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Search Results: 1 - 10 of 331160 matches for " Alex S. Greaves "
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Maximally reliable spatial filtering of steady state visual evoked potentials
Jacek P. Dmochowski,Alex S. Greaves,Anthony M. Norcia
Quantitative Biology , 2014,
Abstract: Due to their high signal-to-noise ratio (SNR) and robustness to artifacts, steady state visual evoked potentials (SSVEPs) are a popular technique for studying neural processing in the human visual system. SSVEPs are conventionally analyzed at individual electrodes or linear combinations of electrodes which maximize some variant of the SNR. Here we exploit the fundamental assumption of evoked responses -- reproducibility across trials -- to develop a technique that extracts a small number of high SNR, maximally reliable SSVEP components. This novel spatial filtering method operates on an array of Fourier coefficients and projects the data into a low-dimensional space in which the trial-to-trial spectral covariance is maximized. When applied to two sample data sets, the resulting technique recovers physiologically plausible components (i.e., the recovered topographies match the lead fields of the underlying sources) while drastically reducing the dimensionality of the data (i.e., more than 90% of the trial-to-trial reliability is captured in the first four components). Moreover, the proposed technique achieves a higher SNR than that of the single-best electrode or the Principal Components. We provide a freely-available MATLAB implementation of the proposed technique, herein termed "Reliable Components Analysis".
Predicting the incidence of planets and debris discs as a function of stellar mass
J. S. Greaves
Physics , 2010, DOI: 10.1111/j.1745-3933.2010.00943.x
Abstract: The mass of solids in a young circumstellar disc may be the key factor in its efficiency in building planetesimals and planetary cores, and dust observed around young T Tauri and Herbig Ae stars can be used as a proxy for this initial solid content. The dust-mass distributions are taken from recent millimetre-wavelength data and fitted using survival analysis to take into account upper limits, and threshold disc-masses for building planets and belts of comets are estimated. Amongst A-stars, 20% gas giant and 55% debris disc systems are predicted, in good agreement with observations. For M-stars, the predicted and observed planet-frequencies agree at ~2-3%, and this low incidence is explained by a lack of massive discs. However, debris is predicted around approx. 14% of M-stars, while only ~2% such systems have so far been found. This suggests that deeper searches such as with Herschel and SCUBA-2 may find a cold disc population previously missed around these low-luminosity stars. Also, an estimate of the efficiency of building millimetre-detected dust into planetary cores suggests that about a third of M-stars could host an Earth-mass planet -- but as the dust is spread over large disc areas, such planets may orbit far from the star.
Debris discs and comet populations around Sun-like stars: the Solar System in context
J. S. Greaves,M. C. Wyatt
Physics , 2010, DOI: 10.1111/j.1365-2966.2010.16415.x
Abstract: Numerous nearby FGK dwarfs possess discs of debris generated by collisions among comets. Here we fit the levels of dusty excess observed by Spitzer at 70$\umu$m and show that they form a rather smooth distribution. Taking into account the transition of the dust removal process from collisional to Poynting-Robertson drag, all the stars may be empirically fitted by a single population with many low-excess members. Within this ensemble, the Kuiper Belt is inferred to be such a low-dust example, among the last 10% of stars, with a small cometary population. Analogue systems hosting gas giant planets and a modest comet belt should occur for only a few per cent of Sun-like stars, and so terrestrial planets with a comparable cometary impact rate to the Earth may be uncommon. The nearest such analogue system presently known is HD154345 at 18pc, but accounting for survey completeness, a closer example should lie at around 10pc.
Have proto-planetary discs formed planets?
