Table of contents

The angular power spectra of the infrared maps obtained by the Diffuse Infrared Background Experiment (DIRBE) instrument on the COBE satellite have been obtained by two methods: (1) the Hauser-Peebles method previously applied to the Differential Microwave Radiometer experiment maps, and (2) by Fourier transforming portions of the all-sky maps projected onto a plane. The two methods give consistent results, and the power spectrum of the high-latitude dust emission is C_ℓ ∝ ℓ^-3 in the range 2 < ℓ < 300.

It is of fundamental importance to determine if and how hierarchical clustering is involved in large-scale structure formation of the universe. Hierarchical evolution is characterized by rules that specify how dark-matter halos are formed by the merging of halos at smaller scales. We show that scale-scale correlations of the matter density field are direct and sensitive measures to quantify this merging tree. Such correlations are most conveniently determined from discrete wavelet transforms. Analyzing two samples of Lyα forests of QSOs absorption spectra, we find significant scale-scale correlations whose scale dependence is typical for branching processes. Therefore, models that predict a "history-independent" evolution are ruled out, and the halos hosting the Lyα clouds must have gone through a "history-dependent" merging process during their formation.

Using the excellent observed correlations among various infrared wave bands with 12 and 60 μm luminosities, we calculate the 2-300 μm spectra of galaxies as a function of luminosity. We then use 12 μm and 60 μm galaxy luminosity functions derived from IRAS data, together with recent data on the redshift evolution of galaxy emissivity, to derive a new, empirically based infrared background spectrum from stellar and dust emission in galaxies. Our best estimate for the infrared background is of order 2-3 nW m^-2 sr^-1 with a peak around 200 μm reaching ~6-8 nW m^-2 sr^-1. Our empirically derived background spectrum is fairly flat in the mid-infrared, as opposed to spectra based on modeling with discrete temperatures that exhibit a pronounced "valley" in the mid-infrared. We also derive a conservative lower limit to the infrared background that is more than a factor of 2 lower than our derived flux.

We have used the infrared Barnes-Evans surface brightness technique to derive the radii and distances of 34 Galactic Cepheid variables. Radius and distance results obtained from both versions of the technique are in excellent agreement. The radii of 28 variables are used to determine the period-radius (PR) relation. This relation is found to have a smaller dispersion than in previous studies, and is identical to the PR relation found by Laney & Stobie from a completely independent method, a fact which provides persuasive evidence that the Cepheid PR relation is now determined at a very high confidence level. We use the accurate infrared distances to determine period-luminosity (PL) relations in the V, I, J, H, and K passbands from the Galactic sample of Cepheids. We derive improved slopes of these relations from updated LMC Cepheid samples and adopt these slopes to obtain accurate absolute calibrations of the PL relation. By comparing these relations to the ones defined by the LMC Cepheids, we derive strikingly consistent and precise values for the LMC distance modulus in each of the passbands that yield a mean value of μ₀(LMC) = 18.46 ± 0.02.

By analyzing the observed dispersions of the PL relations defined by the LMC and Galactic samples of Cepheids, we disentangle the contributions due to uncertainties in the reddenings, in distance measurement, and due to metallicity effects, and we estimate the intrinsic dispersion of the PL relation with the Wesenheit function. Assuming that the Galactic Cepheid distances are typically accurate to ±3% (as shown in a previous paper), and assuming an intrinsic spread in [Fe/H] of ~0.4 dex among the Cepheids of our sample as obtained by Fry & Carney, the observed dispersion of the Galactic Cepheid PL relation suggests a metallicity dependence of Δμ/Δ[Fe/H] ≈ 0.2, about half the value suggested by Sasselov et al. from EROS data. Since this estimate of the metallicity dependence of the PL (V) relation is rather uncertain, however, we prefer to retain μ₀(LMC) = 18.46 as our best value, but with an increased uncertainty of ±0.06, most of which is due to the uncertainty in the appropriate metallicity correction.

Our results show that the infrared Barnes-Evans technique is very insensitive to both Cepheid metallicity and adopted reddening, and therefore is a very powerful tool to derive accurate distances to nearby galaxies by a direct application of the technique to their Cepheid variables, rather than by comparing PL relations of different galaxies, which introduces much more sensitivity to metallicity and absorption corrections that are usually difficult to determine.

In this paper, we report on the morphologically segregated I- and K-band number counts at bright magnitudes from a wide-field deep survey. This survey has been carried out with the first large mosaic CCD camera (8192 × 8192) on the University of Hawaii 88 inch (2.24 m) telescope and the Canada-France-Hawaii Telescope (CFHT). This instrument allows us to take deep wide-field galaxy images efficiently. We have been able to obtain I-band number counts in a wide range from I = 12.5 to I = 22.5, and to classify the morphologies of galaxies down to I = 18.5. Morphologically segregated number counts for ellipticals, spirals, and irregulars are obtained for the first time with magnitudes brighter than I = 18.5. The morphological classifications correlate well with the galaxy colors. By modeling these number counts, we confirm that a no-evolution model can fit both K- and I-band number counts of ellipticals and spirals reasonably well, but the irregular/peculiar galaxies show a substantial excess at faint magnitudes over the no-evolution model. With the number counts at bright magnitudes, we further demonstrate that the rise in this excess begins by I = 20. This rise is consistent with a rapidly increasing star formation rate in low-luminosity galaxies at about z = 0.35.

