Download or read book On the Spectral Evolution of Cool Helium Atmosphere White Dwarfs Detailed Spectroscopic and Photometric Analysis of DZ Stars written by and published by . This book was released on 2007 with total page 19 pages. Available in PDF, EPUB and Kindle. Book excerpt: We present a detailed analysis of a large spectroscopic and photometric sample of DZ white dwarfs based on our latest model atmosphere calculations. We revise the atmospheric parameters of the trigonometric parallax sample of Bergeron, Leggett, & Ruiz (12 stars) and analyze 147 new DZ white dwarfs discovered in the SDSS. The inclusion of metals and hydrogen in our model atmosphere calculations leads to different atmospheric parameters than those derived from pure helium models. Calcium abundances are found in the range from log (Ca/He) = -12 to -8. We also find that fits of the coolest objects show peculiarities, suggesting that our physical models may not correctly describe the conditions of high atmospheric pressure encountered in the coolest DZ stars. We find that the mean mass of the 11 DZ stars with trigonometric parallaxes is significantly lower than that obtained from pure helium models and in much better agreement with the mean mass of other types of white dwarfs. We determine hydrogen abundances for 27% of the DZ stars in our sample, while only upper limits are obtained for objects with low-S/N spectroscopic data. We confirm with a high level of confidence that the accretion rate of hydrogen is at least 2 orders of magnitude smaller than that of metals (and up to 5 in some cases) to be compatible with the observations. We find a correlation between the hydrogen abundance and the effective temperature, suggesting for the first time empirical evidence of a lower temperature boundary for the hydrogen screening mechanism. Finally, we speculate on the possibility that the DZA white dwarfs could be the result of the convective mixing of thin hydrogen rich atmospheres with the underlying helium convection zone.
Download or read book White Dwarfs written by G. Vauclair and published by Springer Science & Business Media. This book was released on 2012-12-06 with total page 460 pages. Available in PDF, EPUB and Kindle. Book excerpt: The European Workshop on White Dwarfs was initiated by Prof. V. Weidemann, with the first meeting organized in Kiel (FRG) in 1974. Almost every two years, an increasing number of astronomers met to share their results and projects in the subsequent workshops: Frascati (1976), Tel Aviv (1978), Paris (1981), Kiel (1984), Frascati (1986). In the mean time, two major IAU colloquia (No. 53 in Rochester, NY, 1979; and No. 114 in Hanover, NH, 1988) emphasized the importance of these stars for our understanding of stellar evolution. The informal organization of the white dwarf community has been the starting point for large cooperative projects of which the Hubble Space Telescope "White Dwarf Consortium" and the "Whole Earth Telescope" are the most spectacular examples. But many other successful collaborations have also been born during the very exciting discussions conducted in the last 16 years on the occasion of our regular meetings. The 7th European Workshop on White Dwarfs took place the year of the seventieth birthday of Prof. Evry Schatzman, whose pioneering work has been the inspiration for many of the new ideas in the white dwarf community. The Scientific Organizing Committee has agreed to dedicate the workshop to him on this occasion. We are pleased to publish as an introduction to the workshop, the text of the talk delivered by Prof. Schatzman at the workshop's banquet.
Download or read book Exoplanets Compositions Mineralogy Evolution written by Natalie R. Hinkel and published by Walter de Gruyter GmbH & Co KG. This book was released on 2024-09-02 with total page 612 pages. Available in PDF, EPUB and Kindle. Book excerpt: The focus for RiMG volume 90 is on rocky exoplanets because the search for truly Earth-like planets is of special interest. The goal is to motivate communication between the disciplines so as to make the best use possible of existing data and data yet to be collected by the James Webb and the Nancy Grace Roman Space Telescopes, since the astronomy community is gathering data on stars and exoplanets at an accelerating rate. Such data now include exoplanet size and mass (i.e., density) as well as their atmospheric compositions, which are collectively telltale of mineralogy and evolution. Much of what is published may still fall in the realm of educated speculation, but our conjectures are metamorphosing into testable hypotheses. There is now a remarkably large amount of astronomical data (with even more on the way) that geochemists and petrologists can make much use of. But just as astronomers may benefit from geologic insights, geologists need our colleagues in astronomy to help interpret their data and their underlying implications to better understand its astronomical context. Our hopes for this volume will be fulfilled if readers initiate their own analyses of what at present may seem like novel or unusual data, and if new collaborations between academic departments and subfields are forged.
