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The search for life in the Universe is a fundamental problem of astrobiology and modern science. The current progress in the detection of terrestrial-type exoplanets has opened a new avenue in the characterization of exoplanetary atmospheres and in the search for biosignatures of life with the upcoming ground-based and space missions. To specify the conditions favourable for the origin, development and sustainment of life as we know it in other worlds, we need to understand the nature of global (astrospheric), and local (atmospheric and surface) environments of exoplanets in the habitable zones (HZs) around G-K-M dwarf stars including our young Sun. Global environment is formed by propagated disturbances from the planet-hosting stars in the form of stellar flares, coronal mass ejections, energetic particles and winds collectively known as astrospheric space weather. Its characterization will help in understanding how an exoplanetary ecosystem interacts with its host star, as well as in the specification of the physical, chemical and biochemical conditions that can create favourable and/or detrimental conditions for planetary climate and habitability along with evolution of planetary internal dynamics over geological timescales. A key linkage of (astro)physical, chemical and geological processes can only be understood in the framework of interdisciplinary studies with the incorporation of progress in heliophysics, astrophysics, planetary and Earth sciences. The assessment of the impacts of host stars on the climate and habitability of terrestrial (exo)planets will significantly expand the current definition of the HZ to the biogenic zone and provide new observational strategies for searching for signatures of life. The major goal of this paper is to describe and discuss the current status and recent progress in this interdisciplinary field in light of presentations and discussions during the NASA Nexus for Exoplanetary System Science funded workshop ‘Exoplanetary Space Weather, Climate and Habitability’ and to provide a new roadmap for the future development of the emerging field of exoplanetary science and astrobiology.
Observations of deuterated species are essential to probing the properties and thermal history of various astrophysical environments, and the ALMA observing facilities will reveal a multitude of new deuterated molecules. To analyze these new vast data we have constructed a new up-to-date network with the largest collection of deuterium chemistry reactions to date. We assess the reliability of the network and probe the role of physical parameters and initial abundances on the chemical evolution of deuterated species. Finally, we perform a sensitivity study to assess the uncertainties in the estimated abundances and D/H ratios.
The study of molecules in space, known as astrochemistry or molecular astrophysics, is a rapidly growing field. Molecules exist in a wide range of environments in both gaseous and solid form, from our own solar system to the distant early universe. To astronomers, molecules are indispensable and unique probes of the physical conditions and dynamics of regions in which they are detected, especially the interstellar medium. In particular, the many stages of both low-mass and high-mass star formation are better understood today thanks to the analysis of molecular observations. Molecules can also yield a global picture of the past and present of sources. Moreover, molecules affect their environment by contributing to the heating and cooling processes that occur.
We show that for ionization-driven chemistry in molecular clouds with very sub-solar (but non-vanishing) metallicities, OH replaces CO as the most abundant molecule containing at least one heavy element. Old Population-II stars, with observed metallicities ≲10-3times solar, may have formed in such OH-dominated star-forming clouds, shortly after the heavy elements were first produced at the epoch of reionization in the early Universe.
We investigate the molecular abundances in protostellar cores by solving the gas-grain chemical reaction network. As a physical model of the core, we adopt a result of one-dimensional radiation-hydrodynamics calculation, which follows the contraction of an initially hydrostatic prestellar core to form a protostellar core. Temporal variation of molecular abundances is solved in multiple infalling shells, which enable us to investigate the spatial distribution of molecules in the evolving core. The shells pass through the warm region of T ~ 20–100 K in several 104 yr and falls onto the central star in ~100 yr after they enter the region of T > 100 K. We found that the complex organic species such as HCOOCH3 are formed mainly via grain-surface reactions at T ~ 20–40 K, and then sublimated to the gas phase when the shell temperature reaches their sublimation temperatures (T ≥ 100 K). Carbon-chain species can be re-generated from sublimated CH4 via gas-phase and grain-surface reactions. HCO2+, which is recently detected towards L1527, are abundant at r = 100–2,000 AU, and its column density reaches ~1011 cm−2 in our model. If a core is isolated and irradiated directly by interstellar UV radiation, photo-dissociation of water ice produces OH, which reacts with CO to form CO2 efficiently. Complex species then become less abundant compared with the case of embedded core in ambient clouds. Although a circumstellar (protoplanetary) disk is not included in our core model, we can expect similar chemical reactions (i.e., production of large organic species, carbon-chains and HCO2+) to proceed in disk regions with T ~ 20–100 K.
