Casimir effect for scalar fields under Robin boundary conditions on plates

We study the Casimir effect for scalar fields with general curvature coupling subject to mixed boundary conditions $(1+\beta_{m}n^{\mu}\partial_{\mu})\phi =0$ at $x=a_{m}$ on one ($m=1$) and two ($m=1,2$) parallel plates at a distance $a\equiv a_{2}-a_{1}$ from each other. Making use of the generalized Abel-Plana formula previously established by one of the authors \cite{Sahrev}, the Casimir energy densities are obtained as functions of $\beta_{1}$ and of $\beta_{1}$,$\beta_{2}$,$a$, respectively. In the case of two parallel plates, a decomposition of the total Casimir energy into volumic and superficial contributions is provided. The possibility of finding a vanishing energy for particular parameter choices is shown, and the existence of a minimum to the surface part is also observed. We show that there is a region in the space of parameters defining the boundary conditions in which the Casimir forces are repulsive for small distances and attractive for large distances. This yields to an interesting possibility for stabilizing the distance between the plates by using the vacuum forces.Comment: 21 pages, 8 figures, consideration of the contribution from complex eigenmodes added, possibility for the stabilization of the distance between the plates is discussed; accepted for publication in J. Phys.

Radial Temperature Profiles of X-Ray--Emitting Gas Within Clusters of Galaxies

Previous analyses of ASCA data of clusters of galaxies have found conflicting results regarding the slope of the temperature profile of the hot X-ray gas within clusters, mainly because of the large, energy-dependent point spread function (PSF) of the ASCA mirrors. We present a summary of all ASCA-determined cluster temperature profiles found in the literature, and find a discrepancy in the radial temperature trend of clusters based on which PSF-correction routine is used. This uncertainty in the cluster temperature profile in turn can lead to large uncertainties in the amount of dark matter in clusters. In this study, we have used ROSAT PSPC data to obtain independent relative temperature profiles for 26 clusters, most of which have had their temperature profiles determined by ASCA. Our aim is not to measure the actual temperature values of the clusters, but to use X-ray color profiles to search for a hardening or softening of the spectra with radius for comparison to ASCA-derived profiles. The radial color profiles indicate that outside of the cooling flow region, the temperature profiles of clusters are in general constant. Within 35% of the virial radius, we find a temperature drop of 20% at 10 keV and 12% at 5 keV can be ruled out at the 99% confidence level. A subsample of non-cooling flow clusters shows that the condition of isothermality applies at very small radii too, although cooling gas complicates this determination in the cooling flow subsample. The colors predicted from the temperature profiles of a series of hydrodynamical cluster simulations match the data very well, although they cannot be used to discriminate among different cosmologies. An additional result is that the color profiles show evidence for a central peak in metallicity in low temperature clusters.Comment: 39 pages, 15 embedded Postscript figures, uses aaspp4.sty, accepted for publication in Astrophysical Journa

Dynamical scaling law in the development of drift wave turbulence

Destabilization and degradation of permafrost carbon in the Arctic regions could constitute a positive feedback to climate change. A better understanding of its fate upon discharge to the Arctic shelf is therefore needed. In this study, bulk carbon isotopes as well as terrigenous and marine biomarkers were used to construct two centennial records in the East Siberian Sea. Differences in topsoil and Pleistocene Ice Complex Deposit permafrost concentrations, modeled using δ13C and δ14C, were larger between inner and outer shelf than the changes over time. Similarly, lignin-derived phenol and cutin acid concentrations differed by a factor of ten between the two stations, but did not change significantly over time, consistent with the dual-carbon isotope model. High molecular weight (HMW) n-alkane and n-alkanoic acid concentrations displayed a smaller difference between the two stations (factor of 3-6). By contrast, the fraction for marine OC drastically decreased during burial with a half-life of 19-27 years. Vegetation and degradation proxies suggested supply of highly degraded gymnosperm wood tissues. Lipid Carbon Preference Index (CPI) values indicated more extensively degraded HMW n-alkanes on the outer shelf with no change over time, whereas n-alkanoic acids appeared to be less degraded toward the core top with no large differences between the stations. Taken together, our results show larger across-shelf changes than down-core trends. Further investigation is required to establish whether the observed spatial differences are due to different sources for the two depositional settings or, alternatively, a consequence of hydrodynamic sorting combined with selective degradation during cross-shelf transport

Molecular and radiocarbon constraints on sources and degradation of terrestrial organic carbon along the Kolyma paleoriver transect, East Siberian Sea

