Protoplanetary Disk Structures in Ophiuchus II: Extension to Fainter Sources

We present new results from a significant extension of our previous high angular resolution (0.3" = 40 AU) Submillimeter Array survey of the 880 um continuum emission from dusty circumstellar disks in the ~1 Myr-old Ophiuchus star-forming region. An expanded sample is constructed to probe disk structures that emit significantly lower millimeter luminosities (hence dust masses), down to the median value for T Tauri stars. Using a Monte Carlo radiative transfer code, the millimeter visibilities and broadband spectral energy distribution for each disk are simultaneously reproduced with a two-dimensional parametric model for a viscous accretion disk. We find wide ranges of characteristic radii (14-198 AU) and disk masses (0.004-0.143 M_sun), but a narrow distribution of surface density gradients (0.4-1.1) that is consistent with a uniform value $\gamma$ = 0.9 +/- 0.2 and independent of mass (or millimeter luminosity). In this sample, we find a correlation between the disk luminosity/mass and characteristic radius, such that fainter disks are both smaller and less massive. We suggest that this relationship is an imprint of the initial conditions inherited by the disks at their formation epoch, compare their angular momenta with those of molecular cloud cores, and speculate on how future observations can help constrain the distribution of viscous evolution timescales. No other correlations between disk and star properties are found. The inferred disk structures are briefly compared with theoretical models for giant planet formation, although resolution limitations do not permit us to directly comment on material inside R = 20 AU. However, there is some compelling evidence for dust evolution in the planet formation region: 4/17 disks in the sample show resolved regions of significantly reduced optical depths within ~20-40 AU of their central stars.Comment: accepted in ApJ, 39 pages, 10 figure

Resolved Images of Large Cavities in Protoplanetary Transition Disks

Circumstellar disks are thought to experience a rapid "transition" phase in their evolution that can have a considerable impact on the formation and early development of planetary systems. We present new and archival high angular resolution (0.3" = 40-75 AU) Submillimeter Array (SMA) observations of the 880 micron dust continuum emission from 12 such transition disks in nearby star-forming regions. In each case, we directly resolve a dust-depleted disk cavity around the central star. Using radiative transfer calculations, we interpret these dust disk structures in a homogeneous, parametric model framework by reproducing their SMA visibilities and SEDs. The cavities in these disks are large (R_cav = 15-73 AU) and substantially depleted of small (~um-sized) dust grains, although their mass contents are still uncertain. The structures of the remnant material at larger radii are comparable to normal disks. We demonstrate that these large cavities are common among the millimeter-bright disk population, comprising at least 20% of the disks in the bright half of the millimeter luminosity (disk mass) distribution. Utilizing these results, we assess some of the physical mechanisms proposed to account for transition disk structures. As has been shown before, photoevaporation models do not produce the large cavity sizes, accretion rates, and disk masses representative of this sample. It would be difficult to achieve a sufficient decrease of the dust optical depths in these cavities by particle growth alone: substantial growth (to meter sizes or beyond) must occur in large (tens of AU) regions of low turbulence without also producing an abundance of small particles. Given those challenges, we suggest instead that the observations are most commensurate with dynamical clearing due to tidal interactions with low-mass companions --young brown dwarfs or giant planets on long-period orbits.Comment: ApJ, in pres

Protoplanetary Disk Structures in Ophiuchus

We present a high angular resolution (0.3" = 40 AU) SMA survey of the 870 micron thermal continuum emission from 9 of the brightest, and therefore most massive, circumstellar disks in the ~1 Myr-old Ophiuchus star-forming region. Using 2-D radiative transfer calculations, we simultaneously fit the observed continuum visibilities and broadband spectral energy distribution for each disk with a parametric structure model. Compared to previous millimeter studies, this survey includes significant upgrades in modeling, data quality, and angular resolution that provide improved constraints on key structure parameters, particularly those that characterize the spatial distribution of mass in the disks. In the context of a surface density profile motivated by similarity solutions for viscous accretion disks, the best-fit models for the sample disks have characteristic radii R_c = 20-200 AU, high disk masses M_d = 0.005-0.14 M_sun, and a narrow range of radial surface density gradients around a median $\gamma$ = 0.9. These density structures are used in conjunction with accretion rate estimates from the literature to help characterize the viscous evolution of the disk material. Using the standard prescription for disk viscosities, those combined constraints indicate that $\alpha$ = 0.0005-0.08. Three of the sample disks show large (R = 20-40 AU) central cavities in their continuum emission morphologies, marking extensive zones where dust has been physically removed and/or has significantly diminished opacities. Based on the current requirements of planet formation models, these emission cavities and the structure constraints for the sample as a whole suggest that these young disks may eventually produce planetary systems, and have perhaps already started. (abridged)Comment: ApJ in press: 51 pages, 13 figure

An approach to the control of disease transmission in pig-to-human xenotransplantation.

Abstract: Although several major immunologic hurdles need to be overcome, the pig is currently considered the most likely source animal of cells, tissues and organs for transplantation into humans. Concerns have been raised with regard to the potential for the transfer of infectious agents with the transplanted organ to the human recipient. This risk is perceived to be increased as it is likely that the patient will be iatrogenically immunocompromised and the organ-source pig may be genetically engineered in such a way to render its organs particularly susceptible to infection with human viruses. Furthermore, the risk may not be restricted to the recipient, but may have consequences for the health of others in the community. The identification of porcine endogenous retroviruses and of hitherto unknown viruses have given rise to the most concern. We document here the agents we believe should be excluded from the organ-source pigs. We discuss the likelihood of achieving this aim and outline the potential means by which it may best be achieved

Temporal and spatial analysis of the 2014-2015 Ebola virus outbreak in West Africa

West Africa is currently witnessing the most extensive Ebola virus (EBOV) outbreak so far recorded. Until now, there have been 27,013 reported cases and 11,134 deaths. The origin of the virus is thought to have been a zoonotic transmission from a bat to a two-year-old boy in December 2013 (ref. 2). From this index case the virus was spread by human-to-human contact throughout Guinea, Sierra Leone and Liberia. However, the origin of the particular virus in each country and time of transmission is not known and currently relies on epidemiological analysis, which may be unreliable owing to the difficulties of obtaining patient information. Here we trace the genetic evolution of EBOV in the current outbreak that has resulted in multiple lineages. Deep sequencing of 179 patient samples processed by the European Mobile Laboratory, the first diagnostics unit to be deployed to the epicentre of the outbreak in Guinea, reveals an epidemiological and evolutionary history of the epidemic from March 2014 to January 2015. Analysis of EBOV genome evolution has also benefited from a similar sequencing effort of patient samples from Sierra Leone. Our results confirm that the EBOV from Guinea moved into Sierra Leone, most likely in April or early May. The viruses of the Guinea/Sierra Leone lineage mixed around June/July 2014. Viral sequences covering August, September and October 2014 indicate that this lineage evolved independently within Guinea. These data can be used in conjunction with epidemiological information to test retrospectively the effectiveness of control measures, and provides an unprecedented window into the evolution of an ongoing viral haemorrhagic fever outbreak.status: publishe