The synthesis of organic molecular anions in TMC-1 and IRC+10216 is investigated. Modelled C2H−, CN−, C3N−, C5N− and C7N− column densities are sufficiently great that these species might be observable in IRC+10216. Density-enhanced shells in the outer envelope of IRC+10216 are found to enhance the C2H− and CN− column densities by shielding these anions from destruction by UV radiation. From a newly-derived upper column density limit of 6.6 × 1010 cm−2 for C2H− in IRC+10216 we deduce the primary production mechanism for this anion to be C2H2 + H− ⇒ C2H− + H2. In TMC-1, due to the low radiative electron attachment rates calculated for C2H−, CN− and CH2CN−, these species have modelled column densities below the detection threshold. They could, however, be produced in reactions we have not yet considered.
We present a chemical model of the central torus around CRL 618 in order to confirm that long-chained hydrocarbons and benzene are formed at detectable abundances. We reproduce the observed abundances of these molecules comparatively well. We infer from our calculations that CRL 618 must be a young PPN, with a post-AGB age of approximately 600 yr.
Molecular hydrogen is formed on interstellar grains by two main processes. In the first, or Langmuir-Hinshelwood, mechanism, hydrogen atoms land on a grain and diffuse over the surface by either tunneling or hopping until they find each other. In the second, or Eley-Rideal, mechanism, hydrogen atoms landing on grains are fixed in position. Reaction occurs only when a gaseous hydrogen atom lands atop an adsorbed one. Based on new experimental results concerning the rate of diffusion of H atoms on interstellar-like surfaces, it is clear that the rate is significantly slower than estimated in the past. The range of temperatures over which diffusive formation of H2 occurs is correspondingly reduced although sites of strong binding can raise the upper temperature limit. The surface formation of molecules heavier than hydrogen is still not well understood.
It is almost certain that H2 and a variety of other molecules are formed on the surfaces of low-temperature interstellar dust particles. On these surfaces, binding sites for adsorbates exist interspersed among regions of higher potential. On a grain of typical radius 0.1 µ there are roughly 106 such binding sites, onto which neutral gas-phase molecules stick with high efficiency. The binding energy, or energy required for desorption (ED), depends on the surface and on the adsorbate. For example, the binding energy of H atoms on olivine (a silicate-type material) has just been measured to be 372 K by Katz et al. (1999), who also measured the binding energy of H on amorphous carbon to be 658 K.
In an attempt to overcome infections associated with central venous catheters, a new antiseptic central venous catheter coated with benzalkonium chloride on the internal and external surfaces has been developed and evaluated in a clinical trial. Patients (235) randomly received either a triple-lumen central venous catheter coated with benzalkonium chloride (117) or a polyurethane non-antiseptic catheter (118). The incidence of microbial colonization of both catheters and retained antiseptic activity of the benzalkonium chloride device following removal were determined. The benzalkonium chloride resulted in a significant reduction of the incidence of microbial colonization on both the internal and external catheter surfaces. The reduction in colonization was detected at both the intradermal (21 benzalkonium chloride catheters vs. 38 controls, P=0.0016) and distal segments of the antiseptic-coated catheters. Following catheter removal retained activity was demonstrated in benzalkonium chloride catheters which had been in place for up to 12 days. No patients developed adverse reactions to the benzalkonium chloride catheters. The findings demonstrate that the benzalkonium chloride catheter significantly reduced the incidence of catheter-associated colonization.
The study described in this chapter was prompted by the report of a decision of the United States Federal Eighth Circuit Court of Appeals in St. Louis (Woo, 1992). In this decision, a child molestation conviction was overturned because the appellate court determined that the presiding trial judge had erred in allowing a 3-year-old child's hearsay statements to a physician to be admitted as testimony under the medical diagnosis and treatment exception. The Federal Court of Appeals ruled that the exception does not apply when the patient is too young to understand the importance of telling the truth to the physician.
When children, especially young children, are excluded from giving testimony, the physician's report of the child's statement given during a physical examination can be of great importance. The child's statements to the physician are known as hearsay, which is a statement made out of court and later used in court to “establish the truth of what was said earlier” (Myers, 1986). While the hearsay rule exists in all English-speaking countries, different countries have different exceptions to the hearsay rule. In several countries, hearsay is more liberally admissible in civil cases (e.g., England, Scotland). In the United States, there has long been a hearsay exception for hearsay statements to health care providers. This exception is based on the premise that patients have an incentive to be truthful with a health care professional, thus rendering the hearsay reliable.
Laser ablation of silicon and germanium was carried out in moderate vacuum with l00fs to 400fs pulses at 248nm and intensities up to 3x1013 W/cm2. Evidence for non-thermal material removal was found. Imaged multishot ablation patterns display the intensity dependent self-structuring effect, forming well-known columnar structures. It is shown that continued irradiation of these structures eventually results in comparatively clean ablation. An increase of ablation rate with depth was observed. The reason is an intensity enhancement inside the pits by reflective focussing to a level where bond-breaking takes place. Furthermore, it was noticed that ablation contours can be significantly improved by electrically grounding the target.
