523 research outputs found
Fractal analysis of CE CT lung tumours images
AIM The fractal dimension (FD) of a structure provides a measure of its complexity. This pilot study aims to determine FD values for lung cancers visualised on Computed Tomography (CT) and to assess the potential for tumour FD measurements to provide an index of tumour aggression. METHOD Pre-and post-contrast CT images of the thorax acquired from 15 patients with lung cancers of greater than 10mm were transformed to fractal dimension images using a box-counting algorithm at various scales. A region of interest (ROI) was determined covering tumour locations, which were more apparent on FD images as compared to images before processing. The average tumour FD (FDavg) was computed and compared with the intensity average before FD processing. FD values were correlated with 2 markers of tumour aggression: tumour stage and tumour uptake of fluorodeoxyglucose (FDG) as determined by Positron Emission Tomography. RESULTS For pre-contrast images, the tumour FDavg correlated with tumour stage (r = 0.537, p = 0.0387) and FDG uptake (r= 0.64, p< 0.001). FDavg decreased following contrast enhancement for most tumours. CONCLUSION Fractal analysis of CT images of lung tumours could potentially provide additional information about likely tumour aggression and so impact on clinical management decisions and choice of treatment
Roche lobe effects on the atmospheric loss of "Hot Jupiters"
Observational evidence of a hydrodynamically evaporating upper atmosphere of
HD209458b (Vidal-Madjar et al. 2003; 2004) and recent theoretical studies on
evaporation scenarios of ``Hot Jupiters'' in orbits around solar-like stars
with the age of the Sun indicate that the upper atmospheres of short-periodic
exoplanets experience hydrodynamic blow-off conditions resulting in loss rates
of the order of about 10^10 - 10^12 g s^-1 (Lammer et al. 2003; Yelle 2004;
Baraffe et al. 2004; Lecavlier des Etangs et al. 2004; Jaritz et al. 2005, Tian
et al. 2005; Penz et al. 2007). By studying the effect of the Roche lobe on the
atmospheric loss from short-periodic gas giants we found, that the effect of
the Roche lobe can enhance the hydrodynamic evaporation from HD209458b by about
2 and from OGLE-TR-56b by about 2.5 times. For similar exoplanets which are
closer to their host star than OGLE-TR-56b, the enhancement of the mass loss
can be even larger. Moreover, we show that the effect of the Roche lobe raises
the possibility that ``Hot Jupiters'' can reach blow-off conditions at
temperatures which are less than expected (< 10000 K) due to the stellar X-ray
and EUV (XUV) heating.Comment: 4 pages, 2 figures, submitted to A&
Estimation of the XUV radiation onto close planets and their evaporation
Context: The current distribution of planet mass vs. incident stellar X-ray
flux supports the idea that photoevaporation of the atmosphere may take place
in close-in planets. Integrated effects have to be accounted for. A proper
calculation of the mass loss rate due to photoevaporation requires to estimate
the total irradiation from the whole XUV range. Aims: The purpose of this paper
is to extend the analysis of the photoevaporation in planetary atmospheres from
the accessible X-rays to the mostly unobserved EUV range by using the coronal
models of stars to calculate the EUV contribution to the stellar spectra. The
mass evolution of planets can be traced assuming that thermal losses dominate
the mass loss of their atmospheres. Methods: We determine coronal models for 82
stars with exoplanets that have X-ray observations available. Then a synthetic
spectrum is produced for the whole XUV range (~1-912 {\AA}). The determination
of the EUV stellar flux, calibrated with real EUV data, allows us to calculate
the accumulated effects of the XUV irradiation on the planet atmosphere with
time, as well as the mass evolution for planets with known density. Results: We
calibrate for the first time a relation of the EUV luminosity with stellar age
valid for late-type stars. In a sample of 109 exoplanets, few planets with
masses larger than ~1.5 Mj receive high XUV flux, suggesting that intense
photoevaporation takes place in a short period of time, as previously found in
X-rays. The scenario is also consistent with the observed distribution of
planet masses with density. The accumulated effects of photoevaporation over
time indicate that HD 209458b may have lost 0.2 Mj since an age of 20 Myr.
Conclusions: Coronal radiation produces rapid photoevaporation of the
atmospheres of planets close to young late-type stars. More complex models are
needed to explain fully the observations.Comment: Accepted by A&A. 10 pages, 8 figures, 7 Tables (2 online). Additional
online material includes 7 pages, 6 figures and 6 tables, all include
Hot-Jupiters and hot-Neptunes: a common origin?
We compare evolutionary models for close-in exoplanets coupling irradiation
and evaporation due respectively to the thermal and high energy flux of the
parent star with observations of recently discovered new transiting planets.
