Convection is a highly turbulent, three dimensional process that is traditionally treated using a simple, local, time independent description. Convection is one of the largest sources of theoretical uncertainty in stellar modeling. We outline recent progress in studies using pulsating white dwarfs to constrain convection and calibrate mixing length theory.

The white dwarfs in close, interacting binaries provide a natural laboratory for exploring the effects of heating and angular momentum from the accreting material arriving on the surface from the companion. This study is even more fruitful when it involves a pulsating white dwarf, which allows an exploration of the effects of the accretion on the interior as well as in the atmosphere. The last decade has seen the accomplishment of UV (HST) and optical (ground) studies of several accreting white dwarfs that have undergone a dwarf nova outburst that heated the white dwarf and subsequently returned to its quiescent temperature. The most recent study involves V386 Ser, which underwent its first known outburst in January 2019, after 19 years at quiescence. V386 Ser is unique in that its quiescent pulsation shows a triplet, with spacing indicating a rotation period of 4.8 days, extremely slow for accreting white dwarfs. This paper presents the result of HST ultraviolet spectra obtained 7 months after its outburst that shows the first clear confirmation of shorter period modes being driven following the heating from a dwarf nova outburst.

An extended magnetosphere of a strongly magnetized accreting white dwarf (known as a polar) prevents the formation of an accretion disk and the matter is channelled to the magnetic pole(s). A few such sources show quasi-periodic oscillations in their optical light curves. These high-frequency oscillations are thought to be generated from the post-shock accretion column. The kinetic energy of the accretion flow is finally emitted from this post-shock region and the involved radiation processes decide the state of the matter. Here we study the structure and the dynamical properties of such accretion columns and compare the results with the observational characteristics.

The shape of the luminosity function of white dwarfs (WDLF) is sensitive to the characteristic cooling time and, therefore, it can be used to test the existence of additional sources or sinks of energy such as those predicted by alternative physical theories. However, because of the degeneracy between the physical properties of white dwarfs and the properties of the Galaxy, the star formation history (SFH) and the IMF, it is almost always possible to explain any anomaly as an artifact introduced by the star formation rate. To circumvent this problem there are at least two possibilities, the analysis of the WDLF in populations with different stories, like disc and halo, and the search of effects not correlated with the SFH. These procedures are illustrated with the case of axions.

Evolution of post-AGB stars is extremely fast. They cross the HR diagram vertically on a timescale of hundreds to some ten thousands of years to reach maximum temperature in their lifetime. This is reflected in an increasing excitation of planetary nebulae on a timescale of years and decades. Since evolutionary timescale of post-AGB stars is very sensitive to their mass, observed changes can be used to determine model dependent central star masses. If an additional parameter is determined (e.g. luminosity or dynamic age), the observed evolution of planetary nebulae can be utilized for observational verification of theoretical models.

In the helium-rich intershell region of asymptotic giant branch (AGB) stars, slow neutron-capture nucleosynthesis produces heavy elements beyond iron. If the stars experience a final-flash of the He-burning shell, a pulse-driven convection zone establishes, the stars become hydrogen-deficient and exhibit former intershell material at their surfaces. In their subsequent evolution towards the white-dwarf cooling sequence, but still at constant luminosity, a strong stellar wind prevents diffusion to wipe out the information about AGB yields. We present and interpret the analysis results of hydrogen-rich and -deficient post-AGB stars, discuss difficulties in their analysis and review the implications on the understanding of post-AGB evolution.

As they evolve, white dwarfs undergo major changes in their atmospheric composition, a phenomenon known as spectral evolution. In particular, most hot He-rich (DO) stars transform into H-rich (DA) stars as they cool off, most likely as a result of the float-up of residual H. We investigate this DO-to-DA transition by taking advantage of the extensive spectroscopic dataset provided by the Sloan Digital Sky Survey (SDSS). Using our new state-of-the-art non-LTE model atmospheres, we perform a spectroscopic analysis of 1882 hot (Teff >30,000 K) white dwarfs identified in the SDSS. We find that at least 15% of all white dwarfs are born with a He-dominated atmosphere. Among these, ∼2/3 turn into H-rich stars before they reach Teff ∼40,000 K, while the remaining ∼1/3 maintain their He-rich surface throughout their entire evolution. We speculate on the origin of these two groups of objects.

Empirically characterizing the spectral evolution of cool white dwarfs is a prerequisite to understanding the physical processes that shape the evolution of these old objects. However, the high photospheric densities of cool helium-rich atmospheres seriously complicate the study of those stars. We have recently developed an updated atmosphere code that is appropriate for high densities and that can model any cool white dwarf (including DZs and DQpecs). Here, we present recent advances in our understanding of the spectral evolution of cool white dwarfs that were made possible thanks to these improved models. We discuss in particular the evolution of the hydrogen-rich to helium-rich ratio at low effective temperatures as well as the DQ→ DQpec transition.

The Gaia DR2 has dramatically increased the ability to detect faint nearby white dwarfs. The census of the local white dwarf population has recently been extended from 25 pc to 50 pc, effectively increasing the sample by roughly an order of magnitude. Here we examine the completeness of this new sample as a function of variables such as apparent magnitude, distance, proper motion, photometric color index, unresolved components, etc.

By comparing two age indicators of high-mass white dwarfs (WDs) derived from Gaia data, two discoveries have been made recently: one is the existence of a cooling anomaly that produces the Q branch structure on the Hertzsprung–Russell diagram, and the other is the existence of high-mass WDs as double-WD merger products. The former poses a challenge for WD cooling models, and the latter has implications on binary evolution and type-Ia supernovae.

