Preliminary Constraints on 12C(alpha,gamma)16O from White Dwarf Seismology

For many years, astronomers have promised that the study of pulsating white dwarfs would ultimately lead to useful information about the physics of matter under extreme conditions of temperature and pressure. In this paper we finally make good on that promise. Using observational data from the Whole Earth Telescope and a new analysis method employing a genetic algorithm, we empirically determine that the central oxygen abundance in the helium-atmosphere variable white dwarf GD 358 is 84+/-3 percent. We use this value to place preliminary constraints on the 12C(alpha,gamma)16O nuclear reaction cross-section. More precise constraints will be possible with additional detailed simulations. We also show that the pulsation modes of our best-fit model probe down to the inner few percent of the stellar mass. We demonstrate the feasibility of reconstructing the internal chemical profiles of white dwarfs from asteroseismological data, and find an oxygen profile for GD 358 that is qualitatively similar to recent theoretical calculations.Comment: Accepted for publication in the Astrophysical Journal, 7 pages, 6 figures, 2 tables, uses emulateapj5.st

Does individual variation in metabolic phenotype predict fish behaviour and performance?

There is increasing interest in documenting and explaining the existence of marked intraspecific variation in metabolic rate in animals, with fishes providing some of the best-studied examples. After accounting for variation due to other factors, there can typically be a two to three-fold variation among individual fishes for both standard and maximum metabolic rate (SMR and MMR). This variation is reasonably consistent over time (provided that conditions remain stable), and its underlying causes may be influenced by both genes and developmental conditions. In this paper, current knowledge of the extent and causes of individual variation in SMR, MMR and aerobic scope (AS), collectively its metabolic phenotype, is reviewed and potential links among metabolism, behaviour and performance are described. Intraspecific variation in metabolism has been found to be related to other traits: fishes with a relatively high SMR tend to be more dominant and grow faster in high food environments, but may lose their advantage and are more prone to risk-taking when conditions deteriorate. In contrast to the wide body of research examining links between SMR and behavioural traits, very little work has been directed towards understanding the ecological consequences of individual variation in MMR and AS. Although AS can differ among populations of the same species in response to performance demands, virtually nothing is known about the effects of AS on individual behaviours such as those associated with foraging or predator avoidance. Further, while factors such as food availability, temperature, hypoxia and the fish's social environment are known to alter resting and MMRs in fishes, there is a paucity of studies examining how these effects vary among individuals, and how this variation relates to behaviour. Given the observed links between metabolism and measures of performance, understanding the metabolic responses of individuals to changing environments will be a key area for future research because the environment will have a strong influence on which animals survive predation, become dominant and ultimately have the highest reproductive success. Although current evidence suggests that variation in SMR may be maintained within populations via context-dependent fitness benefits, it is suggested that a more integrative approach is now required to fully understand how the environment can modulate individual performance via effects on metabolic phenotypes encompassing SMR, MMR and AS

Measuring ^{12}C(&alpha,&gamma)^{16}O from White Dwarf Asteroseismology

During helium burning in the core of a red giant, the relative rates of the 3&alpha and ^{12}C(&alpha,&gamma)^{16}O reactions largely determine the final ratio of carbon to oxygen in the resulting white dwarf star. The uncertainty in the 3&alpha reaction at stellar energies due to the extrapolation from high-energy laboratory measurements is relatively small, but this is not the case for the ^{12}C(&alpha,&gamma)^{16}O reaction. Recent advances in the analysis of asteroseismological data on pulsating white dwarf stars now make it possible to obtain precise measurements of the central ratio of carbon to oxygen, providing a more direct way to measure the ^{12}C(&alpha,&gamma)^{16}O reaction rate at stellar energies. We assess the systematic uncertainties of this approach and quantify small shifts in the measured central oxygen abundance originating from the observations and from model settings that are kept fixed during the optimization. Using new calculations of white dwarf internal chemical profiles, we find a rate for the ^{12}C(&alpha,&gamma)^{16}O reaction that is significantly higher than most published values. The accuracy of this method may improve as we modify some of the details of our description of white dwarf interiors that were not accessible through previous model-fitting methods.Comment: 8 pages, 4 figures, 3 tables, uses emulateapj5.sty, Accepted for publication in the Astrophysical Journa

Variations in Lake Levels during the Holocene in North America: An Indicator of Changes in Atmospheric Circulation Patterns