We use multifiber spectroscopy of 12 poor groups of galaxies to address (1) whether the groups are bound systems or chance projections of galaxies along the line of sight; (2) why the members of each group have not already merged to form a single galaxy, despite the groups' high galaxy densities, short crossing times, and likely environments for galaxy-galaxy mergers; and (3) how galaxies might evolve in these groups, where the collisional effects of the intragroup gas and the tidal influences of the global potential are weaker than in rich clusters. Each of the 12 groups has fewer than about five cataloged members in the literature. Our sample consists of 1002 galaxy velocities, 280 of which are group members. The groups have mean recessional velocities between 1600 and 7600 km s^-1. Nine groups, including three Hickson compact groups, have the extended X-ray emission characteristic of an intragroup medium (see Paper II).

We conclude the following:

1. TheninepoorgroupswithdiffuseX-rayemissionareboundsystemswithatleast ~20-50 groupmemberswithabsolutemagnitudesasfaintasM_B ~ -14 + 5 log₁₀hto -16 + 5 log₁₀h. The large number of group members, the significant early-type population (up to ~55% of the membership) and its concentration in the group center, and the correspondence of the central, giant elliptical with the optical and X-ray group centroids argue that the X-ray groups are not radial superpositions of unbound galaxies. The velocity dispersions of the X-ray groups range from 190 to 460 km s^-1. We are unable to determine if the three non-X-ray groups, which have lower velocity dispersions (<130 km s^-1) and early-type fractions (=0%), are also bound.

2. GalaxiesineachX-ray-detectedgrouphavenotallmergedtogetherbecauseasignificantfractionofthegroupmassliesoutsideofthegalaxiesandinacommonhalo. The velocity dispersion of the combined group sample is constant as a function of radius out to the virial radius of the system (typically ~0.5 h^-1 Mpc). The virial mass of each group (~0.5-1 × 10¹⁴h^-1M_☉) is large compared with the mass in the X-ray gas and in the galaxies (e.g., ~1 × 10¹²h^-5/2M_☉ and ~1 × 10¹³h^-1M_☉, respectively, in NGC 533). These results imply that most of the group mass is in a common, extended halo. The small fraction (~10%-20%) of group mass associated with individual galaxies suggests that the rate of galaxy-galaxy interactions is lower than for a galaxy-dominated system, allowing these groups to virialize before all of their galaxies merge and to survive for more than a few crossing times.

3. Thepositionofthegiant, brightestellipticalineachX-raygroupisindistinguishablefromthecenterofthegrouppotential,asdefinedbythemeanvelocityandtheprojectedspatialcentroidofthegroupgalaxies. This result suggests that dominant cluster ellipticals, such as cD galaxies, may form via the merging of galaxies in the centers of poor group-like environments. Groups with a central, dominant elliptical may then fall into richer clusters. This scenario explains why cD galaxies do not always lie in the spatial and kinematic center of rich clusters but instead occupy the centers of subclusters in nonvirialized clusters.

4. Thefractionofearly-typegalaxiesinthepoorgroupsvariessignificantly,rangingfromthatcharacteristicofthefield (≲25%) tothatofrichclusters (~55%). The high early-type fractions are particularly surprising because all of the groups in this sample have substantially lower velocity dispersions (by a factor of ~2-5) and galaxy number densities (by a factor of ~5-20) than are typical of rich clusters. Hence, the effects of disruptive mechanisms like galaxy harassment on the morphology of poor group galaxies are weaker than in cluster environments. In contrast, the kinematics of poor groups make them preferred sites for galaxy-galaxy mergers, which may alter the morphologies and star formation histories of some group members. If galaxy-galaxy interactions are not responsible for the high early-type fractions, it is possible that the effects of environment are relatively unimportant at the current epoch and that the similarity of the galaxy populations of rich clusters and some poor groups reflects conditions at the time of galaxy formation.

5. Thefractionofearly-typegroupmembersthathaveexperiencedstarformationwithinthelast ~2 h^-1 Gyr isconsistentwiththatinrichclusterswithsignificantsubstructure (~15%). If some of the subclusters in these rich, complex clusters are groups that have recently fallen into the cluster environment, the similarity between the star formation histories of the early types in the subclusters and of those in our sample of field groups indicates that the cluster environment and associated mechanisms like ram pressure stripping are not required to enhance and/or quench star formation in these particular galaxies. If the recent star formation is tied to the external environment of the galaxies and not to internal instabilities, it is more likely that galaxy-galaxy encounters have altered the star formation histories of some early-type galaxies in groups and in subclusters.

We use ROSAT PSPC data to study the X-ray properties of a sample of 12 poor groups that have extensive membership information. Diffuse X-ray emission is detected in nine of these groups. In all but one of the X-ray-detected groups, the X-ray emission is centered on a luminous elliptical galaxy. Fits to the surface brightness profiles of the X-ray emission suggest the presence of two X-ray components in these groups. The first component is centered on the central elliptical galaxy and is extended on scales of 20-40 h^-1 kpc. The location and extent of this component, combined with its X-ray temperature (~0.7- 0.9 keV) and luminosity (~10^41-42h^-2 ergs s^-1), favor an origin in the interstellar medium of the central galaxy. Alternatively, the central component may be the result of a large-scale cooling flow.

