Showing papers in "The Astrophysical Journal in 1999"

Measurements of Omega and Lambda from 42 High-Redshift Supernovae

Abstract: We report measurements of the mass density, Omega_M, and cosmological-constant energy density, Omega_Lambda, of the universe based on the analysis of 42 Type Ia supernovae discovered by the Supernova Cosmology Project. The magnitude-redshift data for these SNe, at redshifts between 0.18 and 0.83, are fit jointly with a set of SNe from the Calan/Tololo Supernova Survey, at redshifts below 0.1, to yield values for the cosmological parameters. All SN peak magnitudes are standardized using a SN Ia lightcurve width-luminosity relation. The measurement yields a joint probability distribution of the cosmological parameters that is approximated by the relation 0.8 Omega_M - 0.6 Omega_Lambda ~= -0.2 +/- 0.1 in the region of interest (Omega_M <~ 1.5). For a flat (Omega_M + Omega_Lambda = 1) cosmology we find Omega_M = 0.28{+0.09,-0.08} (1 sigma statistical) {+0.05,-0.04} (identified systematics). The data are strongly inconsistent with a Lambda = 0 flat cosmology, the simplest inflationary universe model. An open, Lambda = 0 cosmology also does not fit the data well: the data indicate that the cosmological constant is non-zero and positive, with a confidence of P(Lambda > 0) = 99%, including the identified systematic uncertainties. The best-fit age of the universe relative to the Hubble time is t_0 = 14.9{+1.4,-1.1} (0.63/h) Gyr for a flat cosmology. The size of our sample allows us to perform a variety of statistical tests to check for possible systematic errors and biases. We find no significant differences in either the host reddening distribution or Malmquist bias between the low-redshift Calan/Tololo sample and our high-redshift sample. The conclusions are robust whether or not a width-luminosity relation is used to standardize the SN peak magnitudes.

Dark Matter Substructure within Galactic Halos

Abstract: We use numerical simulations to examine the substructure within galactic and cluster mass halos that form within a hierarchical universe. Clusters are easily reproduced with a steep mass spectrum of thousands of substructure clumps that closely matches the observations. However, the survival of dark matter substructure also occurs on galactic scales, leading to the remarkable result that galaxy halos appear as scaled versions of galaxy clusters. The model predicts that the virialized extent of the Milky Way's halo should contain about 500 satellites with circular velocities larger than the Draco and Ursa Minor systems, i.e., bound masses 108 M☉ and tidally limited sizes 1 kpc. The substructure clumps are on orbits that take a large fraction of them through the stellar disk, leading to significant resonant and impulsive heating. Their abundance and singular density profiles have important implications for the existence of old thin disks, cold stellar streams, gravitational lensing, and indirect/direct detection experiments.

Where are the missing galactic satellites

Abstract: According to the hierarchical clustering scenario, galaxies are assembled by merging and accretion of numerous satellites of di†erent sizes and masses. This ongoing process is not 100% efficient in destroying all of the accreted satellites, as evidenced by the satellites of our Galaxy and of M31. Using published data, we have compiled the circular velocity distribution function (VDF) of galaxy satellites in the (V circ ) Local Group. We Ðnd that within the volumes of radius of 570 kpc (400 h~1 kpc assuming the Hubble constant1 h \ 0.7) centered on the Milky Way and Andromeda, the average VDF is roughly approx- imated as km s~1)~1.4B0.4 h3 Mpc~3 for in the range B10E70 km s~1. n( ( V circ ) B 55 ^ 11(V circ /10 V circ The observed VDF is compared with results of high-resolution cosmological simulations. We Ðnd that the VDF in models is very di†erent from the observed one : km s~1)~2.75 h3 n( ( V circ ) B 1200(V circ /10 Mpc~3. Cosmological models thus predict that a halo the size of our Galaxy should have about 50 dark matter satellites with circular velocity greater than 20 km s~1 and mass greater than 3 ) 108 within M _ a 570 kpc radius. This number is signiÐcantly higher than the approximately dozen satellites actually observed around our Galaxy. The di†erence is even larger if we consider the abundance of satellites in simulated galaxy groups similar to the Local Group. The models predict D300 satellites inside a 1.5 Mpc radius, while only D40 satellites are observed in the Local Group. The observed and predicted VDFs cross at B50 km s~1, indicating that the predicted abundance of satellites with km s~1 V circ Z 50 is in reasonably good agreement with observations. We conclude, therefore, that unless a large fraction of the Local Group satellites has been missed in observations, there is a dramatic discrepancy between observations and hierarchical models, regardless of the model parameters. We discuss several possible explanations for this discrepancy including identiÐcation of some satellites with the high-velocity clouds observed in the Local Group and the existence of dark satellites that failed to accrete gas and form stars either because of the expulsion of gas in the supernovae-driven winds or because of gas heating by the intergalactic ionizing background. Subject headings : cosmology : theory E galaxies : clusters : general E galaxies : interactions E Galaxy : formation E Local Group E methods : numerical

Collapsars: Gamma-ray bursts and explosions in 'failed supernovae'

