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Recent advances in time-domain astronomy have uncovered a new class of optical transients with time-scales shorter than typical supernovae and a wide range of peak luminosities. Several subtypes have been identified within this broad class, including Ca-rich transients, Type Ia supernovae (SNe Ia) and fast/bright transients. We examine the predictions from a state-of-the-art grid of three-dimensional simulations of dynamical white dwarf interactions in the context of these fast optical transients. We find that for collisions involving carbon–oxygen or oxygen–neon white dwarfs, the peak luminosities and durations of the light curves in our models are in good agreement with the properties of fast/bright transients. When one of the colliding white dwarfs is made of helium, the properties of the light curves are similar to those of Ca-rich gap transients. The model light curves from our white dwarf collisions are too slow to reproduce those of SNe Ia, and too fast to match any normal or peculiar SNe Ia. [ABSTRACT FROM AUTHOR]

Context. NGC 6397 is one of the most interesting, well-observed, and most thoroughly theoretically studied globular clusters. The existing wealth of observations allows us to study the reliability of the theoretical white dwarf cooling sequences of low-metallicity progenitors, to determine the age of NGC 6397 and the percentage of unresolved binaries. We also assess other important characteristics of the cluster, such as the slope of the initial mass function or the fraction of white dwarfs with hydrogen-deficient atmospheres. Aims. We present a population synthesis study of the white dwarf population of NGC 6397. In particular, we study the shape of the color-magnitude diagram and the corresponding magnitude and color distributions. Methods. To do this, we used an advanced Monte Carlo code that incorporates the most recent and reliable cooling sequences and an accurate modeling of the observational biases. Results. Our theoretical models and the observed data agree well. In particular, we find that this agreement is best for those cooling sequences that take into account residual hydrogen burning. This result has important consequences for the evolution of progenitor stars during the thermally pulsing asymptotic giant branch phase, since it implies that appreciable third dredge-up in low-mass, low-metallicity progenitors is not expected to occur. Using a standard burst duration of 1.0 Gyr, we obtain that the age of the cluster is 12.8+0.50 -0.75 Gyr. Greater ages are also compatible with the observed data, but then unrealistic longer durations of the initial burst of star formation are needed to fit the luminosity function. Conclusions. We conclude that a correct modeling of the white dwarf population of globular clusters, used in combination with the number counts of main-sequence stars, provides a unique tool for modeling the properties of globular clusters. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Context. 47 Tucanae is one of the most interesting, well-observed, and theoretically studied globular clusters. This allows us to determine the reliability of our understanding of white dwarf cooling sequences, to compare different methods of determining its age, and to assess other important characteristics, such as its star formation history. Aims. Here we present a population synthesis study of the cooling sequence of the globular cluster 47 Tucanae. In particular, we study the distribution of effective temperatures, the shape of the color-magnitude diagram, and the corresponding magnitude and color distributions. Methods. To do this, we used an up-to-date population synthesis code based on Monte Carlo techniques that incorporates the most recent and reliable cooling sequences and an accurate modeling of the observational biases. Results. Our theoretical models and the observed data agree well. Thus, our study disproves previous claims that there are still missing physics in the white dwarf cooling models at moderately high effective temperatures. We also derive the age of the cluster using the termination of the cooling sequence and obtain a good agreement with the age determinations made using the main-sequence turnoff. Finally, the star formation history of the cluster is compatible with that obtained using main-sequence stars, which predicts two distinct populations. Conclusions. We conclude that a correct modeling of the white dwarf population of globular clusters, used in combination with the number counts of main -sequence stars, provides a unique tool to model the properties of globular clusters. [ABSTRACT FROM AUTHOR]

Context. 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. Aims. 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. Methods. 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: the first, where the adopted metallicity is constant with time but with a moderate dispersion, and the second where the metallicity increases with time. Results. 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). These results are independent 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 adopted age-metallicity relationship. Conclusions. 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 its location provides a robust indicator of the age of the Galactic disk due to the limited sensitivity of the position of the drop-off to the distribution of metallicities. [ABSTRACT FROM AUTHOR]

