arXiv subjects

Odette Toloza

Publications and source records attributed to Odette Toloza.

At least 19 recordsLinked to original sources

arXiv2025

White Dwarfs in Wide Binary Systems as Reliable Age Calibrators

Deriving precise stellar ages is a challenging task. Consequently, age-dependent relations - such as the age-metallicity and age-velocity dispersion relations of the Milky Way, or the age-rotation-activity relation of low-mass stars - are subject to potentially large uncertainties, despite the well-defined trends observed at the population level. White dwarfs, the most common stellar remnants, follow a relatively simple and well-understood cooling process. When found in wide binary systems with main-sequence companions, they can therefore provide the much-needed precise age estimates. The total age of such systems depends not only on the white dwarf cooling time but also on the lifetime of the main-sequence progenitor. Estimating this lifetime requires knowledge of the progenitor mass, which is typically inferred by adopting an initial-to-final mass relation. However, the observational constraints on this relation are still poorly defined, introducing a source of uncertainty in white dwarf age determinations. To mitigate this issue, we focus on a large sample of massive white dwarfs (>~0.7 Msun), for which the main-sequence progenitor lifetime is negligible. These white dwarfs are intrinsically faint and therefore require specialized facilities for adequate follow-up observations. In this white paper, we outline the instrumentation requirements needed to observe the forthcoming population of massive white dwarfs in our Galaxy.

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arXiv2025

The Future of Evolved Planetary Systems

Understanding the formation, evolution, and chemical diversity of exoplanets are now central areas of astrophysics research. White dwarfs provide a uniquely sensitive laboratory for studying the end stages of planetary-system evolution and for probing the bulk composition of both rocky and volatile-rich exoplanetary material. In the 2030s new facilities will transform our ability to carry out \textit{``industrial-scale''} astrophysics, leading to fundamental results and new challenges for the next decade. By combining the volume of data surveyed by the ESA {\em Gaia} mission and Vera C. Rubin Observatory with the next-generation of spectroscopic facilities, the European Southern Observatory (ESO) community will be in a position to obtain an unbiased census of evolved planetary systems, constrain the composition of thousands of disrupted planetesimals, and connect these signatures to Galactic populations and stellar birth environments. Thus, it is now the time for assessing those challenges and preparing for the future. This white paper outlines key science opportunities arising in the next decade and the technological requirements of future ESO facilities in enabling transformative discoveries in the 2040s. These future facilities will have to combine a number of features that are crucial for studying evolved planetary systems at white dwarfs, such as broad optical to near-infrared coverage, a high sensitivity at blue wavelengths, multi-resolution capability, massive multi-plexing, and time-domain reactivity.

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arXiv2025

White Dwarf Binaries: Probes of Future Astrophysics

White dwarf binaries are fundamental astrophysical probes. They represent ideal laboratories to test the models of binary evolution, which also apply to the sources of gravitational waves, whose detection led to the award of the 2017 Nobel Prize in Physics. Moreover, their final fate is intimately linked to Type Ia Supernovae (SNe Ia), i.e. the thermonuclear explosion of a white dwarf following the interaction with a companion star, which have become the fundamental yardsticks on cosmological distance scales and led to the discovery of dark energy and the award of the 2011 Nobel Prize in Physics. Finally, white dwarf binaries play a crucial role in influencing star formation and chemical evolution of the Galaxy by injecting energy into, and enriching, the interstellar medium with material ejected during nova eruptions and SN Ia explosions. In the next decade, the advent of the Large Synoptic Survey Telescope (LSST) at the Vera Rubin Observatory will lead to the discovery of hundreds of thousands of white dwarf binaries. Nonetheless, the intrinsic faintness of the majority of these systems will prevent their spectroscopic characterisation with the instruments available in the 2030s. Hence ESO's Expanding Horizons call is timely for planning a future transformative facility, capable of delivering phase-resolved spectroscopic observations of faint white dwarf binaries, which are key to advancing our understanding of stellar and Galactic evolution and cosmology.

