Table of contents

We study the prospects for extracting detailed statistical properties of the Sunyaev-Zeldovich (SZ) effect associated with large-scale structure using upcoming multifrequency cosmic microwave background (CMB) experiments. The greatest obstacle to detecting the large-angle signal is the confusion noise provided by the primary anisotropies themselves, and to a lesser degree Galactic and extragalactic foregrounds. We employ multifrequency subtraction techniques and the latest foregrounds models to determine the detection threshold for the Boomerang, Microwave Anisotropy Probe (MAP; several μK), and Planck CMB (sub-μK) experiments. Calibrating a simplified biased-tracer model of the gas pressure from recent hydrodynamic simulations, we estimate the SZ power spectrum, skewness, and bispectrum through analytic scalings and N-body simulations of the dark matter. We show that the Planck satellite should be able to measure the SZ effect with sufficient precision to determine its power spectrum and higher order correlations, e.g., the skewness and bispectrum. Planck should also be able to detect the cross-correlation between the SZ and gravitational lensing effect in the CMB. Detection of these effects will help determine the properties of the as yet undetected gas, including the manner in which the gas pressure traces the dark matter.

The low-redshift structures of the universe act as lenses in a similar way on the cosmic microwave background (CMB) light and on the distant galaxies (say at redshift about unity). As a consequence, the CMB temperature distortions are expected to be statistically correlated with the galaxy shear, exhibiting a nonuniform distribution of the relative angle between the CMB and the galactic ellipticities. Investigating this effect, we find that its amplitude is as high as a 10% excess of alignment between CMB and the galactic ellipticities relative to the uniform distribution. The relatively high signal-to-noise ratio we found should make possible a detection with the planned CMB data sets, provided that a galaxy survey follow-up can be done on a sufficiently large area. It would provide a complementary bias-independent constraint on the cosmological parameters.

The dominant linear contribution to cosmic microwave background (CMB) fluctuations at small angular scales (≲1') is a second-order contribution known as the Vishniac or Ostriker-Vishniac effect. This effect is caused by the scattering of CMB photons off free electrons after the universe has been reionized, and is dominated by linear perturbations near the R_V = 2 Mpc/(hΓ/0.2) scale in the cold dark matter cosmogony. As the reionization of the universe requires that nonlinear objects exist on some scale, however, one can compare the scale responsible for reionization to R_V and ask whether a linear treatment is even feasible in different scenarios of reionization. For an Ω₀ = 1 cosmology normalized to cluster abundances, only ~65% of the linear integral is valid if reionization is due to quasars in halos of mass ~10⁹M_☉, while ~75% of the integral is valid if reionization was caused by stars in halos of ~10⁶M_☉. In Λ or open cosmologies, both the redshift of reionization and z_V are pushed farther back, but still only ~75% to ~85% of the linear integral is valid, independent of the ionization scenario. We point out that all odd higher order moments from Vishniac fluctuations are zero, while even moments are nonzero, regardless of the Gaussianity of the density perturbations. This provides a defining characteristic of the Vishniac effect that differentiates it from other secondary perturbations and may be helpful in separating them.

We use cosmological hydrodynamic simulations to investigate the formation of galactic bulges within the framework of hierarchical clustering in a representative cold dark matter (CDM) cosmological model. We show that the largest objects forming at cosmological redshifts z ~ 4 resemble observed bulges of spiral galaxies or moderate-sized ellipticals in their general properties, such as size, shape, and density profile. This is consistent with observational data indicating the existence of "old" bulges and ellipticals at more moderate redshifts. These bulges are gas dominated at redshift z = 3, with high rates of star formation, and would appear to be good candidates for small blue galaxies seen in the Hubble Deep Field.

Many questions in physical cosmology regarding the thermal history of the intergalactic medium, chemical enrichment, reionization, etc., are thought to be intimately related to the nature and evolution of pregalactic structure. In particular, the efficiency of primordial star formation and the primordial initial mass function are of special interest. We present results from high-resolution three-dimensional adaptive mesh refinement simulations that follow the collapse of primordial molecular clouds and their subsequent fragmentation within a cosmologically representative volume. Comoving scales from 128 kpc down to 1 pc are followed accurately. Dark matter dynamics, hydrodynamics, and all relevant chemical and radiative processes (cooling) are followed self-consistently for a cluster-normalized cold dark matter (CDM) structure formation model. Primordial molecular clouds with ~10⁵ solar masses are assembled by mergers of multiple objects that have formed hydrogen molecules in the gas phase with a fractional abundance of ≲10^-4. As the subclumps merge, cooling lowers the temperature to ~200 K in a "cold pocket" at the center of the halo. Within this cold pocket, a quasi-hydrostatically contracting core with mass ~200 M_☉ and number densities ≳10⁵ cm^-3 are found. We find that less than 1% of the primordial gas in such small-scale structures cools and collapses to sufficiently high densities to be available for primordial star formation. Furthermore, it is worthwhile to note that this study achieved the highest dynamic range covered by structured adaptive mesh techniques in cosmological hydrodynamics to date.

We analyze the three catalogs of nearby loose groups compiled by A. M. Garcia. She identified groups in a magnitude-limited redshift galaxy catalog, which covers about ~ of sky within cz = 5500 km s^-1, using two methods, a percolation method and a hierarchical method. The free parameters of the group-selection algorithms were tuned to obtain similar catalogs of groups. The author also proposed a third catalog of groups, defined as a combination of the two. Each catalog contains almost 500 groups. In agreement with previous works on earlier catalogs, we find that groups can be described as collapsing systems. Their sampled size is in general considerably larger than their expected virialized region. We compute the virial masses and correct them by taking into account the young dynamical status of these groups. We estimate corrected group masses, M, for two reference cosmological models, a flat one with a matter density parameter Ω₀ = 1 and an open one with Ω₀ = 0.2. We calculate the mass function for each of the three catalogs. We find that the amplitude of the mass function is not very sensitive to the choice of the group-identification algorithm. The number density of groups with M > 9 × 10¹²h^-1M_☉, which is the adopted limit of sample completeness, ranges in the interval 1.3-1.9 × 10^-3h³ Mpc^-3 for Ω₀ = 1, and it is about a factor of 15% lower for Ω₀ = 0.2. The mass functions of the hierarchical and combined catalogs have essentially the same shape, while the mass function of the percolation catalog shows a flattening toward large masses. However, the difference decreases if we do not consider the most massive groups, for which reliable results come from galaxy cluster studies. After having estimated the mass contained within the central, presumably virialized, regions of groups by adopting a reduction in mass of ~30%-40%, we make a comparison with the results from the virial analysis of nearby rich clusters. All three group mass functions turn out to be a smooth extrapolation of the cluster mass function at M < 4 × 10¹⁴h^-1M_☉, which is the completeness limit of the cluster sample. The resulting optical virial mass function of galaxy systems, which extends over 2 orders of magnitude, is fitted to a Schechter expression with a slope of ~ 1.5 and a characteristic mass of M_* ~ 3 × 10¹⁴h^-1M_☉. We also verify that our group mass function agrees reasonably well with the Press-Schechter predictions of models which at large masses describe the virial mass function of clusters.

We study the orbits in the modified nonrelativistic dynamics (MOND) theory within a dwarf galaxy of mass M_d ~ 10⁸M_☉ at a distance of ~100 kpc from a neighboring galaxy of mass M_g = 5 × 10¹¹M_☉, such as ours. It is assumed that a second mass m ≪ M_d is gravitationally bound to M_d by a previously calculated potential for the MOND theory. This potential is obtained for a free-falling mass M_d in a constant external gravitational acceleration field ∇ϕ_g. The numerical technique of surfaces of section is used to study the stability of the phase-space orbits in the dwarf galaxy. Equatorial orbits with sufficiently small eccentricities e < 0.65 are found to be stable with respect to small changes in the initial conditions. (The equatorial plane is perpendicular to the direction of ∇ϕ_g, which is along the line joining M_d and M_g.) For decreasing values of the conserved component of the angular momentum, in the direction of ∇ϕ_g, equatorial stability is lost.

We present a simple method for evaluating the nonlinear biasing function of galaxies from a redshift survey. The nonlinear biasing is characterized by the conditional mean of the galaxy density fluctuation given the underlying mass density fluctuation ⟨δ_g|δ⟩, or by the associated parameters of mean biasing, , and nonlinearity, . Using the distribution of galaxies in cosmological simulations, at a smoothing of a few Mpc, we find that ⟨δ_g|δ⟩ can be recovered to a good accuracy from the cumulative distribution functions of galaxies and mass, C_g(δ_g) and C(δ), despite the biasing scatter. Then, using a suite of simulations of different cosmological models, we demonstrate that C(δ) can be approximated in the mildly nonlinear regime by a cumulative lognormal distribution of 1 + δ with a single parameter σ, with deviations that are small compared to the difference between C_g and C. Finally, we show how the nonlinear biasing function can be obtained with adequate accuracy directly from the observed C_g in redshift space. Thus, the biasing function can be obtained from counts in cells once the rms mass fluctuation at the appropriate scale is assumed a priori. The relative biasing function between different galaxy types is measurable in a similar way. The main source of error is sparse sampling, which requires that the mean galaxy separation be smaller than the smoothing scale. Once applied to redshift surveys such as the Point Source Catalog Redshift Survey (PSCz), the Two-Degree Field (2dF), Sloan Digital Sky Survey (SDSS), or the Deep Extragalactic Evolutionary Probe (DEEP), the biasing function can provide valuable constraints on galaxy formation and structure evolution.

