🔭 astrophysics

Millimeter-wave Detections of Symbiotic Stars in SPT and ACT Data

This paper presents a joint millimeter-wave survey of 828 candidate symbiotic stars using South Pole Telescope and Atacama Cosmology Telescope data, resulting in the detection of 31 objects with multi-wavelength light curves and spectral energy distributions to characterize their emission mechanisms and binary system properties.

Original authors: C. Tandoi, A. Foster, T. J. Maccarone, A. J. Anderson, B. Ansarinejad, M. Archipley, L. Balkenhol, D. R. Barron, K. Benabed, A. N. Bender, B. A. Benson, F. Bianchini, L. E. Bleem, S. Bocquet, F. R. Bo

Published 2026-05-05

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CC BY 4.0

Original authors: C. Tandoi, A. Foster, T. J. Maccarone, A. J. Anderson, B. Ansarinejad, M. Archipley, L. Balkenhol, D. R. Barron, K. Benabed, A. N. Bender, B. A. Benson, F. Bianchini, L. E. Bleem, S. Bocquet, F. R. Bouchet, E. Camphuis, M. G. Campitiello, J. E. Carlstrom, J. Carron, C. L. Chang, P. M. Chichura, A. Chokshi, T. -L. Chou, A. Coerver, T. M. Crawford, C. Daley, T. de Haan, K. R. Dibert, M. A. Dobbs, M. Doohan, D. Dutcher, C. Feng, K. R. Ferguson, N. C. Ferree, K. Fichman, S. Galli, A. E. Gambrel, A. K. Gao, F. Ge, F. Guidi, S. Guns, N. W. Halverson, E. Hivon, G. P. Holder, W. L. Holzapfel, J. C. Hood, A. Hryciuk, N. Huang, T. Jhaveri, F. Keruzore, A. R. Khalife, L. Knox, K. Kornoelje, C. -L. Kuo, K. Levy, Y. Li, A. E. Lowitz, C. Lu, G. P. Lynch, A. S. Maniyar, E. S. Martsen, F. Menanteau, M. Millea, J. Montgomery, Y. Nakato, T. Natoli, A. Ouellette, Z. Pan, P. Paschos, K. A. Phadke, A. W. Pollak, K. Prabhu, W. Quan, M. Rahimi, A. Rahlin, C. L. Reichardt, M. Rouble, J. E. Ruhl, A. C. Silva Oliveira, A. Simpson, J. A. Sobrin, A. A. Stark, J. Stephen, C. Trendafilova, J. D. Vieira, A. G. Vieregg, A. Vitrier, Y. Wan, N. Whitehorn, W. L. K. Wu, M. R. Young, J. A. Zebrowski

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, bustling city. Most of the "citizens" are single stars, living quiet lives. But some stars are couples, locked in a cosmic dance where one is a bloated, cool giant (like a red giant) and the other is a tiny, super-hot compact object (like a white dwarf). These couples are called Symbiotic Stars. They are fascinating because the giant star is constantly shedding its skin (mass), and the hot partner is greedily eating it up, creating a messy, glowing cloud of gas and dust around them.

For a long time, astronomers have tried to find these couples using optical telescopes (like giant eyes looking at visible light). But the Milky Way is crowded and dusty, like a city smoggy with fog. This "fog" often hides these star couples, making them hard to spot.

The Big Idea: Listening with "Radio Ears"
This paper is about a new way to find these star couples. Instead of looking with eyes (optical light), the authors used two giant telescopes, the South Pole Telescope (SPT) and the Atacama Cosmology Telescope (ACT). These telescopes are designed to look at the "afterglow" of the Big Bang (the Cosmic Microwave Background), but they are also excellent at listening to the universe in millimeter waves.

Think of millimeter waves like a special kind of radio signal. While the "fog" of dust blocks visible light, it is mostly transparent to these radio signals. It's like trying to see a lighthouse through a thick fog; you can't see the beam, but if you have a radio receiver tuned to the lighthouse's frequency, you can hear it clearly.

