Showing papers in "Revista Mexicana De Astronomia Y Astrofisica in 2000"

Oxygen and Helium Abundances in Galactic H II Regions: Abundance Gradients

Abstract: Absolute integrated uxes have been obtained for 34 H II regions, with Galactocentric distances RG in the 6.6{17.7 kpc range, in the emission lines [O II] 3726 and 3729, [O III] 4363 and 5007, He I 5876, H and H . These uxes are analyzed to derive the physical parameters, the ionic abundances O + /H + , O ++ /H + and He + /H + and the O/H abundances. Accurate electron temperatures have been derived in six H II regions with RG between 6.6 kpc and 14.8 kpc, widening the RG range for which reliable Te measurements exist. Our O/H relationship for 5 kpc < RG < 15 kpc is 12 + log O=H = ( 3:95 0:49) 10 2 RG + (8:82 0:05). The slope is lower, by a factor of two, than that previously obtained by Shaver et al. (1983). No signican t attening of this relation is obtained out to 15 kpc. Our helium observations show that even a region ionized by a star earlier than O 6.5 may contain a signican t amount of neutral helium. We conrm the high Y=(O =H) value ( 180) measured in the Galaxy.

A new, 3d adaptive grid code for astrophysical and geophysical gasdynamics

Abstract: We discuss recent advances in hydrodynamical computations of the circumstellar medium, which are useful to understand, for example, some features observed in...

Cataloguing emission line spectra from fe vii{fe xxiv in the extreme ultraviolet

Abstract: Detailed laboratory astrophysics measurements are in progress to produce spectral tables for the Fe VII { Fe XXIV line emission in the EUV wavelength band. Results for Fe XIII are presented that update line lists used in the Chandra Emission Line Project.

Photoionization of Galactic Halo Gas by Old Supernova Remnants

Abstract: Resumen en: We present new calculations on the contribution from cooling hot gas in old supernova remnants to the photoionization of warm ionized gas in the Galaxy....

Properties of disk galaxies in a hierarchical formation scenario

Abstract: We used galaxy evolutionary models in a hierarchical inside-out disk formation scenario to study the origin of the main local and global properties of disk galaxies as well as some of the correlations between their global properties. We found that most of disk galaxy properties and their correlations are the result of three (cosmological) initial factors of the system and their dispersions: the virial mass, the halo mass aggregation history (MAH), and the angular momentum given through the spin parameter λ. The MAH determines mainly the dark halo structure and the integral color indexes while λ determines mainly the disk surface brightness and the bulge-to-disk ratio. We calculated star formation using a gravitational instability criterion and a self-regulation mechanism driven by the energy balance in the disk turbulent interstellar medium. The efficiency of s formation in this model is almost independent from the mass of the system. Therefore, the galaxy integral color index B V or the stellar mass-to-luminosity ratio, M/LB, do not depend upon mass. We show that the luminosity-dependent dust absorption empirically determined by Wang & Heckman explains the observed color-magnitude and color Tully-Fisher (TF) relations without the necessity of introducing a massdependent star formation efficiency. The star formation history in our models is driven by both the MAH and the surface density (λ). The disks in centrifugal equilibrium form within growing cold dark matter halos with a gas accretion rate proportional to the halo MAH. The disks present exponential surface density and brightness profiles, negative radial color index gradients, and nearly flat rotation curves. We also calculated the secular formation of a bulge due to gravitational instabilities in the stellar disk. The intensive properties of our models agree very well with the observational data and they follow the same trends galaxies follow across the Hubble sequence. The infrared TF relations and the luminosity-radius relation predicted for the cosmological model used here also agree with the observational data. The main shortcomings of the inside-out hierarchical formation models presented in this paper are the excessive radial color gradients and the dark halo dominion in the rotation curve decompositions.

X-ray emission from star forming galaxies

Abstract: Resumen en: In this work, we have linked our evolutionary synthesis models, covering the entire spectral range from radio to X-rays, with photoioniztion codes in or...

The broad line region in active galactic nuclei

Abstract: We review constraints on models of the broad line region that are imposed by observations of the emission lines in Active Galactic Nuclei. Comparison of high and low ionization lines in sources with FWHM H 4000 km s 1 point toward low ionization line emission produced in a attened geometry (the accretion disk?) with an associated high ionization wind. It remains unclear whether these results can be extended to all radio quiet AGN and particularly to radio loud AGN.

