A Submillimeter Array Survey of Protoplanetary Disks in the Orion Nebula Cluster

We present the full results of our 3-year long Submillimeter Array survey of protoplanetary disks in the Orion Nebula Cluster. We imaged 23 fields at 880 microns and 2 fields at 1330 microns, covering an area of ~6.5 arcmin^2 and containing 67 disks. We detected 42 disks with fluxes between 6-135 mJy and at rms noise levels between 0.6 to 5.3 mJy/beam. Thermal dust emission above any free-free component was measured in 40 of the 42 detections, and the inferred disk masses range from 0.003-0.07 Msolar. We find that disks located within 0.3 pc of theta^1 Ori C have a truncated mass distribution, while disks located beyond 0.3 pc have masses more comparable to those found in low-mass star forming regions. The disk mass distribution in Orion has a distance dependence, with a derived relationship max(M_(disk)) = 0.046Msolar(d/0.3pc)^0.33 for the maximum disk masses. We found evidence of grain growth in disk 197-427, the only disk detected at both 880 microns and 1330 microns with the SMA. Despite the rapid erosion of the outer parts of the Orion disks by photoevaporation, the potential for planet formation remains high in this massive star forming region, with approximately 18% of the surveyed disks having masses greater than or equal to 0.01 Msolar within 60 AU.Comment: Accepted for publication in ApJ, 36 pages, 10 figure

On the fidelity of the core mass functions derived from dust column density data

Aims: We examine the recoverability and completeness limits of the dense core mass functions (CMFs) derived for a molecular cloud using extinction data and a core identification scheme based on two-dimensional thresholding. Methods: We performed simulations where a population of artificial cores was embedded into the variable background extinction field of the Pipe nebula. We extracted the cores from the simulated extinction maps, constructed the CMFs, and compared them to the input CMFs. The simulations were repeated using a variety of extraction parameters and several core populations with differing input mass functions and differing degrees of crowding. Results: The fidelity of the observed CMF depends on the parameters selected for the core extraction algorithm for our background. More importantly, it depends on how crowded the core population is. We find that the observed CMF recovers the true CMF reliably when the mean separation of cores is larger than their mean diameter (f>1). If this condition holds, the derived CMF is accurate and complete above M > 0.8-1.5 Msun, depending on the parameters used for the core extraction. In the simulations, the best fidelity was achieved with the detection threshold of 1 or 2 times the rms-noise of the extinction data, and with the contour level spacings of 3 times the rms-noise. Choosing larger threshold and wider level spacings increases the limiting mass. The simulations show that when f>1.5, the masses of individual cores are recovered with a typical uncertainty of 25-30 %. When f=1 the uncertainty is ~60 %. In very crowded cases where f<1 the core identification algorithm is unable to recover the masses of the cores adequately. For the cores of the Pipe nebula f~2.0 and therefore the use of the method in that region is justified.Comment: 9 pages, 6 figures, accepted for publication in A&

The Circumstellar Disk Mass Distribution in the Orion Trapezium Cluster

We present the results of a submillimeter interferometric survey of circumstellar disks in the Trapezium Cluster of Orion. We observed the 880 micron continuum emission from 55 disks using the Submillimeter Array, and detected 28 disks above 3sigma significance with fluxes between 6-70 mJy and rms noise between 0.7-5.3 mJy. Dust masses and upper limits are derived from the submillimeter excess above free-free emission extrapolated from longer wavelength observations. Above our completeness limit of 0.0084 solar masses, the disk mass distribution is similar to that of Class II disks in Taurus-Auriga and rho Ophiuchus but is truncated at 0.04 solar masses. We show that the disk mass and radius distributions are consistent with the formation of the Trapezium Cluster disks ~1 Myr ago and subsequent photoevaporation by the ultraviolet radiation field from Theta-1 Ori C. The fraction of disks which contain a minimum mass solar nebula within 60 AU radius is estimated to be 11-13% in both Taurus and the Trapezium Cluster, which suggests the potential for forming Solar Systems is not compromised in this massive star forming region.Comment: Accepted for publication in ApJL (2009 Feb 3

Hastily Formed Networks (HFN) As an Enabler for the Emergency Response Community

The effects of natural or manmade disasters in communications infrastructures are so severe that immediately after the disaster the emergency responders are unable to use them. In addition, some areas do not have any useful infrastructure at all. To bridge this gap in communications, a need exists for a reliable technology not dependent on the existing infrastructure. This thesis focuses on first identifying the problem of communications gaps during natural or manmade disasters and reviewing the impact and potential benefit of implementing a solution based on the Hastily Formed Networks (HFN) model. The research explores the different technological solutions to solve this problem by evaluating documentation for commercial off-the-shelf technologies (COTS). Additionally, the thesis reviews the results of field experimentation conducted to evaluate the performance of these technologies in the field. The ultimate goal is to introduce the HFN concept as an enabler for the Emergency Response Community (ERC). Throughout this research, the focus revolves around testing COTS technologies. The research provides emergency responders with the background knowledge to make decisions on how to best bridge the gap of lack of communications under austere environments, and therefore enable them to provide better response.http://archive.org/details/hastilyformednet109456762Lieutenant Commander, United States Nav

