Massive star evolution : rotation, winds, and overshooting vectors in the Mass-Luminosity plane I. A calibrated grid of rotating single star models

We aim to constrain massive star evolution models using the unique testbed eclipsing binary HD166734 with new grids of MESA stellar evolution models, adopting calibrated prescriptions of overshooting, mass loss, and rotation. We introduce a novel tool: the "mass-luminosity plane" or "M-L plane", as an equivalent to the traditional HR diagram, utilising it to reproduce the testbed binary HD166734 with newly calibrated MESA stellar evolution models for single stars. We can only reproduce the Galactic binary system with an enhanced amount of core overshooting (alpha = 0.5), mass loss, and rotational mixing. We can utilise the gradient in the M-L plane to constrain the amount of mass loss to 0.5 - 1.5 times the standard Vink et al. 2001 prescriptions, and we can exclude extreme reduction or multiplication factors. The extent of the vectors in the M-L plane leads us to conclude that the amount of core overshooting is larger than is normally adopted in contemporary massive star evolution models. We furthermore conclude that rotational mixing is mandatory to get the nitrogen abundance ratios between the primary and secondary components to be correct (3:1) in our testbed binary system. Our calibrated grid of models, alongside our new M-L plane approach, present the possibility of a widened main sequence due to an increased demand for core overshooting. The increased amount of core overshooting is not only needed to explain the extended main sequence, but the enhanced overshooting is also needed to explain the location of the upper-luminosity limit of the red supergiants. Finally, the increased amount of core overshooting has -- via the compactness parameter -- implications for supernova explodibility.Comment: Revised version, 14 pages, 19 figures, accepted for publication by Astronomy & Astrophysic

An Investigation of the Impact of Multiple Exposures to New Vocabulary Words have on Students with Language Processing Disorders

The purpose of this research aimed to investigate how fifth-grade students diagnosed with processing disorders best-learned new vocabulary and the role motivation played in success. The district, school, and students were given pseudonyms in order to maintain anonymity. Over the course of three, one-week instructional periods, four students participated in six different oral and written vocabulary activities. They completed an assessment immediately following four days of instruction and again at the end of the study to measure growth in their ability to identify and use the word using a four-point scale. Based on the results of the assessments, they showed that, overall, students made gains in their ability to recall the meaning of words and to use the words appropriately in context. Motivation was measured through observations and a rating system. In the final week, when students were familiar with the expectations of the activities, motivation was at its peak and so was quality of work and assessment scores. These findings support the use of explicit vocabulary instruction with the use of various multi-modal activities in order to improve recall, long-term memory, and word retrieval for students with language processing disorders and that motivation does play a role in success

Homoeologous Recombination in Brassica napus

Polyploidy is very common among plants but having multiple sets of chromosomes creates additional challenges for chromosome pairing and recombination during meiosis. Brassica napus is an allotetraploid comprised of the A and C genomes from B. rapa and B. oleracea, respectively. In adapted B. napus lines, the chromosomes of the A and C genomes pair almost exclusively with their true homologue during meiosis, but in newly resynthesized B. napus plants there is a significant increase in pairing between homoeologues, the closely related chromosomes from the other genome, i.e. A/C pairings. This interaction between homoeologues can result in an uneven distribution of chromosomes in the gametes so restricting pairing is important for overall plant fitness. However, this unequal crossing over can also serve to introduce novel variation allowing for species diversification and adaptation. I have developed a new method for detecting homoeologous recombination events in B. napus using a single nucleotide polymorphism (SNP) array. Traditionally scientists have used cytology or restriction fragment length polymorphism markers but the SNP array offers much quicker data generation and a higher density of markers for more precise identification of crossover points and detection of smaller exchanges. Using this new methodology I measured recombination between the A and C genomes in natural B. napus lines and have shown that homoeologous exchanges continue to happen in modern B. napus cultivars at a relatively high frequency. I have also used the SNP array to analyze a resynthesized B. napus population segregating for the level of homoeologous recombination and mapped three quantitative trait loci (QTL) controlling this phenomenon. Further analysis of the genes underlying these QTL can help to identify the mechanisms that have evolved in natural B. napus to control meiotic chromosome pairing and manipulation of those genes could be used to increase homoeologous recombination rates to introduce novel traits from diverse species

