Global 3D radiation-hydrodynamics models of AGB stars. Effects of convection and radial pulsations on atmospheric structures

Context: Observations of asymptotic giant branch (AGB) stars with increasing spatial resolution reveal new layers of complexity of atmospheric processes on a variety of scales. Aim: To analyze the physical mechanisms that cause asymmetries and surface structures in observed images, we use detailed 3D dynamical simulations of AGB stars; these simulations self-consistently describe convection and pulsations. Methods: We used the CO5BOLD radiation-hydrodynamics code to produce an exploratory grid of global "star-in-a-box" models of the outer convective envelope and the inner atmosphere of AGB stars to study convection, pulsations, and shock waves and their dependence on stellar and numerical parameters. Results: The model dynamics are governed by the interaction of long-lasting giant convection cells, short-lived surface granules, and strong, radial, fundamental-mode pulsations. Radial pulsations and shorter wavelength, traveling, acoustic waves induce shocks on various scales in the atmosphere. Convection, waves, and shocks all contribute to the dynamical pressure and, thus, to an increase of the stellar radius and to a levitation of material into layers where dust can form. Consequently, the resulting relation of pulsation period and stellar radius is shifted toward larger radii compared to that of non-linear 1D models. The dependence of pulsation period on luminosity agrees well with observed relations. The interaction of the pulsation mode with the non-stationary convective flow causes occasional amplitude changes and phase shifts. The regularity of the pulsations decreases with decreasing gravity as the relative size of convection cells increases. The model stars do not have a well-defined surface. Instead, the light is emitted from a very extended inhomogeneous atmosphere with a complex dynamic pattern of high-contrast features

Dust-driven winds of AGB stars: The critical interplay of atmospheric shocks and luminosity variations

Winds of AGB stars are thought to be driven by a combination of pulsation-induced shock waves and radiation pressure on dust. In dynamic atmosphere and wind models, the stellar pulsation is often simulated by prescribing a simple sinusoidal variation in velocity and luminosity at the inner boundary of the model atmosphere. We experiment with different forms of the luminosity variation in order to assess the effects on the wind velocity and mass-loss rate, when progressing from the simple sinusoidal recipe towards more realistic descriptions. Using state-of-the-art dynamical models of C-rich AGB stars, a range of different asymmetric shapes of the luminosity variation and a range of phase shifts of the luminosity variation relative to the radial variation are tested. These tests are performed on two stellar atmosphere models. The first model has dust condensation and, as a consequence, a stellar wind is triggered, while the second model lacks both dust and wind. The first model with dust and stellar wind is very sensitive to moderate changes in the luminosity variation. There is a complex relationship between the luminosity minimum, and dust condensation: changing the phase corresponding to minimum luminosity can either increase or decrease mass-loss rate and wind velocity. The luminosity maximum dominates the radiative pressure on the dust, which in turn, is important for driving the wind. These effects of changed luminosity variation are coupled with the dust formation. In contrast there is very little change to the structure of the model without dust. Changing the luminosity variation, both by introducing a phase shift and by modifying the shape, influences wind velocity and the mass-loss rate. To improve wind models it would probably be desirable to extract boundary conditions from 3D dynamical interior models or stellar pulsation models.Comment: 11 pages, 13 figures, accepted for publication in A&

Atmospheres and wind properties of non-spherical AGB stars

The wind-driving mechanism of asymptotic giant branch (AGB) stars is commonly attributed to a two-step process: first, gas in the stellar atmosphere is levitated by shockwaves caused by stellar pulsation, then accelerated outwards by radiative pressure on newly formed dust, inducing a wind. Dynamical modelling of such winds usually assumes a spherically symmetric star. We explore the potential consequences of complex stellar surface structures, as predicted by three-dimensional (3D) star-in-a-box modelling of M-type AGB stars, on the resulting wind properties with the aim to improve the current wind models. Two different modelling approaches are used; the CO$^5$BOLD 3D star-in-a-box code to simulate the convective, pulsating interior and lower atmosphere of the star, and the DARWIN one-dimensional (1D) code to describe the dynamical atmosphere where the wind is accelerated. The gas dynamics of the inner atmosphere region at distances of $R\sim1-2R_\star$, which both modelling approaches simulate, are compared. Dynamical properties and luminosity variations derived from CO$^5$BOLD interior models are used as input for the inner boundary in DARWIN wind models in order to emulate the effects of giant convection cells and pulsation, and explore their influence on the dynamical properties. The CO$^5$BOLD models are inherently anisotropic, with non-uniform shock fronts and varying luminosity amplitudes, in contrast to the spherically symmetrical DARWIN wind models. DARWIN wind models with CO$^5$BOLD-derived inner boundary conditions produced wind velocities and mass-loss rates comparable to the standard DARWIN models, however the winds show large density variations on time-scales of 10-20 years.Comment: 13 pages, 12 figures, Accepted for publication in A&

Pulsation-induced atmospheric dynamics in M-type AGB stars. Effects on wind properties, photometric variations and near-IR CO line profiles