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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

Measurements of three exo-planetesimal compositions: a planetary core, a chondritic body, and an icy Kuiper belt analogue

The study of planetesimal debris accreted by white dwarfs offers unique insights into the composition of exoplanets. Using far-ultraviolet and optical spectroscopy, we have analysed the composition of planetesimals accreted by three metal enriched H-dominated white dwarfs with effective temperatures of T_eff = 20 000 K. WD 0059+257 is accreting an object composed of 71.8 +/- 7.9 per cent Fe and Ni by mass, indicating a large core mass fraction of 69 per cent, similar to that of Mercury. We model this planetesimal as having a differentiated Earth-like composition with 65 per cent of its mantle stripped, and we find this mass loss can be caused by vaporisation of the planetesimal's mantle during post-main sequence evolution. The tentative S detection in WD 0059+257 is a possible clue to the nature of the light element in planetary cores, including that of the Earth. The volatile-rich composition of WD 1943+163 is consistent with accretion of a carbonaceous chondrite-like object, but with an extreme Si depletion. WD 1953-715 accretes a planetesimal which contains 64 +/- 21 per cent of O in the form of ices, likely H2O. This body therefore requires an initial orbit at formation beyond a radial distance of > 100 au for ice survival into the white dwarf phase. These three planetary enriched white dwarfs provide evidence of differing core fractions, volatile budgets, and initial orbital separations of the accreted planetesimals, all of which help us understand their formation and evolutionary history.

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arXiv2025

A ZTF Search for Circumstellar Debris Transits in White Dwarfs: Six New Candidates, one with Gas Disk Emission, identified in a Novel Metric Space

White dwarfs (WDs) showing transits from orbiting planetary debris provide significant insights into the structure and dynamics of debris disks. This is a rare class of objects with only eight published systems. In this work, we perform a systematic search for such systems within 500 pc in the Gaia-eDR3 catalog of WDs using the light curves from the Zwicky Transient Facility (ZTF) and present six new candidates. Our selection process targets the top 1% most photometrically variable sources identified using a combined variability metric from ZTF and Gaia eDR3 photometry, boosted by a metric space we define using von Neumann statistics and Pearson-Skew as a novel discovery tool to identify these systems. This is followed by optical spectroscopic observations of visually selected variables to confirm metal pollution. Four of the six systems show long-timescale photometric variability spanning several months to years, resulting either from long-term evolution of transit activity or dust and debris clouds at wide orbits. Among them, WD J1013-0427 shows an indication of reddening during the long-duration dip. Interpreting this as dust extinction makes it the first system to indicate an abundance of small dust grains (radius $\lesssim$$0.3~{\rm μm}$) in the occulting material. The same object also shows metal emission lines that map an optically thick eccentric gas disk orbiting within the star's Roche limit. For each candidate, we infer the abundances of the photospheric metals and estimate accretion rates. We show that transiting debris systems tend to have higher inferred accretion rates compared to the general population of metal-polluted WDs. Growing the number of these systems will further illuminate such comparative properties in the near future. Separately, we also serendipitously discovered an AM CVn showing a very long-duration outburst $-$ only the fourth such system to be known.

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arXiv2025

V498 Hya, a new candidate for a period bouncer Cataclysmic Variable

V498 Hya (SDSS J084555.07+033929.2) was identified as a short-period cataclysmic variable (CV) by the Catalina Real-Time Transient Survey (CRTS) in 2008. The superhump period was measured during the detected single superoutburst of V498 Hya. The quiescent spectrum subsequently taken by the \SDSSV\ Milky Way Mapper survey suggested that the CV donor may be a brown dwarf. We present time-resolved follow-up spectroscopy of V498 Hya in quiescence, obtained with the GTC OSIRIS spectrograph, from which we derived the 86.053 min spectroscopic period, systemic radial velocity, and the gravitational redshift of the Mg II line. We also modeled the spectral energy distribution to constrain the system parameters, including the > 0.82 Ms mass of the white dwarf and the best-fit value 0.043 +/- 0.004 Ms of the donor star mass. This combination of parameters implies that V498 Hya has evolved past the period minimum and is a relatively rare ``period bouncer''.

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arXiv2024

Simultaneous emission from dust and gas in the planetary debris orbiting a white dwarf

There is increasing evidence for the presence and variability of circumstellar dust and gas around white dwarfs that are polluted with exoplanetary material, although the origin of this dust and gas remains debated. This paper presents the first near-simultaneous observations of both circumstellar dust (via broadband emission) and gas (via emission lines) around a polluted white dwarf. From the optical spectra the gaseous emission lines, notably the calcium infrared triplet and magnesium lines, show significant increases and decreases in their strength over timescales of weeks, while the oxygen and iron lines remain relatively stable. Near-infrared JHKs photometry reveals dust emission changes of up to 0.2 magnitudes in the Ks band over similar timescales, marking the shortest variability timescales observed to date. The two epochs with the strongest emission were correlated between the dust (Ks band brightening) and gas (strengthened calcium and magnesium lines), showing for the first time that the dust and gas must be produced near-simultaneously with a common origin, likely in collisions.

