arXiv subjects

Leandro G. Althaus

Publications and source records attributed to Leandro G. Althaus.

At least 19 recordsLinked to original sources

arXiv2026

Circumstellar interaction in the extreme white dwarf merger remnant ZTF\,J1901+1458: A new class of white dwarf merger remnants with X-ray emission

Double degenerate white dwarf (WD) mergers can exhibit extreme magnetic fields exceeding $10^{8}$ G and rapid rotation, but their spectral-energy distributions and high-energy emission mechanisms remain poorly characterised. ZTF J1901+1458 stands out as the most compact and strongly magnetised object discovered in this class to date. Recent Chandra observations have revealed that the white dwarf is also a source of soft X-ray emission, inconsistent with a photospheric origin. We analyse new phase resolved UV spectroscopy from the HST combined with optical and near-infrared photometry and spectroscopy, with newly developed magnetic atmosphere models to determine its effective temperature, radius, mass, average surface magnetic field strength, and cooling age. Our results demonstrate that the spectral break at $\approx$3000 Å, observed in several highly magnetised WDs, is well-reproduced by our new models, which take into account the effect of magnetic opacities on the structure of the atmosphere. Our best-fit parameters for the WD yield an effective temperature ($T_{\rm{eff}}=28,015\pm 20$ K) and larger radius ($2630\pm10$ km) than previously reported. Furthermore, the near-infrared data exclude the presence of a stellar or brown dwarf companion hotter than $\approx$700 K. We also jointly analyse the previously published Chandra data and new XMM-Newton X-ray spectra. The faint X-ray emission, $L_X =(1.3\pm0.2)\times10^{27}$ erg/s is very soft and highly pulsed on the rotation period of the WD. We suggest that the X-rays are powered by accretion or via the interaction of the WD magnetosphere with CSM. If the rapidly rotating magnetic field could power a weak wind along open field lines, material could be extracted directly from the surface of the WD. Alternatively, accretion of fallback material from the merger or the tidal disruption of a planetary body are possible sources of CSM.

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arXiv2026

Unveiling the properties of pulsating low-mass helium-core white dwarfs through TESS asteroseismology I. First results

Recent space-based photometry, particularly from TESS, has opened new possibilities for probing the internal structure of low-mass (LM) helium (He)-core white dwarfs (WDs). We present a homogeneous asteroseismological analysis of six pulsating LM WD stars, based on new and updated TESS photometry. We processed short- and ultra-short-cadence TESS observations of TIC 290904838 (J1112), TIC 156064657, TIC 33717565, TIC 344130696, TIC 72637474, and TIC 188087204, and analyzed the resulting pulsation spectra. We then carried out a detailed asteroseismological analysis using fully evolutionary models of LM He-core WDs that allow for varying hydrogen (H)-envelope thicknesses. We also estimated spectroscopic/photometric stellar masses when atmospheric parameters are available. We report the first TESS-based frequencies for J1112 and provide revised or expanded frequency solutions for the remaining targets. The asteroseismological analysis yields relatively well-constrained solutions for three stars, a representative but more tentative solution for one target, and constrained ranges for the remaining two. The inferred solutions span a broad range of H-envelope thicknesses, although some of the asteroseismological inferences remain tentative because of the limited number of observed periods available for the analysis. For most objects, the derived spectroscopic/photometric stellar masses are broadly compatible with the asteroseismological values. This is the first homogeneous TESS-based asteroseismological study of a small sample of pulsating LM WDs. Our results suggest that LM WDs can harbor H envelopes with a range of thicknesses, from canonical (thick) to very thin, as in average-mass H-rich pulsating WDs. They also provide a useful reference point for future studies of larger samples, which will hopefully benefit from richer mode sets and improved mode identification.