© The Authors, 2010. This article is distributed under the terms of the Creative Commons Attribution 3.0 License. The definitive version was published in Biogeosciences 7 (2010): 3153-3166, doi:10.5194/bg-7-3153-2010.Climate warming in northeastern Siberia may induce thaw-mobilization of the organic carbon (OC) now held in permafrost. This study investigated the composition of terrestrial OC exported to Arctic coastal waters to both obtain a natural integration of terrestrial permafrost OC release and to further understand the fate of released carbon in the extensive Siberian Shelf Seas. Application of a variety of elemental, molecular and isotopic (δ13C and Δ14C) analyses of both surface water suspended particulate matter and underlying surface sediments along a 500 km transect from Kolyma River mouth to the mid-shelf of the East Siberian Sea yielded information on the sources, degradation status and transport processes of thaw-mobilized soil OC. A three end-member dual-carbon-isotopic mixing model was applied to deduce the relative contributions from riverine, coastal erosion and marine sources. The mixing model was solved numerically using Monte Carlo simulations to obtain a fair representation of the uncertainties of both end-member composition and the end results. Riverine OC contributions to sediment OC decrease with increasing distance offshore (35±15 to 13±9%), whereas coastal erosion OC exhibits a constantly high contribution (51±11 to 60±12%) and marine OC increases offshore (9±7 to 36±10%). We attribute the remarkably strong imprint of OC from coastal erosion, extending up to ~500 km from the coast, to efficient offshoreward transport in these shallow waters presumably through both the benthic boundary layer and ice-rafting. There are also indications of simultaneous selective preservation of erosion OC compared to riverine OC. Molecular degradation proxies and radiocarbon contents indicated a degraded but young (Δ14C ca. −60‰ or ca. 500 14C years) terrestrial OC pool in surface water particulate matter, underlain by a less degraded but old (Δ14C ca. −500‰ or ca. 5500 14C years) terrestrial OC pool in bottom sediments. We suggest that the terrestrial OC fraction in surface water particulate matter is mainly derived from surface soil and recent vegetation fluvially released as buoyant organic-rich aggregates (e.g., humics), which are subjected to extensive processing during coastal transport. In contrast, terrestrial OC in the underlying sediments is postulated to originate predominantly from erosion of mineral-rich Pleistocene coasts (i.e., yedoma) and inland mineral soils. Sorptive association of this organic matter with mineral particles protects the OC from remineralization and also promotes rapid settling (ballasting) of the OC. Our findings corroborate recent studies by indicating that different Arctic surface soil OC pools exhibit distinguishing susceptibilities to degradation in coastal waters. Consequently, the general postulation of a positive feedback to global warming from degradation of permafrost carbon may be both attenuated (by reburial of one portion) and geographically displaced (degradation of released terrestrial permafrost OC far out over the Arctic shelf seas).The ISSS-08 program was supported by the Knut and Alice Wallenberg Foundation, Headquarters of the Far Eastern Branch of the Russian Academy of Sciences, the Swedish Research Council (VR Contract No. 621-2004-4039 and 621-2007-4631), the US National Oceanic and Atmospheric Administration (Siberian Shelf Study), the Russian Foundation of Basic Research (08-05-13572, 08-05-00191-a, and 07-05-00050a), the Swedish Polar Research Secretariat, the Arctic Co-Op Program of the Nordic Council of Ministers (331080-70219) and the National Science Foundation (OPP ARC 0909546). O¨ . G. also acknowledges financial support as an Academy Research Fellow from the Swedish Royal Academy of Sciences, L. S. a Marie Curie grant (contract no. PIEF-GA-2008-220424), T. E. an NSF grant (ARC-0909377) and A. A. support from the Knut and Alice Wallenberg Foundation

Connectivity and Nitrate Uptake Potential of Intermittent Streams in the Northeast USA

Non-perennial streams dominate the extent of stream networks worldwide. Intermittent streams can provide ecosystem services to the entire network—including nitrate uptake to alleviate eutrophication of coastal waters—and are threatened by lack of legal protection. We examined 12 intermittent streams in the temperate, humid climate of the Northeast USA. Over 3 years of monitoring, continuous flow was observed a median of 277 d yr−1, with no-flow conditions from early summer into fall. Estimated median discharge was 2.9 L s−1 or 0.36mm d−1. All intermittent streams originated from source wetlands (median area: 0.27 ha) and the median length of the intermittent stream from the source wetland to the downstream perennial stream was 344m. Through regional geospatial analysis with high resolution orthophotography, we estimated that widely available, “high resolution” (1:24,000) hydrography databases (e.g., NHDPlus HR) only displayed 43% of the total number of intermittent streams. Whole-stream gross nitrate-N uptake rates were estimated at six intermittent streams during continuous flow conditions using pulse additions of nitrate and a conservative tracer. These rates displayed high temporal variability (range: no detect to over 6,000mg N m−1 d−1); hot moments were noted in nine of the 65 pulse additions. Whole-stream gross nitrate-N uptake rates were significantly inversely related to discharge, with no measurable rates above 7 L s−1. Temperature was significantly positively correlated with whole-stream gross nitrate-N uptake rates, with more hot moments in the spring. Microbial assays demonstrated that nitrate cycling in intermittent streams are consistent with results from low order, perennial forested streams and highlighted the importance of debris dams and pools—potential locations for transient storage. Our assessment suggests that intermittent streams in our region may annually contribute 24–47% of the flow to perennial streams and potentially remove 4.1 to 80.4 kg nitrate-N km−2 annually. If development in these areas continues, perennial streams are in danger of losing a portion of their headwaters and potential nitrate uptake areas may become nitrate sources to downstream areas. These results argue to manage fluvial systems with a holistic approach that couples intermittent and perennial components