We present the discovery observations of the optical counterpart of the gamma-ray burst GRB 990712 taken 4.16 hr after the outburst and discuss its light curve observed in the V, R, and I bands during the first ~35 days after the outburst. The observed light curves were fitted with a power-law decay for the optical transient (OT), plus an additional component that was treated in two different ways. First, the additional component was assumed to be an underlying galaxy of constant brightness. The resulting slope of the decay is 0.97, and the magnitudes of the underlying galaxy are V = 22.3 ± 0.05, R = 21.75 ± 0.05, and I = 21.35 ± 0.05. Second, the additional component was assumed to be a galaxy plus an underlying supernova with a time-variable brightness identical to that of GRB 980425, appropriately scaled to the redshift of GRB 990712. The resulting slope of the decay is similar, but the goodness of fit is worse, which would imply that either this GRB is not associated with an underlying supernova or the underlying supernova is much fainter than the supernova associated with GRB 980425. The galaxy in this case is fainter: V = 22.7 ± 0.05, R = 22.25 ± 0.05, and I = 22.15 ± 0.05, and the OT plus the underlying supernova at a given time is brighter. Measurements of the brightnesses of the OT and the galaxy by late-time Hubble Space Telescope observation and ground-based observations can thus assess the presence of an underlying supernova.

We present a detailed analysis of the number count and photometric redshift distribution of faint galaxies in the Hubble Deep Field (HDF), paying special attention to selection effects, including the cosmological dimming of the surface brightness of galaxies, under the observational conditions employed in this field. We find a considerably different result from previous studies that ignore selection effects; these effects should therefore be taken into account in the analysis. We find that the model of pure luminosity evolution (PLE) of galaxies in the Einstein-de Sitter (EdS) universe predicts much smaller counts than those observed at faint magnitude limits, by a factor of more than 10, so that a very strong number evolution of galaxies with η ≳ 3-4 must be invoked to reproduce the I₈₁₄ counts, when parametrized as ϕ* ∝ (1 + z)^η. However, we show that such a strong number evolution under realistic merging processes of galaxies cannot explain the steep slope of the B₄₅₀ and V₆₀₆ counts, and it is seriously inconsistent with their photometric redshift distribution. We find that these difficulties still persist in an open universe with Ω₀ ≳ 0.2, and are resolved only when we invoke a Λ-dominated flat universe, after examining various systematic uncertainties in modeling the formation and evolution of galaxies. The present analysis revitalizes the practice of using faint number counts as an important cosmological test, giving one of the arguments against the EdS universe, and suggests acceleration of the cosmic expansion by vacuum energy density. While a modest number evolution of galaxies with η ≲ 1 is still necessary even in a Λ-dominated universe, a stronger number evolution with η > 1 is rejected from the HDF data, giving a strong constraint on the merger history of galaxies.

We have refined the estimate of the primordial level of ⁷Li abundance to an accuracy better than 10%, based on high-precision Li abundances for metal-poor halo stars and a recent model of post-BBN (big bang nucleosynthesis) chemical evolution that provides a quantitative explanation of the detected gentle ascent of the Spite plateau for stars with metallicities [Fe/H] > -3. Our maximum likelihood analysis obtains an estimate for the primordial Li abundance of A(Li)_p = 2.07 after taking into account possible systematic errors in the estimation of Li abundances, with the exception of a still controversial issue regarding stellar depletion. The inferred value of η (the baryon-to-photon number density ratio in the universe) based on this estimate is more consistent with that derived from the set of reported "low He"+"high D" from extragalactic sites than that derived from reported "high He"+"low D" measurements. Since, within current models of stellar depletion processes, it is difficult to account for the observed very small scatter of Li abundance in metal-poor stars, our estimate of A(Li)_p should be taken as an independent constraint on the baryonic mass density parameter in the universe, giving Ω_bh² = (0.64-1.4) × 10^-2 with h = H₀/100 km s^-1 Mpc^-1.

We have continued our effort to re-reduce archival Q0957+561 brightness monitoring data and present results for 1629 R-band images using the methods of galaxy subtraction and seeing correction that we reported previously. The new data set comes from four observing runs, several nights apiece, with sampling of typically 5 minutes, which allows the first measurement of the structure function for variations in the R band from timescales of hours to years. Comparison of our reductions to previous reductions of the same data and to r-band photometry produced at Apache Point Observatory shows good overall agreement. Two of the data runs, separated by 417 days, allow us for the first time to use intranight variations to estimate the time delay. However, our sharpened value of 417.4 days is valid only if the time delay is close to the now-fashionable 417 day value, and worse, it is highly subject to diurnal windowing effects: our statistical estimators have little discriminatory power where there is no overlap of data, namely, at half-day offsets. Results herein show no unambiguous signature of daily microlensing, though a suggestive feature is found in the data. Because both time delay measurement and microlensing searches suffer from the lack of sampling at half-day offsets, inevitable at a single observatory, we discuss the need for round-the-clock monitoring with participation by multiple observatories.

We examine the origin of clustercentric gradients in the star formation rates and colors of rich cluster galaxies within the context of a simple model where clusters are built through the ongoing accretion of field galaxies. The model assumes that after galaxies enter the cluster their star formation rates decline on a timescale of a few gigayears, the typical gas consumption timescale of disk galaxies in the field. Such behavior might be expected if tides and ram pressure strip off the gaseous envelopes that normally fuel star formation in spirals over a Hubble time. Combining these timescales with mass accretion histories derived from N-body simulations of cluster formation in a ΛCDM universe, we reproduce the systematic differences observed in the color distribution of cluster and field galaxies, as well as the strong suppression of star formation in cluster galaxies and its dependence on clustercentric radius. The simulations also indicate that a significant fraction of galaxies beyond the virial radius of the cluster may have been within the main body of the cluster in the past, a result that explains naturally why star formation in the outskirts of clusters (and as far out as 2 virial radii) is systematically suppressed relative to the field. The agreement with the data beyond the cluster virial radius is also improved if we assume that stripping happens within lower mass systems, before the galaxy is accreted into the main body of the cluster. We conclude that the star formation rates of cluster galaxies depend primarily on the time elapsed since their accretion onto massive virialized systems and that the cessation of star formation may have taken place gradually over a few gigayears.

X-ray photon trains from Seyfert 1 galaxies by ROSAT contain both nondeterministic and deterministic variable components intrinsic to the source. Statistical procedures utilizing principal component analysis and the wavelet transform method permit elimination of the satellite wobble effect and removal of the apparent-luminosity-correlated variable components, which is mostly due to photon arrival time statistics, thus leaving the small source-related nondeterministic and deterministic components. Some of the deterministic structures seen exhibit quasiperiodic behavior in X-ray variability from between 20 and 200 s. We present a detailed analysis of some of the ROSAT data for NGC 5548, in which we find that these structures usually persist for only between 50 and 200 s, but infrequently they last much longer; in one case an ~ 80 s quasiperiodicity persisted for an entire observation period, about 1900 s. Furthermore, during that period the X-ray power in the range between 0.1 and 2.4 keV carried by the deterministic structures was as much as 17.2% of the total. This intrinsic variability—whether deterministic or nondeterministic—can be modeled by the superposition of discrete luminosity "building blocks." In this paper we suggest that these building blocks are due to short super-Eddington ballistic accretion events resulting from small (~ 10 M_☉) black holes passing through accretion disks surrounding the large and intermediate size black holes in a nuclear cluster of compact objects in an AGN. In particular, we show that the luminosity of the deterministic structures in NGC 5548 is within the range of the luminosity such events should produce.

We test accurate models of Comptonization spectra over the high-quality data of the BeppoSAX long look at NGC 5548, allowing for different geometries of the scattering region, different temperatures of the input soft photon field, and different viewing angles. We find that the BeppoSAX data are well represented by a plane-parallel or hemispherical corona viewed at an inclination angle of 30°. For both geometries the best-fit temperature of the soft photons is close to 15 eV. The corresponding best-fit values of the hot plasma temperature and optical depth are kT_e ≃ 250-260 keV and τ ≃ 0.16-0.37 for the slab and hemisphere, respectively. These values are substantially different from those derived fitting the data with a power-law-plus-cutoff approximation to the Comptonization component (kT_e ≲ 60 keV, τ ≃ 2.4). In particular, the temperature of the hot electrons estimated from Comptonization models is much larger. This is due to the fact that accurate Comptonization spectra in anisotropic geometries show "intrinsic" curvature that reduces the necessity of a high-energy cutoff. The Comptonization parameter derived for the slab model is larger than predicted for a two-phase plane-parallel corona in energy balance, suggesting that a more "photon-starved" geometry is necessary. The case of a hemispherical corona is consistent with energy balance but requires a large reflection component. The spectral softening detected during a flare that occurred in the central part of the observation corresponds to a decrease of the Comptonization parameter, probably associated with an increase of the soft photon luminosity, the hard photon luminosity remaining constant. The increased cooling fits in naturally with the derived decrease of the coronal temperature kT_e in the high state.

