Probing discs around massive young stellar objects with CO first overtone emission

We present high resolution (R~50,000) spectroastrometry over the CO 1st overtone bandhead of a sample of seven intermediate/massive young stellar objects. These are primarily drawn from the red MSX source (RMS) survey, a systematic search for young massive stars which has returned a large, well selected sample of such objects. The mean luminosity of the sample is approximately 5 times 10^4 L_\odot, indicating the objects typically have a mass of ~15 solar masses. We fit the observed bandhead profiles with a model of a circumstellar disc, and find good agreement between the models and observations for all but one object. We compare the high angular precision (0.2-0.8 mas) spectroastrometric data to the spatial distribution of the emitting material in the best-fitting models. No spatial signatures of discs are detected, which is entirely consistent with the properties of the best-fitting models. Therefore, the observations suggest that the CO bandhead emission of massive young stellar objects originates in small-scale disks, in agreement with previous work. This provides further evidence that massive stars form via disc accretion, as suggested by recent simulations.Comment: Accepted for publication in MNRA

The Active Stereo Probe: The Design and Implementation of an Active Videometrics System

This thesis describes research leading to the design and development of the Active Stereo Probe (ASP): an active vision based videometrics system. The ASP espouses both definitions of active vision by integrating structured illumination with a steerable binocular camera platform (or head). However, the primary function of the ASP is to recover quantitative 3D surface models of a scene from stereo images captured from the system's stereo pair of CCD video cameras. Stereo matching is performed using a development of Zhengping and Mowforth's Multiple Scale Signal Matcher (MSSM) stereo matcher. The performance of the original MSSM algorithm was dramatically improved, both in terms of speed of execution and dynamic range, by completely re-implementing it using an efficient scale space pyramid image representation. A range of quantitative performance tests for stereo matchers was developed, and these were applied to the newly developed MSSM stereo matcher to verify its suitability for use in the ASP. The performance of the stereo matcher is further improved by employing the ASP's structured illumination device to bathe the imaged scene in textured light. Few previously reported dynamic binocular camera heads have been able to perform any type of quantitative vision task. It is argued here that this failure has arisen mainly from the rudimentary nature of the design process applied to previous heads. Therefore, in order to address this problem, a new rigorous approach, suitable for the design of both dynamic and static stereo vision systems, was devised. This approach relies extensively upon system modelling as part of the design process. In order to support this new design approach, a general mathematical model of stereo imaging systems was developed and implemented within a software simulator. This simulator was then applied to the analysis of the requirements of the ASP and the MSSM stereo matcher. A specification for the imaging and actuation components of the ASP was hence obtained which was predicted to meet its performance requirements. This led directly to the fabrication of the completed ASP sensor head. The developed approach and model has subsequently been used successfully for the design of several other quantitative stereo vision systems. A vital requirement of any vision system that is intended to perform quantitative measurement is calibration. A novel calibration scheme was devised for the ASP by adopting advanced techniques from the field of photogrammetry and adapting them for use in the context of a dynamic computer vision system. The photogrammetric technique known as the Direct Linear Transform was used successfully in the implementation of the first, static stage of this calibration scheme. A significant aspect of the work reported in this thesis is the importance given to integrating the components developed for the ASP, i.e. the sensor head, the stereo matching software and the calibration software, into a complete videometric system. The success of this approach is demonstrated by the high quality of 3D surface models obtained using the integrated videometric system that was developed

Introduction to a Biological Systems Science

Biological systems analysis and biodynamic modelling of physiological and biological interrelationships in human body and mammal

Parasite infections in a social carnivore: Evidence of their fitness consequences and factors modulating infection load