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arXiv2026

Observing bright pulsating white dwarfs with PLATO: A new window into the late stages of stellar evolution

We present the scientific case for exploiting the capabilities of the PLATO mission to study bright pulsating white dwarfs across a wide spectral range, including hydrogen-deficient types (GW Vir and DBV stars) and hydrogen-rich classes (classical DAVs, pulsating extremely low-mass DA white dwarfs, and ultra-massive DA white dwarfs). PLATOs exceptional photometric precision, long-duration continuous monitoring, and extensive sky coverage promise transformative advances in white dwarf asteroseismology. Our key objectives include probing the internal structure and chemical stratification of white dwarfs, detecting secular changes in pulsation modes over extended timescales, and discovering rare or previously unknown classes of pulsators. To assess feasibility, we constructed a sample of 650 white dwarf candidates identified within PLATOs Southern LOPS2 field using the PLATO complementary science catalogue combined with Gaia DR3, and derived atmospheric parameters through photometric modeling. This sample comprises 118 DA white dwarfs (including 23 ZZ Ceti candidates), and 41 non-DAs (including 35 DBV candidates). Simulated observations using PlatoSim demonstrate that PLATO will be capable of detecting white dwarf pulsation modes with amplitudes as low as 0.1 mma depending on stellar magnitude, observation duration, pixel location, and the number of contributing cameras. We provide detailed detection limits and visibility forecasts for known pulsators across a representative range of these parameters. Furthermore, we emphasize strong synergies with Gaia astrometry, TESS photometry, and targeted spectroscopic campaigns, which together will enable robust mode identification and detailed stellar modeling. Collectively, these efforts will unlock unprecedented insights into white dwarf origins, evolution and internal physics, and the fate of their planetary systems.

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arXiv2026

Extreme mass loss during common envelope evolution: the origin of the double low-mass white dwarf system J2102--4145

Eclipsing close double white dwarf (WD) systems provide a unique opportunity to directly constrain hydrogen-envelope retention and test common-envelope (CE) evolution in low-mass stars, since they allow precise determinations of stellar masses and radii. We analyze J2102-4145, an eclipsing binary composed of two low-mass helium-core white dwarfs in a 2.4-hour orbit. By comparing the observed radii and effective temperatures with updated evolutionary models for CE evolution and stable Roche-lobe overflow (SRLOF), we confirm that both stars are helium-core white dwarfs. The primary, with a mass of 0.375 solar masses, is consistent with SRLOF models that retain thick hydrogen envelopes and sustain residual nuclear burning, whereas the secondary, with a mass of 0.314 solar masses, can only be reproduced by CE models in which the hydrogen envelope is almost completely removed. The inferred cooling ages (approximately 220 Myr for the secondary and between about 260 and 510 Myr for the primary, depending on the contribution of residual nuclear burning) support a formation sequence in which the primary formed first through SRLOF, followed by a CE phase that produced the compact secondary. Reconstruction of the CE energy budget yields progenitor and orbital parameters consistent with this evolutionary picture. The unusually small radius of the secondary requires an extremely thin hydrogen envelope, with a mass below about 10e-7 solar masses, well below the values predicted by standard bifurcation criteria. J2102-4145 therefore provides one of the strongest observational constraints on the hydrogen-envelope mass of post-CE low-mass white dwarfs and represents a benchmark challenge for current prescriptions of envelope ejection.

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arXiv2025

Asteroseismology of white dwarfs in the 2040s

White dwarfs, the final evolutionary stage of the vast majority of stars, serve as critical tools for cosmochronology, studies of planetary system evolution, and laboratories for non-standard physics, including exotic cooling channels and weakly interacting particles, as well as crystallization processes. Beyond surface properties accessible via spectroscopy and model atmospheres, global pulsations exhibited by white dwarfs during various evolutionary phases provide a direct window into their deep interiors. Asteroseismology, the comparison of observed pulsation periods with theoretical models, enables us to infer internal chemical stratification, total mass, rotation profiles, and magnetic field strengths. Despite major advances from space missions providing uninterrupted, high-precision photometry, key challenges remain: many predicted pulsators remain quiet, while others oscillate outside theoretical instability strips, highlighting gaps in our understanding of mode excitation, diffusion, and convective mixing. Determining the masses of white dwarfs, particularly for massive and hydrogen-deficient stars, remains uncertain, with discrepancies between spectroscopic, asteroseismic, astrometric, and photometric methods. In the coming decades, large-scale surveys combining high-precision space-based photometry with coordinated ground-based spectroscopic follow-up will dramatically increase both the number and quality of pulsating white dwarf observations.