We present a deep X-ray observation of the low-luminosity active galactic nucleus (AGN) in NGC 4258 (M106) using the Advanced Satellite for Cosmology and Astrophysics (ASCA). Confirming previous results, we find that the X-ray spectrum of this source possesses several components. The soft X-ray spectrum (<2 keV) is dominated by thermal emission from optically thin plasma with kT ~ 0.5 keV. The hard X-ray emission is clearly due to a power-law component with photon index Γ ≈ 1.8 absorbed by a column density of N_H ≈ 8 × 10²² cm^-2. The power law is readily identified with primary X-ray emission from the AGN central engine. Underlying both of these spectral components is an additional continuum, which is possibly due to thermal emission of a very hot gaseous component in the anomalous arms and/or the integrated hard emission of X-ray binaries in the host galaxy. We also clearly detect a narrow iron Kα emission line at ~6.4 keV. No broad component is detected. We suggest that the bulk of this narrow line comes from the accretion disk and, furthermore, that the power-law X-ray source that excites this line emission (which is typically identified with a disk corona) must be at least ~100GM/c² in extent. This is in stark contrast to many higher luminosity Seyfert galaxies that display a broad iron line indicating a small (~10GM/c²) X-ray-emitting region. It must be stressed that this study constrains the size of the X-ray-emitting corona rather than the presence/absence of a radiatively efficient accretion disk in the innermost regions. If, instead, a substantial fraction of the observed narrow line originates from material not associated with the accretion disk, limits can be placed on the parameter space of possible allowed relativistically broad iron lines. We include a discussion of various aspects of iron line limb darkening for highly inclined sources, including the effect of gravitational light bending on the apparent limb-darkening law. By comparing our data with previous ASCA observations, we find marginal evidence for a change in absorbing column density through to the central engine and good evidence for a change in the AGN flux. We conclude with a brief discussion of two serendipitous sources in our field of view: QSO Q1218+472 and a putative z ~ 0.3 cluster of galaxies.

Simulations of the gas flow in a variety of two-dimensional barred spiral galaxies have shown that vortices in the gas appear, when viewed from above, in the corotation frame of the bar. These low-density vortices generally appear at or near the L4 and L5 Lagrangian points. We show that these gas vortices in our models are the hydrodynamic analogs of closed, long-period orbits centered on L4 and L5. Secondary but high-density vortices can appear along the L1-L2 axis. The presence of the vortices at or near L4 and L5 leads to a possible practical application, namely, the determination of the corotation radius. Our models have shown that, when viewed in the rotating frame of the perturbation, vortices are present at or very close to corotation and with position angles approximately 90° with respect to the perturbation. As the viewing frame angular velocity is changed, both the radial positions and the position angles of the vortices change.

If a gas-rich, barred spiral galaxy were observed in H I with sufficient resolution and signal-to-noise ratio, and if the gas velocities in that region are transformed so that they are "observed" in a rigidly rotating frame, having its origin at the galaxy's center, a pair of vortices should appear with position angles approximately 90° with respect to the bar when the angular velocity of the observing frame equals the pattern speed of the bar. Based upon these simulations, we estimate that the pattern speed and corotation radius can be deduced to within approximately 5%-25% of their true values depending upon the quality of the observations.

In the gaseous envelope of protogalaxies, thermal instability leads to the formation of a population of cool fragments that are confined by the pressure of a residual hot background medium. In order to remain in a quasi-hydrostatic equilibrium, the residual gas evolves at approximately the virial temperature of the dark matter halo. Its density is determined by the requirements of thermal equilibrium. The hot gas is heated by compression and shock dissipation. The heating is balanced by direct energy loss due to bremsstrahlung emission and by conductive losses into the cool clouds, which are efficient radiators. The cool fragments are photoionized and heated by the extragalactic UV background and nearby massive stars. Several processes interact to determine the size distribution of the cool fragments. The smallest are evaporated due to conductive heat transfer from the hot gas. All fragments are subject to disruption due to hydrodynamic instabilities. The fragments also gain mass as a result of collisions and mergers and of condensation from the hot gas due to conduction. The size distribution of the fragments in turn determines the rate and efficiency of star formation during the early phase of galactic evolution. We have performed one-dimensional hydrodynamic simulations of the evolution of the hot and cool gas. The cool clouds are assumed to follow a power-law size distribution, and fall into the galactic potential, subject to drag from the hot gas. The relative amounts of the hot and cool gas are determined by the processes discussed above, and star formation occurs at a rate sufficient to maintain the cool clouds at 10⁴ K. We present density distributions for the two phases and also for the stars for several cases, parameterized by the circular speeds of the potentials. Under some conditions, primarily low densities of the hot gas, conduction is more efficient than radiative processes at cooling the hot gas, limiting the X-ray radiation from the halo gas.

The Third EGRET Catalog of High-Energy Gamma-Ray Sources contains 170 unidentified sources, and there is great interest in the nature of these sources. One means of determining source class is the study of flux variability on timescales of days; pulsars are believed to be stable on these timescales, while blazars are known to be highly variable. In addition, previous work has demonstrated that 3EG J0241-6103 and 3EG J1837-0606 are candidates for a new gamma-ray source class. These sources near the Galactic plane display transient behavior but cannot be associated with any known blazars. Although many instances of flaring active galactic nuclei have been reported, the EGRET database has not been systematically searched for occurrences of short-timescale (~1 day) variability. These considerations have led us to conduct a systematic search for short-term variability in EGRET data, covering all viewing periods through proposal cycle 4. Six 3EG catalog sources are reported here to display variability on short timescales; four of them are unidentified. In addition, three noncatalog variable sources are discussed.

We present the results of a series of axisymmetric time-dependent magnetohydrodynamic (MHD) simulations of the propagation of cooling, overdense jets. Our numerical models are motivated by the properties of outflows associated with young stellar objects. A variety of initial field strengths and configurations are explored for both steady and time-variable (pulsed) jets. For the parameters of protostellar jets adopted here, even apparently weak magnetic fields with strengths B ≳ 60 μG in the preshocked jet beam can have a significant effect on the dynamics, for example, by altering the density, width, and fragmentation of thin shells formed by cooling gas. Strong toroidal fields (≥100 μG) with a radial profile that peaks near the surface of the jet result in the accumulation of dense shocked gas in a "nose cone" at the head of jet. We suggest that this structure is unstable in three dimensions. A linear analysis predicts that axisymmetric pinch modes of the MHD Kelvin-Helmholtz instability should grow only slowly for the highly supermagnetosonic jets studied here; we find no evidence for them in our simulations. Some of our models appear unstable to current-driven pinch modes; however, the resulting pressure and density variations induced in the jet beam are not large, making this mechanism an unlikely source of emission knots in the jet beam. In the case of pulsed jets, radial hoop stresses confine shocked jet material in the pulses to the axis, resulting in a higher density in the pulses in comparison to purely hydrodynamic models. In addition, if the magnetic field strength varies with radius, significant radial structure is produced in the pulses (the density is strongly axially peaked, for example) even if the density and velocity in the jet follow a constant "top-hat" profile initially.

We analyze the velocity residuals of 551 carbon stars relative to a rotating-disk model of the inner ~70 deg² of the Large Magellanic Cloud (LMC). We find that the great majority of the stars in this sample are best fitted as being due to two different populations, a young disk population containing 20% of the stars with a velocity dispersion of 8 km s^-1 and an old disk containing the remaining stars with a velocity dispersion of 22 km s^-1. The young disk population has a metallicity ~0.25 dex higher than that of the old disk.

With less certainty, the data also suggest at the 2 σ level that there may be a third kinematically distinct population that is moving toward us at 30 km s^-1 relative to the LMC, consistent with measurements of 21 cm velocities. If real, this population contains about 7% of the carbon stars in the sample. It could be a feature in the disk of the LMC, or it could be tidal debris in the foreground or background. If it is tidal debris, this population could account for some or all of the microlensing events observed toward the LMC.

We analyze the star formation history (SFH) of the Galactic disk by using an infall model. Based on the observed SFH of the Galactic disk, we first determine the timescale of the gas infall into the Galactic disk (t_in) and that of the gas consumption to form stars (t_sf). Since each of the two timescales does not prove to be determined independently from the SFH, we first fix t_sf. Then, t_in is determined so that we minimize χ². Consequently, we choose three parameter sets: [t_sf (Gyr),t_in (Gyr)] = (6.0, 23), (11, 12), and (15, 9.0), where we set the Galactic age as 15 Gyr. All of the three cases predict almost identical star formation history. Next, we test the intermittence (or variability) of the star formation rate (SFR) along with the smooth SFH suggested from the infall model. The large value of the χ² statistic supports the violent time variation of the SFH. If we interpret the observed SFH with smooth and variable components, the amplitude of the variable component is comparable to the smooth component. Thus, intermittent SFH of the Galactic disk is strongly suggested. We also examined the metallicity distribution of G dwarfs. We found that the true parameter set lies between [t_sf (Gyr),t_in (Gyr)] = (6, 23) and (11, 12), though we need a more sophisticated model including the process of metal enrichment within the Galactic halo.

Using the all-sky ROSAT soft X-ray and 408 MHz radio continuum data, we show that the North Polar Spur (NPS) and its western and southern counterspurs draw a giant dumbbell shape necked at the Galactic plane. We interpret these features as due to a shock front originating from a starburst 15 million years ago with a total energy of the order of ~10⁵⁶ ergs or 10⁵ Type II supernovae. We simulate all-sky distributions of radio continuum and soft X-ray intensities based on the bipolar hypershell Galactic center starburst model. The simulations can well reproduce the radio NPS and related spurs, as well as radio spurs in the tangential directions of spiral arms. Simulated X-ray maps in the 0.25, 0.75, and 1.5 keV bands reproduce the ROSAT X-ray NPS, its western and southern counterspurs, and the absorption layer along the Galactic plane. We propose to use the ROSAT all-sky maps to probe the physics of gas in the halo-intergalactic interface, and to directly date and measure the energy of a recent Galactic center starburst.