Download or read book A Detailed Model Atmosphere Analysis of Cool White Dwarfs in the Sloan Digital Sky Survey written by Mukremin Kilic and published by . This book was released on 2010 with total page 23 pages. Available in PDF, EPUB and Kindle. Book excerpt: We present optical spectroscopy and near-infrared photometry of 126 cool white dwarfs (WDs) in the Sloan Digital Sky Survey (SDSS). Our sample includes high proper motion targets selected using the SDSS and USNO-B astrometry and a dozen previously known ultracool WD candidates. Our optical spectroscopic observations demonstrate that a clean selection of large samples of cool WDs in the SDSS (and the SkyMapper, Pan-STARRS, and the Large Synoptic Survey Telescope data sets) is possible using a reduced proper motion diagram and a tangential velocity cut-off (depending on the proper motion accuracy) of 30 km s-1. Our near-infrared observations reveal eight new stars with significant absorption. We use the optical and near-infrared photometry to perform a detailed model atmosphere analysis. More than 80% of the stars in our sample are consistent with either pure hydrogen or pure helium atmospheres. However, the eight stars with significant infrared absorption and the majority of the previously known ultracool WD candidates are best explained with mixed hydrogen and helium atmosphere models. The age distribution of our sample is consistent with a Galactic disk age of 8 Gyr. A few ultracool WDs may be as old as 12-13 Gyr, but our models have problems matching the spectral energy distributions of these objects. There are only two halo WD candidates in our sample. However, trigonometric parallax observations are required for accurate mass and age determinations and to confirm their membership in the halo. Based on observations obtained at the Hobby-Eberly Telescope (HET), Gemini Observatory, and the Infrared Telescope Facility (IRTF).
Download or read book White Dwarf Atmospheres and Circumstellar Environments written by Donald W. Hoard and published by John Wiley & Sons. This book was released on 2012-09-19 with total page 342 pages. Available in PDF, EPUB and Kindle. Book excerpt: Written by selected astronomers at the forefront of their fields, this timely and novel book compiles the latest results from research on white dwarf stars, complementing existing literature by focusing on fascinating new developments in our understanding of the atmospheric and circumstellar environments of these stellar remnants. Complete with a thorough refresher on the observational characteristics and physical basis for white dwarf classification, this is a must-have resource for researchers interested in the late stages of stellar evolution, circumstellar dust and nebulae, and the future of our own Solar System.
Download or read book 15th European Workshop on White Dwarfs written by Ralf Napiwotzki and published by . This book was released on 2007 with total page 718 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book White Dwarfs written by J. Isern and published by Springer Science & Business Media. This book was released on 2012-12-06 with total page 470 pages. Available in PDF, EPUB and Kindle. Book excerpt: The study of white dwarfs has been steadily growing during the last years and now is a mature field extending its influence over many others. Since white dwarfs are long lived objects, they can be used to obtain information about the history of the Galaxy. The simplicity of their structures enables them to act as precise particle physics laboratories and the extreme con ditions reached at their surfaces allow us to test the equation of state and to study the behavior of matter under conditions impossible to be reached in terrestrial laboratories. Nevertheless, white dwarfs are still challenging astrophysicists. Many questions, ranging from the determination of funda mental parameters to the evolution of their outer layers, are still waiting for a satisfactory answer. The European Workshop series on White Dwarfs started in 1974 as a con sequence of the effort and enthusiasm of Professor Volker Weidemann. The existing proceedings of these meetings, together with those corresponding to the lAU Colloquia held in Rochester (1979) and in Hanover (1988), provide a unique opportunity to follow the development of this field. We hope that the present volume will provide a representative snapshot of the state of the art in 1996. In this sense we are very indebted to all the participants that have sent their contributions according to the instructions (this clearly excludes everybody beyond three sigmas from the standards).
