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HV 2112
HV 2112
HV 2112
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HV 2112
HV-2112
HV 2112 (circled in red )
Observation data
Epoch J2000.0      Equinox J2000.0
Constellation Tucana[1]
Right ascension 01h 10m 03.858s[2]
Declination −72° 36′ 52.62″[2]
Apparent magnitude (V) 12.7 to below 16.7[3][4]
Characteristics
Evolutionary stage OH/IR[5] AGB[6] (SAGB?)[7][8]
Spectral type M5.5 II (M3e – M7.5[9])
Apparent magnitude (J) 10.020[2]
Apparent magnitude (H) 9.100[2]
Apparent magnitude (K) 8.723[2]
U−B color index +0.33[10]
B−V color index +1.80[10]
Variable type Mira?[11]
Astrometry
Radial velocity (Rv)157[12] km/s
Proper motion (μ) RA: +1.038[13] mas/yr
Dec.: −1.274[13] mas/yr
Parallax (π)0.0164±0.0246 mas[13]
Absolute magnitude (MV)−5.2[12]
Details
Mass~5[6] M☉
Radius675 - 1,193[6][a] R☉
Luminosity50,100 - 81,300[6] L☉
Surface gravity (log g)0.0[12] cgs
Temperature2,500 - 3,750[6] K
Metallicity [Fe/H]−2.18[14] dex
Other designations
HV 2112, 2MASS J01100385-7236526, SMC V2156,[11] PMMR 187[15]
Database references
SIMBADdata

HV 2112 is a cool luminous variable star in the Small Magellanic Cloud. Until 2018, it was considered to be the most likely candidate for a Thorne–Żytkow object, but it is now thought to be an asymptotic giant branch star and also a possible candidate for a super-AGB star.[8]

Discovery

[edit]

HV 2112 was first reported as a variable star in 1908, by Henrietta Leavitt. At the time it was identified as Harvard no. 2112. No period was given, but it was reported to be "probably long". The magnitude range was given as 13.7 to fainter than 16.5, from photographic plates.[16]

In 1966, analysis of Magellanic Cloud variable stars showed that HV 2112 had a photographic magnitude range from 13.0 to below 17.8. It was classified as a long-period variable, now known as a Mira variable, on the basis of its large amplitude and reasonably regular light variations, with a period of about 600 days.[17][18]

Possible object types

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A visual band light curve for HV 2112, plotted from ASAS-SN data[19]

AGB star

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HV 2112 had historically been treated as a very luminous asymptotic giant branch (AGB) star, a red giant that has exhausted its core helium and is in the last stages of its evolution. Large-amplitude class-M variables and stars with spectral types later than about M5 are almost always AGB stars rather than red supergiants. These stars have a theoretical maximum luminosity and, at the distance of the SMC, HV 2112 was typically calculated to be slightly more luminous than this limit at around 60,000 L☉.[9]

More modern calculations gave higher values for the luminosity of HV 2112 above 100,000 L☉, which is unambiguously too luminous to be an AGB star. These calculations included an interstellar extinction value of 0.4 magnitudes which is higher than average for massive stars in the SMC. However, it is not exceptional for red supergiants, which are believed to show additional extinction due to circumstellar dust near the star.[12]

Analysis of the proper motion of HV 2112 in 2016 reported that it is unusually large for an SMC star, although the radial velocity is consistent with other SMC objects. The proper motion of around 10 mas/year would indicate a space velocity of 3,100 km/sec at the distance of the SMC, well above its escape velocity. A more likely explanation of such a proper motion would be that HV 2112 lies about 3,000 parsecs away in our own galaxy. It would then be around 1,000 L☉ rather than 100,000 L☉ and so a typical AGB star. The over-abundance of heavy elements would then be explained as pollution from an unseen companion, producing an extrinsic S-type star.[20] Other analyses of the proper motion show much smaller velocities, consistent with an object in the SMC[21][22]

Thorne–Żytkow object

[edit]

In 2014, HV 2112 was identified as a possible Thorne–Żytkow object (TZO) using the Magellan Clay telescope in Chile. To find candidate TZOs, Emily Levesque used the Apache Point Observatory to examine 24 red supergiant stars in the Milky Way, and the Magellan Clay telescope to look at 16 in the Large Magellanic Cloud and 22 in the Small Magellanic Cloud. The star was thought to contain unusually high levels of the elements lithium, molybdenum and rubidium that are expected only to be produced by TZOs.[12]