J. S. Greaves,W. K. M. Rice
Physics , 2010, DOI: 10.1111/j.1365-2966.2010.17043.x
Abstract: It has recently been noted that many discs around T Tauri stars appear to comprise only a few Jupiter-masses of gas and dust. Using millimetre surveys of discs within six local star-formation regions, we confirm this result, and find that only a few percent of young stars have enough circumstellar material to build gas giant planets, in standard core accretion models. Since the frequency of observed exo-planets is greater than this, there is a `missing mass' problem. As alternatives to simply adjusting the conversion of dust-flux to disc mass, we investigate three other classes of solution. Migration of planets could hypothetically sweep up the disc mass reservoir more efficiently, but trends in multi-planet systems do not support such a model, and theoretical models suggest that the gas accretion timescale is too short for migration to sweep the disc. Enhanced inner-disc mass reservoirs are possible, agreeing with predictions of disc evolution through self-gravity, but not adding to millimetre dust-flux as the inner disc is optically thick. Finally, the incidence of massive discs is shown to be higher at the {\it proto}stellar stages, Classes 0 and I, where discs substantial enough to form planets via core accretion are abundant enough to match the frequency of exo-planets. Gravitational instability may also operate in the Class 0 epoch, where half the objects have potentially unstable discs of $\ga$30 % of the stellar mass. However, recent calculations indicate that forming gas giants inside 50 AU by instability is unlikely, even in such massive discs. Overall, the results presented suggest that the canonically 'proto-planetary' discs of Class II T Tauri stars {\bf have globally low masses in dust observable at millimetre wavelengths, and conversion to larger bodies (anywhere from small rocks up to planetary cores) must already have occurred.}
Discovery of carbon monoxide in the upper atmosphere of Pluto
J. S. Greaves,Ch. Helling,P. Friberg
Physics , 2011, DOI: 10.1111/j.1745-3933.2011.01052.x
Abstract: Pluto's icy surface has changed colour and its atmosphere has swelled since its last closest approach to the Sun in 1989. The thin atmosphere is produced by evaporating ices, and so can also change rapidly, and in particular carbon monoxide should be present as an active thermostat. Here we report the discovery of gaseous CO via the 1.3mm wavelength J=2-1 rotational transition, and find that the line-centre signal is more than twice as bright as a tentative result obtained by Bockelee-Morvan et al. in 2000. Greater surface-ice evaporation over the last decade could explain this, or increased pressure could have caused the atmosphere to expand. The gas must be cold, with a narrow line-width consistent with temperatures around 50 K, as predicted for the very high atmosphere, and the line brightness implies that CO molecules extend up to approximately 3 Pluto radii above the surface. The upper atmosphere must have changed markedly over only a decade since the prior search, and more alterations could occur by the arrival of the New Horizons mission in 2015.
Measurement of the magnetic field direction in the NGC2024-FIR5 protostellar outflow
J. S. Greaves,W. S. Holland,D. Ward-Thompson
Physics , 2000, DOI: 10.1086/318069
Abstract: Molecular outflows from young protostars are widely believed to be collimated by magnetic fields, but there has been little observational evidence to support this hypothesis. Using the new technique of millimetre-wavelength spectro-polarimetry, we demonstrate the existence of a magnetic field in the NGC2024-FIR5 outflow lobe. The 1.3mm J=2-1 transition of carbon monoxide (CO) is polarized at a level of approximately 1%, in a direction within 10-15 degrees of the outflow axis. This agrees with theoretical models where the magnetic field channels the outflowing gas, and shows that the process can be effective as far as 0.1pc from the protostar.
Fiber intake Inconsistently Alters Gut Hormone Levels in Humans Following Acute or Chronic Intake
Abby S. Klosterbuer,Kathryn A. Greaves,Joanne Slavin
Journal of Food Research (JFR) , 2012, DOI: 10.5539/jfr.v1n2p255
Abstract: Diet composition affects the release of gut hormones involved in the regulation of appetite and energy intake. While some research suggests high fiber foods cause greater satiety than low fiber foods, few studies have measured gut hormone levels as a mechanism by which fiber may influence appetite. A review of the literature was conducted to better understand the effect of fiber on gut hormone concentrations in humans, which specific focus on peptide YY, glucagon-like peptide-1, cholecystokinin, and ghrelin. Considerable variation was found in study design, population, fiber type and level. Few studies reported a significant effect of fiber on gut hormone levels, and data suggest caloric load may have a more significant influence on gut hormone release. While it is possible that circulating gut hormones are not the mechanism by which fibers influence satiety, it is also possible that variability in study design prevents definitive conclusions around this relationship.