We present the results of a 05-09 FWHM imaging survey at K (2.2 μm) and H (1.6 μm) covering ~51 × 51 centered on θ¹C Ori, the most massive star in the Orion Nebula Cluster (ONC). At the age and distance of this cluster, and in the absence of extinction, the hydrogen-burning limit (0.08 M_☉) occurs at K ≈ 13.5 mag, while an object of mass 0.02 M_☉ has K ≈ 16.2 mag. Our photometry is complete for source detection at the 7 σ level to K ≈ 17.5 mag and thus is sensitive to objects as low-mass as 0.02 M_☉ seen through visual extinction values as high as 10 mag. We use the observed magnitudes, colors, and star counts to constrain the shape of the inner ONC stellar mass function across the hydrogen-burning limit. After determining the stellar age and near-infrared excess properties of the optically visible stars in this same inner ONC region, we present a new technique that incorporates these distributions when extracting the mass function from the observed density of stars in the K-(H-K) diagram. We find that our data are inconsistent with a mass function that rises across the stellar/substellar boundary. Instead, we find that the most likely form of the inner ONC mass function is one that rises to a peak around 0.15 M_☉, and then declines across the hydrogen-burning limit with slope N(log M) ∝ M^0.57. We emphasize that our conclusions apply to the inner 0.71 pc × 0.71 pc of the ONC only; they may not apply to the ONC as a whole where some evidence for general mass segregation has been found.

We use our own, recently developed pre-main-sequence evolutionary tracks to investigate the star formation histories of relatively nearby associations and clusters. We first employ published luminosities and effective temperatures to place the known members of each region in the H-R diagram. We then construct age histograms detailing that region's history. The groups studied include Taurus-Auriga, Lupus, Chamaeleon, ρ Ophiuchi, Upper Scorpius, IC 348, and NGC 2264. This study is the first to analyze a large number of star-forming regions with the same set of theoretical tracks.

Our investigation corroborates and extends our previous results on the Orion Nebula Cluster. In all cases, we find that star formation began at a relatively low level some 10⁷ yr in the past and has more recently undergone a steep acceleration. This acceleration, which lasts several million years, is usually continuing through the present epoch. The one clear exception is the OB association Upper Scorpius, where the formation rate climbed upward, peaked, and has now died off. Significantly, this is also the only region of our list that has been largely stripped of molecular gas.

The acceleration represents a true physical phenomenon that cannot be explained away by incompleteness of the samples; nor is the pattern of stellar births significantly affected by observational errors or the presence of unresolved binaries. We speculate that increasing star formation activity arises from contraction of the parent cloud. Despite the short timescale for acceleration, the cloud is likely to evolve quasi-statically. Star formation itself appears to be a critical phenomenon, occurring only in locations exceeding some threshold density. The cloud's contraction must reverse itself, and the remnant gas dissipate, in less than 10⁷ yr, even for aggregates containing no massive stars. In this case, molecular outflows from the stars themselves presumably accomplish the task, but the actual dispersal mechanism is still unclear.

We investigate numerically the role of thermal instability (TI) as a generator of density structures in the interstellar medium (ISM), both by itself and in the context of a globally turbulent medium. We consider three sets of numerical simulations: (1) flows in the presence of the instability only; (2) flows in the presence of the instability and various types of turbulent energy injection (forcing), and (3) models of the ISM including the magnetic field, the Coriolis force, self-gravity and stellar energy injection. Simulations in the first group show that the condensation process that forms a dense phase ("clouds") is highly dynamical and that the boundaries of the clouds are accretion shocks, rather than static density discontinuities. The density histograms (probability density functions [PDFs]) of these runs exhibit either bimodal shapes or a single peak at low densities plus a slope change at high densities. Final static situations may be established, but the equilibrium is very fragile: small density fluctuations in the warm phase require large variations in that of the cold phase, probably inducing shocks in the clouds. Combined with the likely disruption of the clouds by Kelvin-Helmholtz instability, this result suggests that such configurations are highly unlikely. Simulations in the second group show that large-scale turbulent forcing is incapable of erasing the signature of TI in the density PDFs, but small-scale, stellar-like forcing causes the PDFs to transit from bimodal to a single-slope power law, erasing the signature of the instability. However, these simulations do not reach stationary regimes, with TI driving an ever-increasing star formation rate. Simulations in the third group show no significant difference between the PDFs of stable and unstable cases and reach stationary regimes, suggesting that the combination of the stellar forcing and the extra effective pressure provided by the magnetic field and the Coriolis force overwhelm TI as a density-structure generator in the ISM, with TI becoming a second-order effect. We emphasize that a multimodal temperature PDF is not necessarily an indication of a multiphase medium, which must contain clearly distinct thermal equilibrium phases, and that this "multiphase" terminology is often inappropriately used.

A linear triplet isomer of HC₆N has been detected by Fourier transform microwave spectroscopy in a supersonic molecular beam. A total of 85 hyperfine components from six rotational transitions between 8 and 18 GHz were measured to an uncertainty of 5 kHz; a similar set of transitions were detected for the ¹⁵N isotopic species, produced using an isotopically enriched precursor gas sample. The spectroscopic constants for both species, including the fine and hyperfine coupling constants, were determined to very high accuracy, and these allow calculation of the radio spectrum to a fraction of 1 km s^-1 in equivalent radial velocity. Triplet HC₆N is a highly polar, low-lying isomer; measurements show it to be about 10 times more abundant than a ring-chain isomer recently detected with the same spectrometer.

We have extended a simple model of nonlinear diffusive shock acceleration to include the injection and acceleration of electrons and the production of photons from bremsstrahlung, synchrotron, inverse-Compton, and pion-decay processes. We argue that the results of this model, which is simpler to use than more elaborate ones, offer a significant improvement over test-particle, power-law spectra which are often used in astrophysical applications of diffusive shock acceleration. With an evolutionary supernova remnant (SNR) model to obtain shock parameters as functions of ambient interstellar medium parameters and time, we predict broadband continuum photon emission from supernova remnants in general, and SN 1006 in particular, showing that our results compare well with the more complete time-dependent and spherically symmetric nonlinear model of Berezhko, Ksenofontov, & Petukhov. We discuss the implications nonlinear shock acceleration has for X-ray line emission and use our model to describe how ambient conditions determine the TeV/radio flux ratio, an important parameter for γ-ray observations of radio SNRs.

We have observed the ionized gas in the star-forming region W49N with the National Radio Astronomy Observatory Very Large Array (VLA) at 13 mm and 7 mm, and with the Berkeley Illinois Maryland Association (BIMA) Array at 3.3 mm. These observations vary in resolution from 0045 to 035 (500 AU to 4000 AU at a distance of 11.4 kpc). In addition, we have used the VLA to observe water maser emission towards the bright W49N:G sources over a wide velocity range from -435 to 435 km s^-1. The high-resolution continuum observations reveal the morphologies in the ultracompact sources; most of the sources at 0.045'' resolution appear to have shell or ring morphologies. The 3.3 mm emission observed with the BIMA array is dominated by free-free emission in all of the compact sources. There is no evidence for any spectral breaks corresponding to the emergence of a dust component. Of the seven bright sources in W49N for which multifrequency flux densities have been measured, four are observed to have rising spectral indices, with values ranging from α = 0.3-1.1 and three are observed to be flat (S_ν ∝ ν^α). Those sources with rising spectral indices (A, B1, B2, G1, and G2) also have the broadest radio recombination lines, with ΔV_FWHM > 45 km s^-1 in the H66α line (De Pree, Mehringer, & Goss). High-resolution 1.3 cm continuum images made at the same time as the water maser observations have been used to align the maser positions with the high-resolution 7 mm continuum to within 005. The maser positions are closely associated with the G1/G2 sources. The outflow traced by the water masers (Gwinn, Moran & Reid) appears to be centered within 0.2''  of the G2 peak, the brightest continuum source in the region, but it remains unclear whether this source drives the outflow.

Several compact radio continuum sources in W49A show detectable 8-20 μm emission in MIRAC2 images obtained at the IRTF. In general, the infrared morphologies of these sources closely resemble the radio continuum emission. Spectral energy distributions indicate an infrared continuum excess above the level expected from free-free emission, consistent with thermal emission from dust grains heated to a few hundred K. The bright radio continuum sources concentrated at the western end of the ring of ultracompact H II regions are not detected in the mid-infrared, while those at other positions in the ring are detected. This could be due to a localized region of high extinction along the line of sight. In addition, there are a few new infrared sources with no radio continuum counterparts. Finally, several infrared sources show strong 12.8 μm [Ne II] emission, yielding neon abundances that are typically a few percent of the cosmic abundance of neon but are high considering the expected Ne⁺⁺/Ne⁺ ratios for the range of spectral types of the ionizing sources. We conclude that the [Ne II] emission must come from shells around the ultracompact H II regions, where the neon is able to survive as Ne⁺ rather than Ne⁺⁺ because the radiation field has been softened by absorption of hard UV photons within the H II regions.

