Spherically symmetric model stellar atmospheres and limb darkening II: limb-darkening laws, gravity-darkening coefficients and angular diameter corrections for FGK dwarf stars

Limb darkening is a fundamental ingredient for interpreting observations of planetary transits, eclipsing binaries, optical/infrared interferometry and microlensing events. However, this modeling traditionally represents limb darkening by a simple law having one or two coefficients that have been derived from plane-parallel model stellar atmospheres, which has been done by many researchers. More recently, researchers have gone beyond plane-parallel models and considered other geometries. We previously studied the limb-darkening coefficients from spherically symmetric and plane-parallel model stellar atmospheres for cool giant and supergiant stars, and in this investigation we apply the same techniques to FGK dwarf stars. We present limb-darkening coefficients, gravity-darkening coefficients and interferometric angular diameter corrections from Atlas and SAtlas model stellar atmospheres. We find that sphericity is important even for dwarf model atmospheres, leading to significant differences in the predicted coefficients.Comment: 9 pages, 8 figures. Accepted for publication in A&

High Temperature Superfluidity in Double Bilayer Graphene

Exciton bound states in solids between electrons and holes are predicted to form a superfluid at high temperatures. We show that by employing atomically thin crystals such as a pair of adjacent bilayer graphene sheets, equilibrium superfluidity of electron-hole pairs should be achievable for the first time. The transition temperatures are well above liquid helium temperatures. Because the sample parameters needed for the device have already been attained in similar graphene devices, our work suggests a new route towards realizing high-temperature superfluidity in existing quality graphene samples.Comment: 6 pages, 4 figures, effect of screening on superfluidity include

Limb Darkening and Planetary Transits II: Intensity profile correction factors for a grid of model stellar atmospheres

The ability to observe extrasolar planets transiting their stars has profoundly changed our understanding of these planetary systems. However, these measurements depend on how well we understand the properties of the host star, such as radius, luminosity and limb darkening. Traditionally, limb darkening is treated as a parameterization in the analysis, but these simple parameterizations are not accurate representations of actual center-to-limb intensity variations (CLIV) to the precision needed for interpreting these transit observations. This effect leads to systematic errors for the measured planetary radii and corresponding measured spectral features. We compute synthetic planetary transits using model stellar atmosphere CLIV and corresponding best-fit limb-darkening laws for a grid spherically symmetric model stellar atmospheres. From these light curves we measure the differences in flux as a function of the star's effective temperature, gravity, mass, and the inclination of the planet's orbit.Comment: 10 pages, 8 figures, submitted to AAS journals. Comments welcom

Indicators of Mass in Spherical Stellar Atmospheres

Mass is the most important stellar parameter, but it is not directly observable for a single star. Spherical model stellar atmospheres are explicitly characterized by their luminosity ($L_\star$), mass ($M_\star$) and radius ($R_\star$), and observations can now determine directly $L_\star$ and $R_\star$. We computed spherical model atmospheres for red giants and for red supergiants holding $L_\star$ and $R_\star$ constant at characteristic values for each type of star but varying $M_\star$, and we searched the predicted flux spectra and surface-brightness distributions for features that changed with mass. For both stellar classes we found similar signatures of the star's mass in both the surface-brightness distribution and the flux spectrum. The spectral features have been use previously to determine $\log_{10} (g)$, and now that the luminosity and radius of a non-binary red giant or red supergiant can be observed, spherical model stellar atmospheres can be used to determine the star's mass from currently achievable spectroscopy. The surface-brightness variations with mass are slightly smaller than can be resolved by current stellar imaging, but they offer the advantage of being less sensitive to the detailed chemical composition of the atmosphere.Comment: 24 pages, 9 figure

Long-term polarization observations of Mira variable stars suggest asymmetric structures

Mira and semi-regular variable stars have been studied for centuries but continue to be enigmatic. One unsolved mystery is the presence of polarization from these stars. In particular, we present 40 years of polarization measurements for the prototype o Ceti and V CVn and find very different phenomena for each star. The polarization fraction and position angle for Mira is found to be small and highly variable. On the other hand, the polarization fraction for V CVn is large and variable, from 2 - 7 %, and its position angle is approximately constant, suggesting a long-term asymmetric structure. We suggest a number of potential scenarios to explain these observations.Comment: 2 pages, 1 figure, poster presented at IAU Symposium 301, Precision Asteroseismology, August 2013, Wroclaw, Polan

A comparison of plastic collapse and limit loads for single mitred pipe bends under in-plane bending

