arXiv subjects

Alejandro Santos-García

Publications and source records attributed to Alejandro Santos-García.

8 recordsLinked to original sources

arXiv2026

The scarcity of white dwarf-brown dwarf binaries in the solar neighbourhood: A population synthesis study

White dwarf--brown dwarf (WD--BD) binaries are intrinsically rare systems that provide a unique opportunity to study binary evolution and the formation of substellar companions. Despite recent observational progress, only a small number are known in the solar neighbourhood. We investigate whether their scarcity can be explained by binary formation and evolution together with observational selection effects. We extend the MRBIN binary population synthesis code into the substellar regime by incorporating brown dwarf evolutionary models and an extended initial mass function. We simulate the stellar population within 100 pc of the Sun and compare the predicted WD--BD population with the observed sample. Our simulations predict 13+-5 WD--BD systems within 100 pc, consistent with observations. These systems represent only 0.1% of the local white dwarf population, confirming their rarity. The models reproduce the overall distributions of white dwarf masses, temperatures, and orbital periods, although the simulated brown dwarfs are systematically hotter than observed. Close WD--BD binaries are underproduced with the standard common-envelope efficiency 0.3, suggesting that higher efficiencies may be required to reproduce the observed short-period systems. The simulations also reproduce the observed deficit of systems at intermediate orbital periods between post-common-envelope binaries and wide non-interacting systems, as well as a weakened brown dwarf desert in main-sequence--brown dwarf binaries.

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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

The DBL Survey II: towards a mass-period distribution of double white dwarf binaries

Double white dwarf binaries are an important remnant of binary evolution as they are possible type Ia supernova progenitors and strong sources of gravitational waves in the low-frequency regime. The double-lined double white dwarf (DBL) survey searches for compact double white dwarfs where both stars are spectrally disentangleable. Candidates are identified by being overluminous compared to the cooling sequence of a typical mass, single white dwarf. In this second DBL survey instalment, we present full orbital solutions of 15 double white dwarf binaries from our ongoing campaign to accurately measure a magnitude-limited mass-period distribution. 12 of these systems are fully solved for the first time. A long-standing bias in the full population has been evident, favouring systems with orbital periods up to a few hours, with little exploration of the majority of the compact double white dwarf population, whose orbital period distribution centres at approximately 20hr. The 15 systems in this study span the orbital period range 5-75hr, significantly augmenting the number of well-characterised systems over these periods, and in general have two similar mass stars combining to approximately 1.0 solar masses. We witness that the orbitally derived mass ratios generally show an excellent agreement with those deduced from atmospheric fits to double-lined spectra in previous work, emphasising the power of wide-scale spectroscopic surveys to efficiently locate the highest mass, double-lined double white dwarfs in the local Galaxy.

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arXiv2025

Reconstructing post-common envelope white dwarf+main sequence binary histories through inverse population synthesis techniques

The evolution of binary stellar systems involves a wide range of physical processes, many of which are not yet well understood. We aim to build a general-purpose algorithm based on inverse population synthesis techniques, able to reconstruct the past history of binary systems. This algorithm will be applied to a sample of eclipsing binaries, aiming to ascertain their progenitors and past histories. Once validated, it was applied to a sample 30 white dwarf plus main-sequence eclipsing binaries observed by the Zwicky Transient Facility survey. We determined the input space parameters of the progenitors for the 30 eclipsing binary systems to which the algorithm was applied. These parameters included the initial primary and secondary masses, the orbital separation and eccentricity, the common-envelope efficiency ($α_{\rm CE}$), and the age at which the system was formed. Furthermore, the analysis of the global properties revealed some important features: a mild anticorrelation between the common-envelope efficiency parameter and the secondary mass, the absence of a universal value of $α_{\rm CE}$ along with no need for internal energy, although in the low-mass regime, the high values of $α_{\rm CE}$ suggest a possible contribution, and an initial thermalized eccentricity distribution. Although a strong degeneracy among the input parameters exists in the reconstruction of post-common envelope binary systems, the high accuracy obtained for the eclipsing-binary systems analyzed here has allowed our algorithm to make a reasonable determination of the initial parameters without the need to include external constraints. The global properties found here so far, can be substantially improved when analyzing a future volume-complete sample.

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arXiv2025

A population synthesis study of the Gaia 100 pc unresolved white dwarf-main sequence binary population

Binary stars consisting of a white dwarf and a main sequence star (WDMS) are valuable for studying key astrophysical questions. However, observational biases strongly affect the known population, particularly unresolved systems where the main sequence star outshines the white dwarf. This work aims to comprehensively simulate the population of unresolved WDMS binaries within 100 pc of the Sun and to compare the outcome with the currently most complete volume-limited sample available from Gaia data. We employ a population synthesis code, MRBIN, extensively developed by our group and based on Monte Carlo techniques, which uses a standard binary stellar evolutionary code adapted to cover a wide range of stars across all ages, masses, and metallicities. Selection criteria matching those of Gaia observations are applied to generate synthetic populations comparable to the observed WDMS sample. The synthetic data accurately populate the expected regions in the Gaia color-magnitude diagram. However, simulations predict a lower number of extremely low-mass white dwarfs, suggesting potential issues in observed mass derivations. Additionally, our analysis constrains the common envelope efficiency to 0.1-0.4, consistent with previous findings, and estimates a total completeness of about 25% for the observed sample, confirming the strong observational limitations for unresolved WDMS.

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