We present calculations of frictional heating by ion-neutral drift in three-dimensional simulations of turbulent, magnetized molecular clouds. We show that ambipolar drift heating is a strong function of position in a turbulent cloud, and its average value can be significantly larger than the average cosmic-ray heating rate. The heating rate per unit volume due to ambipolar drift, H_AD = | ×|²/ρ_iν_in ~ B⁴/(16π²Lρ_iν_in), is found to depend on the rms Alfvénic Mach number, _A, and on the average field strength, as H_AD ∝ ⟨|B|⟩⁴. This implies that the typical scale of variation of the magnetic field, L_B, is inversely proportional to _A, which we also demonstrate.

We present simulations of the propagation of magnetized jets. This work differs from previous studies in that the cross-sectional distributions of the jets's state variables are derived from analytical models for magnetocentrifugal launching. The source is a magnetized rotator whose properties are specified as boundary conditions. The jets in these simulations are considerably more complex than the "top-hat" constant density, etc. profiles used in previous work. We find that density and magnetic field stratification (with radius) in the jet leads to new behavior including the separation of an inner jet core from a low density collar. We find this "jet within a jet" structure, along with the magnetic stresses, leads to propagation behaviors not observed in previous simulation studies. Our methodology allows us to compare MHD jets from different types of sources whose properties could ultimately be derived from the behavior of the propagating jets.

We present a high-resolution image of the region near HL Tau in ¹³CO (1-0) over a region of 3' made with the BIMA array, supplemented by data from the NRAO 12 m telescope to include emission structures with low spatial frequencies. We find evidence for a shell of dimensions ~2' × 15 (~0.08 × 0.06 pc), with XZ Tau being the closest known source to its center. The ¹³CO map is consistent with an expanding bubble that has blown out on the far side from earth. Portions of the bubble wall are seen in optical scattered light. HL Tau is situated in the bubble wall; the evacuated region corresponds to the truncation in the reflection nebula northeast of HL Tau. Although it is thought that a remnant protostellar envelope is still infalling onto HL Tau, the existence of the expanding bubble makes it difficult to interpret the geometry and kinematics of the HL Tau circumstellar material. Our results demonstrate the importance of including low-spatial frequency emission for the interpretation of interferometer maps.

We investigate shear and buoyancy instabilities in radially stratified, magnetized, cylindrical flows, for application to magnetocentrifugally driven winds—such as those from protostars—and to magnetized accretion disks. Our motivation is to characterize the susceptibility of cold MHD disk winds to growing internal perturbations and to understand the relation of wind instabilities to known accretion disk instabilities. Using four different linear analysis techniques, we identify and study nine principal types of unstable or overstable disturbances, providing numerical and analytic solutions for growth rates for a wide range of parameters. When magnetic fields are predominantly toroidal, as in protostellar winds far from their source, we find the system is susceptible to growth of five different kinds of perturbations: axisymmetric fundamental and toroidal resonance modes, axisymmetric and nonaxisymmetric toroidal buoyancy modes, and nonaxisymmetric magnetorotational modes. Winds having a sufficiently steep field gradient (d ln B/d ln R < -0.75 for a purely toroidal-field case) are globally unstable to the long-wavelength fundamental mode concentrated at small radii; these promote the establishment of narrow dense jets in the centers of wider winds. Long-wavelength outer-wind modes are all stable for power-law wind equilibria. The toroidal buoyancy instabilities promote small-scale radial mixing provided the equilibrium has nonzero magnetic forces. For low-temperature toroidal-B winds, both axisymmetric and nonaxisymmetric magnetorotational instabilities have very low growth rates. The stabilization of buoyancy instabilities by shear and of magnetorotational instabilities by compressibility may be important in allowing cold MHD winds to propagate over vast distances in space. When magnetic fields are predominantly poloidal, as may occur in protostellar winds close to their source or in astrophysical disks, we find the system is susceptible to four additional growing modes: axisymmetric magnetorotational (Balbus-Hawley), axisymmetric poloidal buoyancy, nonaxisymmetric geometric buoyancy, and poloidal resonance modes. The well-known axisymmetric Balbus-Hawley mode has the fastest growth rate. When the magnetic field is nonuniform, the axisymmetric poloidal buoyancy mode promotes radial mixing on small scales. The geometric poloidal buoyancy mode requires high m, thus is readily stabilized by shear. Previous work on magnetorotational instabilities has concentrated on near-incompressible systems (accretion disks or stellar interiors). We extend this analysis to allow for compressibility (important in winds). We introduce a "coherent wavelet" technique (a WKB temporal approximation) and derive closed-form analytic expressions for instantaneous instability criteria, growth rates, and net amplification factors for generalized nonaxisymmetric magnetorotational instabilities in compressible flows with both poloidal and toroidal fields. We confirm that these are in excellent agreement with the results of shearing-sheet temporal integrations and that "locally axisymmetric" perturbations have the largest amplifications only provided ( _A)/Ω ≲ 1.

By employing the equations of mean-square vorticity (enstrophy) fluctuations in strong shear flows, we demonstrate that, unlike energy production of turbulent vorticity in nonrotating shear flows, the turbulent vorticity of weak convection in Keplerian disks cannot gain energy from vortex stretching/tilting by background shear unless the associated Reynolds stresses are negative. This is because the epicyclic motion is an energy sink of the radial component of mean-square turbulent vorticity in Keplerian disks when Reynolds stresses are positive. Consequently, weak convection cannot be self-sustained in Keplerian flows. This agrees with the results implied from the equations of mean-square velocity fluctuations in strong shear flows. Our analysis also sheds light on the explanation of the simulation result in which positive kinetic helicity is produced by the Balbus-Hawley instability in a vertically stratified Keplerian disk. We also comment on the possibility of outward angular momentum transport by strong convection based on azimuthal pressure perturbations and directions of energy cascade.

Some isolated Wolf-Rayet stars present random variability in their optical flux and polarization. We make the assumption that such variability is caused by the presence of regions of enhanced density—i.e., blobs—in their envelopes. In order to find the physical characteristics of such regions, we have modeled the stellar emission using a Monte Carlo code to treat the radiative transfer in an inhomogeneous electron scattering envelope. We are able to treat multiple scattering in the regions of enhanced density as well as in the envelope itself. The finite sizes of the source and structures in the wind are also taken into account. Most of the results presented here are based on a parameter study of models with a single blob. The effects caused by multiple blobs in the envelope are considered to a more limited extent. Our simulations indicate that the density enhancements must have a large geometric cross section in order to produce the observed photopolarimetric variability. The sizes must be of the order of 1 stellar radius, and the blobs must be located near the base of the envelope. These sizes are the same inferred from the widths of the subpeaks in optical emission lines of Wolf-Rayet stars. Other early-type stars show random polarimetric fluctuations with characteristics similar to those observed in Wolf-Rayet stars, which may also be interpreted in terms of a clumpy wind. Although the origin of such structures is still unclear, the same mechanism may be working in different types of hot star envelopes to produce such inhomogeneities.

We have obtained J-band (1.2 μm) polarimetry observations of the circumbinary disk around UY Aurigae. These observations were made possible by the use of the University of Hawaii 36 element adaptive optics instrument, Hokupa'a, at the 3.35 m CFHT. The deep (120 minute), high-resolution (015) polarization images reveal a centrosymmetric polarization signature from the light scattered off the circumbinary dust disk which is ~ 10⁶ times fainter than the stars in the binary system. A comparison with a Mie scattering model of the circumbinary disk in UY Aurigae suggests that the polarization signature is dominated by the smallest grains in the disk (~ 0.03 μm) and further supports the hypothesis that the resolved light seen in the optical and infrared originates from a large flattened disk of dust surrounding both stars.

We present the results of our optical spectropolarimetry of nova V4444 Sagittarii 1999 in its early stage. The observations were performed on four nights from 1999 April 29 (t = 2 days) to 1999 May 7 (t = 10 days). The polarization properties, including the absence of polarization difference across strong emission lines, have remained unchanged through the period. The observed polarization degree increases monotonically toward shorter wavelength. We separate the interstellar polarization component from the observed polarization by assuming that the component intrinsic to this nova has a power-law-type function of wavelength. The decomposed intrinsic polarization can be explained by scattering due to small grains. These results suggest the preexistence of the dust cloud in the vicinity of the nova, at least several dozens of AU from the central star.

I propose that the mechanism behind the formation of concentric semiperiodic shells found in several planetary nebulae (PNs) and proto-PNs and around one asymptotic giant branch (AGB) star is a solar-like magnetic activity cycle in the progenitor AGB stars. The time intervals between consecutive ejection events are ~200-1000 yr, which is assumed to be the cycle period (the full magnetic cycle can be twice as long, as is the 22 yr period in the Sun). The magnetic field has no dynamical effects; it regulates the mass-loss rate by the formation of magnetic cool spots. The enhanced magnetic activity at the cycle maximum results in more magnetic cool spots, which facilitate the formation of dust, hence increasing the mass-loss rate. The strong magnetic activity implies that the AGB star is spun up by a companion, via a tidal or common envelope interaction. The strong interaction with a stellar companion explains the observations that the concentric semiperiodic shells are found mainly in bipolar PNs.

