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It is known that interstellar gas has a fractal structure in a wide range of spatial scales with a fractal dimension that seems to be a constant around D_f 2.7. It is expected that stars forming from this fractal medium exhibit similar fractal patterns. Here we address this issue by quantifying the degree to which star-forming events are clumped. We develop, test, and apply a precise and accurate technique to calculate the correlation dimension D_c of the distribution of H II regions in a sample of disk galaxies. We find that the determination of D_c is limited by the number of H II regions, since if there are 100 regions available, then a bias tending to underestimate the dimension is produced. The reliable results are distributed in the range 1.5 D_c 2.0 with an average value D_c = 1.81. This corresponds to a three-dimensional dimension of D_f = 2.73, very similar to the value measured in the interstellar clouds. However, we get significant variations in the fractal dimension among galaxies, contrary to the universal picture sometimes claimed in literature. The fractal dimension exhibits a weak but significant correlation with the absolute magnitude and, to a lesser extent, with the galactic radius. The faintest galaxies tend to distribute their H II regions in more clustered (less uniform) patterns. The fractal dimension for the brightest H II regions within the same galaxy seems to be smaller than for the faintest ones, suggesting some kind of evolutionary effect, but the obtained correlation remains unchanged if only the brightest regions are taken into account.

We present a new library of fully radiative shock models calculated with the MAPPINGS III shock and photoionization code. The library consists of grids of models with shock velocities in the range v_s = 100–1000 km s⁻¹ and magnetic parameters B/n^1/2 of 10⁻⁴ to 10 μG cm^3/2 for five different atomic abundance sets and for a preshock density of 1.0 cm⁻³. In addition, solar abundance model grids have been calculated for densities of 0.01, 0.1, 10, 100, and 1000 cm⁻³ with the same range in v_s and B/n^1/2. Each model includes components of both the radiative shock and its photoionized precursor, ionized by the extreme ultraviolet (EUV) and soft X-ray radiation generated in the radiative gas. We present the details of the ionization structure, the column densities, and the luminosities of the shock and its precursor. Emission-line ratio predictions are separately given for the shock and its precursor as well as for the composite shock+precursor structure to facilitate comparison with observations in cases in which the shock and its precursor are not resolved. Emission-line ratio grids for shock and shock+precursor are presented on standard line ratio diagnostic diagrams, and we compare these grids to observations of radio galaxies and a sample of AGNs and star-forming galaxies from the Sloan Digital Sky Survey. This library is available online, along with a suite of tools to enable the analysis of the shocks and the easy creation of emission line ratio diagnostic diagrams. These models represent a significant increase in parameter space coverage over previously available models and, therefore, provide a unique tool in the diagnosis of emission by shocks.

The second survey of the molecular clouds in the Large Magellanic Cloud in ¹²CO (J = 1–0) was carried out by NANTEN. The sensitivity of this survey is twice as high as that of the previous NANTEN survey, leading to a detection of molecular clouds with M_CO 2 × 10⁴ M_☉. We identified 272 molecular clouds, 230 of which are detected at three or more observed positions. We derived the physical properties, such as size, line width, and virial mass, of the 164 GMCs that have an extent more than the beam size of NANTEN in both the major and minor axes. The CO luminosity and virial mass of the clouds show a good correlation of M_vir ∝ L_CO^{1.1 ± 0.1}, with a Spearman rank correlation of 0.8, suggesting that the clouds are in nearly virial equilibrium. Assuming the clouds are in virial equilibrium, we derived an X_CO-factor of ~7 × 10²⁰ cm⁻² (K km s⁻¹)⁻¹. The mass spectrum of the clouds is fitted well by a power law of N_cloud(> M_CO) ∝ M_CO^{−0.75 ± 0.06} above the completeness limit of 5 × 10⁴ M_☉. The slope of the mass spectrum becomes steeper if we fit only the massive clouds, e.g., N_cloud(> M_CO) ∝ M_CO^{−1.2 ± 0.2} for M_CO ≥ 3 × 10⁵ M_☉.

