OH yields from the CH3CO+O-2 reaction using an internal standard

Laser flash photolysis of CH3C(O)OH at 248 nm was used to create equal zero time yields of CH3CO and OH. The absolute OH yield from the CH3CO + O2 (+M) reaction was determined by following the OH temporal profile using the zero time OH concentration as an internal standard. The OH yield from CH3CO + O2 (+M) was observed to decrease with increasing pressure with an extrapolated zero pressure yield close to unity (1.1 ± 0.2, quoted uncertainties correspond to 95% confidence limits). The results are in quantitative agreement with those obtained from 248 nm acetone photolysis in the presence of O2

The Galactic IMF: origin in the combined mass distribution functions of dust grains and gas clouds

We present here a theoretical model to account for the stellar IMF as a result of the composite behaviour of the gas and dust distribution functions. Each of these has previously been modelled and the models tested against observations. The model presented here implies a relation between the characteristic size of the dust grains and the characteristic final mass of the stars formed within the clouds containing the grains, folded with the relation between the mass of a gas cloud and the characteristic mass of the stars formed within it. The physical effects of dust grain size are due to equilibrium relations between the efficiency of grains in cooling the clouds, which is a falling function of grain size, and the efficiency of grains in catalyzing the production of molecular hydrogen, which is a rising function of grain size. We show that folding in the effects of grain distribution can yield a reasonable quantitative account of the IMF, while gas cloud mass function alone cannot do so.Comment: 8 pages, 6 figures, MNRAS accepted for publicatio

Regulating Drones Under the First and Fourth Amendments

The FAA Modernization and Reform Act of 2012 requires the Federal Aviation Administration to integrate unmanned aerial vehicles (UAVs), or drones, into the national airspace system by September 2015. Yet perhaps because of their chilling accuracy in targeted killings abroad, perhaps because of an increasing consciousness of diminishing privacy more generally, and perhaps simply because of a fear of the unknown, divergent UAV-restrictive legislation has been proposed in Congress and enacted in a number of states. Given UAV utility and cost-effectiveness over a vast range of tasks, however, widespread commercial use ultimately seems certain. Consequently, it is imperative to understand the constitutional restraints on public flight and constitutional protections afforded to private flight. Unfortunately, although there are a few Fourth Amendment precedents in manned aviation, they are mired not only in 1980s technology but also in the 1980s third party doctrine, and therefore do not reflect more recent Fourth Amendment developments and doctrinal fissures. There is also considerable uncertainty over First Amendment protection of information-gathering—for example, is there a right to record? Further, there is no judicial or scholarly analysis of how UAV flight fits within contemporary First Amendment forum doctrine, a framework that provides a useful starting point for analyzing speech restrictions in government-controlled airspace, but that comes with some uncertainties of its own. It is into this thicket that we dive, and fortunately some clarity emerges. Although the Fourth Amendment third party doctrine hopelessly misunderstands privacy and therefore under-protects our security and liberty interests, the Supreme Court’s manned flyover cases can be mined for a sensible public disclosure doctrine that seems agnostic as to the various Fourth Amendment conceptions: we do not typically require only law enforcement to shield its eyes. Of course, both constitutions and legislation can place special restrictions upon law enforcement, and sometimes doing so makes good sense. But as a general Fourth Amendment matter, the officer may do and see as the citizen would. Hence to understand Fourth Amendment regulation, we must understand how the First Amendment limits government restraint on speech-relevant private UAV flight. Here we analyze the developing right to record and apply contemporary forum doctrine to this novel means of speech and information-gathering. If navigable airspace is treated as a limited public forum, as we propose with some qualification, then the Federal Aviation Administration will have significant—though not unlimited—regulatory leeway to evenhandedly burden speech-related UAV activities where doing so would reasonably promote safe unmanned and manned flight operations. The Agency, however, would likely need further congressional action before it can restrict UAV flight based on privacy rather than safety concerns. As the legality and norms of private flight correspondingly take shape, they will inform Fourth Amendment restrictions on government use

The Perils of Clumpfind: The Mass Spectrum of Sub-structures in Molecular Clouds

We study the mass spectrum of sub-structures in the Perseus Molecular Cloud Complex traced by 13CO (1-0), finding that $dN/dM\propto M^{-2.4}$ for the standard Clumpfind parameters. This result does not agree with the classical $dN/dM\propto M^{-1.6}$. To understand this discrepancy we study the robustness of the mass spectrum derived using the Clumpfind algorithm. Both 2D and 3D Clumpfind versions are tested, using 850 $\mu$m dust emission and 13CO spectral-line observations of Perseus, respectively. The effect of varying threshold is not important, but varying stepsize produces a different effect for 2D and 3D cases. In the 2D case, where emission is relatively isolated (associated with only the densest peaks in the cloud), the mass spectrum variability is negligible compared to the mass function fit uncertainties. In the 3D case, however, where the 13CO emission traces the bulk of the molecular cloud, the number of clumps and the derived mass spectrum are highly correlated with the stepsize used. The distinction between "2D" and "3D" here is more importantly also a distinction between "sparse" and "crowded" emission. In any "crowded" case, Clumpfind should not be used blindly to derive mass functions. Clumpfind's output in the "crowded" case can still offer a statistical description of emission useful in inter-comparisons, but the clump-list should not be treated as a robust region decomposition suitable to generate a physically-meaningful mass function. We conclude that the 13CO mass spectrum depends on the observations resolution, due to the hierarchical structure of MC.Comment: 5 pages, 3 figures. Accepted for publication in ApJ Letter

