arXiv subjects

Alberto Rebassa-Mansergas

Publications and source records attributed to Alberto Rebassa-Mansergas.

At least 37 records · Page 2Linked to original sources

arXiv2025

Discovery of the richest pulsating ultra-massive white dwarf

The discovery of pulsations in ultra-massive white dwarfs can help to probe their interiors and unveil their core composition and crystallized mass fraction through asteroseismic techniques. To date, the richest pulsating ultra-massive white dwarf known is BPM 37093 with 8 modes detected, for which detailed asteroseismic analysis has been performed in the past. In this work, we report the discovery of 19 pulsation modes in the ultra-massive white dwarf star WD~J0135+5722, making it the richest pulsating hydrogen-atmosphere ultramassive white dwarf known to date. %In this work, we report the discovery of pulsations in WD~J0135+5722, a hydrogen-rich ultra-massive white dwarf observed with the Gran Telescopio Canarias. This object exhibits multi-periodic luminosity variations with periods ranging from 137 to 1345 s, typical of pulsating white dwarfs in the ZZ Ceti instability strip, which is centered at $T_{\rm eff} \sim 12\,000$ K. We estimate the stellar mass of WD J0135+5722 by different methods, resulting in $M_{\star} \sim 1.12-1.14 M_{\odot}$ if the star's core is made of oxygen and neon, or $M_{\star} \sim 1.14-1.15 M_{\odot}$ if the star hosts a carbon oxygen core. Future analysis of the star periods could shed light on the core chemical composition through asteroseismology.

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arXiv2024

Main sequence dynamo magnetic fields emerging in the white dwarf phase

Recent observations of volume-limited samples of magnetic white dwarfs (WD) have revealed a higher incidence of magnetism in older WDs. Specifically, these studies indicate that magnetism is more prevalent in WDs with fully or partially crystallized cores compared to those with entirely liquid cores. This has led to the recognition of a crystallization-driven dynamo as an important mechanism for explaining magnetism in isolated WDs. However, recent simulations challenged the capability of this mechanism to match both the incidence of magnetism and the field strengths detected in WDs. In this letter, we explore an alternative hypothesis for the surface emergence of magnetic fields in isolated WDs. WDs with masses $\gtrsim 0.55 M_\odot$ are the descendants of main-sequence stars with convective cores capable of generating strong dynamo magnetic fields. This idea is supported by asteroseismic evidence of strong magnetic fields buried within the interiors of red giant branch stars. Assuming that these fields are disrupted by subsequent convective zones, we have estimated magnetic breakout times for WDs. Due to the significant uncertainties in breakout times stemming from the treatment of convective boundaries and mass loss rates, we cannot provide a precise prediction for the emergence time of the main-sequence dynamo field. However, we can predict that this emergence should occur during the WD phase for WDs with masses $\gtrsim 0.65 M_\odot$. We also find that the magnetic breakout is expected to occur earlier in more massive WDs, consistently with observations from volume-limited samples and the well-established fact that magnetic WDs tend to be more massive than non-magnetic ones. Moreover, within the uncertainties of stellar evolutionary models, we find that the emergence of main-sequence dynamo magnetic fields can account for a significant portion of the magnetic WDs.

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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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arXiv2023

Exsolution process in white dwarf stars

White dwarf (WD) stars are considered cosmic laboratories to study the physics of dense plasma. Furthermore, the use of WD stars as cosmic clocks to date stellar populations and main sequence companions demands an appropriate understanding of the WD physics in order to provide precise ages for these stars. We aim at studying exsolution in the interior of WD stars, a process in which a crystallized ionic binary mixture separates into two solid solutions with different fractions of the constituents. Depending on the parent solid mixture composition, this process can release or absorb heat, thus leading to a delay or a speed-up of WD cooling. Relying on accurate phase diagrams for exsolution, we have modeled this process in hydrogen-rich WDs with both carbon-oxygen and oxygen-neon core composition, with masses ranging from 0.53 to 1.29Msun and from 1.10 to 1.29Msun, respectively. Exsolution is a slow process that takes place at low luminosities (log(L/Lsun)$\lesssim$-2.75) and effective temperatures (Teff$\lesssim$18 000K) in WDs. We find that exsolution begins at brighter luminosities in CO than in ONe WDs of the same mass. Massive WDs undergo exsolution at brighter luminosities than their lower-mass counterparts. The net effect of exsolution on the WD cooling times depends on the stellar mass and the exact chemical profile. For standard core chemical profiles and preferred assumptions regarding miscibility gap microphysics, the cooling delay can be as large as ~0.35 Gyrs at L/Lsun ~ -5. We have neglected a chemical redistribution possibly associated with this process, which could lead to a further cooling delay. Exsolution has a marginal effect on the WD cooling times and, accordingly, we find no WD branches on the Gaia color magnitude diagram associated with it. However, exsolution in massive WDs can alter the faint end of the WD luminosity function, thus impacting WD cosmochronology.

