Table of contents

We calculate perturbatively the normalized spatial skewness, S₃, and full three-point correlation function (3PCF), ζ, induced by gravitational instability of Gaussian primordial fluctuations for a biased tracer-mass distribution in flat and open cold dark matter (CDM) models. We take into account the explicit dependence on cosmological parameters, the shape and evolution of the CDM power spectrum, and we allow the bias to be nonlinear and/or evolving in time, using an extension of Fry's bias evolution model. We derive a scale-dependent, leading-order correction to the standard perturbative expression for S₃ in the case of nonlinear biasing, as defined for the unsmoothed galaxy and dark-matter fields, and find that this correction becomes large when probing positive effective power-spectrum indices, i.e., scales above 100 h^-1 Mpc for reasonable CDM models. This term implies that the inferred nonlinear-bias parameter, as usually defined in terms of the smoothed density fields, might in general depend on the chosen smoothing scale and could allow better constraints on both the linear- and nonlinear-bias parameters on the basis of skewness measurements alone (or at least distinguish between the smoothed and unsmoothed bias pictures), if S₃ could be measured over very large scales. In general, we find that the dependence of S₃ on the biasing scheme can substantially outweigh that on the adopted cosmology, with linear and nonlinear bias separately giving rise to distinct signatures in the skewness, but degenerate ones in combination. We demonstrate that the normalized 3PCF, Q, is an ill-behaved quantity, and speculate that reported discrepancies between perturbative and N-body predictions for Q may arise in part from systematic errors associated with the poor choice of normalization. To avoid this problem we investigate Q_V, the variance-normalized 3PCF. The configuration dependence of Q_V shows similarly strong sensitivities to the bias scheme as S₃, but also exhibits significant dependence on the form of the CDM power spectrum. Though the degeneracy of S₃ with respect to the cosmological parameters and constant linear- and nonlinear-bias parameters can be broken by the full configuration dependence of Q_V, neither statistic can distinguish well between evolving and nonevolving bias scenarios, since an evolving bias is found to effectively mimic a smaller but constant bias. We show that this can be resolved, in principle, by considering the redshift dependence of ζ, which can also yield direct constraints on Ω₀ and the epoch of galaxy formation.

This paper treats the energetic and nucleosynthetic history of a Friedmann model with low photon-baryon ratio η_γ and positive lepton-baryon ratio η_L. At early times, such a universe is in equilibrium and is energetically dominated by particle Fermi energies; at later times photon release from elementary particle decays probably lifts degeneracy of nucleons but not of leptons. After outlining the early history I present the results of full nucleosynthesis calculations for nondegenerate baryons and degenerate electrons and neutrinos. The results show that, for η_γ ≲ 0.01 (consistent with the particle-decay heating), the neutron-proton ratio depends strongly on η_L and that production of sufficient helium to match observations creates large primordial metallicity, in agreement with a previous argument by Carr for the η_γ = 0 case. Excessive metallicity in the cold model is avoided only if η_L is sufficiently high (η_L > 1.5 for η_γ = 0; η_L ≳ 5 for η_γ = 0.01) to suppress all nucleosynthesis before the Population III epoch. The calculations also reveal certain combinations of η_γ ≳ 100 (corresponding to T₀ ~ 0.02) and η_L ≳ 5 that produce acceptable helium yields. This result contradicts the widespread notion that astrophysical observations of helium abundance together with nucleosynthesis calculations within a big bang model predict the current radiation background temperature.

Two supernovae detected in the Hubble Deep Field (HDF) using the original 1995 December epoch and data from a shorter (63,000 s in F814W) 1997 December visit with HST are discussed. The supernovae (SNe) are both associated with distinct galaxies at redshifts of 0.95 (spectroscopic) from Cohen et al. and 1.32 (photometric) from the work of Fernández-Soto, Lanzetta, & Yahil. These redshifts are near, in the case of 0.95, and well beyond, for 1.32, the greatest distance reported previously for SNe. We show that our observations are sensitive to supernovae to z ≲ 1.8 in either epoch for an event near peak brightness. Detailed simulations are discussed that quantify the level at which false events from our search phase would start to arise and the completeness of our search as a function of both SN brightness and host galaxy redshift. The number of Type Ia and Type II SNe expected as a function of redshift in the two HDF epochs are discussed in relation to several published predictions and our own detailed calculations. A mean detection frequency of one SN per epoch for the small HDF area is consistent with expectations from current theory.

In this paper we derive the galaxy luminosity function from the nearby optical galaxy (NOG) sample, which is a nearly complete, magnitude-limited (B ≤ 14 mag), all-sky sample of nearby optical galaxies (~6400 galaxies with cz < 5500 km s^-1). For this local sample, we use galaxy distance estimates based on different peculiar velocity models. Therefore, the derivation of the luminosity function is carried out using the locations of field and grouped galaxies in real distance space. The local field galaxy luminosity function in the B system is well described by a Schechter function with a slope of α ~ -1.1, a low normalization factor (Φ* ~ 0.006 Mpc^-3), and a particularly bright characteristic magnitude (M^*_B ~ -20.6) (H₀ = 75 km^-1 Mpc^-1). The exact values of the Schechter parameters depend slightly on the adopted peculiar velocity field models. Peculiar motion effects are of the order of statistical errors and cause at most variations of 0.08 in α and 0.2 mag in M^*_B. Our M^*_B value is brighter by a few tenths of a magnitude than previous corresponding values, because using total corrected blue magnitudes better represent the galaxy light. In addition, the selection function, evaluated in terms of the luminosity function, appears to be not very sensitive to the adopted peculiar velocity field models, which, however, have a large impact on the local galaxy density on the smallest scales. The shape of the luminosity function of spiral galaxies does not differ significantly from that of E-S0 galaxies. On the other hand, the late-type spirals and irregulars have a very steeply rising luminosity function toward the faint end (α ~ -2.3 to -2.4), whereas the ellipticals appreciably decrease in number toward low luminosities. The presence of galaxy systems in the NOG sample does not significantly affect the field galaxy luminosity function, since environmental effects on the total luminosity function appear to be marginal. The luminosity function of the members of the richest galaxy systems tends to show a slightly brighter M^*_B value than the norm. In light of constraints imposed by the observed galaxy number counts, the low normalization of the luminosity function suggests that the nearby universe (cz ≲ 5000 km s^-1) examined in this paper may be underdense by a factor of ~1.5.

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 β (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 F_FIR/F₁₆₀₀ versus β relation in terms of A₁₆₀₀ (the dust absorption at 1600 Å) and the transformation from broadband photometric color to β. 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 M_1600,0 correlates with β: 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 M_1600,0 ≈ -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 ρ_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 Å.

We present an unbiased method for evaluating the ranges of ages and metallicities that are allowed by the photometric properties of the stellar populations that dominate the light of early-type galaxies in clusters. The method is based on the analysis of morphologically classified early-type galaxies in 17 clusters at redshifts 0.3 ≲ z ≲ 0.9 and in the nearby Coma Cluster using recent stellar population synthesis models that span a wide range of metallicities. We confirm that metallicity effects must play a role in the origin of the slope of the color-magnitude relation for cluster early-type galaxies. We show, however, that the small scatter of the color-magnitude relation out to redshifts z ~ 1 does not formally imply a common epoch of major star formation for all early-type galaxies. Instead, it requires that galaxies more recently assembled should be, on average, more metal-rich than older galaxies of similar luminosity. Regardless of the true ages and metallicities of early-type galaxies within the allowed range, their photometric properties and the implied strengths of several commonly used spectral indices are found to be consistent with the apparently passive evolution of the stellar populations. Also, the implied dependence of the mass-to-light ratio on galaxy luminosity is consistent with the observed trend. The results of our unbiased analysis define the boundaries in age and metallicity that must be satisfied by theoretical studies aimed at explaining the formation and evolution of early-type galaxies in clusters.

We present the ASCA spectral diagnostics of the X-ray-emitting gas of five clusters of galaxies in the Shapley concentration. X-rays from two rich clusters, A3562 and A3558, nicely follow the standard L_X-k_BT relation, while the X-ray luminosity of a poor cluster, A3556, comes below the relation line. The X-ray spectrum of another poor cluster, SC 1329-313, is found to be peculiar; the gas temperature is higher than that estimated from the velocity dispersion of its member galaxies. The energy of the iron Kα line is also higher than that expected from the temperature of the plasma considered in ionization equilibrium and hence is attributable to a plasma in the recombination-dominant phase. We infer that high-temperature gas and highly ionized iron are relics of past major merger events in the formation phase of poor clusters at several times 10⁸ yr ago. The putative merger heated the gas to high temperature by the associated shock and produced highly ionized iron as well. The high-temperature gas is gradually cooled down by adiabatic expansion, but the temperature is presently still higher than that expected from the velocity dispersion of galaxies. Iron atoms in the plasma, on the other hand, remain in highly ionized states with irregularities in the temperature distribution. We propose a dynamical sequence of clusters in the Shaply concentration; SC 1329-313 is an ongoing merger, A3556 is a candidate for postmerger, SC 1327-312 is a relaxed poor cluster, while A3562 and A3558 are already developed and rich clusters even with some mergers in succession. The Shapley concentration thus can be called a kindergarten of clusters of galaxies.

