Absolute continuity of symmetric Markov processes

We study Girsanov's theorem in the context of symmetric Markov processes, extending earlier work of Fukushima-Takeda and Fitzsimmons on Girsanov transformations of ``gradient type.'' We investigate the most general Girsanov transformation leading to another symmetric Markov process. This investigation requires an extension of the forward-backward martingale method of Lyons-Zheng, to cover the case of processes with jumps.Comment: Published by the Institute of Mathematical Statistics (http://www.imstat.org) in the Annals of Probability (http://www.imstat.org/aop/) at http://dx.doi.org/10.1214/00911790400000043

Occupation time limits of inhomogeneous Poisson systems of independent particles

We prove functional limits theorems for the occupation time process of a system of particles moving independently in $R^d$ according to a symmetric $\alpha$-stable L\'evy process, and starting off from an inhomogeneous Poisson point measure with intensity measure $\mu(dx)=(1+|x|^{\gamma})^{-1}dx,\gamma>0$, and other related measures. In contrast to the homogeneous case $(\gamma=0)$, the system is not in equilibrium and ultimately it vanishes, and there are more different types of occupation time limit processes depending on arrangements of the parameters $\gamma, d$ and $\alpha$. The case $\gamma<d<\alpha$ leads to an extension of fractional Brownian motion.Comment: 22 page

Physical and dynamical characterisation of low Delta-V NEA (190491) 2000 FJ10

We investigated the physical properties and dynamical evolution of Near Earth Asteroid (NEA) (190491) 2000 FJ10 in order to assess the suitability of this accessible NEA as a space mission target. Photometry and colour determination were carried out with the 1.54 m Kuiper Telescope and the 10 m Southern African Large Telescope during the object's recent favourable apparition in 2011-12. During the earlier 2008 apparition, a spectrum of the object in the 6000-9000 Angstrom region was obtained with the 4.2 m William Herschel Telescope. Interpretation of the observational results was aided by numerical simulations of 1000 dynamical clones of 2000 FJ10 up to 10^6 yr in the past and in the future. The asteroid's spectrum and colours determined by our observations suggest a taxonomic classification within the S-complex although other classifications (V, D, E, M, P) cannot be ruled out. On this evidence, it is unlikely to be a primitive, relatively unaltered remnant from the early history of the solar system and thus a low priority target for robotic sample return. Our photometry placed a lower bound of 2 hrs to the asteroid's rotation period. Its absolute magnitude was estimated to be 21.54+-0.1 which, for a typical S-complex albedo, translates into a diameter of 130+-20 m. Our dynamical simulations show that it has likely been an Amor for the past 10^5 yr. Although currently not Earth-crossing, it will likely become so during the period 50 - 100 kyr in the future. It may have arrived from the inner or central Main Belt > 1 Myr ago as a former member of a low-inclination S-class asteroid family. Its relatively slow rotation and large size make it a suitable destination for a human mission. We show that ballistic Earth-190491-Earth transfer trajectories with Delta-V < 2 km s^-1 at the asteroid exist between 2052 and 2061.Comment: 2 Tables, 11 Figures, accepted for publication in Astronomy & Astrophysic

Sputtering of Oxygen Ice by Low Energy Ions

Naturally occurring ices lie on both interstellar dust grains and on celestial objects, such as those in the outer solar system. These ices are continu- ously subjected to irradiation by ions from the solar wind and/or cosmic rays, which modify their surfaces. As a result, new molecular species may form which can be sputtered off into space or planetary atmospheres. We determined the experimental values of sputtering yields for irradiation of oxygen ice at 10 K by singly (He+, C+, N+, O+ and Ar+) and doubly (C2+, N2+ and O2+) charged ions with 4 keV kinetic energy. In these laboratory experiments, oxygen ice was deposited and irradiated by ions in an ultra high vacuum chamber at low temperature to simulate the environment of space. The number of molecules removed by sputtering was observed by measurement of the ice thickness using laser interferometry. Preliminary mass spectra were taken of sputtered species and of molecules formed in the ice by temperature programmed desorption (TPD). We find that the experimental sputtering yields increase approximately linearly with the projectile ion mass (or momentum squared) for all ions studied. No difference was found between the sputtering yield for singly and doubly charged ions of the same atom within the experimental uncertainty, as expected for a process dominated by momentum transfer. The experimental sputter yields are in good agreement with values calculated using a theoretical model except in the case of oxygen ions. Preliminary studies have shown molecular oxygen as the dominant species sputtered and TPD measurements indicate ozone formation.Comment: to be published in Surface Science (2015

Costs and benefits of a subtype-specific surveillance system for identifying Escherichia coli O157:H7 outbreaks.

