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DOE OSTI2023

From Inside Out: How the Buried Interface, Shell Defects, and Surface Chemistry Conspire to Determine Optical Performance in Nonblinking Giant Quantum Dots

“Giant” or core/thick‐shell quantum dots (gQDs) are an important class of solid‐state quantum emitter characterized by strongly suppressed blinking and photobleaching under ambient conditions, and reduced nonradiative Auger processes. Together, these qualities provide distinguishing and useful functionality as single‐ and ensemble‐photon sources. For many applications, operation at elevated temperatures and under intense photon flux is desired, but performance is strongly dependent on the synthetic method employed for thick‐shell growth. Here, a comprehensive analysis of gQD structural properties “from the inside out” as a function of shell‐growth method is reported: successive ionic layer adsorption and reaction (SILAR) and high‐temperature continuous injection (HT‐CI), or sequential combinations of the two. Key correlations across synthesis methods, structural features (interfacial alloying, stacking‐fault density and surface‐ligand identity), and performance metrics (quantum yield, single‐gQD photoluminescence under thermal/photo stress, charging behavior and quantum‐optical properties) are identified. Surprisingly, it is found that interfacial alloying is the strongest indicator of gQD stability under stress, but this parameter is not the determining factor for Auger suppression. Furthermore, quantum yield is strongly influenced by surface chemistry and can approach unity even in the case of high shell‐defect density, while introduction of zinc‐blende stacking faults increases the likelihood that a gQD exhibits charged‐state emission.

77 NANOSCIENCE AND NANOTECHNOLOGY↗

DOE OSTI2018

Streptococcus gallolyticus conspires myeloid cells to promote tumorigenesis of inflammatory bowel disease

Highlights: • Streptococcus gallolyticus shows a distinctive enrichment at colonic mucosa in colitis-associated carcinoma. • Introduction of Streptococcus gallolyticus could aggravate the development of colitis-associated carcinoma. • CD11b{sup +} myeloid cells recruitment are initialed by Streptococcus gallolyticus to promote colitis-associated carcinoma progress. Metagenomic analyses indicate that streptococcus gallolyticus is enriched at carcinoma in colitis associated colorectal cancer compared with sporadic colorectal cancer. But the particular mechanism of streptococcus gallolyticus in Inflammatory Bowel Disease malignant progression remains undefined yet. We aim to explore the biological carcinogenesis efficacy of streptococcus gallolyticus and its potential mechanism in azoxymethane and dextran sodium sulphate-induced colitis associated colorectal cancer in mice. Oral pretreatment of streptococcus gallolyticus was adopted to evaluate its detrimental effect. The colorectums of experimental C57BL/6 mice were collected and examined for the degree of tumorigenesis. Comparative 16S rRNA sequencing was carried out to observe streptococcus gallolyticus alterations in abundance. We found that streptococcus gallolyticus are enriched in colonic carcinoma compared to adenoma and healthy mice. Pretreatment of Streptococcus gallolyticus aggravated tumor formation, with more colonic obstruction, larger number and diameter of tumor node. Furthermore, Streptococcus gallolyticus selectively recruits tumor-infiltrating myeloid cells but not mast cells, including marrow-derived suppressor cells, tumor-associated macrophages and dendritic cells, which can inhibit competence of T cells. Moreover, several myeloid-cell-derived proinflammatory cytokines (IL-6, IL-1β, IL-8, CCL2, COX-2, TNF-α) were increased with the formation of carcinoma in IBD. Collectively, these data suggest that, through recruitment of tumor-infiltrating immune cells, Streptococcus gallolyticus generate an immune suppressive microenvironment that is conducive for neoplasia progression of Inflammatory Bowel Disease.