We present hard X-ray observations of the nearby radio galaxy M87 and the core of the Virgo Cluster using the Rossi X-Ray Timing Explorer. These are the first hard X-ray observations of M87 not affected by contamination from the nearby Seyfert 2 galaxy NGC 4388. Thermal emission from Virgo's intracluster medium is clearly detected and has a spectrum indicative of kT ≈ 2.5 keV plasma with approximately 25% cosmic abundances. No nonthermal (power-law) emission from M87 is detected in the hard X-ray band, with fluctuations in the cosmic X-ray background being the limiting factor. Combining with ROSAT data, we infer that the X-ray spectrum of the M87 core and jet must be steep (Γ_core > 1.90 and Γ_jet > 1.75), and we discuss the implications of this result. In particular, these results are consistent with M87 being a misaligned BL Lac object.

We present results of VLA and VLBA observations of the 1.420 GHz neutral hydrogen absorption associated with the compact symmetric object 1946+708 (z = 0.101). We find significant structure in the gas on parsec scales. The peak column density in the H I [N_{H I} ≃ 2.2 × 10²³ cm^-2 (T_s/8000 K)] occurs toward the center of activity of the source, as does the highest velocity dispersion (ΔV_FWHM ≃ 350 km s^-1). In addition, we find that the continuum spectra of the various radio components associated with these jets strongly indicate free-free absorption. This effect is particularly pronounced toward the core and inner components of the receding jet, suggesting the presence of a screen local to the source, perhaps part of an obscuring torus.

The γ-ray blazar 3C 279 was monitored on a nearly daily basis with IUE, ROSAT, and EGRET for 3 weeks between 1992 December and 1993 January. During this period, the blazar was at a historical minimum at all wavelengths. Here we present the UV data obtained during this multiwavelength campaign. A maximum UV variation of ~50% is detected, while during the same period the X-ray flux varied by no more than 13%. At the lowest UV flux level, the average spectrum in the 1230-2700 Å interval is unusually flat for this object (⟨α_UV⟩ ~ 1). The flattening could represent the lowest energy tail of the inverse Compton component responsible for the X-ray emission, or it could be due to the presence of a thermal component at ~20,000 K, possibly associated with an accretion disk. The presence of an accretion disk in this blazar object, likely observable only in very low states and otherwise hidden by the beamed, variable synchrotron component, would be consistent with the scenario in which the seed photons for the inverse Compton mechanism producing the γ-rays are external to the relativistic jet. We further discuss the long-term correlation of the UV flux with the X-ray and γ-ray fluxes obtained at various epochs. All UV archival data are included in the analysis. Both the X-ray and γ-ray fluxes are generally well correlated with the UV flux, with approximately square root and quadratic dependences, respectively.

PKS 0349-27 is a classical FR II radio galaxy whose active galactic nucleus host has a spectacular, spiral-like structure in its extended emission line gas (EELG). We have measured the velocity field in this gas and find that it splits into two cloud groups separated by radial velocities that at some points approach 400 km s^-1. Measurements of the diagnostic emission line ratios [O III] 5007/Hβ, [S II] 6716+6731/Hα, and [N II] 6583/Hα in these clouds show no evidence for the type of H II region emission associated with starburst activity in either velocity system. The measured emission line ratios are similar to those found in the nuclei of narrow-line radio galaxies, but the extended ionization/excitation cannot be produced by continuum emission from the active nucleus alone. We present arguments that suggest that the velocity disturbances seen in the EELG are most likely the result of a galaxy-galaxy collision or merger, but we cannot completely rule out the possibility that the gas has been disrupted by the passage of a radio jet.

We present Hubble Space Telescope (HST) observations of seven unusual objects from the HST "snapshot survey" of BL Lacertae objects, of which four are gravitational lens candidates. In three cases a double point source is observed: 0033+595, with 158 separation, and 0502+675 and 1440+122, each with ~03 separation. The last two also show one or more galaxies, which could be either host or lensing galaxies. If any are confirmed as lenses, these BL Lac objects are excellent candidates for measuring H₀ via gravitational time delay because of their characteristic rapid, high-amplitude variability. An additional advantage is that, like other blazars, they are likely superluminal radio sources, in which case the source plane is mapped out over a period of years, providing strong additional constraints on the lensing mass distribution. The fourth gravitational lens candidate is 1517+656, which is surrounded by three arclets forming an almost perfect ring of radius 24. If this is indeed an Einstein ring, it is most likely a background source gravitationally lensed by the BL Lac object host galaxy and possibly a surrounding group or cluster. In the extreme case that all four candidates are true lenses, the derived frequency of gravitational lensing in this BL Lac sample would be an order of magnitude higher than in comparable quasar samples. We also report on three other remarkable BL Lac objects: 0138-097, which is surrounded by a large number of close companion galaxies; 0806+524, whose host galaxy contains an uncommon arclike structure; and 1959+650, which is hosted by a gas-rich elliptical galaxy with a prominent dust lane of ~5 × 10⁵M_☉.

This paper reports on the X-ray emission from BL Lacertae during its 1997 July outburst as observed with the Rossi X-Ray Timing Explorer (RXTE), compares the RXTE data with previous measurements, and interprets the overall electromagnetic emission in the context of the currently popular theoretical models. The source is bright and variable, with the 2-10 keV flux approximately 2-3.5 times higher than measured in 1995 November by ASCA. The spectrum is also harder, with power-law energy indices of ~0.4-0.6, compared with ~0.9 in 1995 November. Both in the optical band, where BL Lacertae now shows broad emission lines, and in the X-ray band, where the spectrum is hard, the overall electromagnetic distribution of BL Lacertae is similar to that observed in blazars associated with quasars rather than to that seen in the more common high-energy peaked BL Lac-type objects. We argue that the high-energy (X-ray and γ-ray) emission from BL Lacertae consists of two spectral components: X-rays are produced by Comptonization of synchrotron radiation, while the γ-rays are produced by Comptonization of the broad emission line flux.

We report on the discovery of Cepheids in the Virgo spiral galaxy NGC 4535, based on observations made with the Wide Field and Planetary Camera 2 on board the Hubble Space Telescope (HST). NGC 4535 is one of 18 galaxies observed as a part of The HST Key Project on the Extragalactic Distance Scale, which aims to measure the Hubble constant to 10% accuracy. NGC 4535 was observed over 13 epochs using the F555W filter, and over 9 epochs using the F814W filter. The HST F555W and F814W data were transformed to the Johnson V and Kron-Cousins I magnitude systems, respectively. Photometry was performed using two independent programs, DoPHOT and DAOPHOT II/ALLFRAME. Period-luminosity relations in the V and I bands were constructed using 39 high-quality Cepheids present in our set of 50 variable candidates. We obtain a distance modulus of 31.02 ± 0.26 mag, corresponding to a distance of 16.0 ± 1.9 Mpc. Our distance estimate is based on values of μ₀ = 18.50 ± 0.10 mag and E(V-I) = 0.13 mag for the distance modulus and reddening of the LMC, respectively.

The recent discovery of the unusual supernova SN 1998bw and its apparent correlation with the γ-ray burst GRB 980425 has raised new issues concerning both γ-ray bursts and supernovae. Although the spectra of SN 1998bw resemble those of Type Ic supernovae (SNe Ic), there are distinct differences. At early times the expansion velocities inferred by the Doppler shift of (unidentified) absorption features were very high, and SN 1998bw was unusually bright and red at maximum light (Galama et al.). These distinctions make SN 1998bw a candidate of a "hypernova," with explosion energies exceeding normal supernovae by a factor in excess of 10. We present an alternative picture that allows SN 1998bw to have an explosion energy and ejecta mass consistent with core-collapse supernovae, although at the bright end of the typical range. We specifically propose that all SNe Ic are significantly asymmetric and that SN 1998bw is a SN Ic that is distinguished principally by being viewed close to the symmetry axis.

We investigate the hypothesis that SNe Ic and SN 1998bw are the results of aspherical explosions along the rotational axis of basically spherical, nondegenerate C/O cores of massive stars. Light curves for aspherical explosions are computed assuming an ellipsoidal geometry for the ejecta. Guided by the polarization observations of "normal" SN Ic and related events, we assume an axis ratio of 2 near maximum light. The evolution of the isodensity contours with time is discussed. We show that the light curve of SN 1998bw may be understood with an explosion energy of 2 × 10⁵¹ ergs, a total ejecta mass of 2 M_☉, and a mass of ⁵⁶Ni of 0.2 M_☉ if it is observed at a large angle (≥60°) with respect to the plane of symmetry. In this picture, the high expansion velocities are a direct consequence of an aspherical explosion mechanism which, in turn, produces oblate isodensity contours. Prolate isodensity contours are ruled out. This interpretation suggests that the fundamental core-collapse explosion process itself is strongly asymmetric.

