VLT near- to mid-IR imaging and spectroscopy of the M17 UC1-IRS5 region

We investigate the surroundings of the hypercompact HII region M17 UC1 to probe the physical properties of the associated young stellar objects and the environment of massive star formation. Five of the seven point sources in this region show $L$-band excess emission. Geometric match is found between the H_2 emission and near-IR polarized light in the vicinity of IRS5A, and between the diffuse mid-IR emission and near-IR polarization north of UC1. The H_2 emission is typical for dense PDRs, which are FUV pumped initially and repopulated by collisional de-excitation. The spectral types of IRS5A and B273A are B3-B7 V/III and G4-G5 III, respectively. The observed infrared luminosity L_IR in the range 1-20 micron is derived for three objects; we obtain 2.0x10^3 L_\sun for IRS5A, 13 L_\sun for IRS5C, and 10 L_\sun for B273A. IRS5 might be a young quadruple system. Its primary star IRS5A is confirmed to be a high-mass protostellar object (~ 9 M_\sun, ~1x10^5 yrs); it might have terminated accretion due to the feedback from the stellar activities (radiation pressure, outflow) and the expanding HII region of M17. UC1 might also have terminated accretion because of the expanding hypercompact HII region ionized by itself. The disk clearing process of the low-mass YSOs in this region might be accelerated by the expanding HII region. The outflows driven by UC1 are running in south-north with its northeastern side suppressed by the expanding ionization front of M17; the blue-shifted outflow lobe of IRS5A is seen in two types of tracers along the same line of sight in the form of H_2 emission filament and mid-emission. The H_2 line ratios probe the properties of M17 SW PDR, which is confirmed to have a clumpy structure with two temperature distributions: warm, dense molecular clumps with n_H>10^5 cm^-3 and T~575 K and cooler atomic gas with n_H~3.7x10^3-1.5x10^4 cm-3 and T~50-200 K.Comment: accepted for publication in A&A, 19 pages, 15 figures, 5 table

Probing the centre of the large circumstellar disc in M17

We investigated the nature of the hitherto unresolved elliptical infrared emission in the centre of the ~20000 AU disc silhouette in M 17. We combined high-resolution JHKsL'M' band imaging carried out with NAOS/CONICA at the VLT with [Fe II] narrow band imaging using SOFI at the NTT. The analysis is supported by Spitzer/GLIMPSE archival data and by already published SINFONI/VLT Integral Field Spectroscopy data. For the first time, we resolve the elongated central infrared emission into a point-source and a jet-like feature that extends to the northeast in the opposite direction of the recently discovered collimated H2 jet. They are both orientated almost perpendicular to the disc plane. In addition, our images reveal a curved southwestern emission nebula whose morphology resembles that of the previously detected northeastern one. Both nebulae are located at a distance of 1500 AU from the disc centre. We describe the infrared point-source in terms of a protostar that is embedded in circumstellar material producing a visual extinction of 60 <= Av <= 82. The observed Ks band magnitude is equivalent to a stellar mass range of 2.8 Msun <= Mstar <= 8 Msun adopting conversions for a main-sequence star. Altogether, we suggest that the large M 17 accretion disc is forming an intermediate to high-mass protostar. Part of the accreted material is expelled through a symmetric bipolar jet/outflow.Comment: 6 pages, 3 figures, accepted by MNRAS (16 May 2008

Finite element inversion of DInSAR data from the Mw 6.3 L’Aquila earthquake, 2009 (Italy)

Fault slip distribution is usually retrieved from geodetic data assuming that the local crust is an elastic, homogeneous and isotropic half‐space. In the last decades spatially dense geodetic data (e.g., DInSAR maps) have highlighted complex patterns of coseismic deformation that require new modeling tools, such as numerical methods, able to represent rheological and geometrical complexities of the Earth’s crust. In this work, we develop a procedure to perform inversion of geodetic data based on the finite element method, accounting for a more realistic description of the local crust. The method is applied to the 2009 L’Aquila earthquake (Mw 6.3), using DInSAR images of the coseismic displacement. Results highlight the non‐negligible influence of the medium structure: homogeneous and heterogeneous models show discrepancies up to 20% in the fault slip distribution values. Furthermore, in the heterogeneous models a new area of slip appears above the hypocenter. We also perform a resolution study, showing that the information about fault slip distributions retrieved from geodetic data should be considered as averaged on surrounding patches