Abstract: Using a two-dimensional hydrodynamics code (PROMETHEUS), we explore the continued evolution of rotating helium stars, Mα 10 M☉, in which iron-core collapse does not produce a successful outgoing shock but instead forms a black hole of 2-3 M☉. The model explored in greatest detail is the 14 M☉ helium core of a 35 M☉ main-sequence star. The outcome is sensitive to the angular momentum. For j16 ≡ j/(1016 cm2 s-1) 3, material falls into the black hole almost uninhibited. No outflows are expected. For j16 20, the infalling matter is halted by centrifugal force outside 1000 km where neutrino losses are negligible. The equatorial accretion rate is very low, and explosive oxygen burning may power a weak equatorial explosion. For 3 j16 20, however, a reasonable value for such stars, a compact disk forms at a radius at which the gravitational binding energy can be efficiently radiated as neutrinos or converted to beamed outflow by magnetohydrodynamical (MHD) processes. These are the best candidates for producing gamma-ray bursts (GRBs). Here we study the formation of such a disk, the associated flow patterns, and the accretion rate for disk viscosity parameter α ≈ 0.001 and 0.1. Infall along the rotational axis is initially uninhibited, and an evacuated channel opens during the first few seconds. Meanwhile the black hole is spun up by the accretion (to a ≈ 0.9), and energy is dissipated in the disk by MHD processes and radiated by neutrinos. For the α = 0.1 model, appreciable energetic outflows develop between polar angles of 30° and 45°. These outflows, powered by viscous dissipation in the disk, have an energy of up to a few times 1051 ergs and a mass ~1 M☉ and are rich in 56Ni. They constitute a supernova-like explosion by themselves. Meanwhile accretion through the disk is maintained for approximately 10-20 s but is time variable (±30%) because of hydrodynamical instabilities at the outer edge in a region where nuclei are experiencing photodisintegration. Because the efficiency of neutrino energy deposition is sensitive to the accretion rate, this instability leads to highly variable energy deposition in the polar regions. Some of this variability, which has significant power at 50 ms and overtones, may persist in the time structure of the burst. During the time followed, the average accretion rate for the standard α = 0.1 and j16 = 10 model is 0.07 M☉ s-1. The total energy deposited along the rotational axes by neutrino annihilation is (1-14) × 1051 ergs, depending upon the evolution of the Kerr parameter and uncertain neutrino efficiencies. Simulated deposition of energy in the polar regions, at a constant rate of 5 × 1050 ergs s-1 per pole, results in strong relativistic outflow jets beamed to about 1% of the sky. These jets may be additionally modulated by instabilities in the sides of the "nozzle" through which they flow. The jets blow aside the accreting material, remain highly focused, and are capable of penetrating the star in ~10 s. After the jet breaks through the surface of the star, highly relativistic flow can emerge. Because of the sensitivity of the mass ejection and jets to accretion rate, angular momentum, and disk viscosity, and the variation of observational consequences with viewing angle, a large range of outcomes is possible, ranging from bright GRBs like GRB 971214 to faint GRB-supernovae like SN 1998bw. X-ray precursors are also possible as the jet first breaks out of the star. While only a small fraction of supernovae make GRBs, we predict that collapsars will always make supernovae similar to SN 1998bw. However, hard, energetic GRBs shorter than a few seconds will be difficult to produce in this model and may require merging neutron stars and black holes for their explanation.

Lyman Break Galaxies at z>4 and the Evolution of the UV Luminosity Density at High Redshift

Abstract: We present initial results of a survey for star-forming galaxies in the redshift range 3.8 z 4.5. This sample consists of a photometric catalog of 244 galaxies culled from a total solid angle of 0.23 deg2 to an apparent magnitude of IAB = 25.0. Spectroscopic redshifts in the range 3.61 ? z ? 4.81 have been obtained for 48 of these galaxies; their median redshift is z = 4.13. Selecting these galaxies in a manner entirely analogous to our large survey for Lyman-break galaxies at smaller redshift (2.7 z 3.4) allows a relatively clean differential comparison between the populations and integrated luminosity density at these two cosmic epochs. Over the same range of UV luminosity, the spectroscopic properties of the galaxy samples at z ~ 4 and z ~ 3 are indistinguishable, as are the luminosity function shapes and the total integrated UV luminosity densities [?UV(z = 3)/?UV(z = 4) = 1.1 ? 0.3]. We see no evidence at these bright magnitudes for the steep decline in the star formation density inferred from fainter photometric Lyman-break galaxies in the Hubble deep field (HDF). The HDF provides the only existing data on Lyman-break galaxy number densities at fainter magnitudes. We have reanalyzed the z ~ 3 and z ~ 4 Lyman-break galaxies in the HDF using our improved knowledge of the spectral energy distributions of these galaxies, and we find, like previous authors, that faint Lyman-break galaxies appear to be rarer at z ~ 4 than z ~ 3. This might signal a large change in the faint-end slope of the Lyman-break galaxy luminosity function between redshifts z ~ 3 and z ~ 4, or, more likely, be due to significant variance in the number counts within the small volumes probed by the HDF at high redshifts (~160 times smaller than the ground-based surveys discussed here). If the true luminosity density at z ~ 4 is somewhat higher than implied by the HDF, as our ground-based sample suggests, then the emissivity of star formation as a function of redshift would appear essentially constant for all z > 1 once internally consistent corrections for dust are made. This suggests that there may be no obvious peak in star formation activity and that the onset of substantial star formation in galaxies might occur at z 4.5.

A Model for Solar Coronal Mass Ejections

Abstract: We propose a new model for the initiation of a solar coronal mass ejection (CME). The model agrees with two properties of CMEs and eruptive flares that have proved to be very difficult to explain with previous models: (1) very low-lying magnetic field lines, down to the photospheric neutral line, can open toward infinity during an eruption; and (2) the eruption is driven solely by magnetic free energy stored in a closed, sheared arcade. Consequently, the magnetic energy of the closed state is well above that of the posteruption open state. The key new feature of our model is that CMEs occur in multipolar topologies in which reconnection between a sheared arcade and neighboring flux systems triggers the eruption. In this "magnetic breakout" model, reconnection removes the unsheared field above the low-lying, sheared core flux near the neutral line, thereby allowing this core flux to burst open. We present numerical simulations that demonstrate our model can account for the energy requirements for CMEs. We discuss the implication of the model for CME/flare prediction.

