337 research outputs found
BK15-16
No full textThis book has a page (5538) of three illustrated fables: The Lark and Her Young, The Fox and the Wolf, and The Donkey's Wish. The Fox and the Wolf is told differently; its illustration is done by Harry Neilson. On 5826, AD and DM are told without illustration.This is a hardbound book (hard cover)Holland Thompson and Arthur Me
Exploring the evolution of classical cepheids with MESA
No full textClassical Cepheid stars play an important role in the rst step of the distance ladder
determination in the Milky Way and in nearby galaxies. The period-luminosity rela-
tionship of classical Cepheid stars is used to calculate the Hubble constant which is
a measurement of the expansion of the universe. Using Modules for Experiments in
Stellar Astrophysics (MESA), new evolution models with and without convective core
overshooting are used to probe for the period rate change at the di erent crossing of
the instability strip. A population synthesis analysis shows that 63.4% of classical
Cepheids have positive period rate change while the rest have negative period rate
change. With overshooting, 91.1% of classical Cepheid stars have positive period rate
change. To account for the di erence in the number of classical Cepheid stars that are
observed versus those that are modeled with MESA, mass-loss, rotation and metallic-
ity need to be added to the models to help explain the di erence with observations
The Dynamic Atmospheres of Classical Cepheids: Studies of Atmospheric Extension, Mass Loss, and Shocks
No full textIn this dissertation, we develop new tools for the study of stellar atmospheres, pulsating stellar atmospheres and mass loss from pulsating stars. These tools provide new insights into the structure and evolution of stars and complement modern observational techniques such as optical interferometry and high resolution spectroscopy. In the first part, a new spherically symmetric version of the Atlas program is developed for modelling extended stellar atmospheres. The program is used to model interferometric observations from the literature and to study limb-darkening for stars with low gravity. It is determined that stellar limb-darkening can be used to constrain fundamental properties of stars. When this is coupled with interferometric or microlensing observations, stellar limb-darkening can predict the masses of isolated stars. The new SAtlas program is combined with the plane-parallel hydrodynamic program Hermes to develop a new spherically-symmetric radiative hydrodynamic program that models radial pulsation in the atmosphere of a star to depths including the pulsation-driving regions of the stars. Preliminary tests of this new program are discussed.
In the second part, we study the recent observations of circumstellar envelopes surrounding Cepheids and develop a mass-loss hypothesis to explain their formation. The hypothesis is studied using a modified version of the Castor, Abbott, & Klein theory for radiative-driven winds to contain the effects of pulsation. In the theory, pulsation is found to be a driving mechanism that increases the mass-loss rates of Cepheids by up to four orders of magnitude. These mass-loss rates are large enough to explain the formation of the envelopes from dust forming in the wind at large distances from the surface of the star. The mass-loss rates are found to be plausible explanation for the Cepheid mass discrepancy. We also compute mass-loss rates from optical and infrared observations of Large Magellanic Cloud Cepheids from the infrared excess and find mass loss to be an important phenomena in these stars. The amount of infrared excess is found to potentially affect the structure of the infrared Leavitt law.Ph
The Dynamic Atmospheres of Classical Cepheids: Studies of Atmospheric Extension, Mass Loss, and Shocks
No full textIn this dissertation, we develop new tools for the study of stellar atmospheres, pulsating stellar atmospheres and mass loss from pulsating stars. These tools provide new insights into the structure and evolution of stars and complement modern observational techniques such as optical interferometry and high resolution spectroscopy. In the first part, a new spherically symmetric version of the Atlas program is developed for modelling extended stellar atmospheres. The program is used to model interferometric observations from the literature and to study limb-darkening for stars with low gravity. It is determined that stellar limb-darkening can be used to constrain fundamental properties of stars. When this is coupled with interferometric or microlensing observations, stellar limb-darkening can predict the masses of isolated stars. The new SAtlas program is combined with the plane-parallel hydrodynamic program Hermes to develop a new spherically-symmetric radiative hydrodynamic program that models radial pulsation in the atmosphere of a star to depths including the pulsation-driving regions of the stars. Preliminary tests of this new program are discussed.