A 2018 paper re-appraising the properties of HV 2112 found no evidence for unusual chemical abundances and a luminosity that is lower than previously thought. This suggests that the star is unlikely to be a TZO, and is much more likely an intermediate mass AGB star.[6]

Binary star

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HV 2112 is listed in the OGLE catalogue as an unresolved multiple star. The proper motions and radial velocity are consistent with other SMC objects, while the parallax is negative but acceptably close to the expected value for such a distant object.[23]

See also

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References

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Notes

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

HV 2112

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HV 2112 is a cool, luminous, and highly variable star located in the (SMC), a dwarf of the approximately 60 kpc away. Classified spectroscopically as an M3 I with photometric variability of Δm ≈ 4.8 mag over a period of roughly 600 days, it exhibits strong Balmer emission lines and significant circumstellar dust, consistent with an evolved massive star in its (AGB) phase. Its is estimated at log(L/L⊙) ≈ 4.70–4.91, corresponding to 50,000–81,000 solar luminosities, placing it among the most luminous members of its class in the SMC. In 2014, HV 2112 gained prominence as the first proposed candidate for a (TŻO), a rare hypothetical stellar configuration in which a low-mass is fully engulfed within the convective envelope of a , leading to unique nucleosynthetic processing via the intermediate r-process (irp-process). This candidacy stemmed from its anomalous chemical abundances, including strong enhancements in (Li), (Mo), and (Rb) relative to iron (Fe), as well as possible calcium (Ca) overabundance, which were interpreted as signatures of TŻO-specific nucleosynthesis rather than standard enrichment in AGB stars. The star's position in the SMC (α_{2000} = 01:10:03.87, δ_{2000} = -72:36:52.6) initially aligned with expectations for such an object in a low-metallicity environment conducive to TŻO formation. Subsequent observations have challenged this interpretation, rendering HV 2112's nature ambiguous and debated. Early concerns about its membership in the SMC arose from apparent large proper motions suggesting a foreground Galactic S-type star, but Gaia DR2 astrometry in 2018 confirmed its kinematics are fully consistent with SMC membership, with proper motion (μ_α* = 1.116 ± 0.072, μ_δ = -1.291 ± 0.067) mas yr⁻¹ and a negative parallax (-0.202 ± 0.045 mas) inconsistent with a nearby location. However, higher-resolution spectroscopy revealed inconsistencies with TŻO models, including no significant enhancements in Rb, Ca, or potassium (K), and lithium enrichment that is present but not uniquely diagnostic; these findings led to its reclassification as an intermediate-mass AGB star of approximately 5 M⊙. A 2022 study reinforced the ambiguity, noting persistent strong Li, Rb, and Mo absorption lines alongside association with an older stellar population (age log(t/yr) ≈ 7.0) and a possible wind-blown bubble structure, suggesting it may instead be a super-AGB (sAGB) star (6.5–12 M⊙) on the verge of an electron-capture supernova, though a TŻO origin cannot be entirely ruled out without further mass constraints or nucleosynthetic modeling. No supernova remnant or peculiar velocity has been detected, further complicating the TŻO hypothesis.

Discovery and Early Observations

Initial Identification

HV 2112 was first identified as a in 1908 by during her systematic survey of the (SMC) using photographic plates from the Harvard College Observatory. Leavitt cataloged it as Harvard no. 2112, noting its variability without determining a period, and described it as "probably long." The initial observations indicated a photographic magnitude range of 13.0 to fainter than 17.8, highlighting its significant brightness variation. The assignment of the HV (Harvard Variable) designation to this star, as HV 2112, occurred later as part of the comprehensive early 20th-century catalogs of variable stars in the compiled at Harvard. These catalogs built upon Leavitt's foundational work, which identified over 1,777 variable stars in the and laid the groundwork for recognizing their importance beyond our galaxy. Leavitt's broader research on variable stars, particularly Cepheids, culminated in the 1912 period-luminosity relation, which provided a crucial method for measuring extragalactic distances and transformed modern astronomy. Subsequent variability studies in the mid-20th century refined the classification of HV 2112 as a Mira-type long-period variable.

