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1. Phosphine Gas in the Cloud Decks of Venus

Author

Greaves, Jane S., Richards, Anita M. S., Bains, William, Rimmer, Paul B., Sagawa, Hideo, Clements, David L., Seager, Sara, Petkowski, Janusz J., Sousa-Silva, Clara, Ranjan, Sukrit, Drabek-Maunder, Emily, Fraser, Helen J., Cartwright, Annabel, Mueller-Wodarg, Ingo, Zhan, Zhuchang, Friberg, Per, Coulson, Iain, Lee, E'lisa, and Hoge, Jim

Subjects
Astrophysics - Earth and Planetary Astrophysics
Abstract

Measurements of trace-gases in planetary atmospheres help us explore chemical conditions different to those on Earth. Our nearest neighbor, Venus, has cloud decks that are temperate but hyper-acidic. We report the apparent presence of phosphine (PH3) gas in Venusian atmosphere, where any phosphorus should be in oxidized forms. Single-line millimeter-waveband spectral detections (quality up to ~15 sigma) from the JCMT and ALMA telescopes have no other plausible identification. Atmospheric PH3 at ~20 parts-per-billion abundance is inferred. The presence of phosphine is unexplained after exhaustive study of steady-state chemistry and photochemical pathways, with no currently-known abiotic production routes in Venusian atmosphere, clouds, surface and subsurface, or from lightning, volcanic or meteoritic delivery. Phosphine could originate from unknown photochemistry or geochemistry, or, by analogy with biological production of phosphine on Earth, from the presence of life. Other PH3 spectral features should be sought, while in-situ cloud and surface sampling could examine sources of this gas.

Published
2020
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2. The Venus Hypothesis

Author

Cartwright, Annabel

Subjects
Astrophysics - Earth and Planetary Astrophysics, Physics - Popular Physics
Abstract

Current models indicate that Venus may have been habitable. Complex life may have evolved on the highly irradiated Venus, and transferred to Earth on asteroids. This model fits the pattern of pulses of highly developed life appearing, diversifying and going extinct with astonishing rapidity through the Cambrian and Ordovician periods, and also explains the extraordinary genetic variety which appeared over this period.

Published
2016

3. Addendum: Phosphine gas in the cloud deck of Venus

Author

Greaves, Jane S., Richards, Anita M. S., Bains, William, Rimmer, Paul B., Sagawa, Hideo, Clements, David L., Seager, Sara, Petkowski, Janusz J., Sousa-Silva, Clara, Ranjan, Sukrit, Drabek-Maunder, Emily, Fraser, Helen J., Cartwright, Annabel, Mueller-Wodarg, Ingo, Zhan, Zhuchang, Friberg, Per, Coulson, Iain, Lee, E’lisa, and Hoge, Jim

Published
2021
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4. The intrinsic shapes of starless cores in Ophiuchus

Author

Lomax, Oliver, Whitworth, Anthony P., and Cartwright, Annabel

Subjects
Astrophysics - Solar and Stellar Astrophysics
Abstract

Using observations of cores to infer their intrinsic properties requires the solution of several poorly constrained inverse problems. Here we address one of these problems, namely to deduce from the projected aspect ratios of the cores in Ophiuchus their intrinsic three-dimensional shapes. Four models are proposed, all based on the standard assumption that cores are randomly orientated ellipsoids, and on the further assumption that a core's shape is not correlated with its absolute size. The first and simplest model, M1, has a single free parameter, and assumes that the relative axes of a core are drawn randomly from a log-normal distribution with zero mean and standard deviation \sigma o. The second model, M2a, has two free parameters, and assumes that the log-normal distribution (with standard deviation \sigma o) has a finite mean, \mu o, defined so that \mu o<0 means elongated (prolate) cores are favoured, whereas \mu o>0 means flattened (oblate) cores are favoured. Details of the third model (M2b, two free parameters) and the fourth model (M4, four free parameters) are given in the text. Markov chain Monte Carlo sampling and Bayesian analysis are used to map out the posterior probability density functions of the model parameters, and the relative merits of the models are compared using Bayes factors. We show that M1 provides an acceptable fit to the Ophiuchus data with \sigma o ~ 0.57+/-0.06; and that, although the other models sometimes provide an improved fit, there is no strong justification for the introduction of their additional parameters., Comment: 10 pages, 8 figures. Accepted by MNRAS