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arXiv2025

The Galactic White Dwarf Population

The ESA Gaia mission has revolutionized our understanding of the white dwarf population, delivering an unprecedented census of these nearby remnants and revealing previously unseen structures in the Hertzsprung-Russell (HR) diagram. However, while Gaia has expanded the scope of white dwarf astrophysics, it has also exposed new questions related to atmospheric composition, spectral evolution, crystallization, magnetism, and merger-driven pathways. Many of these open problems are encoded in the detailed morphology of the Gaia HR diagram, where precise spectroscopic characterization is essential for interpreting the underlying physical processes. Spectroscopic characterization, obtainable with current and future ESO facilities, can provide the effective temperatures and surface gravities that are required to derive accurate white dwarf masses, cooling ages, and luminosities. These fundamental parameters not only enable studies of spectral evolution, interior physics, and the origin of magnetic and high-mass white dwarfs, but also guarantee the construction of robust mass distributions and luminosity functions, essential for constraining the initial-to-final mass relation, probing the initial mass function, and reconstructing the star formation history of the local Galaxy, among other applications. Looking toward the 2040s, future multi-fiber spectrographs operating in survey mode on 10--15 meter class telescopes will be able to collect a complete spectroscopic sample of white dwarf, enabling the detailed characterization of their population. Achieving spectroscopic completeness for the nearby Galactic population and securing high signal-to-noise, moderate-to-high resolution spectra across the HR diagram with ESO instrumentation will be critical steps toward resolving these longstanding questions in white dwarf astrophysics.

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arXiv2025

Post-common Envelope Evolution of Helium-core White Dwarfs

Helium-core white dwarfs (He WDs) formed through common envelope (CE) evolution offer valuable insight into binary interaction channels and compact remnant formation. Their cooling rates critically impact both detectability and age estimates in close binaries. Compared to He WDs formed via stable Roche-lobe overflow (SRLOF), those from the CE channel undergo markedly different mass-loss histories, resulting in distinct post-CE evolutionary behavior. We explore how the mass of the residual hydrogen envelope (Mh) shapes the cooling evolution of CE He WDs, focusing on the role of the bifurcation point in setting Mh and enabling residual hydrogen burning. Using the LPCODE stellar evolution code, we computed models of He WDs with masses from 0.20 to 0.42 solar masses, evolving from post-CE conditions to the white dwarf cooling track. Two evolutionary branches emerge: (i) non-flashing sequences, which cool rapidly with negligible hydrogen burning, and (ii) flashing sequences, where hydrogen shell flashes alter the envelope structure prior to cooling. Minimal-envelope models cool within 5-130 million years for effective temperatures between 12,000 and 27,000 K, and reach in approx. 300 million years at lower temperatures, remaining much younger than SRLOF counterparts. In contrast, models with more hydrogen retain active nuclear burning, delaying cooling and yielding ages of several billion years. Flashing sequences prolong the pre-white dwarf phase, though still shorter than in SRLOF evolution. The value of Mh also affects WD mass and surface gravity estimates, introducing systematic shifts with respect to SRLOF WDs. Our results show that CE He WDs follow distinct evolutionary paths, with important implications for interpreting the nature and fate of compact binaries hosting He WDs.

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arXiv2024

A second candidate magnetic helium core white dwarf and 3 other variable white dwarfs in the globular cluster NGC 6397

Using archival Hubble Space Telescope observations, we report the discovery of four variable low-mass white dwarfs ($0.18 \, M_\odot \leq M \leq 0.5 \,M_\odot$) in the globular cluster NGC 6397. One source exhibits a periodic optical modulation of $5.21 \pm 0.02$ hours, which we interpret as potentially due to the rotation of a magnetic helium core WD (He WD). This makes this candidate the second magnetic He WD in NGC 6397, and one of the few He WDs with a measured rotation period. The other three candidates show aperiodic variability, with a change in magnitude ranging from $\sim 0.11-0.6$. These discoveries highlight the importance of high-cadence photometric surveys in dense stellar environments. Follow-up spectroscopic observations are needed to confirm the nature of the variability of these systems.