This paper reports the star count predictions of warped and flaring models of the outer Milky Way disk. These have been suggested as possible locations of the lenses responsible for the microlensing events toward the Large Magellanic Cloud (LMC). Three such models are investigated in detail, and the theoretical predictions are confronted with Hubble Space Telescope star count data in seven low-latitude fields (30° < |b| < 40°). If the warped and flaring disk population has the same characteristics as disk stars in the solar neighborhood, then the models can be unambiguously ruled out. Metallicity gradients are well known in disk galaxies and may cause the outer disk population to differ in colors and luminosity from the local population. This effect is studied using a simple Ansatz for the variation in the color-magnitude relation with position, and while it is shown to lead to better agreement with the star counts, upper limits on the contribution of the warped and flaring disk to the optical depth are still below the value measured toward the LMC. Only if the warp is very asymmetric or if the luminosity function changes strongly with Galactocentric radius can the models be made consistent with the star counts.

Rapid neutron-capture (i.e., r-process) nucleosynthesis calculations, employing internally consistent and physically realistic nuclear physics input (quasi-particle random-phase approximation [QRPA] β-decay properties and the recent extended Thomas-Fermi with Strutinsky integral and quenching (ETFSI-Q) nuclear mass model), have been performed. These theoretical computations assume the classical waiting-point approximation of (n,γ) ⇄ (γ,n) equilibrium. The calculations reproduce the solar isotopic r-abundances in detail, including the heaviest stable Pb and Bi isotopes. These calculations are then compared with ground-based and Hubble Space Telescope observations of neutron-capture elements in the metal-poor halo stars CS 22892-052, HD 115444, HD 122563, and HD 126238. The elemental abundances in all four metal-poor stars are consistent with the solar r-process elemental distribution for the elements Z ≥ 56. These results strongly suggest, at least for those elements, that the relative elemental r-process abundances have not changed over the history of the Galaxy. This indicates also that it is unlikely that the solar r-process abundances resulted from a random superposition of varying abundance patterns from different r-process nucleosynthesis sites. This further suggests that there is one r-process site in the Galaxy, at least for elements Z ≥ 56. Employing the observed stellar abundances of stable elements, in conjunction with the solar r-process abundances to constrain the calculations, we present predictions for the zero decay-age abundances of the radioactive elements Th and U. We compare these predictions (obtained with the mass model ETFSI-Q, which reproduces solar r-abundances best) with newly derived observational values in three very metal-poor halo stars: HD 115444, CS 22892-052, and HD 122563. Within the observational errors the ratio of [Th/Eu] is the same in both CS 22892-052 and HD 115444. Comparing with the theoretical ratio suggests an average age of these two very metal-poor stars to be 15.6 ± 4.6 Gyr, consistent with earlier radioactive age estimates and recent globular and cosmological age estimates. Our upper limit on the uranium abundance in HD 115444 also implies a similar age. Such radioactive age determinations of very low metallicity stars avoid uncertainties in Galactic chemical evolution models. They still include uncertainties due to the involved nuclear physics far from β-stability. However, we give an extensive overview of the possible variations expected and come to the conclusion that this aspect alone should not exceed limits of 3 Gyr. Therefore this method shows promise as an independent dating technique for the Galaxy.

We compare (V-I)₀-(V-K)₀ color-color and (V-I)₀-log P period-color diagrams for Baade's window and local RRab Lyrae stars. We find that for a fixed log P, the Baade's window RR Lyrae stars are ~0.17 mag redder in (V-I)₀ than the local RR Lyrae stars. We also show that there is no such effect observed in (V-K)₀. We argue that an extinction misestimate toward Baade's window is not a plausible explanation of the discrepancy. Unlike Baade's window RR Lyrae stars, the local ones follow a blackbody color-color relation and are well approximated by theoretical models. We test two parameters, iron metallicity and surface gravity, and find that their effects are too small to explain the (V-I)₀ discrepancy between the two groups of stars. We suggest that the anomalous (V-I)₀ behavior of the Baade's window RR Lyrae stars may be caused by an enhancement in α-element abundance. We note that a similar color discrepancy for clump giant stars has been recently reported by Paczyn´ski, and we caution that RR Lyrae stars and clump giants, often used as standard candles, can be subject to the same type of systematics.

Recent abundance determinations using faint optical recombination lines suggest that the heavy element abundances in many ionized nebulae might be much higher than previously deduced from the standard collisional line method. In the case of the O²⁺/H⁺ abundance ratio, the collisional line [O III] (λ4959 + λ5007) involved in the comparison is about 1000 times stronger than even the strongest O II optical recombination line λ4649. An alternative comparison is to the [O III] λ4931 line, with about the same strength as λ4649. The I(λ4931)/I(λ4959) ratio, completely independent of all physical conditions because both lines arise from the same upper level, can be determined from both theory and nebular observations. We present new observations of the λ4931/λ4959 and other line ratios by Liu et al. and discuss older published values. The agreement of observations and theory is excellent, and I(λ4931)/I(λ4959) ≈ 4.1 × 10^-4. In the Orion Nebula (Esteban et al. 1998), the λ4931.08 line is partially blended with the [Fe III] λ4930.5 line. After correction, the observed O II I(λ4649) line in two positions is, respectively, 15% lower and 40% higher than expected from [O III] λ4931, which are rather modest discrepancies. The λ4649 line in many planetary nebulae is significantly stronger than predicted from the λ4931 line, corroborating results from comparison with the much stronger λλ4959, 5007 lines. The observations of invariant line ratios presented here show that for lines close in wavelength, their intensity ratios can be measured with a precision better than 5%, even if their strengths differ by orders of magnitude.

This paper provides detailed descriptions of the X-ray emission from supernova remnants (SNRs) evolving in warm, low-density, nonthermal pressure-dominated regions (T₀ = 10⁴ K, n = 0.001 cm^-3, P_nt = 1800 or 7200 K cm^-3). Nonequilibrium ionization hydrocode simulations are used to predict the high-resolution spectra, 1/4 and 3/4 keV ROSAT PSPC count rates, spatial appearance, color temperatures, and ratios of O VII to O VIII emission-line fluxes as a function of time. If undisturbed, the remnants are quite long lived, surviving for ~1.2 × 10⁷ to ~1.6 × 10⁷ yr. During their brief energy-conserving phases, the hot, highly pressurized gas behind their shock fronts copiously emits X-rays. Thus, their 1/4 keV surface brightnesses are thousands of ROSAT PSPC counts s^-1 arcmin^-2 and the remnants appear strongly edge brightened. The onset of the radiative phase heralds the end of the extreme X-ray luminosities but not the end of the X-ray emission. After the cool shell forms behind the shock front, the hot SNR bubble slowly radiates away the remaining energy, with a diminutive fraction released in the form of X-rays. Thus, the 1/4 keV surface brightnesses are tens to hundreds of ROSAT PSPC counts s^-1 arcmin^-2 and the remnants appear "centrally filled." The hot plasma within the remnant bubbles is always out of collisional ionizational equilibrium. Early on, the ionization states are much lower than expected for such hot gas. Later on, the ionization states are higher. This paper also applies the standard observational analyses for determining the color temperature, electron density, and thermal pressure to ROSAT "observations" of one of the simulated remnants, thus providing a map between observational results and physical conditions. The paper reports the O⁺⁵, N⁺⁴, and C⁺³ column densities for the simulated remnants. The simulations may be of interest and are applied to studies of the Galactic halo and Local Bubble. They may also be of interest to studies of external galaxies and interarm regions of the Milky Way.

Spectroscopic observations at 2.4-45 μm of the young supernova remnant Cas A with the Infrared Space Observatory (ISO) Short Wavelength Spectrometer reveal strong emission lines of O, Ne, Si, S, and Ar. These lines are observed at high velocities (several 10³ km s^-1) and are therefore associated with the supernova ejecta known as the fast-moving knots (FMKs). Continuum emission from dust is also seen in the Cas A spectrum. The continuum strength is spatially well correlated with the O and Ar line strengths, which indicates that the dust emission also arises from the FMKs. The dust continuum has an emission feature at ~22 μm which cannot be fitted by typical astronomical silicates but can be fitted with a particular class of silicate minerals. This suggests that the dust in Cas A is silicate material that has freshly condensed from the Cas A ejecta into a mineral form that is uncharacteristic of typical ISM dust grains.