In the second part, we study the recent observations of circumstellar envelopes surrounding Cepheids and develop a mass-loss hypothesis to explain their formation. The hypothesis is studied using a modified version of the Castor, Abbott, & Klein theory for radiative-driven winds to contain the effects of pulsation. In the theory, pulsation is found to be a driving mechanism that increases the mass-loss rates of Cepheids by up to four orders of magnitude. These mass-loss rates are large enough to explain the formation of the envelopes from dust forming in the wind at large distances from the surface of the star. The mass-loss rates are found to be plausible explanation for the Cepheid mass discrepancy. We also compute mass-loss rates from optical and infrared observations of Large Magellanic Cloud Cepheids from the infrared excess and find mass loss to be an important phenomena in these stars. The amount of infrared excess is found to potentially affect the structure of the infrared Leavitt law.Ph
The Drake Equation and the Search for Life in the Galaxy through Indigenist Lenses
No full text<p>Abstract:</p>
<p>So much of the modern search for life in beyond our Solar System is motivated by the interpretation of the Drake Equation. However, much of this discourse has arisen with colonial and Eurocentric biases that limit the interpretation of the various terms of the equation. In this talk, I will present an Indigenist view of the Drake Equation and show that this perspective leads to a significantly different view of life in the Galaxy. I will also show that this perspective suggests that the current binary discussion of bio-signatures and techno-signatures in the search for intelligent life can be very limiting.</p>
Period Change and Stellar Evolution of β Cephei Stars
No full textThe β Cephei stars represent an important class of massive star pulsators that probe the evolution of B-type stars and the transition from main sequence to hydrogen-shell burning evolution. By understanding β Cep stars, we gain insights into the detailed physics of massive star evolution, including rotational mixing, convective core overshooting, magnetic fields, and stellar winds, all of which play important roles. Similarly, modeling their pulsation provides additional information into their interior structures. Furthermore, measurements of the rate of change of pulsation period offer a direct measure of β Cephei stellar evolution. In this work, we compute state-of-the-art stellar evolution models assuming different amounts of initial rotation and convective core overshoot and measure the theoretical rates of period change, that we compare to rates previously measured for a sample of β Cephei stars. The results of this comparison are mixed. For three stars, the rates are too low to infer any information from stellar evolution models, whereas for three other stars the rates are too high. We infer stellar parameters, such as mass and age, for two β Cephei stars: ξ1 CMa and δ Cet, which agree well with independent measurements. We explore ideas for why models may not predict the higher rates of period change. In particular, period drifts in β Cep stars can artificially lead to overestimated rates of secular period change
Convection, Granulation, and Period Jitter in Classical Cepheids.
No full textAnalyses of recent observations of the sole classical Cepheid in the Kepler field, V1154 Cygni, found random changes of about 30 min in the pulsation period. These period changes challenge standard theories of pulsation and evolution because the period change is non-secular, and explaining this period jitter is necessary for understanding stellar evolution and the role of Cepheids as precise standard candles. We suggest that convection and convective hot spots can explain the observed period jitter. Convective hot spots alter the timing of flux maximum and minimum in the Cepheid light curve, hence change the measured pulsation period. We present a model of random hot spots that generate a localized flux excess that perturbs the Cepheid light curve and consequently the pulsation period, which is consistent with the observed jitter. This result demonstrates how important understanding convection is for modeling Cepheid stellar structure and evolution, how convection determines the red edge of the instability strip, and just how sensitive Cepheid light curves are to atmospheric physics
Cepheid in the Eclipsing Binary System OGLE-LMC-CEP1812 is a Stellar Merger
No full textClassical Cepheids and eclipsing binary systems are powerful probes for measuring stellar fundamental parameters and constraining stellar astrophysics. A Cepheid in an eclipsing binary system is even more powerful, constraining stellar physics, the distance scale and the Cepheid mass discrepancy. However, these systems are rare, only three have been discovered. One of these, OGLE-LMC-CEP1812, presents a new mystery: where the Cepheid component appears to be younger than its red giant companion. In this work, we present stellar evolution models and show that the Cepheid is actually product of a stellar merger during main sequence evolution that causes the Cepheid to be a rejuvenated star. This result raises new questions into the evolution of Cepheids and their connections to smaller-mass anomalous Cepheids
Current awareness on a shoe string: RSS at the HQC
No full textChristine Neilson ([email protected]) served as the Librarian for the Health
Quality Council from September 2004 to December 2006. She is currently the Outreach
Services Librarian for the Saskatchewan Health Information Resources Partnership
(SHIRP), Health Sciences Library, 107 Wiggins Road, Saskatoon SK, S7N 5E5. The
author would like to thank Maureen Bingham, the Health Quality Council's former
Director of Linkage and Exchange, for offering her insight and supporting the HQC
current awareness projectThere is no shortage of articles describing the nature of blogs and RSS feeds and their
potential use in libraries. However, articles describing the implementation and evaluation
of RSS for library current awareness services and the lessons learned along the way are
harder to find. This case study relates the experience of implementing an RSS feed-based
current awareness service in a small special library in Canada, and the preliminary uptake
and feedback of the staff it serves
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