1,707 research outputs found
The evolution of cataclysmic variables as revealed by their donor stars
No full textWe present an attempt to reconstruct the complete evolutionary path followed by cataclysmic variables (CVs), based on the observed mass-radius relationship of their donor stars. Along the way, we update the semi-empirical CV donor sequence presented previously by one of us, present a comprehensive review of the connection between CV evolution and the secondary stars in these systems, and reexamine most of the commonly used magnetic braking (MB) recipes, finding that even conceptually similar ones can differ greatly in both magnitude and functional form. The great advantage of using donor radii to infer mass-transfer and angular-momentum-loss (AML) rates is that they sample the longest accessible timescales and are most likely to represent the true secular (evolutionary average) rates. We show explicitly that if CVs exhibit long-term mass-transfer-rate fluctuations, as is often assumed, the expected variability timescales are so long that other tracers of the mass-transfer rate—including white dwarf (WD) temperatures—become unreliable. We carefully explore how much of the radius difference between CV donors and models of isolated main-sequence stars may be due to mechanisms other than mass loss. The tidal and rotational deformation of Roche-lobe-filling stars produces sime 4.5% radius inflation below the period gap and sime 7.9% above. A comparison of stellar models to mass-radius data for non-interacting stars suggests a real offset of sime 1.5% for fully convective stars (i.e., donors below the gap) and sime 4.9% for partially radiative ones (donors above the gap). We also show that donor bloating due to irradiation is probably smaller than, and at most comparable to, these effects. After calibrating our models to account for these issues, we fit self-consistent evolution sequences to our compilation of donor masses and radii. In the standard model of CV evolution, AMLs below the period gap are assumed to be driven solely by gravitational radiation (GR), while AMLs above the gap are usually described by an MB law first suggested by Rappaport et al. We adopt simple scaled versions of these AML recipes and find that these are able to match the data quite well. The optimal scaling factors turn out to be f GR = 2.47 ± 0.22 below the gap and f MB = 0.66 ± 0.05 above (the errors here are purely statistical, and the standard model corresponds to f GR = f MB = 1). This revised model describes the mass-radius data significantly better than the standard model. Some of the most important implications and applications of our results are as follows. (1) The revised evolution sequence yields correct locations for the minimum period and the upper edge of the period gap; the standard sequence does not. (2) The observed spectral types of CV donors are compatible with both standard and revised models. (3) A direct comparison of predicted and observed WD temperatures suggests an even higher value for f GR, but this comparison is sensitive to the assumed mean WD mass and the possible existence of mass-transfer-rate fluctuations. (4) The predicted absolute magnitudes of donor stars in the near-infrared form a lower envelope around the observed absolute magnitudes for systems with parallax distances. This is true for all of our sequences, so any of them can be used to set firm lower limits on (or obtain rough estimates of) the distances toward CVs based only on P orb and single epoch near-IR measurements. (5) Both standard and revised sequences predict that short-period CVs should be susceptible to dwarf nova (DN) eruptions, consistent with observations. However, both sequences also predict that the fraction of DNe among long-period CVs should decline with P orb above the period gap. Observations suggest the opposite behavior, and we discuss the possible explanations for this discrepancy. (6) Approximate orbital period distributions constructed from our evolution sequences suggest that the ratio of long-period CVs to short-period, pre-bounce CVs is about 3 × higher for the revised sequence than the standard one. This may resolve a long-standing problem in CV evolution. Tables describing our donor and evolution sequences are provided in electronically readable form
Cut-elimination, substitution and normalisation
No full textDate of Acceptance: 01/2015We present a proof (of the main parts of which there is a formal version, checked with the Isabelle proof assistant) that, for a G3-style calculus covering all of intuitionistic zero-order logic, with an associated term calculus, and with a particular strongly normalising and confluent system of cut-reduction rules, every reduction step has, as its natural deduction translation, a sequence of zero or more reduction steps (detour reductions, permutation reductions or simplifications). This complements and (we believe) clarifies earlier work by (e.g.) Zucker and Pottinger on a question raised in 1971 by Kreisel.Peer reviewe
SpS1-The evolution of brown dwarf infrared spectroscopic properties
