The Physics of Crystallization from Globular Cluster White Dwarf Stars in NGC 6397

We explore the physics of crystallization in the deep interiors of white dwarf stars using the color-magnitude diagram and luminosity function constructed from proper motion cleaned Hubble Space Telescope photometry of the globular cluster NGC 6397. We demonstrate that the data are consistent with the theory of crystallization of the ions in the interior of white dwarf stars and provide the first empirical evidence that the phase transition is first order: latent heat is released in the process of crystallization as predicted by van Horn (1968). We outline how this data can be used to observationally constrain the value of Gamma = E_{Coulomb}/E_{thermal} near the onset of crystallization, the central carbon/oxygen abundance, and the importance of phase separation.Comment: 5 pages, 5 figures, accepted for publication in the Astrophysical Journal Letter

A Determination of the Local Density of White Dwarf Stars

The most recent version of the Catalog of Spectroscopically Identified White Dwarfs lists 2249 white dwarf stars. Among these stars are 118 white dwarfs that have either reliable trigonometric parallaxes or color-based distance moduli which place them at a distance within 20 pc of the Sun. Most of these nearby white dwarfs are isolated stars, but 35 (30 % of the sample) are in binary systems, including such well known systems as Sirius A/B, and Procyon A/B. There are also three double degenerate systems in this sample of the local white dwarf population. The sample of local white dwarfs is largely complete out to 13 pc and the local density of white dwarf stars is found to be (5.5 +/- 0.8) x 10^-3 pc^-3 with a corresponding mass density of (3.7 +/- 0.5) x10^-3 M pc^-3.Comment: 20 pages, 3 Postscript figure

The galactic population of white dwarfs

Original paper can be found at: http://www.iop.org/EJ/conf DOI: 10.1088/1742-6596/172/1/012004 [16th European White Dwarfs Workshop]The contribution of white dwarfs of the different Galactic populations to the stellar content of our Galaxy is only poorly known. Some authors claim a vast population of halo white dwarfs, which would be in accordance with some investigations of the early phases of Galaxy formation claiming a top-heavy initial– mass– function. Here, I present a model of the population of white dwarfs in the Milky Way based on observations of the local white dwarf sample and a standard model of Galactic structure. This model will be used to estimate the space densities of thin disc, thick disc and halo white dwarfs and their contribution to the baryonic mass budget of the Milky Way. One result of this investigation is that white dwarfs of the halo population contribute a large fraction of the Galactic white dwarf number count, but they are not responsible for the lion's share of stellar mass in the Milky Way. Another important result is the substantial contribution of the – often neglected – population of thick disc white dwarfs. Misclassification of thick disc white dwarfs is responsible for overestimates of the halo population in previous investigations.Peer reviewe

Lippen-Kiefer-Gaumen-Spalten: Individuelle Analyse der Lippenspalte durch 3-D-Lasertopometrie

Zusammenfassung: Hintergrund: Mit Gipsmodellen und Fotografien ist die dreidimensionale Analyse einer Lippen-Kiefer-Gaumen-Spalte meist nur unzureichend möglich. Ziel der vorliegenden Studie war es daher, die 3-D-Lasertopometrie auf ihre Anwendbarkeit zur dreidimensionalen Weichgewebserfassung bei Patienten mit Lippen-Kiefer-Gaumen-Spalten zu testen. Patienten und Methode: Bei 20 Patienten (3-35 Jahre), die eine einseitige, nicht operierte Lippen-, Lippen-Kiefer- oder Lippen-Kiefer-Gaumen-Spalte aufwiesen, wurde mit einem 3-D-Laserscanner die Gesichtsoberfläche prä- und postoperativ dreidimensional erfasst. Die dabei erzeugten digitalen Datensätze wurden in einer virtuellen Umgebung metrisch analysiert und anhand von Quotienten größenunabhängig wiedergegeben. Sie dienten der Auswahl der Operationstechnik und der Beurteilung des Operationsergebnisses. Ergebnisse: Mit dem 3-D-Laserscanner wurden 3-D-Oberflächen guter Qualität erstellt, die sich im Millimeterbereich ausmessen ließen. Die dreidimensionale Spaltmorphologie konnte in den Datensätzen reproduzierbar mit Landmarks versehen und vermessen werden. Auch die postoperative Symmetrie ließ sich so kontrollieren und objektivieren. Als nachteilig erwiesen sich die relativ lange Messzeit und die Notwendigkeit zur Kombination mehrerer Ansichten. Schlussfolgerung: Das vorgestellte 3-D-Laserverfahren ermöglicht eine präzise dreidimensionale Weichteilanalyse der Lippen- und Nasenregion bei Spaltpatienten. Es eignet sich jedoch nur bedingt für lebhafte Säuglinge und unkooperative Patiente