Introducing zeus-mp: a 3d, parallel, multiphysics code forastrophysical fluid dynamics

Abstract: We describe ZEUS-MP: a Multi-Physics, Massively-Parallel, Message-Passing code for astrophysical uid dynamics simulations in 3 dimensions. ZEUS-MP is a follow-on to the sequential ZEUS-2D and ZEUS-3D codes developed and disseminated by the Laboratory for Computational Astrophysics (lca.ncsa.uiuc.edu) at NCSA. V1.0 released 1/1/2000 includes the following physics modules: ideal hydrodynamics, ideal MHD, and self-gravity. Future releases will include ux-limited radiation diusion, thermal heat conduction, two-temperature plasma, and heating and cooling functions. The covariant equations are cast on a moving Eulerian grid with Cartesian, cylindrical, and spherical polar coordinates currently supported. Parallelization is done by domain decomposition and implemented in F77 and MPI. The code is portable across a wide range of platforms from networks of workstations to massively parallel processors. Some parallel performance results are presented as well as an application to turbulent star formation.

Coronal physics and the chandra emission line project

Abstract: With the launch of the Chandra X-ray Observatory, high resolution X-ray spectroscopy of cosmic sources has begun. Early, deep observations of three stellar coronal sources|Capella, Procyon, and HR 1099|are providing not only invaluable calibration data, but also benchmarks for plasma spectral models. These models are needed to interpret data from stellar coronae, galaxies and clusters of galaxies, supernova remnants and other astrophysical sources. They have been called into question in recent years as problems with understanding low resolution ASCA and moderate resolution EUVE data have arisen. The Emission Line Project is a collaborative eort to improve the models, with Phase 1 being the comparison of models with observed spectra of Capella, Procyon, and HR 1099. Goals of these comparisons are (1) to determine and verify accurate and robust diagnostics and (2) to identify and prioritize issues in fundamental spectroscopy which will require further theoretical and/or laboratory work. A critical issue in exploiting the coronal data for these purposes is to understand the extent to which common simplifying assumptions (coronal equilibrium, negligible optical depth) apply. We will discuss recent advances in our understanding of stellar coronae in this context.

Ultracompact H II regions: new challenges

Abstract: The designation \ultracompact H II region" rst appeared in the astronomical literature over 30 years ago. Since that time, of order 10 3 actual or candidate ultracompact (UC) H II regions have been identied, and no fewer than seven theoretical models have been proposed to describe them. Recent observational evidence suggests that the conventional \UC H II" classication may not be adequate to describe all of the small, dense nebulae surrounding young, massive stars. In particular, it appears that some UC H II regions may be small, high density regions that are integral parts of much larger structures; these we call \UC H II regions with extended emission". Other H II regions appear to be an order of magnitude smaller and two orders of magnitude denser than traditional UC H II regions, and hence are deserving of a new classication. We designate these as \super-ultracompact H II regions", (though we suspect that \hypercompact" will prevail). We provide brief scientic and historical perspectives, present the new observational data and arguments for the new classications, and briey discuss possible implications for theoretical models of UC H II regions.

Fast reconnection of magnetic fields in turbulent fluids

Abstract: RESUMEN La reconecci on es el proceso mediante el cual los campos magn eticos cambian de topolog a en un medio conductor, y es fundamental para entender diferentes procesos, incluyendo la turbulencia interestelar y los dinamos estelares y gal acticos. Para explicar el campo gal actico, las r afagas y el ciclo solar, la reconecci on debe ser r apida y propagarse a la velocidad de Alfv en. Trabajos anteriores consideraban uidos laminares y obten an tasas de reconecci on peque~ nas. Mostramos que la presencia de una componente aleatoria del campo magn etico permite una reconnecci on r apida ya que, a diferencia del caso laminar donde el proceso avanza l nea por l nea, en el caso turbulento participan muchas l neas simult aneamente. Una fracci on importante de la energ a magn etica se va a la turbulencia MHD, lo cual aumenta la tasa de reconecci on al aumentar la parte aleatoria del campo. Como consecuencia, los dinamos solares y gal acticos tambi en se vuelven r apidos.

CHIANTI--An Atomic Database for EUV Emission Lines

Abstract: Resumen en: CHIANTI consists of a critically evaluated set of atomic data and transition probabilities necessary to calculate the emission line spectrum of astrophy...

Is the Plasma Within Bubbles and Superbubbles Hot or Cold

Abstract: Resumen en: I review what is known about the temperature of the plasma within stellar wind bubbles and superbubbles. Classical theory suggests that it should be ho...

Plasma simulator for rotating astrophysical objects

Abstract: Resumen en: We are developing an astrophysical rotating plasma simulator, which consists of reusable modules plugged-in to a platform of a 3-dimensional magnetohydr...

