The effects of metallicity on the Galactic disk population of white dwarfs

It has been known for a long time that stellar metallicity plays a significant role in the determination of the ages of the different Galactic stellar populations, when main sequence evolutionary tracks are employed. Here we analyze the role that metallicity plays on the white dwarf luminosity function of the Galactic disk, which is often used to determine its age. We employ a Monte Carlo population synthesis code that accounts for the properties of the population of Galactic disk white dwarfs. Our code incorporates the most up-to-date evolutionary cooling sequences for white dwarfs with hydrogen-rich and hydrogen-deficient atmospheres for both carbon-oxygen and oxygen-neon cores. We use two different models to assess the evolution of the metallicity, one in which the adopted metallicity is constant with time, but with a moderate dispersion, and a second one in which the metallicity increases with time. We found that our theoretical results are in a very satisfactory agreement with the observational luminosity functions obtained from the Sloan Digital Sky Survey (SDSS) and from the SuperCOSMOS Sky Survey (SSS), independently of the adopted age-metallicity law. In particular, we found that the age-metallicity law has no noticeable impact in shaping the bright branch of the white dwarf luminosity function, and that the position of its cut-off is almost insensitive to the adopoted age-metallicity relationship. Because the shape of the bright branch of the white dwarf luminosity function is insensitive to the age-metallicity law, it can be safely employed to test the theoretical evolutionary sequences, while due to the limited sensitivity of the position of the drop-off to the distribution of metallicities, its location provides a robust indicator of the age of the Galactic disk.Comment: 7 pages, 5 figures, accepted for publication in A&

Population synthesis studies of the white dwarfs of the galactic disk and halo

As the evolutionary end-point of most main-sequence stars, white dwarfs can be used to infer important properties concerning the formation and evolution of the different Galactic stellar populations. Population synthesis methods, when correlated with the excellent quality of modern white dwarf cooling tracks, can prove to be a powerful tool for further exploring the information offered by observed samples of white dwarfs. Possible applications may include estimating the age and history of stellar populations, measuring the local stellar formation rate and stellar density, constraining the local content of baryonic matter, examining intrinsic aspects of single and binary stellar evolution and exploring different progenitor scenarios for type Ia supernovae. In this thesis we studied the white dwarf populations of the Galactic disk and halo and our work was focused on four main objectives. The first was to analyze the effects of stellar metallicity on the white dwarf luminosity function of the Galactic disk. We use our Monte Carlo population synthesis code to produce synthetic samples of white dwarfs which we later compare to two relatively large observational samples, from the Sloan Digital Sky Survey (SDSS) and SuperCosmos Sky Survey (SSS). For each of the two cases, we implement the corresponding observational selection criteria. We first explore the impact of varying the fraction of white dwarfs with hydrogen-deficient atmospheres and of using alternative sets of cooling tracks. We then test different age-metallicity relations, considering the impact of progenitor metallicity throughout the entire evolution of the star. Our second objective was to determine the luminosity, mass and cumulative age functions derived from a sample of disk white dwarfs identified by the LAMOST Sky Survey of the Galactic Anti-Center. We initially compute the observed functions and then the theoretical ones, according to series of initial assumptions, using our population synthesis code. We derive values for the local space density and formation rate of hydrogen-rich white dwarfs and analyze the possible causes behind the apparent excess of massive white dwarfs identified in the observed mass function. The next objective of this thesis was to revisit the halo white dwarf luminosity function, modeling the halo white dwarf population using a self-consistent prescription for very low metallicity progenitors. We pass the synthetic sample through a series of filters that reproduce the selection criteria employed in culling the observed sample of halo white dwarfs from the SSS. Given that for very low-metallicity progenitors residual hydrogen burning can become a significant source of energy, we test two different sets of cooling tracks, one that incorporates residual hydrogen burning and another where this phenomenon is artificially ignored. Finally, our last study consisted in modeling the sample of white dwarf-main sequence binaries in the Galactic disk. We compare our simulations to the largest and most recent catalog of white dwarf-main sequence binaries from the SDSS and we provide a detailed account of observational biases and errors. We use this sample to constrain several important inputs for binary formation and evolution, namely the initial mass ratio distribution and the common envelope efficiency parameter. To conclude, the work presented in this thesis testifies to the versatility of using white dwarf populations for constraining different properties of the Galactic disk and halo. The methodology described here can be used in future studies of this kind, with a myriad of new applications, especially given than in the near future a noticeable improvement in terms of the size and quality of observed white dwarf catalogs is expected.Como estadio final en la evolución de la mayoría de estrellas de la secuencia principal, las enanas blancas se pueden utilizar para inferir propiedades importantes relativas a la formación y evolución de la Galaxia. Los métodos de síntesis de población, junto con la excelente calidad ofrecida por las secuencias de enfriamiento actuales, constituyen una herramienta versátil para el estudio detallado de las muestras observadas de enanas blancas. Entre otras posibles aplicaciones cabe citar: estimar las edades de diversas poblaciones estelares, medir el ritmo local de formación estelar y la densidad estelar, acotar el contenido local de materia bariónica, analizar la evolución estelar para estrellas tanto individuales como binarias o explorar diversos escenarios para los progenitores de Supernovas tipo Ia. En esta tesis doctoral se presentan varios estudios sobre las poblaciones de enanas blancas del disco y del halo de la Galaxia centrados en torno a cuatro objetivos principales. El primero consistió en analizar de forma teórica los efectos de la metalicidad sobre la función de luminosidad de las enanas blancas del disco Galáctico. Para ello se empleó un código Monte Carlo de síntesis de población. La muestra simulada obtenida se comparó con dos muestras observadas de enanas blancas: la del Sloan Digital Sky Survey (SDSS) y la del SuperCosmos Sky Survey (SSS). En ambos casos se tuvo en cuenta los correspondientes efectos de selección. En una primera fase, se analizaron los efectos de la proporción de enanas blancas con atmósferas escasas en hidrógeno. A continuación, se comprobaron diferentes relaciones edad-metalicidad, teniendo en cuenta el efecto de ésta a lo largo de la evolución de la estrella. Nuestro segundo objetivo consintió en determinar las funciones de luminosidad, de masa y de edad acumulativa, derivadas de la muestra de enanas blancas identificadas por LAMOST. Primero se calcularon las funciones observadas y luego las simuladas en base a nuestro código de síntesis de poblaciones. Se calcularon también los valores tanto de la densidad estelar como del ritmo local de formación para las enanas blancas con atmósferas ricas en hidrógeno. Por último, se analizaron las posibles causas del exceso de enanas blancas masivas observado. El tercer objetivo de esta tesis se centró en realizar un análisis actualizado de la función de luminosidad de las enanas blancas del halo, utilizando una prescripción auto-consistente para enanas blancas con progenitores de muy baja metalicidad. La población simulada se pasó por una serie de filtros creados para reproducir los criterios de selección utilizados en la muestra observacional del SSS. Dado que, para enanas blancas provenientes de estrellas de muy baja metalicidad, la quema residual de hidrógeno se convierte en una fuente importante de energía, se compararon los resultados usando secuencias de enfriamiento con quema residual de hidrógeno y secuencias calculadas sin tener en cuenta este fenómeno. Finalmente, nuestro último objetivo fue reproducir la muestra de binarias compuestas por una enana blanca y una estrella de la secuencia principal en el contexto del disco Galáctico. Las simulaciones fueron comparadas con la muestra más extensa y reciente para este tipo de objetos obtenida del SDSS. El estudio también incorporó un tratamiento muy detallado de los sesgos y errores observacionales. La muestra resultante simulada se utilizó para restringir varios parámetros importantes en la formación y evolución de sistemas binarios, tales como la distribución inicial del cociente de masa y el parámetro de eficiencia de la fase de envoltura común. Para concluir, el trabajo presentado en esta tesis demuestra la versatilidad del uso de los modelos de síntesis de poblaciones de enanas blancas en la estimación de diferentes propiedades del disco y halo de la Galaxia. La metodología aquí descrita puede ser utilizada en multitud de estudios futuros de este tipoPostprint (published version

