The FGS is a white-light shearing interferometer capable of resolving binaries with angular separations down to $\sim$ 10 milli-arcseconds (mas).
Sample: Six Be stars were observed (HD 33232, HD 35345, HD 50737, HD 52244, HD 239618, HD 206135). Targets were selected to be fainter than magnitude 8 to avoid saturating the photomultiplier tubes without requiring filter wheel moves (which were avoided to protect aging instruments).
Observations:
- Observations used the F583W filter (central wavelength 5830 Å).
- Data consisted of "S-curves" (interferograms) generated by scanning the telescope at a 45° angle.
- 15 scans per target were co-added and smoothed.
Data Analysis & Detection Techniques:
1. Calibration: S-curves were rectified to remove photomultiplier sensitivity variations. HD 50737 was used as a point-source calibrator due to its similar color (B-V) to the target HD 52244.
2. STScI Pipeline (Least Squares Fit): A FORTRAN/C code fitted a model S-curve (superposition of two point sources) to the observed data to derive separation ( $\rho$ ) and differential magnitude ( $\Delta m$ ).
3. Cross-Correlation Function (CCF): Sensitive to larger separations ( $\gtrsim$ 25 mas), this method compares the target S-curve against a calibrator to identify residuals indicating a companion.
4. Second-Derivative Test: Sensitive to smaller separations ( $\lesssim$ 25 mas). It mathematically analyzes the difference between the binary and single-star S-curves, which is proportional to the second derivative of the calibrator curve.
5. Disk Exclusion: The team calculated the expected angular size of the circumstellar disk for HD 52244 ( $\sim$ 0.88 mas) and confirmed it was well below the FGS resolution limit, ensuring the detected signal was due to a companion star, not the disk.

3. Key Contributions

Operational Innovation: Successfully repurposed the HST FGS for a multiplicity survey during a period of instrument downtime, demonstrating the utility of FGS TRANS mode for high-contrast binary detection.
Methodological Rigor: Applied a multi-pronged detection approach (Least Squares, CCF, and Second Derivative tests) to robustly confirm a companion detection and rule out false positives from circumstellar disks or instrumental noise.
Gaia RUWE Context: Provided a critical analysis of Gaia DR3 Renormalized Unit Weight Error (RUWE) values for Be stars, highlighting that standard binary thresholds (RUWE > 1.4) are unreliable for Be stars due to disk-induced astrometric noise, and proposed a higher threshold (RUWE > 2.5) for disk-hosting stars.

4. Results

Primary Detection (HD 52244):
- Status: A new companion was detected around the Be star HD 52244 (Spectral Class B2IVnpe).
- Separation: $\rho = 42.7 \pm 1.1$ mas (projected physical separation of $\sim$ 74 AU at a distance of 1734 pc).
- Position Angle: $144.2 \pm 0.3^\circ$ (East of North).
- Brightness: Differential magnitude in F583W filter is $\Delta m = 1.91 \pm 0.02$ mag.
- Characterization: Assuming the primary is on the main sequence, the companion is estimated to be an early B8 V star ( $\sim$ 3.38 $M_\odot$ ). The total system mass is $\sim$ 10.68 $M_\odot$ .
- Orbital Parameters: Assuming a circular, face-on orbit, the estimated orbital period is $\sim$ 110 years.
Non-Detections:
- The other five stars in the sample (HD 33232, HD 35345, HD 50737, HD 239618, HD 206135) showed no evidence of companions with separations $>10$ mas (9–13 AU).
- The study noted that while some targets had proper motion anomalies or high RUWE values in Gaia data, these were likely due to circumstellar disks or sub-stellar companions below the detection limit of this survey.

5. Significance

Filling the Multiplicity Gap: This study provides crucial data on the multiplicity of massive B/Be stars at wide separations, a regime often missed by spectroscopic surveys.
Angular Momentum Transfer: The detection of a wide companion supports the theoretical framework (Tokovinin 1997) where angular momentum is transferred from a close binary interaction to a wide third companion, potentially explaining the rapid rotation of Be stars.
Instrument Validation: The paper validates the HST/FGS as a powerful tool for high-angular-resolution astrometry, capable of resolving binaries that other HST instruments (like WFC3 or ACS) cannot (which are limited to $\sim$ 200 mas).
Future Directions: The work lays the groundwork for combining FGS data with other high-resolution surveys (e.g., Gemini Speckle) to better constrain the true multiplicity fraction of Be stars and understand the role of binaries in stellar evolution.