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Spectroscopy of a sample of RV Tauri stars without IR excess

This study analyzes high-resolution optical spectra of 11 RV Tauri stars lacking IR excess to identify chemical depletion patterns, revealing that only V457 Cyg shows depletion and likely hosts a circumbinary disc, while V894 Per is reclassified as an eclipsing binary with sodium overabundance due to mass transfer.

Original authors: Karlis Pukitis, Karina Korenika

Published 2026-07-23

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CC BY 4.0

Original authors: Karlis Pukitis, Karina Korenika

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 universe as a giant, cosmic recycling plant. In this factory, stars are the workers that forge heavy elements like gold and iron, but sometimes, they get messy. When a star like our Sun runs out of fuel and swells up into a giant, it can puff off its outer layers, creating a dusty cloud around itself. This is where the story gets interesting: some of these dying stars, known as RV Tauri variables, seem to have a peculiar "diet." They are missing certain ingredients—specifically, elements that love to stick together and form dust grains. Scientists call this "chemical depletion." It's like walking into a bakery and finding the flour and sugar missing, but the eggs and butter are still there. Usually, this happens because the star is sipping up gas from a dusty disc around it, leaving the dust behind. But here's the mystery: some of these stars don't seem to have a dusty disc anymore. So, did they lose the disc? Did they never have one? Or is there a hidden trick going on? Understanding these stars helps us figure out how binary star systems (pairs of stars dancing together) evolve and how they interact with the cosmic dust they create.

In this study, two astronomers from Latvia, Karlis Putis and Karina Korenika, decided to take a closer look at 11 of these "disc-less" RV Tauri stars using powerful telescopes. Their goal was to act like cosmic detectives, peering into the starlight to see if any of these stars still showed signs of that missing-ingredient diet, even without a visible dusty cloud. They grabbed high-resolution snapshots of the stars' light (spectra) and measured the strength of the dark lines in the light, which act like fingerprints for different chemical elements.

Out of the 11 stars they studied, they focused their detailed analysis on five: HD 172810, V399 Cyg, AA Ari, V457 Cyg, and V894 Per. The results were a mix of "nothing special," "very interesting," and "wait, that's not what we thought."

First, three of the stars—HD 172810, V399 Cyg, and AA Ari—turned out to be fairly normal in terms of their chemical makeup. They didn't show the missing-ingredient pattern. HD 172810 had a slight extra amount of heavy elements, but the others looked pretty much like standard stars, just a bit older.

Then, they found a star that really stood out: V457 Cyg. This star was a smoking gun. Its chemical fingerprint showed clear signs of depletion; it was missing the dust-loving elements, just like the stars with dusty discs. Even though V457 Cyg doesn't have a dusty disc right now, the authors suggest it likely used to have one. It's probably a binary system where the star once ate from a disc that has since evaporated or disappeared. Adding to the mystery, this star's light showed emission lines (glowing lines instead of dark ones) from metals and even molecules called TiO, which is a rare and exciting sight. It seems this star is a binary system that once had a circumbinary disc, and the chemical leftovers are still visible on its surface.

Finally, there was V894 Per, which turned out to be a case of mistaken identity. When the astronomers looked at its light, they saw two sets of spectral lines moving at different speeds. It's as if they were looking at a star, but then realized there was a second, invisible star dancing right next to it, messing up the view. This suggests V894 Per isn't a standard RV Tauri star at all, but rather an eclipsing binary system where one star passes in front of the other. The light from this second star made the chemical analysis tricky, but one thing was clear: this star had a huge overabundance of sodium. The authors suggest this salty taste might be the result of the two stars swapping material (mass transfer) while they orbit each other.

In short, this paper tells us that even without a visible dusty disc, some RV Tauri stars still carry the scars of one, like V457 Cyg. It also reminds us that the universe loves to play tricks, and sometimes what looks like a single pulsating star is actually a binary pair in disguise, like V894 Per. By studying these stars, we are slowly piecing together the complex history of how stars and their surrounding discs live, die, and interact.

Technical Summary: Spectroscopy of a Sample of RV Tauri Stars without IR Excess

Problem and Motivation
RV Tauri stars are post-asymptotic giant branch (post-AGB) or post-red giant branch (post-RGB) variables characterized by alternating deep and shallow minima in their light curves. A defining feature of many of these objects is "depletion," a photospheric abundance pattern where elements with high dust condensation temperatures are systematically underabundant. This phenomenon is generally attributed to the accretion of gas from a dusty circumbinary disc, which also produces an infrared (IR) excess. While depletion is typically observed in stars with IR excess, a subset of RV Tauri stars lacks this IR signature. The evolutionary status of these IR-excess-free objects remains ambiguous; they may be single, low-luminosity post-AGB stars or binary post-RGB stars where the disc has already dispersed. Identifying depletion in stars without current IR excess is crucial, as it provides evidence that a disc existed at an earlier evolutionary stage, offering insights into the longevity and dispersal mechanisms of such structures.