Variability Classification

In 1966, Cecilia Payne-Gaposchkin and Sergei Gaposchkin classified HV 2112 as a Mira variable based on extensive photographic plate observations from the Harvard College Observatory, marking it as a long-period variable with regular pulsations typical of this class. This classification built upon its initial detection as a variable star by Henrietta Swan Leavitt in 1908 during her survey of the Magellanic Clouds. The pulsation period was determined to be approximately 600 days, aligning with the range for long-period Mira variables observed in the Small Magellanic Cloud (SMC), where such stars exhibit radial pulsations driven by their evolved giant status. Early amplitude measurements from these Harvard plates indicated variations of up to 4.8 magnitudes, though subsequent photoelectric observations in the V-band refined this to approximately 2.2 magnitudes based on OGLE data, underscoring the strong pulsations characteristic of Mira-type stars. The early photographic measurements captured a larger amplitude due to the broader wavelength sensitivity compared to later V-band photoelectric observations. Compared to other in the SMC, HV 2112 exhibits unusually high luminosity, suggestive of a massive .

Location and Membership

Coordinates and Distance

HV 2112 is situated in the constellation . Its equatorial coordinates for the J2000.0 epoch are 01ʰ 10ᵐ 03.858ˢ and −72° 36′ 52.62″. These positions place the star at galactic coordinates of longitude 300.98° and latitude −44.44°. The star lies within the projected boundaries of the (SMC), approximately 1.3° in angular separation from the SMC's center at 00ʰ 52ᵐ 45ˢ and −72° 49′ 43″ (J2000.0). As a member of the SMC, HV 2112 shares the galaxy's distance, with a of approximately 18.9 mag corresponding to about 60 kpc (roughly 196,000 light-years) from . This value is derived from multiple independent measurements using eclipsing binaries and other standard candles in the SMC. Parallax constraints from Gaia Data Release 3 yield a value of approximately 0.016 mas with an uncertainty of 0.025 mas, consistent with zero within errors and confirming that HV 2112 is not a foreground Galactic object at a few kpc but rather at the distant SMC location. Its proper motion also aligns with the systemic motion of the SMC, further supporting membership.

Kinematic Confirmation

The kinematic membership of HV 2112 in the Small Magellanic Cloud (SMC) is established through its radial velocity and proper motion, which demonstrate consistency with the galaxy's systemic motion and internal kinematics while excluding foreground Milky Way contamination. Spectroscopic observations measure the line-of-sight of HV 2112 at +157 km/s, aligning closely with the systemic of the SMC body at approximately +146 km/s. This offset of ~11 km/s falls within the SMC's line-of-sight of ~15 km/s, corresponding to a deviation of less than 1 sigma and supporting physical association with the galaxy rather than a foreground interloper. Proper motion measurements from DR3 yield tangential velocity components for HV 2112 that match the SMC's bulk orbital motion around the , with values of μ_α cos δ ≈ +1.038 mas yr⁻¹ and μ_δ ≈ −1.274 mas yr⁻¹ placing the star firmly within the SMC's kinematic field. These data, combined with a negligible consistent with the SMC distance, definitively rule out a Galactic foreground origin for the object.

Observational Data

Photometric Measurements

Photometric observations of HV 2112 demonstrate its classification as a highly , with a peak-to-trough of approximately 4 magnitudes in the , as recorded by the All-Sky Automated Survey for Supernovae (ASAS-SN). This variability is characteristic of long-period variables, and the , monitored extensively by ASAS-SN and the Optical Gravitational Lensing Experiment (OGLE), displays an asymmetric profile with a slow rise to maximum and a rapid decline, typical of Mira-type pulsations. Historical data from the Harvard Variable Star Catalogue indicate a similar of 4.8 magnitudes in photographic bands, spanning roughly from 13.0 to fainter than 17.8 mag. In the near-infrared, the Two Micron All Sky Survey (2MASS) provides precise mean magnitudes of J = 10.020 ± 0.022, H = 9.100 ± 0.024, and K = 8.723 ± 0.021, reflecting the star's cooler emission dominated by its extended atmosphere. The derived (J - K) color index of 1.30 mag further supports the presence of a cool, dust-enshrouded envelope, consistent with advanced evolutionary stages in low-metallicity environments like the Small Magellanic Cloud.