White dwarfs (WDs) in open star clusters are a highly useful ensemble of stars. While numerous researchers use open cluster WDs to study the initial-final mass relation, numerous other evolutionary studies are also enabled by this sample of stars, including searches for stochastic mass loss, studies of binary star evolution, and measurements of metallicity impacts on WD formation and evolution. However, it is crucial to use astrometric data such as proper motions to remove contaminating field WDs from open cluster samples; multi-epoch ground based imaging is needed for most open cluster WDs. Also, the strongly correlated errors in the initial mass - final mass plane must be considered; we illustrate the importance of this consideration using a large open cluster WD sample and Monte Carlo techniques.

The cornerstone mission of the European Space Agency, Gaia, has revealed properties of 260 000 white dwarfs in the Galaxy, allowing us for the first time to constrain the evolution of white dwarfs with a large sample. Complementary surveys (CoRoT, Kepler, K2, APOGEE and Gaia-ESO), will revolutionize our understanding of the formation and history of our Galaxy, providing accurate stellar masses, radii, ages, distances, and chemical properties for very large samples of stars across different Galactic stellar populations. To exploit the potential of the combination of spectroscopic, seismic and astrometric observations, the population synthesis approach is a very crucial and efficient tool. We develop the Besançon Galaxy model (BGM, Lagarde et al.2017) for which stellar evolution predictions are included, providing the global asteroseismic properties and the surface chemical abundances along the evolution of low- and intermediate-mass stars. For the first time, the BGM can explore the effects of an extra-mixing occurring in red-giant stars Lagarde et al.2019, changing their stellar properties. The next step is to model a consistent treatment of giant stars and their remnants (e.g., white dwarfs). This kind of improvement would help us to constrain stellar and Galactic evolutions.

We employ Pan-STARRS photometry, Gaia trigonometric parallaxes, modern stellar evolution and atmosphere models, and our Bayesian fitting approach to determine cooling and total ages for 159,238 white dwarfs. In many cases we are able to derive precise ages (better than 5%) for individual white dwarfs. These results are meant for broad use within the white dwarf and stellar astrophysics communities and we plan to make available on-line the posterior distributions for cooling age, total age, initial stellar mass, and other parameters.

We analyze individual white dwarfs in open clusters observed by Gaia. In particular, we determine ages when different model ingredients are used. We also explore fundamental properties of the white dwarfs, including temperature and mass, when using different filter combinations. Such tests are important to understanding any systematic effects when applying similar techniques to field stars.

We present a sample of nearly 650 widely separated double white dwarf binaries found using Gaia DR2 astrometry. We derive preliminary total ages for each white dwarf in our sample using Gaia photometry and compare the total ages of both components of each binary in our sample. We find agreement within 3 sigma between the two ages ∼85% of the time with median age uncertainties of ∼3.5 Gyr depending on which initial-final mass relation is used. When a subsample with the most precise ages is used, the agreement within 3 sigma drops to ∼70% with median age uncertainties of 300-600 Myr.

Many intermediate polars are hard X-ray sources. The theory of their hard X-ray radiation is well developed and allows us to determine white dwarf masse in this kind of cataclysmic variables. Here we present the results of determination the masses of 35 white dwarfs in the intermediate polars observed by observatories NuSTAR (10 sources) and Swift/BAT (25 sources). The corresponding mass accrerion rates and the luminosity function were also derived due to accurate distance to the sources well known now after Gaia DR2.

We present some results from the DRAGON simulations, a set of four direct N-body simulations of globular clusters (GCs) with a million stars and five percent initial (primordial) binaries. These simulations were undertaken with the NBODY6++GPU code, which allowed us to follow dynamical and stellar evolution of individual stars and binaries, formation and evolution of white dwarfs, neutron stars, and black holes, and the effect of a galactic tidal field. The simulations are the largest existing models of a realistic globular cluster over its full lifetime of 12 billion years. In particular we will show here an investigation of the population of binaries including compact objects (such as white dwarfs - cataclysmic variables and merging black hole binaries in the model as counterparts of LIGO/Virgo sources); their distribution in the cluster and evolution with time.

Masses have been computed for the white dwarfs (WDs) in eclipsing, mass exchange (symbiotic), WD–red giant (RG) binaries by using single-lined spectroscopic orbits, orbital inclinations, and the RG masses. Inclinations have been measured for 13 eclipsing symbiotic binaries. Using Gaia data the mass of the RG can be found from evolutionary tracks. Since the WD evolved from the more massive star in the binary, the WD should be more massive than predicted from the mass of the current RG. Typically the WD has a lower mass than expected implying a previous mass exchange stage for these systems.

In Kawahara et al. (2018) and Masuda et al. (2019), we reported the discovery of four self-lensing binaries consisting of F/G-type stars and (most likely) white dwarfs whose masses range from 0.2 to 0.6 solar masses. Here we present their updated system parameters based on new radial velocity data from the Tillinghast Reflector Echelle Spectrograph at the Fred Lawrence Whipple Observatory, and the Gaia parallaxes and spectroscopic parameters of the primary stars. We also briefly discuss the astrophysical implications of these findings.

White dwarfs are useful objects with which to study the local interstellar medium (ISM). High ionisation state absorption features that cannot be attributed to the photosphere or the ISM have been observed along the line-of-sight to a number of white dwarf stars. Suggested origins of these lines include ionisation from past supernovae, stellar winds, circumstellar disks, photoionisation from nearby hot stars or also from the white dwarf itself. In this study we consider the origin of these non-photospheric highly ionised lines in two stars towards a rarefied region of the galaxy known as the extended β CMa Tunnel. We present preliminary results from our analysis of the first of these two stars.