The Search
The team took a "shopping list" of 828 potential star couples (candidates) from a database called NODSV. They pointed their telescopes at these specific locations and asked: "Are you there? Are you glowing in millimeter waves?"

The Results: Finding the Hidden Gems
Out of the 828 candidates, they successfully "heard" 31 unique star couples.

18 were confirmed as genuine symbiotic stars.
13 were suspected (they looked like the real thing, but needed more proof).

They found that these stars glow in millimeter waves for two main reasons:

The Hot Gas: The hot partner ionizes the gas around it, creating a "free-free" emission (think of it as the gas humming a specific radio tune).
The Cold Dust: The giant partner is covered in a thick coat of dust. This dust gets warm and glows like a blackbody radiator (like a toaster heating up).

The "Slow Transient" Surprise
One of the most exciting discoveries involves a star called CN Cha.

The Story: This star had a massive explosion (a nova) around 2012/2013.
The Mystery: In 2013, a space telescope called Herschel looked at this star and saw almost nothing. But when the authors looked at the same star in 2024 using SPT and ACT, it was blazing bright.
The Analogy: Imagine a firework that explodes. You expect the smoke to be visible immediately. But in this case, the "smoke" (the millimeter glow) took over a decade to appear and get bright. The paper suggests the explosion happened, but the dust and gas took a long time to heat up and start glowing in radio waves. This makes CN Cha a rare "slow transient"—a cosmic event that reveals itself very slowly.

Why This Matters
The paper shows that using these powerful telescopes (originally built for cosmology) is a fantastic way to find these star couples, especially the ones hidden behind dust.

They found that the "dusty" types of star couples (D-types) are much easier to spot in these radio waves than the "clean" types (S-types), because the dust itself is what makes the radio signal.
They provided a detailed "ID card" for every star they found, including how bright they are, how far away they are, and how they have changed over time.

In Summary
This paper is a successful "treasure hunt." By using telescopes that listen to radio waves instead of looking at visible light, the team found 31 hidden star couples in our galaxy. They discovered that these stars often glow brightly in radio waves due to their dusty coats and hot gas, and they caught a rare glimpse of a star that took years to "turn on" its radio signal after a massive explosion. This proves that looking at the universe through "radio ears" is a powerful tool for finding the hidden, dusty couples of the Milky Way.

Problem
Symbiotic stars (SySts) are interacting binary systems consisting of a cool giant donor and a hot compact accretor (typically a white dwarf). While the population of confirmed SySts is growing, identification remains challenging due to confusion with other objects like planetary nebulae (PNe) and H II regions, particularly in the crowded and highly extincted Galactic Plane. Traditional identification relies on optical spectroscopy (e.g., Raman-scattered O vi lines) and infrared spectral energy distributions (SEDs), but these methods are often time-consuming or impossible for highly reddened sources. Furthermore, theoretical estimates suggest the Milky Way contains between $1.2 \times 10^4$ and $4 \times 10^5$ SySts, far exceeding the few hundred currently confirmed. There is a need for alternative detection methods that can probe high-extinction regions and distinguish SySts from imposters using multi-wavelength data.

Methodology
The authors conducted a targeted search for candidate symbiotic stars using millimeter-wave (mm) data from two Cosmic Microwave Background (CMB) experiments: the South Pole Telescope (SPT-3G) and the Atacama Cosmology Telescope (ACT DR6).