Dust Absorption and the Ultraviolet Luminosity Density at z ≈ 3 as Calibrated by Local Starburst Galaxies*

Abstract: We refine a technique to measure the absorption-corrected ultraviolet (UV) luminosity of starburst galaxies using rest-frame UV quantities alone and apply it to Lyman-limit U dropouts at z ? 3 found in the Hubble Deep Field (HDF). The method is based on an observed correlation between the ratio of far-infrared (FIR) to UV fluxes with spectral slope ? (a UV color). A simple fit to this relation allows the UV flux absorbed by dust and reprocessed to the FIR to be calculated, and hence the dust-free UV luminosity to be determined. International Ultraviolet Explorer spectra and Infrared Astronomical Satellite fluxes of local starbursts are used to calibrate the FFIR/F1600 versus ? relation in terms of A1600 (the dust absorption at 1600 ?) and the transformation from broadband photometric color to ?. Both calibrations are almost completely independent of theoretical stellar-population models. We show that the recent marginal and nondetections of HDF U dropouts at radio and submillimeter wavelengths are consistent with their assumed starburst nature and our calculated A1600. This is also true of recent observations of the ratio of optical emission-line flux to UV flux density in the brightest U dropouts. This latter ratio turns out not to be a good indicator of dust extinction. In U dropouts, absolute magnitude M1600,0 correlates with ?: brighter galaxies are redder, as is observed to be the case for local starburst galaxies. This suggests that a mass-metallicity relationship is already in place at z ? 3. The absorption-corrected UV luminosity function of U dropouts extends up to M1600,0 ? -24 AB mag, corresponding to a star formation rate ~200 ? yr-1 (H0 = 50 km s-1 Mpc-3 and q0 = 0.5 are assumed throughout). The absorption-corrected UV luminosity density at z ? 3 is ?1600,0 ? 1.4 ? 1027 ergs-1 Hz-1 Mpc-1. It is still a lower limit since completeness corrections have not been done and because only galaxies with A1600 3.6 mag are blue enough in the UV to be selected as U dropouts. The luminosity-weighted mean dust-absorption factor of our sample is 5.4 ? 0.9 at 1600 ?.

The NextGen Model Atmosphere Grid for 3000 ≤ Teff ≤ 10,000 K

Abstract: We present our NextGen Model Atmosphere grid for low-mass stars for effective temperatures larger than 3000 K. These LTE models are calculated with the same basic model assumptions and input physics as the VLMS part of the NextGen grid so that the complete grid can be used, e.g., for consistent stellar evolution calculations and for internally consistent analysis of cool star spectra. This grid is also the starting point for a large grid of detailed NLTE model atmospheres for dwarfs and giants. The models were calculated from 3000 to 10,000 K (in steps of 200 K) for 3.5{le}logthinspg{le}5.5 (in steps of 0.5) and metallicities of {minus}4.0{le}[M/H]{le}0.0. We discuss the results of the model calculations and compare our results to the Kurucz grid. Some comparisons to standard stars like Vega and the Sun are presented and compared with detailed NLTE calculations. {copyright} {ital {copyright} 1999.} {ital The American Astronomical Society}

Where Are the Baryons

Abstract: New high-resolution, large-scale cosmological hydrodynamic galaxy formation simulations of a standard cold dark matter model (with a cosmological constant) are utilized to predict the distribution of baryons at the present and at moderate redshift. It is found that the average temperature of baryons is an increasing function of time, with most of the baryons at the present time having a temperature in the range of 105-107 K. Thus not only is the universe dominated by dark matter, but more than one-half of the normal matter is yet to be detected. Detection of this warm/hot gas poses an observational challenge, which requires sensitive EUV and X-ray satellites. Signatures include a soft cosmic X-ray background, apparent warm components in hot clusters due to both intrinsic warm intracluster and intercluster gas projected onto clusters along the line of sight, absorption lines in X-ray and UV quasar spectra [e.g., O VI (1032, 1038) A lines, O VII 574 eV line], strong emission lines (e.g., O VIII 653 eV line), and low-redshift, broad, low column density Lyα absorption lines. We estimate that approximately one-fourth of the extragalactic soft X-ray background (at 0.7 keV) arises from the warm/hot gas, half of it coming from z<0.65, and three-quarters coming from z<1.00, so the source regions should be identifiable on deep optical images.

Starburst-driven Mass Loss from Dwarf Galaxies: Efficiency and Metal Ejection

Abstract: We model the effects of repeated supernova (SN) explosions from starbursts in dwarf galaxies on the interstellar medium of these galaxies, taking into account the gravitational potential of their dominant dark matter halos. We explore SN rates from one every 30,000 yr to one every 3 Myr, equivalent to steady mechanical luminosities of L=0.1-10×1038 ergs s−1, occurring in dwarf galaxies with gas masses Mg=106-109 M☉. We address in detail, both analytically and numerically, the following three questions: 1. When do the SN ejecta blow out of the disk of the galaxy? 2. When blowout occurs, what fraction of the interstellar gas is blown away, escaping the potential of the galactic halo? 3. What happens to the metals ejected from the massive stars of the starburst? Are they retained or blown away?

