On the ionisation fraction in protoplanetary disks III. The effect of X-ray flares on gas-phase chemistry

Context. Recent observations of the X-ray emission from T Tauri stars in the Orion nebula have shown that they undergo frequent outbursts in their X-ray luminosity. These X-ray flares are characterised by increases in luminosity by two orders of magnitude, a typical duration of less than one day, and a significant hardening of the X-ray spectrum. Aims. It is unknown what effect these X-ray flares will have on the ionisation fraction and dead-zone structure in protoplanetary disks. We present the results of calculations designed to address this question. Methods. We have performed calculations of the ionisation fraction in a standard $\alpha$-disk model using two different chemical reaction networks. We include in our models ionisation due to X-rays from the central star, and calculate the time-dependent ionisation fraction and dead--zone structure for the inner 10 AU of a protoplanetary disk model. Results. We find that the disk response to X-ray flares depends on whether the plasma temperature increases during flares and/or whether heavy metals (such as magnesium) are present in the gas phase. Under favourable conditions the outer disk dead--zone can disappear altogether,and the dead-zone located between 0.5 < R < 2 AU can disappear and reappear in phase with the X-ray luminosity. Conclusions. X-ray flares can have a significant effect on the dead-zone structure in protoplanetary disks. Caution is required in interpreting this result as the duration of X-ray bursts is considerably shorter than the growth time of MHD turbulence due to the magnetorotational instability.Comment: 12 pages, 8 figures, accepted by A &

Grain charging in protoplanetary discs

Recent work identified a growth barrier for dust coagulation that originates in the electric repulsion between colliding particles. Depending on its charge state, dust material may have the potential to control key processes towards planet formation such as MHD (magnetohydrodynamic) turbulence and grain growth which are coupled in a two-way process. We quantify the grain charging at different stages of disc evolution and differentiate between two very extreme cases: compact spherical grains and aggregates with fractal dimension D_f = 2. Applying a simple chemical network that accounts for collisional charging of grains, we provide a semi-analytical solution. This allowed us to calculate the equilibrium population of grain charges and the ionisation fraction efficiently. The grain charging was evaluated for different dynamical environments ranging from static to non-stationary disc configurations. The results show that the adsorption/desorption of neutral gas-phase heavy metals, such as magnesium, effects the charging state of grains. The greater the difference between the thermal velocities of the metal and the dominant molecular ion, the greater the change in the mean grain charge. Agglomerates have more negative excess charge on average than compact spherical particles of the same mass. The rise in the mean grain charge is proportional to N**(1/6) in the ion-dust limit. We find that grain charging in a non-stationary disc environment is expected to lead to similar results. The results indicate that the dust growth and settling in regions where the dust growth is limited by the so-called "electro-static barrier" do not prevent the dust material from remaining the dominant charge carrier.Comment: 18 pages, 10 figures, accepted for publication in Astronomy and Astrophysic

The Beam Conditions Monitor of the LHCb Experiment

The LHCb experiment at the European Organization for Nuclear Research (CERN) is dedicated to precision measurements of CP violation and rare decays of B hadrons. Its most sensitive components are protected by means of a Beam Conditions Monitor (BCM), based on polycrystalline CVD diamond sensors. Its configuration, operation and decision logics to issue or remove the beam permit signal for the Large Hadron Collider (LHC) are described in this paper.Comment: Index Terms: Accelerator measurement systems, CVD, Diamond, Radiation detector

On the Ionisation Fraction in Protoplanetary Disks II: The Effect of Turbulent Mixing on Gas--phase Chemistry

We calculate the ionisation fraction in protostellar disk models using two different gas-phase chemical networks, and examine the effect of turbulent mixing by modelling the diffusion of chemical species vertically through the disk. The aim is to determine in which regions of the disk gas can couple to a magnetic field and sustain MHD turbulence. We find that the effect of diffusion depends crucially on the elemental abundance of heavy metals (magnesium) included in the chemical model. In the absence of heavy metals, diffusion has essentially no effect on the ionisation structure of the disks, as the recombination time scale is much shorter than the turbulent diffusion time scale. When metals are included with an elemental abundance above a threshold value, the diffusion can dramatically reduce the size of the magnetically decoupled region, or even remove it altogther. For a complex chemistry the elemental abundance of magnesium required to remove the dead zone is 10(-10) - 10(-8). We also find that diffusion can modify the reaction pathways, giving rise to dominant species when diffusion is switched on that are minor species when diffusion is absent. This suggests that there may be chemical signatures of diffusive mixing that could be used to indirectly detect turbulent activity in protoplanetary disks. We find examples of models in which the dead zone in the outer disk region is rendered deeper when diffusion is switched on. Overall these results suggest that global MHD turbulence in protoplanetary disks may be self-sustaining under favourable circumstances, as turbulent mixing can help maintain the ionisation fraction above that necessary to ensure good coupling between the gas and magnetic field.Comment: 11 pages, 7 figures; accepted for publication in A &