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arXiv2024

Impact of current uncertainties in the 12C+12C nuclear reaction rate on intermediate-mass stars and massive white dwarfs

Recent determinations of the total rate of the 12C+12C nuclear reaction show non-negligible differences with the reference reaction rate commonly used in previous stellar simulations. In addition, the current uncertainties in determining each exit channel constitute one of the main uncertainties in shaping the inner structure of super asymptotic giant branch stars that could have a measurable impact on the properties of pulsating ultra-massive white dwarfs (WDs). We explore how new determinations of the nuclear reaction rate and its branching ratios affect the evolution of WD progenitors. We show that the current uncertainties in the branching ratios constitute the main uncertainty factor in determining the inner composition of ultra-massive WDs and their progenitors. We found that the use of extreme branching ratios leads to differences in the central abundances of 20Ne of at most 17%, which are translated into differences of at most 1.3 and 0.8% in the cooling times and size of the crystallized core. However, the impact on the pulsation properties is small, less than 1 s for the asymptotic period spacing. We found that the carbon burns partially in the interior of ultra-massive WD progenitors within a particular range of masses, leaving a hybrid CONe-core composition in their cores. The evolution of these new kinds of predicted objects differs substantially from the evolution of objects with pure CO cores. Differences in the size of the crystallized core and cooling times of up to 15 and 6%, respectively leading to distinct patterns in the period spacing distribution.

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arXiv2024

An analysis of spectroscopic, seismological, astrometric, and photometric masses of pulsating white dwarf stars

A central challenge in the field of stellar astrophysics lies in accurately determining the mass of isolated stars. However, for pulsating white dwarf (WD) stars, the task becomes more tractable due to the availability of multiple approaches such as spectroscopy, asteroseismology, astrometry, and photometry. The objective of this work is to compare the asteroseismological and spectroscopic mass values of WDs in detail and, in turn, to compare them with the masses derived using astrometric parallaxes/distances and photometry. Our analysis encompasses a selection of pulsating WDs with different surface chemical abundances that define the main classes of variable WDs. We calculated their spectroscopic masses, compiled seismological masses, and determined astrometric masses. We also derived photometric masses, when possible. Subsequently, we compared all the sets of stellar masses obtained through these different methods. To ensure consistency and robustness in our comparisons, we used identical WD models and evolutionary tracks across all four methods. The analysis suggests a general consensus among the these methods regarding the masses of pulsating WD with H-rich atmospheres, known as DAV or ZZ Ceti stars, especially for objects with masses below approximately $0.75 M_{\sun}$, although notable disparities emerge for certain massive stars. For pulsating WD stars with He-rich atmospheres, called DBV or V777 Her stars, we find that astrometric masses generally exceed seismological, spectroscopic, and photometric masses. Finally, while there is agreement among the sets of stellar masses for pulsating WDs with C-, O-, and He-rich atmospheres (designated as GW Vir stars), outliers exist where mass determinations by various methods show significant discrepancies.

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arXiv2024

Pulsating hydrogen-deficient white dwarfs and pre-white dwarfs observed with TESS VI. Asteroseismology of the GW Vir-type central star of the Planetary Nebula NGC 246