The models provide an overall good agreement with observations, although at the
very limit of the quoted error bars of OGLE-TR-10, depending on its age. Using
the same general theory, we show that the three recently detected hot-Neptune
planets (GJ436, Cancri, Ara) may originate from more massive gas
giants which have undergone significant evaporation. We thus suggest that
hot-Neptunes and hot-Jupiters may share the same origin and evolution history.
Our scenario provides testable predictions in terms of the mass-radius
relationships of these hot-Neptunes.Comment: 5 pages, 2 figures, accepted in A&A Lette
Planetary evaporation by UV & X-ray radiation: basic hydrodynamics
We consider the evaporation of close in planets by the star's intrinsic EUV
and X-ray radiation. We calculate evaporation rates by solving the
hydrodynamical problem for planetary evaporation including heating from both
X-ray and EUV radiation. We show that most close-in planets (a<0.1 AU) are
evaporating hydrodynamically, with the evaporation occurring in two distinct
regimes: X-ray driven, in which the X-ray heated flow contains a sonic point,
and EUV driven, in which the X-ray region is entirely sub-sonic. The mass-loss
rates scale as L_X/a^2 for X-ray driven evaporation, and as Phi_*^{1/2}/a for
EUV driven evaporation at early times, with mass-loss rates of order
10e10-10e14 g/s. No exact scaling exists for the mass-loss rate with planet
mass and planet radius, however, in general evaporation proceeds more rapidly
for planets with lower densities and higher masses. Furthermore, we find that
in general the transition from X-ray driven to EUV driven evaporation occurs at
lower X-ray luminosities for planets closer to their parent stars and for
planets with lower densities.
Coupling our evaporation models to the evolution of the high energy radiation
- which falls with time - we are able to follow the evolution of evaporating
planets. We find that most planets start off evaporating in the X-ray driven
regime, but switch to EUV driven once the X-ray luminosity falls below a
critical value. The evolution models suggest that while `hot Jupiters' are
evaporating, they are not evaporating at a rate sufficient to remove the entire
gaseous envelope on Gyr time-scales. However, we do find that close in Neptune
mass planets are more susceptible to complete evaporation of their envelopes.
Thus we conclude that planetary evaporation is more important for lower mass
planets, particularly those in the `hot Neptune'/`super Earth' regime.Comment: 18 Pages, Accepted for publication in MNRA
Service evaluation of the GOALS family-based childhood obesity treatment intervention during the first 3â years of implementation.
Objectives: To evaluate the impact of the GOALS (Getting Our Active Lifestyles Started) family-based childhood obesity treatment intervention during the first 3 years of implementation. Design: Single-group repeated measures with qualitative questionnaires. Setting: Community venues in a socioeconomically deprived, urban location in the North-West of England. Participants: 70 overweight or obese children (mean age 10.5 years, 46% boys) and their parents/carers who completed GOALS between September 2006 and March 2009. Interventions: GOALS was a childhood obesity treatment intervention that drew on social cognitive theory to promote whole family lifestyle change. Sessions covered physical activity (PA), diet and behaviour change over 18 2 h weekly group sessions (lasting approximately 6 months). A Template for Intervention Description and Replication (TIDieR) checklist of intervention components is provided. Primary and secondary outcome measures: The primary outcome measure was child body mass index (BMI) z-score, collected at baseline, post-intervention and 12 months. Secondary outcome measures were child selfperceptions, parent/carer BMI and qualitative changes in family diet and PA (parent/carer questionnaire). Results: Child BMI z-score reduced by 0.07 from baseline to post-intervention (p<0.001) and was maintained at 12 months (p<0.05). There was no change in parent/carer BMI or child self-perceptions, other than an increase in perceived social acceptance from baseline to post-intervention (p<0.05). Parents/carers reported positive changes to family PA and dietary behaviours after completing GOALS. Conclusions: GOALS completion was associated with small improvements in child BMI z-score and improved family PA and dietary behaviours. Several intervention modifications were necessary during the implementation period and it is suggested childhood obesity treatment interventions need time to embed before a definitive evaluation is conducted. Researchers are urged to use the TIDieR checklist to ensure transparent reporting of interventions and facilitate the translation of evidence to practice
Risky Business: Motivations for Markets in Programmable Networks
Abstract. We believe that the problems of safety, security and resource usage combine to make it unlikely that programmable networks will ever be viable without mechanisms to transfer risk from the platform provider to the user and the programmer. However, we have well established mechanisms for managing risk -markets. In this paper we argue for the establishment of markets to manage the risk in running a piece of software and to ensure that the risk is reflected on all the stakeholders. We describe a strawman architecture for third party computation in the programmable network. Within this architecture, we identify two major novel features:-Dynamic price setting, and a reputation service. We investigate the feasibility of these features and provide evidence that a practical system can indeed be built. Our contributions are in the argument for markets providing a risk management mechanism for programmable networks, the development of an economic model showing incentives for developing better software, and in the first analysis of a real transaction graph for reputation systems from an Internet commerce site
A scenario of planet erosion by coronal radiation
Context: According to theory, high-energy emission from the coronae of cool
stars can severely erode the atmospheres of orbiting planets. No observational
tests of the long term effects of erosion have yet been made. Aims: To analyze
the current distribution of planetary mass with X-ray irradiation of the
atmospheres in order to make an observational assessment of the effects of
erosion by coronal radiation. Methods: We study a large sample of
planet-hosting stars with XMM-Newton, Chandra and ROSAT; make a careful
identification of X-ray counterparts; and fit their spectra to make accurately
measurements of the stellar X-ray flux. Results: The distribution of the
planetary masses with X-ray flux suggests that erosion has taken place: most
surviving massive planets, (M_p sin i >1.5 M_J), have been exposed to lower
accumulated irradiation. Heavy erosion during the initial stages of stellar
evolution is followed by a phase of much weaker erosion. A line dividing these
two phases could be present, showing a strong dependence on planet mass.