Debris discs around nearby Solar analogues
J. S. Greaves,M. C. Wyatt,G. Bryden
Physics , 2009, DOI: 10.1111/j.1365-2966.2009.15048.x
Abstract: An unbiased search for debris discs around nearby Sun-like stars is reported. Thirteen G-dwarfs at 12-15 parsecs distance were searched at 850 $\umu$m wavelength, and a disc is confirmed around HD 30495. The estimated dust mass is 0.008 M$_{\oplus}$ with a net limit $\la 0.0025$ M$_{\oplus}$ for the average disc of the other stars. The results suggest there is not a large missed population of substantial cold discs around Sun-like stars -- HD 30495 is a bright rather than unusually cool disc, and may belong to a few hundred Myr-old population of greater dust luminosity. The far-infared and millimetre survey data for Sun-like stars are well fitted by either steady state or stirred models, provided that typical comet belts are comparable in size to that in the Solar System.
Steady-state evolution of debris disks around solar-type stars
N. Kains,M. C. Wyatt,J. S. Greaves
Physics , 2011,
Abstract: We present an analysis of debris disk data around Solar-type stars (spectral types F0-K5) using the steady-state analytical model of Wyatt et al. (2007). Models are fitted to published data from the FEPS (Meyer et al. 2006) project and various GTO programs obtained with MIPS on the Spitzer Space Telescope at 24 micron and 70 micron, and compared to a previously published analysis of debris disks around A stars using the same evolutionary model. We find that the model reproduces most features found in the data sets, noting that the model disk parameters for solar-type stars are different to those of A stars. Although this could mean that disks around Solar-type stars have different properties from their counterparts around earlier-type stars, it is also possible that the properties of disks around stars of different spectral types appear more different than they are because the blackbody disk radius underestimates the true disk radius by a factor $X_r$ which varies with spectral type. We use results from realistic grain modelling to quantify this effect for solar-type stars and for A stars. Our results imply that planetesimals around solar-type stars are on average larger than around A stars by a factor of a few but that the mass of the disks are lower for disks around FGK stars, as expected. We also suggest that discrepancies between the evolutionary timescales of 24 micron statistics predicted by our model and that observed in previous surveys could be explained by the presence of two-component disks in the samples of those surveys, or by transient events being responsible for the 24 micron emission of cold disks beyond a few Myr. Further study of the prevalence of two component disks, and of constraints on $X_r$, and increasing the size of the sample of detected disks, are important for making progress on interpreting the evolution of disks around solar-type stars.
SCUBA Observations of Dust around Lindroos Stars: Evidence for a Substantial Submillimetre Disc Population
M. C. Wyatt,W. R. F. Dent,J. S. Greaves
Physics , 2003, DOI: 10.1046/j.1365-8711.2003.06595.x
Abstract: We have observed 22 Lindroos stars with SCUBA on the JCMT to search for evidence of dust discs. Stars in this sample are the less massive companions of B-type primaries and have ages of 10-170Myr. Dust was detected around three of these stars (HD74067, HD112412 and HD99803B). The emission around HD74067 is centrally peaked and is approximately symmetrically distributed out to ~70". This emission either arises from a two component disc, one circumstellar and the other circumbinary with dust masses of 0.3 and >27Mearth respectively, or an unrelated background object. The other two detections we attribute to circumsecondary discs with masses of 0.04 and 0.3Mearth; a circumprimary disc is also present around HD112413 with a similar mass to that around the companion HD112412. Cross-correlation of our sample with the IRAS catalogs only showed evidence for dust emission at 25um and 60um toward one star (HD1438); none of the sub-mm detections were evident in the far-IR data implying that these discs are cold (<40K assuming beta=1). Our sub-mm detections are some of the first of dust discs surrounding evolved stars that were not detected by IRAS or ISO and imply that 9-14% of stars could harbour previously undetected dust discs that await discovery in unbiased sub-mm surveys. If these discs are protoplanetary remnants, rather than secondary debris discs, dust lifetime arguments show that they must be devoid of small <0.1mm grains. Thus it may be possible to determine the origin of these discs from their spectral energy distributions. The low dust masses for this sample support the picture that protoplanetary dust discs are depleted to the levels of the brightest debris discs (~1Mearth) within 10Myr.
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