Dwarfs Cooler Than M: The Definition of Spectral Type L Using Discoveries from the 2-Micron All-Sky Survey (2MASS)

Abstract: Before the 2-Micron All-Sky Survey (2MASS) began, only six objects were known with spectral types later than M9.5 V. In the first 371 sq. deg. of actual 2MASS survey data, we have identified another twenty such objects spectroscopically confirmed using the Low Resolution Imaging Spectrograph (LRIS) at the W.M. Keck Observatory.

Reconnection in a weakly stochastic field

Abstract: We examine the effect of weak, small-scale magnetic field structure on the rate of reconnection in a strongly magnetized plasma. This affects the rate of reconnection by reducing the transverse scale for reconnection flows and by allowing many independent flux reconnection events to occur simultaneously. Allowing only for the first effect and using Goldreich & Sridhar's model of strong turbulence in a magnetized plasma with negligible intermittency, we find a lower limit for the reconnection speed ~VA-3/16L3/4, where VA is the Alfven speed, L is the Lundquist number, and is the large-scale magnetic Mach number of the turbulence. We derive an upper limit of ~VA2 by invoking both effects. We argue that generic reconnection in turbulent plasmas will normally occur at close to this upper limit. The fraction of magnetic energy that goes directly into electron heating scales as -2/5L8/5, and the thickness of the current sheet scales as -3/5L-2/5. A significant fraction of the magnetic energy goes into high-frequency Alfven waves. The angle between adjacent field lines on the same side of the reconnection layer is ~-1/5L6/5 on the scale of the current sheet thickness. We claim that the qualitative sense of these conclusions, that reconnection is fast even though current sheets are narrow, is almost independent of the local physics of reconnection and the nature of the turbulent cascade. As the consequence of this the Galactic and solar dynamos are generically fast, i.e., do not depend on the plasma resistivity.

Jets in Gamma-Ray Bursts

Abstract: In the afterglows of several gamma-ray bursts (GRBs), rapid temporal decay, which is inconsistent with spherical (isotropic) blast-wave models, is observed. In particular, GRB 980519 had the most rapidly fading of the well-documented GRB afterglows, with t(sup -2.05 +/- 0.04) in optical as well as in X-rays. We show that such temporal decay is more consistent with the evolution of a jet after it slows down and spreads laterally, for which t(sup -P) decay is expected (where p is the index of the electron energy distribution). Such a beaming model would relax the energy requirements on some of the more extreme GRBs by a factor of several hundred. It is likely that a large fraction of the weak- (or no-) afterglow observations are also due to the common occurrence of beaming in GRBs and that their jets have already transitioned. to the spreading phase before the first afterglow observations were made. With this interpretation, a universal value of p approx. = 2.4 is consistent with all data.

Differential Galaxy Evolution in Cluster and Field Galaxies at z ≈ 0.3

Abstract: We measure spectral indexes for 1823 galaxies in the Canadian Network for Observational Cosmology 1 (CNOC1) sample of 15 X-ray luminous clusters at 0.18 5 A] but no [O II] emission [W0(O ) < 5 A], perhaps indicative of recently terminated star formation. The observed fraction of 4.4% ± 0.7% in the cluster sample is an overestimate due to a systematic effect that results from the large uncertainties on individual spectral index measurements. Corrected for this bias, we estimate that K+A galaxies make up only 2.1% ± 0.7% of the cluster sample and 0.1% ± 0.7% of the field. From the subsample of galaxies more luminous than Mr = -18.8 + 5 log h, which is statistically representative of a complete sample to this limit, the corrected fraction of K+A galaxies is 1.5% ± 0.8% in the cluster and 1.2% ± 0.8% in the field. Compared with the z ≈ 0.1 fraction of 0.30%, the fraction of K+A galaxies in the CNOC1 field sample is greater by perhaps a factor of 4, but with only 1 σ significance; no further evolution of this fraction is detectable over our redshift range. We compare our data with the results of PEGASE and GISSEL96 spectrophotometric models and conclude, from the relative fractions of red and blue galaxies with no [O II] λ3727 emission and strong Hδ absorption, that up to 1.9% ± 0.8% of the cluster population may have had its star formation recently truncated without a starburst. However, this is still not significantly greater than the fraction of such galaxies in the field, 3.1% ± 1.0%. Furthermore, we do not detect an excess of cluster galaxies that have unambiguously undergone starbursts within the last 1 Gyr. In fact, at 6.3% ± 2.1%, the A+em galaxies that Poggianti et al. have recently suggested are dusty starbursts are twice as common in the field as in the cluster environment. Our results imply that these cluster environments are not responsible for inducing starbursts; thus, the increase in cluster blue galaxy fraction with redshift may not be a strictly cluster-specific phenomenon. We suggest that the truncation of star formation in clusters may largely be a gradual process, perhaps due to the exhaustion of gas in the galactic disks over fairly long timescales; in this case differential evolution may result because field galaxies can refuel their disks with gas from extended halos, thus regenerating star formation, while cluster galaxies may not have such halos and so continue to evolve passively.