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arXiv2023

The unusual planetary nebula nucleus in the Galactic open cluster M37 and six further hot white dwarf candidates

Planetary nebulae in Galactic open star clusters are rare objects; only three are known to date. They are of particular interest because their distance can be determined with high accuracy, allowing one to characterize the physical properties of the planetary nebula and its ionizing central star with high confidence. Here we present the first quantitative spectroscopic analysis of a central star in an open cluster, namely the faint nucleus of IPHASX J055226.2$+$323724 in M37. This cluster contains 14 confirmed white dwarf members, which were previously used to study the initial-to-final-mass relation of white dwarfs, and six additional white dwarf candidates. We performed an atmosphere modeling of spectra taken with the 10m Gran Telescopio Canarias. The central star is a hot hydrogen-deficient white dwarf with an effective temperature of 90,000 K and spectral type PG1159 (helium- and carbon-rich). We know it is about to transform into a helium-rich DO white dwarf because the relatively low atmospheric carbon abundance indicates ongoing gravitational settling of heavy elements. The star belongs to a group of hot white dwarfs that exhibit ultrahigh-excitation spectral lines possibly emerging from shock-heated material in a magnetosphere. We find a relatively high stellar mass of $M= 0.85^{+0.13}_{-0.14}$ M$_\odot$. This young white dwarf is important for the semi-empirical initial-final mass relation because any uncertainty related to white-dwarf cooling theory is insignificant with respect to the pre-white-dwarf timescale. Its post-asymptotic-giant-branch age of $170,000-480,000$ yr suggests that the extended planetary nebula is extraordinarily old. We also performed a spectroscopic analysis of the six other white dwarf candidates of M37, confirming one as a cluster member.

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arXiv2023

An irradiated-Jupiter analogue hotter than the Sun

Planets orbiting close to hot stars experience intense extreme-ultraviolet radiation, potentially leading to atmosphere evaporation and to thermal dissociation of molecules. However, this extreme regime remains mainly unexplored due to observational challenges. Only a single known ultra-hot giant planet, KELT-9b, receives enough ultraviolet radiation for molecular dissociation, with a day-side temperature of ~4,600K. An alternative approach uses irradiated brown dwarfs as hot-Jupiter analogues. With atmospheres and radii similar to those of giant planets, brown dwarfs orbiting close to hot Earth-sized white-dwarf stars can be directly detected above the glare of the star. Here we report observations revealing an extremely irradiated low-mass companion to the hot white dwarf WD0032-317. Our analysis indicates a day-side temperature of ~8,000K, and a day-to-night temperature difference of ~6,000K. The amount of extreme-ultraviolet radiation (with wavelengths 100-912Å) received by WD0032-317B is equivalent to that received by planets orbiting close to stars as hot as a late B-type stars, and about 5,600 times higher than that of KELT-9b. With a mass of ~75-88 Jupiter masses, this near-hydrogen-burning-limit object is potentially one of the most massive brown dwarfs known.

