The impact of element diffusion on the formation and evolution of helium white dwarf stars

The aim of this work is to investigate the effect of element diffusion on the evolution of helium white dwarfs. To this end, we couple the multicomponent flow equations that describe gravitational settling, chemical and thermal diffusion to an evolutionary code. We compute the evolution of a set of helium white dwarf models with masses ranging from 0.169 to 0.406 M_⊙. In particular, several low-mass white dwarfs have been found in binary systems as companion to millisecond pulsars. In these systems, pulsar emission is activated by mass transfer episodes so that, if we place the zero-age point at the end of such mass transfer, then the pulsar and the white dwarf ages should be equal. Interestingly enough, available models of helium white dwarfs neglect element diffusion. Using such models, good agreement has been found between the ages of the components of the PSR J1012+5307 system. However, recent observations of the PSR B1855+09 system cast doubts on the correctness of such models, which predict a white dwarf age twice as long as the spin-down age of the pulsar. In this work, we find that element diffusion induces thermonuclear hydrogen shell flashes for models in the mass interval 0.18≲ M /M_⊙ ≲ 0.41 . We show, in particular, that the occurrence of these diffusion-induced flashes eventually leads to white dwarf models with hydrogen envelope masses too small to support any further nuclear burning, thus implying much shorter cooling ages than in the case when diffusion is neglected. In particular, excellent agreement is found between the ages of PSR B1855+09 system components, solving the age discrepancy from first principles.     

Der Stabilitätsbereich des Pyrophyllits unter dem Einfluß von Säuren

Zusammenfassung Isochemische Experimente bei 2000 Bar Gesamtdruck wurden mit Gemischen aus natürlichem Kaolinit und Quarz bei verschiedenen Molenbrüchen verschiedener anorganischer Säuren durchgeführt. Die Reaktionen, welche zur Bildung und zum Abbau von Pyrophyllit führen, werden durch die Anwesenheit von Säuren im System bivariant, d.h. die Reaktionstemperatur ist nicht allein vom Druck, sondern auch von der Zusammensetzung der fluiden Phase abhängig. Durch starke Säuren (HCl, H₂SO₄) wird sowohl die obere als auch die untere Stabilitätstemperatur des Pyrophyllits stark erniedrigt; schwache Säuren (HF, H₃BO₃) haben keinen nachweisbaren Einfluß auf diese Temperaturen. Die Gründe für die Erniedrigung der Reaktionstemperaturen werden diskutiert; einige Beobachtungen über die Bildung von Zunyit und Topas werden mitgeteilt.

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Mixtures of kaolinite and quartz were subjected to isochemical hydrothermal experiments under 2000 bars fluid pressure and different mole fractions of various inorganic acids. The reactions — leading to the formation of pyrophyllite and its subsequent breakdown — become bivariant in presence of acids, so that not only the fluid pressure but also the composition of the fluid phase need be specified to define the equilibrium temperature uniquely. While strong acids (HCl and H₂SO₄) bring about a remarkable shift of the lower and upper stability limits of pyrophyllite towards lower temperatures, weaker acids like HF and H₃BO₃ do not have any significant influence on the same. Some observations bearing on the genesis of zunyite and topaz are presented and the reasons of the lowering of the equilibrium temperatures are discussed.

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Herrn Professor Dr. H. G. F. Winkler danke ich für sein förderndes Interesse an dieser Arbeit sowie für kritische Diskussionen und viele nützliche Ratschläge bei der Abfassung des Manuskripts. Ferner danke ich ihm für die Erlaubnis zur Benutzung der Apparaturen, die zum Teil mit Hilfe der Deutschen Forschungsgemeinschaft aufgebaut wurden.Den Kollegen vom Mineralogisch-Petrologischen Institut bin ich für wertvolle Kritik zu Dank verpflichtet, ebenso Herrn Feinmechanikermeister Baumbach für die Betreuung der Apparaturen.     

River discharges of water and nutrients to the Mediterranean and Black Sea: Major drivers for ecosystem changes during past and future decades?

Rivers are important sources of freshwater and nutrients for the Mediterranean and Black Sea. We present a reconstruction of the spatial and temporal variability of these inputs since the early 1960s, based on a review of available data on water discharge, nutrient concentrations and climatic parameters. Our compilation indicates that Mediterranean rivers suffer from a significant reduction in freshwater discharge, contrary to rivers of the Black Sea, which do not have clear discharge trends. We estimate this reduction to be at least about 20% between 1960 and 2000. It mainly reflects recent climate change, and dam construction may have reduced discharge even further. A similar decrease can also be expected for the fluxes of dissolved silica (Si), strongly controlled by water discharge and potentially reduced by river damming as well. This contrasts with the fluxes of nitrogen (N) and phosphorus (P) in Mediterranean and Black Sea rivers, which were strongly enhanced by anthropogenic sources. Their total inputs to the Mediterranean Sea could have increased by a factor of >5. While N still remained at elevated levels in 2000, P only increased up to the 1980–1990s, and then rapidly dropped down to about the initial values of the 1960s. With respect to the marine primary production that can be supported by the riverine nutrient inputs, Mediterranean and the Black Sea rivers were mostly phosphorus limited during the study period. Their anthropogenic nutrient enrichment could only have had a fertilizing effect before the general decline of the P loads. When also considering Si as a limiting element, which is the case for siliceous primary producers such as diatoms, silica limitation may have become a widespread phenomenon in the Mediterranean rivers since the early 1980s. For the Black Sea rivers, this already started the late 1960s. Gross primary production sustained by rivers (PPR) represents only less than 2% of the gross production (PP) in the Mediterranean, and less than 5% in the Black Sea. Possible ecological impacts of the changing river inputs should therefore be visible only in productive coastal areas, such as the Gulf of Lions, where PPR can reach more than two thirds of PP. Reported ecosystem changes both in the Adriatic Sea and the Black Sea are concomitant with major changes in the reconstructed river inputs. Further work combining modelling and data collection is needed to test whether this may also have been the case for coastal ecosystems at other places in the Mediterranean and Black Sea.     