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arXiv2024

Expansion properties of the young supernova type Iax remnant Pa 30 revealed

The recently discovered Pa 30 nebula, the putative type Iax supernova remnant associated with the historical supernova of 1181 AD, shows puzzling characteristics that make it unique among known supernova remnants. In particular, Pa 30 exhibits a complex morphology, with a unique radial and filamentary structure, and it hosts a hot stellar remnant at its center, which displays oxygen-dominated, ultra-fast winds. Because of the surviving stellar remnant and the lack of hydrogen and helium in its filaments, it has been suggested that Pa 30 is the product of a failed thermonuclear explosion in a near- or super-Chandrasekhar white dwarf, which created a sub-luminous transient, a rare sub-type of the Ia class of supernovae called type Iax. We here present a detailed study of the 3D structure and velocities of a full radial section of the remnant. The Integral Field Unit (IFU) observations, obtained with the new red channel of the Keck Cosmic Web Imager spectrograph, reveal that the ejecta are consistent with being ballistic, with velocities close to the free-expansion velocity. Additionally, we detect a large cavity inside the supernova remnant and a sharp inner edge to the filamentary structure, which coincides with the outer edge of a bright ring detected in infrared images. Finally, we detect a strong asymmetry in the amount of ejecta along the line of sight, which might hint to an asymmetric explosion. Our analysis provides strong confirmation that the explosion originated from SN 1181.

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arXiv2024

Rotation plays a role in the generation of magnetic fields in single white dwarfs

Recent surveys of close white dwarf binaries as well as single white dwarfs have provided evidence for the late appearance of magnetic fields in white dwarfs, and a possible generation mechanism a crystallization and rotation-driven dynamo has been suggested. A key prediction of this dynamo is that magnetic white dwarfs rotate, at least on average, faster than their non-magnetic counterparts and/or that the magnetic field strength increases with rotation. Here we present rotation periods of ten white dwarfs within 40 pc measured using photometric variations. Eight of the light curves come from TESS observations and are thus not biased towards short periods, in contrast to most period estimates that have been reported previously in the literature. These TESS spin periods are indeed systematically shorter than those of non-magnetic white dwarfs. This means that the crystallization and rotation-driven dynamo could be responsible for a fraction of the magnetic fields in white dwarfs. However, the full sample of magnetic white dwarfs also contains slowly rotating strongly magnetic white dwarfs which indicates that another mechanism that leads to the late appearance of magnetic white dwarfs might be at work, either in addition to or instead of the dynamo. The fast-spinning and massive magnetic white dwarfs that appear in the literature form a small fraction of magnetic white dwarfs, and probably result from a channel related to white dwarf mergers.

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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

An HST COS ultra-violet spectroscopic survey of 311 DA white dwarfs.I. Fundamental parameters and comparative studies

White dwarf studies carry significant implications across multiple fields of astrophysics, including exoplanets, supernova explosions, and cosmological investigations. Thus, accurate determinations of their fundamental parameters (Teff and log g) are of utmost importance. While optical surveys have provided measurements for many white dwarfs, there is a lack of studies utilising ultraviolet (UV) data, particularly focusing on the warmer ones that predominantly emit in the UV range. Here, we present the medium-resolution far-UV spectroscopic survey of 311 DA white dwarfs obtained with Cosmic Origins Spectrograph (COS) onboard Hubble Space Telescope confirming 49 photometric Gaia candidates. We used 3D extinction maps, parallaxes, and hydrogen atmosphere models to fit the spectra of the stars that lie in the range 12 000 < Teff < 33 000 K, and 7 <= log g < 9.2. To assess the impact of input physics, we employed two mass-radius relations in the fitting and compared the results with previous studies. The comparisons suggest the COS Teff are systematically lower by 3 per cent on average than Balmer line fits while they differ by only 1.5 per cent from optical photometric studies. The mass distributions indicate that the COS masses are smaller by approximately 0.05 Msol and 0.02 Msol than Balmer lines and photometric masses, respectively. Performing several tests, we find that the discrepancies are either arising due to issues with the COS calibration, broadening theories for hydrogen lines, or interstellar reddening which needs further examination. Based on comparative analysis, we identify 30 binary candidates drawing attention for follow-up studies to confirm their nature.

