The dynamical properties of dense filaments in the infrared dark cloud G035.39-00.33

Infrared Dark Clouds (IRDCs) are unique laboratories to study the initial conditions of high-mass star and star cluster formation. We present high-sensitivity and high-angular resolution IRAM PdBI observations of N2H+ (1-0) towards IRDC G035.39-00.33. It is found that G035.39-00.33 is a highly complex environment, consisting of several mildly supersonic filaments (sigma_NT/c_s ~1.5), separated in velocity by <1 km s^-1 . Where multiple spectral components are evident, moment analysis overestimates the non-thermal contribution to the line-width by a factor ~2. Large-scale velocity gradients evident in previous single-dish maps may be explained by the presence of substructure now evident in the interferometric maps. Whilst global velocity gradients are small (<0.7 km s^-1 pc^-1), there is evidence for dynamic processes on local scales (~1.5-2.5 km s^-1 pc^-1 ). Systematic trends in velocity gradient are observed towards several continuum peaks. This suggests that the kinematics are influenced by dense (and in some cases, starless) cores. These trends are interpreted as either infalling material, with accretion rates ~(7 \pm 4)x10^-5 M_sun yr^-1 , or expanding shells with momentum ~24 \pm 12 M_sun km s^-1 . These observations highlight the importance of high-sensitivity and high-spectral resolution data in disentangling the complex kinematic and physical structure of massive star forming regions.Comment: 25 pages, 23 figures, accepted for publication in MNRA

Models of Mating System Evolution: Gamete Competition, Hermaphroditism and Sexual Selection

To understand how mating systems evolve, we depend on both (i) theoretical explanations and predictions, supported by mathematical modelling, and (ii) quantitative tools to test predictions rigorously. This thesis is divided equally between these two aims. The first three papers explore the evolution of mating systems using analytic and simulation models. I begin by considering a long-standing puzzle in marine invertebrate systems: the widespread association between a species' body size, whether it is hermaphroditic or has separate sexes, and its mode of fertilisation (i.e. whether eggs and sperm are released into the water or retained by the adult until fertilisation). I argue that local competition among eggs for fertilisation can explain these patterns, which arise in taxa as diverse as sea stars, corals and polychaete worms (Paper 1). I then turn to egg trading – the alternating exchange of egg parcels during mating by simultaneous hermaphrodites – which is one of the best-supported cases of reciprocity between non-relatives. I show that egg trading is under positive frequency-dependent selection and should evolve most easily when potential mates are encountered frequently (Paper 2). Once evolved, egg trading selects for female-biased sex allocation. I explain how this bias allows simultaneous hermaphroditism to persist stably, even in motile species living at high population densities, where simultaneous hermaphroditism is otherwise predicted to be unstable (Paper 3). These three papers contribute to the resolution of Williams' paradox – the mismatch between the empirical distribution of hermaphroditism and our theoretical expectations – by providing finer-grained predictions for when hermaphroditism should occur. The last three papers provide quantitative tools for the measurement of natural (and particularly sexual) selection. I argue that pre-mating sexual selection should be understood as a two-step causal process: traits affect mating success, which in turn affects reproductive success. Most previous work has focussed too narrowly on one or the other of these steps, leading to confusion about what sexual selection is and how we should measure it. I provide a new statistical framework that integrates both steps in the sexual selection pathway, and includes a third path whereby traits directly affect reproductive success. This leads to better estimates of the strength of sexual selection on traits (Paper 4). I then compare various indices of sexual selection, often used in comparative work, that do not rely on trait measurements. I show that the recently defined Jones index outperforms all others in predicting the actual strength of sexual selection, because it accounts for both steps in the sexual selection pathway (Paper 5). Lastly, I provide a new way to quantify the total selection acting on a trait, including both directional and non-directional selection (Paper 6)

Kinematics and physical properties of a highly filamentary Infrared Dark Cloud

This thesis contains a detailed study of the kinematics and physical properties of a potential site of massive star formation; the IRDC G035.39-00.33. The gas kinematics are first of all investigated using high-spectral resolution and high-sensitivity data from the IRAM 30 m telescope. The primary focus of this work is the J = 1 → 0 transition of both N2H+ and C18O, as well as N2H+ (3 − 2). Dense gas is found to be extended over ∼ 3 pc scales within G035.39-00.33. The C18 O observations confirm the presence of at least three morphologically distinct filamentary components. It is speculated that the merging of filaments may be responsible for the formation of localised density enhancements at their interface; the potential sites for massive star and star-cluster formation. The kinematic properties of the dense gas are then probed at high-angular resolution, using observations of N2H+ (1−0) from the Plateau de Bure Interferometer. It is revealed that the dense gas of G035.39-00.33 is organised into a complex network of mildly supersonic filaments separated in velocity by < 1 km s−1 . Whilst global velocity gradients throughout each filament are small, there is evidence for dynamic processes on local scales. This suggests that the kinematics are influenced by the dense (and in some cases, starless) cores. The physical properties of the embedded core population are derived in the final study of this thesis. A total of 14 continuum peaks are identified, representative of the pre- and protostellar core population covering two main clumps within G035.39-00.33. The derived core masses are found to be between 2.4-12.3 solar masses, with sizes and densities between 0.03-0.07 pc and 1.6×10^5-7.3×10^5 cm^-3, respectively. Some of the cores exhibit irregular boundaries, which may imply the presence of unresolved sub-structure. Although the dynamical state of each core is dependent on both its geometry and density profile (which are both sources of uncertainty) it is found that many of the identified cores are unstable to collapse. Cores which are well represented by monolithic, centrally condensed structures, exhibiting low virial parameters and many Jeans masses, are good candidates for the progenitors of intermediate-to-high-mass stars. Within the selected area of G035.39-00.33, two of the identified cores meet this criteria

