Barkhausen Noise and Critical Scaling in the Demagnetization Curve

The demagnetization curve, or initial magnetization curve, is studied by examining the embedded Barkhausen noise using the non-equilibrium, zero temperature random-field Ising model. The demagnetization curve is found to reflect the critical point seen as the system's disorder is changed. Critical scaling is found for avalanche sizes and the size and number of spanning avalanches. The critical exponents are derived from those related to the saturation loop and subloops. Finally, the behavior in the presence of long range demagnetizing fields is discussed. Results are presented for simulations of up to one million spins.Comment: 4 pages, 4 figures, submitted to Physical Review Letter

Multisite Binding of Drugs and Natural Products in an Entropically Favorable, Heteroleptic Receptor.

The cavities of artificial receptors are defined by how their components fit together. The encapsulation of specific molecules can thus be engineered by considering geometric principles; however, intermolecular interactions and steric fit scale with receptor size, such that the ability to bind multiple guests from a specific class of compounds remains a current challenge. By employing metal-organic self-assembly, we have prepared a triangular prism from two different ligands that is capable of binding more than 20 different natural products, drugs, and steroid derivatives within its prolate cavity. Encapsulation inflates the host, enhancing its ability to bind other guests in peripheral pockets and thus enabling our system to bind combinations of different drug and natural product cargoes in different locations simultaneously. This new mode of entropically favorable self-assembly thus enables central encapsulation to amplify guest-binding events around the periphery of an artificial receptor.This work was supported by the UK Engineering and Physical Sciences Research Council (EPSRC EP/P027067/1) and the European Research Council (695009). We thank Cambridge Australia Scholarships (FJR) and the European Union’s Horizon 2020 research and innovation program, Marie Sklodowska-Curie Grant 642192 (JPC) for Ph.D. funding

Constraints on the Stellar/Sub-stellar Mass Function in the Inner Orion Nebula Cluster

We present the results of a 0.5-0.9" FWHM imaging survey at K (2.2 micron) and H (1.6 micron) covering 5.1' x 5.1' centered on Theta 1C Ori, the most massive star in the Orion Nebula Cluster (ONC). At the age and distance of this cluster, and in the absence of extinction, the hydrogen burning limit (0.08 Mo) occurs at K~13.5 mag while an object of mass 0.02 Mo has K~16.2 mag. Our photometry is complete for source detection at the 7 sigma level to K~17.5 mag and thus is sensitive to objects as low-mass as 0.02 Mo seen through visual extinction values as high as 10 magnitudes. We use the observed magnitudes, colors, and star counts to constrain the shape of the inner ONC stellar mass function across the hydrogen burning limit. After determining the stellar age and near-infrared excess properties of the optically visible stars in this same inner ONC region, we present a new technique that incorporates these distributions when extracting the mass function from the observed density of stars in the K-(H-K) diagram. We find that our data are inconsistent with a mass function that rises across the stellar/sub-stellar boundary. Instead, we find that the most likely form of the inner ONC mass function is one that rises to a peak around 0.15 Mo, and then declines across the hydrogen-burning limit with slope N(log M) ~ M^(0.57+/-0.05). We emphasize that our conclusions apply to the inner 0.71 pc x 0.71 pc of the ONC only; they may not apply to the ONC as a whole where some evidence for general mass segregation has been found.Comment: Accepted for publication in the Astrophysical Journal. Preprints/tables also available at http://phobos.caltech.edu/~jmc/papers/onc

An ALMA Constraint on the GSC 6214-210 B Circum-Substellar Accretion Disk Mass

We present Atacama Large Millimeter/submillimeter Array (ALMA) observations of GSC 6214-210 A and B, a solar-mass member of the 5-10 Myr Upper Scorpius association with a 15 $\pm$ 2 Mjup companion orbiting at $\approx$330 AU (2.2"). Previous photometry and spectroscopy spanning 0.3-5 $\mu$m revealed optical and thermal excess as well as strong H$\alpha$ and Pa~$\beta$ emission originating from a circum-substellar accretion disk around GSC 6214-210 B, making it the lowest mass companion with unambiguous evidence of a subdisk. Despite ALMA's unprecedented sensitivity and angular resolution, neither component was detected in our 880 $\mu$m (341 GHz) continuum observations down to a 3-$\sigma$ limit of 0.22 mJy/beam. The corresponding constraints on the dust mass and total mass are <0.15 Mearth and <0.05 Mjup, respectively, or <0.003% and <0.3% of the mass of GSC 6214-210 B itself assuming a 100:1 gas-to-dust ratio and characteristic dust temperature of 10-20 K. If the host star possesses a putative circum-stellar disk then at most it is a meager 0.0015% of the primary mass, implying that giant planet formation has certainly ceased in this system. Considering these limits and its current accretion rate, GSC 6214-210 B appears to be at the end stages of assembly and is not expected to gain any appreciable mass over the next few Myr.Comment: Accepted to ApJ