Published
2012
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5. Controlling Artificial Viscosity in SPH simulations of accretion disks

Author

Cartwright, Annabel and Stamatellos, Dimitrios

Subjects
Astrophysics - Instrumentation and Methods for Astrophysics
Abstract

We test the operation of two methods for selective application of Artificial Viscosity (AV) in SPH simulations of Keplerian Accretion Disks, using a ring spreading test to quantify effective viscosity, and a correlation coefficient technique to measure the formation of unwanted prograde alignments of particles. Neither the Balsara Switch nor Time Dependent Viscosity work effectively, as they leave AV active in areas of smooth shearing flow, and do not eliminate the accumulation of alignments of particles in the prograde direction. The effect of both switches is periodic, the periodicity dependent on radius and unaffected by the density of particles. We demonstrate that a very simple algorithm activates AV only when truly convergent flow is detected and reduces the unwanted formation of prograde alignments. The new switch works by testing whether all the neighbours of a particle are in Keplerian orbit around the same point, rather than calculating the divergence of the velocity field, which is very strongly affected by Poisson noise in the positions of the SPH particles., Comment: 8 pages, 5 figures

Published
2010
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6. The statistical Analysis of Star Clusters

Author

Cartwright, Annabel and Whitworth, Anthony P

Subjects
Astrophysics
Abstract

We review a range of stastistical methods for analyzing the structures of star clusters, and derive a new measure ${\cal Q}$ which both quantifies, and distinguishes between, a (relatively smooth) large-scale radial density gradient and multi-scale (fractal) sub-clustering. Q is derived from the normalised correlation length and the normalised edge length of the minimal spanning tree for each cluster.

Published
2004
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7. SPH simulation of the formation and evolution of protoplanetary disks

Author

Cartwright, Annabel

Subjects
523.01
Abstract

The formation and evolution of protoplanetary disks is simulated by computer modelling, using the Smoothed Particle Hydrodynamics (SPH) method. The suitability of SPH for modelling disks is investigated, and problems are identified with the SPH implementation of Artificial Viscosity in disks with Keplerian velocity profiles. Analytical and experimental results reveal that the resultant viscous force for a simulated Keplerian disk is in the opposite direction to that produced by linear shear. Applying Artificial Viscosity only to approaching particles results in a radial force four times larger than the force in the direction of the orbit. The viscous force can change direction if temperature, and therefore sound speed, decreases. Techniques for activating Artificial Viscosity only when convergence is detected are found to fail in differentially rotating disks. Both the Balsara Switch and Time Dependent Artificial Viscosity use the SPH estimate of V v sph, which has a low freqency time varying component which is independent of h, and so cannot be removed by increasing the number of SPH particles. An alternative method, based on pattern recognition, is shown to reduce the viscous spread of a differentially rotating ring by an order of magnitude. We also identify problems associated with the gravitational field of disks. The use of an annulus to represent a portion of a much larger, continuous disk, may yield unrepresentative results. The edge effects can cause preferential accretion zones, where the Toomre Q parameter is not the same as it would be for the same region of an extended disk. SPH simulations of Protoplanetary disks produce condensations which do not persist long enough to collapse. The high tidal shearing forces in a Keplerian accretion disk disrupt the condensations before they accumulate enough mass to collapse. Including a more realistic treatment of the thermal physics, and reducing the effective shear viscosity, makes the situation worse.