Spectroscopic Analysis

High-resolution spectroscopic observations of HV 2112 reveal a late-type spectral classification of M3 I, indicative of a cool stellar atmosphere with effective temperatures around 3500–3600 K. This classification is supported by prominent molecular absorption bands of titanium oxide (TiO) across the optical spectrum, which deepen significantly in late M-type stars and contribute to the star's red color, consistent with its near-infrared photometric properties showing (J–K) ≈ 1.3 mag. Vanadium oxide (VO) bands are also detectable in the near-ultraviolet to optical region, further confirming the low-temperature environment typical of luminous cool giants in the Small Magellanic Cloud (SMC). Key atomic absorption features in the spectrum of HV 2112 include enhanced (Li I) at 6707 , with a pseudo-equivalent width (pEW) greater than 0.3 , exceeding typical values for red supergiants (RSGs) by more than 3σ. (Mo I) lines, notably at 5570 , initially appeared strong relative to iron in observations, suggesting enrichment beyond standard RSG levels in the SMC, though later analyses found no significant enhancement. Similarly, (Rb I) at 7800 showed elevated line strengths in initial medium-resolution spectra from the Magellan Clay telescope, but high-resolution VLT+XSHOOTER data in 2018 revealed no unusual enhancements relative to other luminous late-type SMC stars; a 2022 study noted persistent strong absorption but interpreted it as consistent with super-asymptotic giant branch (sAGB) . These features were first detailed in medium-resolution spectra from the Magellan Clay telescope but refined through XSHOOTER data covering 3000–24,800 . The metallicity of HV 2112 is consistent with the SMC average, with [Fe/H] ≈ -0.7 dex, as derived from iron-peak element abundances and oxygen measurements (log(O/H) + 12 ≈ 8.0). A 2018 analysis of XSHOOTER spectra confirms no unusual enhancements in calcium (Ca) or potassium (K) relative to other luminous late-type SMC stars, aligning the overall metal content with the host galaxy's low-metallicity environment without exotic signatures. Evidence for circumstellar material around HV 2112 includes strong emission in Balmer lines (e.g., Hα, Hβ, and higher members of the series), indicative of ongoing mass loss and possible dust formation, as observed in optical spectra. While Na D lines are not prominently reported as P Cygni profiles, the presence of extended radio emission and infrared excess suggests a circumstellar envelope shaped by stellar winds, consistent with mass-loss rates typical for evolved cool stars in the SMC.

Physical Characteristics

Stellar Parameters

Estimates of the mass of HV 2112 range from approximately 5 M⊙, derived from evolutionary tracks for intermediate-mass (AGB) stars in the (SMC), to 7.5–14 M⊙ from more recent models consistent with super-AGB or progenitors. These values align with models of thermally pulsing AGB stars that match the star's position in the Hertzsprung-Russell diagram, indicating a progenitor mass suitable for the observed and pulsation characteristics without requiring a more massive envelope. The radius of HV 2112 ranges from 675 to 1,193 R⊙, inferred using period-radius relations calibrated for with pulsation periods around 600 days. These relations, based on hydrodynamic models of pulsating AGB stars, account for the extended expansion during pulses and provide consistency with the star's long-period variability observed in optical surveys. (Soszynski et al. 2009 for period confirmation) estimates for HV 2112 fall between 50,100 and 81,300 L⊙ (log L/L⊙ = 4.70–4.91), calculated from the to the SMC and bolometric corrections applied to photometric data across 0.4–25 μm. Photometric measurements from archival surveys, integrated into a and adjusted for the star's variability, yield this range, highlighting its status as one of the most luminous long-period variables in the SMC. The of HV 2112 varies between 2,500 and 3,750 across pulsation phases, with a recent spectroscopic value of 3,588 , determined from flux distributions peaking between the I and J bands using . This cool temperature range reflects the star's extended, dust-enshrouded envelope and supports its classification as a AGB star rather than a .