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arXiv2023

White dwarf Random Forest classification through Gaia spectral coefficients

The third data release of Gaia has provided approximately 220 million low resolution spectra. Among these, about 100,000 correspond to white dwarfs. The magnitude of this quantity of data precludes the possibility of performing spectral analysis and type determination by human inspection. In order to tackle this issue, we explore the possibility of utilising a machine learning approach, based on a Random Forest algorithm. We aim to analyze the viability of the Random Forest algorithm for the spectral classification of the white dwarf population within 100 pc from the Sun, based on the Hermite coefficients of Gaia spectra. We utilized the assigned spectral type from the Montreal White Dwarf Database for training and testing our Random Forest algorithm. Once validated, our algorithm model is applied to the rest of unclassified white dwarfs within 100 pc. First, we started by classifying the two major spectral type groups of white dwarfs: hydrogen-rich (DA) and hydrogen-deficient (non-DA). Next, we explored the possibility of classifying the various spectral subtypes, including in some cases the secondary spectral types. Our Random Forest classification presented a very high recall (>80%) for DA and DB white dwarfs, and a very high precision (>90%) for DB, DQ and DZ white dwarfs. As a result we have assigned a spectral type to 9,446 previously unclassified white dwarfs: 4,739 DAs, 76 DBs (60 of them DBAs), 4,437 DCs, 132 DZs and 62 DQs (9 of them DQpec). Despite the low resolution of Gaia spectra, the Random Forest algorithm applied to the Gaia spectral coefficients proves to be a highly valuable tool for spectral classification.

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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

Photometric follow-up of 43 new eclipsing white dwarf plus main-sequence binaries from the ZTF survey

Wide-field time-domain photometric sky surveys are now finding hundreds of eclipsing white dwarf plus M dwarf binaries, a population encompassing a wealth of information and potential insight into white dwarf and close binary astrophysics. Precise follow-up observations are essential in order to fully constrain these systems and capitalise on the power of this sample. We present the first results from our program of high-speed, multi-band photometric follow-up. We develop a method to measure temperatures, (model-dependent) masses, and radii for both components from the eclipse photometry alone and characterize 34 white dwarf binaries, finding general agreement with independent estimates using an alternative approach while achieving around a factor of two increase in parameter precision. In addition to these parameter estimates, we discover a number of interesting systems -- finding four with sub-stellar secondaries, doubling the number of eclipsing examples, and at least six where we find the white dwarf to be strongly magnetic, making these the first eclipsing examples of such systems and key to investigating the mechanism of magnetic field generation in white dwarfs. We also discover the first two pulsating white dwarfs in detached and eclipsing post-common-envelope binaries -- one with a low-mass, likely helium core, and one with a relatively high mass, towards the upper end of the known sample of ZZ Cetis. Our results demonstrate the power of eclipse photometry, not only as a method of characterising the population, but as a way of discovering important systems that would have otherwise been missed by spectroscopic follow-up.

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arXiv2022

Can we reveal the core-chemical composition of ultra-massive white dwarfs through their magnetic fields?

Ultra-massive white dwarfs ($ 1.05 \rm M_\odot \lesssim M_{WD}$) are particularly interesting objects that allow us to study extreme astrophysical phenomena such as type Ia supernovae explosions and merger events. Traditionally, ultra-massive white dwarfs are thought to harbour oxygen-neon (ONe) cores. However, recent theoretical studies and new observations suggest that some ultra-massive white dwarfs could harbour carbon-oxygen (CO) cores. Although several studies have attempted to elucidate the core composition of ultra-massive white dwarfs, to date, it has not been possible to distinguish them through their observed properties. Here, we present a new method for revealing the core-chemical composition in ultra-massive white dwarfs that is based on the study of magnetic fields generated by convective mixing induced by the crystallization process. ONe white dwarfs crystallize at higher luminosities than their CO counterparts. Therefore, the study of magnetic ultra-massive white dwarfs in the particular domain where ONe cores have reached the crystallization conditions but CO cores have not, may provide valuable support to their ONe core-chemical composition, since ONe white dwarfs would display signs of magnetic fields and CO would not. We apply our method to eight white dwarfs with magnetic field measurements and we suggest that these stars are candidate ONe white dwarfs.

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arXiv2022

Random Forest classification of Gaia DR3 white dwarf-main sequence spectra: a feasibility study