The population of single and binary white dwarfs of the Galactic bulge

Recent Hubble Space Telescope observations have unveiled the white dwarf cooling sequence of the Galactic bulge. Although the degenerate sequence can be well fitted employing the most up-to-date theoretical cooling sequences, observations show a systematic excess of red objects that cannot be explained by the theoretical models of single carbon–oxygen white dwarfs of the appropriate masses. Here, we present a population synthesis study of the white dwarf cooling sequence of the Galactic bulge that takes into account the populations of both single white dwarfs and binary systems containing at least one white dwarf. These calculations incorporate state-of-the-art cooling sequences for white dwarfs with hydrogen-rich and hydrogen-deficient atmospheres, for both white dwarfs with carbon–oxygen and helium cores, and also take into account detailed prescriptions of the evolutionary history of binary systems. Our Monte Carlo simulator also incorporates all the known observational biases. This allows us to model with a high degree of realism the white dwarf population of the Galactic bulge. We find that the observed excess of red stars can be partially attributed to white dwarf plus main sequence binaries, and to cataclysmic variables or dwarf novae. Our best fit is obtained with a higher binary fraction and an initial mass function slope steeper than standard values, as well as with the inclusion of differential reddening and blending. Our results also show that the possible contribution of double degenerate systems or young and thick-discbulge stars is negligible.Peer ReviewedPostprint (published version