60 APPLIED LIFE SCIENCES↗

DOE OSTI2024

Mixed gauge-global symmetries, elliptic modes, and black hole thermodynamics in Hořava-Lifshitz gravity

In Hořava-Lifshitz gravity, a putative consistent theory of quantum gravity for which there is evidence for both black hole thermodynamics and a holographic construction, spacetime is endowed with a preferred dynamical spacelike foliation. The theory has a leaf reparameterization symmetry that is neither global nor local gauge, hyperbolic and elliptic equations of motion, a lack of splittability, and universal horizon black hole solutions. The reparameterization symmetry is “mixed”: it is a local symmetry in one coordinate yet global on each leaf. More broadly it is an example of both unfree and projectable gauge symmetries. The mixed symmetry and associated charge has not yet been accounted for in calculations of universal horizon thermodynamics in Hořava-Lifshitz gravity. This has led to problems, in particular the failure of the first law in a class of asymptotically AdS solutions where the normal to the leaves of the foliation is not aligned with the time translation Killing vector at infinity. We show how the dynamics of the charge corresponding to this symmetry coupled with the other features above resolves this issue. We then briefly comment how this mixed symmetry, the corresponding charge, and the elliptic equations of motion also conspire to evade recent holographic arguments for only local gauge fields in consistent theories of quantum gravity due to the lack of splittability of the elliptic equation and associated mode.

Global Symmetries↗

DOE OSTI2018

Inter-annual variability of net and gross ecosystem carbon fluxes: A review

As the lifetime of regional flux networks approach twenty years, there is a growing number of papers that have published long term records (5 years or more) of net carbon fluxes between ecosystems and the atmosphere. Unanswered questions from this body of work are: 1) how variable are carbon fluxes on a year to year basis?; 2) what are the biophysical factors that may cause interannual variability and/or temporal trends in carbon fluxes?; and 3) how does the biophysical control on this carbon flux variability differ by climate and ecological spaces? To address these questions, we surveyed published data from 59 sites that reported on five or more years of continuous measurements, yielding 544 site-years of data. We found that the standard deviation of the interannual variability in net ecosystem carbon exchange (162 gC m -2 y -1 ) is large relative to its population mean (-200 gC m -2 y -1 ). Broad-leaved evergreen forests and crops experienced the greatest absolute variability in interannual net carbon exchange (greater than ±300 gC m -2 y -1 ) and boreal evergreen forests and maritime wetlands were among the least variable (less than ±40 gC m -2 y -1 ). A disproportionate fraction of the yearly variability in net ecosystem exchange was associated with biophysical factors that modulated ecosystem photosynthesis rather than ecosystem respiration. Yet, there was appreciable and statistically significant covariance between ecosystem photosynthesis and respiration. Consequently, biophysical conditions that conspired to increase ecosystem photosynthesis to from one year to the next were associated with an increase in ecosystem respiration, and vice versa; on average, the year to year change in respiration was 40% as large as the year to year change in photosynthesis. The analysis also identified sets of ecosystems that are on the verge of switching from being carbon sinks to carbon sources. These include sites in the Arctic tundra, the evergreen forests in the Pacific northwest and some grasslands, where year to year changes in respiration are outpacing those in photosynthesis. While a select set of climatic and ecological factors (e.g. light, rainfall, temperature, phenology) played direct and indirect roles on this variability, their impact differed conditionally, as well as by climate and ecological spaces. For example, rainfall had both positive and negative effects. Deficient rainfall caused a physiological decline in photosynthesis in temperate and semi-arid regions. Too much rain, in the humid tropics, limited photosynthesis by limiting light. In peatlands and tundra, excess precipitation limited ecosystem respiration when it raised the water table to the surface. For deciduous forests, warmer temperatures lengthened the growing season, increasing photosynthesis, but this effect also increased soil respiration. Finally, statistical analysis was performed to evaluate the detection limit of trends; we computed the confidence intervals of trends in multi-year carbon fluxes that need to be resolved to conclude whether the differences are to be attributed to randomness or biophysical forcings. Ongoing studies and reports on interannual variations need to consider the role of the duration of the time series on random errors when quantifying potential trends and extreme events.