Magnetic fields in molecular clouds: Observations confront theory

Abstract: This paper presents a summary of all 27 available sensitive Zeeman measurements of magnetic field strengths in molecular clouds together with other relevant physical parameters. From these data input parameters to magnetic star formation theory are calculated, and predictions of theory are compared with observations. Results for this cloud sample are the following: (1) Internal motions are supersonic but approximately equal to the Alfv?n speed, which suggests that supersonic motions are likely MHD waves. (2) The ratio of thermal to magnetic pressures ?p ? 0.04, implying that magnetic fields are important in the physics of molecular clouds. (3) The mass-to-magnetic flux ratio is about twice critical, which suggests but does not require that static magnetic fields alone are insufficient to support clouds against gravity. (4) Kinetic and magnetic energies are approximately equal, which suggests that static magnetic fields and MHD waves are roughly equally important in cloud energetics. (5) Magnetic field strengths scale with gas densities as |B| ?? with ? ? 0.47; this agrees with the prediction of ambipolar diffusion driven star formation, but this scaling may also be predicted simply by Alfv?nic motions. The measurements of magnetic field strengths in molecular clouds make it clear that magnetic fields are a crucial component of the physics governing cloud evolution and star formation.

Power Spectra for Cold Dark Matter and its Variants

Abstract: The bulk of recent cosmological research has focused on the adiabatic cold dark matter model and its simple extensions. Here we present an accurate —tting formula that describes the matter transfer func- tions of all common variants, including mixed dark matter models. The result is a function of wavenum- ber, time, and six cosmological parameters: the massive neutrino density, number of neutrino species degenerate in mass, baryon density, Hubble constant, cosmological constant, and spatial curvature. We show how observational constraintse.g., the shape of the power spectrum, the abundance of clusters and damped Lya systems, and the properties of the Lya forestcan be extended to a wide range of cosmologies, which includes variations in the neutrino and baryon fractions in both high-density and low-density universes. Subject headings: cosmology: theorydark matterlarge-scale structure of universe

Coronal Loop Oscillations Observed with the Transition Region and Coronal Explorer

Abstract: We report here, for the —rst time, on spatial oscillations of coronal loops, which were detected in extreme-ultraviolet wavelengths (171 with the T ransition Region and Coronal Explorer, in the tem- Ae ) perature range of MK. The observed loop oscillations occurred during a —are that began at T e B 1.0¨1.5 1998 July 14, 12:55 UT and are most prominent during the —rst 20 minutes. The oscillating loops connect the penumbra of the leading sunspot to the —are site in the trailing portion. We identi—ed —ve oscillating loops with an average length of L \ 130,000 ^ 30,000 km. The transverse amplitude of the oscillations is A \ 4100 ^ 1300 km, and the mean period is T \ 280 ^ 30 s. The oscillation mode appears to be a standing wave mode (with —xed nodes at the footpoints). We investigate diUerent MHD wave modes and —nd that the fast kink mode with a period q \ 205(L /1010 cm~3)1@2 cm)(n e /109 (B/10 G)~1 s provides the best agreement with the observed period. We propose that the onset of loop oscillations in distant locations is triggered by a signal or disturbance that propagates from the central —are site with a radial speed of B700 km s~1. Because the observed loop oscillation periods are compa- rable to photospheric 5 minute oscillations, a resonant coupling between the two systems is possible. We further —nd evidence for global extreme-UV dimming in the entire active region possibly associated with a coronal mass ejection. Subject headings: Sun: coronaSun: —aresSun: oscillationsSun: UV radiation

Neutron Capture in Low Mass Asymptotic Giant Branch Stars: Cross Sections and Abundance Signatures

Abstract: Recently improved information on the stellar (n, γ) cross sections of neutron magic nuclei at N = 82, and in particular of 142Nd, turn out to represent a sensitive test for models of s-process nucleosynthesis. While these data were found to be incompatible with the classical approach based on an exponential distribution of neutron exposures, they provide significantly better agreement between the solar abundance distribution of s nuclei and the predictions of models for low-mass asymptotic giant branch (AGB) stars. The origin of this phenomenon is identified as lying in the high neutron exposures at low neutron density obtained between thermal pulses when 13C burns radiatively in a narrow layer of a few 10-4 M☉. This effect is studied in some detail, and the influence of the currently available nuclear physics data is discussed with respect to specific further questions. In this context, particular attention is paid to a consistent description of s-process branchings in the region of the rare earth elements. It is shown that, in certain cases, the nuclear data are sufficiently accurate that the resulting abundance uncertainties can be completely attributed to stellar modeling. Thus, the s-process becomes important for testing the role of different stellar masses and metallicities as well as for constraining the assumptions used in describing the low neutron density provided by the 13C source.

Generation of Magnetic Fields in the Relativistic Shock of Gamma-Ray Burst Sources

Abstract: We show that the relativistic two-stream instability can naturally generate strong magnetic fields with 10-5-10-1 of the equipartition energy density, in the collisionless shocks of gamma-ray burst (GRB) sources. The generated fields are parallel to the shock front and fluctuate on the very short scale of the plasma skin depth. The synchrotron radiation emitted from the limb-brightened source image is linearly polarized in the radial direction relative to the source center. Although the net polarization vanishes under circular symmetry, GRB sources should exhibit polarization scintillations as their radio afterglow radiation gets scattered by the Galactic interstellar medium. Detection of polarization scintillations could therefore test the above mechanism for magnetic field generation.