Download or read book White Dwarfs Cosmological and Galactic Probes written by E. Sion and published by Springer Science & Business Media. This book was released on 2005-12-08 with total page 260 pages. Available in PDF, EPUB and Kindle. Book excerpt: The emphasis on white dwarf stars and cosmology arises from the most recent advances in cosmological and galactic structure research in which white dwarf stars are playing a very prominent role. Examples are Type Ia supernovae (i.e. white dwarf supernovae), the origin and evolution of the universe, the age of the galactic disk, cosmochronology using white dwarfs in globular clusters and galactic clusters, and the physics of accretion onto compact (very dense) stars. As an assisting guide to the reader, we have included, by invitation, comprehensive review articles in each of the four major areas of the book, white dwarf supernovae, cosmology, accretion physics and galactic structure. The reviews include introductory material that they build upon. The book is suitable and most useful to advanced undergraduates, graduate students and scientific professionals (e.g. astronomers, astrophysicists, cosmologists, physicists).
Download or read book White Dwarfs Advances in Observation and Theory written by M.A. Barstow and published by Springer Science & Business Media. This book was released on 2012-12-06 with total page 555 pages. Available in PDF, EPUB and Kindle. Book excerpt: The European Workshop on White Dwarfs was initiated by Prof. V. Weidemann, with the first meeting held in Kiel (FRG) in 1974. Since then a similar workshop has been held almost every two years: Frascati (1976), Tel Aviv (1978), Paris (1981), Kiel (1984), Frascati (1986) and Toulouse (1990). Two major IAU colloquia have also been devoted to the study of white dwarfs (No. 53, Rochester NY, 1979; No. 114, Hanover, NH, 1988). Our most recent meeting, the 8th Workshop, marks a number of important advances in both observational and theoretical studies of white dwarfs. This coincides with a significant expansion in the size of the community active in the field, as was clear from an increase in the number of participants between this workshop (77) and that in Toulouse (46), just two years ago. Key parts of any workshop are the discussion sessions following the pa pers. However, these are difficult to record and, very often, the 'live' flavour of the debates is lost. During this workshop we carried out an experiment, recording the discussion sessions on video tape and then transcribing these later for the proceedings. Although the recordings were made in non-studio conditions with regard to lighting and noise, only a very small part was unin telligible. All participants were given an opportunity to check their questions and answers for scientific accuracy and to correct any errors arising in the transcription process.
Download or read book Pulsational Oddities at the Extremes of the DA White Dwarf Instability Strip written by Keaton John Bell and published by . This book was released on 2017 with total page 542 pages. Available in PDF, EPUB and Kindle. Book excerpt: White dwarf stars are the remnant products of the vast majority of Galactic stellar evolution. They are compact objects that serve as remote laboratories for studying high energy/density physics. The outer regions of hydrogen-atmosphere (DA) white dwarfs become convective and able to drive global, nonradial, gravity-mode pulsations below roughly 12,500 K. The pulsations propagate through and are affected by the interior structures of these stars. The oscillations cause a pulsating star to exhibit brightness variations at its characteristic frequencies as a physical system. These frequencies can be measured through Fourier analysis of time series photometric observations. I have focused my studies on new pulsational phenomena near the cool and low-mass edges of the DA white dwarf instability strip, using extensive space-based data from the Kepler spacecraft and the K2 mission, as well as high-speed ground-based photometry from the 2.1-meter Otto Struve Telescope at McDonald Observatory (where I have personally observed 225 nights). The extensive short-cadence (1-min exposures) light curve of the first DAV (DA variable) identified within the original Kepler field of view provided one of the most complete and sensitive records of white dwarf pulsations ever. The light curve also revealed a new, completely unexpected outburst-like phenomenon. I detected 178 instances of significant brightness enhancement in 20 months of observations of the cool DAV KIC 4552982. Recurring with a quasi-period of 2.7 days, the outbursts last 4–25 hours and increase the stellar flux by up to 17%. I estimate the energy of each outburst to be of-order 1033 ergs. After the Kepler spacecraft suffered the loss of a second reaction wheel in May 2013, it began the K2 mission, visiting new fields along the ecliptic roughly every 80 days. This allowed