How Efficient is Rotational Mixing in Massive Stars ?

The VLT-Flames Survey for Massive Stars (Evans05,Evans06) provides recise measurements of rotational velocities and nitrogen surface abundances of massive stars in the Magellanic Clouds. Specifically, for the first time, such abundances have been estimated for stars with significant rotational velocities. This extraordinary data set gives us the unique possibility to calibrate rotationally and magnetically induced mixing processes. Therefore, we have computed a grid of stellar evolution models varying in mass, initial rotational velocity and chemical composition. In our models we find that although magnetic fields generated by the Spruit-Taylor dynamo are essential to understand the internal angular momentum transport (and hence the rotational behavior), the corresponding chemical mixing must be neglected to reproduce the observations. Further we show that for low metallicities detailed initial abundances are of prime importance, as solar-scaled abundances may result in significant calibration errors.Comment: To appear in the proceedings of "First Stars III", Santa Fe, New Mexico, July 16-20, 2007, 3 pages, 3 figure

How Efficient is Rotational Mixing in Massive Stars ?

The VLT-Flames Survey for Massive Stars (Evans05,Evans06) provides recise measurements of rotational velocities and nitrogen surface abundances of massive stars in the Magellanic Clouds. Specifically, for the first time, such abundances have been estimated for stars with significant rotational velocities. This extraordinary data set gives us the unique possibility to calibrate rotationally and magnetically induced mixing processes. Therefore, we have computed a grid of stellar evolution models varying in mass, initial rotational velocity and chemical composition. In our models we find that although magnetic fields generated by the Spruit-Taylor dynamo are essential to understand the internal angular momentum transport (and hence the rotational behavior), the corresponding chemical mixing must be neglected to reproduce the observations. Further we show that for low metallicities detailed initial abundances are of prime importance, as solar-scaled abundances may result in significant calibration errors.Comment: To appear in the proceedings of "First Stars III", Santa Fe, New Mexico, July 16-20, 2007, 3 pages, 3 figure

Stellar Winds on the Main-Sequence II: the Evolution of Rotation and Winds

Aims: We study the evolution of stellar rotation and wind properties for low-mass main-sequence stars. Our aim is to use rotational evolution models to constrain the mass loss rates in stellar winds and to predict how their properties evolve with time on the main-sequence. Methods: We construct a rotational evolution model that is driven by observed rotational distributions of young stellar clusters. Fitting the free parameters in our model allows us to predict how wind mass loss rate depends on stellar mass, radius, and rotation. We couple the results to the wind model developed in Paper I of this series to predict how wind properties evolve on the main-sequence. Results: We estimate that wind mass loss rate scales with stellar parameters as $\dot{M}_\star \propto R_\star^2 \Omega_\star^{1.33} M_\star^{-3.36}$. We estimate that at young ages, the solar wind likely had a mass loss rate that is an order of magnitude higher than that of the current solar wind. This leads to the wind having a higher density at younger ages; however, the magnitude of this change depends strongly on how we scale wind temperature. Due to the spread in rotation rates, young stars show a large range of wind properties at a given age. This spread in wind properties disappears as the stars age. Conclusions: There is a large uncertainty in our knowledge of the evolution of stellar winds on the main-sequence, due both to our lack of knowledge of stellar winds and the large spread in rotation rates at young ages. Given the sensitivity of planetary atmospheres to stellar wind and radiation conditions, these uncertainties can be significant for our understanding of the evolution of planetary environments.Comment: 26 pages, 14 figures, 2 tables, to be published in A&