Anomalous archaeomagnetic directions and site formation processes at archaeological sites in Israel

As part of a program to investigate archaeomagnetic secular variation in Israel and implications for archaeomagnetic dating, we have identified “anomalous” results that have yielded information about site formation processes. Stereonets, vector demagnetization diagrams, and sample location maps are most useful in examining stability of magnetization and consistency of archaeomagnetic directions with each other and with what would be expected from normal secular variation. A collection of examples is presented, including: strong and hard remanence due to vitrification, unstable magnetization due to lightning strikes, superimposed thermal magnetizations, chemical remagnetization, magnetization of a hearth fragment before falling, magnetization of a hot brick after falling, and mechanical deformation of an archaeomagnetic feature. © 1999 John Wiley & Sons, Inc.     

Evolution of a 3-M⊙ star from the main sequence to the ZZ Ceti stage: the role played by element diffusion

The purpose of this paper is to present new full evolutionary calculations for DA white dwarf stars with the major aim of providing a physically sound reference frame for exploring the pulsation properties of the resulting models in future communications. Here, white dwarf evolution is followed in a self-consistent way with the predictions of time-dependent element diffusion and nuclear burning. In addition, full account is taken of the evolutionary stages prior to white dwarf formation. In particular, we follow the evolution of a 3-M_⊙ model from the zero-age main sequence (the adopted metallicity is   Z =0.02)  , all the way from the stages of hydrogen and helium burning in the core up to the thermally pulsing phase. After experiencing 11 thermal pulses, the model is forced to evolve towards its white dwarf configuration by invoking strong mass loss episodes. Further evolution is followed down to the domain of the ZZ Ceti stars on the white dwarf cooling branch.
Emphasis is placed on the evolution of the chemical abundance distribution caused by diffusion processes and the role played by hydrogen burning during the white dwarf evolution. We find that discontinuities in the abundance distribution at the start of the cooling branch are considerably smoothed out by diffusion processes by the time the ZZ Ceti domain is reached. Nuclear burning during the white dwarf stage does not represent a major source of energy, as expected for a progenitor star of initially high metallicity. We also find that thermal diffusion lessens even further the importance of nuclear burning.
Furthermore, the implications of our evolutionary models for the main quantities relevant for adiabatic pulsation analysis are discussed. Interestingly, the shape of the Ledoux term is markedly smoother compared with previous detailed studies of white dwarfs. This is translated into a different behaviour of the Brunt–Väisälä frequency.     

Evolutionary and pulsational properties of white dwarf stars

White dwarf stars are the final evolutionary stage of the vast majority of stars, including our Sun. Since the coolest white dwarfs are very old objects, the present population of white dwarfs contains a wealth of information on the evolution of stars from birth to death, and on the star formation rate throughout the history of our Galaxy. Thus, the study of white dwarfs has potential applications in different fields of astrophysics. In particular, white dwarfs can be used as independent reliable cosmic clocks, and can also provide valuable information about the fundamental parameters of a wide variety of stellar populations, such as our Galaxy and open and globular clusters. In addition, the high densities and temperatures characterizing white dwarfs allow these stars to be used as cosmic laboratories for studying physical processes under extreme conditions that cannot be achieved in terrestrial laboratories. Last but not least, since many white dwarf stars undergo pulsational instabilities, the study of their properties constitutes a powerful tool for applications beyond stellar astrophysics. In particular, white dwarfs can be used to constrain fundamental properties of elementary particles such as axions and neutrinos and to study problems related to the variation of fundamental constants. These potential applications of white dwarfs have led to renewed interest in the calculation of very detailed evolutionary and pulsational models for these stars. In this work, we review the essentials of the physics of white dwarf stars. We enumerate the reasons that make these stars excellent chronometers, and we describe why white dwarfs provide tools for a wide variety of applications. Special emphasis is placed on the physical processes that lead to the formation of white dwarfs as well as on the different energy sources and processes responsible for chemical abundance changes that occur along their evolution. Moreover, in the course of their lives, white dwarfs cross different pulsational instability strips. The existence of these instability strips provides astronomers with a unique opportunity to peer into their internal structure that would otherwise remain hidden from observers. We will show that this allows one to measure stellar masses with unprecedented precision and to infer their envelope thicknesses, to probe the core chemical stratification, and to detect rotation rates and magnetic fields. Consequently, in this work, we also review the pulsational properties of white dwarfs and the most recent applications of white dwarf asteroseismology.