Unusually Luminous Giant Molecular Clouds in the Outer Disk of M33

We use high spatial resolution (~7pc) CARMA observations to derive detailed properties for 8 giant molecular clouds (GMCs) at a galactocentric radius corresponding to approximately two CO scale lengths, or ~0.5 optical radii (r25), in the Local Group spiral galaxy M33. At this radius, molecular gas fraction, dust-to-gas ratio and metallicity are much lower than in the inner part of M33 or in a typical spiral galaxy. This allows us to probe the impact of environment on GMC properties by comparing our measurements to previous data from the inner disk of M33, the Milky Way and other nearby galaxies. The outer disk clouds roughly fall on the size-linewidth relation defined by extragalactic GMCs, but are slightly displaced from the luminosity-virial mass relation in the sense of having high CO luminosity compared to the inferred virial mass. This implies a different CO-to-H2 conversion factor, which is on average a factor of two lower than the inner disk and the extragalactic average. We attribute this to significantly higher measured brightness temperatures of the outer disk clouds compared to the ancillary sample of GMCs, which is likely an effect of enhanced radiation levels due to massive star formation in the vicinity of our target field. Apart from brightness temperature, the properties we determine for the outer disk GMCs in M33 do not differ significantly from those of our comparison sample. In particular, the combined sample of inner and outer disk M33 clouds covers roughly the same range in size, linewidth, virial mass and CO luminosity than the sample of Milky Way GMCs. When compared to the inner disk clouds in M33, however, we find even the brightest outer disk clouds to be smaller than most of their inner disk counterparts. This may be due to incomplete sampling or a potentially steeper cloud mass function at larger radii.Comment: Accepted for Publication in ApJ; 7 pages, 4 figure

A Search for Kinematic Evidence of Radial Gas Flows in Spiral Galaxies

CO and HI velocity fields of seven nearby spiral galaxies, derived from radio-interferometric observations, are decomposed into Fourier components whose radial variation is used to search for evidence of radial gas flows. Additional information provided by optical or near-infrared isophotes is also considered, including the relationship between the morphological and kinematic position angles. To assist in interpreting the data, we present detailed modeling that demonstrates the effects of bar streaming, inflow, and a warp on the observed Fourier components. We find in all of the galaxies evidence for either elliptical streaming or a warped disk over some range in radius, with deviations from pure circular rotation at the level of ~20-60 km/s. Evidence for kinematic warps is observed in several cases well inside R_{25}. No unambiguous evidence for radial inflows is seen in any of the seven galaxies, and we are able to place an upper limit of ~5-10 km/s (3-5% of the circular speed) on the magnitude of any radial inflow in the inner regions of NGC 4414, 5033 and 5055. We conclude that the inherent non-axisymmetry of spiral galaxies is the greatest limitation to the direct detection of radial inflows.Comment: 22 emulateapj pages with bitmapped colour figures, to appear in ApJ (April 2004). For full resolution figures go to http://www.atnf.csiro.au/people/twong/preprints

Evidence for a Weak Galactic Center Magnetic Field from Diffuse Low Frequency Nonthermal Radio Emission

New low-frequency 74 and 330 MHz observations of the Galactic center (GC) region reveal the presence of a large-scale (6\arcdeg\times 2\arcdeg) diffuse source of nonthermal synchrotron emission. A minimum energy analysis of this emission yields a total energy of $\sim (\phi^{4/7}f^{3/7})\times 10^{52}$ ergs and a magnetic field strength of $\sim 6(\phi/f)^{2/7}$ \muG (where $\phi$ is the proton to electron energy ratio and $f$ is the filling factor of the synchrotron emitting gas). The equipartition particle energy density is $1.2(\phi/f)^{2/7}$ \evcm, a value consistent with cosmic-ray data. However, the derived magnetic field is several orders of magnitude below the 1 mG field commonly invoked for the GC. With this field the source can be maintained with the SN rate inferred from the GC star formation. Furthermore, a strong magnetic field implies an abnormally low GC cosmic-ray energy density. We conclude that the mean magnetic field in the GC region must be weak, of order 10 \muG (at least on size scales \ga 125\arcsec).Comment: 12 pages, 1 JPEG figure, uses aastex.sty; Accepted for publication, ApJL (2005, published

Physical Properties of Giant Molecular Clouds in the Large Magellanic Cloud

The Magellanic Mopra Assessment (MAGMA) is a high angular resolution CO mapping survey of giant molecular clouds (GMCs) in the Large and Small Magellanic Clouds using the Mopra Telescope. Here we report on the basic physical properties of 125 GMCs in the LMC that have been surveyed to date. The observed clouds exhibit scaling relations that are similar to those determined for Galactic GMCs, although LMC clouds have narrower linewidths and lower CO luminosities than Galactic clouds of a similar size. The average mass surface density of the LMC clouds is 50 Msol/pc2, approximately half that of GMCs in the inner Milky Way. We compare the properties of GMCs with and without signs of massive star formation, finding that non-star-forming GMCs have lower peak CO brightness than star-forming GMCs. We compare the properties of GMCs with estimates for local interstellar conditions: specifically, we investigate the HI column density, radiation field, stellar mass surface density and the external pressure. Very few cloud properties demonstrate a clear dependence on the environment; the exceptions are significant positive correlations between i) the HI column density and the GMC velocity dispersion, ii) the stellar mass surface density and the average peak CO brightness, and iii) the stellar mass surface density and the CO surface brightness. The molecular mass surface density of GMCs without signs of massive star formation shows no dependence on the local radiation field, which is inconsistent with the photoionization-regulated star formation theory proposed by McKee (1989). We find some evidence that the mass surface density of the MAGMA clouds increases with the interstellar pressure, as proposed by Elmegreen (1989), but the detailed predictions of this model are not fulfilled once estimates for the local radiation field, metallicity and GMC envelope mass are taken into account.Comment: 28 pages, 10 figures, accepted by MNRA