The role of CaSR signalling in adipocytes and its regulation by ADMA

The amino acid sensitive calcium-sensing receptor (CaSR) plays a critical role in regulating extracellular calcium ([Ca2+]o) levels. In addition to maintaining [Ca2+] homeostasis, the CaSR is involved in a variety of non-calciotropic functions, among which it has emerged as a mediator of adipocyte (patho)physiology. Recent work in our lab has revealed that adipocyte hypertrophy is stimulated by the amino acid asymmetric dimethylarginine (ADMA) via a nitric oxideindependent, CaSR pathway. However, the underpinning mechanisms and relevance in adipocytes was not clearly identified. This thesis aimed to delineate CaSR signalling pathways in vitro, and subsequently test the effect of ADMA on CaSR downstream signalling and functions. CaSR was investigated first in HEK293 cells overexpressing CaSR, which showed CaSR-dependent ERK1/2 signalling in response to pharmacological CaSR stimulation and exogenous ADMA. Next, CaSR/ADMA signalling pathways were examined using 3T3-L1 cells modelling mature adipocytes. Agonist-induced CaSR signalling caused adipocyte hypertrophy, leptin secretion and lipogenic gene expression. ADMA enlarged adipocytes, and ADMA-induced lipogenic gene expression was perturbed by CaSR blockade indicating ADMA/CaSR crosstalk in adipocytes. However, ADMA was unable to fully replicate the effects of CaSR acting in adipocytes highlighting the complexity of CaSR signalling pathways requiring further investigation. Adipose tissue (AT) dysfunction is a key mechanism linking metabolic disorders and cardiovascular disease (CVD). The CaSR is associated with CVD, but the relevance of adipocyte CaSR in cardiovascular health has not been directly examined. Therefore, this thesis also aimed to investigate the function of adipocyte CaSR, and determine its significance in AT and cardiovascular biology. To investigate the role of adipocyte CaSR in vivo, this thesis generated a murine model of adipocyte-specific CaSR deficiency. In female mice specifically, body weight, adipocyte size and AT-induced vascular contractility was reduced. This thesis establishes ADMA as a potentially novel ligand of CaSR, and demonstrates that adipocyte CaSR can impact on AT biology as well as cardiometabolic health in the intact animal

The hydrogen clock to infer the upper stellar mass

The most massive stars dominate the chemical enrichment, mechanical and radiative feedback, and energy budget of their host environments. Yet how massive stars initially form and how they evolve throughout their lives is ambiguous. The mass loss of the most massive stars remains a key unknown in stellar physics, with consequences for stellar feedback and populations. In this work, we compare grids of very massive star (VMS) models with masses ranging from 80-1000Msun, for a range of input physics. We include enhanced winds close to the Eddington limit as a comparison to standard O-star winds, with consequences for present-day observations of ~50-100Msun stars. We probe the relevant surface H abundances (Xs) to determine the key traits of VMS evolution compared to O stars. We find fundamental differences in the behaviour of our models with the enhanced-wind prescription, with a convergence on the stellar mass at 1.6 Myr, regardless of the initial mass. It turns out that Xs is an important tool in deciphering the initial mass due to the chemically homogeneous nature of VMS above a mass threshold. We use Xs to break the degeneracy of the initial masses of both components of a detached binary, and a sample of WNh stars in the Tarantula nebula. We find that for some objects, the initial masses are unrestricted and, as such, even initial masses of the order 1000Msun are not excluded. Coupled with the mass turnover at 1.6 Myr, Xs can be used as a 'clock' to determine the upper stellar mass.Comment: Accepted for publication in MNRAS, 14 figure

Stellar Wind Yields of Very Massive Stars

The most massive stars provide an essential source of recycled material for young clusters and galaxies. While very massive stars (VMS, M>100M) are relatively rare compared to O stars, they lose disproportionately large amounts of mass already from the onset of core H-burning. VMS have optically thick winds with elevated mass-loss rates in comparison to optically thin standard O-star winds. We compute wind yields and ejected masses on the main sequence, and we compare enhanced mass-loss rates to standard ones. We calculate solar metallicity wind yields from MESA stellar evolution models in the range 50 - 500M, including a large nuclear network of 92 isotopes, investigating not only the CNO-cycle, but also the Ne-Na and Mg-Al cycles. VMS with enhanced winds eject 5-10 times more H-processed elements (N, Ne, Na, Al) on the main sequence in comparison to standard winds, with possible consequences for observed anti-correlations, such as C-N and Na-O, in globular clusters. We find that for VMS 95% of the total wind yields is produced on the main sequence, while only ~5% is supplied by the post-main sequence. This implies that VMS with enhanced winds are the primary source of 26Al, contrasting previous works where classical Wolf-Rayet winds had been suggested to be responsible for Galactic 26Al enrichment. Finally, 200M stars eject 100 times more of each heavy element in their winds than 50M stars, and even when weighted by an IMF their wind contribution is still an order of magnitude higher than that of 50M stars.Comment: Accepted for publication in MNRAS. 14 pages, 10 figure

ADMA: A key player in the relationship between vascular dysfunction and inflammation in atherosclerosis

Atherosclerosis is a chronic cardiovascular disease which increases risk of major cardiovascular events including myocardial infarction and stroke. Elevated plasma concentrations of asymmetric dimethylarginine (ADMA) have long been recognised as a hallmark of cardiovascular disease and are associated with cardiovascular risk factors including hypertension, obesity and hypertriglyceridemia. In this review, we discuss the clinical literature that link ADMA concentrations to increased risk of the development of atherosclerosis. The formation of atherosclerotic lesions relies on the interplay between vascular dysfunction, leading to endothelial activation and the accumulation of inflammatory cells, particularly macrophages, within the vessel wall. Here, we review the mechanisms through which elevated ADMA contributes to endothelial dysfunction, activation and reactive oxygen species (ROS) production; how ADMA may affect vascular smooth muscle phenotype; and finally whether ADMA plays a regulatory role in the inflammatory processes occurring within the vessel wall