Significant advances have been achieved through the latest improvements in the photometric observations accomplished by the recent space missions, substantially boosting the study of pulsating stars via asteroseismology. The TESS mission has already proven to be of relevance for pulsating white dwarf and pre-white dwarf stars. We report a detailed asteroseismic analysis of the pulsating PG 1159 star NGC 246 (TIC3905338), the central star of the planetary nebula NGC 246, based on high-precision photometric data gathered by the TESS space mission. We reduced TESS observations of NGC 246 and performed a detailed asteroseismic analysis using fully evolutionary PG 1159 models computed accounting for the complete prior evolution of their progenitors. We constrained the mass of this star by comparing the measured mean period spacing with the average of the computed period spacings of the models and also employed the observed individual periods to search for a seismic stellar model. We extracted 17 periodicities from the TESS light curves from the two sectors where NGC246 was observed. All the oscillation frequencies are associated with g-mode pulsations, with periods spanning from ~1460 to ~1823s. We found a constant period spacing of $ΔΠ= 12.9$s, allowing us to deduce that the stellar mass is larger than ~0.87 Mo if the period spacing is assumed to be associated with l= 1 modes, and ~ 0.568 Mo if it is associated with l= 2 modes. The less massive models are more consistent with the distance constraint from Gaia parallax. Although we were not able to find a unique asteroseismic model for this star, the period-to-period fit analyses suggest a high-stellar mass ($\gtrsim$0.74 Mo) when the observed periods are associated with modes with l= 1 only, and both a high ($\gtrsim$ 0.74 Mo) and intermediate (~0.57 Mo) stellar mass when the observed periods are associated with modes with l= 1 and 2.

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arXiv2024

J0526+5934: a peculiar ultra-short period double white dwarf

Ultra-short period compact binaries are important sources of gravitational waves, which include e.g. the progenitors of type Ia supernovae or the progenitors of merger episodes that may lead to massive and magnetic single white dwarfs. J0526+5934 is an unresolved compact binary star with an orbital period of 20.5 minutes that belongs to this category. The visible component of J0526+5934 has been recently claimed to be a hot sub-dwarf star with a CO white dwarf companion. Our aim is to provide strong observational plus theoretical evidence that the primary star is rather an extremely-low mass white dwarf, although the hot subdwarf nature cannot be completely ruled out. We analyse optical spectra together with time-series photometry of the visible component of J0526+5934 to constrain its orbital and stellar parameters. We also employ evolutionary sequences for low-mass white dwarfs to derive independent values of the primary mass. From the analysis of our observational data, we find a stellar mass for the primary star in J0526+5934 of 0.26+-0.05 Msun, which perfectly matches the 0.237+-0.035 Msun independent measurement we derived from the theoretical evolutionary models. This value is considerably lower than the theoretically expected and generally observed mass range of hot subdwarf stars, but falls well within the mass limit values of extremely low-mass white dwarfs. We conclude J0526+5934 is the fifth ultra-short period detached double white dwarf currently known.

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arXiv2024

The impact of breathing pulses during core-helium burning on the core chemical structure and pulsations of hydrogen-rich atmosphere white dwarfs

Breathing pulses are mixing episodes that could develop during the core-helium burning phase of low- and intermediate-mass stars. The occurrence of breathing pulses is expected to bear consequences on the formation and evolution of white dwarfs, particularly on the core chemical structure, which can be probed by asteroseismology. We aim to explore the consequences of breathing pulses on the chemical profiles and pulsational properties of variable white-dwarf stars with hydrogen-rich envelopes, known as ZZ Ceti stars. We compute stellar models with masses of $1.0 M_{\odot}$ and $2.5 M_{\odot}$ in the zero-age main sequence, and evolve them through the core-helium burning phase to the thermal pulses on the asymptotic giant branch, and finally to advanced stages of white-dwarf cooling. We compare the chemical structure of the core of white dwarfs whose progenitors have experienced breathing pulses during the core-helium burning phase with the case in which breathing pulses have not occurred. We find that, when breathing pulses occur, the white-dwarf cores are larger and the central abundances of oxygen are higher than for the case in which the breathing pulses are suppressed, in line with previous studies. However, the occurrence of breathing pulses is not sufficient to explain the large cores and the excessive oxygen abundances that characterize recently derived asteroseismological models of pulsating white dwarfs. We find absolute differences of up to $\sim 30$ seconds when we compare pulsation periods of white dwarfs coming from progenitors that have experienced breathing pulses with the case in which the progenitors have not suffered breathing pulses.