Published
2006

9. Modelling Star Formation in Ophiuchus

Author

Lomax, Oliver, Whitworth, Anthony Peter, Cartwright, Annabel, Stamatellos, Dimitris, Walch, Stefanie K., Stamatellos, Dimitris, editor, Goodwin, Simon, editor, and Ward-Thompson, Derek, editor

Published
2014
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10. Video assignments: A powerful way to introduce creativity and fun into your program – A physics education story

Author

Cartwright, Annabel and Lewis, Richard

Subjects
LB2300, QC
Abstract

The School of Physics and Astronomy has been setting video assessments for several years and they have been very popular with students. Now that video equipment and editing software is so easily available to students, and videos can now be uploaded to Learning Central, there are no technical barriers. This can enable students, particularly in technical subjects, to practice communication skills as well as show creativity and flair.

Published
2022

12. Phosphine gas in the cloud decks of Venus

Author

Massachusetts Institute of Technology. Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology. Department of Physics, MIT Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology. Department of Aeronautics and Astronautics, Greaves, Jane S, Richards, Anita MS, Bains, William, Rimmer, Paul B, Sagawa, Hideo, Clements, David L, Seager, Sara, Petkowski, Janusz J, Sousa-Silva, Clara, Ranjan, Sukrit, Drabek-Maunder, Emily, Fraser, Helen J, Cartwright, Annabel, Mueller-Wodarg, Ingo, Zhan, Zhuchang, Friberg, Per, Coulson, Iain, Lee, E’lisa, Hoge, Jim, Massachusetts Institute of Technology. Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology. Department of Physics, MIT Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology. Department of Aeronautics and Astronautics, Greaves, Jane S, Richards, Anita MS, Bains, William, Rimmer, Paul B, Sagawa, Hideo, Clements, David L, Seager, Sara, Petkowski, Janusz J, Sousa-Silva, Clara, Ranjan, Sukrit, Drabek-Maunder, Emily, Fraser, Helen J, Cartwright, Annabel, Mueller-Wodarg, Ingo, Zhan, Zhuchang, Friberg, Per, Coulson, Iain, Lee, E’lisa, and Hoge, Jim

Published
2021

16. WHY DON'T YOU SEE OIL 'GUSHING' OUT OF OIL WELLS ANYMORE

Author

Cartwright, Annabel

Subjects
Oil wells, Science and technology
Abstract

An oil well has a blowout, or 'gushes', when crude oil is accidentally released too quickly. Modern wells are equipped with pressure release systems which make incidents far less likely [...]

Published
2021

17. Phosphine gas in the cloud decks of Venus

Author

Greaves, Jane S., primary, Richards, Anita M. S., additional, Bains, William, additional, Rimmer, Paul B., additional, Sagawa, Hideo, additional, Clements, David L., additional, Seager, Sara, additional, Petkowski, Janusz J., additional, Sousa-Silva, Clara, additional, Ranjan, Sukrit, additional, Drabek-Maunder, Emily, additional, Fraser, Helen J., additional, Cartwright, Annabel, additional, Mueller-Wodarg, Ingo, additional, Zhan, Zhuchang, additional, Friberg, Per, additional, Coulson, Iain, additional, Lee, E’lisa, additional, and Hoge, Jim, additional

Published
2020
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18. Phosphine gas in the cloud decks of Venus.

Author

Greaves, Jane S., Richards, Anita M. S., Bains, William, Rimmer, Paul B., Sagawa, Hideo, Clements, David L., Seager, Sara, Petkowski, Janusz J., Sousa-Silva, Clara, Ranjan, Sukrit, Drabek-Maunder, Emily, Fraser, Helen J., Cartwright, Annabel, Mueller-Wodarg, Ingo, Zhan, Zhuchang, Friberg, Per, Coulson, Iain, Lee, E'lisa, and Hoge, Jim

Published
2021
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25. Four-parameter fits to the initial mass function using stable distributions.