54 ENVIRONMENTAL SCIENCES↗

DOE OSTI2021

An asymptotically compatible treatment of traction loading in linearly elastic peridynamic fracture

Meshfree discretizations of state-based peridynamic models are attractive due to their ability to naturally describe fracture of general materials. However, two factors conspire to prevent meshfree discretizations of state-based peridynamics from converging to corresponding local solutions as resolution is increased: quadrature error prevents an accurate prediction of bulk mechanics, and the lack of an explicit boundary representation presents challenges when applying traction loads. Herein, we develop a reformulation of the linear peridynamic solid (LPS) model to address these shortcomings, using improved meshfree quadrature, a reformulation of the nonlocal dilatation, and a consistent handling of the nonlocal traction condition to construct a model with rigorous accuracy guarantees. In particular, these improvements are designed to enforce discrete consistency in the presence of evolving fractures, whose a priori unknown location render consistent treatment difficult. In the absence of fracture, when a corresponding classical continuum mechanics model exists, our improvements provide asymptotically compatible convergence to corresponding local solutions, eliminating surface effects and issues with traction loading which have historically plagued peridynamic discretizations. When fracture occurs, our formulation automatically provides a sharp representation of the fracture surface by breaking bonds, avoiding the loss of mass. We provide rigorous error analysis and demonstrate convergence for a number of benchmarks, including manufactured solutions, free-surface, nonhomogeneous traction loading, and composite material problems. Finally, we validate simulations of brittle fracture against a recent experiment of dynamic crack branching in soda-lime glass, providing evidence that the scheme yields accurate predictions for practical engineering problems.

42 ENGINEERING↗

DOE OSTI2016

Methodology for the structural design of single spoke accelerating cavities at Fermilab

Fermilab is planning to upgrade its accelerator complex to deliver a more powerful and intense proton-beam for neutrino experiments. In the framework of the so-called Proton Improvement Plan-II (PIP-II), we are designing and developing a cryomodule containing superconducting accelerating cavities, the Single Spoke Resonators of type 1 (SSR1). In this paper, we present the sequence of analysis and calculations performed for the structural de- sign of these cavities, using the rules of the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code (BPVC). The lack of an accepted procedure for addressing the design, fabrication, and inspection of such unique pressure vessels makes the task demanding and challenging every time. Several factors such as exotic materials, unqualified brazing procedures, limited nondestructive examination, and the general R&D nature of these early generations of cavity design, conspire to make it impractical to obtain full compliance with all ASME BPVC requirements. However, the presented approach allowed us to validate the design of these new generation of single spoke cavities with values of maximum allowable working pressure that exceed the safety requirements. This set of rules could be used as a starting point for the structural design and development of similar objects.

42 ENGINEERING↗

DOE OSTI2022

Suppression of Superconductivity and Nematic Order in Fe 1–y Se 1–x S x (0 ≤ x ≤ 1; y ≤ 0.1) Crystals by Anion Height Disorder

Connections between crystal chemistry and critical temperature T c have been in the focus of superconductivity, one of the most widely studied phenomena in physics, chemistry and materials science alike. In most Fe-based superconductors materials chemistry and physics conspire so that T c correlates with the average anion height above the Fe plane, i.e. with the geometry of the FeAs 4 or FeCh 4 (Ch = Te, Se, S) tetrahedron. Here, by synthesizing Fe 1-y Se 1-x S x (0 ≤ x ≤ 1, y ≤ 0.1) we find that in alloyed crystals T c is not correlated with the anion height as most other Fe superconductors. Instead, changes in T c (x) and tetragonal-to-orthorombic (nematic) transition T s (x) on cooling are correlated with disorder in Fe vibrations in direction orthogonal to Fe planes, along the crystallographic c-axis. The disorder stems from the random nature of S substitution, causing deformed Fe(Se,S) 4 tetrahedra with different Fe-Se and Fe-S bond distances. Our results provide evidence of T c and T s suppression by disorder in anion height. The connection to local crystal chemistry may be exploited in computational prediction of new superconducting materials with FeSe/S building blocks.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2019