Wind-driving in asymptotic giant branch (AGB) stars is commonly attributed to a two-step process. First, matter in the stellar atmosphere is levitated by shock waves, induced by stellar pulsation, and second, this matter is accelerated by radiation pressure on dust, resulting in a wind. In dynamical atmosphere and wind models the effects of the stellar pulsation are often simulated by a simplistic prescription at the inner boundary. We test a sample of dynamical models for M-type AGB stars, for which we kept the stellar parameters fixed to values characteristic of a typical Mira variable but varied the inner boundary condition. The aim was to evaluate the effect on the resulting atmosphere structure and wind properties. The results of the models are compared to observed mass-loss rates and wind velocities, photometry, and radial velocity curves, and to results from 1D radial pulsation models. Dynamical atmosphere models are calculated, using the DARWIN code for different combinations of photospheric velocities and luminosity variations. The inner boundary is changed by introducing an offset between maximum expansion of the stellar surface and the luminosity and/or by using an asymmetric shape for the luminosity variation. Models that resulted in realistic wind velocities and mass-loss rates, when compared to observations, also produced realistic photometric variations. For the models to also reproduce the characteristic radial velocity curve present in Mira stars (derived from CO $\Delta v = 3$ lines), an overall phase shift of 0.2 between the maxima of the luminosity and radial variation had to be introduced. We find that a group of models with different boundary conditions (29 models, including the model with standard boundary conditions) results in realistic velocities and mass-loss rates, and in photometric variations

Easier material management - at what cost? : Librarians meet IMMS

Intelligent Material Management System, IMMS, was developed in a collaboration between Lyngsoe Systems, a commercial company, and public libraries in Aarhus and Copenhagen, Denmark, with the aim to reduce the time staff spend on managing library materials. The aim of this article is to shed light on what IMMS means for the library practices and hence for the librarian profession. Two research questions will guide the analysis: How do librarians and IMMS interplay at the public library in Copenhagen, Denmark? How does the implementation of IMMS impact the library practices at the branch libraries in Copenhagen, Denmark? With the theoretical lens of practice theory, the article shows how new norms and rules as well as new tools and objects are implemented with IMMS. Librarians need to be able to work with the new objects and tools, the new norms and to create an inspiring library room for library users. Their relation to collection management is changed, and their ability to evaluate materials is not needed in the same way when it comes to selection of titles for the collection. This sometimes creates a tension between the librarian and the system, especially when the librarians’ role in the practice is to perform the decision-making by the algorithm, and not to use their skills to evaluate resources

A grammar of Palula

This grammar provides a grammatical description of Palula, an Indo-Aryan language of the Shina group. The language is spoken by about 10,000 people in the Chitral district in Pakistan’s Khyber Pakhtunkhwa Province. This is the first extensive description of the formerly little-documented Palula language, and is one of only a few in-depth studies available for languages in the extremely multilingual Hindukush-Karakoram region. The grammar is based on original fieldwork data, collected over the course of about ten years, commencing in 1998. It is primarily in the form of recorded, mainly narrative, texts, but supplemented by targeted elicitation as well as notes of observed language use. All fieldwork was conducted in close collaboration with the Palula-speaking community, and a number of native speakers took active part in the process of data gathering, annotation and data management. The main areas covered are phonology, morphology and syntax, illustrated with a large number of example items and utterances, but also a few selected lexical topics of some prominence have received a more detailed treatment as part of the morphosyntactic structure. Suggestions for further research that should be undertaken are given throughout the grammar. The approach is theory-informed rather than theory-driven, but an underlying functional-typological framework is assumed. Diachronic development is taken into account, particularly in the area of morphology, and comparisons with other languages and references to areal phenomena are included insofar as they are motivated and available. The description also provides a brief introduction to the speaker community and their immediate environment

Outstanding Fields of Corn

Students will construct a diorama illustrating a selected geographic theme. Students will write a short informational paragraph relating to the theme. Students will include both elements of the past and present in their display. Students will give evidence of basic knowledge agriculture

A grammar of Palula

This grammar provides a grammatical description of Palula, an Indo-Aryan language of the Shina group. The language is spoken by about 10,000 people in the Chitral district in Pakistan’s Khyber Pakhtunkhwa Province. This is the first extensive description of the formerly little-documented Palula language, and is one of only a few in-depth studies available for languages in the extremely multilingual Hindukush-Karakoram region. The grammar is based on original fieldwork data, collected over the course of about ten years, commencing in 1998. It is primarily in the form of recorded, mainly narrative, texts, but supplemented by targeted elicitation as well as notes of observed language use. All fieldwork was conducted in close collaboration with the Palula-speaking community, and a number of native speakers took active part in the process of data gathering, annotation and data management. The main areas covered are phonology, morphology and syntax, illustrated with a large number of example items and utterances, but also a few selected lexical topics of some prominence have received a more detailed treatment as part of the morphosyntactic structure. Suggestions for further research that should be undertaken are given throughout the grammar. The approach is theory-informed rather than theory-driven, but an underlying functional-typological framework is assumed. Diachronic development is taken into account, particularly in the area of morphology, and comparisons with other languages and references to areal phenomena are included insofar as they are motivated and available. The description also provides a brief introduction to the speaker community and their immediate environment