There are substantial individual differences in parasite composition and infection load in wildlife populations. Few studies have investigated the factors shaping this heterogeneity in large wild mammals or the impact of parasite infections on Darwinian fitness, particularly in juveniles. A host's parasite composition and infection load can be shaped by factors that determine contact with infective parasite stages and those that determine the host's resistance to infection, such as abiotic and social environmental factors, and age. Host–parasite interactions and synergies between coinfecting parasites may also be important. We test predictions derived from these different processes to investigate factors shaping infection loads (fecal egg/oocyte load) of two energetically costly gastrointestinal parasites: the hookworm Ancylostoma and the intracellular Cystoisospora, in juvenile spotted hyenas (Crocuta crocuta) in the Serengeti National Park, in Tanzania. We also assess whether parasite infections curtail survival to adulthood and longevity. Ancylostoma and Cystoisospora infection loads declined as the number of adult clan members increased, a result consistent with an encounter‐reduction effect whereby adults reduced encounters between juveniles and infective larvae, but were not affected by the number of juveniles in a clan. Infection loads decreased with age, possibly because active immune responses to infection improved with age. Differences in parasite load between clans possibly indicate variation in abiotic environmental factors between clan den sites. The survival of juveniles (<365 days old) to adulthood decreased with Ancylostoma load, increased with age, and was modulated by maternal social status. High‐ranking individuals with low Ancylostoma loads had a higher survivorship during the first 4 years of life than high‐ranking individuals with high Ancylostoma loads. These findings suggest that high infection loads with energetically costly parasites such as hookworms during early life can have negative fitness consequences

ATLASGAL-selected massive clumps in the inner Galaxy: VI. Kinetic temperature and spatial density measured with formaldehyde

We aim to directly determine the kinetic temperature and spatial density with formaldehyde for the $\sim$100 brightest ATLASGAL-selected clumps at 870 $\mu$m representing various evolutionary stages of high-mass star formation. Ten transitions ($J$ = 3-2 and 4-3) of ortho- and para-H$_2$CO near 211, 218, 225, and 291 GHz were observed with the APEX 12 m telescope. Using non-LTE models with RADEX, we derive the gas kinetic temperature and spatial density using the measured p-H$_2$CO 3$_{21}$-2$_{20}$/3$_{03}$-2$_{02}$, 4$_{22}$-3$_{21}$/4$_{04}$-3$_{03}$, and 4$_{04}$-3$_{03}$/3$_{03}$-2$_{02}$ ratios. The gas kinetic temperatures derived from the p-H$_2$CO 3$_{21}$-2$_{20}$/3$_{03}$-2$_{02}$ and 4$_{22}$-3$_{21}$/4$_{04}$-3$_{03}$ line ratios are high, ranging from 43 to $>$300 K with an unweighted average of 91 $\pm$ 4 K. Deduced $T_{\rm kin}$ values from the $J$ = 3-2 and 4-3 transitions are similar. Spatial densities of the gas derived from the p-H$_2$CO 4$_{04}$-3$_{03}$/3$_{03}$-2$_{02}$ line ratios yield 0.6-8.3 $\times$ 10$^6$ cm$^{-3}$ with an unweighted average of 1.5 ($\pm$0.1) $\times$ 10$^6$ cm$^{-3}$. A comparison of kinetic temperatures derived from p-H$_2$CO, NH$_3$, and the dust emission indicates that p-H$_2$CO traces a distinctly higher temperature than the NH$_3$ (2,2)/(1,1) transitions and the dust, tracing heated gas more directly associated with the star formation process. The H$_2$CO linewidths are found to be correlated with bolometric luminosity and increase with the evolutionary stage of the clumps, which suggests that higher luminosities tend to be associated with a more turbulent molecular medium. It seems that the spatial densities measured with H$_2$CO do not vary significantly with the evolutionary stage of the clumps. However, averaged gas kinetic temperatures derived from H$_2$CO increase with time through the evolution of the clumps.Comment: Accepted for publication in A&

CO bandhead emission of massive young stellar objects: determining disc properties

Massive stars play an important role in many areas of astrophysics, but numerous details regarding their formation remain unclear. In this paper we present and analyse high-resolution (R~30 000) near-infrared 2.3 μm spectra of 20 massive young stellar objects (MYSOs) from the Red MSX Source (RMS) data base, in the largest such study of CO first overtone bandhead emission to date. We fit the emission under the assumption it originates from a circumstellar disc in Keplerian rotation. We explore three approaches to modelling the physical conditions within the disc-a disc heated mainly via irradiation from the central star, a disc heated mainly via viscosity, and a disc in which the temperature and density are described analytically. We find that the models described by heating mechanisms are inappropriate because they do not provide good fits to the CO emission spectra. We therefore restrict our analysis to the analytic model, and obtain good fits to all objects that possess sufficiently strong CO emission, suggesting circumstellar discs are the source of this emission. On average, the temperature and density structure of the discs correspond to geometrically thin discs, spread across a wide range of inclinations. Essentially all the discs are located within the dust sublimation radius, providing strong evidence that the CO emission originates close to the central protostar, on astronomical unit scales. In addition, we show that the objects in our sample appear no different to the general population of MYSOs in the RMS data base, based on their near- and mid-infrared colours. The combination of observations of a large sample of MYSOs with CO bandhead emission and our detailed modelling provide compelling evidence of the presence of small-scale gaseous discs around such objects, supporting the scenario in which massive stars form via disc accretion. © 2013 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society