Fluctuations in the extent of closed lakes provide a detailed record of regional and continental variations in mean annual water budget. The temporal sequence of hydrological fluctuations during the Holocene in North America has been reconstructed using information from the Oxford Lake-Level Data Bank. This data base includes 67 basins from the Americas north of the equator. Maps of lake status, an index of relative depth, are presented for the period 10,000 to 0 yr BP. The early Holocene was characterised by increasingly arid conditions, which led to widespread low lake levels in the mid-latitudes by 9,000 yr BP. By 6,000 yr BP this zone of low lakes extended from 32o to 51oN. Many of the features of the present day lake-level pattern, particularly high lake levels north of 46oN and along the eastern seaboard, were established by 3.000 yr BP. Four distinctive regional patterns of lake behaviour through time are apparent. Histograms of lake status from 20,000 to 0 yr BP are presented for each of these regions. They illustrate the temporal patterns of lake-level fluctuations on a time scale of 103 — 104 yr. Changes in lake status over North America are interpreted as indicating displacements in major features of the general circulation, specifically the zonal Westerlies and the Equatorial Trough, as reflected by changes in air mass trajectories and hence the position of air mass boundaries over the continent.Les fluctuations de superficie enregistrées dans les lacs fermés fournissent des données détaillées quant aux variations, aux échelles régionale et continentale, du bilan annuel moyen de l'eau. On a reconstitué, en Amérique du Nord, la chronologie des fluctuations survenues à l'Holocène, en se fondant sur les données fournies par le Oxford Lake-Level Data Bank. Cette banque de données renseigne sur 67 bassins lacustres, situés en Amérique au nord de l'équateur. On présente ici des cartes illustrant les niveaux lacustres entre 10 000 et 0 ans BP. L'Holocène inférieur se caractérisait par une progression de l'aridité qui a entraîné un abaissement général du niveau des lacs situés sous les moyennes latitudes, environ 9000 ans BP. Environ 6000 ans BP. cette zone passait du 32o de latitude nord au 51o. Un grand nombre de particularités qui caractérisent aujourd'hui les niveaux lacustres, notamment les hauts niveaux qu'on observe au nord du 46o N et le long de la côte est, étaient déjà acquises vers 3000 ans BP. Quatre types d'évolution générale se dégagent au cours des millénaires. Par ailleurs, on présente des histogrammes illustrant les niveaux lacustres selon une échelle temporelle de 103 — 104 ans. On considère que les changements qu'ont connu, en Amérique du Nord, les niveaux lacustres révèlent qu'il y a eu déplacement des composantes principales de la circulation atmosphérique générale, surtout en ce qui a trait aux vents d'ouest et à la dépression équatoriale. Ils se sont traduits par des modifications de la trajectoire des masses d'air, ce qui explique la position des limites de masses d'air sur le continent.Schwankungen in der Ausdehnung geschlossener Seen liefern einen detaillierten Nachweis ùber régionale und kontinentale Variationen im durchschnittlichen jàhrlichen Wasser-Budget. Man hat die zeitliche Abfolge von Wasserschwankungen wàhrend des Holozàn in Nordamerika rekonstruiert, gestùtzt auf Informationen von der Oxford Lake-Level Data Bank. Diese Daten-Bank umfaBt 67 Wasserbecken Amerikas, die nôrlich vom Aquator liegen. Fur die Zeit von 10 000 bis O v.u.Z. werden Karten \/orgelegt. die ùber die relative Tiefe der Seen informieren. Das frùhe Holozàn zeichnete sich durch zunehmend trockene Bedingungen aus, was urn 9 000 v.u.Z. in den mittleren Breiten zu weit verbreiteten niedrigen Seewasserspiegeln fùhrte. Um 6 000 v.u.Z. dehnte sich diese Zone niedriger Wasserspiegel von 32° bis 51o nôrdlicher Breite aus. Viele Erscheinungsformen, die die heutigen Seen-Wasserspiegel charakterisieren. insbesondere hohe Seen-Wasserspiegel nôrdlich von 46° nôrdlicher Breite und entlang der Ostkùste wurden um 3 000 v.u.Z. festgelegt. Durch die Jahrtausende sind vier unterschiedliche régionale Muster des Seen-Verhaltens fests-tellbar. Fur jedes dieser Gebiete werden Histogramme des Seen-Zustandes von 20 000 bis O Jahren v.u.Z. vorgelegt. Sie illustrieren die zeitliche Anordnung der Seen-Wasserspiegelschwankungen auf einer Zeit-Skala von 103-104 Jahren. Verànderungen im Seenzustand Nord-amerikas werden interpretiert als Hinweis auf eine Verlagerung der Hauptmerkmale der allgemeinen atmosphàrischen Strômungen, insbesondere was die Westwinde des Gebiets und die àquatoriale Windstille betrifft. Das spiegelt sich in Verànderungen der Zugrichtung der Luftmassen und folglich der Position der Luftmassengrenzen ùber dem Kontinent