Author

Cartwright, Annabel and Whitworth, Anthony P.

Subjects
*STAR formation, *DISTRIBUTION (Probability theory), *PARETO principle, *STELLAR populations, *CENTRAL limit theorem, *STELLAR mass, *STELLAR luminosity function
Abstract

ABSTRACT We propose the use of the statistically well-founded Pareto-Levy family of stable distributions for the characterization of the mass distributions of stellar and pre-stellar populations. This family of distributions, which includes the Gaussian, Levy and Cauchy distributions, can be shown to obey the central limit theorem. This makes them statistically justifiable models for populations resulting from the combination of many independent variables. Applying publicly available software to raw data sets we find probability distributions which fit the data to 95 per cent confidence levels. This automatic process is much simpler, faster and more objective than the normal procedure of deciding upon bin sizes and thresholds, and only four parameters are required to define the best-fitting stable distribution. As there is only one function to describe the whole mass range, there is no need to speculate upon the physical significance of 'thresholds' between different subranges. Using stable distribution fits we demonstrate that the modal peaks and widths of the peaks for core mass functions from Ophiuchus, Aquila and Orion are measurably different, width increasing with peak mass. The slopes of the high-mass power laws are indistinguishable. [ABSTRACT FROM AUTHOR]

Published
2012
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26. Methods for analysing structure in molecular clouds.

Author

Cartwright, Annabel, Whitworth, Anthony P., and Nutter, David

Subjects
*MOLECULAR clouds, *INTERSTELLAR molecules, *STAR clusters, *STATISTICAL correlation, *FRACTALS
Abstract

We have previously reported a dimensionless measure, , which can both quantify, and distinguish between, the extent to which a star cluster is centrally concentrated, and the extent to which it contains small-scale subclusters. is the ratio of the normalized correlation length, , (i.e. the mean projected separation between stars, divided by the overall radius of the cluster), to the mean length, , of the segments of a minimal spanning tree (MST) joining all star positions: . In this paper, we attempt to adapt the correlation-length method to the characterization of gas clouds, with a view to comparing directly the structures of gas clouds and star clusters. We also compare the results of the correlation-length method with fractal dimensions estimated using the more familiar perimeter–area method whereby the lengths of closed contours are plotted against the areas they enclose, on a log–log plot. We find that the normalized correlation length, when modified to deal with pixellated grey-scale data, is a robust indicator of either central concentration or fractal subclustering of gas clouds, but cannot distinguish between the two types of structure. It is, however, extremely reliable, easy to implement and works accurately at all scales and over all dynamic ranges, even with poorly sampled data. It implicitly incorporates edge effects, so all the data in the complete cloud are used, and it therefore provides a useful method for comparing the structures of molecular clouds and star clusters. The normalized correlation length produces comparable results to the perimeter–area method when used on molecular cloud data. However, the perimeter–area method is unable to distinguish the degree of clustering in three-dimensional objects with fractal dimensions greater than 2.0. It also suffers from measurement noise and lack of objectivity, particularly if only a few contours are selected for analysis. It cannot be used to compare clouds with star clusters. It is not found possible to construct an MST algorithm which works reliably for grey-scale data and is immune to scaling problems. The previously reported parameter is therefore not useful when considering gas clouds. [ABSTRACT FROM AUTHOR]

Published
2006
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27. SPH simulation of the formation and evolution of protoplanetary disks

Author

Cartwright, Annabel

Subjects
Physics::Fluid Dynamics, QA75, Astrophysics::Earth and Planetary Astrophysics, Astrophysics::Galaxy Astrophysics
Abstract