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arXiv2023

Systematic uncertainties in the characterisation of helium-dominated metal-polluted white dwarf atmospheres

White dwarf photospheric parameters are usually obtained by means of spectroscopic or photometric analysis. These results are not always consistent with each other, with the published values often including just the statistical uncertainties. The differences are more dramatic for white dwarfs with helium-dominated photospheres, so to obtain realistic uncertainties we have analysed a sample of 13 of these white dwarfs, applying both techniques to up to three different spectroscopic and photometric data sets for each star. We found mean standard deviations of < $σT_{\mathrm{eff}}$ > = 524 K, < $σ\log g$ > = 0.27 dex and < $σ\log(\mathrm{H/He})$ > = 0.31 dex for the effective temperature, surface gravity and relative hydrogen abundance, respectively, when modelling diverse spectroscopic data. The photometric fits provided mean standard deviations up to < $σT_{\mathrm{eff}}$ > = 1210 K and < $σ\log g$ > = 0.13 dex. We suggest these values to be adopted as realistic lower limits to the published uncertainties in parameters derived from spectroscopic and photometric fits for white dwarfs with similar characteristics. In addition, we investigate the effect of fitting the observational data adopting three different photospheric chemical compositions. In general, pure helium model spectra result in larger $T_{\mathrm{eff}}$ compared to those derived from models with traces of hydrogen. The $\log g$ shows opposite trends: smaller spectroscopic values and larger photometric ones when compared to models with hydrogen. The addition of metals to the models also affects the derived atmospheric parameters, but a clear trend is not found.

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arXiv2022

Modules for Experiments in Stellar Astrophysics (MESA): Time-Dependent Convection, Energy Conservation, Automatic Differentiation, and Infrastructure

We update the capabilities of the open-knowledge software instrument Modules for Experiments in Stellar Astrophysics (MESA). The new auto_diff module implements automatic differentiation in MESA, an enabling capability that alleviates the need for hard-coded analytic expressions or finite difference approximations. We significantly enhance the treatment of the growth and decay of convection in MESA with a new model for time-dependent convection, which is particularly important during late-stage nuclear burning in massive stars and electron degenerate ignition events. We strengthen MESA's implementation of the equation of state, and we quantify continued improvements to energy accounting and solver accuracy through a discussion of different energy equation features and enhancements. To improve the modeling of stars in MESA we describe key updates to the treatment of stellar atmospheres, molecular opacities, Compton opacities, conductive opacities, element diffusion coefficients, and nuclear reaction rates. We introduce treatments of starspots, an important consideration for low-mass stars, and modifications for superadiabatic convection in radiation-dominated regions. We describe new approaches for increasing the efficiency of calculating monochromatic opacities and radiative levitation, and for increasing the efficiency of evolving the late stages of massive stars with a new operator split nuclear burning mode. We close by discussing major updates to MESA's software infrastructure that enhance source code development and community engagement.

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arXiv2022

Testing Lyman alpha emission line reconstruction routines at multiple velocities in one system

The 1215.67A HI Lyman alpha emission line dominates the ultraviolet flux of low mass stars, including the majority of known exoplanet hosts. Unfortunately, strong attenuation by the interstellar medium (ISM) obscures the line core at most stars, requiring the intrinsic Lyman alpha flux to be reconstructed based on fits to the line wings. We present a test of the widely-used Lyman alpha emission line reconstruction code LYAPY using phase-resolved, medium-resolution STIS G140M observations of the close white dwarf-M dwarf binary EG UMa. The Doppler shifts induced by the binary orbital motion move the Lyman alpha emission line in and out of the region of strong ISM attenuation. Reconstructions to each spectrum should produce the same Lyman alpha profile regardless of phase, under the well-justified assumption that there is no intrinsic line variability between observations. Instead, we find that the reconstructions underestimate the Lyman alpha flux by almost a factor of two for the lowest-velocity, most attenuated spectrum, due to a degeneracy between the intrinsic Lyman alpha and ISM profiles. Our results imply that many stellar Lyman alpha fluxes derived from G140M spectra reported in the literature may be underestimated, with potential consequences for, for example, estimates of extreme-ultraviolet stellar spectra and ultraviolet inputs into simulations of exoplanet atmospheres.