We discuss the excess gamma-ray emission seen from point sources in the vicinity of the supernova remnant (SNR) G312.4-0.4 with the EGRET instrument on board the Compton Gamma Ray Observatory in the energy range of 100 MeV to 10 GeV. Two gamma-ray sources have been significantly detected. The best-fit position for the source 2EG J1412-6211, which was originally discovered by COS B (catalog name 2CG 311-01), is located approximately 17' southwest of the center of the supernova remnant G312.4-0.4. We find the gamma-ray flux level of this source to be relatively constant over a period of 4 yr. The origin of the gamma-ray emission with respect to a SNR-pulsar source is discussed. The best-fit position for the other source, 2EGS J1418-6049, lies 13 northeast of the center of G312.4-0.4. It has a variable flux level, with a spectrum extending up to a few GeV. We argue that 2EGS J1418-6049 is transient in nature and that its variability makes it unlikely to be associated with G312.4-0.4 or any other SNR or isolated pulsar. We suggest that this source may belong to a new class of unidentified Galactic sources which includes the EGRET source 2EG J0241+6119 (2CG 135+01).

Methyl formate (HCOOCH₃), a threefold internal rotor, is a well-known interstellar molecule located in hot cores of giant molecular clouds. Over a decade ago, we published separate analyses of the rotational-torsional spectra of methyl formate in its A and E ground torsional substates. The analysis of the A substate was undertaken with a standard asymmetric top Hamiltonian, while that of the E substate was undertaken with a principal axis method (PAM), which was less than entirely satisfactory. In the interim, we have measured many new lines of the rotational-torsional spectrum of both the ground A and E torsional substates of this species through the rotational quantum number J = 50 using two spectrometers: our standard klystron-based system and a new fast scan spectrometer (FASSST). The newly measured lines have been combined with previous data to form a data set consisting of over 2000 lines that has been fitted to the accuracy of our experiments using a modification of the internal axis method (IAM) we have previously applied to methanol. The spectral constants obtained from the fit have allowed us to predict the frequencies of many additional lines up to 700 GHz in frequency and up to J = 50 in rotational quantum number.

Recent ground-based mid-infrared spectra of 29 late-type stars, most with substantial dust shells, are compared to ground-based spectra of these stars from the 1960s and 1970s and to IRAS-LRS spectra obtained in 1983. The spectra of about half the stars show no detectable changes, implying that their distributions of circumstellar material and associated dust grain properties have changed little over this time interval. However, many of the stars with strong silicate features showed marked changes. In nearly all cases the silicate peak has strengthened with respect to the underlying continuum, although there is one case (VY CMa) in which the silicate feature has almost completely disappeared. This suggests that, in general, an oxygen-rich star experiences long periods of gradual silicate feature strengthening, punctuated by relatively rare periods when the feature weakens. We discuss various mechanisms for producing the changes, favoring the slow evolution of the intrinsic dust properties (i.e., the chemical composition or grain structure). Although most IRAS spectra agree well with ground-based spectra, there are a number of cases in which they fall well outside the expected range of uncertainty. In almost all such cases, the slopes of the red and blue LRS spectra do not match in their region of overlap.

The recent temperature measurements of the two older isolated neutron stars PSR 1929+10 and PSR 0950+08 (ages of 3 × 10⁶ and 2 × 10⁷ yr, respectively) indicate that these objects are heated. A promising candidate heat source is friction between the neutron star crust and the superfluid it is thought to contain. We study the effects of superfluid friction on the long-term thermal and rotational evolution of a neutron star. Differential rotation velocities between the superfluid and the crust (averaged over the inner crust moment of inertia) of ~ 0.6 rad s^-1 for PSR 1929+10 and ~0.02 rad s^-1 for PSR 0950+08 would account for their observed temperatures. These differential velocities could be sustained by the pinning of superfluid vortices to the inner crust lattice with strengths of ~1 MeV per nucleus. Pinned vortices can creep outward through thermal fluctuations or quantum tunneling. For thermally activated creep, the coupling between the superfluid and crust is highly sensitive to temperature. If pinning maintains large differential rotation (~30 rad s^-1), a feedback instability could occur in stars younger than ~ 10⁵ yr causing oscillations of the temperature and spin-down rate over a period of ~ 0.3t_age. For stars older than ~ 10⁶ yr, however, vortex creep occurs through quantum tunneling and the creep velocity is too insensitive to temperature for a thermal-rotational instability to occur. These older stars could be heated through a steady process of superfluid friction.

The inner crust of a neutron star comprises lattices of neutron-rich nuclei and a neutron superfluid, in which many quantized vortex lines exist. The vortex lines are pinned by nuclei but can jump from one pinning site to another. In previous papers by Mochizuki & Izuyama and Mochizuki, Oyamatsu, & Izuyama, it was pointed out that such vortex lines may induce nuclear matter rods along the vortex cores. Such an exotic nuclear rod along a vortex line is formed by captures of nuclei from outside the vortex core and by subsequent fusion reactions of the captured nuclei with the nuclei inside the core. The nuclear rod was shown to have a lower energy than the original nuclear lattice. As the next step, dynamics for the rod formation is required. Namely, to conclude that the nuclear rods are actually formed in the neutron star, we need to confirm that the formation time of the nuclear rod is shorter than the time of the vortex sojourn at a pinning site. It is the purpose of this paper to confirm this. We find that under conditions appropriate to the Vela pulsar, the rod formation time is indeed much shorter than the vortex sojourn time in the frontier region (the region where a vortex line is tangential to the central core of the star). Rod formation is attained by pycnonuclear reactions rather than by thermonuclear ones. The resulting exotic nuclear rod formation in the frontier region can lead to a pulsar glitch, as proposed by Mochizuki & Izuyama. A brief summary of our microscopic glitch model is also presented.

We have compared stellar evolution models with the three intermediate-mass binary systems V2291 Oph, α Aur, and η And, whose masses are determined accurately within 7%. These systems were recently regarded by Schröder and coworkers as systems that favor evolution with moderate overshoot mixing from the convective cores. While they assumed a standard Population I metallicity for these binaries, we have taken into account the available heavy-element abundance data in the literature. We have taken two approaches for each system. First, assuming no core overshooting, we have adjusted helium abundance for each binary to obtain the best fit. Second, adopting a helium abundance from a standard metallicity-helium relation, we have estimated a required extent of core overshooting l_ov. Our results indicate that the required extent of core overshooting is less than ~0.15H_p, which is smaller than the extent estimated by Schröder and his collaborators. The main reason for the difference is attributed to the fact that we have taken into account published estimates of metallicity for each system. We have found that to obtain a good fit with red components we have to modify the ratio of mixing length to pressure scale height for each system. The required ratio is found to be smaller for a metal-poor system and vice versa.

We report molecular emission from the circumstellar envelopes of two carbon-rich stars with oxygen-rich envelopes, EU And and BM Gem. We find a narrow (FWHM ~ 5 km s^-1) CO (2-1) emission line from EU And and an even narrower (FWHM ~ 1 km s^-1) ¹³CO emission line from BM Gem. We also place upper limits to the emission of HCN, SiO, SO, HCO⁺, and CS from BM Gem. We argue that the narrow CO emission lines are signatures of long-lived reservoirs of orbiting gas and that standard models for CO emission from red giant winds are not appropriate for these two stars. By including the Red Rectangle and AC Her, narrow CO emission characteristic of gravitationally bound gas has been detected from four post-main-sequence systems, and we can begin to characterize these apparently similar environments. Some common characteristics are the following: (1) Their diameters are typically between ~100 and ~1000 AU. (2) The masses of CO are near 10²⁷ g. (3) Unlike the envelopes around mass-losing carbon stars where M_CO/M_dust ~ 2, the circumstellar orbiting reservoirs often appear to have M_CO < M_dust. (4) Molecules in addition to CO seem to be rare; we have yet to detect any other abundant gas-phase molecule besides CO. (5) Grains from 20 μm to 0.2 cm in radius may be common in these systems. (6) The reservoirs can possess large clumps. These properties can be understood if substantial chemical and dynamical evolution has occurred during the long lifetime of the orbiting reservoirs which are probably produced during mass loss in a binary system.

We present a method for constructing equilibrium disks with net angular momentum in general relativity. The method solves the relativistic Vlasov equation coupled to Einstein's equations for the gravitational field. We apply the method to construct disks that are relativistic versions of Newtonian Kalnajs disks. In Newtonian gravity these disks are analytic and are stable against ring formation for certain ranges of their velocity dispersion. We investigate the existence of fully general relativistic equilibrium sequences for differing values of the velocity dispersion. These models are the first rotating, relativistic disk solutions of the collisionless Boltzmann equation.

The nonlinear hydrodynamic stability of thin, compressible, Keplerian disks is studied on the large two-dimensional compressible scale, using a high-order-accuracy spectral method. We find that purely hydrodynamical perturbations can develop initially into either sheared disturbances or coherent vortices. However, the perturbations decay and do not evolve into a self-sustained turbulence. Temporarily, because of an inverse cascade of energy, which is characteristic of two-dimensional flows, energy is being transferred to the largest-scale mode before being dissipated. In the case of an inner reflecting boundary condition, it is found that the innermost disk is globally unstable to nonaxisymmetric modes which can evolve into turbulence. However, this turbulence cannot play a significant role in angular momentum transport in disks.