The formation and evolution of protoplanetary disks is simulated by computer modelling, using the Smoothed Particle Hydrodynamics (SPH) method. The suitability of SPH for modelling disks is investigated, and problems are identified with the SPH implementation of Artificial Viscosity in disks with Keplerian velocity profiles. Analytical and experimental results reveal that the resultant viscous force for a simulated Keplerian disk is in the opposite direction to that produced by linear shear. Applying Artificial Viscosity only to approaching particles results in a radial force four times larger than the force in the direction of the orbit. The viscous force can change direction if temperature, and therefore sound speed, decreases. Techniques for activating Artificial Viscosity only when convergence is detected are found to fail in differentially rotating disks. Both the Balsara Switch and Time Dependent Artificial Viscosity use the SPH estimate of V v sph, which has a low freqency time varying component which is independent of h, and so cannot be removed by increasing the number of SPH particles. An alternative method, based on pattern recognition, is shown to reduce the viscous spread of a differentially rotating ring by an order of magnitude. We also identify problems associated with the gravitational field of disks. The use of an annulus to represent a portion of a much larger, continuous disk, may yield unrepresentative results. The edge effects can cause preferential accretion zones, where the Toomre Q parameter is not the same as it would be for the same region of an extended disk. SPH simulations of Protoplanetary disks produce condensations which do not persist long enough to collapse. The high tidal shearing forces in a Keplerian accretion disk disrupt the condensations before they accumulate enough mass to collapse. Including a more realistic treatment of the thermal physics, and reducing the effective shear viscosity, makes the situation worse

28. Phosphine gas in the cloud decks of Venus

Author

Greaves, Jane S., Richards, Anita M. S., Bains, William, Rimmer, Paul B., Sagawa, Hideo, Clements, David L., Seager, Sara, Petkowski, Janusz J., Sousa-Silva, Clara, Ranjan, Sukrit, Drabek-Maunder, Emily, Fraser, Helen J., Cartwright, Annabel, Mueller-Wodarg, Ingo, Zhan, Zhuchang, Friberg, Per, Coulson, Iain, Lee, E’lisa, Hoge, Jim, Greaves, Jane S., Richards, Anita M. S., Bains, William, Rimmer, Paul B., Sagawa, Hideo, Clements, David L., Seager, Sara, Petkowski, Janusz J., Sousa-Silva, Clara, Ranjan, Sukrit, Drabek-Maunder, Emily, Fraser, Helen J., Cartwright, Annabel, Mueller-Wodarg, Ingo, Zhan, Zhuchang, Friberg, Per, Coulson, Iain, Lee, E’lisa, and Hoge, Jim

29. Video assignments: A powerful way to introduce creativity and fun into your program – A physics education story

Author

Cartwright, Annabel, Lewis, Richard, Cartwright, Annabel, and Lewis, Richard

Abstract

The School of Physics and Astronomy has been setting video assessments for several years and they have been very popular with students. Now that video equipment and editing software is so easily available to students, and videos can now be uploaded to Learning Central, there are no technical barriers. This can enable students, particularly in technical subjects, to practice communication skills as well as show creativity and flair.

30. Video assignments: A powerful way to introduce creativity and fun into your program – A physics education story

Author

Cartwright, Annabel, Lewis, Richard, Cartwright, Annabel, and Lewis, Richard

Abstract

The School of Physics and Astronomy has been setting video assessments for several years and they have been very popular with students. Now that video equipment and editing software is so easily available to students, and videos can now be uploaded to Learning Central, there are no technical barriers. This can enable students, particularly in technical subjects, to practice communication skills as well as show creativity and flair.