Recent radiative lifetime measurements accurate to ±5% (Stockett et al. 2007, J. Phys. B 40, 4529) using laser-induced fluorescence (LIF) on 7 even-parity and 63 odd-parity levels of Er II have been combined with new branching fractions measured using a Fourier transform spectrometer (FTS) to determine transition probabilities for 418 lines of Er II. This work moves Er II onto the growing list of rare-earth spectra with extensive and accurate modern transition probability measurements using LIF plus FTS data. This improved laboratory data set has been used to determine a new solar photospheric Er abundance, log ε = 0.96 ± 0.03 (σ = 0.06 from 8 lines), a value in excellent agreement with the recommended meteoritic abundance, log ε = 0.95 ± 0.03. Revised Er abundances have also been derived for the r-process-rich metal-poor giant stars CS 22892–052, BD +17 3248, HD 221170, HD 115444, and CS 31082–001. For these five stars the average Er/Eu abundance ratio, log ε (Er/Eu) = 0.42, is in very good agreement with the solar-system r-process ratio. This study has further strengthened the finding that r-process nucleosynthesis in the early Galaxy, which enriched these metal-poor stars, yielded a very similar pattern to the r-process, which enriched later stars including the Sun.

The ever-expanding depth and quality of photometric and spectroscopic observations of stellar populations increase the need for theoretical models in regions of age-composition parameter space that are largely unexplored at present. Stellar evolution models that employ the most advanced physics and cover a wide range of compositions are needed to extract the most information from current observations of both resolved and unresolved stellar populations. The Dartmouth Stellar Evolution Database is a collection of stellar evolution tracks and isochrones that spans a range of [Fe/H] from –2.5 to +0.5, [α/Fe] from –0.2 to +0.8 (for [Fe/H] ≤ 0) or +0.2 (for [Fe/H] > 0), and initial He mass fractions from Y = 0.245 to 0.40. Stellar evolution tracks were computed for masses between 0.1 and 4 M_☉, allowing isochrones to be generated for ages as young as 250 Myr. For the range in masses where the core He flash occurs, separate He-burning tracks were computed starting from the zero age horizontal branch. The tracks and isochrones have been transformed to the observational plane in a variety of photometric systems including standard UBV(RI)_C, Stromgren uvby, SDSS ugriz, 2MASS JHK_s, and HST ACS/WFC and WFPC2. The Dartmouth Stellar Evolution Database is accessible through a Web site at http://stellar.dartmouth.edu/~models/ where all tracks, isochrones, and additional files can be downloaded.

We construct partially ionized hydrogen atmosphere models for magnetized neutron stars in radiative equilibrium with fixed surface fields between B = 10¹² and 2 × 10¹³ G and effective temperatures log T_eff = 5.5–6.8, as well as with surface B and T_eff distributions around these values. The models are based on the latest equation of state and opacity results for magnetized partially ionized hydrogen plasmas. The atmospheres directly determine the characteristics of thermal emission from the surface of neutron stars. We also incorporate these model spectra into XSPEC, under the model name NSMAX, thus allowing them to be used by the community to fit X-ray observations of neutron stars.

Type I X-ray bursts are violent stellar events that take place in the H/He-rich envelopes of accreting neutron stars. We have investigated the role played by uncertainties in nuclear processes on the nucleosynthesis accompanying these explosive phenomena. Two different approaches have been adopted, in the framework of postprocessing calculations. In the first one, nuclear rates are varied individually within uncertainties. Ten different models, covering the characteristic parameter space for these stellar events, have been considered. The second, somewhat complementary approach involves a Monte Carlo code in which all nuclear rates are randomly varied within uncertainty limits simultaneously. All in all, about 50,000 postprocessing calculations, with a network containing 606 nuclides (H to ¹¹³Xe) and more than 3500 nuclear processes, have been performed in this work. A brief comparison between both procedures is outlined together with an overall account of the key nuclear reactions whose uncertainties have the largest impact in our X-ray burst nucleosynthesis studies.

A new code for astrophysical magnetohydrodynamics (MHD) is described. The code has been designed to be easily extensible for use with static and adaptive mesh refinement. It combines higher order Godunov methods with the constrained transport (CT) technique to enforce the divergence-free constraint on the magnetic field. Discretization is based on cell-centered volume averages for mass, momentum, and energy, and face-centered area averages for the magnetic field. Novel features of the algorithm include (1) a consistent framework for computing the time- and edge-averaged electric fields used by CT to evolve the magnetic field from the time- and area-averaged Godunov fluxes, (2) the extension to MHD of spatial reconstruction schemes that involve a dimensionally split time advance, and (3) the extension to MHD of two different dimensionally unsplit integration methods. Implementation of the algorithm in both C and FORTRAN95 is detailed, including strategies for parallelization using domain decomposition. Results from a test suite which includes problems in one-, two-, and three-dimensions for both hydrodynamics and MHD are given, not only to demonstrate the fidelity of the algorithms, but also to enable comparisons to other methods. The source code is freely available for download on the web.