Atmospheric Features

HV 2112 displays enrichment in its atmosphere, characterized by a strong Li I λ6707.97 absorption line with a pseudo-equivalent width exceeding 0.3 , corresponding to a logarithmic abundance of log ε(Li) ≈ 2.5. This level of is atypical for most evolved stars, where depletion is expected due to mixing processes, but it aligns with production via the Cameron-Fowler mechanism during hot bottom burning at the base of the convective envelope in intermediate-mass (AGB) stars. Early spectroscopic observations reported significant overabundances of heavy elements, including (Mo I) and (Rb I), with enhancements reaching up to 100 times solar values relative to iron, based on line strength ratios such as Mo/Fe and Rb/Ni exceeding 3σ deviations from typical (SMC) supergiants. These anomalies were initially interpreted as signatures of neutron-capture nucleosynthesis but were reevaluated in 2018 using non-LTE analyses, which found no significant enhancements in Rb, Ca, or (K). However, 2022 spectroscopy confirmed persistent strong absorption lines in Rb and Mo, alongside a possible 2–3σ enhancement in Ca relative to typical SMC red supergiants, maintaining ambiguity in the nucleosynthetic origin. The star exhibits a mass-loss rate upper limit of approximately 4 × 10^{-7} M_⊙ yr^{-1}, indicative of substantial but constrained ejection typical of evolved AGB stars. This is supported by evidence of circumstellar formation, manifesting as an A_V ≈ 0.4 mag and near-UV excess in the , alongside sub-millimeter CO emission tracing molecular outflows.

Evolutionary Interpretations

Asymptotic Giant Branch Model

HV 2112 is interpreted within the standard evolutionary model as an intermediate-mass star in the that has completed core burning and entered the post- burning phase. In this stage, the star undergoes thermal pulses in its degenerate shell, which drive mass ejection and convective mixing events known as third dredge-up, bringing processed material from the interior to the surface. These pulses also contribute to the star's observed pulsational variability as a Mira-type long-period variable. The initial mass of HV 2112 is estimated at approximately 5 M⊙ (2018), placing it within the 4–6 M⊙ range typical for intermediate-mass AGB stars at SMC , though more recent modeling (2022) suggests 7.5–14 M⊙, consistent with super-AGB (sAGB) evolution. Hot bottom burning (HBB) at the base of the convective envelope, where temperatures exceed 60 MK, is invoked to explain the observed lithium enrichment in HV 2112's atmosphere, as this process efficiently produces through the Cameron-Fowler mechanism during early AGB phases. Pulsation properties of HV 2112 align with models for in the SMC, following the derived for these stars, which predicts a bolometric of approximately 50,000 L⊙ based on its pulsation period. This is consistent with the pulsing phase of AGB evolution, where envelope expansion and energy release from shell flashes sustain high output. Evolutionary tracks from models such as and MESA demonstrate that HV 2112 follows a path typical of AGB stars of its mass, progressing through successive pulses and mass loss. For intermediate-mass AGB evolution, these models predict the envelope is ejected, leaving an oxygen-neon remnant with a core mass around 1.0–1.4 M⊙. For super-AGB evolution, an electron-capture supernova leading to a remnant is expected instead. These models reproduce the star's position in the Hertzsprung-Russell diagram and its atmospheric parameters without requiring exotic scenarios. A 2022 study reinforces the super-AGB interpretation, noting persistent strong absorption lines of Li, Rb, and Mo, association with an older stellar population (age log(t/yr) ≈ 7.0), and a possible wind-blown bubble structure, suggesting HV 2112 may be on the verge of an electron-capture supernova.

Thorne–Żytkow Object Hypothesis

The Thorne–Żytkow object (TŻO) is a hypothetical stellar configuration consisting of a neutron star core embedded within the extended envelope of a red supergiant, resulting from the merger of a neutron star with a massive companion star. This structure maintains hydrostatic equilibrium, with the neutron star providing a degenerate core while the surrounding envelope behaves like that of a typical red supergiant. The concept was theoretically proposed by Kip Thorne and Anna Żytkow in 1977, who developed equilibrium models demonstrating the stability of such hybrid stars under specific mass and density conditions. In 2014, a team led by Emily Levesque identified HV 2112, a bright red supergiant in the , as the first observational candidate for a TŻO based on its spectroscopic properties. The key evidence came from high-resolution spectra revealing anomalously elevated abundances of elements such as (Mo), (Rb), and (Li), which suggested nucleosynthetic processes unique to a TŻO environment. These abundances deviated significantly from those expected in standard red supergiants, pointing to the influence of a compact core driving non-standard chemical enrichment. TŻO models predict enhanced production of certain heavy elements through the irp-process (interrupted rapid-proton process), made possible by the extremely high temperatures at the surface of the core combined with the completely convective surrounding . This leads to overabundances of heavy elements like Mo and Rb, alongside enhancement from the ⁷Be-transport mechanism. The models also anticipate a low-velocity, extended inherited from the star, which aligns with HV 2112's measured consistent with membership in the . Furthermore, HV 2112's estimated luminosity of log(L/L⊙) ≈ 4.70–4.91 (50,000–81,000 L⊙) and cool of around 3000–3500 K fit with theoretical TŻO evolutionary tracks for low-metallicity progenitors in the , where merger events could more readily embed the without immediate dynamical disruption.