Photophysical Processes in Rhenium(I) Diiminetricarbonyl Arylisocyanides Featuring Three Interacting Triplet Excited States

We present here a series of four transition-metal complexes based on the rhenium(I) tricarbonyl 1,10-phenanthroline (phen) template, with a lone ancillary arylisocyanide (CNAr) ligand to yield metal–organic chromophores of the generic molecular formula [Re(phen)(CO) 3 (CNAr)] + [CNAr = 2,6-diisopropylphenyl isocyanide ( 1 ), 4-phenyl-2,6-diisopropylphenyl isocyanide ( 2 ), 4-phenylethynyl-2,6-diisopropylphenyl isocyanide ( 3 ), and 4-biphenyl-2,6-diisopropylphenyl isocyanide ( 4 )]. This particular series features varied degrees of π-conjugation length in the CNAr moiety, resulting in significant modulation in the resultant photophysical properties. All molecules possess long-lived [8–700 μs at room temperature (RT)], strongly blue-green photoluminescent and highly energetic excited states (λ max,em = 500–518 nm; Φ = 14–64%). Each of these chromophores has been photophysically investigated using static and dynamic spectroscopic techniques, the latter probed from ultrafast to supra-nanosecond time scales using transient absorption and photoluminescence (PL). Time-resolved PL intensity decays recorded as a function of the temperature were consistent with the presence of at least two emissive states lying closely spaced in energy with a third nonemissive state lying much higher in energy and likely ligand-field in character. The combined experimental evidence, along with the aid of electronic structure calculations (density functional theory and time-dependent density functional theory performed at the M06/Def2-SVP/SDD level), illustrates that the CNAr ligand is actively engaged in manipulating the excited-state decay in three of these molecules ( 2 – 4 ), wherein the triplet metal-to-ligand charge-transfer ( 3 MLCT) state along with two distinct triplet ligand-centered ( 3 LC) excited-state configurations (phen and CNAr) conspire to produce the resultant photophysical properties. Because the π conjugation within the CNAr ligand was extended, an interesting shift in the dominant photophysical processes was observed. When the CNAr conjugation length is short, as in 1 , the phenanthroline 3 LC state dominates, resulting in a configurationally mixed triplet excited state of both LC and MLCT character. With more extended π conjugation in the CNAr subunit ( 2 – 4 ), the initially generated 3 LC(phen)/ 3 MLCT excited state ultimately migrates to the CNAr 3 LC state on the order of tens of picoseconds. Molecules 3 and 4 in this series also feature unique examples of inorganic excimer formation, as demonstrated by dynamic self-quenching in the corresponding PL intensity decays accompanied by the observation of a short-lived low-energy emission feature.

14 SOLAR ENERGY↗

DOE OSTI2022

Vibrational signatures of HNO 3 acidity when complexed with microhydrated alkali metal ions, M + ∙(HNO 3 )(H 2 O) n =5 (M = Li, K, Na, Rb, Cs), at 20 K

The speciation of strong acids like HNO 3 under conditions of restricted hydration is an important factor in the rates of chemical reactions at the air-water interface. Here we explore the trade-offs in play when HNO 3 is attached to alkali ions (Li + -Cs + ) with four water molecules in their primary hydration shells. This is achieved by analyzing the vibrational spectra of the M + ∙(HNO 3 )(H 2 O) 5 clusters cooled to about 20 K in a cryogenic photofragmentation mass spectrometer. The local acidity of the acidic OH group is estimated by the extent of the red shift in its stretching frequency when attached to a single water molecule. Here, the persistence of this local structural motif (HNO 3 -H 2 O) in all of these alkali metal clusters enables us to determine the competition between the effect of the direct complexation of the acid with the cation, which acts to enhance acidity, and the role of the water network in the first hydration shell around the ions, which acts to counter (screen) the intrinsic effect of the ion. Analysis of the vibrational features associated with the acid molecule as well as those of the water network reveal how cooperative interactions in the micro hydration regime conspire to effectively offset the intrinsic enhancement of HNO 3 acidity afforded by attachment to the smaller cations.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2020