us to increase the number of DAVs with extensive space-based photometry, and we quickly discovered a second, more dramatic example of this new outburst behavior in PG 1149+057 (Hermes et al. 2015b). I have led the efforts to characterize the outbursts in DAVs ever since and have detected these events in eight DAVs through K2 Campaign 10. Notably, spectroscopic effective temperature constraints place all known members of this new outbursting class of DAV near the cool (red) edge of the instability strip. With a growing outbursting class of DAV, we begin to study their ensemble outburst properties to inform a theory of their physical mechanism. Much of my work from McDonald Observatory has continued in the recent tradition of discovering and characterizing new pulsating extremely low-mass (ELM) white dwarfs. After identifying candidate ELM variables (ELMVs) from the ELM Survey catalog and parameters from model fits to the Sloan Digital Sky Survey spectroscopic data, I obtained time series photometric observations on the 2.1-meter Otto Struve telescope. I published SDSS J1618+3854 as the sixth member of this new class of variable star. However, most of the variability that I measured for this project was inconsistent with expectations for cooling track ELM white dwarfs. This includes long pulsation periods, high pulsation amplitudes, long eclipse timescales, and an overabundance of photometric variables that are not in confirmed short-period binaries from time series radial velocity measurements. Either the surface gravities of another class of star are being systematically overestimated from model fits to hydrogen line profiles in stellar spectra, or these observations are revealing an unexpectedly large population of recently formed pre-ELM white dwarfs. In total, I have discovered and characterized the variability of nine new pulsating stars in the spectroscopic parameter space of ELM white dwarfs, and I also developed an improved framework for interpreting measurements of tidally induced ellipsoidal variations in photometric binaries. Beyond these main results of my thesis on extreme pulsating white dwarfs, I have also explored the limits of the detectability of stellar pulsations in extreme photometric data sets. I analyze long-cadence (30-minute) K2 observations of two fairly typical DAVs in one such study, where the pulsations are severely undersampled. While accurate frequency determinations are nontrivial in such cases, I am able to recover the super-Nyquist frequencies of some pulsation modes with full K2 precision with the help of a few hours of ground-based observations. The space-based data, in turn, enables me to select the intrinsic frequency from the complex alias structure of multi-night ground-based data, providing a practical demonstration of the importance of carefully considering the spectral window. I apply what I have learned about undersampled data to anticipate upcoming pulsating star science in the next generation of synoptic time domain photometric surveys such as the Zwicky Transient Facility and the Large Synoptic Survey Telescope
Download or read book White Dwarfs written by International Astronomical Union and published by Springer Science & Business Media. This book was released on 1971-06-30 with total page 180 pages. Available in PDF, EPUB and Kindle. Book excerpt: Proceedings of IAU Symposium 42, held at St. Andrews, Fife, Scotland, August 11-13, 1970
Download or read book From Pulsations to Planetary Debris written by Zachary Philip Vanderbosch and published by . This book was released on 2021 with total page 390 pages. Available in PDF, EPUB and Kindle. Book excerpt: The final fates of stars and their planetary systems are in most cases embodied by white dwarfs. These are the compact remnants of stellar evolution for all low- to intermediate-mass stars in our Galaxy, including the Sun. The relative simplicity of white dwarfs makes them valuable tools in a variety of astrophysical contexts, from placing limits on the age of our Galaxy, to measuring the bulk abundances of exoplanets via the presence of actively accreted metals in their atmospheres. White dwarfs can also exhibit photometric variability for a variety of reasons, both as isolated stars and in binary systems, providing insights into white dwarf structure and circumstellar environments. In this dissertation we first present novel techniques to identify variable white dwarfs in modern time-domain photometric surveys. We then focus on identifying and characterizing two types of variable white dwarfs: those that pulsate and those being transited by circumstellar planetary debris. Using the unprecedented number of white dwarfs observed by Gaia and the Zwicky Transient Facility (ZTF), we define a set of variability metrics based on these publicly available catalogues to efficiently identify variable white dwarfs across the Gaia color magnitude diagram. We then present a dedicated search for pulsating