We assessed the societal costs and benefits of a subtype-specific surveillance system for identifying outbreak-associated Escherichia coli O157:H7 infections. Using data from Colorado, we estimated that if it averted five cases annually, the system would recover all its costs

Spectroscopy and thermal modelling of the first interstellar object 1I/2017 U1 ‘Oumuamua

During the formation and evolution of the Solar System, significant numbers of cometary and asteroidal bodies were ejected into interstellar space. It is reasonable to expect that the same happened for planetary systems other than our own. Detection of such interstellar objects would allow us to probe the planetesimal formation processes around other stars, possibly together with the effects of long-term exposure to the interstellar medium. 1I/2017 U1 ‘Oumuamua is the first known interstellar object, discovered by the Pan-STARRS1 telescope in October 2017. The discovery epoch photometry implies a highly elongated body with radii of ~ 200 × 20 m when a comet-like geometric albedo of 0.04 is assumed. The observable interstellar object population is expected to be dominated by comet-like bodies in agreement with our spectra, yet the reported inactivity of 'Oumuamua implies a lack of surface ice. Here, we report spectroscopic characterization of ‘Oumuamua, finding it to be variable with time but similar to organically rich surfaces found in the outer Solar System. We show that this is consistent with predictions of an insulating mantle produced by long-term cosmic ray exposure. An internal icy composition cannot therefore be ruled out by the lack of activity, even though ‘Oumuamua passed within 0.25 au of the Sun

Does the Type of Assessment Feedback I Give Make a Difference?: The Impact of Qualitative and Quantitative Assessment Feedback

Feedback provided to postgraduate students about their assessment tasks influences the way in which they reflect on their learning and themselves personally. In particular, the nature of the feedback and the way in which its dissemination is sequenced and timed can further impact how students incorporate, or don\u27t incorporate, assessment advice into their future learning, a process referred to by Duncan (2007) as feed-forward . Despite the value placed on assessment feedback by academic teaching staff, it often has minimal impact on students\u27 learning (Sadler, 2010). Past research into the impact of qualitative and quantitative feedback on student learning established that quantitative feedback (for example, marks, grades, scores) impacts students\u27 egos whereas qualitative feedback (for example, advice-giving comments) impacts students\u27 understanding of the learning task (Butler, 1987, 1988; Butler & Nisan, 1986). The impact of assessment feedback continues to be investigated in higher education (The Higher Education Academy, 2012) but it remains a contentious issue (Boud & Molloy, 2013). Pulfrey, Darnon and Butera (2013) suggest that the practice of giving grades to students continues to be an area of disagreement among educators. The research project, reported here, extends on previous research about the nature of assessment feedback and its impact on students\u27 learning and students\u27 self-perceptions. This study investigates students’ responses to assessment feedback (Bell, Miadenovic, & Price, 2013), but specifically focusing on postgraduate students enrolled in online courses. The study investigated how postgraduate students, in an online course, responded to the sequenced release of qualitative and quantitative assessment feedback using online assessment tools within the institution\u27s Learning Management System (LMS). Students responded to a series of online prompts that elicited their comments about what they did and how they felt when they received, firstly, qualitative feedback and, secondly, quantitative feedback about their assessment tasks. To gather both holistic data and in-depth information about each case (Flick, 2004), a qualitative research approach was adopted in which students\u27 responses to varied types of data were gathered. Data gathered from each student enabled the consideration of each participant as an individual case study. These data were then analysed through a process of constant comparison to determine how each type of feedback influenced the students\u27 perceptions of their learning and of themselves. The findings of the study revealed that students focused on themselves and their learning, depending on the feedback they were given about their assessment tasks. Furthermore, the type of feedback received tended to influence the nature and specificity of their responses to the feedback. Full details of the findings will be presented in the conference paper. The findings of this study have implications for the way in which: a) assessment tasks are designed; and b) assessment feedback is provided to students. Findings from this research may interest university lecturers, course designers and supervisors of postgraduate students who are looking to promote deeper engagement of their students with their assessment feedback