Reduced description of exact coherent states in parallel shear flows

A reduced description of exact coherent structures in the transition regime of plane parallel shear flows is developed, based on the Reynolds number scaling of streamwise-averaged (mean) and streamwise-varying (fluctuation) velocities observed in numerical simulations. The resulting system is characterized by an effective unit Reynolds number mean equation coupled to linear equations for the fluctuations, regularized by formally higher-order diffusion. Stationary coherent states are computed by solving the resulting equations simultaneously using a robust numerical algorithm developed for this purpose. The algorithm determines self-consistently the amplitude of the fluctuations for which the associated mean flow is just such that the fluctuations neither grow nor decay. The procedure is used to compute exact coherent states of a flow introduced by Drazin and Reid [Hydrodynamic Stability (Cambridge University Press, Cambridge, UK, 1981)] and studied by Waleffe [Phys. Fluids 9, 883 (1997)]: a linearly stable, plane parallel shear flow confined between stationary stress-free walls and driven by a sinusoidal body force. Numerical continuation of the lower-branch states to lower Reynolds numbers reveals the presence of a saddle node; the saddle node allows access to upper-branch states that are, like the lower-branch states, self-consistently described by the reduced equations. Both lower- and upper-branch states are characterized in detail

Exact coherent structures in an asymptotically reduced description of parallel shear flows

A reduced description of shear flows motivated by the Reynolds number scaling of lower-branch exact coherent states in plane Couette flow (Wang J, Gibson J and Waleffe F 2007 Phys. Rev. Lett. 98 204501) is constructed. Exact time-independent nonlinear solutions of the reduced equations corresponding to both lower and upper branch states are found for a sinusoidal, body-forced shear flow. The lower branch solution is characterized by fluctuations that vary slowly along the critical layer while the upper branch solutions display a bimodal structure and are more strongly focused on the critical layer. The reduced equations provide a rational framework for investigations of subcritical spatiotemporal patterns in parallel shear flows

A Radio Perspective on the Wet Merger Remnant NGC 34

We present VLA observations of the neutral hydrogen and radio continuum of NGC 34 (= NGC 17 = Mrk 938). This object is an ideal candidate to study the fate of gas in mergers, since, as shown by an optical study done by Schweizer & Seitzer (2007), it is a gas-rich ("wet") merger remnant of two disk galaxies of unequal mass hosting a strong central starburst and a weak AGN. We detect HI emission from both tidal tails and from nearby galaxies, suggesting that NGC 34 is actually part of a gas-rich group and might have recently interacted with one of its companions. The kinematics of the gas suggests this remnant is forming an outer disk of neutral hydrogen from the gas of the northern tail. We also detect broad HI absorption (514 +/- 21 km/s wide) at both negative and positive velocities with respect to the systemic velocity. This absorption could be explained by the motions of the tidal tails or by the presence of a circumnuclear disk. In addition, we present radio-continuum images that show both nuclear (62.4 +/- 0.3 mJy) and extra-nuclear emission (26.5 +/- 3.0 mJy). The extra-nuclear component is very diffuse and in the shape of two radio lobes, spanning 390 kpc overall. This emission could be a signature of an AGN that has turned off, or it could originate from a starburst-driven superwind. We discuss the possible scenarios that explain our observations, and what they tell us about the location of the gas and the future evolution of NGC 34.Comment: 29 pages, 12 figures, 2 tables. Accepted for publication in the Astronomical Journal. Figs. 1, 2 & 6 degraded to reduce file size