Radiative Transfer in a Clumpy Universe. III. The Nature of Cosmological Ionizing Sources

Abstract: The history of the transition from a neutral intergalactic medium (IGM) to one that is almost fully ionized can reveal the character of cosmological ionizing sources. We study the evolution of the volume filling factors of H II and He III regions in a clumpy IGM and discuss the implications for rival reionization scenarios of the rapid decline observed at z3 in the space density of optical and radio-loud quasars and of the large population of star-forming galaxies recently discovered at the same epoch. The hydrogen component in a highly inhomogeneous universe is completely reionized when the number of photons emitted above 1 ryd in one recombination time equals the mean number of hydrogen atoms. If stellar sources are responsible for keeping the IGM ionized at z=5, the rate of star formation at this epoch must be comparable or greater than the one inferred from optical observations of galaxies at z≈3 and the mean metallicity per baryon in the universe 0.002 solar. An early generation of stars in dark matter halos with circular velocities, vcirc≈50 km s-1, possibly one of the main sources of UV photons at high z, could be detectable with the Next Generation Space Telescope. Models in which the quasar emissivity declines rapidly at z3 predict a late He II reionization epoch, a feature that could explain the recent detection of patchy He II Lyα at z=2.9 by Reimers et al. and the abrupt change observed by Songaila at about the same epoch of the Si IV /C IV ratio, but appear unable to provide the required number of hydrogen-ionizing photons at z≈5.

The Dynamics and Light Curves of Beamed Gamma-Ray Burst Afterglows

Abstract: The energy requirements of gamma-ray bursts have in past been poorly constrained because of three major uncertainties: the distances to bursts, the degree of burst beaming, and the efficiency of gamma-ray production. The first of these has been resolved, with both indirect evidence (the distribution of bursts in flux and position) and direct evidence (redshifted absorption features in the afterglow spectrum of GRB 970508) pointing to cosmological distances. We now wish to address the second uncertainty. Afterglows allow a statistical test of beaming, described in an earlier paper. In this paper, we modify a standard fireball afterglow model to explore the effects of beaming on burst remnant dynamics and afterglow emission. If the burst ejecta are beamed into angle ζm, the burst remnant's evolution changes qualitatively once its bulk Lorentz factor Γ 1/ζm: before this, Γ declines as a power law of radius, while afterward, it declines exponentially. This change results in a broken power-law light curve whose late-time decay is faster than expected for a purely spherical geometry. These predictions disagree with afterglow observations of GRB 970508. We explored several variations on our model, but none seems to be able to change this result. We therefore suggest that this burst is unlikely to have been highly beamed and that its energy requirements were near those of isotropic models. More recent afterglows may offer the first practical applications for our beamed models.

The Star Formation Histories of Galaxies in Distant Clusters

Abstract: We present a detailed analysis of the spectroscopic catalog of galaxies in 10 distant clusters from Dressler et al. We investigate the nature of the different spectral classes defined by Dressler et al., including star-forming, poststarburst, and passive galaxy populations, and reproduce their basic properties using our spectral synthesis model. We attempt to identify the evolutionary pathways between the various spectral classes in order to search for the progenitors of the numerous poststarburst galaxies. The comparison of the spectra of the distant galaxy populations with samples drawn from the local universe leads us to identify a significant population of dust-enshrouded starburst galaxies, showing both strong Balmer absorption and relatively modest [O II] emission, that we believe are the most likely progenitors of the poststarburst population. We present the differences between the field and cluster galaxies at z=0.4-0.5. We then compare the spectral and morphological properties of the distant cluster galaxies, exploring the connection between the quenching of star formation inferred from the spectra and the strong evolution of the S0 population discussed by Dressler et al. We conclude that either two different timescales and/or two different physical processes are responsible for the spectral and morphological transformation.

r-Process in Neutron Star Mergers

Abstract: The production site of the neutron-rich heavy elements that are formed by rapid neutron capture (the r-process) is still unknown despite intensive research. Here we show detailed studies of a scenario that has been proposed earlier by Lattimer & Schramm, Symbalisty & Schramm, Eichler et al., and Davies et al., namely the merger of two neutron stars. The results of hydrodynamic and full network calculations are combined in order to investigate the relevance of this scenario for r-process nucleosynthesis. Sufficient material is ejected to explain the amount of r-process nuclei in the Galaxy by decompression of neutron star material. Provided that the ejecta consist of matter with a proton-to-nucleon ratio of Ye approximately 0.1, the calculated abundances fit the observed solar r-pattern excellently for nuclei that include and are heavier than the A approximately 130 peak.

Hyperaccreting black holes and gamma-ray bursts

Abstract: A variety of current models of gamma-ray bursts (GRBs) suggest a common engine: a black hole of several solar masses accreting matter from a disk at a rate of 0.01 to 10 M☉ s-1. Using a numerical model for relativistic disk accretion, we have studied steady state accretion at these high rates. Outside about 108 cm, the disk is advection dominated; energy released by dissipation is carried in by the optically thick gas, and the disk does not cool. Inside this radius, for accretion rates greater than about 0.01 M☉ s-1 a global state of balanced power comes to exist between neutrino losses, chiefly pair capture on nucleons, and dissipation. As a result of these losses, the temperature is reduced, the density is raised, and the disk scale height is reduced compared to the advective solution. The sudden onset of neutrino losses (due to the high temperature dependence) and photodisintegration leads to an abrupt thinning of the disk that may provide a favorable geometry for jet production. The inner disk remains optically thin to neutrinos for accretion rates of up to about 1 M☉ s-1. The energy emitted in neutrinos is less, and in the case of low accretion rates, very much less, than the maximum efficiency factor for black hole accretion (0.057 for no rotation; 0.42 for extreme Kerr rotation) times the accretion rate, c2. Neutrino temperatures at the last stable orbit range from 2 MeV (no rotation, slow accretion) to 13 MeV (Kerr geometry, rapid accretion), and the density ranges from 109 to 1012 g cm-3. The efficiency for producing a pair fireball along the rotational axis by neutrino annihilation is calculated and found to be highly variable and very sensitive to the accretion rate. For some of the higher accretion rates studied, it can be several percent or more; for accretion rates less than 0.05 M☉ s-1, it is essentially zero. The efficiency of the Blandford-Znajek mechanism in extracting rotational energy from the black hole is also estimated. In light of these results, the viability of various gamma-ray burst models is discussed, and the sensitivity of the results to disk viscosity, black hole rotation rate, and black hole mass is explored. A diverse range of GRB energies seems unavoidable, and neutrino annihilation in hyperaccreting black hole systems can explain bursts of up to 1052 ergs. Larger energies can be inferred for beaming systems.