The second X-ray component is detected out to a radius of at least ~100-300 h^-1 kpc. This component follows the same relationships found among the X-ray temperature (T), X-ray luminosity (L_X), and optical velocity dispersion (σ_r) of rich clusters. This result suggests that the X-ray-detected groups are low-mass versions of clusters and that the extended gas component can properly be called the intragroup medium, by analogy to the intracluster medium in clusters. The failure to detect an intragroup medium in the three groups with very low velocity dispersions is consistent with their predicted X-ray luminosities and temperatures based on the relationships derived for clusters and X-ray-detected groups. The best-fit value of β derived from the σ_r-T relationship for groups and clusters is ~0.99 ± 0.08, implying that the galaxies and the hot gas trace the same potential with equal energy per unit mass and that the groups are dynamically relaxed.

We also find a trend for the position angle of the optical light in the central elliptical galaxy to align with the position angle of the large-scale X-ray emission. This trend is consistent with that found for some rich clusters containing cD galaxies. The alignment of the central galaxy with the extended X-ray emission suggests that the formation and/or evolution of the central galaxy is linked to the shape of the global group potential. One possible scenario is that the central galaxy formed via galaxy-galaxy mergers early in the lifetime of the group and has not been subject to significant dynamical evolution recently.

An analysis of the environment of a sample of 33 CfA Seyfert galaxies and a control sample of 45 nonactive galaxies matched in luminosity, redshift, and morphology to the Seyfert galaxies as reported in Paper I is presented. The covariance function amplitudes of the Seyfert and control samples are not statistically significantly different from one another and from the general field. Moreover, the companion frequency of the Seyfert galaxies, the probability of finding a companion galaxy brighter than -17.5 in R within 50 kpc (0.30 ± 0.11), is not statistically significantly different from that for the nonactive control sample (0.23 ± 0.09).

The mean environment of Seyfert 1 galaxies is found to be different from that of Seyfert 2 galaxies at greater than the 95% confidence level, in the sense that the latter have a larger covariance amplitude. Such evidence is problematic for the Unified Model, which attributes spectroscopic differences between the classes to purely geometric effects on the order of parsec scales. It cannot, however, account for differences on the order of 100 kpc scales.

It is argued that triggering of activity in galactic nuclei may involve a variety of mechanisms and may depend on the luminosity of the class. That is, while there is excellent evidence that QSOs, radio galaxies, and BL Lac objects inhabit environments significantly richer than the field, the same does not seem to be true for Seyfert galaxies and perhaps for LINERs.

Finally, because a significant fraction of Seyfert host galaxies show little or no evidence for a recent merger, it is suggested that "minor mergers," mergers that involve a gas-rich disk galaxy and a bound companion or satellite galaxy, may play a significant role in triggering activity in Seyfert galaxies.

We have isolated a sample of 14 candidate variable objects with extended image structure to B_J = 22.5 in 0.284 deg² of Selected Area 57. The majority of candidates are blue (U-B < 0) and relatively compact. At fainter magnitudes, there is a steep rise in the number of variable objects. These objects are also compact and blue, and some of them are likely to be truly stellar. Twelve of the B_J ≤ 22.5 candidates have been observed spectroscopically over limited ranges of wavelength and a variety of resulting signal-to-noise ratios. Three of the four brightest have broad emission lines characteristic of Seyfert 1 galaxies. The fourth has a highly variable spectrum consistent with Seyfert-like activity. A fifth candidate has emission-line ratios nominally consistent with a Seyfert 2 galaxy. In most cases where we have not been able to confirm a Seyfert spectroscopic type, the spectra are of insufficient quality or coverage to rule out such a classification. Based on spectroscopic and photometric redshift information, the majority of candidates have luminosities less than 10% of the nominal demarcation between QSOs and active galactic nuclei (M_B = -23, H₀ = 50 km s^-1 Mpc^-1, q₀ = 0.5). The surface density of confirmedM_B > -23 active galactic nuclei (AGNs) to B_J = 22, including stellar sources, is ~40 deg^-2, in good agreement with other surveys at this depth. The confirmed AGNs in extended sources make up 36% of this population. Thus, the application of a variability criterion to images with extended structure enhances the completeness of the census of active nuclei. If the majority of our candidates are bona fide AGNs, the surface density could be as high as 82 deg^-2 for M_B > -23 and 162 deg^-2 for all luminosities to B_J = 22, with extended sources contributing up to 33% of the total.

For an extended sample of Seyfert galaxies we compile from the literature the infrared fluxes in the four IRAS bands, the ground-based small-beam (~5''-10'') fluxes in the standard Q, N, M, L (or L') bands, and the nuclear (nonstellar) estimated fluxes in the JHK bands. We estimate nuclear fluxes in the L band by applying a correction for stellar light.