Stellar Winds on the Main-Sequence I: Wind Model

Aims: We develop a method for estimating the properties of stellar winds for low-mass main-sequence stars between masses of 0.4 and 1.1 solar masses at a range of distances from the star. Methods: We use 1D thermal pressure driven hydrodynamic wind models run using the Versatile Advection Code. Using in situ measurements of the solar wind, we produce models for the slow and fast components of the solar wind. We consider two radically different methods for scaling the base temperature of the wind to other stars: in Model A, we assume that wind temperatures are fundamentally linked to coronal temperatures, and in Model B, we assume that the sound speed at the base of the wind is a fixed fraction of the escape velocity. In Paper II of this series, we use observationally constrained rotational evolution models to derive wind mass loss rates. Results: Our model for the solar wind provides an excellent description of the real solar wind far from the solar surface, but is unrealistic within the solar corona. We run a grid of 1200 wind models to derive relations for the wind properties as a function of stellar mass, radius, and wind temperature. Using these results, we explore how wind properties depend on stellar mass and rotation. Conclusions: Based on our two assumptions about the scaling of the wind temperature, we argue that there is still significant uncertainty in how these properties should be determined. Resolution of this uncertainty will probably require both the application of solar wind physics to other stars and detailed observational constraints on the properties of stellar winds. In the final section of this paper, we give step by step instructions for how to apply our results to calculate the stellar wind conditions far from the stellar surface.Comment: 24 pages, 13 figures, 2 tables, Accepted for publication in A&

Rotational Mixing in Magellanic Clouds B Stars - Theory versus Observation

We have used VLT FLAMES data to constrain the uncertain physics of rotational mixing in stellar evolution models. We have simulated a population of single stars and find two groups of observed stars that cannot be explained: (1) a group of fast rotating stars which do not show evidence for rotational mixing and (2) a group of slow rotators with strong N enrichment. Binary effects and fossil magnetic fields may be considered to explain those two groups. We suggest that the element boron could be used to distinguish between rotational mixing and the binary scenario. Our single star population simulations quantify the expected amount of boron in fast and slow rotators and allow a comparison with measured nitrogen and boron abundances in B-stars.Comment: to appear in Comm. in Astroseismology - Contribution to the Proceedings of the 38th LIAC, 200

The impact of stroke: are people with aphasia different to those without?

Purpose. Stroke rehabilitation programmes aim to improve functional outcomes and quality of life. This study explored long-term outcomes in a cohort of people admitted to two acute stroke units with stroke. Comparisons were drawn between people with aphasia (PWA) and people without aphasia. Methods. People admitted to hospital with a first stroke were assessed at 2-weeks, 3-months and 6-months post-stroke. Measures included: the Barthel Index for Activities of Daily Living (ADL), the Frenchay Aphasia Screening Test, the General Health Questionnaire-12 for emotional well-being and the Stroke and Aphasia Quality of Life Scale-39g. Extended ADL and social support were also measured at 3 and 6 months, with the Frenchay Activities Index and the Social Support Survey, respectively. Results. Of 126 eligible participants, 96(76%) took part and 87(69%) were able to self-report. Self-report data are reported here. Although outcomes improved significantly across time, at 6 months people continued to experience substantial functional limitations (16% aphasic; 32% dependent on basic ADL); participation limitations (79% ≤30 on the FAI); high psychological distress (45%) and compromised quality of life (54% ≤4 on the SAQOL-39g). Levels of social support remained relatively stable. Though at 3-months post-stroke PWA were significantly more likely to experience high psychological distress (93% versus 50% for those without), across time, there were no significant differences between PWA and those without on psychological distress and also ADL and social support. There were, however, significant differences on extended ADL (F(1,68) = 7.80, p < 0.01) and quality of life (F(1,69) = 6.30, p < 0.05). Conclusion. PWA participated in fewer activities and reported worse quality of life after stroke than people without aphasia, even when their physical abilities, well-being and social support were comparable. Implications for clinical practice and future research are discussed

Broad-band photometric colors and effective temperature calibrations for late-type giants. II. Z<0.02