The disc white dwarf luminosity function is an important tool for studying the solar neighbourhood, since it allows the determination of several Galactic parameters, the most important one being the age of the Galactic disc. However, only the method has been employed so far for observationally determining the white dwarf luminosity function, whereas for other kind of luminosity functions several other methods have been frequently used. Moreover, the procedures to determine the white dwarf luminosity function are not free of biases. These biases have two different origins: they can either be of statistical nature or a consequence of the measurement errors. In a previous paper we carried out an in-depth study of the first category of biases for several luminosity function estimators. In this paper we focus on the biases introduced by the measurement errors and on the effects of the degree of contamination of the input sample used to build the disc white dwarf luminosity function by different kinematical populations. To assess the extent of these biases we use a Monte Carlo simulator to generate a controlled synthetic population and analyse the behaviour of the disc white dwarf luminosity function for several assumptions about the magnitude of the measurement errors and for several degrees of contamination, comparing the performances of the most robust luminosity function estimators under such conditions. [ABSTRACT FROM AUTHOR]

The white dwarf luminosity function is an important tool for the study of the solar neighbourhood, since it allows the determination of the age of the Galactic disc. Over the years, several methods have been proposed to compute galaxy luminosity functions, from the most simple ones – counting sample objects inside a given volume – to very sophisticated ones – like the C− method, the STY method or the Choloniewski method, among others. However, only the method is usually employed in computing the white dwarf luminosity function and other methods have not been applied so far to the observational sample of spectroscopically identified white dwarfs – in sharp contrast with the situation when galaxy luminosity functions are derived from a large variety of samples. Moreover, the statistical significance of the white dwarf luminosity function has also received little attention and a thorough study still remains to be done. In this paper, we study, using a controlled synthetic sample of white dwarfs generated using a Monte Carlo simulator, which is the statistical significance of the white dwarf luminosity function and which are the expected biases. We also present a comparison between different estimators for computing the white dwarf luminosity function. We find that for sample sizes large enough the method provides a reliable characterization of the white dwarf luminosity function, provided that the input sample is selected carefully. Particularly, the method recovers well the position of the cut-off of the white dwarf luminosity function. However, this method turns out to be less robust than the Choloniewski method when the possible incompletenesses of the sample are taken into account. We also find that the Choloniewski method performs better than the method in estimating the overall density of white dwarfs, but misses the exact location of the cut-off of the white dwarf luminosity function. [ABSTRACT FROM AUTHOR]

One of the most promising space missions of the European Space Agency is the astrometric satellite Gaia, which will provide very precise astrometry and multicolour photometry, for all 1.3 billion objects to , and radial velocities with accuracies of a few km s−1 for most stars brighter than . Consequently, full homogeneous six-dimensional phase-space information for a huge number of stars will become available. Our Monte Carlo simulator has been used to estimate the number of white dwarfs potentially observable by Gaia. From this we assess the white dwarf luminosity functions that Gaia will obtain and discuss in depth the scientific returns of Gaia in the specific field of white dwarf populations. Scientifically attainable goals include, among others, a reliable determination of the age of the Galactic disc, a better knowledge of the halo of the Milky Way and the reconstruction of the star formation history of the Galactic disc. Our results also demonstrate the potential impact of a mission such as Gaia within the context of current understanding of white dwarf cooling theory. [ABSTRACT FROM AUTHOR]

In this paper we analyse the consequences in the white dwarf population of a hypothetical merger episode in our Galactic disc. We have studied several different merging scenarios with our Monte Carlo simulator. For each one of these scenarios we have derived the main characteristics of the resulting white dwarf population and we have compared them with the available observational data, namely the white dwarf luminosity function and the kinematic properties of the white dwarf population. Our results indicate that very recent (less than ∼6 Gyr ago) and massive (∼16 per cent of the mass of our Galaxy) merger episodes are quite unlikely in view of the available kinematical properties of the disc white dwarf population. Smaller merger episodes (of the order of ∼4 per cent of the mass of our Galaxy) are, however, compatible with our current knowledge of those kinematical properties. Finally, we prove that the white dwarf luminosity function is quite insensitive to such a merger episode. [ABSTRACT FROM AUTHOR]

NGC 6791 is a well studied open cluster that it is so close to us that can be imaged down to very faint luminosities. The main-sequence turn-off age (∼8 Gyr) and the age derived from the termination of the white dwarf cooling sequence (∼6 Gyr) are very different. One possible explanation is that as white dwarfs cool, one of the ashes of helium burning, 22Ne, sinks in the deep interior of these stars. At lower temperatures, white dwarfs are expected to crystallize and phase separation of the main constituents of the core of a typical white dwarf (12C and 16O) is expected to occur. This sequence of events is expected to introduce long delays in the cooling times, but has not hitherto been proven. Here we report that, as theoretically anticipated, physical separation processes occur in the cores of white dwarfs, resolving the age discrepancy for NGC 6791. [ABSTRACT FROM AUTHOR]