Faint NUV/FUV Standards from Swift/UVOT, GALEX and SDSS Photometry

At present, the precision of deep ultraviolet photometry is somewhat limited by the dearth of faint ultraviolet standard stars. In an effort to improve this situation, we present a uniform catalog of eleven new faint (u sim17) ultraviolet standard stars. High-precision photometry of these stars has been taken from the Sloan Digital Sky Survey and Galaxy Evolution Explorer and combined with new data from the Swift Ultraviolet Optical Telescope to provide precise photometric measures extending from the Near Infrared to the Far Ultraviolet. These stars were chosen because they are known to be hot (20,000 < T_eff < 50,000 K) DA white dwarfs with published Sloan spectra that should be photometrically stable. This careful selection allows us to compare the combined photometry and Sloan spectroscopy to models of pure hydrogen atmospheres to both constrain the underlying properties of the white dwarfs and test the ability of white dwarf models to predict the photometric measures. We find that the photometry provides good constraint on white dwarf temperatures, which demonstrates the ability of Swift/UVOT to investigate the properties of hot luminous stars. We further find that the models reproduce the photometric measures in all eleven passbands to within their systematic uncertainties. Within the limits of our photometry, we find the standard stars to be photometrically stable. This success indicates that the models can be used to calibrate additional filters to our standard system, permitting easier comparison of photometry from heterogeneous sources. The largest source of uncertainty in the model fitting is the uncertainty in the foreground reddening curve, a problem that is especially acute in the UV.Comment: Accepted for publication in Astrophysical Journal. 31 pages, 13 figures, electronic tables available from ApJ or on reques

Hubble Space Telescope Astrometry of the Procyon System

The nearby star Procyon is a visual binary containing the F5 IV-V subgiant Procyon A, orbited in a 40.84 yr period by the faint DQZ white dwarf Procyon B. Using images obtained over two decades with the Hubble Space Telescope, and historical measurements back to the 19th century, we have determined precise orbital elements. Combined with measurements of the parallax and the motion of the A component, these elements yield dynamical masses of 1.478 +/- 0.012 Msun and 0.592 +/- 0.006 Msun for A and B, respectively. The mass of Procyon A agrees well with theoretical predictions based on asteroseismology and its temperature and luminosity. Use of a standard core-overshoot model agrees best for a surprisingly high amount of core overshoot. Under these modeling assumptions, Procyon A's age is ~2.7 Gyr. Procyon B's location in the H-R diagram is in excellent agreement with theoretical cooling tracks for white dwarfs of its dynamical mass. Its position in the mass-radius plane is also consistent with theory, assuming a carbon-oxygen core and a helium-dominated atmosphere. Its progenitor's mass was 1.9-2.2 Msun, depending on its amount of core overshoot. Several astrophysical puzzles remain. In the progenitor system, the stars at periastron were separated by only ~5 AU, which might have led to tidal interactions and even mass transfer; yet there is no direct evidence that these have occurred. Moreover the orbital eccentricity has remained high (~0.40). The mass of Procyon B is somewhat lower than anticipated from the initial-to-final-mass relation seen in open clusters. The presence of heavy elements in its atmosphere requires ongoing accretion, but the place of origin is uncertain.Comment: Accepted by Astrophysical Journa