(Abridged) We investigate the effects of metallicity on the broad-band photometric colors of late-type giants, and make a comparison of synthetic colors with observed photometric properties of late-type giants over a wide range of effective temperatures (T_eff=3500-4800 K) and gravities (log g=0.0-2.5), at [M/H]=-1.0 and -2.0. The influence of metallicity on the synthetic photometric colors is generally small at effective temperatures above \~3800 K, but the effects grow larger at lower T_eff, due to the changing efficiency of molecule formation which reduces molecular opacities at lower [M/H]. To make a detailed comparison of the synthetic and observed photometric colors of late type giants in the T_eff--color and color--color planes, we derive a set of new T_eff--log g--color relations based on synthetic photometric colors, at [M/H]=-0.5, -1.0, -1.5, and -2.0. While differences between the new T_eff--color relations and those available from the literature are typically well within ~100 K, effective temperatures predicted by the scales based on synthetic colors tend to be slightly higher than those resulting from the T_eff--color relations based on observations, with the offsets up to ~100 K. This is clearly seen both at [M/H]=-1.0 and -2.0, especially in the T_eff--(B-V) and T_eff--(V-K) planes. The consistency between T_eff--log g--color scales based on synthetic colors calculated with different stellar atmosphere codes is very good, with typical differences being well within \Delta T_eff~70 K at [M/H]=-1.0 and \Delta T_eff~40 K at [M/H]=-2.0.Comment: 20 pages, 11 figures, A&A accepte

Photometric colors of late-type giants: theory versus observations

To assess the current status in the theoretical modeling of the spectral properties of late-type giants, we provide a comparison of synthetic photometric colors of late-type giants (calculated with PHOENIX, MARCS and ATLAS model atmospheres) with observations, at [M/H]=0.0 and -2.0. Overall, there is a good agreement between observed and synthetic colors, and synthetic colors and published Teff-color relations, both at [M/H]=0.0 and -2.0. Deviations from the observed trends in Teff-color planes are generally within \pm 150K (or less) in the effective temperature range Teff=3500-4800K. Synthetic colors calculated with different stellar atmosphere models typically agree to ~100K, within a large range of effective temperatures and gravities. Some discrepancies are seen in the Teff-(B-V) plane below Teff~3800K at [M/H]=0.0, due to difficulties in reproducing the 'turn-off' to the bluer colors which is seen in the observed data at Teff~3600K. Note that at [M/H]=-2.0 effective temperatures given by the scale of Alonso et al. (1999) are generally lower than those resulting from other Teff-color relations based both on observed and synthetic colors.Comment: 2 pages, 1 figure. Proceedings of the IAU Symposium 232 "The Scientific Requirements for Extremely Large Telescopes", eds. P. Whitelock, B. Leibundgut, and M. Dennefel

The VLT-FLAMES survey of massive stars: Nitrogen abundances for Be-type stars in the Magellanic Clouds

Aims. We compare the predictions of evolutionary models for early-type stars with atmospheric parameters, projected rotational velocities and nitrogen abundances estimated for a sample of Be-type stars. Our targets are located in 4 fields centred on the Large Magellanic Cloud cluster: NGC 2004 and the N 11 region as well as the Small Magellanic Cloud clusters: NGC 330 and NGC 346. Methods. Atmospheric parameters and photospheric abundances have been determined using the non-LTE atmosphere code tlusty. Effective temperature estimates were deduced using three different methodologies depending on the spectral features observed; in general they were found to yield consistent estimates. Gravities were deduced from Balmer line profiles and microturbulences from the Si iii spectrum. Additionally the contributions of continuum emission from circumstellar discs were estimated. Given its importance in constraining stellar evolutionary models, nitrogen abundances (or upper limits) were deduced for all the stars analysed. Results. Our nitrogen abundances are inconsistent with those predicted for targets spending most of their main sequence life rotating near to the critical velocity. This is consistent with the results we obtain from modelling the inferred rotational velocity distribution of our sample and of other investigators. We consider a number of possibilities to explain the nitrogen abundances and rotational velocities of our Be-type sample.Comment: 14 pages, 9 figures, submitted to A&