Distances of galactic planetary nebulae based on a relationship between the central star mass and the n/o abundance

Abstract: In this paper, we propose a method to determine distances of Galactic planetary nebulae on the basis of a relationship between the central star mass M c and the nebular N/O abundance ratio. This relationship is used in combination with some basic parameters of the central stars, such as the λ5480 flux, surface gravity and visual magnitude in order to obtain distances to a sample of a hundred Galactic planetary nebulae.

Ground-state alignment of atoms and ions: New Diagnostics of Astrophysical Magnetic Field in Diffuse Medium

Abstract: We discuss a new technique of studying magnetic fields in diffuse astrophysical media, e.g. interstellar and intergalactic gas/plasma. This technique is based on the angular momentum alignment of atoms and ions in their ground or metastable states. As the life-time of atoms in such states is long, the alignment induced by anisotropic radiation is susceptible to weak magnetic fields (1G ∼ B ∼ 0.1μG). The alignment reveals itself in terms of the polarization of the absorbed and emitted light. A variety of atoms with fine or hyperfine splitting of the ground or metastable states exhibit the alignment and the resulting polarization degree in some cases exceeds 20%. We show that in the case of absorption the polarization direction is either parallel or perpendicular to magnetic field, while more complex dependencies emerge for the case of emission of aligned atoms. We show that the corresponding studies of magnetic fields can be performed with optical and UV polarimetry. A unique feature of these studies is that they can reveal the 3D orientation of magnetic field. In addition, we point out that the polarization of the radiation arising from the transitions between fine and hyperfine states of the ground level can provide yet another promising diagnostic of magnetic fields, including the magnetic fields in the Early Universe. We mention several cases of interplanetary, circumstellar and interstellar magnetic fields for which the studies of magnetic fields using ground state atomic alignment effect are promising.

Emission Line Spectra from Low-Density Laboratory Plasmas

Abstract: Using spectroscopic equipment optimized for laboratory astrophysics, we are performing systematic measurements of the line emission from astrophysically relevant ions in the wavelength band between 1 and 400 A important to X-ray missions such as Chandra, XMM, Astro-E, and EUVE. Obtained in a controlled laboratory setting at electron densities similar to those found in stellar coronae, the data are used to test spectral modeling codes for accuracy and completeness. Our eort includes the compilation of the iron L-shell emission lines from 6{18 A and the iron M-shell emission lines from 50{200 A. Many lines have been identied for the rst time, and the uxes from lines missing in the spectral modeling codes are assessed. Our measurements also assess the accuracy of line excitation calculations, including direct electron-impact excitation, dielectronic recombination, and resonance excitation. These measurements yield a calibration of specic diagnostic line ratios. Examples of our current measurements are given.

Dielectronic recombination: an overview of theory and experiment, and some astrophysical implications

Abstract: The status of dielectronic recombination (DR) rate coecien ts used for modeling cosmic plasmas is discussed. A brief overview of theoretical and experimental studies of DR is given. Results are shown which demonstrate the astrophysical importance of accurate DR rates for studies of the intergalactic medium.

Error estimates for emission lines in the hydrogen and helium isosequences

Abstract: Emission lines from hydrogen and helium isosequence are among tile strongest in X-ray spectra; they will soon be used to measure the temperature, density, and equilibrium state of collisionally excited, astrophysical plasmas. We have created a new plasma code, APEC, which calculates the emission from such a plasma. APEC calculates the line emission from the direct electron and proton excitation rate and the radiative and dielectronic recombination rate. We show how different collisional plasma codes give varying emissivities for some strong lines of O VII and Fe XXVI, where direct excitation is the. primary effect. This variation is partly due to simple differences in the plasma code. However, the primary reason is that much work remains to be done on experimental and theoretical calculations of the, atomic rates. Large (approximately 50%) differences exist even for excitation rates for hydrogenic ions.

Reionization of the universe and the photoevaporation of cosmological minihalos

Abstract: The rst sources of ionizing radiation to condense out of the dark and neutral IGM sent ionization fronts sweeping outward through their surroundings, overtaking other condensed objects and photoevaporating them. This feedback of universal reionization on cosmic structure formation is demonstrated here by gas dynamical simulations, including radiative transfer, for a cosmological minihalo of dark matter and baryons exposed to an external source of ionizing radiation, either starlight or quasar light, just after the passage of the global ionization front created by the source.

Magnetic fields and the polarization of astrophysical maser radiation: a review

Abstract: Resumen en: Basic aspects of the relationship between the magnetic eld and polarized maser radiation are described with the emphasis on interpreting the observed sp...