From the statistical study of the infrared colors and luminosities, we derive the typical spectral energy distributions (SEDs) of Seyfert 1 and 2 nuclei and the typical differences in luminosities between the two types of objects in the mid- and near-infrared spectral ranges. The analyses of colors and luminosities agree with the fact that in general Seyfert 2 nuclei become increasingly fainter with respect to Seyfert 1 nuclei as we go toward shorter infrared wavelengths. This behavior is consistent with growing anisotropy of a dusty torus emission toward shorter wavelengths, but the degree of anisotropy is low (the radiation appears to be substantially isotropic at λ ≳ 25 μm). For Seyfert 2 galaxies having Compton-thin obscuring structures at hard X-ray energies, we find correlations between the absorbing hydrogen columns and some infrared colors and luminosities.

The observational data appear to severely challenge many models of dusty tori, which hardly account for the shapes of the SEDs and the degree of anisotropy observed in Seyfert galaxies. In particular, at variance with some earlier claims, very thick and compact tori are basically inconsistent with these observational constraints. The most successful models, though having problems in accounting for several details, can fit the major infrared observational data of both Seyfert 1 and Seyfert 2 nuclei with tori that extend up to several hundred parsecs and have fairly low optical thickness.

We have obtained HubbleSpaceTelescope Wide Field and Planetary Camera 2 ultraviolet (UV) 2200 Å and optical V-band images of 20 low-luminosity active galactic nuclei, most of which are spectroscopically classified as LINERs, in order to search for a possible photoionizing continuum. Six (30%) of the galaxies are detected in the UV. Two of the detected galaxies (NGC 3642 and NGC 4203) have compact, unresolved nuclear UV sources, while the remaining four UV sources (in NGC 4569, NGC 5005, NGC 6500, and NGC 7743) are spatially extended. Combining our data set with the earlier sample of Maoz et al., we find that the probability of detection of a nuclear UV source is greatest for galaxies having low internal reddening and low inclination, and we conclude that dust obscuration is the dominant factor determining whether or not a UV source is detected. Large emission-line equivalent widths and the presence of broad-line emission also increase the likelihood of detection of nuclear UV emission. Our results suggest that the majority of LINERs harbor obscured nuclear UV sources, which may be either accretion-powered active nuclei or young star clusters. Under the assumption that the compact UV sources in NGC 3642 and NGC 4203 have nonstellar spectra of slope f_ν ∝ ν^-1 extending into the extreme ultraviolet, the extrapolated ionizing fluxes are sufficiently strong to photoionize the narrow-line regions of these objects. The V-band images of many galaxies in our sample reveal remarkably strong dust lanes that may be responsible for obscuring some UV sources.

We have compared the metallicity (represented by oxygen abundance), X_o, and the dust-to-gas ratio, , in a sample of dwarf galaxies. For dwarf irregulars (dIrrs) we find a good correlation between the two quantities, with a power-law index of 0.52 ± 0.25. Blue compact dwarf (BCD) galaxies do not show such a good correlation; in addition, both the dust-to-gas ratio and the metallicity tend to be higher than for dIrrs. We have then developed a simple but physical analytical model for the above relation. Comparing the model results with the data, we conclude that (i) for low values of , the -X_o relation is quasi-linear, whereas for higher values, the curve strongly deviates from the linear behavior, implying that the commonly used power-law approximation is very poor; (ii) the deviation from the linear behavior depends critically on the parameter χ, the "differential" mass outflow rate from the galaxy in units of the star formation rate, ψ; (iii) the shape of the -X_o curve does not depend on ψ, but only on χ; however, the specific location of a given galaxy on the curve does depend on ψ; and (iv) the BCD metallicity segregation is the result of a higher ψ, together with a significant differential mass outflow. Thus, the lack of correlation can be produced by largely different star formation rates and values of χ in these objects.

Checking stellar yields with galactic chemical evolution models is an important test of stellar nucleosynthesis simulations. This is generally done by applying a very simple galactic model with a large number of free parameters. It is, however, at least questionable whether such a model can serve as a dependable test of stellar nucleosynthesis. A self-consistent galactic model, on the other hand, can be used to determine yields that are consistent with the galactic chemical evolution. The chemodynamical model introduced in this paper includes a detailed description of the stellar metal-dependent enrichment and serves to determine the chemical yields of massive stars. It shows that the metal dependency of stellar nucleosynthesis, gas flows in the galaxy, mixing processes in the interstellar medium, and the energy release of supernovae (SNe) have a great influence on the distribution of chemical elements. From the chemodynamical model and the observed evolution of [α/Fe], I find that on average, one out of 195 intermediate-mass stars explodes as a SN of Type Ia. This yields a ratio of Type Ia SN to Type II + Ib SN of 1:8.5. Furthermore, the model predicts present SN rates in our Galaxy for Types Ia and II + Ib of 1/107 yr and 1/41 yr, respectively. I present Galactic yields of 23 different chemical elements from C to Ni. One important result is that the yields of Cr, Fe, and Ni are a factor of 2 too high, and odd-z element yields are systematically underestimated in the nucleosynthesis models of massive stars. A comparison of the stellar with the Galactic yields suggests that the lower mass limit for stars to become Type II SNe may be 11 M_☉, and that the core collapse of stars more massive than 30-40 M_☉ produces a black hole. The model predicts 1.2-1.5 × 10⁹ stellar remnants of massive stars (neutron stars and black holes) in our Galaxy.