Observations of Hercules X-1 by the Extreme-Ultraviolet Explorer at the end of the X-ray short high state are reported here. Her X-1 is found to exhibit a strong orbital modulation of the EUV flux, with a large dip superposed on a broad peak around orbital phase 0.5 when the neutron star is closest the observer. Alternate mechanisms for producing the observed EUV light curve are modeled. We conclude that (1) the X-ray-heated surface of the companion is too cool to produce enough emission, (2) the accretion disk can produce enough emission but does not explain the orbital modulation, and (3) the reflection of X-rays off the companion can produce the shape and intensity of the observed light curve. The only viable cause for the large dip at orbital phase 0.5 is shadowing of the companion by the accretion disk.

The recent discovery of the first known accretion-powered millisecond pulsar with the Rossi X-Ray Timing Explorer provides the first direct probe of the interaction of an accretion disk with the magnetic field of a weakly magnetic (B ≲ 10¹⁰ G) neutron star. We demonstrate that the presence of coherent pulsations from a weakly magnetic neutron star over a wide range of accretion rates places strong constraints on models of the disk-magnetosphere interaction. We argue that the simple ^3/7 scaling law for the Keplerian frequency at the magnetic interaction radius, widely used to model disk accretion onto magnetic stars, is not consistent with observations of SAX J1808.4-3658 for most proposed equations of state for stable neutron stars. We show that the usually neglected effects of multipole magnetic moments, radiation drag forces, and general relativity must be considered when modeling such weakly magnetic systems. Using only very general assumptions, we obtain a robust estimate of μ ≃ (1-10) × 10²⁶ G cm³ for the dipole magnetic moment of SAX J1808.4-3658, implying a surface dipole field of ~10⁸-10⁹ G at the stellar equator. We therefore infer that after the end of its accretion phase, this source will become a normal millisecond radio pulsar. Finally, we compare the physical properties of this pulsar with those of the nonpulsing, weakly magnetic neutron stars in low-mass X-ray binaries and argue that the absence of coherent pulsations from the latter does not necessarily imply that these neutron stars have significantly different magnetic field strengths from SAX J1808.4-3658.

We report on optical and RXTE observations of a new high-latitude bursting X-ray transient, XTE J2123-058. We identified the optical counterpart and discovered a 5.9573 ± 0.0016 hr periodic optical modulation, which was subsequently shown to be the same as the spectroscopic orbital period. From the absence of orbital X-ray modulation and the presence of partial optical eclipses we conclude that the binary inclination is between 55° and 73°. From the optical magnitude in quiescence and from the X-ray flux of type I X-ray bursts, we estimate that the source distance is between 4.5 and 15 kpc, which implies that the source is unusually far from the Galactic plane, since b = -362. Optical bursts with properties consistent with being reprocessed X-ray bursts occurred.

We detected a pair of high-frequency quasi-periodic oscillations (QPOs) at 847.1 ± 5.5 Hz and 1102 ± 13 Hz simultaneously. According to the beat-frequency model, this QPO separation implies a neutron star spin period of 3.92 ± 0.22 ms. A change in the energy spectrum occurred during the decay of the outburst, which may have been due to the onset of the propeller mechanism. If so, then the neutron star magnetic field strength is between 2 and 8 × 10⁸ G for an assumed distance of 10 kpc. However, the changes in the timing and spectral properties observed during the decay are typical of atoll sources, which may indicate that the changes are due solely to the dynamics of the accretion disk. As the phase-averaged V-band magnitude declined from 17.26 at the peak of the outburst to 19.24, and the X-ray flux decreased from 9.6 × 10^-10 to 7.3 × 10^-11 ergs cm^-2 s^-1, the peak-to-peak amplitude of the V-band modulation increased from 0.75 to 1.49 mag. This behavior can be explained if the size of the accretion disk decreases during the decay of the outburst.

Relativistic plasma emission due to a beam instability in the polar cap regions is examined critically as a pulsar radio emission mechanism. Wave dispersion in the pulsar plasma is discussed, based on the use of a relativistic plasma dispersion function. The growth rate for the beam instability is estimated in the rest frame of the plasma for parallel Langmuir waves, L-O mode waves, and oblique Alfvén waves. The first two of these imply frequencies that are much higher than the observed frequencies for plausible parameters, suggesting that they are not viable as pulsar radio emission mechanisms. Growth of Alfvén waves requires that the beam speed equal the phase speed of the Alfvén waves, and this condition cannot be satisfied within the light cylinder, except for an extremely high energy beam. It is suggested that either the plasma parameters in the source region are quite different from what is currently considered plausible or the emission mechanism does not involve a beam instability. Alternative pulsar radio emission mechanisms should be explored further.

A complete 45 day supercycle of the cataclysmic variable V1159 Ori comprising a superoutburst and eight normal outbursts was observed. Coverage included ground-based optical observations as well as observations with RXTE for 38 days, ROSAT for 34 days, IUE for 27 days, and Extreme Ultraviolet Explorer (EUVE) for 10 days. The resulting light curves reveal that the optical and UV light variations are inversely correlated with the RXTE and ROSAT fluxes, with the largest change in intensity occurring in the ROSAT bandpass. There is no evidence for a strong EUV/soft X-ray component during outburst. An outflowing wind is evident from the C IV line profile during each brief outburst as well as the superoutburst. The transitions from outburst states of the disk to quiescent states take place on timescales of hours. Accretion disk models can fit the UV line and continuum energy distributions near outburst only if the disk radial temperature profile is modified from the standard case to produce a hotter distribution in the outer annuli. The high mass transfer rate, the hot disk distribution, and the similarity of outbursts and superoutbursts argue for outside-in outbursts in this system.

We have investigated the possibility of nonexplosive core contraction of massive stars in the framework of general relativity by constructing equilibrium sequences of rapidly rotating compact stars with phenomenological equations of state for the high-density region. Since there are no satisfactory equations of state for the high-temperature/high-density region, we have devised a simplified equation of state that can be considered as representing an extremely soft equation of state for the high-density region. By analyzing the stabilities of equilibrium sequences with constant total angular momentum, we have shown that there is a chance of nonexplosive core contraction in from white dwarf to neutron star density regions. It is important to note that the pressure due to a high electron fraction and/or high temperature plays an essential role in making rapidly rotating stars dynamically stable against axisymmetric collapse. In general, both a very soft equation of state and general relativity tend to make the formation of "fizzlers" more difficult. Nevertheless, since there are parameter regions that allow for fizzlers even from general relativistic computations, the possibility of their existence becomes greater than ever in spite of our oversimplified equation of state.

UV spectra of λ Velorum taken with the Goddard High Resolution Spectrograph (GHRS) on the Hubble Space Telescope are used to probe the structure of the outer atmospheric layers and wind and to estimate the mass-loss rate from this K5 Ib-II supergiant. VLA radio observations at λ = 3.6 cm are used to obtain an independent check on the wind velocity and mass-loss rate inferred from the UV observations. Parameters of the chromospheric structure are estimated from measurements of UV line widths, positions, and fluxes and from the UV continuum flux distribution. The ratios of optically thin C II] emission lines indicate a mean chromospheric electron density of log N_e ≈ 8.9 ± 0.2 cm^-3. The profiles of these lines indicate a chromospheric turbulence (v₀ ≈ 25-36 km s^-1), which greatly exceeds that seen in either the photosphere or wind. The centroids of optically thin emission lines of Fe II and of the emission wings of self-reversed Fe II lines indicate that they are formed in plasma approximately at rest with respect to the photosphere of the star. This suggests that the acceleration of the wind occurs above the chromospheric regions in which these emission line photons are created. The UV continuum detected by the GHRS clearly traces the mean flux-formation temperature as it increases with height in the chromosphere from a well-defined temperature minimum of 3200 K up to about 4600 K. Emission seen in lines of C III] and Si III] provides evidence of material at higher than chromospheric temperatures in the outer atmosphere of this noncoronal star. The photon-scattering wind produces self-reversals in the strong chromospheric emission lines, which allow us to probe the velocity field of the wind. The velocities to which these self-absorptions extend increase with intrinsic line strength, and thus height in the wind, and therefore directly map the wind acceleration. The width and shape of these self-absorptions reflect a wind turbulence of ≈9-21 km s^-1. We further characterize the wind by comparing the observations with synthetic profiles generated with the Lamers et al. Sobolev with Exact Integration (SEI) radiative transfer code, assuming simple models of the outer atmospheric structure. These comparisons indicate that the wind in 1994 can be described by a model with a wind acceleration parameter β ~ 0.9, a terminal velocity of 29-33 km s^-1, and a mass-loss rate ~ 3 × 10^-9M_☉ yr^-1. Modeling of the 3.6 cm radio flux observed in 1997 suggests a more slowly accelerating wind (higher β) and/or a higher mass-loss rate than inferred from the UV line profiles. These differences may be due to temporal variations in the wind or from limitations in one or both of the models. The discrepancy is currently under investigation.