The core/envelope symmetry in pulsating stars

We demonstrate that there is an inherent symmetry in the way high-overtone stellar pulsations sample the core and the envelope, which can potentially lead to an ambiguity in the asteroseismologically derived locations of internal structures. We provide an intuitive example of the source of this symmetry by analogy with a vibrating string. For the stellar case, we focus on the white dwarf stars, establishing the practical consequences for high-order white dwarf pulsations both analytically and numerically. In addition, we verify the effect empirically by cross-fitting two different structural models, and we discuss the consequences that this approximate symmetry may have for past and present asteroseismological fits of the pulsating DBV, GD 358. Finally, we show how the signatures of composition transition zones that are brought about by physically distinct processes may be used to help alleviate this potential ambiguity in our asteroseismological interpretation of the pulsation frequencies observed in white dwarf stars.Comment: Accepted for publication in MNRAS; 8 pages, 8 figure

New Pulsating DB White Dwarf Stars from the Sloan Digital Sky Survey

We are searching for new He atmosphere white dwarf pulsators (DBVs) based on the newly found white dwarf stars from the spectra obtained by the Sloan Digital Sky Survey. DBVs pulsate at hotter temperature ranges than their better known cousins, the H atmosphere white dwarf pulsators (DAVs or ZZ Ceti stars). Since the evolution of white dwarf stars is characterized by cooling, asteroseismological studies of DBVs give us opportunities to study white dwarf structure at a different evolutionary stage than the DAVs. The hottest DBVs are thought to have neutrino luminosities exceeding their photon luminosities (Winget et al. 2004), a quantity measurable through asteroseismology. Therefore, they can also be used to study neutrino physics in the stellar interior. So far we have discovered nine new DBVs, doubling the number of previously known DBVs. Here we report the new pulsators' lightcurves and power spectra.Comment: 15 pages, 2 figures, 3 tables, ApJ accepte

Producing Birdsfoot Trefoil Seed

Supplies of birdsfoot trefoil seed haven\u27t met the demand in Iowa. Seed is scarce and relatively high priced. Seed production can be profitable for those willing to take the precautions necessary for good seed yields

Evolutionary Timescale of the DAV G117-B15A: The Most Stable Optical Clock Known

We observe G117-B15A, the most precise optical clock known, to measure the rate of change of the main pulsation period of this blue-edge DAV white dwarf. Even though the obtained value is only within 1 sigma, Pdot = (2.3 +/- 1.4) x 10^{-15} s/s, it is already constraining the evolutionary timescale of this cooling white dwarf star.Comment: Accepted for publication in ApJ

An asteroseismic test of diffusion theory in white dwarfs

The helium-atmosphere (DB) white dwarfs are commonly thought to be the descendants of the hotter PG1159 stars, which initially have uniform He/C/O atmospheres. In this evolutionary scenario, diffusion builds a pure He surface layer which gradually thickens as the star cools. In the temperature range of the pulsating DB white dwarfs (T_eff ~ 25,000 K) this transformation is still taking place, allowing asteroseismic tests of the theory. We have obtained dual-site observations of the pulsating DB star CBS114, to complement existing observations of the slightly cooler star GD358. We recover the 7 independent pulsation modes that were previously known, and we discover 4 new ones to provide additional constraints on the models. We perform objective global fitting of our updated double-layered envelope models to both sets of observations, leading to determinations of the envelope masses and pure He surface layers that qualitatively agree with the expectations of diffusion theory. These results provide new asteroseismic evidence supporting one of the central assumptions of spectral evolution theory, linking the DB white dwarfs to PG1159 stars.Comment: 7 pages, 3 figures, 3 tables, accepted for publication in A&