Hydrogen Two-Photon Continuum Emission from the Horseshoe Filament in NGC 1275

Far ultraviolet emission has been detected from a knot of Halpha emission in the Horseshoe filament, far out in the NGC 1275 nebula. The flux detected relative to the brightness of the Halpha line in the same spatial region is very close to that expected from Hydrogen two-photon continuum emission in the particle heating model of Ferland et al. (2009) if reddening internal to the filaments is taken into account. We find no need to invoke other sources of far ultraviolet emission such as hot stars or emission lines from CIV in intermediate temperature gas to explain these data.Comment: 9 pages, 8 figures. Accepted for publication in MNRA

A sighting of a Luth (Dermochelys coriacea (L): Chelonia) in Bass Strait, Tasmania

At 09.00 h Eastern Daylight Saving Time on 8 December 1973 we observed a large turtle from the bow of the M. V. NELLA DAN. It was floating at the surface of the water with part of its carapace exposed, and swam slowly southwards out of the ship's path. According to the ship's log, NELLA DAN was at that time in Bass Strait in about 400 00' s, 147 degrees 02'E, travelling on a course of 309 degrees west of the Furneaux Group, Tasmania

Antiretroviral therapy initiated soon after HIV diagnosis as standard care: potential to save lives?

In 2008, an estimated 33.4 million people were infected with human immunodeficiency virus (HIV) and ~4 million people were receiving antiretroviral therapy (ART). However, in 2007, an estimated 6.7 million people were in need of ART under the current World Health Organization guidelines, and 2.7 million more people became infected with HIV. Most of those not currently eligible for ART will become eligible within the next decade, making the current treatment strategy unsustainable. The development of cheaper, less toxic, and more potent antiretrovirals over the past decade has made it possible to consider novel strategies of arresting the HIV/AIDS epidemic. Evidence is growing that ART can be used to prevent HIV transmission and that earlier initiation of treatment is beneficial for those infected with HIV. A mathematical model predicts that by testing whole communities annually and treating all who are infected immediately, up to 7.2 million AIDS-related deaths could be prevented in the next 40 years, long-term funding required to fight the HIV epidemic could be reduced, and, most importantly, control of the HIV/ AIDS epidemic could be regained within 1–2 years of full-scale implementation of the strategy. We discuss the development of the concept of ART for the prevention of HIV transmission and the modeled impact that a test-and-treat strategy could have on the HIV epidemic, and consequently argue that a field trial should be carried out to confirm model parameters, highlight any practical problems, and test the model’s predictions

4-(3-Azaniumylpropyl)morpholin-4-ium chloride hydrogen oxalate: An unusual example of a dication with different counter-anions

© 2014 International Union of Crystallography. The mixed organic-inorganic title salt, C7H18N2O2+·C2HO4-·Cl-, forms an assembly of ionic components which are stabilized through a series of hydrogen bonds and charge-assisted intermolecular interactions. The title assembly crystallizes in the monoclinic C2/c space group with Z = 8. The asymmetric unit consists of a 4-(3-azaniumylpropyl)morpholin-4-ium dication, a hydrogen oxalate counter-anion and an inorganic chloride counter-anion. The organic cations and anions are connected through a network of N - H⋯O, O - H⋯O and C - H⋯O hydrogen bonds, forming several intermolecular rings that can be described by the graph-set notations R33(13), R21(5), R12(5), R21(6), R23(6), R22(8) and R33(9). The 4-(3-azaniumylpropyl)morpholin-4-ium dications are interconnected through N - H⋯O hydrogen bonds, forming C(9) chains that run diagonally along the ab face. Furthermore, the hydrogen oxalate anions are interconnected via O - H⋯O hydrogen bonds, forming head-to-tail C(5) chains along the crystallographic b axis. The two types of chains are linked through additional N - H⋯O and O - H⋯O hydrogen bonds, and the hydrogen oxalate chains are sandwiched by the 4-(3-azaniumylpropyl)morpholin-4-ium chains, forming organic layers that are separated by the chloride anions. Finally, the layered three-dimensional structure is stabilized via intermolecular N - H⋯Cl and C - H⋯Cl interactions

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

Constraints on star formation theories from the Serpens molecular cloud and protocluster

We have mapped the large-scale structure of the Serpens cloud core using moderately optically thick (13CO(1--0) and CS(2--1)) and optically thin tracers (C18O(1--0), C34S(2--1), and N2H+(1--0)), using the 16-element focal plane array operating at a wavelength of 3mm at the Five College Radio Astronomy Observatory. Our main goal was to study the large-scale distribution of the molecular gas in the Serpens region and to understand its relation with the denser gas in the cloud cores, previously studied at high angular resolution. All our molecular tracers show two main gas condensations, or sub-clumps, roughly corresponding to the North-West and South-East clusters of submillimeter continuum sources. We also carried out a kinematical study of the Serpens cloud. The 13CO and C18O(1--0) maps of the centroid velocity show an increasing, smooth gradient in velocity from East to West, which we think may be caused by a global rotation of the Serpens molecular cloud whose rotation axis is roughly aligned in the SN direction. Although it appears that the cloud angular momentum is not sufficient for being dynamically important in the global evolution of the cluster, the fact that the observed molecular outflows are roughly aligned with it may suggest a link between the large-scale angular momentum and the circumstellar disks around individual protostars in the cluster. We also used the normalized centroid velocity difference as an infall indicator. We find two large regions of the map, approximately coincident with the SE and NW sub-clumps, which are undergoing an infalling motion. Although our evidence is not conclusive, our data appear to be in qualitative agreement with the expectation of a slow contraction followed by a rapid and highly efficient star formation phase in localized high density regions.Comment: 17 pages, A&A in press, full resolution figures available at http://www.arcetri.astro.it/~lt/preprints/preprints.htm