This paper presents a comparison of the plastic collapse loads from experimental in-plane bending tests on three 90 degree single un-reinforced mitred pipe bends, with the results from various 3D solid finite element models. The bending load applied reduced the bend angle and in turn, the resulting cross-sectional ovalisation led to a recognised weakening mechanism, which is only observable by testing or by including large displacement effects in the plastic finite element solution. A small displacement limit solution with an elastic-perfectly-plastic material model overestimated the collapse load by 40%. The plastic collapse finite element solution produced excellent agreement with experiment

The occurrence of classical Cepheids in binary systems

Classical Cepheids, like binary stars, are laboratories for stellar evolution and Cepheids in binary systems are especially powerful ones. About one-third of Galactic Cepheids are known to have companions and Cepheids in eclipsing binary systems have recently been discovered in the Large Magellanic Cloud. However, there are no known Galactic binary Cepheids with orbital periods less than one year. We compute population synthesis models of binary Cepheids to compare to the observed period and eccentricity distributions of Galactic Cepheids as well as to the number of observed eclipsing binary Cepheids in the LMC. We find that our population synthesis models are consistent with observed binary properties of Cepheids. Furthermore, we show that binary interaction on the red giant branch prevents some red giant stars from becoming classical Cepheids. Such interactions suggest that the binary fraction of Cepheids should be significantly less than that of their main-sequence progenitors, and that almost all binary Cepheids have orbital periods longer than one year. If the Galactic Cepheid spectroscopic binary fraction is about 35%, then the spectroscopic binary fraction of their intermediate mass main sequence progenitors is about 40-45%.Comment: 7 pages, 3 figures, resubmitted to A&

School leavers into nursing: a study of high academic achieving school pupils in Scottish schools