Infrared L Band Observations of the Trapezium Cluster: A Census of Circumstellar Disks and Candidate Protostars

We report the results of a sensitive near-infrared JHKL imaging survey of the Trapezium cluster in Orion. We use the JHKL colors to obtain a census of infrared excess stars in the cluster. Of (391) stars brighter than 12th magnitude in the K and L bands, 80 +/- 7% are found to exhibit detectable infrared excess on the J-H, K-L color-color diagram. Examination of a subsample of 285 of these stars with published spectral types yields a slightly higher infrared excess fraction of 85%. We find that 97% of the optical proplyds in the cluster exhibit excess in the JHKL color-color diagram indicating that the most likely origin of the observed infrared excesses is from circumstellar disks. We interpret these results to indicate that the fraction of stars in the cluster with circumstellar disks is between 80-85%. Moreover, we find that the probability of finding an infrared excess/protoplanetary disk around a star is independent of stellar mass over essentially the entire range of the stellar mass function down to the hydrogen burning limit. We identify 78 stars in our sample characterized by K-L colors suggestive of deeply embedded protostellar objects. If even a modest fraction fraction (i.e., ~ 50%) of these objects are protostars, then star formation could be continuing in the molecular ridge at a rate comparable to that which produced the foreground Trapezium cluster.Comment: 33 pages plus 3 separate color figures. For higher resolution color figures and a single file containing the entire paper, figures and tables see http://cfa-www.harvard.edu/~gmuench/thesis/clusters/TRAP/traplband.html Used AASTEX macros v 5.0. Paper will appear in December A

Taking gradients through experiments: LSTMs and memory proximal policy optimization for black-box quantum control

In this work we introduce the application of black-box quantum control as an interesting rein- forcement learning problem to the machine learning community. We analyze the structure of the reinforcement learning problems arising in quantum physics and argue that agents parameterized by long short-term memory (LSTM) networks trained via stochastic policy gradients yield a general method to solving them. In this context we introduce a variant of the proximal policy optimization (PPO) algorithm called the memory proximal policy optimization (MPPO) which is based on this analysis. We then show how it can be applied to specific learning tasks and present results of nu- merical experiments showing that our method achieves state-of-the-art results for several learning tasks in quantum control with discrete and continouous control parameters

Why are the K dwarfs in the Pleiades so Blue?

The K dwarfs in the Pleiades fall nearly one half magnitude below a main sequence isochrone when plotted in a color-magnitude diagram utilizing V magnitude as the luminosity index and B-V as the color index. This peculiarity has been known for forty years but has gone unexplained and mostly ignored. When compared to Praesepe members, the Pleiades K dwarfs again are subluminous (or blue) in a color-magnitude diagram using B-V as the color index. However, using V-I as the color index, stars in the two clusters are coincident to M_V ~ 10; using V-K as the color index, Pleiades late K and M stars fall above the main sequence locus defined by Praesepe members. We believe that the anomalous spectral energy distributions for the Pleiades K dwarfs, as compared to older clusters, are a consequence of rapid stellar rotation and may be primarily due to spottedness. If so, the required areal filling factor for the cool component has to be very large (=> 50%). Weak-lined T Tauri stars have similar color anomalies, and we suspect this is a common feature of all very young K dwarfs (sp. type > K3). The peculiar spectral energy distribution needs to be considered in deriving accurate pre-main sequence isochrone-fitting ages for clusters like the Pleiades since the age derived will depend on the temperature index used.Comment: 41 pages, 15 figures, AASTeX5.0. Accepted 05 May 2003; Scheduled for publication in the Astronomical Journal (August 2003

GUTs in Curved Spacetime: Running Gravitational Constants, Newtonian Potential and the Quantum Corrected Gravitational Equations

The running coupling constants (in particular, the gravitational one) are studied in asymptotically free GUTs and in finite GUTs in curved spacetime, with explicit examples. The running gravitational coupling is used to calculate the leading quantum GUT corrections to the Newtonian potential, which turn out to be of logarithmic form in asymptotically free GUTs. A comparison with the effective theory for the conformal factor ---where leading quantum corrections to the Newtonian potential are again logarithmic--- is made. A totally asymptotically free $O(N)$ GUT with quantum higher derivative gravity is then constructed, using the technique of introducing renormalization group (RG) potentials in the space of couplings. RG equations for the cosmological and gravitational couplings in this theory are derived, and solved numerically, showing the influence of higher-derivative quantum gravity on the Newtonian potential. The RG-improved effective gravitational Lagrangian for asymptotically free massive GUTs is calculated in the strong (almost constant) curvature regime, and the non-singular De Sitter solution to the quantum corrected gravitational equations is subsequently discussed. Finally, possible extensions of the results here obtained are briefly outlined.Comment: LaTeX, 27 pages, 2 uu-figure