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arXiv2023

Asteroseismological analysis of the polluted ZZ Ceti star G29-38 with TESS

G\,29$-$38 (TIC~422526868) is one of the brightest ($V=13.1$) and closest ($d = 17.51$\,pc) pulsating white dwarfs with a hydrogen-rich atmosphere (DAV/ZZ Ceti class). It was observed by the {\sl TESS} spacecraft in sectors 42 and 56. The atmosphere of G~29$-$38 is polluted by heavy elements that are expected to sink out of visible layers on short timescales. The photometric {\sl TESS} data set spans $\sim 51$ days in total, and from this, we identified 56 significant pulsation frequencies, that include rotational frequency multiplets. In addition, we identified 30 combination frequencies in each sector. The oscillation frequencies that we found are associated with $g$-mode pulsations, with periods spanning from $\sim$ 260 s to $\sim$ 1400 s. We identified %three distinct rotational frequency triplets with a mean separation $δν_{\ell=1}$ of 4.67 $μ$Hz and a quintuplet with a mean separation $δν_{\ell=2}$ of 6.67 $μ$Hz, from which we estimated a rotation period of about $1.35 \pm 0.1$ days. We determined a constant period spacing of 41.20~s for $\ell= 1$ modes and 22.58\,s for $\ell= 2$ modes. We performed period-to-period fit analyses and found an asteroseismological model with $M_{\star}/M_{\odot}=0.632 \pm 0.03$, $T_{\rm eff}=11\, 635\pm 178$ K, and $\log{g}=8.048\pm0.005$ (with a hydrogen envelope mass of $M_{\rm H}\sim 5.6\times 10^{-5}M_{\star}$), in good agreement with the values derived from spectroscopy. We obtained an asteroseismic distance of 17.54 pc, which is in excellent agreement with that provided by {\sl Gaia} (17.51 pc).

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arXiv2023

General relativistic pulsations of ultra-massive ZZ Ceti stars

Ultra-massive white dwarf stars are currently being discovered at a considerable rate, thanks to surveys such as the {\it Gaia} space mission. These dense and compact stellar remnants likely play a major role in type Ia supernova explosions. It is possible to probe the interiors of ultra-massive white dwarfs through asteroseismology. In the case of the most massive white dwarfs, General Relativity could affect their structure and pulsations substantially. In this work, we present results of relativistic pulsation calculations employing relativistic ultra-massive ONe-core white dwarf models with hydrogen-rich atmospheres and masses ranging from $1.29$ to $1.369 M_{\odot}$ with the aim of assessing the impact of General Relativity on the adiabatic gravity ($g$)-mode period spectrum of very-high mass ZZ Ceti stars. Employing the relativistic Cowling approximation for the pulsation analysis, we find that the critical buoyancy (Brunt-Väisälä) and acoustic (Lamb) frequencies are larger for the relativistic case, compared to the Newtonian case, due to the relativistic white dwarf models having smaller radii and higher gravities for a fixed stellar mass. In addition, the $g$-mode periods are shorter in the relativistic case than in the Newtonian computations, with relative differences of up to $\sim 50$ \% for the highest-mass models ($1.369 M_{\odot}$) and for effective temperatures typical of the ZZ Ceti instability strip. Hence, the effects of General Relativity on the structure, evolution, and pulsations of white dwarfs with masses larger than $\sim 1.29 M_{\odot}$ cannot be ignored in the asteroseismological analysis of ultra-massive ZZ Ceti stars.