We reexamine a time series of spectroscopic data of the Be star ζ Tauri obtained by Kaye & Gies. When the difference spectra are folded with the 0.777 day period, a distinctive pattern emerges. This pattern is not the standard "barber pole" diagram expected from nonradial pulsation but is instead a pattern of two sinusoidal variations of enhanced absorption having different amplitudes. We interpret these results as additional evidence for corotating clouds, recently suggested as an explanation of the variations observed in several other Be stars and as indirect evidence for the possible presence of magnetic fields.

Hg II abundances have been determined for 42 mercury-manganese (HgMn) stars by fitting synthetic spectra to observed spectra of the 3984 Å Hg II line. Twenty of the stars had lines sharp enough to allow their Hg isotopic abundance mixes to be estimated. The Hg abundance is reported for more HgMn stars here than in any other single work. No correlation was found between Hg II abundance and T_eff or the mean central wavelength of HgMn λ3984 stars. The mean central wavelength of λ3984 , an indicator of the Hg isotopic mix, is loosely correlated with T_eff: stars with primarily heavy Hg isotopes tend to be cooler, although one star, 46 Aql, has almost pure ²⁰⁴Hg and T_eff in about the middle of the temperature range for HgMn stars. We find that there is no evidence that any of the HgMn stars have ¹⁹⁶Hg or ¹⁹⁸Hg. For the very sharp-lined stars, the ²⁰⁴Hg abundance decreases with increasing T_eff. No correlation is seen between the mean central wavelength and the surface gravity. No correlation was found between the projected rotational velocity and the Hg II abundance or the central wavelength of λ3984, although this result may be biased by the selection of stars with low reported v sin i. Hg I λ4358 was measured at high spectral resolution for seven HgMn stars. The isotopic shifts are too small, and the hyperfine components are too weak to allow unambiguous isotopic abundance ratios to be found. Hg I abundances correlate fairly well with Hg II abundances. Some of the Hg isotopic mixtures are difficult to explain using only diffusion. HR 7245 has approximately equal abundances of ¹⁹⁹Hg, ²⁰⁰Hg, ²⁰²Hg, and ²⁰⁴Hg but very little ²⁰¹Hg, and 11 Per has Hg that is mostly ¹⁹⁹Hg and ²⁰⁴Hg. Calculations show that hyperfine splitting of ²⁰¹Hg changes the radiative forces it feels compared with other isotopes, which may alter diffusion of that isotope enough to explain its absence in HR 7245, but we have found no possible explanation for the Hg isotopic mix found in 11 Per. These are the first very high resolution measurements of Hg II λ3984 for HR 7245 and 11 Per. Although diffusion may be acting in HgMn stars, either there are one or more other mechanisms acting to help produce the overabundances and isotopic mixtures seen or our understanding of diffusion is lacking on some important point.

The coupling of radiation transport and hydrodynamics is discussed for the Eulerian frame. The discussion is aimed at developing a suitable set of equations for nonrelativistic radiation hydrodynamics (RHD) that can be numerically integrated using high-resolution methods for conservation laws. We outline how numerical methods based on a wave decomposition may be developed, along with the importance of conservation, particularly in the equilibrium regime. The properties of the RHD equations are examined through asymptotic and dispersion analyses. The conditions required to obtain the classical equilibrium limit are rigorously studied. The results show that a simple coupling term developed recently by Morel, which retains a minimum of relativistic corrections, may be sufficient for nonrelativistic flows. We also give two constraints on the relativistic corrections that result in retaining terms on the order of the truncation. In addition, the dispersion results for the P₁ approximation are studied in detail and are compared with both the exact-transport results and a full relativistic treatment. We also examine some nonintuitive behavior in the dispersion results.

Coronal heating is clearly related to the coronal magnetic field. This may be due to a passive role of the magnetic field in modifying wave propagation and dissipation or to an active role resulting from the liberation of magnetic energy by reconnection or in some other way. The purpose of this article is to examine the consequences of reconnection at the chromospheric level rather than in the corona. We note that the chromosphere is indeed a favorable site for reconnection to occur, since the resistivity is greatest in that region—specifically at the temperature-minimum location. Chromospheric reconnection can lead to coronal heating by Joule heating, by the generation and subsequent dissipation of high-frequency Alfvén and magnetacoustic waves, or by the response of the coronal magnetic field to a sudden change in connectivity. The second process could also contribute to heating of the solar wind, since high-frequency Alfvén waves can be absorbed by cyclotron damping. We note also that chromospheric reconnection could inject sufficient chromospheric gas into the corona to balance the known steady downflow of coronal gas through the transition region. It is also possible that chromospheric reconnection plays a role in the first ionization potential effect.

We perform three-dimensional simulations of the rise of twisted magnetic flux tubes in an adiabatically stratified model solar convection zone. The initial flux tube in our simulations is a uniformly twisted, buoyant, horizontal tube located near the bottom of the stratified layer. The twist of the initial flux tube is described by a parameter α, which is defined as the angular rate of field-line rotation about the tube axis per unit length of the tube. We study the nonlinear evolution of the helical kink instability of the flux tube as it rises through the stratified layer. We find from our simulations that in order for the tube to develop significant kinking during its rise, the initial twist of the tube needs to be close to or greater than the critical limit (α_c) for the onset of the kink instability. If the initial twist is significantly below the critical limit (α below about 50% of α_c), we find essentially no kink development and the evolution is similar to the results from previous two-dimensional simulations of the rise of twisted, horizontal flux tubes. On the other hand, if the initial twist is sufficiently greater than the critical limit such that the e-folding period of the fastest growing kink mode is small compared to the rise time of the tube, we find sharp bending and distortion of the tube as a result of the nonlinear evolution of the kink instability. In this case, we find that due to the effect of gravitational stratification, the kinked flux tube arches upward and evolves into a buckled loop with a local change of tube orientation at the loop apex that exceeds 90° from the original direction of the tube. The emergence of this buckled loop can give rise to a compact magnetic bipole with polarity order inverted from the Hale polarity law, similar to the configuration often seen in δ spots. Furthermore, our simulations show that the writhing of the tube axis as a result of the kink instability stretches the flux tube and increases its buoyancy. Hence, the development of the kink instability can speed up the overall rise of the flux tube.

We report here the probable detection of an emission line of Si IX that was observed from an open C130 aircraft over the Pacific Ocean during the 1998 total solar eclipse. Although the IR data themselves are inconclusive because of the uncertainty in the precise central wavelengths of the narrowband filters during the eclipse, the consistency of the measured IR limb excess with simultaneous EUV emission measured by SOHO/Coronal Diagnostic Spectrometer and the EUV Imager Telescope support our detection claim. This line appears to be the brightest IR coronal line yet observed, and its existence may significantly improve future prospects for obtaining optical coronal magnetic field measurements.

Impulsive dynamical processes in the solar corona, such as the flaring of magnetic loops, can be modeled by compressible one-dimensional magnetohydrodynamic (MHD) equations with a forcing that mimics the erratic motion of the magnetic footpoints. Two different kinds of dissipative events are observed in such a model, the study of which sustains the image of an efficient heating by a swarm of small events.

Stark width simultaneous dependence on the electron temperature and rest core charge of the emitter has been evaluated and discussed. It has been verified that the found relations, connecting Stark broadening parameters with upper level ionization potential and rest core charge of the emitters for a particular electron temperature and density, can be used for the prediction of Stark line width and shift data in the case of ions for which observed data, or more detailed calculations, are not yet available.

We present simple analytic approximations for the linear and fully evolved nonlinear mass power spectrum of matter density fluctuations for spatially flat cold dark matter (CDM) cosmological models with quintessence (Q). Quintessence is a time-evolving, spatially inhomogeneous energy component with negative pressure and an equation of state w_Q < 0. It clusters gravitationally on large length scales but remains smooth like the cosmological constant on small length scales. We show that the clustering scale is determined by the Compton wavelength of the Q-field and derive a shape parameter, Γ_Q, to characterize the linear mass power spectrum. The growth of linear perturbations as functions of redshift, w_Q, and matter density, Ω_m, is also quantified. Calibrating to N-body simulations, we construct a simple extension of Ma's 1998 formula that closely approximates the nonlinear power spectrum for a range of plausible QCDM models.

It is now well established that galaxies are biased tracers of the distribution of matter, although it is still not known what form this bias takes. In local bias models, the propensity for a galaxy to form at a point depends only on the overall density of matter at that point. Hierarchical scaling arguments allow one to build a fully specified model of the underlying distribution of matter and to explore the effects of local bias in the regime of strong clustering. Using a generating function method developed by Bernardeau & Schaeffer, we show that hierarchical models lead one directly to the conclusion that a local bias does not alter the shape of the galaxy correlation function relative to the matter correlation function on large scales. This provides an elegant extension of a result first obtained by Coles for Gaussian underlying fields and confirms the conclusions of Scherrer & Weinberg obtained using a different approach. We also argue that particularly dense regions in a hierarchical density field display a form of bias that is different from that obtained by selecting such peaks in Gaussian fields: they are themselves hierarchically distributed with scaling parameters S_p = p^(p-2). This kind of bias is also factorizable, thus in principle furnishing a simple test of this class of models.