No full textBrown dwarfs (hereafter BDs) are formed, like stars, by interstellar cloud collapse, but attaining masses of less then 0.075 M⊙ (Baraffe et al. 1998), i.e. too low core temperatures (< 3.5 × 106 K) to stabilize the nuclear burning of the hydrogen PP chain. Therefore, even the most massive BDs begin cooling after some 109 yrs. However, for masses above 0.06 M⊙, core temperatures become hotter than the lithium burning temperature (2.4 x 106 K). All BDs above 0.013 M⊙ (13 MJup) reach core temperatures above the 1.0 x 106 K necessary to burn deuterium from about 107 yrs. The IAU has adopted the definition of the planetary regime as objects having masses below the deuterium burning conditions. But BDs are likely to form well below this limit into the planetary mass regime down to some 5 MJup. It is therefore convenient, in the absence of indices on their formation mechanisms, to call them planetary mass objects or planemos.</jats:p
I remember teaching English at Seabrook
No full textIn this "I remember" memoir, Isabell Waugh, a former teacher at Seabrook, compares and constrasts the different groups of students she taught. She remembers that native-born American teenagers tended to be more concerned with athletics and social activities, than academic matters. In comparison, Estonian and Japanese parents did not tolerate low academic performance, so students from the two groups often competed intensely with each other for academic achievement and recognition. Isabelle recalls that the Estonians were, in general, more sophisticated and better educated. Most of the children knew 3-5 languages, and were more advanced in math and science. She sensed that some Estonian parents felt that their homes at Seabrook were temporary, and that they would be returning to Estonia at some point. The Seabrook Educational and Cultural Center has been soliciting current and past residents of Seabrook Farms for an "I remember" project. Residents are asked to create narratives regarding their experiences at Seabrook Farms. These memories help preserve the history and multi-cultural heritage of Seabrook Farms
Is multiplicity universal? A study of multiplicity in the young moving groups
No full textThe young moving groups are collections of nearby (<200 pc), young (5-150 Myr) pre-main
sequence stars; these stars offer us one of the best opportunities to characterise
stellar multiplicity, sub-stellar phenomena, disc evolution and planet formation.
Here we present results from a series of multiplicity studies aimed at producing comprehensive
multiplicity statistics of the young moving groups. The aim was to compare
the derived statistics of the young moving groups to other populations in order to investigate
whether the abundance and properties of multiple systems are environment independent.
We have combined high-resolution spectroscopy, AO-imaging and direct imaging to
identify and characterise multiple systems across a huge range of orbital periods (1-
10e10 day). The observational techniques also allow us to constrain the abundance of
multiple systems in these populations by calculating detection limits.
We found many similarities (frequency of spectroscopic binaries; frequency, mass-ratio
and physical separation of visual binaries) between the young moving groups and both
younger and older regions, for multiple systems with physical separations smaller than
1000 au. We did, however, identify a significant number of new wide (>1000 au) companions.
We reconciled the apparent excess of wide binary systems, when compared
to the field population, by arguing that the wide systems are weakly bound and most
likely decaying. By comparing the multiplicity statistics in one particular moving group
we showed that the dynamical evolution of non-hierarchical protostars could lead to the
population of wide binaries we can observe today.
Our results indicate that the majority of low-mass stars form in small groups with 3 or
4 components that undergo significant dynamical evolution. The multiplicity properties
of the young nearby moving groups are statistically similar to many other populations,
supporting the environment-independent formation of multiple systems
Isabelle Bell to Susan Niemcewicz, December 23, 1800
No full textIsabelle Bell wrote to Susan U. Niemcewicz in Elizabethtown, New Jersey. Bell expressed her disappointment in not receiving a line from Susan. She sent Bell Lucretia Rephans subscription epistle, but Susan refrained from writing a letter to her. Bell did not execute any of Susan’s commissions in New York because her time there was short. Miss Resham heard that Mr. B Livingston told his sister, Mrs. J. Livingston that he would offer Bell a salary to live in his house and take charge of his children’s education. Asked if Susan what she thought of her being an author and if Susan would subscribe to a small volume that may have the good fortune to rival the poems of the immortal Scarron.https://digitalcommons.kean.edu/lhc_1800s/1143/thumbnail.jp
Climate Simulations of Hot Jupiters: Developing and Applying an Accurate Radiation Scheme
No full textTo date more than 1500 exoplanets have been discovered. A large number of these are hot Jupiters, Jupiter-sized planets orbiting < 0.1 au from their parent stars, due to limitations in observational techniques making them easier to detect than smaller planets in wider orbits. This is also, for the same reasons, the class of exoplanets with the most observational constraints. Due to the very large interaction between these planets and their parent stars they are believed to be tidally locked, causing a large temperature contrast between the permanently hot day side and colder night side.
