1,721,013 research outputs found
A Submillimetre Study of Nearby Star Formation using Molecular Line Data
No full textThis thesis primarily uses submillimetre molecular line data from HARP, a heterodyne array on the James Clerk Maxwell Telescope (JCMT), to further investigate star formation in the Ophiuchus L1688 cloud. HARP was used to observe CO J = 3-2 isotopologues: 12CO, 13CO and C18O; and the dense gas tracer HCO+ J = 4-3.
A method for calculating molecular line contamination in the SCUBA-2 450 and 850 μm dust continuum data was developed, which can be used to convert 12CO J =6-5and J =3-2 maps of integrated intensity (K km s−1) to molecular line flux (mJy beam−1) contaminating the continuum emission. Using HARP maps of 12CO J = 3-2, I quantified the amount of molecular line contamination found in the SCUBA-2 850 μm maps of three different regions, including NGC 1333 of Perseus and NGC 2071 and NGC 2024 of Orion B. Regions with ‘significant’ (i.e. > 20%) molecular line contamination correspond to molecular outflows. This method is now being used to remove molecular line contamination from regions with both SCUBA-2 dust continuum and HARP 12CO map coverage in the Gould Belt Legacy Survey (GBS).
The Ophiuchus L1688 cloud was observed in all three CO J = 3-2 isotopologues. I carried out a molecular outflow analysis in the region on a list of 30 sources from the Spitzer ‘c2d’ survey [Evans et al., 2009]. Out of the 30 sources, 8 had confirmed bipolar outflows, 20 sources had ‘confused’ outflow detections and 2 sources did not have outflow detections.
The Ophiuchus cloud was found to be gravitationally bound with the turbulent kinetic energy a factor of 7 lower than the gravitational binding energy. The high-velocity outflowing gas was found to be only 21% of the turbulence in the cloud, suggesting outflows are significant but not the dominant source of turbulence in the region. Other factors were found to influence the global high-velocity outflowing gas in addition to molecular outflows, including hot dust from nearby B-type stars, outflow remnants from less embedded sources and stellar winds from the Upper Scorpius OB association.
To trace high density gas in the Ophiuchus L1688 cloud, HCO+ J = 4-3 was observed to further investigate the relationship between high column density and high density in the molecular cloud. Non-LTE codes RADEX and TORUS were used to develop density models corresponding to the HCO+ emission. The models involved both constant density and peaked density profiles. RADEX [van der Tak et al., 2007] models used a constant density model along the line-of-sight and indicated the HCO+ traced densities that were predominantly subthermally excited with den- sities ranging from 10^3–10^5 cm^−3. Line-of-sight estimates ranged from several parsecs to 90 pc, which was unrealistic for the Ophiuchus cloud. This lead to the implementation of peaked density profiles using the TORUS non-LTE radiative transfer code. Initial models used a ‘triangle’ density profile and a more complicated log-normal density probability density function (PDF) profile was subsequently implemented. Peaked density models were relatively successful at fitting the HCO+ data. Triangle models had density fits ranging from 0.2–2.0×10^6 cm^−3 and 0.1–0.3×10^6 cm^−3 for the 0.2 and 0.3 pc cloud length models re- spectively. Log-normal density models with constant-σ had peak density ranges from 0.2–1.0 ×10^5 cm^−3 and 0.6–2.0×10^5 cm^−3 for 0.2 and 0.3 pc models respectively. Similarly, log-normal models with varying-σ had lower and upper density limits corresponding to the range of FWHM velocities. Densities (lower and upper limits) ranged from 0.1–1.0 ×10^6 and 0.5–3.0 ×10^5 cm^-3 for the 0.2 and 0.3 pc models respectively.
The result of the HCO+ density modelling indicated the distributions of starless, prestellar and protostellar cores do not have a preference for higher densities with respect to the rest of the cloud. This is contrary to past research suggesting the probability of finding a submillimetre core steeply rises as a function of column density (i.e. density; Belloche et al. 2011; Hatchell et al. 2005). Since the majority of sources are less embedded (i.e Class II/III), it is possible the evolutionary state of Ophiuchus is the main reason the small sample of Class 0/I protostars do not appear to have a preference for higher densities in the cloud.University of Exeter studentshi
The influence of radiative feedback on star formation observed by the James Clerk Maxwell Telescope Gould Belt Survey of nearby star-forming regions.