The Transport of Cosmic Rays across a Turbulent Magnetic Field

Abstract: We present a new analysis of the transport of cosmic rays in a turbulent magnetic field that varies in all three spatial dimensions. The analysis utilizes a numerical simulation that integrates the trajectories of an ensemble of test particles from which we obtain diffusion coefficients based on the particle motions. We find that the diffusion coefficient parallel to the mean magnetic field is consistent with values deduced from quasi-linear theory, in agreement with earlier work. The more interesting and less understood transport perpendicular to the average magnetic field is found to be enhanced (above the classical scattering result) by the random walk, or braiding, of the magnetic field. The value of κ⊥ obtained is generally larger than the classical scattering value but smaller than the quasi-linear value. The computed values of κ⊥/κ∥, for a representation of the interplanetary magnetic field, are 0.02-0.04; these values are of the same general magnitude as those assumed in recent numerical simulations of cosmic-ray modulation and transport in the heliosphere, and give reasonable agreement with spacecraft observations of cosmic rays. Some consequences of these results for the interpretation of heliospheric observations are discussed.

A Babcock-Leighton Flux Transport Dynamo with Solar-like Differential Rotation

Abstract: We investigate the properties of a kinematic —ux transport solar dynamo model. The model is charac- terized by a solar-like internal diUerential rotation pro—le, a single-cell meridional —ow in the convective envelope that is directed poleward at the surface, and a magnetic diUusivity that is constant within the envelope but decreases sharply at the core-envelope interface. As in earlier —ux transport models of the Babcock-Leighton type, we assume that the poloidal —eld is regenerated as a consequence of the emer- gence at the surface, and subsequent decay, of bipolar active regions exhibiting a systematic tilt with respect to the east-west direction. Inspired by recent simulations of the rise of toroidal magnetic —ux ropes across the solar convective envelope, we model this poloidal —eld regeneration mechanism as a nonlocal source term formulated in such a way as to account for some of the properties of rising —ux ropes revealed by the simulations. For a broad range of parameter values the model leads to solar cycle¨ like oscillatory solutions. Because of the solar-like internal diUerential rotation pro—le used in the model, solutions tend to be characterized by time-latitude (butter—y) diagrams that exhibit both poleward- and equatorward-propagating branches. We demonstrate that the latitudinal shear in the envelope, often omitted in other —ux transport models previously published in the literature, actually has a dominant eUect on the global morphology and period of the solutions, while the radial shear near the core- envelope interface leads to further intensi—cation of the toroidal —eld. On the basis of an extensive parameter space study, we establish a scaling law between the time period of the cycle and the primary parameters of the model, namely the meridional —ow speed, source coefficient, and turbulent diUusion coefficient. In the parameter regime expected to characterize the Sun, we show that the time period of the cycle is most signi—cantly in—uenced by the circulation —ow speed and, unlike for conventional mean —eld a) dynamos, is little aUected by the magnitude of the source coefficient. Finally, we present one speci—c solution that exhibits features that compare advantageously with the observed properties of the solar cycle. Subject headings: diUusionSun: interiorSun: magnetic —eldsSun: rotation

Population Diagram Analysis of Molecular Line Emission

Abstract: We develop the use of the population diagram method to analyze molecular emission in order to derive physical properties of interstellar clouds. We focus particular attention on how the optical depth affects the derived total column density and the temperature. We derive an optical depth correction factor that can be evaluated based on observations and that incorporates the effect of saturation on derived upper level populations. We present analytic results for linear molecules in local thermodynamic equilibrium (LTE). We investigate numerically how subthermal excitation influences the population diagram technique, studying how the determination of kinetic temperature is affected when the local density is insufficient to achieve LTE. We present results for HC3N and CH3OH, representative of linear and nonlinear molecules, respectively. In some cases, alternative interpretations to the standard optically thin and thermalized picture yield significantly different results for column density and kinetic temperature, and we discuss this behavior. The population diagram method can be a very powerful tool for determining physical conditions in dense clouds if proper recognition is given to effects of saturation and subthermal excitation. We argue that the population diagram technique is, in fact, superior to fitting intensities of different transitions directly, and we indicate how it can be effectively employed.