3-D Models of Embedded High-Mass Stars: Effects of a Clumpy Circumstellar Medium

We use 3-D radiative transfer models to show the effects of clumpy circumstellar material on the observed infrared colors of high mass stars embedded in molecular clouds. We highlight differences between 3-D clumpy and 1-D smooth models which can affect the interpretation of data. We discuss several important properties of the emergent spectral energy distribution (SED): More near-infrared light (scattered and direct from the central source) can escape than in smooth 1-D models. The near- and mid-infrared SED of the same object can vary significantly with viewing angle, depending on the clump geometry along the sightline. Even the wavelength-integrated flux can vary with angle by more than a factor of two. Objects with the same average circumstellar dust distribution can have very different near-and mid-IR SEDs depending on the clump geometry and the proximity of the most massive clump to the central source. Although clumpiness can cause similar objects to have very different SEDs, there are some observable trends. Near- and mid-infrared colors are sensitive to the weighted average distance of clumps from the central source and to the magnitude of clumpy density variations (smooth-to-clumpy ratio). Far-infrared emission remains a robust measure of the total dust mass. We present simulated SEDs, colors, and images for 2MASS and Spitzer filters. We compare to observations of some UCHII regions and find that 3-D clumpy models fit better than smooth models. In particular, clumpy models with fractal dimensions in the range 2.3-2.8, smooth to clumpy ratios of <50%, and density distributions with shallow average radial density profiles fit the SEDs best.Comment: accepted to ApJ; version with full-res figures: http://www.astro.virginia.edu/~ri3e/clumpy3d.pd

A Review of Energy-for-water Data in Energy-water Nexus Publications

Published literature on the energy-water nexus continues to increase, yet much of the supporting data, particularly regarding energy-for-water, remains obscure or inaccessible. We perform a systematic review of literature that describes the primary energy and electricity demands for drinking water and wastewater systems in urban environments. This review provides an analysis of the underlying data and other properties of over 170 published studies by systematically creating metadata on each study. Over 45% of the evaluated studies utilized primary data sources (data collected directly from utilities), potentially enabling large-scale data sharing and a more comprehensive understanding of global water-related energy demand. The most prevalent geographic scale of the existing literature was at the individual city scale (39%), limiting comparisons between utilities. Additionally, energy-for-water studies span 34 different countries with 11 countries having at least 4 published studies. The analyzed literature often considered greenhouse gas emissions of energy demand as an important input for life cycle analysis, highlighting the broader impact of the energy-water nexus. As a result of the review, we identify several common practices for filling data gaps, discover that research and data are primarily concentrated in three countries (Australia, China, and the United States), and offer suggestions for the future of the energy-water nexus, specifically regarding energy-for-water

Time-stepping approach for solving upper-bound problems: Application to two-dimensional Rayleigh-Benard convection

An alternative computational procedure for numerically solving a class of variational problems arising from rigorous upper-bound analysis of forced-dissipative infinite-dimensional nonlinear dynamical systems, including the Navier-Stokes and Oberbeck-Boussinesq equations, is analyzed and applied to Rayleigh-Benard convection. A proof that the only steady state to which this numerical algorithm can converge is the required global optimal of the relevant variational problem is given for three canonical flow configurations. In contrast with most other numerical schemes for computing the optimal bounds on transported quantities (e.g., heat or momentum) within the "background field" variational framework, which employ variants of Newton's method and hence require very accurate initial iterates, the new computational method is easy to implement and, crucially, does not require numerical continuation. The algorithm is used to determine the optimal background-method bound on the heat transport enhancement factor, i.e., the Nusselt number (Nu), as a function of the Rayleigh number (Ra), Prandtl number (Pr), and domain aspect ratio L in two-dimensional Rayleigh-Benard convection between stress-free isothermal boundaries (Rayleigh's original 1916 model of convection). The result of the computation is significant because analyses, laboratory experiments, and numerical simulations have suggested a range of exponents alpha and beta in the presumed Nu similar to (PrRa beta)-Ra-alpha scaling relation. The computations clearly show that for Ra <= 10(10) at fixed L = 2 root 2, Nu <= 0.106Pr(0)Ra(5/12), which indicates that molecular transport cannot generally be neglected in the "ultimate" high-Ra regime.NSF DMS-0928098 DMS-1515161 DMS-0927587 PHY-1205219Simons FoundationNSFONRInstitute for Computational Engineering and Sciences (ICES