Although a larger sample will be required to establish a well-defined erosion
line, the distribution found is very suggestive. Conclusions: The distribution
of planetary mass with X-ray flux is consistent with a scenario in which planet
atmospheres have suffered the effects of erosion by coronal X-ray and EUV
emission. The erosion line is an observational constraint to models of
atmospheric erosion.Comment: A&A 511, L8 (2010). 4 pages, 3 figures, 1 online table (included).
Language edited; corrected a wrong unit conversion (g/s -> M_J/Gyr);
corrected values in column 12 of Table 1 (slightly underestimated in first
version), and Figure 2 updated accordingl
Solar Particle Acceleration at Reconnecting 3D Null Points
Context: The strong electric fields associated with magnetic reconnection in
solar flares are a plausible mechanism to accelerate populations of high
energy, non-thermal particles. One such reconnection scenario occurs at a 3D
magnetic null point, where global plasma flows give rise to strong currents in
the spine axis or fan plane. Aims: To understand the mechanism of charged
particle energy gain in both the external drift region and the diffusion region
associated with 3D magnetic reconnection. In doing so we evaluate the
efficiency of resistive spine and fan models for particle acceleration, and
find possible observables for each. Method: We use a full orbit test particle
approach to study proton trajectories within electromagnetic fields that are
exact solutions to the steady and incompressible magnetohydrodynamic equations.
We study single particle trajectories and find energy spectra from many
particle simulations. The scaling properties of the accelerated particles with
respect to field and plasma parameters is investigated. Results: For fan
reconnection, strong non-uniform electric drift streamlines can accelerate the
bulk of the test particles. The highest energy gain is for particles that enter
the current sheet, where an increasing "guide field" stabilises particles
against ejection. The energy is only limited by the total electric potential
energy difference across the fan current sheet. The spine model has both slow
external electric drift speed and weak energy gain for particles reaching the
current sheet. Conclusions: The electromagnetic fields of fan reconnection can
accelerate protons to the high energies observed in solar flares, gaining up to
0.1 GeV for anomalous values of resistivity. However, the spine model, which
gave a harder energy spectrum in the ideal case, is not an efficient
accelerator after pressure constraints in the resistive model are included.Comment: 15 pages, 14 figures. Submitted to Astronomy and Astrophysic
Mass-loss rates for transiting exoplanets
Exoplanets at small orbital distances from their host stars are submitted to
intense levels of energetic radiations, X-rays and extreme ultraviolet (EUV).
Depending on the masses and densities of the planets and on the atmospheric
heating efficiencies, the stellar energetic inputs can lead to atmospheric mass
loss. These evaporation processes are observable in the ultraviolet during
planetary transits. The aim of the present work is to quantify the mass-loss
rates (dm/dt), heating efficiencies (eta), and lifetimes for the whole sample
of transiting exoplanets, now including hot jupiters, hot neptunes, and hot
super-earths. The mass-loss rates and lifetimes are estimated from an "energy
diagram" for exoplanets, which compares the planet gravitational potential
energy to the stellar X/EUV energy deposited in the atmosphere. We estimate the
mass-loss rates of all detected transiting planets to be within 10^6 to 10^13
g/s for various conservative assumptions. High heating efficiencies would imply
that hot exoplanets such the gas giants WASP-12b and WASP-17b could be
completely evaporated within 1 Gyr. We further show that the heating efficiency
can be constrained when dm/dt is inferred from observations and the stellar
X/EUV luminosity is known. This leads us to suggest that eta ~ 100% in the
atmosphere of the hot jupiter HD209458b, while it could be lower for HD189733b.
Simultaneous observations of transits in the ultraviolet and X-rays are
necessary to further constrain the exospheric properties of exoplanets.Comment: 10 pages, 5 figures, 2 tables. Accepted for publication in Astronomy
& Astrophysic
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