We report the discovery of extended X-ray emission from the planetary nebula BD +30^°3639. Analysis of the ROSAT HRI image shows clearly that BD +30^°3639 has extended X-ray emission which is well fitted with a Gaussian source of σ = 17-28 (95.4% confidence interval). This size is consistent with being the same as that of the optical nebula as imaged by the Hubble Space Telescope. BD +30^°3639 therefore represents the best case for the detection of a shocked-stellar-wind bubble in planetary nebulae.

A theoretical light curve for the 1987 outburst of V394 Coronae Australis (V394 CrA) is modeled to obtain various physical parameters of this recurrent nova. We then apply the same set of parameters to a quiescent phase and confirm that these parameters give a unified picture of the binary. Our V394 CrA model consists of a very massive white dwarf (WD), with an accretion disk (ACDK) having a flaring-up rim, and a lobe-filling, slightly evolved, main-sequence star (MS). The model includes irradiation effects of the MS and the ACDK by the WD. The early visual light curve (t ~ 1-10 days after the optical maximum) is well reproduced by a thermonuclear runaway model on a very massive WD close to the Chandrasekhar limit (1.37 ± 0.01 M_☉). The ensuing plateau phase (t ~ 10-30 days) is also reproduced by the combination of a slightly irradiated MS and a fully irradiated flaring-up disk with a radius ~1.4 times the Roche lobe size. The best-fit parameters are the WD mass ~1.37 M_☉, the companion mass ~1.5 M_☉ (0.8-2.0 M_☉ is acceptable), the inclination angle of the orbit i ~ 65^°-68°, and the flaring-up rim ~0.30 times the disk radius. The envelope mass at the optical peak is estimated to be ~6 × 10^-6M_☉, which indicates an average mass accretion rate of ~1.5 × 10^-7M_☉ yr^-1 during the quiescent phase between the 1949 and 1987 outbursts. In the quiescent phase, we properly include the accretion luminosity of the WD and the viscous luminosity of the ACDK as well as the irradiation effects of the ACDK and MS by the WD. The observed light curve can be reproduced with a disk size of 0.7 times the Roche lobe size and a rather slim thickness of 0.05 times the accretion disk size at the rim. About 0.5 mag sinusoidal variation of the light curve requires a mass accretion rate higher than ~1.0 × 10^-7M_☉ yr^-1, which is consistent with the above estimation from the 1987 outburst. These newly obtained quantities are exactly the same as those predicted in a new progenitor model of Type Ia supernovae.

Low-resolution spectra of SN 1999dn at early times are presented and compared with synthetic spectra generated with the parameterized supernova synthetic spectrum code SYNOW. We find that the spectra of SN 1999dn strongly resemble those of SN 1997X and SN 1984L, and hence we classify it as a Type Ib event. Line identifications are established through spectrum synthesis. Strong evidence of both Hα and C II λ6580 is found. We infer that Hα appears first, before the time of maximum brightness, and then is blended with and finally overwhelmed by the C II line after maximum; this favors a thin high-velocity hydrogen skin in this Type Ib supernova.

We report high-resolution, high signal-to-noise observations of the extremely metal-poor double-lined spectroscopic binary CS 22876-032. The system has a long period: P = 424.7 ± 0.6 days. It comprises two main-sequence stars having effective temperatures 6300 and 5600 K, with a ratio of secondary to primary mass of 0.89 ± 0.04. The metallicity of the system is [Fe/H] = -3.71 ± 0.11 ± 0.12 (random and systematic errors)—somewhat higher than previous estimates.

We find [Mg/Fe] = 0.50, typical of values of less extreme halo material. [Si/Fe], [Ca/Fe], and [Ti/Fe], however, all have significantly lower values, ~0.0-0.1, suggesting that the heavier elements might have been underproduced relative to Mg in the material from which this object formed. In the context of both the hypothesis that the abundance patterns of extremely metal-poor stars are driven by individual enrichment events and the models of Woosley & Weaver, the data for CS 22876-032 are consistent with its having been enriched by a zero-metallicity supernova of mass 30 M_☉.

As the most metal-poor near-main-sequence turnoff star currently known, the primary of the system has the potential to strongly constrain the primordial lithium abundance. We find A(Li) {= log[N(Li)/N(H)] + 12.00} = 2.03 ± 0.07, which is consistent with the finding of Ryan et al. that, for stars of extremely low metallicity, A(Li) is a function of [Fe/H].

The luminosities, effective temperatures, and metallicities that are derived empirically by Kovács & Jurcsik from the light curves of a large number of globular cluster and field RRab and RRc stars are compared to theoretical RR Lyrae models. The strong luminosity dependence of the empirical blue and red edges (log L vs. log T_eff diagram) is in disagreement with that of both radiative and convective models. A reexamination of the theoretical uncertainties in the modeling leads us to conclude that the disagreement appears irreconcilable.

In this paper we discuss X-ray observations of γ Cas obtained in 1998 November with the Rossi X-Ray Timing Explorer (RXTE). The data were obtained nearly continuously over 54 hr, which is about twice the expected rotational period. An earlier RXTE light curve obtained in 1996 March over a 27 hr period showed X-ray flux arising from short-duration shots (flares) superimposed on an undulating "basal" component that was anticorrelated with fluctuations of the UV continuum over a timescale of ~10 hr. The object of the present study was to (1) examine the long-term variations of the X-ray characteristics through comparisons with this earlier data and (2) to determine whether variations of the basal flux repeat during a second rotation period. A comparison of the results with the 1996 data set shows a number of similarities and differences in the X-ray behavior: (a) the mean X-ray level in 1998 was only 60% of the 1996 level, (b) the basal fluxes in 1998 vary over shorter timescales (less than 2 hr) than in 1996, (c) the shots were found to show a slightly softer (cooler) mean color than the basal component in 1998, although they were slightly hotter in 1996, (d) fluctuations in the colors of the shot and basal fluxes generally track one another in both data sets, (e) cyclical patterns of X-ray flux decrease with a period of about 7.5 hr occurred in both data sets, and (f) the frequency of shots with a given integrated energy was found to decrease exponentially with energy, although the rate of decrease in 1996 was slower than in 1998. There was only marginal evidence for a repetition during the second half of the time sequence of long-term basal flux variations seen during the first half of the observations. We suspect, however, that the large intrinsic variability of the X-ray source would have masked a true replication. We also present archival IUE data that shows the presence of UV continuum variations in 1982 with similar characteristics to those seen in 1996. This suggests that the regions responsible for the UV variability are very long lived. The data also provide the basis for a refined but still tentative rotational period of 1.12277 days. Assuming a flare paradigm and a very simple electron beam model, we examine the atmospheric heating expected for the shot events. We conclude that it is possible to explain how the measured shot temperature can be smaller than the temperature deduced for the basal X-ray emission. We also discover that if the beam model is correct then the electrons within the beam have relatively high energies (>200 keV) and are nearly monoenergetic. In three appendices we discuss arguments, first, against the idea that the X-ray emission from γ Cas arises from mass accretion onto a hypothetical white dwarf companion or from an active late-type star and, second, in favor of its origin from near the surface of γ Cas.

The rotation of horizontal-branch stars places important constraints on angular momentum evolution in evolved stars and therefore on rotational mixing on the giant branch. Prompted by new observations of rotation rates of horizontal-branch stars, we calculate simple models for the angular momentum evolution of a globular cluster giant star from the base of the giant branch to the star's appearance on the horizontal branch. We include mass loss and infer the accompanied loss of angular momentum for each of four assumptions about the internal angular momentum profile. Mass loss is found to have important implications for angular momentum evolution. These models are compared to observations of horizontal-branch rotation rates in M13. We find that rapid rotation on the horizontal branch can be reconciled with slow solid body main-sequence rotation if giant-branch stars have differential rotation in their convective envelopes and a rapidly rotating core, which is then followed by a redistribution of angular momentum on the horizontal branch. We discuss the physical reasons that these very different properties relative to the solar case may exist in giants. Rapid rotation in the core of the main-sequence precursors of the rapidly rotating horizontal-branch star or an angular momentum source on the giant branch is required for all cases if the rotational velocity of turnoff stars is less than 4 km s^-1. We suggest that the observed range in rotation rates on the horizontal branch is caused by internal angular momentum redistribution, which occurs on a timescale comparable to the evolution of the stars on the horizontal branch. The apparent lack of rapid horizontal-branch rotators hotter than 12,000 K in M13 could be a consequence of gravitational settling, which inhibits internal angular momentum transport. Alternative explanations and observational tests are discussed.

The close-in extrasolar giant planets (CEGPs), ≲0.05 AU from their parent stars, may have a large component of optically reflected light. We present theoretical optical photometric light curves and polarization curves for the CEGP systems from reflected planetary light. Different particle sizes of three condensates are considered. In the most reflective case, the variability is ≈100 μmag, which will be easily detectable by the upcoming satellite missions Microvariability and Oscillations of Stars (MOST), COROT, and Measuring Oscillations in Nearby Stars (MONS), and possibly from the ground in the near future. The least reflective case is caused by small, highly absorbing grains such as solid Fe, with variation of much less than 1 μmag. Polarization for all cases is lower than current detectability limits. We also discuss the temperature-pressure profiles and resulting emergent spectra of the CEGP atmospheres. We discuss the observational results of τ Boo b by Cameron et al. and Charbonneau et al. in context of our model results. The predictions—the shape and magnitude of the light curves and polarization curves—are highly dependent on the sizes and types of condensates present in the planetary atmosphere.