Candidate Selection: The search targeted 828 candidate SySts from the New Online Database of Symbiotic Variables (NODSV) located within the SPT and ACT footprints, covering most of the southern hemisphere up to a declination of +20°.
Data Processing: The team performed forced photometry on the candidate locations in SPT and ACT maps at three frequency bands: 95, 150, and 220 GHz.
- For SPT, they used coadded maps from the Ext-10k survey and the Galactic Plane survey, applying transient-detection pipelines and flux density corrections.
- For ACT, they utilized "depth-1" maps from the Advanced ACTpol instrument, applying matched filtering and forced photometry using the pixell software.
Detection Criteria: A source was considered detected if it showed a significance greater than $3\sigma$ in at least two of the three frequency bands. Additional cuts were applied to remove extragalactic contaminants, non-point sources, and misidentified objects.
Multi-wavelength Analysis: The study incorporated external datasets, including Gaia DR3 for astrometry and distance estimation, 2MASS and WISE for infrared photometry, and optical light curves from ASAS-SN and ATLAS. Spectral Energy Distributions (SEDs) and light curves were constructed to analyze variability and emission mechanisms.
Special Case Analysis: The authors specifically analyzed CN Cha using Herschel SPIRE data from 2013 to compare with their 2024 SPT data, investigating millimeter variability during a nova event.

Key Contributions

Detection of 31 Unique Systems: The study successfully detected 31 unique SySts (18 confirmed and 13 suspected) out of the 828 candidates. This includes 15 detections in SPT data and 23 in ACT data, with seven objects detected by both.
Comprehensive Catalog: The paper provides a detailed catalog of these detections, including coadded flux measurements, light curves from 2016–2026 across mm, optical, and infrared bands, and SEDs.
Data Release: The authors release coadded thumbnails and light curves for all 828 candidate SySts (including non-detections) to facilitate future analysis and threshold adjustments.
Reclassification and Characterization: The study reclassified CN Cha and R Aqr from S-type to D-type based on literature and data, and provided updated distance estimates and physical parameters for the detected systems.

Results

Detection Rates: The detection rate was significantly higher for D-type (dusty) and D'-type SySts (52.2% and 33.3% of detected candidates, respectively) compared to S-type (stellar) SySts (2.4%). This aligns with the expectation that dusty systems emit more strongly in the mm regime due to thermal dust emission. Suspected SySts were detected at lower rates than their confirmed counterparts.
Emission Mechanisms: The mm emission is primarily a combination of optically thick blackbody emission from cool dust components and free-free (FF) emission from the ionized region. The spectral indices generally agree with models where D/D'-types show a mix of optically thin FF ( $\alpha \approx -0.1$ ) and blackbody emission ( $\alpha \approx 2$ ).
CN Cha as a Millimeter Slow Transient: A significant finding involves CN Cha, which underwent a symbiotic nova in 2012/2013. Herschel SPIRE data from 2013 (near the start of the optical rise) placed strict upper limits on mm flux, while SPT data from 2024 shows a massive increase in flux. This indicates a strong variability and a significant lag in the mm rise compared to the optical rise, marking CN Cha as a rare Galactic mm slow transient.
Synchrotron Re-evaluation: The paper re-examines the non-outbursting SySt H 1-36. While previous studies suggested synchrotron emission was responsible for mm excess, the authors suggest that source variability and large measurement uncertainties in older data may explain the apparent excess without requiring a significant synchrotron component.
Distance and Variability: The study highlights that geometric distances derived from Gaia parallax may have systematic uncertainties for binaries with specific orbital periods, particularly for S-type systems like AG Peg.

Significance
The paper demonstrates that CMB experiments like SPT and ACT are powerful tools for Galactic astronomy, capable of detecting and characterizing symbiotic stars in regions of high extinction where optical spectroscopy fails. The results confirm that mm-wave observations are particularly sensitive to dusty (D- and D'-type) systems, offering a complementary method to optical surveys for growing the SySt population. By providing multi-wavelength light curves and SEDs, the study enables the testing of binary models and offers insights into the geometry and physical properties of these systems, including mass loss and accretion processes. The detection of CN Cha as a mm slow transient underscores the potential of these surveys to capture rare evolutionary phases of symbiotic novae. The authors conclude that future CMB surveys (e.g., Simons Observatory) and upgrades to existing instruments will further enhance the detection of Galactic mm transients and SySts, particularly in the crowded Galactic Center.

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