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arXiv2023

Carbon-oxygen ultra-massive white dwarfs in general relativity

We employ the La Plata stellar evolution code, LPCODE, to compute the first set of constant rest-mass carbon-oxygen ultra-massive white dwarf evolutionary sequences for masses higher than 1.29 Msun that fully take into account the effects of general relativity on their structural and evolutionary properties. In addition, we employ the LP-PUL pulsation code to compute adiabatic g-mode Newtonian pulsations on our fully relativistic equilibrium white dwarf models. We find that carbon-oxygen white dwarfs more massive than 1.382 Msun become gravitationally unstable with respect to general relativity effects, being this limit higher than the 1.369 Msun we found for oxygen-neon white dwarfs. As the stellar mass approaches the limiting mass value, the stellar radius becomes substantially smaller compared with the Newtonian models. Also, the thermo-mechanical and evolutionary properties of the most massive white dwarfs are strongly affected by general relativity effects. We also provide magnitudes for our cooling sequences in different passbands. Finally, we explore for the first time the pulsational properties of relativistic ultra-massive white dwarfs and find that the period spacings and oscillation kinetic energies are strongly affected in the case of most massive white dwarfs. We conclude that the general relativity effects should be taken into account for an accurate assessment of the structural, evolutionary, and pulsational properties of white dwarfs with masses above 1.30 Msun.

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arXiv2023

A hidden population of white dwarfs with atmospheric carbon traces in the Gaia bifurcation

The ESA Gaia space mission has revealed a bifurcation of the white dwarf (WD) sequence on the color magnitude diagram in two branches: A and B. While the A branch consists mostly of WDs with H-rich atmospheres, the B branch is not completely understood. Although invoked to be populated mainly by He-rich WDs, the B branch overlaps a $\sim 0.8M_\odot$ evolutionary track with a pure He envelope, fact that would imply an unexpected peak in the WD mass distribution. In cold He-rich WDs, it is expected that the outer convective zone penetrates into deep C-rich layers, thus leading to a slight C contamination in their surfaces at $\sim 10,000$K. Here we aim at studying the Gaia bifurcation as the natural consequence of C dredge-up by convection in cold He-dominated WDs. Relying on accurate atmosphere models, we provide a new set of evolutionary models for He-rich WDs employing different prescriptions for the C enrichment. On the basis of these models, we made a population synthesis study of the Gaia 100pc WD sample to constrain the models that best fit the bifurcation. Our study shows that He-rich WD models with a slight C contamination below the optical detection limit can accurately reproduce the Gaia bifurcation. We refer to these stars as stealth DQ WDs because they do not exhibit detectable C signatures in their optical spectra, but the presence of C in their atmosphere produces a continuum absorption favouring the emission in bluer wavelengths, thereby creating the B branch of the bifurcation. Also, we show that the mass distribution for He-rich WDs obtained when a stealth C contamination is considered is consistent with the mass distribution for H-rich WDs and with the standard evolutionary channels for their formation. We conclude that stealth DQ WDs can account for the lower branch in the Gaia bifurcation. The C signatures of these stars could be detectable in Ultra-Violet spectra.

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arXiv2023

The "canonical" White Dwarf Cooling Sequence of M5

Recently, a new class of white dwarfs (dubbed ``slowly cooling WDs'') has been identified in two globular clusters (namely M13 and NGC 6752) showing a horizontal branch (HB) morphology with an extended blue tail. The cooling rate of these WDs is reduced by stable thermonuclear hydrogen burning in their residual envelope, and they are thought to be originated by stars that populate the blue tail of the HB and then skip the asymptotic giant branch phase. Consistently, no evidence of such kind of WDs has been found in M3, a similar cluster with no blue extension of the HB. To further explore this phenomenon, we took advantage of deep photometric data acquired with the Hubble Space Telescope in the near-ultraviolet and investigate the bright portion of the WD cooling sequence in M5, another Galactic globular cluster with HB morphology similar to M3. The normalized WD luminosity function derived in M5 turns out to be impressively similar to that observed in M3, in agreement with the fact that the stellar mass distribution along the HB of these two systems is almost identical. The comparison with theoretical predictions is consistent with the fact that the cooling sequence in this cluster is populated by canonical (fast cooling) WDs. Thus, the results presented in this paper provide further support to the scenario proposing a direct causal connection between the slow cooling WD phenomenon and the horizontal branch morphology of the host stellar cluster.

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