There are still many open questions about these planets. Many are observed to have inflated radii, i.e. the observed radius is larger for a given mass than evolutionary models predict. A mechanism that can transport some of the stellar heating into the interior of the planet may be able to explain this. The presence of hazes or clouds has been inferred on some planets, but their composition and distribution remain unknown. According to chemical equilibrium models TiO and VO should be present on the day side of the hottest of these planets, but these molecules have not yet been detected. Cold traps, where these molecules condense out on the night side, have been suggested to explain this. The efficiency of the heat redistribution from the day side to the night side has been found to vary significantly between different planets; the mechanism behind this is still unknown.
To begin to answer many of these questions we need models capturing the three-dimensional nature of the atmospheres of these planets. General circulation models (GCMs) do this by solving the equations of fluid dynamics for the atmosphere coupled to a radiative transfer scheme. GCMs have previously been applied to several exoplanets, but many solve simplified fluid equations (shallow water or primitive equations) or highly parametrised radiation schemes (temperature-forcing, gray or band-averaged opacities). We here present an adaptation of the Met Office Unified Model (UM), a GCM used for weather predictions and climate studies for the Earth, to hot Jupiters. The UM solves the full 3D Euler equations for the fluid, and the radiation scheme uses the two-stream approximation and correlated-k method, which are state of the art for both Earth and exoplanet GCMs. This makes it ideally suited for the study of hot Jupiters.
An important part of this work is devoted to the adaptation of the radiation scheme of the UM to hot Jupiters. This includes calculation of opacities for the main absorbers in these atmospheres from state-of-the-art high temperature line lists, the calculation of k-coefficients from these opacities, and making sure all aspects of the scheme perform satisfactorily at high temperatures and pressures. We have tested approximations made in previous works such as the two-stream approximation, use of band-averaged opacities and different treatments of gaseous overlap. Uncertainties in current models, such as the lack of high temperature line broadening parameters for these atmospheres, are discussed.
We couple the adapted radiation scheme to the UM dynamical core, which has been tested independently. Our first application is devoted to one of the most well-observed hot Jupiters, HD 209458b. Differences between previous modelling works and our model are discussed, and we compare results from the full coupled model with results obtained using a temperature-forcing scheme.
We have also developed a tool to calculate synthetic phase curves, and emission and transmission spectra from the output of our 3D model. This enables us to directly compare our model results to observations and test the effect of various parameters and model choices on observable quantities
Interviews with Carl T. Bode, Isabelle Fritschen, Joseph H. Hirt, Mary G. Hirt, and Minnie Campbell
No full textInterviews with Carl T. Bode, Isabelle Fritschen, Joseph H. Hirt, Mary G. Hirt, and Minnie Campbell. The recording includes a variety of German-language songs. The last half of the recording is dedicated to Minnie Campbell telling about her time working for Mother Bickerdyke. The first few minutes of the recording are missing. 00:00:13 - Song, The Messenger Bird sung by Joseph H. Hirt and translated by Isabelle Fritschen 00:01:35 - Song, Birdie in the Window, sung by Mary Gertrude Hirt 00:02:59 - Story of Peter John Thielen\u27s experience in the Franco-Prussian War told by Joseph Hirt 00:05:27 - Grandfather\u27s experience with wild cattle told by Isabelle Fritschen 00:07:31 - Carl T. Bode introduction 00:08:46 - Nursery rhyme about hands 00:09:09 - The Cuckoo and the Donkey 00:09:42 - Sleep Baby Sleep 00:10:24 - Golden Evening Sun 00:11:00 - Beautiful Moon 00:12:10 - My Homeland 00:13:50 - Minnie Campbell Introduction 00:14:05 - Experiences as Mother Bickerdyke\u27s secretary 00:14:35 - Mother Bickerdyke\u27s 81st birthday celebration in Bunker Hill, KS 00:19:59 - Mother Bickerdyke\u27s portrait 00:23:55 - How Lydia Foster, Mother Bickerdyke\u27s Black maid came to live with her. 00:26:34 - Mother Bickerdyke\u27s death 00:29:34 - Mother Bickerdyke\u27s burial in Galesburg, Illinois 00:30:28 - Working for Mother Bickerdyke 00:34:01 - Going to School as a student of James Bickerdyke, Mother Bickerdyke\u27s son 00:35:26 - Decline of Bunker Hill, KS 00:37:15 - Russell stealing the county seat from Bunker Hill 00:38:09 - Closing of the Dorrance, KS bank 00:39:00 - Mother Bickerdyke\u27s personality 00:42:34 - Experience with Nina Brown Baker author of Cyclone in Calico 00:48:24 - Mother Bickerdyke Home for Widows and Children in Ellsworth, KS 00:51:13 - Post scripthttps://scholars.fhsu.edu/sackett/1014/thumbnail.jp
The Chemistry of Hot Exoplanet Atmospheres: Developing and Applying Chemistry Schemes in 1D and 3D Models
No full textThe focus of this work is the development and improvement of chemistry schemes in both 1D and 3D atmosphere models, applied to exoplanets. With an ever increasing number of known exoplanets, planets orbiting stars other than the Sun, the diversity in the physical and chemical nature of planets and their atmospheres is becoming more apparent. One of the prime targets, and the focus of many observational and theoretical studies, are the subclass of exoplanets termed hot Jupiters, Jovian sized planets on very short period orbits around their host star.