Central Masses and Broad-Line Region Sizes of Active Galactic Nuclei. I. Comparing the Photoionization and Reverberation Techniques

Abstract: The masses and emission-line region sizes of active galactic nuclei (AGNs) can be measured by "reverberation-mapping" techniques, and we use these results to calibrate similar determinations made by photoionization models of the AGN line-emitting regions. Reverberation mapping uses the light travel-time delayed emission-line response to continuum variations to determine the size and kinematics of the emission-line region. We compile a sample of 17 Seyfert 1 galaxies and two quasars with reliable reverberation and spectroscopy data, twice the number available previously. The data provide strong evidence that the broad-line region (BLR) size (as measured by the lag of the emission-line luminosity after changes in the continuum) and the emission-line width measure directly the central mass: the virial assumption is tested with long-term UV and optical monitoring data on NGC 5548. Two methods are used to estimate the distance of the broad emission-line region from the ionizing source: the photoionization method (which is available for many AGNs but has large intrinsic uncertainties) and the reverberation method (which gives very reliable distances but is available for only a few objects). The distance estimate is combined with the velocity dispersion, derived from the broad Hβ line width (in the photoionization method) or from the variable part (rms) of the line profile, in the reverberation-rms method, to estimate the virial mass. Comparing the central masses calculated with the reverberation-rms method to those calculated using a photoionization model, we find a highly significant, nearly linear correlation. This provides a calibration of the photoionization method on the objects with presently available reverberation data, which should enable mass estimates for all AGNs with measured Hβ line width. We find that the correlation between the masses is significantly better than the correlation between the corresponding BLR sizes calculated by the two methods, which further supports the conclusion that both methods measure the mass of the central black hole . Comparing the BLR sizes given by the two methods also enables us to estimate the ionizing EUV luminosity Lion, which is not directly observable. Typically it is 10 times the monochromatic luminosity at 5100 A (Lv). The Eddington ratio for the objects in our sample is in the range Lv/LEdd ~ 0.001-0.03 and Lion/LEdd ≈ 0.01-0.3.

Far-Infrared and Submillimeter Emission from Galactic and Extragalactic Photodissociation Regions

Abstract: Photodissociation region (PDR) models are computed over a wide range of physical conditions, from those appropriate to giant molecular clouds illuminated by the interstellar radiation field to the conditions experienced by circumstellar disks very close to hot massive stars. These models use the most up-to-date values of atomic and molecular data, the most current chemical rate coefficients, and the newest grain photoelectric heating rates, which include treatments of small grains and large molecules. In addition, we examine the effects of metallicity and cloud extinction on the predicted line intensities. Results are presented for PDR models with densities over the range n = 101-107 cm-3 and for incident far-ultraviolet radiation fields over the range G0 = 10-0.5-106.5 (where G0 is the far-ultravioliet [FUV] flux in units of the local interstellar value), for metallicities Z = 1 and 0.1 times the local Galactic value, and for a range of PDR cloud sizes. We present line strength and/or line ratio plots for a variety of useful PDR diagnostics: [C II] 158 μm, [O I] 63 μm and 145 μm, [C I] 370 μm and 609 μm, CO J = 1-0, J = 2-1, J = 3-2, J = 6-5, and J = 15-14, as well as the strength of the far-infrared continuum. These plots will be useful for the interpretation of Galactic and extragalactic far-infrared and submillimeter spectra observable with the Infrared Space Observatory (ISO), the Stratospheric Observatory for Infrared Astronomy, the Submillimeter Wave Astronomy Satellite, the Far Infrared and Submillimeter Telescope, and other orbital and suborbital platforms. As examples, we apply our results to ISO and ground-based observations of M82, NGC 278, and the Large Magellanic Cloud. Our comparison of the conditions in M82 and NGC 278 show that both the gas density and FUV flux are enhanced in the starburst nucleus of M82 compared with those in the normal spiral NGC 278. We model the high [C II]/CO ratio observed in the 30 Doradus region of the LMC and find that it can be explained either by lowering the average extinction through molecular clouds or by enhancing the density contrast between the atomic layers of PDRs and the CO-emitting cloud cores. The ratio L[CO]/M[H2] implied by the low extinction model gives cloud masses too high for gravitational stability. We therefore rule out low-extinction clouds as an explanation for the high [C II]/CO ratio and instead appeal to density contrast in AV = 10 clouds.

Properties of the intracluster medium in an ensemble of nearby galaxy clusters

Abstract: We present a systematic analysis of the intracluster medium (ICM) in an X-ray flux limited sample of 45 galaxy clusters. Using archival ROSAT Position-Sensitive Proportional Counter (PSPC) data and published ICM temperatures, we present best-fit double and single β model profiles, and extract ICM central densities and radial distributions. We use the data and an ensemble of numerical cluster simulations to quantify sources of uncertainty for all reported parameters. We examine the ensemble properties within the context of models of structure formation and feedback from galactic winds. We present best-fit ICM mass-temperature MICM-TX relations for MICM calculated within r500 and 1 h-150 Mpc. These relations exhibit small scatter (17%), providing evidence of regularity in large, X-ray flux limited cluster ensembles. Interestingly, the slope of the MICM-TX relation (at limiting radius r500) is steeper than the self-similar expectation by 4.3 σ. We show that there is a mild dependence of ICM mass fraction fICM on TX; the clusters with ICM temperatures below 5 keV have a mean ICM mass fraction fICM=0.160±0.008, which is significantly lower than that of the hotter clusters fICM=0.212 ± 0.006 (90% confidence intervals). In apparent contradiction with previously published analyses, our large, X-ray flux limited cluster sample provides no evidence for a more extended radial ICM distribution in low-TX clusters down to the sample limit of 2.4 keV. By analyzing simulated clusters we find that density variations enhance the cluster X-ray emission and cause MICM and fICM to be overestimated by ~12%. Additionally, we use the simulations to estimate an fICM depletion factor at r500. We use the bias corrected mean fICM within the hotter cluster subsample as a lower limit on the cluster baryon fraction. In combination with nucleosynthesis constraints this measure provides a firm upper limit on the cosmological density parameter for clustered matter ΩM≤(0.36 ± 0.01) h-½50.