The progenitors of magnetic white dwarfs in open clusters

According to the fossil-field hypothesis magnetic fields are remnants of the previous stages of evolution. However, population synthesis calculations are unable to reproduce the magnetic white dwarf (MWD) sample without binary interaction or inclusion of a population of progenitor with unobservable small-scale fields. One necessary ingredient in population synthesis is the initial-to-final-mass relation (IFMR) which describes the mass-loss processes during the stellar evolution. When white dwarfs are members of open clusters, their evolutionary histories can be assessed through the use of cluster properties. In this work, we assess the cluster membership by correlating the proper-motion of MWDs with the cluster proper-motion and by analyzing the candidates spectroscopically with our magnetic model spectra in order to estimate the effective temperature and radii. We identified SDSS J085523.87+164059.0 to be a proper-motion member of Praesepe. We also included the data of the formerly identified cluster members NGC 6819-8, WD 0836+201 and estimated the mass, cooling age and the progenitor masses of the three probable MWD members of open clusters. According to our analysis, the newly identified cluster member SDSS J085523.87+164059.0 is an ultra-massive MWD of mass 1.12 $\pm$ 0.11 Msolar. We increase the sample of MWDs with known progenitor masses to ten, with the rest of the data coming from the common proper motion binaries. Our investigations show that, when effects of the magnetic fields are included in the diagnostics, the estimated properties of these cluster MWDs do not show evidence for deviations from the IFMR. Furthermore, we estimate the precision of the magnetic diagnostics which would be necessary to determine quantitatively whether magnetism has any effect on the mass-loss.Comment: 8 pages, 4 figures, accepted for publication in A&

White Dwarf Cosmochronology in the Solar Neighborhood

The study of the stellar formation history in the solar neighborhood is a powerful technique to recover information about the early stages and evolution of the Milky Way. We present a new method which consists of directly probing the formation history from the nearby stellar remnants. We rely on the volume complete sample of white dwarfs within 20 pc, where accurate cooling ages and masses have been determined. The well characterized initial-final mass relation is employed in order to recover the initial masses (1 < M/Msun < 8) and total ages for the local degenerate sample. We correct for moderate biases that are necessary to transform our results to a global stellar formation rate, which can be compared to similar studies based on the properties of main-sequence stars in the solar neighborhood. Our method provides precise formation rates for all ages except in very recent times, and the results suggest an enhanced formation rate for the solar neighborhood in the last 5 Gyr compared to the range 5 < Age (Gyr) < 10. Furthermore, the observed total age of ~10 Gyr for the oldest white dwarfs in the local sample is consistent with the early seminal studies that have determined the age of the Galactic disk from stellar remnants. The main shortcoming of our study is the small size of the local white dwarf sample. However, the presented technique can be applied to larger samples in the future.Comment: 25 pages, 10 figures, accepted for publication in the Astrophysical Journa

The Field White Dwarf Mass Distribution

We revisit the properties and astrophysical implications of the field white dwarf mass distribution in preparation of Gaia applications. Our study is based on the two samples with the best established completeness and most precise atmospheric parameters, the volume-complete survey within 20 pc and the Sloan Digital Sky Survey (SDSS) magnitude-limited sample. We explore the modelling of the observed mass distributions with Monte Carlo simulations, but find that it is difficult to constrain independently the initial mass function (IMF), the initial-to-final-mass relation (IFMR), the stellar formation history (SFH), the variation of the Galactic disk vertical scale height as a function of stellar age, and binary evolution. Each of these input ingredients has a moderate effect on the predicted mass distributions, and we must also take into account biases owing to unidentified faint objects (20 pc sample), as well as unknown masses for magnetic white dwarfs and spectroscopic calibration issues (SDSS sample). Nevertheless, we find that fixed standard assumptions for the above parameters result in predicted mean masses that are in good qualitative agreement with the observed values. It suggests that derived masses for both studied samples are consistent with our current knowledge of stellar and Galactic evolution. Our simulations overpredict by 40-50% the number of massive white dwarfs (M > 0.75 Msun) for both surveys, although we can not exclude a Salpeter IMF when we account for all biases. Furthermore, we find no evidence of a population of double white dwarf mergers in the observed mass distributions.Comment: 15 pages, 16 figures, accepted for publication in MNRA