Protoplanetary Disk Masses in the Young NGC 2024 Cluster

We present the results from a Submillimeter Array survey of the 887 micron continuum emission from the protoplanetary disks around 95 young stars in the young cluster NGC 2024. Emission was detected from 22 infrared sources, with flux densities from ~5 to 330 mJy; upper limits (at 3sigma) for the other 73 sources range from 3 to 24 mJy. For standard assumptions, the corresponding disk masses range from ~0.003 to 0.2Msolar, with upper limits at 0.002--0.01Msolar. The NGC 2024 sample has a slightly more populated tail at the high end of its disk mass distribution compared to other clusters, but without more information on the nature of the sample hosts it remains unclear if this difference is statistically significant or a superficial selection effect. Unlike in the Orion Trapezium, there is no evidence for a disk mass dependence on the (projected) separation from the massive star IRS2b in the NGC 2024 cluster. We suggest that this is due to either the cluster youth or a comparatively weaker photoionizing radiation field.Comment: ApJ, in pres

Anion Pairs Template a Trigonal Prism with Disilver Vertices.

Here we describe the formation of a trigonal prismatic cage, utilizing 2-formyl-1,8-naphthyridine subcomponents to bind pairs of silver(I) ions in close proximity. This cage is the first example of a new class of subcomponent self-assembled polyhedral structures having bimetallic vertices, as opposed to the single metal centers that typically serve as structural elements within such cages. Our new cage self-assembles around a pair of anionic templates, which are shown by crystallographic and solution-phase data to bind within the central cavity of the structure. Many different anions serve as competent templates and guests. Elongated dianions, such as the strong oxidizing agent peroxysulfate, also serve to template and bind within the cavity of the prism. The principle of using subcomponents that have more than one spatially close, but nonchelating, binding site may thus allow access to other higher-order structures with multimetallic vertices.European Research Council (695009) and the UK Engineering and Physical Sciences Re‐ search Council (EPSRC, EP/P027067/1). European Union's Horizon 2020 research and innovation program, Marie Sklodowska‐Curie Grant (642192)

2MASS Studies of Differential Reddening Across Three Massive Globular Clusters

J, H, and K_S band data from the Two Micron All-Sky Survey (2MASS) are used to study the effects of differential reddening across the three massive Galactic globular clusters Omega Centauri, NGC 6388, and NGC 6441. Evidence is found that variable extinction may produce false detections of tidal tails around Omega Centauri. We also investigate what appears to be relatively strong differential reddening towards NGC 6388 and NGC 6441, and find that differential extinction may be exaggerating the need for a metallicity spread to explain the width of the red giant branches for these two clusters. Finally, we consider the implications of these results for the connection between unusual, multipopulation globular clusters and the cores of dwarf spheroidal galaxies (dSph).Comment: 40 pages, 14 figures. Accepted for publication in Oct. 2003 A

Ascospore release and survival in Sclerotinia sclerotiorum

The release and survival of ascospores of a UK Sclerotinia sclerotiorum isolate were studied. Apothecia placed in a spore clock apparatus with different lighting regimes at 15 °C released ascospores continuously with an increasing rate for the duration of experiments (72–84 h). Spore release was not confined to light or dark periods in alternating regimes and occurred in continuous dark or light. Ascospores were released in both saturated air (90–95% rh) and at 65–75% rh. High temperature and rh were detrimental to ascospore survival but spore viability was maintained for longer periods than previously reported. The significance of these results in relation to disease control is discussed

Debris Disks of Members of the Blanco 1 Open Cluster

We have used the Spitzer Space Telescope to obtain Multiband Imaging Photometer for Spitzer (MIPS) 24 um photometry for 37 members of the ~100 Myr old open cluster Blanco 1. For the brightest 25 of these stars (where we have 3sigma uncertainties less than 15%), we find significant mid-IR excesses for eight stars, corresponding to a debris disk detection frequency of about 32%. The stars with excesses include two A stars, four F dwarfs and two G dwarfs. The most significant linkage between 24 um excess and any other stellar property for our Blanco 1 sample of stars is with binarity. Blanco 1 members that are photometric binaries show few or no detected 24 um excesses whereas a quarter of the apparently single Blanco 1 members do have excesses. We have examined the MIPS data for two other clusters of similar age to Blanco 1 -- NGC 2547 and the Pleiades. The AFGK photometric binary star members of both of these clusters also show a much lower frequency of 24 um excesses compared to stars that lie near the single-star main sequence. We provide a new determination of the relation between V-Ks color and Ks-[24] color for main sequence photospheres based on Hyades members observed with MIPS. As a result of our analysis of the Hyades data, we identify three low mass Hyades members as candidates for having debris disks near the MIPS detection limit.Comment: Accepted to Ap