On the Ionisation Fraction in Protoplanetary Disks I: Comparing Different Reaction Networks

We calculate the ionisation fraction in protostellar disk models using a number of different chemical reaction networks, including gas-phase and gas-grain reaction schemes. The disk models we consider are conventional alpha-disks, which include viscous heating and radiative cooling. The primary source of ionisation is assumed to be X-ray irradiation from the central star. We consider a number of gas-phase chemical networks. In general we find that the simple models predict higher fractional ionisation levels and more extensive active zones than the more complex models. When heavy metal atoms are included the simple models predict that the disk is magnetically active throughout. The complex models predict that extensive regions of the disk remain magnetically uncoupled even with a fractional abundance of magnesium of 10(-8). The addition of submicron sized grains with a concentration of 10(-12) causes the size of the dead zone to increase dramatically for all kinetic models considered. We find that the simple and complex gas-grain reaction schemes agree on the size and structure of the resulting dead zone. We examine the effects of depleting the concentration of small grains as a crude means of modeling the growth of grains during planet formation. We find that a depletion factor of 10(-4) causes the gas-grain chemistry to converge to the gas-phase chemistry when heavy metals are absent. 10(-8) is required when magnesium is included. This suggests that efficient grain growth and settling will be required in protoplanetary disks, before a substantial fraction of the disk mass in the planet forming zone between 1 - 10 AU becomes magnetically active and turbulent.Comment: 21 pages, 23 figures, accepted for publication in A & A Includes correction to our implementation of the Umebayashi-Nakano reaction networ

Composition of LHB Comets and Their Influence on the Early Earth Atmosphere Composition

Two main processes were responsible for the composition of this atmosphere: chemical evolution of the volatile fraction of the accretion material forming the planet and the delivery of gasses to the planetary surface by impactors during the late heavy bombardment (LHB). The amount and composition of the volatile fraction influences the outgassing of the Earth mantle during the last planetary formation period. A very weakened form of outgassing activity can still be observed today by examining the composition of volcanic gasses. An enlightenment of the second process is based on the sparse records of the LHB impactors resulting from the composition of meteorites, observed cometary comas, and the impact material found on the Moon. However, for an assessment of the influence of the outgassing on the one hand and the LHB event on the other, one has to supplement the observations with numerical simulations of the formation of volatiles and their incorporation into the accretion material which is the precursors of planetary matter, comets and asteroids. These simulations are performed with a combined hydrodynamic-chemical model of the solar nebula (SN). We calculate the chemical composition of the gas and dust phase of the SN. From these data, we draw conclusions on the upper limits of the water content and the amount of carbon and nitrogen rich volatiles incorporated later into the accretion material. Knowing these limits we determine the portion of major gas compounds delivered during the LHB and compare it with the related quantities of the outgassed species

Turbulent transport and its effect on the dead zone in protoplanetary discs

Protostellar accretion discs have cool, dense midplanes where externally originating ionisation sources such as X-rays or cosmic rays are unable to penetrate. This suggests that for a wide range of radii, MHD turbulence can only be sustained in the surface layers where the ionisation fraction is sufficiently high. A dead zone is expected to exist near the midplane, such that active accretion only occurs near the upper and lower disc surfaces. Recent work, however, suggests that under suitable conditions the dead zone may be enlivened by turbulent transport of ions from the surface layers into the dense interior. In this paper we present a suite of simulations that examine where, and under which conditions, a dead zone can be enlivened by turbulent mixing. We use three-dimensional, multifluid shearing box MHD simulations, which include vertical stratification, ionisation chemistry, ohmic resistivity, and ionisation due to X-rays from the central protostar. We compare the results of the MHD simulations with a simple reaction-diffusion model. The simulations show that in the absence of gas-phase heavy metals, such as magnesium, turbulent mixing has essentially no effect on the dead zone. The addition of a relatively low abundance of magnesium, however, increases the recombination time and allows turbulent mixing of ions to enliven the dead zone completely beyond a distance of 5 AU from the central star, for our particular disc model. During the late stages of protoplanetary disc evolution, when small grains have been depleted and the disc surface density has decreased below its high initial value, the structure of the dead zone may be significantly altered by the action of turbulent transport.Comment: 20 pages, 11 figures, accepted for publication in A&A, high resolution pdf available at http://www.maths.qmul.ac.uk/~rpn/preprints/index.htm