It is currently unknown whether the universe was reionized by quasars or stars at z ≳ 5. We point out that quasars can be best distinguished from stellar systems by their X-ray emission. Based on a simple hierarchical CDM model, we predict the number counts and X-ray fluxes of quasars at high redshifts. The model is consistent with available data on the luminosity function of high-redshift quasars in the optical and soft X-ray bands. The cumulative contribution of faint, undetected quasars in our model is consistent with the unresolved fraction of the X-ray background. We find that the Chandra X-ray Observatory might detect ~10² quasars from redshifts z ≳ 5 per its 17' × 17' field of view at the flux threshold of ~2 × 10^-16 ergs s^-1 cm^-2. The redshifts of these faint point sources could be identified by follow-up infrared observations from the ground or with the Next Generation Space Telescope.

By considering the radiative force by a circumnuclear starburst as well as an active galactic nucleus (AGN), we analyze the equilibrium configuration and the stability of dusty gas in the circumnuclear regions. It is found that the radiative force by an intensive starburst can support a stable gaseous wall with a scale height of several hundred parsecs. Moreover, by taking the simple stellar evolution in the starburst into account, we find that the covering factor of the wall decreases on a timescale of several times 10⁷ yr. The large-scale wall, if formed, works to obscure the nucleus as a result of the dust opacity. Hence, it is anticipated that the index of AGN type tends to shift from higher to lower in several times 10⁷ yr as the circumnuclear starburst becomes dimmer. On the other hand, if the AGN itself is brighter than the circumnuclear starburst (e.g., the quasar case), no stable large-scale wall forms. In that case, the AGN is most probably identified as type 1. The present mechanism may provide a physical explanation for the putative correlation between AGN type and host properties whereby Seyfert type 2 galaxies are more frequently associated with circumnuclear starbursts than type 1 galaxies, whereas quasars are mostly observed as type 1 regardless of star-forming activity in the host galaxies.

Distant galaxies produce ~1, 0.1, and 0.01 gravitationally lensed X-ray sources per square degree with soft X-ray fluxes exceeding 10^-15, 10^-14, and 10^-13 ergs s^-1 cm^-2, respectively. These sources will be detected serendipitously with the Chandra X-Ray Observatory at a rate of one to three lenses per year of high-resolution imaging. The low detection rate is due to the small area over which the High Resolution Camera and AXAF CCD Imaging Spectrometer instruments have the smaller than 15 FWHM resolution necessary to find gravitational lenses produced by galaxies. Deep images of rich clusters at intermediate redshifts should yield one wide separation (Δθ ≳ 50) multiply imaged background X-ray source for every ~10, 30, and 300 clusters imaged to the same flux limits.

We present here initial results on the X-ray cluster luminosity function (XCLF) from the Bright Serendipitous High-Redshift Archival Cluster (SHARC) sample of distant X-ray clusters of galaxies. This sample is 97% complete in its optical identifications and contains 12 X-ray-luminous clusters in the redshift range 0.3 ≤ z ≤ 0.83 (median z = 0.42) and 1.1 × 10⁴⁴ ≤ L_X ≤ 8.3 × 10⁴⁴ ergs s^-1 (0.5-2.0 keV). We present a preliminary selection function for the Bright SHARC Survey based on Monte Carlo simulations. Using this selection function, we have computed the Bright SHARC Survey XCLF and find it to be fully consistent with a nonevolving XCLF to L_X ≃ 5 × 10⁴⁴ ergs s^-1 and z ≃ 0.7. At L_X > 5 × 10⁴⁴ ergs s^-1, we find evidence for a deficit of clusters compared to that expected from a nonevolving XCLF. We detect only one such cluster in the redshift range 0.3 ≤ z ≤ 0.7 when we would expect 4.9 clusters based on the local XCLF of De Grandi et al. The statistical significance of this deficit is ≃96%. To increase the statistical significance of this possible deficit, we have combined the Bright SHARC Survey and the 160 deg² survey of Vikhlinin et al. This joint survey covers ≃260 deg² and contains only one confirmed 0.3 ≤ z ≤ 0.7, L_X > 5 × 10⁴⁴ ergs s^-1 cluster, while we would expect 7.6 such clusters based on the local XCLF (De Grandi et al.). The statistical significance of the deficit in this joint survey increases to 99.5%. These results remain preliminary because of incompletenesses in the optical follow-up and uncertainties in the local XCLF.

We have detected redshifted CO (2-1) emission at 43 GHz and radio continuum emission at 1.47 and 4.86 GHz from the z = 4.4 QSO BRI 1335-0417 using the Very Large Array. The CO data imply optically thick emission from warm (>30 K) molecular gas with a total mass, M(H₂), of (1.5 ± 0.3) × 10¹¹M_☉, using the standard Galactic gas mass-to-CO luminosity conversion factor. We set an upper limit to the CO source size of 11 and a lower limit of 023(T_ex/50 K)^-1/2, where T_ex is the gas excitation temperature. We derive an upper limit to the dynamical mass of 2 × 10¹⁰sin⁻²iM_☉, where i is the disk inclination angle. Reconciling the gas mass with the dynamical mass requires either a nearly face-on disk (i < 25°) or a gas mass-to-CO luminosity conversion factor significantly lower than the Galactic value. The spectral energy distribution from the radio to the rest-frame infrared of BRI 1335-0417 is consistent with that expected from a nuclear starburst galaxy, with an implied massive star formation rate of 2300 ± 600 M_☉ yr^-1.

We present the first 86 GHz VLBI observations of the radio galaxy 3C 120, together with contemporaneous 43 and 22 GHz polarimetric VLBA observations. The very high angular resolution obtained at 86 GHz provides an upper limit to the size of the core of 54 μas (0.025 h^-1 pc). This represents a direct determination of the base of the jet that is independent of variability arguments (which depend on uncertain estimates of the Doppler factor) and places it below approximately 1 lt-month. Comparison with previous VLBA observations after a 1 yr interval shows pronounced changes in the structure and polarization of the jet. Most of the components are found to follow a curved path while undergoing a steepening of their spectra accompanied by a decrease in total and polarized emission. However, at least one component is observed to follow a quasi-ballistic motion, accompanied by a flattening of its spectrum as well as an increase in total and polarized flux. This may be explained by its interaction with the external medium, resulting in a shock that enhances the emission and aligns the magnetic field perpendicular to the component motion, thereby producing an increase of the degree of polarization from undetected values to as high as 15%. A second strong component, with the highest degree of polarization (23%), is found to have experienced a displacement from the ridge line of the structural position angle of the jet as it moved downstream. We have found a mean swing to the south of the position angle of the innermost components of ~6° between late 1996 and 1997, which may be responsible for the jet curvature observed at parsec and kiloparsec scales.

The last years have seen a revolution in ground-based γ-ray detectors. We can now detect the spectra of nearby TeV blazars like Mrk 421 and Mrk 501 out to ~20 TeV, and during the strongest flares, we can now follow fluctuations in these spectra on timescales close to the shortest ones likely in these objects. We point out that this represents a unique opportunity. Using these and future detectors in combination with broadband X-ray satellites like BeppoSAX and Rossi X-Ray Timing Explorer, we will be able to simultaneously follow all significant X-ray/γ-ray variations in a blazar's emission. This will provide the most stringent test yet of the synchrotron-Compton emission model for these objects. In preparation for the data to come, we present sample synchrotron self-Compton model calculations using a fully self-consistent, accurate code to illustrate the variability behavior one might see and to show how good timing information can probe physical conditions in the source. If the model works, i.e., if X-ray/TeV variations are consistent with being produced by a common electron distribution, then we show it is possible to robustly estimate the blazar's intrinsic TeV spectrum from its X-ray spectrum. Knowing this spectrum, we can then determine the level of absorption in the observed spectrum. Constraining this absorption, which is due to γ-ray pair production on diffuse radiation, provides an important constraint on the infrared extragalactic background intensity. Without the intrinsic spectrum, we show that detecting absorption is very difficult and argue that Mrk 421 and Mrk 501, as close as they are, may already be absorbed by a factor 2 at ~3 TeV. This should not be ignored when fitting emission models to the spectra of these objects.

We present an analysis of ground-based and Hubble Space Telescope images of three early-type barred galaxies. The first, NGC 2681, may be the clearest example yet of a galaxy with three concentric bars. The two other galaxies were previously suggested to be triple barred. Our analysis shows that while NGC 3945 is probably double barred, NGC 4371 has only one bar; but both have intriguing central structures. NGC 3945 has a large, extremely bright disk inside its primary bar, with patchy dust lanes, a faint nuclear ring or pseudoring within the disk, and an apparent secondary bar crossing the ring. NGC 4371 has a bright nuclear ring that is only marginally bluer than the surrounding bulge and bar. There is no evidence for significant dust or star formation in either of these nuclear rings. The presence of stellar nuclear rings suggests that the centers of these galaxies are dynamically cool and disklike.