3D Surface Measurement for Medical Application—Technical Comparison of Two Established Industrial Surface Scanning Systems

In 3D mapping of flexible surfaces (e.g. human faces) measurement errors due to movement or positioning occur. Aggravated by equipment- or researcher-caused mistakes considerable deviations can result. Therefore first the appliances' precision handling and reliability in clinical environment must be established. Aim of this study was to investigate accuracy and precision of two contact-free 3D measurement systems (white light vs. laser). Standard specimens of known diameter for sphere deviation, touch deviation and plane deviation were tested. Both systems are appropriate for medical application acquiring solid data (<mm). The more complex white-light system shows better accuracy at 0.2s measuring time. The laser system is superior concerning robustness, while accuracy is poorer and input time (1.5-2.5s) longer. Due to the clinical demand the white-light system is superior in a laboratory environment, while the laser system is easier to handle under non-laboratory condition

Spectroscopic confirmation of UV-bright white dwarfs from the Sandage Two-Color Survey of the Galactic Plane

We present spectroscopic observations confirming the identification of hot white dwarfs among UV-bright sources from the Sandage Two-color Survey of the Galactic plane and listed in the Lanning (Lan) catalog of such sources. A subsample of 213 UV bright Lan sources have been identified as candidate white dwarfs based on the detection of a significant proper motion. Spectroscopic observations of 46 candidates with the KPNO 2.1m telescope confirm 30 sources to be hydrogen white dwarfs with subtypes in the DA1-DA6 range, and with one of the stars (Lan 161) having an unresolved M dwarf as a companion. Five more sources are confirmed to be helium white dwarfs, with subtypes from DB3 to DB6. One source (Lan 364) is identified as a DZ 3 white dwarf, with strong lines of calcium. Three more stars are found to have featureless spectra (to within detection limits), and are thus classified as DC white dwarfs. In addition, three sources are found to be hot subdwarfs: Lan 20 and Lan 480 are classified as sdOB, and Lan 432 is classified sdB. The remaining four objects are found to be field F star interlopers. Physical parameters of the DA and DB white dwarfs are derived from model fits.Comment: To appear in the Astronomical Journa

The White Dwarf Population in NGC 1039 (M34) and the White Dwarf Initial-Final Mass Relation

We present the first detailed photometric and spectroscopic study of the white dwarfs (WDs) in the field of the ~225 Myr old (log tau_cl = 8.35) open cluster NGC 1039 (M34) as part of the ongoing Lick-Arizona White Dwarf Survey. Using wide-field UBV imaging, we photometrically select 44 WD candidates in this field. We spectroscopically identify 19 of these objects as WDs; 17 are hydrogen-atmosphere DA WDs, one is a helium-atmosphere DB WD, and one is a cool DC WD that exhibits no detectable absorption lines. We find an effective temperature (T_eff) and surface gravity (log g) for each DA WD by fitting Balmer-line profiles from model atmospheres to the observed spectra. WD evolutionary models are then invoked to derive masses and cooling times for each DA WD. Of the 17 DAs, five are at the approximate distance modulus of the cluster. Another WD with a distance modulus 0.45 mag brighter than that of the cluster could be a double-degenerate binary cluster member, but is more likely to be a field WD. We place the five single cluster member WDs in the empirical initial-final mass relation and find that three of them lie very close to the previously derived linear relation; two have WD masses significantly below the relation. These outliers may have experienced some sort of enhanced mass loss or binary evolution; however, it is quite possible that these WDs are simply interlopers from the field WD population. Eight of the 17 DA WDs show significant CaII K absorption; comparison of the absorption strength with the WD distances suggests that the absorption is interstellar, though this cannot be confirmed with the current data.Comment: 24 pages, 13 figures. Accepted for publication in the Astronomical Journal. Figures 1, 2 and 3 reduced in resolutio