A model of a precessing jet obtained from radio data was used to explain the optical light curve of the OVV quasar 3C 279. The geometrical parameters of the model and the bulk motion of the jet were obtained from the position angles, velocities, and epoch of formation of the superluminal features in the radio jet. As a consequence, the model provides the Doppler factor time dependence. This factor reached its maximum value at the epoch in which the very strong and fast optical flare was observed. Therefore, the large increases in the emission at optical wavelengths could be the result of beaming, as the jet approaches its minimum separation to the line of sight. Fast variability observed at these epochs would be naturally explained by the shortening of the timescale in the observer's reference frame due to relativistic effects. The precessing jet model could also explain the variability in the γ-ray emission assuming that the high-energy radiation is produced by the inverse Compton process on external seed photons. The variability would occur when the precessing jet sweeps the broadline region clouds, which would provide the seed photons.

The intervening Lyα forest and metal-line absorbers toward the "Cloverleaf" gravitational lens H1413+1143 (z_em = 2.54, m_v ≈ 17) are examined along the four individual sight lines using HST-FOS spectra. The observations probe the Lyα forest systems on size scales of up to 4 h^-1 kpc (H₀ = 100 h km s^-1 Mpc^-1, q₀ = 0.5) and metal-line systems on scales up to 6 h^-1 kpc. The equivalent widths of Lyα forest lines in general are well correlated along the four different lines of sight and are consistent with the sight lines piercing the same absorbers. The heavy-element systems clearly show evidence for differences. One striking difference is seen in a system at z_abs = 2.0969 in which the Lyα line possesses a column density approaching log N(H I) = 19.7 in component B but not exceeding log N(H I) = 18.4 in any of the other three components. Simulations show that a circular absorber responsible for such a scenario has a radius of ≤10 h^-1 kpc with a 90% confidence level, assuming a lens redshift of z_lens = 1.55. The z_abs = 1.6595 system contains a probable damped Lyα line and shows evidence for rotation. A system at z_abs = 1.4377 exhibits very significant Fe II absorption in only two of the components at the level of sensitivity of the observations. Examination of how the absorption-line equivalent widths are correlated between the component spectra can provide information about whether all four lines of sight to the QSO are passing through the same absorbers, placing limits on absorber sizes and shapes. The four sight lines toward H1413+1143 provide a unique geometry for this type of study.

We have imaged the classical Seyfert galaxy NGC 4151 with the VLBA at wavelengths of 6 and 18 cm, achieving resolutions of ≃2 and 5 mas (≃0.16 and 0.40 pc), respectively. At 18 cm, four radio components, spread over ≃05 (≃40 pc) are detected. The easternmost pair comprises a bright component elongated in position angle ≃20° (component E) and an extension to the northeast (component F) which curves into position angle ≃75°, similar to that of the 35 (280 pc) scale radio jet and the narrow-line region. At the higher resolution achieved at 6 cm, component E is found to be a linear radio source with a length of ~13 mas (1.0 pc). This source has a length/width ratio of ≳4, and therefore fulfills one of the classical criteria for a radio jet, but its radio luminosity is only ~10³⁸ ergs s^-1, several orders of magnitude less than the parsec-scale jets in radio galaxies. It is also misaligned by ≃55° from the arcsecond-scale radio jet. Possible reasons for the misalignment include deflection by the inner narrow-line region, buoyancy forces, and a change in the plane of the accretion disk presumably responsible for jet collimation. We also discuss an alternative interpretation, in which component E is a disk or torus viewed edge-on and emitting flat-spectrum, possibly thermal, radio emission.

Component E appears to be the only part of the radio emission that contains a significant flat-spectrum component, and we favor identification of the central, brightest, unresolved subcomponent of E as the ultraviolet (UV) and optical nucleus. In order to reconcile the much smaller column density of H I toward the nucleus found by Lyman absorption than by 21 cm absorption measurements, we argue that a ≃0.01 pc thick gas disk surrounds the nucleus and is ionized out to a radius of ≃2 pc. The large 21 cm absorption column observed then results from off-nuclear radio components shining through the outer, neutral part of this disk. The flat spectrum of the nuclear radio source may indicate synchrotron self-absorption or free-free absorption by the inner, ionized part of the accretion disk. Interestingly, the apparent nuclear source and a radio subcomponent ≃0.2 pc to the southeast align precisely perpendicular to the arcsec-scale radio jet, suggesting that they may outline the large-scale structure of the accretion disk responsible for jet collimation.

Comparison of the 18 cm image with the European VLBI network image acquired by Harrison et al. in 1984 provides upper limits of 0.14c and 0.25c for the apparent speeds of the radio components at distances of 7 and 36 pc, respectively, from the galaxy nucleus.

We report the discovery of a new optical synchrotron jet in the radio galaxy 3C 15. This brings to nine the number of known optical synchrotron jets in nearby radio galaxies. The jet is well resolved in both length and width and extends to a projected metric distance of ~5 kpc from the galaxy nucleus. The host galaxy shows complex inner structure, including a dust lane and what appears to be two, or possibly three, emission filaments or arms. On average the optical jet is bluer than the host galaxy by about 0.4 mag. Unlike other galaxies hosting optical jets, the core of the galaxy does not possess a bright, unresolved nucleus, suggesting that the active galactic nucleus is presently inactive or buried in dust. The radio-optical spectral index, α_ro ~ 1, is the softest yet observed for radio galaxies with optical jets. The observed disturbances in the nuclear regions are indicative of a recent merger or infall possibly resulting in the ejection of material along the jet.