The third Gaia data release provides low-resolution spectra for around 200 million sources. It is expected that a sizeable fraction of them contain a white dwarf (WD), either isolated, or in a binary system with a main-sequence (MS) companion, i.e. a white dwarf-main sequence (WDMS) binary. Taking advantage of a consolidated Random Forest algorithm used in the classification of WDs, we extend it to study the feasibility of classifying Gaia WDMS binary spectra. The Random Forest algorithm is first trained with a set of synthetic spectra generated by combining individual WD and MS spectra for the full range of effective temperatures and surface gravities. Moreover, with the aid of a detailed population synthesis code, we simulate the Gaia spectra for the above mentioned populations. For evaluating the performance of the models, a set of metrics are applied to our classifications. Our results show that for resolving powers above ~300 the accuracy of the classification depends exclusively on the SNR of the spectra, while below that value the SNR should be increased as the resolving power is reduced to maintain a certain accuracy. The algorithm is then applied to the already classified SDSS WDMS catalogue, revealing that the automated classification exhibits an accuracy comparable (or even higher) to previous classification methods. Finally, we simulate the {\it Gaia} spectra, showing that our algorithm is able to correctly classify nearly 80% the synthetic WDMS spectra. Our algorithm represents a useful tool in the analysis and classification of real Gaia WDMS spectra. Even for those spectra dominated by the flux of the MS stars, the algorithm reaches a high degree of accuracy (60%).

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arXiv2022

Gaia0007-1605: an old triple system with an inner brown dwarf-white dwarf binary and an outer white dwarf companion

We identify Gaia0007-1605AC as the first inner brown dwarf-white dwarf binary of a hierarchical triple system in which the outer component is another white dwarf (Gaia0007-1605B). From optical/near-infrared spectroscopy obtained at the Very Large Telescope with the X-Shooter instrument and/or from Gaia photometry plus SED fitting, we determine the effective temperatures and masses of the two white dwarfs (12018+-68 K, 0.54+-0.01 Msun for Gaia0007-1605A and 4445+-116 K, 0.56+-0.05 Msun for Gaia0007-1605B) and the effective temperature of the brown dwarf (1850+-50 K; corresponding to a spectral type L3+-1). By analysing the available TESS light curves of Gaia0007-1605AC we detect a signal at 1.0446+-0.0015 days with an amplitude of 6.25 ppt, which we interpret as the orbital period modulated from irradiation effects of the white dwarf on the brown dwarf's surface. This drives us to speculate that the inner binary evolved through a common envelope phase in the past. Using the outer white dwarf as a cosmochronometer and analysing the kinematic properties of the system, we conclude that the triple system is about 10 Gyr old.

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arXiv2022

White dwarfs as Physics laboratories: lights and shadows

The evolution of white dwarfs is essentially a gravothermal process of cooling in which the basic ingredients for predicting their evolution are well identified, although not always well understood. There are two independent ways to test the cooling rate. One is the luminosity function of the white dwarf population, and another is the secular drift of the period of pulsation of those individuals that experience variations. Both scenarios are sensitive to the cooling or heating time scales, for which reason, the inclusion of any additional source or sink of energy will modify these properties and will allow to set bounds to these perturbations. These studies also require complete and statistical significant samples for which current large data surveys are providing an unprecedented wealth of information. In this paper we review how these techniques are applied to several cases like the secular drift of the Newton gravitational constant, neutrino magnetic moments, axions and weakly interacting massive particles (WIMPS).

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arXiv2021

Infrared Excesses around Bright White Dwarfs from Gaia and unWISE. II

Infrared excesses around white dwarf stars indicate the presence of various astrophysical objects of interest, including companions and debris disks. In this second paper of a series, we present follow-up observations of infrared excess candidates from Gaia and unWISE discussed in the first paper, Paper I. We report space-based infrared photometry at 3.6 and 4.5 micron for 174 white dwarfs from the Spitzer Space Telescope and ground-based near-infrared J, H, and K photometry of 235 white dwarfs from Gemini Observatory with significant overlap between Spitzer and Gemini observations. This data is used to confirm or rule-out the observed unWISE infrared excess. From the unWISE-selected candidate sample, the most promising infrared excess sample comes from both colour and flux excess, which has a Spitzer confirmation rate of 95%. We also discuss a method to distinguish infrared excess caused by stellar or sub-stellar companions from potential dust disks. In total, we confirm the infrared excess around 62 white dwarfs, 10 of which are likely to be stellar companions. The remaining 52 bright white dwarf with infrared excess beyond two microns has the potential to double the known sample of white dwarfs with dusty exoplanetary debris disks. Follow-up high-resolution spectroscopic studies of a fraction of confirmed excess white dwarfs in this sample have discovered emission from gaseous dust disks. Additional investigations will be able to expand the parameter space from which dust disks around white dwarfs are found.