On the Radial Distribution of White Dwarfs in the Globular Cluster NGC 6397

We have examined the radial distribution of white dwarfs over a single HST/ACS field in the nearby globular cluster NGC 6397. In relaxed populations, such as in a globular cluster, stellar velocity dispersion, and hence radial distribution, is directly dependent on stellar masses. The progenitors of very young cluster white dwarfs had a mass of ~0.8 solar masses, while the white dwarfs themselves have a mass of ~0.5 solar masses. We thus expect young white dwarfs to have a concentrated radial distribution (like that of their progenitors) that becomes more extended over several relaxation times to mimic that of ~0.5 solar mass main-sequence stars. However, we observe young white dwarfs to have a significantly extended radial distribution compared to both the most massive main sequence stars in the cluster and also to old white dwarfs.Comment: 13 pages including 1 table and 3 figures. Accepted for publication in the MNRAS Letter

The Binary Fraction of Low Mass White Dwarfs

We describe spectroscopic observations of 21 low-mass (<0.45 M_sun) white dwarfs (WDs) from the Palomar-Green Survey obtained over four years. We use both radial velocities and infrared photometry to identify binary systems, and find that the fraction of single, low-mass WDs is <30%. We discuss the potential formation channels for these single stars including binary mergers of lower-mass objects. However, binary mergers are not likely to explain the observed number of single low-mass WDs. Thus additional formation channels, such as enhanced mass loss due to winds or interactions with substellar companions, are likely.Comment: 9 pages, accepted to Ap

Deep HST Imaging in NGC 6397: Stellar Dynamics

Multi-epoch observations with ACS on HST provide a unique and comprehensive probe of stellar dynamics within NGC 6397. We are able to confront analytic models of the globular cluster with the observed stellar proper motions. The measured proper motions probe well along the main sequence from 0.8 to below 0.1 M$_\odot$ as well as white dwarfs younger than one gigayear. The observed field lies just beyond the half-light radius where standard models of globular cluster dynamics (e.g. based on a lowered Maxwellian phase-space distribution) make very robust predictions for the stellar proper motions as a function of mass. The observed proper motions show no evidence for anisotropy in the velocity distribution; furthermore, the observations agree in detail with a straightforward model of the stellar distribution function. We do not find any evidence that the young white dwarfs have received a natal kick in contradiction with earlier results. Using the observed proper motions of the main-sequence stars, we obtain a kinematic estimate of the distance to NGC 6397 of $2.2^{+0.5}_{-0.7}$ kpc and a mass of the cluster of $1.1 \pm 0.1 \times 10^5 \mathrm{M}_\odot$ at the photometric distance of 2.53 kpc. One of the main-sequence stars appears to travel on a trajectory that will escape the cluster, yielding an estimate of the evaporation timescale, over which the number of stars in the cluster decreases by a factor of e, of about 3 Gyr. The proper motions of the youngest white dwarfs appear to resemble those of the most massive main-sequence stars, providing the first direct constraint on the relaxation time of the stars in a globular cluster of greater than or about 0.7 Gyr.Comment: 25 pages, 20 figures, accepted for publication in Astrophysical Journa

The Spectral Energy Distributions of White Dwarfs in 47 Tucanae: The Distance to the Cluster

We present a new distance determination to the Galactic globular cluster 47 Tucanae by fitting the spectral energy distributions of its white dwarfs to pure hydrogen atmosphere white dwarf models. Our photometric dataset is obtained from a 121 orbit Hubble Space Telescope program using the Wide Field Camera 3 UVIS/IR channels, capturing F390W, F606W, F110W, and F160W images. These images cover more than 60 square arcmins and extend over a radial range of 5-13.7 arcmin (6.5-17.9 pc) within the globular cluster. Using a likelihood analysis, we obtain a best fitting unreddened distance modulus of (m - M)o=13.36+/-0.02+/-0.06 corresponding to a distance of 4.70+/-0.04+/-0.13 kpc, where the first error is random and the second is systematic. We also search the white dwarf photometry for infrared excess in the F160W filter, indicative of debris disks or low mass companions, and find no convincing cases within our sample.Comment: Accepted to The Astronomical Journal, 13 Figures, 2 Tables. Figures 3 and 6 are figure sets, each composed of 59 subfigures (to appear in the electronic journal). This is a Companion paper to the article ID: submit/037561