Uncertainty in Model Predictions of Vibrio Vulnificus Response to Climate Variability and Change: A Chesapeake Bay Case Study

The effect that climate change and variability will have on waterborne bacteria is a topic of increasing concern for coastal ecosystems, including the Chesapeake Bay. Surface water temperature trends in the Bay indicate a warming pattern of roughly 0.3-0.4 C per decade over the past 30 years. It is unclear what impact future warming will have on pathogens currently found in the Bay, including Vibrio spp. Using historical environmental data, combined with three different statistical models of Vibrio vulnificus probability, we explore the relationship between environmental change and predicted Vibrio vulnificus presence in the upper Chesapeake Bay. We find that the predicted response of V. vulnificus probability to high temperatures in the Bay differs systematically between models of differing structure. As existing publicly available datasets are inadequate to determine which model structure is most appropriate, the impact of climatic change on the probability of V. vulnificus presence in the Chesapeake Bay remains uncertain. This result points to the challenge of characterizing climate sensitivity of ecological systems in which data are sparse and only statistical models of ecological sensitivity exist

ATLASGAL-selected massive clumps in the inner Galaxy: VIII. Chemistry of photodissociation regions

Aims. We study ten molecular transitions obtained from an unbiased 3 mm molecular line survey using the IRAM 30 m telescope toward 409 compact dust clumps identified by the APEX Telescope Large Area Survey of the Galaxy (ATLASGAL) to understand photodissociation regions (PDRs) associated with the clumps. The main goal of this study is to investigate whether the abundances of the selected molecules show any variations resulting from the PDR chemistry in different clump environments. Methods. We selected HCO, HOC$^+$, C$_2$H, c-C$_3$H$_2$, CN, H$^{13}$CN, HC$^{15}$N, and HN$^{13}$C as PDR tracers, and H$^{13}$CO$^+$ and C$^{18}$O as dense gas tracers. By using estimated optical depths of C$_2$H and H$^{13}$CN and assuming optically thin emission for other molecular transitions, we derived column densities of those molecules and their abundances. To assess the influence of the presence and strength of ultraviolet radiation, we compare abundances of three groups of the clumps: Hii regions, infrared bright non-Hii regions, and infrared dark non-Hii regions. Results. We detected C$^{18}$O, H$^{13}$CO$^+$, C$_2$H, c-C$_3$H$_2$, CN and HN$^{13}$C toward most of the observed dust clumps (detection rate > 94%), and H$^{13}$CN is also detected with a detection rate of 75%. On the other hand, HCO and HC$^{15}$N show detection rates of 32% and 39%, respectively, toward the clumps, which are mostly associated with Hii region sources: detection rates of HCO and HC$^{15}$N toward the Hii regions are 66% and 79%. We find that the abundances of HCO, CN, C$_2$H, and c-C$_3$H$_2$ decrease as the H$_2$ column density increase, indicating high visual extinction, while those of high density tracers (i.e., H$^{13}$CO$^+$ and HC$^{15}$N) are constant. In addition, N(HCO)/N(H$^{13}$CO$^+$) ratios significantly decrease as H$_2$ column density increase, and in particular, 82 clumps have X(HCO) $\gtrapprox$ 10$^{−10}$ and N(HCO)/N(H$^{13}$CO$^+$+) $\gtrapprox$ 1, which are the indication of far-ultraviolet (FUV) chemistry. This suggests the observed HCO abundances are likely associated with FUV radiation illuminating the PDRs. We also find that high N(c-C$_3$H$_2$)/N(C$_2$H) ratios found for Hii regions having high HCO abundances ($\gtrapprox$ 10$^{−10}$) are associated with more evolved clumps with high L$_{bol}$/M$_{clump}$. This trend might be associated with gain-surface processes, which determine initial abundances of these molecules, and time-dependent effects in the clumps corresponding to the envelopes around dense PDRs and Hii regions. In addition, some fraction of the measured abundances of the small hydrocarbons of the Hii sources can be the result of the photodissociation of PAH molecules