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arXiv2022

A white dwarf accreting planetary material determined from X-ray observations

The atmospheres of a large proportion of white dwarf stars are polluted by heavy elements that are expected to sink out of visible layers on short timescales. This has been interpreted as a signature of ongoing accretion of debris from asteroids, comets, and giant planets. This scenario is supported by the detection of debris discs and transits of planetary fragments around some white dwarfs. However, photospheric metals are only indirect evidence for ongoing accretion, and the inferred accretion rates and parent body compositions heavily depend on models of diffusion and mixing processes within the white dwarf atmosphere. Here we report a 4.4$σ$ detection of X-rays from a polluted white dwarf, G29$-$38, using a 106 ks exposure with the Chandra X-ray Observatory, demonstrating directly that the star is currently accreting. From the measured X-ray luminosity, we find an instantaneous accretion rate of $\dot{M_{\rm X}}=1.63^{+1.29}_{-0.40}\times 10^{9}\mathrm{\,g\,s^{-1}}$. This is the first direct measurement of the accretion rate onto the white dwarf, which is independent of stellar atmosphere models. This rate exceeds estimates based on past studies of the photospheric abundances by more than a factor two, and implies that convective overshoot has to be accounted for in modelling the spectra of debris-accreting white dwarfs. We measure a low plasma temperature of $kT=0.5\pm0.2\,\mathrm{keV}$, corroborating the predicted bombardment solution for white dwarfs accreting at low accretion rates. Offering a new method for studying evolved planetary systems, these observations provide the opportunity to independently measure the instantaneous accretion rate of planetary material, and therefore investigate the timescale of accretion onto white dwarfs, and the evolution and replenishment of debris disks.

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arXiv2021

Discovery of a young pre-intermediate polar

We present the discovery of a magnetic field on the white dwarf component in the detached post common envelope binary (PCEB) CC Cet. Magnetic white dwarfs in detached PCEBs are extremely rare, in contrast to the high incidence of magnetism in single white dwarfs and cataclysmic variables. We find Zeeman-split absorption lines in both ultraviolet Hubble Space Telescope (HST) spectra and archival optical spectra of CC Cet. Model fits to the lines return a mean magnetic field strength of approximately 600-700 kG. Differences in the best-fit magnetic field strength between two separate HST observations and the high v sin i of the lines indicate that the white dwarf is rotating with a period ~0.5 hours, and that the magnetic field is not axisymmetric about the spin axis. The magnetic field strength and rotation period are consistent with those observed among the intermediate polar class of cataclysmic variable, and we compute stellar evolution models that predict CC Cet will evolve into an intermediate polar in 7-17 Gyr. Among the small number of known PCEBs containing a confirmed magnetic white dwarf, CC Cet is the hottest (and thus youngest), with the weakest field strength, and cannot have formed via the recently proposed crystallisation/spin-up scenario. In addition to the magnetic field measurements, we update the atmospheric parameters of the CC Cet white dwarf via model spectra fits to the HST data and provide a refined orbital period and ephemeris from TESS photometry.

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arXiv2021

Horizontal spreading of planetary debris accreted by white dwarfs

White dwarfs with metal-polluted atmospheres have been studied widely in the context of the accretion of rocky debris from evolved planetary systems. One open question is the geometry of accretion and how material arrives and mixes in the white dwarf surface layers. Using the 3D radiation-hydrodynamics code CO$^5$BOLD, we present the first transport coefficients in degenerate star atmospheres which describe the advection-diffusion of a passive scalar across the surface-plane. We couple newly derived horizontal diffusion coefficients with previously published vertical diffusion coefficients to provide theoretical constraints on surface spreading of metals in white dwarfs. Our grid of 3D simulations probes the vast majority of the parameter space of convective white dwarfs, with pure-hydrogen atmospheres in the effective temperature range 6000-18000 K and pure-helium atmospheres in the range 12000-34000 K. Our results suggest that warm hydrogen-rich atmospheres (DA; $\gtrsim$13000 K) and helium-rich atmospheres (DB, DBA; $\gtrsim$30000 K) are unable to efficiently spread the accreted metals across their surface, regardless of the time dependence of accretion. This result may be at odds with the current non-detection of surface abundance variations at white dwarfs with debris discs. For cooler hydrogen- and helium-rich atmospheres, we predict a largely homogeneous distribution of metals across the surface within a vertical diffusion timescale. This is typically less than 0.1 per cent of disc lifetime estimates, a quantity which is revisited in this paper using the overshoot results. These results have relevance for studies of the bulk composition of evolved planetary systems and models of accretion disc physics.

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