We present an ASCA observation of the nearby spiral galaxy M51 (NGC 5194). We detected hard X-ray emission with a photon index of ~1.4 and a luminosity of L_X ~ 1.1 × 10⁴⁰ ergs s^-1 in the 2-10 keV band (assuming a distance of 9.6 Mpc). A strong fluorescent iron K line (equivalent width ~900 eV) was detected at 6.4 keV in the X-ray spectra. Such an intense iron line is characteristic of Seyfert 2 galaxies and strongly suggests the presence of a heavily obscured active galactic nucleus (AGN). However, the X-ray image is extended even at energies above 2 keV. From the strong iron line and the extended hard X-ray image we speculate that the AGN is obscured by matter with a hydrogen column density more than several times 10²³ cm^-2 and that the observed 2-10 keV X-ray flux is not dominated by emission from the AGN but rather by other components, such as low-mass X-ray binaries, which typically dominate the X-ray emission of normal spiral galaxies. Emission lines from O K, Ne K, Fe L, Mg K and Si K were detected in the soft energy spectra, which indicate the presence of hot gas. The soft component is well represented by a Raymond-Smith thermal plasma model (with kT ~ 0.4 keV), which suggests a lower iron abundance (<0.1 solar) than other elements (~0.1-0.4 solar), or by a two temperature (kT ~ 0.3 keV and kT ~ 0.8 keV) model with ~0.1 solar abundance, which is reminiscent of the X-ray-emitting gas in starburst galaxies.

Future detection of a supernova neutrino burst by large underground detectors would give important information for the explosion mechanism of collapse-driven supernovae. We studied the statistical analysis for the future detection of a nearby supernova by using a numerical supernova model and realistic Monte Carlo simulations of detection by the Super-Kamiokande detector. We mainly discuss the detectability of the signatures of the delayed explosion mechanism in the time evolution of the _e luminosity and spectrum. For a supernova at 10 kpc away from the Earth, we find not only that the signature is clearly discernible but also that the deviation of the energy spectrum from the Fermi-Dirac (FD) distribution can be observed. The deviation from the FD distribution would, if observed, provide a test for the standard picture of neutrino emission from collapse-driven supernovae. For the D = 50 kpc case, the signature of the delayed explosion is still observable, but statistical fluctuation is too large to detect the deviation from the FD distribution. We also propose a method for statistical reconstruction of the time evolution of _e luminosity and spectrum from data, by which we can get a smoother time evolution and smaller statistical errors than by a simple, time-binning analysis. This method is useful especially when the available number of events is relatively small, e.g., a supernova in the LMC or SMC. A neutronization burst of ν_e's produces about five scattering events when D = 10 kpc, and this signal is difficult to distinguish from _ep events.

We have observed the ¹²CO J = 2-1 and J = 3-2 lines at a few locations in the dwarf irregular galaxies IC 10 and NGC 6822 using the James Clerk Maxwell Telescope. In addition, we have observed the ¹³CO J = 2-1 line for IC 10 and the first detection of the ¹³CO J = 3-2 transition in a Local Group galaxy. The CO line ratios in IC 10 are uniform and are consistent with the average line ratios observed in M33 at the 1 σ level. These low-metallicity environments appear to be porous to UV radiation and allow for more efficient heating of molecular gas by nearby H II regions. The ¹²CO J = 3-2/J = 2-1 ratio for the molecular cloud in NGC 6822 is higher than those found for IC 10 and M33 and suggests that the ¹²CO emission is optically thin in this region. This high line ratio is likely the result of its location inside a large H II region with low metallicity and low gas content, and requires a hydrogen density greater than 10⁴ cm^-3 and a kinetic temperature greater than 100 K. The ¹²CO/¹³CO J = 3-2 line ratio in one of the molecular clouds in IC 10 indicates that the gas must have a rather high kinetic temperature of about 100 K. In IC 10 we observe structures on a variety of size scales that all appear to be gravitationally bound. This effect may help explain the rather high star formation rate in IC 10.

Using the Goddard High-Resolution Spectrograph (GHRS) of the HubbleSpaceTelescope (HST) we have detected the neutron-capture elements osmium, platinum, and lead in the very metal poor ([Fe/H] ≃ -2.8) Galactic halo star HD 115444. This star joins the metal-poor giant HD 126238 ([Fe/H] ≃ -1.7) as the second Galactic halo star for which "third neutron-capture peak" elements have been detected. We have also determined upper limits on these same elements for the neutron-capture deficient star HD 122563 ([Fe/H] ≃ - 2.7). We have identified zirconium and germanium features in the HST spectra of all three stars; this marks the initial identification of the latter element in halo stars.