Stable isotopic evidence for nutrient rejuvenation and long-term resilience on Tikopia Island (Southeast Solomon Islands)

Tikopia Island, a small and relatively isolated Polynesian Outlier in the Southeast Solomon Islands, supports a remarkably dense human population with minimal external support. Examining long-term trends in human land use on Tikopia through archaeological datasets spanning nearly 3000 years presents an opportunity to investigate pathways to long-term sustainability in a tropical island setting. Here, we trace nutrient dynamics across Tikopia’s three pre-European contact phases (Kiki, Sinapupu, Tuakamali) via stable carbon and nitrogen isotope analysis of commensal Pacific rat (Rattus exulans) and domestic pig (Sus scrofa) bone and tooth dentine collagen. Our results show a decline in δ15N values from the Kiki (c. 800 BC-AD 100) to Sinapupu (c. AD 100–1200) phases, consistent with long-term commensal isotope trends observed on other Polynesian islands. However, increased δ15N coupled with lower δ13C values in the Tuakamali Phase (c. AD 1200–1800) point to a later nutrient rejuvenation, likely tied to dramatic transformations in agriculture and land use at the Sinapupu-Tuakamali transition. This study offers new, quantifiable evidence for deep-time land and resource management decisions on Tikopia and subsequent impacts on island nutrient status and long-term sustainability.1. Introduction 2. Materials and Methods 2.1. Sample Selection 2.2. Taxonomic Identification via Zooarchaeology by Mass Spectrometry (ZooMS) 2.3. Carbon and Nitrogen Stable Isotope Analysis of Bone and Tooth Dentine Collagen 3. Result

The Soft X-ray Lightcurves of Partially Eclipsed Stellar Flares

Most stellar flares' soft X-ray lightcurves possess a `typical' morphology, which consists of a rapid rise followed by a slow exponential decay. However, a study of 216 of the brightest flares on 161 pre-main sequence stars, observed during the Chandra Orion-Ultradeep Project (COUP), showed that many flare lightcurves depart from this typical morphology. While this can be attributed to the superposition of multiple typical flares, we explore the possibility that the time-variable eclipsing of flares by their host stars may also be an important factor. We assume each flare is contained within a single, uniform plasma density magnetic loop and specify the intrinsic variation of the flare's emission measure with time. We consider rotational eclipse by the star itself, but also by circumstellar discs and flare-associated prominences. Based on this simple model, we generate a set of flares similar to those observed in the COUP database. Many eclipses simply reduce the flare's maximum emission measure or decay time. We conclude therefore that eclipses often pass undetected, but usually have only a modest influence on the flare emission measure profile and hence the derived loop lengths. We show that eclipsing can easily reproduce the observed atypical flare morphologies. The number of atypical modelled flare morphologies is however much less than that found in the COUP sample. The large number of observed atypical flare morphologies, therefore, must be attributed to other processes such as multiple flaring loops.Comment: 11 pages, 9 figure

Cardiac Autonomic Effects of Acute Exposures to Airborne Particulates in Men and Women

The aim of this research was to investigate cardiac autonomic changes associated with acute exposures to airborne particulates. Methods: High fidelity 12-lead ECG (CardioSoft, Houston, TX) was acquired from 19 (10 male / 9 female) non-smoking volunteers (age 33.6 +/- 6.6 yrs) during 10 minutes pre-exposure, exposure and post-exposure to environmental tobacco smoke (ETS), cooking oil fumes, wood smoke and sham (water vapor). To control exposure levels, noise, subject activity, and temperature, all studies were conducted inside an environmental chamber. Results: The short-term fractal scaling exponent (Alpha-1) and the ratio of low frequency to high frequency Heart Rate Variability (HRV) powers (LF/HF, a purported sympathetic index) were both higher in males (p<0.017 and p<0.05, respectively) whereas approximate entropy (ApEn) and HF/(LF+HF) (a purported parasympathetic index) were both lower in males (p<0.036, and p<0.044, respectively). Compared to pre-exposure (p<0.0002) and sham exposure (p<0.047), male heart rates were elevated during early ETS post-exposure. Our data suggest that, in addition to tonic HRV gender differences, cardiac responses to some acute airborne particulates are gender related