The Effect of Piston Errors on the Image Quality of Ground-Based Segmented Mirror Telescopes

Abstract: The errors associated with a segmented primary mirror limit the image quality obtainable with those astronomical telescopes which employ them. It is necessary to derive the effects of segmentation errors on image quality in order to determine realistic error budgets for such telescopes. The main segmentation errors are misalignment and piston of the segments as well as figure errors. Here, the effect of segmentation on image quality is studied analytically by considering a two-ring telescope. Approximate expressions for the MTF and CIR (Central Intensity Ratio) are derived following the technique described by Dierickx (1992). It is found by numerical simulation that the results may be applied to telescopes with realistic segmentation schemes for piston errors up to ≈ 80nm.

Hot gas in the galaxy: what do we know for sure?

Abstract: Resumen en: Major advances have been made in the past decade in our knowledge of the hot interstellar gas of our Milky Way galaxy. The Di use X-ray Spectrometer obt...

Cloudy 94 and Applications to Quasar Emission Line Regions

Abstract: This review discusses the most recent developments of the plasma simulation code Cloudy and its application to the, emission-line regions of quasars. The longterm goal is to develop the tools needed to determine the chemical composition of the emitting gas and the luminosity of the central engine for any emission line source. Emission lines and the underlying thermal continuum are formed in plasmas that are far from thermodynamic equilibrium. Their thermal and ionization states are the result of a balance of a vast set of microphysical processes. Once produced, radiation must, propagate out of the (usually) optically thick source. No analytic solutions are possible, and recourse to numerical simulations is necessary. I am developing the large-scale plasma simulation code Cloudy as an investigative tool for this work, much as an observer might build a spectrometer. This review describes the current version of Cloudy, version 94. It describes improvements made since the, release of the previous version, C90. The major recent, application has been the development of the "Locally Optimally-Emitting Cloud" (LOC) model of AGN emission line regions. Powerful selection effects, introduced by the atomic physics and line formation process, permit individual lines to form most efficiently only near certain selected parameters. These selection effects, together with the presence of gas with a wide range of conditions, are enough to reproduce the spectrum of a typical quasar with little dependence on details. The spectrum actually carries little information to the identity of the emitters. I view this as a major step forward since it provides a method to handle accidental details at the source, so that we can concentrate on essential information such as the luminosity or chemical composition of the quasar.

Photoionization models of NGC 2363

Abstract: Calculamos modelos de fotoionización para la región H II extragaláctica gigante NGC 2363, y los comparamos con datos observacionales ópticos. Mostramos que modelos calculados con el valor de metalicidad que se observa en la región (Z = 0.10 Z ) no reproducen el espectro observacional, y que el desacuerdo puede ser resuelto suponiendo fluctuaciones espaciales de temperatura en la nebulosa. Mostramos que la metalicidad de NGC 2363 ha sido subestimada, y que un valor de Z ' 0.25 Z está en mejor acuerdo con los datos observacionales, respecto al valor comúnmente adoptado de Z ' 0.10 Z .

The interaction of multiple shell planetary nebulae withthe ism

Abstract: Resumen en: Multiple shell planetary nebulae (MSPNe) are those which, in addition to the main bright inner shell, show one or two additional external shells These

Temperature variations and non-photoionization heating in the warm ionized medium of galaxies

Abstract: Optical emission lines observed in the gaseous halo of the Milky Way and other galaxies suggest the existence of a supplemental heat source|in addition to photoionization|that increases the electron temperature in regions of low density. Such heat sources are in fact predicted to exit in the interstellar medium. For example, both the dissipation of turbulence through ion-neutral dampening and photoelectric heating by small grains could heat the warm ionized medium in the Milky Way at the rate of about 10 25 ne ergs cm 3 s 1 . If such a source were present, it would dominate over photoionization heating in regions where ne < 0.1 cm 3 , producing the observed increases in the [S II]/H and [N II]/H intensity ratios at large distances from the Galactic midplane. Magnetic reconnection and Coulomb collisions by cosmic rays are other potential sources. This heating rate would also account for the emission line ratio variations observed in NGC 891, including the increase in [O III] 5007/H with distance from the midplane, which cannot be explained by pure photoionization models.

X-Ray Line Spectroscopy of Massive X-Ray Binaries

Abstract: Spectra from ASCA have provided the most, detailed view to date of the X-ray spectral properties of stellar winds in massive X-ray binaries. Using detailed atomic models that account for recombination cascade kinetics, we have reexamined archival data from Vela X-1 and Cen X-3 in the context of simple models of their wind geometries and velocity distributions. Our approach emphasizes apparent differential emission measure (DEM) distributions, and their dependence on orbital phase and wind parameters. A grid of theoretical DEM distributions is used to generate model spectra, which are compared to the data. We obtain good fits, and derive constraints oil the stellar wind parameters. We provide a summary of the method, and show that, even though the companion stars in Vela X-1 and Cen X-3 have comparable mass-loss rates, the winds in these two systems are dramatically different, in character.