We compare and contrast the spectra and abundances of the neutron-capture elements in these three stars. Combining the new HST observations of the third neutron-capture peak elements of HD 115444 with ground-based data, we find the stellar abundance distribution, over a wider atomic number range than ever before possible, is consistent with the solar system r-process abundance curve. Comparison of this star to the more metal-rich HD 126238 finds that the ratio of the third-peak element abundances to that of the pure r-process element europium is thesameinbothstars. Earlier ground-based studies have found that abundances of the lighter elements, such as barium and europium, in HD 115444 exceed those of HD 122563 by ~0.7 dex. Our osmium, platinum, and lead upper limits show a similar difference and are not inconsistent with either the scaled solar r-process or the solar total distribution. Thus, the second and third neutron-capture peak element (Z ≥ 56) abundances in all three stars are consistent with scaled solar system r-process abundances. However, the zirconium abundance is approximately the same in HD 122563 and HD 115444, so this element (near the first neutron-capture peak) is overabundant (somewhat) in HD 122563 and (slightly) in HD 115444 with respect to the solar r-process abundances. Germanium (synthesized in approximately equal amounts by the r- and the s-process in solar material) is underabundant in all three stars, but does seem to scale with metallicity—it is identical in HD 115444 and HD 126238, but significantly greater in the higher metallicity HD 126238. These new results support previous observations that demonstrate the operation of the r-process, including the synthesis of the heaviest such elements, early in the history of the Galaxy. Implications of these results for early Galactic chemical evolution are discussed.

Period changes in RR Lyrae variables and Cepheids, known for more than 60 years, can possibly be explained by small changes in a helium composition gradient below the hydrogen and helium convection zones. The particular cases for the globular cluster M15 double-mode RR Lyrae variable V53 and the Cepheid Polaris are studied. For the last 80 years, the fundamental mode period of V53 has been decreasing while the overtone mode period in this same star has been increasing. The rather steady overtone mode period increase for Polaris stopped very recently, and the period now seems constant. Diffusive settling of helium in these kinds of stars has been known to be slight because of the two convection zones and the long diffusion timescale below them. But a small amount of helium settling, even before the star begins to pulsate, and then a dredge-up of just a little helium by an occasional overshooting can change surface layer structures and periods. This dredge-up can have a timescale as short as the convection turnover time, i.e., a few days. A slight helium dredge-up episode may now have temporarily stopped the decaying pulsations and period increase of Polaris. Such an episode cannot explain the double-mode V53 case, but possibly the helium composition gradient is deepened enough by matter accretion in only 80 years to explain its observed opposite period changes. Another mechanism that might be important for period changes is tidal mixing of the small composition gradients caused by occasional close encounters of stars in clusters. Significant stellar rotation would keep the surface layer composition homogeneous and not allow the anomalous period changes discussed here.

We present the geometrical optics for refraction of a distant background radio source by an interstellar plasma lens, with specific application to a lens with a Gaussian profile of free-electron column density. The refractive properties of the lens are specified completely by a dimensionless parameter α, which is a function of the wavelength of observation, the free-electron column density through the lens, the lens-observer distance, and the diameter of the lens transverse to the line of sight. A lens passing between the observer and a background source, due to the relative motions of the observer, lens, and source, produces modulations in the light curve of the background source. Because plasma lenses are diverging, the light curve displays a minimum in the background source's flux density, formed when the lens is on-axis, surrounded by enhancements above the nominal (unlensed) flux density. The exact form of the light curve depends only upon the parameter α and the relative angular sizes of the source and lens as seen by the observer. Other effects due to lensing include the following: (1) the formation of caustic surfaces, upon which the apparent brightness of the background source becomes very large; (2) the possible creation of multiple images of the background source; and (3) angular position wander of the background source. If caustics are formed, the separation of the outer caustics can be used to constrain α, while the separation of the inner caustics can constrain the size of the lens. We apply our analysis to two sources, which have undergone extreme scattering events: (1) 0954+658, a source for which we can identify multiple caustics in its light curve, and (2) 1741-038, for which polarization observations were obtained during and after the scattering event. We find general agreement between modeled and observed light curves at 2.25 GHz, but poor agreement at 8.1 GHz. The discrepancies between the modeled and observed light curves may result from some combination of substructure within the lens, an anisotropic lens shape, a lens which only grazes the source rather than passing completely over it, or unresolved substructure within the extragalactic sources. Our analysis also allows us to place constraints on the physical characteristics of the lens. The inferred properties of the lens responsible for the scattering event toward 0954+658 (1741-038) are that it was 0.38 AU (0.065 AU) in diameter with a peak column density of 0.24 pc cm^-3 (10^-4 pc cm^-3), an electron density within the lens of 10⁵ cm^-3 (300 cm^-3), and a mass of 6.5 × 10^-14M_☉ (10^-18M_☉). The angular position wander caused by the lens was 250 mas (0.4 mas) at 2.25 GHz. In the case of 1741-038, we can place an upper limit of only 100 mG on the magnetic field within the lens.