Economic analysis of the effect of flood disaster on food security of arable farming households in Southern Guinea Savanna Zone, Nigeria

The study assessed the economic analysis of the effect of flood disaster on food security of arable farming households in southern guinea savanna&nbsp; zone, Nigeria. A multistage sampling technique was used to select 120 farmers who experienced and didn’t experience flooding in Kwara and Niger States from whom data were collected with the aid of a structured questionnaire. The data were analyzed using descriptive statistics, food security index and logistics regression model. This revealed that majority of the farming household heads in the study area were predominantly young people, who were still in their active age and had at least secondary education. Also, the food security result shows that majority of the farming households who experienced flood were not food secure. The regression result shows that the coefficients which were significant were years of schooling, household size, off farm income, household expenditure and flooding. The adaptation practices adopted against flooding by households includes seasonal migration, diversification of livelihood, terracing and early harvesting. The study recommends that climatic information especially the seasonal rainfall prediction annual reports be made available early enough to farmers in the flood plain areas to reduce the effect of food insecurity. Keywords: Flood, Food Security, Logistic Regression and Mitigat

A probable Keplerian disk feeding an optically revealed massive young star.

The canonical picture of star formation involves disk-mediated accretion, with Keplerian accretion disks and associated bipolar jets primarily observed in nearby, low-mass young stellar objects (YSOs). Recently, rotating gaseous structures and Keplerian disks have been detected around several massive (M > 8 M ) YSOs (MYSOs) , including several disk-jet systems . All the known MYSO systems are in the Milky Way, and all are embedded in their natal material. Here we report the detection of a rotating gaseous structure around an extragalactic MYSO in the Large Magellanic Cloud. The gas motion indicates that there is a radial flow of material falling from larger scales onto a central disk-like structure. The latter exhibits signs of Keplerian rotation, so that there is a rotating toroid feeding an accretion disk and thus the growth of the central star. The system is in almost all aspects comparable to Milky Way high-mass YSOs accreting gas from a Keplerian disk. The key difference between this source and its Galactic counterparts is that it is optically revealed rather than being deeply embedded in its natal material as is expected of such a massive young star. We suggest that this is the consequence of the star having formed in a low-metallicity and low-dust content environment. Thus, these results provide important constraints for models of the formation and evolution of massive stars and their circumstellar disks. [Abstract copyright: © 2023. The Author(s).

A probable Keplerian disk feeding an optically revealed massive young star

The canonical picture of star formation involves disk-mediated accretion, with Keplerian accretion disks and associated bipolar jets primarily observed in nearby, low-mass young stellar objects (YSOs). Recently, rotating gaseous structures and Keplerian disks have been detected around a number of massive (M > 8 solar masses) YSOs (MYSOs) including several disk-jet systems. All of the known MYSO systems are located in the Milky Way, and all are embedded in their natal material. Here we report the detection of a rotating gaseous structure around an extragalactic MYSO in the Large Magellanic Cloud. The gas motions show radial flow of material falling from larger scales onto a central disk-like structure, the latter exhibiting signs of Keplerian rotation, i.e., a rotating toroid feeding an accretion disk and thus the growth of the central star. The system is in almost all aspects comparable to Milky Way high-mass young stellar objects accreting gas via a Keplerian disk. The key difference between this source and its Galactic counterparts is that it is optically revealed, rather than being deeply embedded in its natal material as is expected of such a young massive star. We suggest that this is the consequence of the star having formed in a low-metallicity and low-dust content environment, thus providing important constraints for models of the formation and evolution of massive stars and their circumstellar disks.Comment: 20 pages, 9 page

Multicomponent kinematics in a massive filamentary IRDC

To probe the initial conditions for high-mass star and cluster formation, we investigate the properties of dense filaments within the infrared dark cloud G035.39-00.33 (IRDC G035.39) in a combined Very Large Array (VLA) and the Green Bank Telescope (GBT) mosaic tracing the NH3 (1,1) and (2,2) emission down to 0.08 pc scales. Using agglomerative hierarchical clustering on multiple line-of-sight velocity component fitting results, we identify seven extended velocity-coherent components in our data, likely representing spatially coherent physical structures, some exhibiting complex gas motions. The velocity gradient magnitude distribution peaks at its mode of 0.35 km/s/pc and has a long tail extending into higher values of 1.5 - 2 km/s/pc, and is generally consistent with those found toward the same cloud in other molecular tracers and with the values found towards nearby low-mass dense cloud cores at the same scales. Contrary to observational and theoretical expectations, we find the non-thermal ammonia line widths to be systematically narrower (by about 20%) than those of N2H+ (1-0) line transition observed with similar resolution. If the observed ordered velocity gradients represent the core envelope solid-body rotation, we estimate the specific angular momentum to be about 2 x 10^21 cm^2/s, similar to the low-mass star-forming cores. Together with the previous finding of subsonic motions in G035.39, our results demonstrate high levels of similarity between kinematics of a high-mass star-forming IRDC and the low-mass star formation regime.Comment: 22 pages, 14 figures, accepted to Ap