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arXiv2021

Forever young white dwarfs: when stellar ageing stops

White dwarf stars are the most common end point of stellar evolution. Of special interest are the ultramassive white dwarfs, as they are related to type Ia Supernovae explosions, merger events, and Fast Radio Bursts. Ultramassive white dwarfs are expected to harbour oxygen-neon (ONe) cores as a result of single standard stellar evolution. However, a fraction of them could have carbon-oxygen (CO) cores. Recent studies, based on the new observations provided by the {\it Gaia} space mission, indicate that a small fraction of the ultramassive white dwarfs experience a strong delay in their cooling, which cannot be attributed only to the occurrence of crystallization, thus requiring an unknown energy source able to prolong their life for long periods of time. In this study we find that the energy released by $^{22}$Ne sedimentation in the deep interior of ultramassive white dwarfs with CO cores and high $^{22}$Ne content is consistent with the long cooling delay of these stellar remnants. On the basis of a synthesis study of the white dwarf population, based on Monte Carlo techniques, we find that the observations revealed by {\it Gaia} can be explained by the existence of these prolonged youth ultramassive white dwarfs. Although such a high $^{22}$Ne abundance is not consistent with the standard evolutionary channels, our results provide sustain to the existence of CO-core ultramassive white dwarfs and to the occurrence of $^{22}$Ne sedimentation.

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arXiv2021

The Gaia DR2 halo white dwarf population: the luminosity function, mass distribution and its star formation history

We analyze the volume-limited nearly complete 100 pc sample of 95 halo white dwarf candidates identified by the second data release of Gaia. Based on a detailed population synthesis model, we apply a method that relies on Gaia astrometry and photometry to accurately derive the individual white dwarf parameters (mass, radius, effective temperature, bolometric luminosity and age). This method is tested with 25 white dwarfs of our sample for which we took optical spectra and performed spectroscopic analysis. We build and analyse the halo white dwarf luminosity function, for which we find for the first time possible evidences of the cut-off at its faintest end, leading to an age estimate of $\simeq12\pm0.5 $Gyr. The mass distribution of the sample peaks at $0.589\,M_{\odot}$, with $71\%$ of the white dwarf masses below $0.6\,M_{\odot}$ and just two massive white dwarfs of more than $0.8\,M_{\odot}$. From the age distribution we find three white dwarfs with total ages above 12 Gyr, of which J1312-4728 is the oldest white dwarf known with an age of $12.41\pm0.22 $Gyr. We prove that the star formation history is mainly characterised by a burst of star formation that occurred from 10 to 12 Gyr in the past, but extended up to 8 Gyr. We also find that the peak of the star formation history is centered at around 11 Gyr, which is compatible with the current age of the Gaia-Enceladus encounter. Finally, $13\%$ of our halo sample is contaminated by high-speed young objects (total age<7 Gyr). The origin of these white dwarfs is unclear but their age distribution may be compatible with the encounter with the Sagittarius galaxy.

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arXiv2020

The formation of ultra-massive carbon-oxygen core white dwarfs and their evolutionary and pulsational properties

(Abridged abstract) We explore the formation of ultra-massive (M_{\rm WD} \gtrsim 1.05 M_\sun$), carbon-oxygen core white dwarfs resulting from single stellar evolution. We also study their evolutionary and pulsational properties and compare them with those of the ultra-massive white dwarfs with oxygen-neon cores resulting from carbon burning in single progenitor stars, and with binary merger predictions. We consider two single-star evolution scenarios for the formation of ultra-massive carbon-oxygen core white dwarfs that involve rotation of the degenerate core after core helium burning and reduced mass-loss rates in massive asymptotic giant-branch stars. We compare our findings with the predictions from ultra-massive white dwarfs resulting from the merger of two equal-mass carbon-oxygen core white dwarfs, by assuming complete mixing between them and a carbon-oxygen core for the merged remnant. The resulting ultra-massive carbon-oxygen core white dwarfs evolve markedly slower than their oxygen-neon counterparts. Our study strongly suggests the formation of ultra-massive white dwarfs with carbon-oxygen core from single stellar evolution. We find that both the evolutionary and pulsation properties of these white dwarfs are markedly different from those of their oxygen-neon core counterparts and from those white dwarfs with carbon-oxygen core that might result from double degenerate mergers. This can eventually be used to discern the core composition of ultra-massive white dwarfs and their formation scenario.

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