ABSTRACT The central objective of the study was to better understand 5th and 6th year school pupils’ perceptions of nursing as a career choice and to obtain current data regarding the recruitment situation pertaining to school leavers pursuing nursing as a career choice. This was achieved by utilising a multi-strategy approach which involved the use of a self-administered questionnaire (n = 1062) and the interviews of 20 paradigmatic cases. The school pupils came from 11 schools, with different socio-demographic profiles, from one educational area in Scotland. The study found that only 8.8% (n = 56) of school pupils who had made a career choice had chosen to pursue nursing as a career, despite the fact that 82.4% of the school pupils in the sample had the necessary academic qualifications to enter the student nurse education programme. Merely 21.2% of all the pupils had actually considered nursing as a career choice. The findings would appear to suggest that those pupils who had chosen to pursue nursing as a career choice are committed to this choice. As 93% of the pupils would still continue to pursue nursing as a career choice even if they obtained better grades in their examinations than they expected. Equally resolute in their choice were the pupils who had chosen not to pursue nursing. Of the school pupils in the sample who had not chosen to pursue nursing as a career choice 88.5% of the school pupils stated that even though they did not get the exam grades that they expected they still would not pursue nursing as a career. What was also significant was that of the school pupils who had not chosen nursing as a career choice 86.9% of the pupils stated that they would not consider nursing as a career option in the future. The gender breakdown of school pupils who had made a career choice showed that 2.5% of male pupils who had made a career choice had chosen to pursue nursing. 14% of female school pupils who had made a career choice had chosen to pursue nursing. Further examination in relation to the gender breakdown of those pupils who had chosen to follow nursing as a career revealed that males only accounted for 12.5% whereas females accounted for 87.5%. This would seem to propose that a gender bias still exists in relation to nursing as a career choice. Nursing was not ranked particularly highly by the pupils as a career choice. Male pupils ranked nursing 13th out of the 14 main career choice categories. While female pupils ranked nursing 8th out of the 14 main career choice categories, this being only slightly better than secretarial/administration. Parents appeared to have a major influence on the pupils’ career choice and there was evidence that parents as well as further significant others were influencing against a career in nursing. Nursing does not seem to be a popular career choice among school pupils. There is strong evidence from the questionnaire data to suggest that there are a number of problem indicators which could propose that recruitment of school pupils into nursing could prove extremely difficult even with the proposed increase in the number of school leavers entering higher education. This could have a serious impact on nursing care delivery within the United Kingdom as the nursing population continues to age. The paradigmatic cases interviewed were 20 high academic achieving school pupils who at one stage in their career choice process had considered nursing as a career choice. Despite considering nursing as a possible career choice none of the 20 pupils went on to pursue nursing as a career. The qualitative interview findings appeared to suggest that nursing was not a credible career choice consideration for high achieving pupils. There was a very strong feeling amongst the pupils that nursing would be a waste of their academic qualifications with a belief that nursing was not a career choice for intelligent pupils with good examination grades. Also that they could help people at a much higher level than nursing by becoming a doctor with a prevailing belief that knowledge and caring are polarised – doctors cure and nurses only care for patients. The status of nursing as a career choice was not high amongst the pupils and this was influenced by the type of person that they had observed who were nurses mostly, in their opinion, women who were weak and had no power within society. Also the perceived nature of the job that nurses do with the prominent belief that what nurses do is principally practical in nature having no intellectual aspects, for example making beds, washing and feeding patients. The influence of significant others, these being specifically parents, guardians, guidance teachers and careers advisors was very apparent in the data in that they had a very negative view regarding nursing as a career choice for high academic achieving school pupils. Participants reported that their parents were actively and vigorously discouraging them away from a career in nursing because of the pupils’ good examination grades and the belief that the pupils could do something better than nursing. Also the participants reported that their guidance teachers and careers advisors were assertively steering them away from nursing as a career choice because of their perception that nursing was a low status career choice requiring little intellectual ability and was on a par with hairdressing, office work and being a secretary. In addition the school pupils stated that guidance teachers and careers advisors were more interested in and attempted to have a greater influence on the career choice of the high academic achieving school pupils opposed to other less academic pupils. Only two of the pupils stated that they believed that their career choice was their own decision. With the majority of school pupils valuing the opinion of their parents regarding career choice and with parents along with associated significant others being a major influence on the school pupils’ career choice and advising against a career in nursing. There was strong evidence from the interview data to suggest that the chances of recruiting high academic achieving school pupils into nursing would appear to be negligible. There was also evidence that the image of nursing as an occupation was an important determining factor in the school pupils’ career choice. The pupils displayed a negative image of nursing and this was influenced by a number of factors. The negative image of nursing depicted in television programmes; the negative image of nursing portrayed by people who are nurses; the sexual stereotype image of female and male nurses; and the image that it is very easy to get into train to be a nurse. What was also concerning was that the pupils had few positive and contemporary images of nursing. Also with regard to the image of nursing the data showed that the pupils considered it important to join a profession. Nursing was deemed not to be a profession and did not merit being a profession because it was believed that the entry to student nurse education programmes is not strictly controlled therefore it is easy to get into nursing. There was a strong consensus among the high achieving school pupils regarding their image of the archetypical school pupil who would select nursing as a career choice – their view was of a predominantly unexceptional individual who was mostly female, no more than average intelligence, kind, caring, good listener, good practically and can follow task orders. This view of the typical school pupil who would enter nursing as a career, that of a person with a low academic achievement record, conflicts with their own personal typology and thus became a further important dissuading factor regarding nursing as a career choice for them. In addition they also witness a certain non academic school pupil type being encouraged towards a career in nursing which again reinforces their image perception that nursing is not a career choice for high academic achieving school pupils. The level of education required to be a nurse was thought, by many high achieving school pupils, not required to be at degree level within a university. This was again based mostly on their perceptions of what nurses’ do. The high academic achieving school pupils were extremely suspicious of nursing as a university programme and doubtful as to the value of a degree in nursing. This again had an adverse effect on their consideration of nursing as a career choice. These doubts and suspicions were manifest in the following main areas – nursing has much lower entry requirements than the entry requirements for other university degree programmes; a degree in nursing does not have the same value as other degrees; nursing is an easy way to get into university – school pupils that universities would never have given admission to in the past are entering nursing degree programmes with much less qualifications than those required to enter any other university degree programme; and nursing should not be a university programme. When the high achieving school pupils were asked to think about how nursing could be made more appealing as a career choice for them four main themes emerged: engagement with nurses also detailed and comprehensive information about nursing as a career; proper nursing work experience; links between schools and schools/departments of nursing within universities; changing/improving/losing the stereotypical image of nursing. Choice of career is one of the major areas of concern for young people nearing the end of their schooling (Alberts et al 2003). In the current discourse on the transition from school to work, career decision making has a pivotal position. With the United Kingdom government’s goal of increasing access to higher education to 50 percent of the 18 – 30 year old population by 2010 (DfES 2003) and the rise in the number of school pupils with good examination results at all grades (SQA 2006; DfES 2007). This will boost the number of school leavers entering higher education and the number of pupils trying to decide on which programme of education to choose (Douglass 2003). Even with this substantial increase in student numbers entering higher education the research data would suggest that the vast majority of these school leavers will not pursue nursing as a career