No full textThe aim of this thesis is to investigate evidence of heating and radiative feedback in local Gould Belt star-forming regions. I discuss what impact, if any, radiative feedback is having on the star formation. I primarily use Submillimeter Common-User Bolometer Array 2 (SCUBA-2) observations from the James Clerk Maxwell Telescope (JCMT) Gould Belt legacy Survey (GBS) of nearby star-forming regions. I analyse this data in conjunction with catalogues of candidate young stellar objects (YSOcs) from mid-infrared surveys with Spitzer IRAC and MIPS surveys. I use the ratio of SCUBA-2 fluxes to calculate dust temperature, given a constant value of dust opacity spectral index, following the method of Reid & Wilson (2005). I employ a two-component beam (2CB) cross convolution to map the temperature of the Serpens MWC 297 region, achieving a resolution of 19.9′′. I employ a convolution kernel to map the temperature of the majority of the JCMT GBS, including the Aquila W40 complex, achieving a resolution of 14.8′′. I use the fellwalker clump finding algorithm to produce a global catalogue of 619 SCUBA-2 850 μm clumps across 26 distinct sub-regions of the JCMT GBS, calculating real temperatures where available. I was the PI of a proposal to observe 12CO 3-2 line emission, with the aim of decontaminating the SCUBA-2 850 μm band. I find 12CO 3-2 line contamination has a significant impact, increasing the dust temperatures calculated per pixel, on average, by 3 K where contamination is less than 10%, and by 16 K where contamination is greater than 10% (in the Aquila W40 complex). I find evidence for 12 outflows in this region, associated with active star formation. I also use archival VLA data to decontaminate both SCUBA-2 bands of free-free emission associated with massive star formation. Where compact free-free sources are sufficiently bright and optically thick, for example the B1.5Ve star MWC 297, their contribution can lead to prominent bright sources at the submillimeter wavelengths detected by SCUBA-2 and lower temperatures around Herbig stars. I present published studies of the Serpens MWC 297 region and the Aquila W40 complex. In both cases I find evidence that the presence of young OB stars is raising the temperatures of nearby clumps. Examining clumps across the JCMT GBS, I find that those clumps isolated from OB stars have a mean temperature of 15±2 K, a value that is consistent with gas temperatures (Friesen et al., 2009) and Bonnor-Ebert sphere models (Kirk et al., 2006). I find no evidence of heating from embedded low-to-medium mass YSOs. Clumps that lie within 3 pc of OB stars have a mean temperature of 21±4 K and O type stars heat clumps over the greatest range. By remodelling the heated clumps with a temperature of 15 K, I calculate that up to 10% of clumps in the JCMT GBS are no longer Jeans unstable, indicating that radiative feedback from OB stars is potentially suppressing fragmentation and allowing for the formation of more massive stars.Science and Technology Facilities Counci
Solving the Catalogue Cross-Match Problem in the Galactic Plane
Full text linkIn this thesis the cross-matching problem is considered. One of the most fundamental processes in astrophysics, the cross-matching of two photometric catalogues is the assignment of an object in one catalogue and one object from the second catalogue as pairs, i.e., different detections of the same physical source in the sky. I present here new methods for considering such a problem, including the additional magnitude information available in photometric catalogues to break degeneracies between astrometric matches. I also generalise the Astrometric Uncertainty Function (AUF), usually assumed to be a Gaussian, to allow for the inclusion of systematic astrometric perturbations, such as those from blended sources of contamination.
The separations of sources in several widely used photometric catalogues with respect to the much more precise Gaia positions are considered. I find that the separations are described by a combination of a Gaussian distribution and a large non-Gaussian wing, and show that this is caused by flux contamination from blended stars not treated separately. At least one in three of the stars in the faint half of a given catalogue will suffer from flux contamination above the 1% level when the density of catalogue objects per point-spread function area is above approximately 0.005.
I then introduce a new method to use the additional photometric information from both catalogues in the process of accepting or rejecting counterparts, providing approximately a factor 10 improvement in Bayes' factor with its inclusion. The method uniquely combines photometric information from both catalogues while avoiding the use of prior astrophysical knowledge. Additionally, I formally describe the probability of two sources being the same astrometric object, allowing systematic effects of astrometric perturbation (by, e.g., contaminant objects) to be accounted for. I apply this method to two key match cases, of two catalogues of similar wavelength coverage but differing dynamic ranges, and of two catalogues with approximately equal astrometric precision, discussing the importance of the inclusion of the magnitude information in each case.
Finally, the extension to the inclusion of perturbations due to faint contaminant stars in the AUFs of catalogues is combined with the improved cross-matching method for the specific case of the Wide-field Infrared Survey Explorer (WISE) catalogue. I describe the rigorous construction of the description of astrometric offsets due to faint stars, and then apply the method to Gaia-WISE matches in the Galactic plane. I analyse several test cases and discuss the photometric effects of the blended star contamination, showing that stars with significant astrometric perturbation are detectably photometrically compromised. I discuss the implications this has on derived parameters in several areas of astrophysics
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
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