31. SPH simulation of the formation and evolution of protoplanetary disks

Author

Cartwright, Annabel and Cartwright, Annabel

Abstract

The formation and evolution of protoplanetary disks is simulated by computer modelling, using the Smoothed Particle Hydrodynamics (SPH) method. The suitability of SPH for modelling disks is investigated, and problems are identified with the SPH implementation of Artificial Viscosity in disks with Keplerian velocity profiles. Analytical and experimental results reveal that the resultant viscous force for a simulated Keplerian disk is in the opposite direction to that produced by linear shear. Applying Artificial Viscosity only to approaching particles results in a radial force four times larger than the force in the direction of the orbit. The viscous force can change direction if temperature, and therefore sound speed, decreases. Techniques for activating Artificial Viscosity only when convergence is detected are found to fail in differentially rotating disks. Both the Balsara Switch and Time Dependent Artificial Viscosity use the SPH estimate of V v sph, which has a low freqency time varying component which is independent of h, and so cannot be removed by increasing the number of SPH particles. An alternative method, based on pattern recognition, is shown to reduce the viscous spread of a differentially rotating ring by an order of magnitude. We also identify problems associated with the gravitational field of disks. The use of an annulus to represent a portion of a much larger, continuous disk, may yield unrepresentative results. The edge effects can cause preferential accretion zones, where the Toomre Q parameter is not the same as it would be for the same region of an extended disk. SPH simulations of Protoplanetary disks produce condensations which do not persist long enough to collapse. The high tidal shearing forces in a Keplerian accretion disk disrupt the condensations before they accumulate enough mass to collapse. Including a more realistic treatment of the thermal physics, and reducing the effective shear viscosity, makes the situation worse

32. Highly accurate video coordinate generation for automatic 3-D trajectory calculation

Author

Walton, James S., Cartwright, Annabel, Morris, Julian R. W., Lyster, M., Walton, James S., Cartwright, Annabel, Morris, Julian R. W., and Lyster, M.

Abstract

Most TV-based motion analysis systems, including the original version of 1/ICON, produce 3D coordinates by combining pre-tracked 2D trajectories from each camera. The latest version of the system, VICON-VX, uses totally automatic 3D trajectory calculation using the Geometric Self Identification (GSI) technique. This is achieved by matching unsorted 2D image coordinates from all cameras, looking for intersecting marker 'rays', and matching intersections into 3D trajectories. Effective GSI, with low false-positive intersection rates is only possible with highly accurate 2D data, produced by stable, high-resolution coordinate generators, and incorporating appropriate compensation for lens distortions. Data capture software and hardware have been completely redesigned to achieve this accuracy, together with higher throughput rates and better resistance to errors. In addition, a new ADC facility has been incorporated to allow very high speed analog data acquisition, synchronised with video measurements.

33. SPH simulation of the formation and evolution of protoplanetary disks

Author

Cartwright, Annabel and Cartwright, Annabel

Abstract

The formation and evolution of protoplanetary disks is simulated by computer modelling, using the Smoothed Particle Hydrodynamics (SPH) method. The suitability of SPH for modelling disks is investigated, and problems are identified with the SPH implementation of Artificial Viscosity in disks with Keplerian velocity profiles. Analytical and experimental results reveal that the resultant viscous force for a simulated Keplerian disk is in the opposite direction to that produced by linear shear. Applying Artificial Viscosity only to approaching particles results in a radial force four times larger than the force in the direction of the orbit. The viscous force can change direction if temperature, and therefore sound speed, decreases. Techniques for activating Artificial Viscosity only when convergence is detected are found to fail in differentially rotating disks. Both the Balsara Switch and Time Dependent Artificial Viscosity use the SPH estimate of V v sph, which has a low freqency time varying component which is independent of h, and so cannot be removed by increasing the number of SPH particles. An alternative method, based on pattern recognition, is shown to reduce the viscous spread of a differentially rotating ring by an order of magnitude. We also identify problems associated with the gravitational field of disks. The use of an annulus to represent a portion of a much larger, continuous disk, may yield unrepresentative results. The edge effects can cause preferential accretion zones, where the Toomre Q parameter is not the same as it would be for the same region of an extended disk. SPH simulations of Protoplanetary disks produce condensations which do not persist long enough to collapse. The high tidal shearing forces in a Keplerian accretion disk disrupt the condensations before they accumulate enough mass to collapse. Including a more realistic treatment of the thermal physics, and reducing the effective shear viscosity, makes the situation worse

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