Severe Dirac Mass Gap Suppression in Sb 2 Te 3 -Based Quantum Anomalous Hall Materials

The quantum anomalous Hall (QAH) effect appears in ferromagnetic topological insulators (FMTIs) when a Dirac mass gap opens in the spectrum of the topological surface states (SSs). Unaccountably, although the mean mass gap can exceed 28 meV (or ~320 K), the QAH effect is frequently only detectable at temperatures below 1 K. Using atomic-resolution Landau level spectroscopic imaging, we compare the electronic structure of the archetypal FMTI Cr 0.08 (Bi 0.1 Sb 0.9 ) 1.92 Te 3 to that of its nonmagnetic parent (Bi 0.1 Sb 0.9 ) 2 Te 3 , to explore the cause. In (Bi 0.1 Sb 0.9 ) 2 Te 3 , we find spatially random variations of the Dirac energy. Statistically equivalent Dirac energy variations are detected in Cr 0.08 (Bi 0.1 Sb 0.9 ) 1.92 Te 3 with concurrent but uncorrelated Dirac mass gap disorder. Additionally, these two classes of SS electronic disorder conspire to drastically suppress the minimum mass gap to below 100 μeV for nanoscale regions separated by <1 μm. This fundamentally limits the fully quantized anomalous Hall effect in Sb 2 Te 3 -based FMTI materials to very low temperatures.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2025

Leveraging Intramolecular π-Stacking to Access an Exceptionally Long-Lived 3 MC Excited State in an Fe(II) Carbene Complex

The ability to manipulate excited-state decay cascades using molecular structure is essential to the application of abundant-metal photosensitizers and chromophores. Ligand design has yielded some spectacular results elongating charge-transfer excited state lifetimes of Fe(II) coordination complexes, but triplet metal-centered ( 3 MC) excited states - recently demonstrated to be critical to the photoactivity of isoelectronic Co(III) polypyridyls - have to date remained elusive, with temporally isolable examples limited to the picosecond regime. Here, with this report, we show how strong-field donors and intramolecular π-stacking can conspire to stabilize a long-lived 3 MC excited state for a remarkable 4.1 ± 0.3 ns in fluid solution at ambient temperature. Analysis of variable-temperature time-resolved absorption data using theoretical models ranging from Arrhenius to semiclassical Marcus theory, combined with computational modeling and X-ray crystallography, reveal a Jahn−Teller stabilized excited state with a high activation barrier for ground-state recovery. The net result is a chromophore with a 3 MC excited-state lifetime that is orders of magnitude longer than anything yet observed for an Fe(II) complex.

carbene compounds↗

DOE OSTI2026

A Unified Theory for the Global Thunderstorm Distribution and Land–Sea Contrast

This article evaluates Entraining CAPE (ECAPE) as a thunderstorm proxy in climate studies using Global Precipitation Measurement satellite observations. ECAPE modifies traditional CAPE to account for the dependence of entrainment on the vertical wind shear, the lifted condensation level (LCL) height, and the properties of a cloud's surrounding atmosphere. ECAPE shows stronger pattern correlations with global regions of intense thunderstorms than previous metrics for updraft speed. In these regions, large CAPE, large shear, and high LCLs conspire to produce wide updrafts that are shielded from the negative effects of dry-air entrainment. ECAPE more skillfully discriminates intense thunderstorms from their less intense counterparts than other metrics commonly used in climatology and climate change studies of thunderstorms. We provide evidence that the well-known land-sea contrast in thunderstorm intensity is a consequence of larger CAPE and higher LCL heights over land than over the ocean.

entraining CAPE↗

DOE OSTI2016

Thermodynamic origins of the solvent-dependent stability of lithium polysulfides from first principles