The population of white dwarf-main sequence binaries in the SDSS DR 12

We present a Monte Carlo population synthesis study of white dwarf-main sequence (WD+MS) binaries in the Galactic disc aimed at reproducing the ensemble properties of the entire population observed by the Sloan Digital Sky Survey (SDSS) Data Release 12. Our simulations take into account all known observational biases and use the most up-to-date stellar evolutionary models. This allows us to perform a sound comparison between the simulations and the observational data. We find that the properties of the simulated and observed parameter distributions agree best when assuming low values of the common envelope efficiency (0.2-0.3), a result that is in agreement with previous findings obtained by observational and population synthesis studies of close SDSSWD+MS binaries.We also show that all synthetic populations that result from adopting an initial mass ratio distribution with a positive slope are excluded by observations. Finally, we confirm that the properties of the simulated WD+MS binary populations are nearly independent of the age adopted for the thin disc, on the contribution of WD+MS binaries from the thick disc (0-17 per cent of the total population) and on the assumed fraction of the internal energy that is used to eject the envelope during the common envelope phase (0.1-0.5).Peer ReviewedPostprint (published version

Axions and the luminosity function of white dwarfs : the thin and thick discs, and the halo

The evolution of white dwarfs is a simple gravothermal process of cooling. Since the shape of their luminosity function is sensitive to the characteristic cooling time, it is possible to use its slope to test the existence of additional sources or sinks of energy, such as those predicted by alternative physical theories. The aim of this paper is to study if the changes in the slope of the white dwarf luminosity function around bolometric magnitudes ranging from 8 to 10 and previously attributed to axion emission are, effectively, a consequence of the existence of axions and not an artefact introduced by the star formation rate. We compute theoretical luminosity functions of the thin and thick disc, and of the stellar halo including axion emission and we compare them with the existing observed luminosity functions. Since these stellar populations have different star formation histories, the slope change should be present in all of them at the same place if it is due to axions or any other intrinsic cooling mechanism. The signature of an unexpected cooling seems to be present in the luminosity functions of the thin and thick discs, as well as in the halo luminosity function. This additional cooling is compatible with axion emission, thus supporting to the idea that DFSZ axions, with a mass in the range of 4–10 meV, could exist. If this were the case, these axions could be detected by the future solar axioscope IAXO.Peer ReviewedPostprint (author's final draft

The RefinedWeb Dataset for Falcon LLM: Outperforming Curated Corpora with Web Data, and Web Data Only

Large language models are commonly trained on a mixture of filtered web data and curated high-quality corpora, such as social media conversations, books, or technical papers. This curation process is believed to be necessary to produce performant models with broad zero-shot generalization abilities. However, as larger models requiring pretraining on trillions of tokens are considered, it is unclear how scalable is curation and whether we will run out of unique high-quality data soon. At variance with previous beliefs, we show that properly filtered and deduplicated web data alone can lead to powerful models; even significantly outperforming models from the state-of-the-art trained on The Pile. Despite extensive filtering, the high-quality data we extract from the web is still plentiful, and we are able to obtain five trillion tokens from CommonCrawl. We publicly release an extract of 600 billion tokens from our RefinedWeb dataset, and 1.3/7.5B parameters language models trained on it

Revisiting the luminosity function of single halo white dwarfs

Context. White dwarfs are the fossils left by the evolution of low- and intermediate-mass stars, and have very long evolutionary timescales. This allows us to use them to explore the properties of old populations, like the Galactic halo. Aims. We present a population synthesis study of the luminosity function of halo white dwarfs, aimed at investigating which information can be derived from the currently available observed data. Methods. We employ an up-to-date population synthesis code based on Monte Carlo techniques, which incorporates the most recent and reliable cooling sequences for metal-poor progenitors as well as an accurate modeling of the observational biases. Results. We find that because the observed sample of halo white dwarfs is restricted to the brightest stars, only the hot branch of the white dwarf luminosity function can be used for these purposes, and that its shape function is almost insensitive to the most relevant inputs, such as the adopted cooling sequences, the initial mass function, the density profile of the stellar spheroid, or the adopted fraction of unresolved binaries. Moreover, since the cutoff of the observed luminosity has not yet been determined only the lower limits to the age of the halo population can be placed. Conclusions. We conclude that the current observed sample of the halo white dwarf population is still too small to obtain definite conclusions about the properties of the stellar halo, and the recently computed white dwarf cooling sequences, which incorporate residual hydrogen burning, should be assessed using metal-poor globular clusters.Facultad de Ciencias Astronómicas y GeofísicasInstituto de Astrofísica de La Plat

The age-metallicity relation in the solar neighbourhood from a pilot sample of white dwarf-main sequence binaries

The age-metallicity relation (AMR) is a fundamental observational constraint for understanding how the Galactic disc formed and evolved chemically in time. However, there is not yet an agreement on the observational properties of the AMR for the solar neighbourhood, primarily due to the difficulty in obtaining accurate stellar ages for individual field stars. We have started an observational campaign for providing the much needed observational input by using wide white-dwarf-main-sequence (WDMS) binaries. White dwarfs are `natural' clocks and can be used to derive accurate ages. Metallicities can be obtained from the main-sequence companions. Since the progenitors of white dwarfs and the main-sequence stars were born at the same time, WDMS binaries provide a unique opportunity to observationally constrain in a robust way the properties of the AMR. In this work we present the AMR derived from analysing a pilot sample of 23 WDMS binaries and provide clear observational evidence for the lack of correlation between age and metallicity at young and intermediate ages (0-7 Gyr)