Models of Stars, Brown Dwarfs and Exoplanets

Within the next few years, GAIA and several instruments aiming at imag- ing extrasolar planets will see first light. In parallel, low mass planets are being searched around red dwarfs which offer more favourable conditions, both for radial velocity de- tection and transit studies, than solar-type stars. Authors of the model atmosphere code which has allowed the detection of water vapour in the atmosphere of Hot Jupiters re- view recent advancement in modelling the stellar to substellar transition. The revised solar oxygen abundances and cloud model allow for the first time to reproduce the pho- tometric and spectroscopic properties of this transition. Also presented are highlight results of a model atmosphere grid for stars, brown dwarfs and extrasolar planets.Comment: Refereed paper submitted to the british journal Philosophical Transactions A as an invited review to the Theo Murphy Meeting entitled "Water in the gas phase" held by the Kavli Royal Society in Chichely, GB, June 13-14th 201

The role of convection, overshoot, and gravity waves for the transport of dust in M dwarf and brown dwarf atmospheres

Observationally, spectra of brown dwarfs indicate the presence of dust in their atmospheres while theoretically it is not clear what prevents the dust from settling and disappearing from the regions of spectrum formation. Consequently, standard models have to rely on ad hoc assumptions about the mechanism that keeps dust grains aloft in the atmosphere. We apply hydrodynamical simulations to develop an improved physical understanding of the mixing properties of macroscopic flows in M dwarf and brown dwarf atmospheres, in particular of the influence of the underlying convection zone. We performed 2D radiation hydrodynamics simulations including a description of dust grain formation and transport with the CO5BOLD code. The simulations cover the very top of the convection zone and the photosphere including the dust layers for effective temperatures between 900K and 2800K, all with logg=5 assuming solar chemical composition. Convective overshoot occurs in the form of exponentially declining velocities with small scale heights, so that it affects only the region immediately above the almost adiabatic convective layers. From there on, mixing is provided by gravity waves that are strong enough to maintain thin dust clouds in the hotter models. With decreasing effective temperature, the amplitudes of the waves become smaller but the clouds become thicker and develop internal convective flows that are more efficient in mixing material than gravity waves. The presence of clouds leads to a highly structured appearance of the stellar surface on short temporal and small spatial scales. We identify convectively excited gravity waves as an essential mixing process in M dwarf and brown dwarf atmospheres. Under conditions of strong cloud formation, dust convection is the dominant self-sustaining mixing component

Developing a Web 2.0 technology for hazard response simulation

Published version used with permission from Higher Education Academy. Miller, S., & France, D. Proceedings of the HEA STEM Learning and Teaching Conference, 2012.Students studying disaster/hazard management in UK Higher Education institutions (HEi's) traditionally focus on hazard mapping and process analysis, but have limited opportunities to develop their risk communication skills which are required during emergency response situations. These skills are vital for the real world and employment. Opportunities to develop risk communication skills are not readily available to students during their studies as employers are reluctant to offer placements due to legal barriers. Therefore, universities have to develop tools to provide students with this vital ‘real-world’ experience. Over the last two years, the department of Geography & Development Studies at the University of Chester has begun to explore and evaluate the role of the Web 2.0 tool, Yammer (microblogging/communication tool) for natural hazard (volcano) simulation exercises. This paper highlights the continuing development of the natural hazard simulation exercise through input from external emergency/contingency practitioners locally and internationally to enhance its usability. Input from practitioners has resulted in the adaptation of the tool to flooding hazard emergency response and to other geographically based scenarios (e.g. crime analysis). The input from professionals in the field has enhanced the quality of the exercise/tool as well as providing students with vital employability skills currently used in the workplace of hazard management. Feedback from students highlighted their feeling of a ‘real-life’ pressure situation in which ‘real-time’ decisions have to be made in response to a rapidly changing environment. At the same time they indicated that their experience was stimulating, fun, innovative and enabled networking and interactive opportunities between tutors and students. The development of the Web 2.0 simulation tool through contributions from practitioners and an assessment as to whether the use of such technologies enhances student-learning experience is the focus of this paper.Higher Education Academ