An understanding of the complex solution phase chemistry of dissolved lithium polysulfides is critical to approaches aimed at improving the cyclability and commercial viability of lithium sulfur batteries. Experimental measurements are frustrated by the versatile sulfur–sulfur bond, with spontaneous disproportionation and interconversion leading to unknown equilibrium distributions of polysulfides with varying lengths and charge states. The solubility of isolated lithium polysulfides is calculated from first-principles molecular dynamics simulations. We explore the associated changes in the dissolution free energy, enthalpy and entropy in two regimes: liquid-phase monodentate solvation in dimethylformamide (DMF) and polymer-like chelation in bis(2-methoxyethyl) ether (diglyme). In both of these technologically relevant solvents, we show that the competition between enthalpy and entropy, related to specific interfacial atomic interactions, conspires to increase the relative stability of long chain dianionic species, which exist as Li + –LiS x - contact-ion-pairs. Further, we propose a mechanism of radical polysulfide stabilization in simple solvents through the reorientation of the 1 st shell solvent molecules to screen electrostatic fields emanating from the solute and explain nonmonotonicity of the dissolution entropy with polysulfide length in terms of a three-shell solvation model. Finally, our analysis provides statistical dynamics insights into polylsulfide stability, useful to understand or predict the relevant chemical species present in the solvent at low concentrations.

36 MATERIALS SCIENCE↗

DOE OSTI2018

A fast low-to-high confinement mode bifurcation dynamics in the boundary-plasma gyrokinetic code XGC1

Here, a fast edge turbulence suppression event has been simulated in the electrostatic version of the gyrokinetic particle-in-cell code XGC1 in a realistic diverted tokamak edge geometry under neutral particle recycling. The results show that the sequence of turbulent Reynolds stress followed by neoclassical ion orbit-loss driven together conspire to form the sustaining radial electric field shear and to quench turbulent transport just inside the last closed magnetic flux surface. As a result, the main suppression action is located in a thin radial layer around ψ N ≃0.96–0.98, where ψ N is the normalized poloidal flux, with the time scale ~0.1 ms.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2019

The purported square ice in bilayer graphene is a nanoscale, monolayer object

The phase diagram of water is complex, and interfacial effects can stabilize unusual structures at the nanoscale. In this paper, we employ bond order accelerated molecular dynamics simulations to show that upon encapsulation within bilayer graphene, water can spontaneously adopt a two-dimensional (monomolecular) layer of “square ice” at ambient conditions, instead of an encapsulated water droplet. Free energy calculations show that this motif is thermodynamically stable up to diameters of approximately 15 nm due to enhanced hydrogen bonding and favorable binding to the graphene sheets. Entropic losses due to solidification and reduced graphene–graphene binding enthalpy are opposing thermodynamic forces that conspire to limit the maximum size, but modification of any of these thermodynamic factors should change the range of stability. Finally, simulated core-level spectroscopy reveals unambiguous orientation dependent signatures of square ice that should be discernable in experiments.

36 MATERIALS SCIENCE↗

DOE OSTI2020

Temperature dependent inverse spin Hall effect in Co/Pt spintronic emitters

In bilayers of ferromagnets and heavy metals, which form the so-called spintronic emitters, the phenomena of ultrafast demagnetization and the inverse spin Hall effect (ISHE) conspire to yield remarkably efficient emission of electric pulses in the THz band. Light-induced demagnetization of the ferromagnet launches a pulse of spin current into the heavy metal, wherein it bifurcates into a radiative charge transient due to the ISHE. The influence of temperature on this combined effect should depend on both the magnetic phase diagram and the microscopic origin of spin Hall conductivity, but its exact dependence remains to be clarified. Here, we experimentally study the temperature dependence of an archetypal spintronic emitter, the Co/Pt bilayer, using electro-optic sampling of the emitted THz pulses in the time domain. The emission amplitude is attenuated with decreasing temperature, consistent with an inverse spin Hall effect in platinum of predominantly intrinsic origin.