Due to their close orbit, with orbital periods of just a few days, the atmospheres of such planets are heated to very high temperatures (~1000-2000 K) by the intense irradiation from the star. In addition, it is expected that these planets should have synchronised their rotation with their orbital period, a phenomenon called tidal-locking, that leads to a permanently illuminated dayside and a perpetually dark nightside. This combination of intense heating and tidal-locking leads to an exotic type of atmosphere that is without analogue in our own Solar system.
Observational constraints suggest that some of these atmospheres may be clear whilst others may be cloudy or contain haze. Some hot Jupiters appear to be inflated with radii larger than is expected for their mass. For the warmest hot Jupiters optical absorbing species TiO and VO are expected to be present, due to the thermodynamical conditions, where they can strongly influence the thermal structure of the atmosphere, yet so far these species have remained elusive in observations. Theoretical simulations of these planets appear to provide poor matches to the observed emission flux from the nightside of the planet whilst providing a much better agreement with the observed dayside flux.
These outstanding questions can be tackled in two complimentary ways. Firstly, the number of exoplanets subject to intense observational scrutiny must be increased to improve the statistical significance of observed trends. Secondly, and in tandem, the suite of available theoretical models applied to such atmospheres must be improved to allow for a more comprehensive understanding of the potential physical and chemical processes that occur in these atmospheres, as well as for better comparison of model predictions with observations.
In this thesis we present the development and application of one-dimensional (1D) and three-dimensional (3D) models to the atmospheres of hot exoplanets, with a focus on improving the representation of chemistry. One of the concerns of this work is to couple the radiative transfer and chemistry calculations in a one-dimensional model to allow for a self-consistent model that includes feedback between the chemical composition and the thermal structure. We apply this model to the atmospheres of two typical hot Jupiters to quantify this effect. Implications for previous models that do not include this consistency are discussed.
Another major focus is to improve the representation of chemistry in the Met Office Unified Model (UM) for exoplanet applications, a three-dimensional model with its heritage in modelling the Earth atmosphere that has recently been applied to exoplanets. We discuss the coupling of two new chemistry schemes that improve both the flexibility and capabilities of the UM applied to exoplanets. Ultimately these developments will allow for a consistent approach to calculate the 3D chemical composition of the atmosphere taking into account the effect of large scale advection, one of the processes currently hypothesised to cause the discrepancy between model predictions and observations of the nightside emission flux of many hot Jupiters
Formalization of Isabelle Meta Logic in NuPRL
No full textNuPRL and Isabelle are two general purpose theorem provers. Both of them are based on a version of Constructive Higher Order Type Theory. In an earlier work the author has proposed an informal semantics of Isabelle Meta Logic in an extension of NuPRL Type Theory. An automated converter, based on this semantics, has been developed, that translates Isabelle theorem statements into NuPRL. This work presents a formalization of the above semantics in NuPRL. It starts with a deep embedding of Isabelle type and term syntax into NuPRL Constructive Type Theory. Next, two internal NuPRL functions are defined. One of them maps Isabelle types into NuPRL types and the other maps Isabelle terms into elements of appropriate NuPRL types. These two functions provide an interpretation of Isabelle in NuPRL. Finally, interpretations of all Isabelle Meta Logic rules are proven as theorems in some classical extension of NuPRL Type Theory. This formalization is aimed to provide a more secure foundation for the interaction between two systems
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