Securing field learning using a twenty-first century Cook's Tour

This is an Accepted Manuscript of an article published by Taylor & Francis in Journal of Geography in Higher Education on 22/01/2015, available online: http://wwww.tandfonline.com/http://dx.doi.org/10.1080/03098265.2014.1003801This paper evaluates the effectiveness of incorporating digital video into a traditional Cook’s Tour as part of a 7-day road trip around the east coast of New Zealand’s North Island over a 4-year period

Alkali Line Profiles in Degenerate Dwarfs

Ultracool stellar atmospheres show absorption by alkali resonance lines severely broadened by collisions with neutral perturbers. In the coolest and densest atmospheres, such as those of T dwarfs, Na I and K I broadened by molecular hydrogen and helium can come to dominate the entire optical spectrum. Their profiles have been successfully modelled with accurate interaction potentials in the adiabatic theory, computing line profiles from the first few orders of a density expansion of the autocorrelation function. The line shapes in the emergent spectrum also depend on the distribution of absorbers as a function of depth, which can be modelled with improved accuracy by new models of dust condensation and settling. The far red K I wings of the latest T dwarfs still show missing opacity in these models, a phenomenon similar to what has been found for the Na I line profiles observed in extremely cool, metal-rich white dwarfs. We show that the line profile in both cases is strongly determined by multiple-perturber interactions at short distances and can no longer be reproduced by a density expansion, but requires calculation of the full profile in a unified theory. Including such line profiles in stellar atmosphere codes will further improve models for the coolest and densest dwarfs as well as for the deeper atmosphere layers of substellar objects in general.Comment: VI Serbian Conference on Spectral Line Shapes in Astrophysics; to be published by the American Institute of Physics, eds. Milan S. Dimitrijevic and Luka C. Popovic; 6 pages, 6 figure

Podcasting: A tool for enhancing assessment feedback?

This presentation discusses how audio feedback to student assignments have been given by podcast at the University of Chester for some Geography modules. There is an overview of the process of producing the podcast and a discussion of the advantages and disadvantages of giving feedback in this form

A physically motivated and empirically calibrated method to measure effective temperature, metallicity, and Ti abundance of M dwarfs

The ability to perform detailed chemical analysis of Sun-like F-, G-, and K-type stars is a powerful tool with many applications including studying the chemical evolution of the Galaxy and constraining planet formation theories. Unfortunately, complications in modeling cooler stellar atmospheres hinders similar analysis of M-dwarf stars. Empirically-calibrated methods to measure M dwarf metallicity from moderate-resolution spectra are currently limited to measuring overall metallicity and rely on astrophysical abundance correlations in stellar populations. We present a new, empirical calibration of synthetic M dwarf spectra that can be used to infer effective temperature, Fe abundance, and Ti abundance. We obtained high-resolution (R~25,000), Y-band (~1 micron) spectra of 29 M dwarfs with NIRSPEC on Keck II. Using the PHOENIX stellar atmosphere modeling code (version 15.5), we generated a grid of synthetic spectra covering a range of temperatures, metallicities, and alpha-enhancements. From our observed and synthetic spectra, we measured the equivalent widths of multiple Fe I and Ti I lines and a temperature-sensitive index based on the FeH bandhead. We used abundances measured from widely-separated solar-type companions to empirically calibrate transformations to the observed indices and equivalent widths that force agreement with the models. Our calibration achieves precisions in Teff, [Fe/H], and [Ti/Fe] of 60 K, 0.1 dex, and 0.05 dex, respectively and is calibrated for 3200 K < Teff < 4100 K, -0.7 < [Fe/H] < +0.3, and -0.05 < [Ti/Fe] < +0.3. This work is a step toward detailed chemical analysis of M dwarfs at a similar precision achieved for FGK stars.Comment: accepted for publication in ApJ, all synthetic spectra available at http://people.bu.edu/mveyette/phoenix