We report multifrequency radio observations of GRO J1655-40 obtained with the Australia Telescope Compact Array, the Molonglo Observatory Synthesis Telescope and the Hartebeesthoek Radio Astronomy Observatory at the time of the major hard X-ray and radio outbursts in 1994 August-September. The radio emission reached levels of the order of a few Jy and was found to be linearly polarized by up to 10%, indicating a synchrotron origin. The light curves are in good agreement with those measured with the VLA, but our closer time sampling has revealed two new short-lived events and significant deviations from a simple exponential decay. The polarization data show that the magnetic field is well ordered and aligned at right angles to the radio jets for most of the monitoring period. The time evolution of the polarization cannot be explained solely in terms of a simple synchrotron bubble model, and we invoke a hybrid "core-lobe" model with a core which contributes both synchrotron and free-free emission and "lobes," which are classical synchrotron emitters.

We present, for the first time, the Fourier-Doppler Imaging (FDI) analysis of periodic line profile variations in a ζ Oph-type star. For this purpose we obtained, in the period from 1996 May 3 to May 5, a total of 242 high-resolution, high signal-to-noise ratio spectra of the Be star ζ Oph itself. Using the FDI technique, we examine the variations in both time and wavelength and complement it with time series analysis. This kind of analysis is valid for both the nonradial pulsator model and the rotation modulation model, but we discuss the results in terms of the former model, considering it the more likely explanation for the observed line profile variability. Two distinct groups of modes are detected: medium (4 ≤ l ≈ |m| ≤ 8) and high-degree modes (which could be associated with 13 ≤ l ≈ |m| ≤ 17). It is shown that the high-frequency oscillations were strongly confined to an equatorial belt narrower than 20° and that the line profile variability was caused predominantly by sectoral modes, although tesseral modes |m| = l - 1 are not excluded in taking into account the effect of fast rotation. We discuss the modal nature of the waves with respect to the characteristic oscillation periods in the corotating frame and the high amplitude of the projected rotational velocity variations (≈20 km s^-1).

We present three-dimensional numerical simulations of the rise and fragmentation of twisted, initially horizontal magnetic flux tubes that evolve into emerging Ω-loops. The flux tubes rise buoyantly through an adiabatically stratified plasma that represents the solar convection zone. The MHD equations are solved in the anelastic approximation, and the results are compared with studies of flux-tube fragmentation in two dimensions. We find that if the initial amount of field line twist is below a critical value, the degree of fragmentation at the apex of a rising Ω-loop depends on its three-dimensional geometry: the greater the apex curvature of a given Ω-loop, the lesser the degree of fragmentation of the loop as it approaches the photosphere. Thus, the amount of initial twist necessary for the loop to retain its cohesion can be reduced substantially from the two-dimensional limit. The simulations also suggest that, as a fragmented flux tube emerges through a relatively quiet portion of the solar disk, extended crescent-shaped magnetic features of opposite polarity should form and steadily recede from one another. These features eventually coalesce after the fragmented portion of the Ω-loop emerges through the photosphere.

We have investigated the effects of resonant scattering of emission lines on the image morphology and intensity from coronal loop structures. It has previously been shown that line-of-sight effects in optically thin line emission can yield loop images that appear uniformly bright at one viewing angle but show "looptop sources" at other viewing angles. For optically thick loops where multiple resonant scattering is important, we use a three-dimensional Monte Carlo radiation transfer code. Our simulations show that the intensity variation across the image is more uniform than the optically thin simulation, and depending on viewing angle the intensity may be lower or higher than that predicted from optically thin simulations due to scattering out of or into the line of sight.

This paper describes a new scenario for type III solar radio emission. While the conventional theories emphasize Langmuir waves propagating along the ambient magnetic field, the present model stresses extraordinary-mode Bernstein waves with quasi-perpendicular wave vectors. The present model relies on plasma inhomogeneity and wave reflection to produce the electromagnetic radiation, in contrast to the conventional theories, which rely on nonlinear wave-wave and wave-particle interactions. The essence of the model is that Bernstein waves with frequencies above the X-mode cutoff frequency are first excited. Initially these waves can only propagate toward regions with higher density and magnetic field. During this process, quasi-electrostatic Bernstein waves spontaneously convert to electromagnetic waves and rapidly reach the local X-mode cutoff frequency. Subsequently, these waves are reflected and propagate away from the source region. The escaping radiation is observed as the type III emission. It is pointed out that in general, the observed radiation should possess one band in the dynamic spectrum. However, under certain conditions, the present theory is also capable of explaining the appearance of a pair of bands similar to those commonly called the "fundamental" and "harmonic" components in the literature. The present model resolves some of the inconsistencies between the usual plasma emission hypothesis and the observed pair emissions.

We have studied 8 yr of active region observations from the United States Air Force/Mount Wilson data set, supplied by the NOAA World Data Center, to confirm the relation between δ spots and large flares. We found that after correcting some errors we were able to describe relationships among active region size, peak flare soft X-ray (SXR) flux (measured by GOES 1-8 Å flux), and magnetic classification. We found the Solar Optical Observing Network magnetic classification to be reasonably accurate but its area measures to be inaccurate for many of the regions. This is due partly to transcription errors and partly to wrong correction for limb foreshortening. Errors could, however, be repaired by intercomparison of multiple observations. We confirm Künzel's original idea that regions classified βγδ produce many more large flares than other regions of comparable size. Almost all substantial flares occurred in regions classified βγδ by the Air Force sites. Each region larger than 1000 μh and classified βγδ had nearly 40% probability of producing flares classified X1 or greater. Yet only a half-dozen of those, showing the "island delta" configuration, produced great activity. There is a general trend for large regions to produce large flares, but it is less significant than the dependence on magnetic class.

The carbonaceous chondrite meteorites contain a record of the formation of the solar system, part of which is present within organic matter. This organic matter is predominantly aromatic, and its sources remain controversial. The δ¹³C values for individual free and macromolecular aromatic moieties from Cold Bokkeveld and Murchison suggest that these units originate from radiation-induced "circle" reactions involving simultaneous bond synthesis and cracking. Large carbon isotope fractionations and a deuterium enrichment for these entities suggest that these reactions occurred in a dense interstellar cloud. The juxtaposition of the synthesis and cracking products implies that the reactions occurred in a restricting medium, the most likely candidate for which is the icy organic mantles of interstellar grains. In contrast, the δ¹³C record in aromatic moieties from Orgueil is mostly obscured, possibly due to the increased levels of parent body aqueous alteration experienced by this meteorite. These novel observations are consistent with the interstellar-parent body hypothesis, where the final form of meteoritic organic matter results from the transfiguration of interstellar organic precursors by aqueous reactions on the meteorite parent body.

The central densities of dark matter (DM) halos are much lower than predicted in cold DM models of structure formation. Confirmation that they have cores with a finite central density would allow us to rule out many popular types of collisionless particles as candidates for DM. Any model that leads to cusped halos (such as cold DM) is already facing serious difficulties on small scales, and hot DM models have been excluded. Here I show that fermionic warm DM is inconsistent with the wide range of phase-space densities in the DM halos of well-observed nearby galaxies.

The possibility that Galactic halo MACHOs are white dwarfs has recently attracted much attention. Using the known properties of white dwarf binaries in the Galactic disk as a model, we estimate the possible contribution of halo white dwarf binaries to the low-frequency (10^-5 Hz < f < 10^-1 Hz) gravitational wave background. Assuming the fraction of white dwarfs in binaries is the same in the halo as in the disk, we find the confusion background from halo white dwarf binaries could be 5 times stronger than the expected contribution from Galactic disk binaries, dominating the response of the proposed space-based interferometer LISA. Low-frequency gravitational wave observations will be the key to discovering the nature of the dark MACHO binary population.

The last few months have seen the measurements of the radial velocities of all of the dwarf spheroidal companions to the Andromeda galaxy (M31) using the spectrographs (HIRES and LRIS) on the Keck Telescope. This Letter summarizes the data on the radial velocities and distances for all the companion galaxies and presents new dynamical modeling to estimate the mass of the extended halo of M31. The best-fit values for the total mass of M31 are ~(7-10) × 10¹¹M_☉, depending on the details of the modeling. The mass estimate is accompanied by considerable uncertainty caused by the small size of the data set; for example, the upper bound on the total mass is ~24 × 10¹¹M_☉, while the lower bound is ~3 × 10¹¹M_☉. These values are less than the most recent estimates of the most likely mass of the Milky Way halo. Bearing in mind all the uncertainties, a fair conclusion is that the M31 halo is roughly as massive as that of the Milky Way halo. There is no dynamical evidence for the widely held belief that M31 is more massive—it may even be less massive.

Emission-line variability data for Seyfert 1 galaxies provide strong evidence for the existence of supermassive black holes in the nuclei of these galaxies and that the line-emitting gas is moving in the gravitational potential of that black hole. The time-delayed response of the emission lines to continuum variations is used to infer the size of the line-emitting region, which is then combined with measurements of the Doppler widths of the variable line components to estimate a virial mass. In the case of the best-studied galaxy, NGC 5548, various emission lines spanning an order of magnitude in distance from the central source show the expected V ∝ r^-1/2 correlation between distance and line width and are thus consistent with a single value for the mass. Two other Seyfert galaxies, NGC 7469 and 3C 390.3, show a similar relationship. We compute the ratio of luminosity to mass for these three objects and the narrow-line Seyfert 1 galaxy NGC 4051 and find that the gravitational force on the line-emitting gas is much stronger than radiation pressure. These results strongly support the paradigm of gravitationally bound broad emission line region clouds.