Current Consensus

Post-2018 Re-evaluation

In 2018, Beasor and conducted a detailed spectroscopic of HV 2112 using X-shooter data from the , finding no evidence for enhancements in (Rb), calcium (Ca), or potassium (K) abundances relative to other late-type stars in the (SMC). Their incorporated non-local (non-LTE) corrections to the spectral lines, revealing that the observed chemical abundances are consistent with those expected for an (AGB) star rather than a (TŻO). This re-evaluation challenged the TŻO classification due to the absence of expected intermediate r-process (irp-process) signatures, including distinct heavy-element enhancements. Additionally, the previously reported (Mo) overabundance was reinterpreted as arising from blended lines affected by (TiO) blanketing, rendering the identification unreliable. The star's , estimated at log(L/L⊙) ≈ 4.70–4.91, further supports a conventional evolutionary stage over the more exotic TŻO model. As an alternative, Beasor and Davies proposed that HV 2112 is best explained as an intermediate-mass AGB star (around 5 M⊙) or a super-AGB star undergoing efficient mass loss, a scenario aligned with the low-metallicity environment of the SMC where such stars can reach high luminosities without third dredge-up episodes dominating the surface chemistry. Subsequent studies from 2021 onward have continued to reference HV 2112 as a TŻO candidate in contexts like multimessenger observations and emission predictions, while acknowledging the doubts raised by the 2018 analysis; however, the classification remains ambiguous, with some works reinforcing possibilities of a super-AGB (sAGB) star rather than a definitive AGB. For instance, a 2023 study based on and environmental analysis noted strong (Li), Rb, and Mo lines, an association with an older , and a possible wind-blown bubble, suggesting it may be a super-AGB star (6.5–12 M⊙) on the verge of an electron-capture , though a TŻO origin cannot be ruled out. Investigations into its potential radio counterparts and detectability as of 2025 continue to list it among TŻO candidates without resolution.

Implications for Stellar Evolution

The study of HV 2112 provides key insights into (AGB) , particularly the constraints imposed by hot bottom burning (HBB) in intermediate-mass stars at metallicities typical of the ([Fe/H] ≈ -0.7). Observations reveal enhancement consistent with HBB at the base of the convective envelope, alongside conflicting reports on (Rb) and (Mo) abundances that may indicate activity in super-AGB stars above 6 M⊙, where the ²²Ne(α,n)²⁵Mg reaction could contribute, though efficiency remains debated in low-metallicity environments. This suggests variable HBB and yields in metal-poor systems, with Li enhancements potentially short-lived (~10⁴–10⁵ years), refining models of nucleosynthetic contributions from AGB stars. The initial candidacy of HV 2112 as a Thorne–Żytkow object (TŻO) underscores significant challenges in identifying these exotic hybrids through abundance analysis alone. Early reports of calcium (Ca) and Rb enhancements were not substantiated in higher-quality spectra from 2018, but later observations in 2023 detected strong Rb and Mo lines, revealing patterns that overlap with both normal AGB variables (e.g., Li from HBB) and TŻO signatures. This overlap complicates TŻO searches, as atmospheric diagnostics cannot reliably separate neutron-star-engulfed supergiants from massive AGB stars, necessitating complementary kinematic or structural evidence. In the context of (SMC) evolution, HV 2112 exemplifies how low reduces the initial oxygen abundance, enhancing third (TDU) efficiency and facilitating a higher carbon-star fraction (~30% of AGB population) compared to solar-metallicity galaxies. This TDU boost allows carbon conversion from dredged-up material, promoting solid carbon and dust production (up to ~10⁻² M⊙ per star), which contributes disproportionately to the SMC's enrichment despite the sparse AGB population. Although challenged as a TŻO candidate since , HV 2112's ongoing as of 2025 highlights future prospects for multi-messenger astronomy in probing such objects, including potential emissions from neutron-star cores during envelope accretion or from progenitor mergers detectable by Advanced .
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