Methodology
The authors conducted high-resolution optical spectroscopy of 11 RV Tauri stars lacking IR excess, selected from a previous compilation based on brightness and lack of prior high-resolution study. Observations were performed using the FIES spectrograph at the Nordic Optical Telescope and the VUES spectrograph at the Molėtai Astronomical Observatory.

The study focused on the initial analysis of five specific targets: HD 172810, V399 Cyg, AA Ari, V457 Cyg, and V894 Per. The analysis involved:

Data Reduction: Spectra were reduced using automatic pipelines and normalized via spline fitting.
Radial Velocity and Phase: Radial velocities were estimated via cross-correlation of absorption lines. Light curves from the ASAS-SN survey were used to determine the pulsation phase at the time of observation, though the RV Tauri nature of some targets (HD 172810, AA Ari) could not be confirmed due to problematic light curves.
Atmospheric Parameters: Photospheric parameters ( $T_{\text{eff}}$ , $\log g$ , $V_{\text{mic}}$ ) were derived using the standard excitation and ionization balance method for iron lines (Fe I and Fe II), employing the SPECTRUM code and ATLAS model atmospheres.
Abundance Analysis: Chemical abundances were calculated using equivalent widths (EWs) of absorption lines and the VALD atomic database.

Key Results
The analysis yielded distinct findings for the five stars:

HD 172810, V399 Cyg, and AA Ari: These stars showed no evidence of chemical depletion.
- HD 172810 exhibited a slight overabundance of neutron-capture elements ( $[n/H] \approx 0.4\text{--}0.5$ ) but solar abundances for lighter elements.
- V399 Cyg displayed a uniform underabundance of approximately $-1.5$ dex for most elements, excluding oxygen and magnesium.
- AA Ari presented broad spectral lines (FWHM $\approx 28$ km/s), leading to high uncertainties in abundance calculations, though titanium and vanadium appeared overabundant relative to iron.
V457 Cyg: This star is the primary discovery of the study.
- Depletion Signature: The abundance pattern indicates depletion, with $[Zn/Ti] = 0.67$ and $[S/Ti] = 1.18$.
- Spectral Features: The spectrum shows numerous weak metal emission lines (vanadium and titanium) and TiO molecular lines in emission, alongside broad absorption lines (FWHM $\approx 23$ km/s).
- Interpretation: The authors conclude V457 Cyg is likely a binary system that was once surrounded by a circumbinary disc which has since dissipated. The surface composition reflects accretion from this former disc. However, the high abundances of silicon and magnesium suggest the star's initial composition was $\alpha$ -enhanced, complicating the simple depletion model.
V894 Per:
- Binary Nature: The spectrum is contaminated by a second set of absorption lines shifted by $\approx 73$ km/s, indicating the presence of a companion.
- Classification: The authors suggest V894 Per is likely an eclipsing variable rather than an RV Tauri star.
- Abundance: The derived abundances are uncertain due to contamination, but a significant sodium overabundance ( $[Na/H] \approx 1.6$ ) is observed, potentially resulting from mass transfer within the binary system.

Significance and Claims
The paper claims that V457 Cyg is the fourth known RV Tauri star without IR excess to exhibit chemical depletion, joining SS Gem, AZ Sgr, and EQ Cas. This finding supports the hypothesis that these stars possessed circumbinary discs in the past, which have since dispersed. The detection of depletion in the absence of current IR excess provides a critical link in understanding the evolutionary timeline of post-AGB/post-RGB binaries and the dispersal mechanisms of their surrounding discs.

Furthermore, the study highlights the complexity of interpreting abundance patterns in these evolved stars. In V457 Cyg, the coexistence of depletion signatures with $\alpha$ -enhanced elements challenges simple depletion models, suggesting that initial stellar composition and potential FIP (First Ionization Potential) effects must be considered. The identification of V894 Per as a likely eclipsing binary rather than an RV Tauri variable underscores the necessity of high-resolution spectroscopy to correctly classify these objects and avoid misinterpretation of their evolutionary status.

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