A new scenario for extracting energy from a Kerr black hole is proposed. With magnetic field lines connecting plasma particles inside the ergosphere with remote loads, the frame dragging twists the field lines so that energy and angular momentum are extracted from the plasma particles. If the magnetic field is strong enough, the energy extracted from the particles can be so large that the particles have negative energy as they fall into the black hole. So, effectively, the energy is extracted from the black hole. The particles inside the ergosphere can be continuously replenished with accretion from a disk surrounding the black hole, so a transition region with a sufficient amount of plasma is formed between the black hole's horizon and the inner edge of the disk. Thus, the energy can be continuously extracted from the black hole through the transition region. This may be the most efficient way for extracting energy from a Kerr black hole: in principle, almost all of the rotational energy (up to ≈29% of the total energy of the black hole) can be extracted.

An off-line scan for nontriggered gamma-ray bursts (GRBs) in the BATSE daily records at 1024 ms time resolution covering about 7 yr of observations gave 1353 nontriggered and 1581 triggered GRBs. The scan efficiency was measured by adding artificial test bursts to the data. The log N- log P distribution could be extended down to peak fluxes, P ~ 0.1 photons cm^-2 s^-1. Previous indications of a turnover at small P are not confirmed. The log N- log P distribution cannot be fitted with a standard candle model with a nonevolving GRB source population, assuming that there are no large non-GRB contaminations. It is likely that the intrinsic luminosity function of GRBs is wide.

The central source of the supernova remnant PKS 1209-51/52 was observed with the Advanced CCD Imaging Spectrometer aboard Chandra X-Ray Observatory on 2000 January 6-7. The use of the continuous clocking mode allowed us to perform the timing analysis of the data with a time resolution of 2.85 ms and to find a period P = 0.42412924 s ± 0.23 μs. The detection of this short period proves that the source is a neutron star. It may be either an active pulsar with an unfavorably directed radio beam or a truly radio-silent neutron star whose X-ray pulsations are caused by a nonuniform distribution of surface temperature. To infer the actual properties of this neutron star, the period derivative should be measured.

We report the discovery of a third kilohertz quasi-periodic oscillation (kHz QPO) in the power spectra of the low-mass X-ray binaries 4U 1608-52 (6.3 σ), 4U 1728-34 (6.0 σ), and 4U 1636-53 (3.7 σ) which is present simultaneously with the previously known kHz QPO pair. The new kHz QPO is found at a frequency that is 52.8 ± 0.9, 64 ± 2, and 58.4 ± 1.9 Hz higher than the frequency of the lower kHz QPO in 4U 1608-52, 4U 1728-34, and 4U 1636-53, respectively. The difference between the frequency of the new kHz QPO and the lower kHz QPO increased in 4U 1608-52 from 49.6 ± 1.4 to 53.9 ± 0.5 Hz when the frequency of the lower kHz QPO increased from 672 to 806 Hz. Simultaneously the difference between the frequency of the new kHz QPO and the upper kHz QPO increased by ~60 Hz, suggesting that the new kHz QPO is unrelated to the upper kHz QPO. In 4U 1636-53 a fourth, weaker, kHz QPO is simultaneously detected (3 σ) at the same frequency separation below the lower kHz QPO, suggesting that the new kHz QPOs are sidebands to the lower kHz QPO. We discuss the nature of this new kHz QPO and its implications on the models for the kHz QPOs.

We show that, unlike the results presented previously in the literature, the transition from an outer Shakura-Sunyaev disk (SSD) to an advection-dominated accretion flow (ADAF) is possible for large values of the viscosity parameter α > 0.5. The transition is triggered by the thermal instability of a radiation pressure-supported SSD. The transition radius is close to the central black hole. We confirm our qualitative prediction by actually constructing global bimodal SSD-ADAF solutions.

We present full-disk X-ray reflection spectra for two currently popular accretion flow geometries for active galactic nuclei (AGNs): (1) the lamppost model, which is frequently used to discuss the iron-line reverberation in AGNs, and (2) the model in which X-rays are produced in magnetic flares above a cold accretion disk. The lamppost spectra contain several spectroscopic features that are characteristic of highly ionized material that is not seen in the X-ray spectra of most AGNs. The magnetic flare model, on the other hand, produces reflected spectra that are roughly a superposition of a power law and a neutral-like reflection and an iron Kα line; thus, these spectra are more in line with typical AGN X-ray spectra. Furthermore, because of the difference in the ionization structure of the illuminated material in the two models, the line equivalent width increases with the X-ray luminosity L_X for the lamppost and decreases with L_X for the flare model. In light of these theoretical insights, the recent iron-line reverberation studies of AGNs, the X-ray Baldwin effect, and the general lack of X-ray reflection features in distant quasars all suggest that, for high accretion rates, the cold accretion disk is covered by a thick, completely ionized Thomson skin. Because the latter is only possible when the X-rays are concentrated in small emitting regions, we believe that this presents strong evidence for the magnetic flare origin of X-rays in AGNs.

Multifrequency observations of the planet pulsar PSR B1257+12 have been made to examine a possibility that the 25.3 day periodicity observed in its pulse timing residuals represents a variable delay generated by solar rotation-induced density fluctuations in the solar wind. New timing measurements of the pulsar show that the amplitude of these residuals is frequency independent, implying that the periodicity cannot be caused by any arrival time delays related to the pulse propagation through an ionized medium. Therefore, in agreement with the original assumption, the observed 25.3 day pulse timing periodicity is most plausibly explained in terms of the orbital motion of a low-mass planet around the pulsar.

We present BVRIZ photometric observations of HD 209458 during the transit by its planetary companion on UT 1999 November 15 with the University of Hawaii 0.6 and 2.2 m telescopes and the High Altitude Observatory STARE telescope. The detailed shape of the transit curve is predicted to vary with color primarily as a result of the color-dependent limb darkening of the star but potentially due as well to the effect of color-dependent opacity in the planetary atmosphere. We model the light curves and present refined values for the transit timing and orbital period, useful for planning future observations of the planetary transit. We also derive significantly improved measurements of the planetary radius, R_p = 1.55 ± 0.10 R_Jup, stellar radius, R_s = 1.27 ± 0.05 R_☉, and orbital inclination, i = 859 ± 05. The derived planetary radius favors evolutionary models in which the planet has a low albedo.

We analyze the nucleosynthesis implications of the recent discovery by M. J. Pellin and collaborators that two odd isotopes of molybdenum, ⁹⁵Mo and ⁹⁷Mo, are overabundant in type X SiC grains: X grains condensed within expanding supernova interiors. We find that a rapid release of neutrons (on a timescale of seconds) with fluence τ = 0.07-0.08 neutrons mbarn^-1 produces the observed pattern by way of abundant production of progenitor radioactive Zr isotopes. This suggests that the condensing matter was in a supernova shell in which rapid burning was occurring at the time of ejection, probably owing to the passage of the shock wave from the core. Which shell, and the exact source of the neutrons, is still unknown, but we present a model based on the shock of an He shell.

We present high-resolution interferometric and single-dish observations of molecular gas in the Serpens cluster-forming core. Star formation does not appear to be homogeneous throughout the core, but is localized in spatially and kinematically separated subclusters. The stellar (or protostellar) density in each of the subclusters is much higher than the mean for the entire Serpens cluster. This is the first observational evidence for the hierarchical fragmentation of protocluster cores suggested by cluster formation models.

We present Hubble Space Telescope Hα and [S II] images of HH 29. The proximity of HH 29 (140 pc) and the high resolution of the Planetary Camera has resulted in the most detailed images obtained so far of any Herbig-Haro object. The most prominent feature is a linear Hα ridge leading the working surface of a bow shock with a chaotic trailing [S II] bright region. The high-excitation ridge is perpendicular to a line extending toward the class 0 protostar L1551-NE, supporting its recent identification as the driving source. Previous studies have identified several low-velocity features within the working surface. Our images reveal them to be miniature bow shocks facing upstream. Evidently a cluster of dense quasi-stationary clumps have been overrun by a faster, lower density flow. The shock front impacted the front of the largest clump several decades ago, and during the 1990s, a prominent gap appeared in the advancing bow shock in the wake of the obstacle. The Hubble Space Telescope images show that by 1998 the shock front had wrapped around the back of the clump, closing the shock shadow it produced.

The MgNH₂ radical has been detected in the laboratory for the first time in its²A₁ ground state using techniques of millimeter-wave spectroscopy. This short-lived species was created using Broida oven methods. Multiple K_a asymmetry components were recorded for MgNH₂ in 12 rotational transitions in the frequency range 129-527 GHz, and spin-rotation splittings were resolved in every component. A total of 251 separate lines were measured. An intensive search for a possible inversion state in MgNH₂ was also conducted but proved negative. This data set has been modeled with an S-reduced Hamiltonian and very accurate rotational, centrifugal distortion, and spin-rotation parameters have been determined for magnesium amide. This study strongly suggests that the radical is planar with C_2v symmetry, although small barriers to planarity cannot be ruled out. MgNH₂ could be produced in interstellar/circumstellar gas from the association reaction of Mg⁺ + NH₃, as predicted by theory.