Physics↗

DOE OSTI2011

THE OUTER DISKS OF DWARF IRREGULAR GALAXIES

In order to explore the properties of extreme outer stellar disks, we obtained ultra-deep V and GALEX ultraviolet (UV) images of four dwarf irregular galaxies and one blue compact dwarf galaxy, and ultra-deep B images of three of these. Our V-band surface photometry extends to 29.5 mag arcsec{sup -2}. We convert the FUV and V-band photometry, along with H{alpha} photometry obtained in a larger survey, into radial star formation rate profiles that are sensitive to timescales from 10 Myr to the lifetime of the galaxy. We also obtained H I-line emission data and compare the stellar distributions, surface brightness profiles, and star formation rate profiles to H I-line emission maps, gas surface density profiles, and gas kinematics. Our data lead us to two general observations. First, the exponential disks in these irregular galaxies are extraordinarily regular. We observe that the stellar disks continue to decline exponentially as far as our measurements extend. In spite of lumpiness in the distribution of young stars and H I distributions and kinematics that have significant unordered motions, sporadic processes that have built the disks-star formation, radial movement of stars, and perhaps even perturbations from the outside-have, nevertheless, conspired to produce standard disk profiles. Second, there is a remarkable continuity of star formation throughout these disks over time. In four out of five of our galaxies the star formation rate in the outer disk measured from the FUV tracks that determined from the V-band, to within factors of five, requiring star formation at a fairly steady rate over the galaxy's lifetime. Yet, the H I surface density profiles generally decline with radius more shallowly than the stellar light, and the gas is marginally gravitationally stable against collapse into clouds. Outer stellar disks are challenging our concepts of star formation and disk growth and provide a critical environment in which to understand processes that mold galaxy disks.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2011

ESTIMATES OF THE PLANET YIELD FROM GROUND-BASED HIGH-CONTRAST IMAGING OBSERVATIONS AS A FUNCTION OF STELLAR MASS

We use Monte Carlo simulations to estimate the number of extrasolar planets that are directly detectable in the solar neighborhood using current and forthcoming high-contrast imaging instruments. Our calculations take into consideration the important factors that govern the likelihood for imaging a planet, including the statistical properties of stars in the solar neighborhood, correlations between star and planet properties, observational effects, and selection criteria. We consider several different ground-based surveys, both biased and unbiased, and express the resulting planet yields as a function of stellar mass. Selecting targets based on their youth and visual brightness, we find that strong correlations between star mass and planet properties are required to reproduce high-contrast imaging results to date (i.e., HR 8799, {beta} Pic). Using the most recent empirical findings for the occurrence rate of gas-giant planets from radial velocity (RV) surveys, our simulations indicate that naive extrapolation of the Doppler planet population to semimajor axes accessible to high-contrast instruments provides an excellent agreement between simulations and observations using present-day contrast levels. In addition to being intrinsically young and sufficiently bright to serve as their own beacon for adaptive optics correction, A-stars have a high planet occurrence rate and propensity to form massive planets in wide orbits, making them ideal targets. The same effects responsible for creating a multitude of detectable planets around massive stars conspire to reduce the number orbiting low-mass stars. However, in the case of a young stellar cluster, where targets are approximately the same age and situated at roughly the same distance, MK-stars can easily dominate the number of detections because of an observational bias related to small number statistics. The degree to which low-mass stars produce the most planet detections in this special case depends upon whether multiple formation mechanisms are at work. Upon relaxing our assumption that planets in ultra-wide (a > 100 AU) orbits resemble the RV sample, our simulations suggest that the companions found orbiting late-type stars (AB Pic, 1RXSJ1609, GSC 06214, etc.) are consistent with a formation channel distinct from that of RV planets. These calculations explain why planets have thus far been imaged preferentially around A-stars and K-, M-stars, but no spectral types in between, despite concerted efforts targeting F-, G-stars.

79 ASTRONOMY AND ASTROPHYSICS↗