We report the discovery of a new nonthermal filament, G358.85+0.47, the "Pelican," located ~225 pc in projection from Sagittarius A and oriented parallel to the Galactic plane. VLA continuum observations at 20 cm reveal that this 7' (17.5 pc) structure bends at its northern extension and is comprised of parallel strands, which are most apparent at its ends. Observations at 6 and 3.6 cm reveal that the Pelican is a synchrotron-emitting source and is strongly linearly polarized over much of its extent. The spectral index of the filament changes from α_20/6 = -0.8 to α_6/3.6 = -1.5. The rotation measures exhibit a smooth gradient, with values ranging from -1000 to 500 rad m^-2. The intrinsic magnetic field is well aligned along the length of the filament. Based on these properties, we classify the Pelican as one of the nonthermal filaments unique to the Galactic center. Since these filaments (most of which are oriented perpendicular to the Galactic plane) are believed to trace the overall magnetic field in the inner Galaxy, the Pelican is the first detection of a component of this field parallel to the plane. The Pelican may thus mark a transition region of the magnetic field orientation in the inner 1 kpc of the Galaxy.

In X-ray binaries, several percent of the compact object luminosity is intercepted by the surface of the normal companion and reradiated through Compton reflection and the K fluorescence. This reflected emission follows the variability of the compact object with a delay approximately equal to the orbital radius divided by the speed of light. This provides the possibility of measuring the orbital radius and thus substantially refining the compact object mass determination compared to using optical data alone. We demonstrate that it may be feasible to measure the time delay between the direct and reflected emission using cross-correlation of the light curves observed near the Kα line and above the K edge of neutral iron. In the case of Cygnus X-1, the time delay measurement is feasible with a 3 × 10⁵-10⁶ s observation by a telescope with a 1000 cm² effective area near 6.4 keV and with a ~5 eV energy resolution. With longer exposures, it may be possible to obtain mass constraints even if an X-ray source in the binary system lacks an optical counterpart.

We have discovered a third harmonic cyclotron resonance scattering feature (CRSF) in observations of the recent outburst of 4U 0115+63 with the Rossi X-Ray Timing Explorer (RXTE). The spectrum in a narrow pulse phase range shows CRSFs at 12.40, 21.45, and 33.56 keV. With centroid energy ratios to the fundamental of 1.73 ± 0.08 and 2.71 ± 0.13, the CRSFs are not harmonically spaced. Strong variability of the continuum and CRSFs with pulse phase indicate a complex emission geometry near the neutron star polar cap. In addition, one RXTE observation, which spanned periastron passage, revealed a strong 2 mHz quasi-periodic oscillation (QPO). This is slower by 2 orders of magnitude than the beat-frequency QPO expected in this system and slower by a factor of more than 5 compared with other QPOs seen in accreting X-ray pulsars.

We investigate the influences of the outer boundary conditions on the structure of an optically thin accretion flow. We find that the outer boundary condition plays an important role in determining the topological structure and the profiles of the density and temperature of the solutions. Therefore, it should be regarded as a new parameter in the accretion model.

Hydrogen-rich matter has been added to a carbon-oxygen white dwarf of initial mass 0.516 M_☉ at the rates 10^-8 and 2 × 10^-8M_☉ yr^-1, and results are compared with those for a white dwarf of the same initial mass that accretes pure helium at the same rates. For the chosen accretion rates, hydrogen burns in a series of recurrent mild flashes and the ashes of hydrogen burning build up a helium layer at the base of which a helium flash eventually occurs. In previous studies involving accretion at higher rates and including initially more massive white dwarfs, the diffusion of energy inward from the hydrogen shell-flashing region contributes to the increase in the temperature at the base of the helium layer, and the mass of the helium layer when the helium flash begins is significantly smaller than in a comparison model accreting pure helium; the helium shell flash is strong enough to cause the model to expand beyond its Roche lobe, but not strong enough to develop into a supernova explosion. In contrast, for the conditions adopted here, the temperature at the base of the helium layer becomes gradually independent of the deposition of energy by hydrogen shell flashes, and the mass of the helium layer when the helium flash occurs is a function only of the accretion rate, independent of the hydrogen content of the accreted matter. Several thousand hydrogen shell flashes must be followed before the helium flash takes place. Because of the high degeneracy at the base of the helium layer, temperatures in the flashing zone will rise without a corresponding increase in pressure, nuclear burning will continue until nuclear statistical equilibrium is achieved, and structural evolution will proceed hydrodynamically; the model will become a supernova, but not of the classical Type Ia variety.

We present the first detection of the low-lying pure rotational emission lines of H₂ from circumstellar disks around T Tauri stars, using the Short Wavelength Spectrometer on the Infrared Space Observatory. These lines provide a direct measure of the total amount of warm molecular gas in disks. The J = 2 → 0 S(0) line at 28.218 μm and the J = 3 → 1 S(1) line at 17.035 μm have been observed toward the double binary system GG Tau. Together with limits on the J = 5 → 3 S(3) and J = 7 → 5 S(5) lines, the data suggest the presence of gas at T_kin ≈ 110 ± 10 K with a mass of (3.6 ± 2.0) × 10^-3M_☉ (±3 σ). This amounts to ~3% of the total gas + dust mass of the circumbinary disk as imaged by millimeter interferometry, but it is larger than the estimated mass of the circumstellar disk(s). Possible origins for the warm gas seen in H₂ are discussed in terms of photon and wind-shock heating mechanisms of the circumbinary material, and comparisons with model calculations are made.

We report the detection of the 1₁₀-1₁₁ ground-state transition of ortho-H₂D⁺ at 372.421 GHz in emission from the young stellar object NGC 1333 IRAS 4A. Detailed excitation models with a power-law temperature and density structure yield a beam-averaged H₂D⁺ abundance of 3 × 10^-12 with an uncertainty of a factor of 2. The line was not detected toward W33A, GL 2591, and NGC 2264 IRS (in the latter source at a level that is 3-8 times lower than previous observations). The H₂D⁺ data provide direct evidence in support of low-temperature chemical models in which H₂D⁺ is enhanced by the reaction of H and HD. The H₂D⁺ enhancement toward NGC 1333 IRAS 4A is also reflected in the high DCO⁺/HCO⁺ abundance ratio. Simultaneous observations of the N₂H⁺ 4-3 line show that its abundance is about 50-100 times lower in NGC 1333 IRAS 4A than in the other sources, suggesting significant depletion of N₂. The N₂H⁺ data provide independent lower limits on the H abundance that are consistent with the abundances derived from H₂D⁺. The corresponding limits on the H column density agree with recent near-infrared absorption measurements of H toward W33A and GL 2591.

We report the detection of the 62 μm feature of crystalline water ice in emission toward the bow-shaped Herbig-Haro object HH 7. Significant amounts of far-infrared continuum emission are also detected between 10 and 200 μm, so that Herbig-Haro objects cease to be pure emission-line objects at far-infrared wavelengths. The formation of crystalline water ice mantles requires grain temperatures T_gr ≳ 100 K at the time of mantle formation, suggesting that we are seeing material processed by the HH 7 shock front. The deduced ice mass is ~2 × 10^-5M_☉, corresponding to a water column density N(H₂O) ~ 10¹⁸ cm^-2; an estimate of the [H₂O]/[H] abundance yields values close to the interstellar gas-phase oxygen abundance. The relatively high dust temperature and the copious amounts of gas-phase water needed to produce the observed quantity of crystalline water ice suggest a scenario in which both dissociative and nondissociative shocks coexist. The timescale for ice mantle formation is of the order of ~400 yr, so that the importance of gas-phase water cooling as a shock diagnostic may be greatly diminished.

Using extreme-ultraviolet (EUV) spectroheliograms from the first intentional postflare observations with the Coronal Diagnostic Spectrometer (CDS) on board SOHO, we determine relative line-of-sight velocities and their temporal evolution during the gradual flare phase of an M6.8 two-ribbon flare that occurred on 1998 April 29. Dopplergrams in lines of O V, Fe XVI, and Fe XIX, with formation temperatures T_max of, respectively, 0.25, 2.0, and 8.0 MK show strong velocity gradients coincident with the Hα ribbons, visible in Big Bear Solar Observatory (BBSO) images. These gradients are perpendicular to and moving with the Hα ribbons. Bright downflowing plasma seems to be prevalent in the regions, between the ribbons and the magnetic neutral line, that coincide with the ends of postflare loops seen with the Extreme-Ultraviolet Imaging Telescope (EIT) on board SOHO. The plasma on the outer side of the ribbons is less bright in the EUV but shows strong relative blueshifts. This pattern of upflows and downflows demonstrates, for the first time in transition region and coronal lines, the existence of chromospheric evaporation during the late gradual phase of a flare and provides evidence for ongoing reconnection.