He-atmosphere white dwarfs (DBVs) using McDonald Observatory follow-up observations, in an attempt to better define the observed extent of the helium-atmosphere white dwarf instability strip. We increase the number of DBVs from 28 to 47 using Sloan Digital Sky Survey spectroscopy to initially identify candidate DBVs. We find the observed blue and red edges of the DB instability strip are still hotter than theoretical expectations by about 1500K and 600K, respectively. Using our set of Gaia and ZTF variability metrics, we identify and characterize the second and third cases of white dwarfs with recurring planetary debris transits, with debris orbital periods differing by more than two orders of magnitude. These objects, in combination with the first known system, probe a unique parameter space constraining earlier stages of the planetary debris disruption and migration process
Download or read book Scientific and Technical Aerospace Reports written by and published by . This book was released on 1991 with total page 562 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book White Dwarfs written by Evry L. Schatzman and published by . This book was released on 1958 with total page 196 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book Unlocking the Secrets of White Dwarf Stars written by Hugh M. Van Horn and published by Springer. This book was released on 2014-11-14 with total page 335 pages. Available in PDF, EPUB and Kindle. Book excerpt: White dwarfs, each containing about as much mass as our Sun but packed into a volume about the size of Earth, are the endpoints of evolution for most stars. Thousands of these faint objects have now been discovered, though only a century ago only three were known. They are among the most common stars in the Milky Way Galaxy, and they have become important tools in understanding the universe. Yet a century ago only three white dwarfs were known. The existence of these stars completely baffled the scientists of the day, and solving the mysteries of these strange objects required revolutionary advances in science and technology, including the development of quantum physics, the construction and utilization of large telescopes, the invention of the digital computer, and the ability to make astronomical observations from space. This book tells the story of the growth in our understanding of white dwarf stars, set within the context of the relevant scientific and technological advances. Part popular science, part historical narrative, this book is authored by one of the astrophysicists who participated directly in uncovering some of the secrets of white dwarf stars.
Download or read book International Aerospace Abstracts written by and published by . This book was released on 1990 with total page 1154 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book Ultracool Companions to White Dwarfs written by Paul Raymond Steele and published by . This book was released on 2010 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: In this thesis I present a new near-infrared photometric search for unresolved ultracool companions and debris disks to white dwarfs in UKIDSS DR5. 24 DA white dwarfs were found with multiple excesses indicative of a low mass companion, with 7 of these having a predicted mass in the range associated with brown dwarfs. The results of this survey show that the unresolved brown dwarf companion fraction to DA white dwarfs is 0.5? fWD+BD? 1.8? 0.7%. I also calculate the unresolved L-dwarf companion fraction to be 0.5? fWD+dL? 1.5? 0.6%, which is much higher than the previous estimate of Farihi, Becklin & Zuckerman (2005). However, the results are consistent with previous estimates of the brown dwarf companion fraction to main sequence stars. I analyse the near-infrared spectra of eleven known DA white dwarf + M-dwarf binaries, and compare the spectral types assigned using optical photometry against those assigned using near-infrared spectra. I search for evidence that the known short period systems once existed in a common envelope phase. No such evidence was found. I also present the spectroscopic analysis of two particularly interesting binaries; PG 1234+ 482 and PHL 5038. PG 1234+482 is the hottest and youngest DA white dwarf with a cool companion on the stellar-substellar borderline. I discuss the possibility that the companion is a brown dwarf, and the possibility that the secondary is the source of contamination by heavy metals in the white dwarf's atmosphere. PHL 5038 was identified to have a near-infrared excess in UKIDSS, and the spectroscopic analysis in this thesis confirms that the secondary in this system can be resolved at a projected orbital separation of 55AU, and is a brown dwarf with spectral type L8-L9. This is only the second such pair found (over 20 years later) after GD165AB. The secondary in this system has the potential to be used as a benchmark brown dwarf for testing substellar atmospheric models.