We discuss the parameters needed to model chemistry in extragalactic clouds. While density and temperature can be constrained by multiline observations, molecular abundances may be severely affected by the adopted elemental abundances. While the observations of the Magelllanic Clouds can be reasonably interpreted in terms of dark cloud models, molecular gas in starburst galaxies could well be dominated by photoeffects. The detection of deuterium in extragalactic molecules would provide a valuable diagnostic.
Ultraviolet photons are created in the interior of dense interstellar clouds by the impact excitation of molecular hydrogen by secondary electrons generated by cosmic ray ionization. The resulting photodissociation and photoionization rates of a wide range of interstellar molecules are calculated. The effects on the equilibrium chemical composition of dense clouds are briefly discussed.
The bulk magnetic properties of (ErxY1-x)2Fe14B and (ErxPr1-x)2Fe14B systems were studied over the temperature range 4.2-1100 K. Lattice parameters, saturation magnetizations, Curie temperatures and spin reorientation temperatures were determined. Theoretical description of the detailed magnetic behavior is presented, based on a crystal field model. The (ErxY1-x)2Fe14B compounds were all found to exhibit plane-to-axis spin reorientations similar to that observed for Er2Fe14B, with the transition temperature decreasing with increasing Y content. In contrast, the spin reorientations in the (ErxPr1-x)2Fe14B systems appear to be of the cone-to-axis type. Since higher order crystal field terms appear to be significant only in the cases of Nd3+ and Ho3+, the results are discussed in terms of a crystal field Hamiltonian involving only 2nd order terms. Using known values of the exchange field, Fe anisotropy and the ratios of the crystal field coefficients, the multi-ion cr 6.tal field problem was formulated in terms of a single adjustable parameter (B02(f). It is shown that 2nd order crystal field terms are capable, not only of explaining the conical anisotropy of the (ErxPr1-x)2Fe14B systems, but also the decrease in the Er moment upon passing through the spin reorientation (as has been observed for Er2Fe14B). The magnetic structure of Er1.5Pr0.5Fe14B is also predicted.
Thin Films (≃4000 A) of R2Fe14B compounds have been synthesized by d.c. triode sputtering for R=Nd,Sm and Er. Deposition onto single-crystal substrates heated to 600”C and with rates of 1.3 A/s results in nearly epitaxial film growth such that the c-axis of the tetragonal structure is perpendicular to the film plane. As a consequence, intrinsic anisotropy effects are observed in the magnetic properties of the as-made films. Although sapphire and quartz substrates give the best results, similar directed growth is observed on recrystallized Nb foils. Magneto-optical measurements on the Nd-based compounds give a remnant polar Kerr angle θ=0.22° and coercive field Hc=5.5 k0e. The easy axis is in the film plane for R=Sm and Er and a remnant magnetization greater than 80% of saturation is observed for H applied parallel to the film plane. Electrical resistivity measurements on Nd2Fe14B films indicate that spin-disorder scattering from the rare earth site is important and features from both the Curie and spin reorientation temperatures are observed.
The effect of small aluminum additions on the temperature dependence of remanence and intrinsic coercivity in Co- and Dy-containing Nd-Fe-B was studied. Sintered magnets were prepared and demagnetization curves measured at temperatures between −50 to +200°C. Curie temperatures and irreversible flux losses in open circuit were determined. Al increases the coercivity while decreasing remanence, energy product and Curie temperature. Other unfavorable side effects are the increase in temperature coefficients of Br and, especially, MHc. Substitution of Al is not beneficial for magnets used at elevated temperature.
Initial magnetization and demagnetization data are reported for three forms of rapidly solidified Nd-Fe-B permanent magnet materials: melt-spun ribbons, hot pressed magnets, and die upset magnets. In all three materials the results are consistent with domain wall pinning at grain boundary phases as the coercivity mechanism. Optimally quenched ribbons are comprised of randomly oriented single domain Nd2Fe14B grains, and both initial magnetization and demagnetization are controlled by strong domain wall pinning at grain boundaries. Maximum coercivity is accompanied by a low initial permeability. Coercivity is reduced in overquenched ribbons by partial retention of a magnetically soft amorphous or very finely crystalline microstructure. Coercivity decreases in underquenched ribbons because wall pinning weakens as the grain size increases above optimum. Correlation of magnetization and demagnetization behaviors suggests that maximum coercivity in ribbons is largely determined by the resistance to domain wall formation in grains smaller than the single domain particle limit. Grain size is much less important in the aligned die upset magnets. Domain walls are initially free to move until they become strongly pinned at grain edges, and complete magnetization requires an applied field greater than the coercive field. Hot pressed magnets show a mixture of ribbon and die upset behavior.