The central region of the Orion A cloud complex has been observed in the submillimeter range, using the French balloon-borne 2 m telescope PRONAOS-SPM during its first flight, in 1994 September. An area covering 50' × 10' and including the M42 Nebula has been mapped in four submillimeter photometric bands: λ180-240, 240-340, 340-560, and 560-1050 μm, with high sensitivity (≤2 MJy sr^-1) and an angular resolution from 2' to 35. Four brightness enhancements are visible and have been identified with the following sources: (1) the brightest peak corresponding to the central core of the nebula, in the BN/KL source direction; (2) an extended emission region around 5' × 8', to the southeast of BN/KL, correlated with 100 μm IRAS and ¹³CO emission (this has been identified as a gas-density enhancement associated with a shock region); (3) the edge of an ionized part of the cloud, correlated with the 100 μm emission; and (4) a very cold and extended condensation, discovered at 16' northwest of BN/KL. The emission spectra obtained for these sources show a variation of the dust emissivity spectral index within a large range, from 1 ± 0.1 to 2.2 ± 0.2. The cold condensation discovered (T = 12.5 ± 3 K) has a very low brightness emission, undetected on the 100 μm IRAS map. It extends over approximately 0.7 pc (FWHM), with a total mass of 11 M_☉ and a total luminosity of 2.4 L_☉.

We present VLA-A radio continuum observations at 3.6 cm and 2 cm of the extremely young, double-shell planetary nebula IC 4997. A comparison of the new 3.6 cm map with that obtained in 1995 July shows that morphological changes have occurred in the nebula in ≃1.3 yr. These changes reveal themselves by the presence in the outer shell of several new bright compact regions, most of which are located along the major nebular axis. The observed changes suggest that a variable highly collimated stellar wind impinges on the outer shell and causes variation of the physical conditions in compact nebular regions. This mechanism is probably related to the origin of the microstructure in IC 4997. The inner shell has been resolved at 2 cm and shows an elliptical morphology with a deconvolved size of ≃012 × 009 (P.A. ≃ 56°). Evidence for an extended, flat equatorial disk (size ≃14 × 022, P.A. ≃ 125°) is found at 2 cm. The derived spectral index α_{(3.6-2 cm)} map of the nebula reveals a compact (size ≃ 05 × 02, P.A. ≃ 125°), dense [N_e ≃ (2-6) × 10⁵ cm^-3], optically thick (τ_{3.6 cm} ≃ 1-8) band that probably represents the innermost, densest regions of the extended disk. This disk can be identified as the collimating agent of both the inner and outer shells. In addition, extremely young bipolar jetlike features are observed along the major axis of the inner shell, exhibiting the typical properties of jetlike outflows in planetary nebulae.

The star formation region GL 2136 has been imaged using narrowband filters at 2.2, 3.08, and 3.45 μm. Water ice absorption at 3.08 μm is commonly seen in molecular clouds toward embedded objects such as GL 2136. By constructing and analyzing a water ice absorption map and a M_2.2 - M_3.45 color map, we are able to set constraints upon the location, size, and scattering properties of grains in the GL 2136 region. Through detailed modeling of the composition and size distribution of the grains in the reflection nebula, we calculate a dust photospheric temperature of 660 K for IRS 1. Our analysis indicates that absorption is occurring within a circumstellar disk in addition to absorption in the molecular cloud. For the density of water ice in the molecular cloud, we find an upper limit of 7 H₂O molecules cm^-3 and a water ice abundance of 1.2 × 10^-5 by number with respect to hydrogen. The south and east lobes of the reflection nebula have small values of M_2.2 - M_3.45 and τ_ice compared to those of IRS 1, and appear to line a conical cavity created by the blueshifted molecular outflow. From the apparent positions of the reflection lobes, the estimated opening angle of the outflow is 45°. We also present a schematic diagram of the geometry of the region as determined by the results of this study.

We present the first detection of the "blue-bulge" infall signature toward a protostellar source. The blue-bulge infall signature can be observed in the centroid velocity maps of protostellar objects when infall dominates over rotation. This infall signature can be detected under a wide variety of source conditions. The detection of the blue-bulge infall signature toward a protobinary system such as IRAS 16293-2422 suggests that detailed studies of gravitational collapse toward a large number of protostellar sources may now be possible.

Our CS J = 7 → 6 data appear to be tracing gaseous material in the inner circumbinary core, while the CS J = 5 → 4 and HCO⁺J = 4 → 3 appear to trace the outer envelope and static core in addition to the inner circumbinary core. The mass accretion rate through the infall region appears to be consistent with an inside-out collapse model for the source. Using three-dimensional radiative transfer models based on the rotating, collapse solution of Terebey, Shu, & Cassen, we derive the infall parameters of the IRAS 16293-2422 cloud core. Our best-fit model suggests that the infall radius of the IRAS 16293-2422 cloud core is ~39'' (0.03 pc).

Optical Transients from gamma-ray burst sources, in addition to offering a distance determination, convey important information about the physics of the emission mechanism, and perhaps also about the underlying energy source. As the gamma-ray phenomenon is extremely diverse, with timescales spanning several orders of magnitude, some diversity in optical counterpart signatures appears plausible.

We have studied the optical transient that accompanied the gamma-ray burst of 1997 May 8, GRB 970508. Observations conducted at the 2.5 m Nordic Optical Telescope (NOT) and the 2.2 m telescope at the German-Spanish Calar Alto observatory (CAHA) cover the time interval starting 3 hr 5 minutes to 96 days after the high-energy event. This brackets all other published observations, including radio. When analyzed in conjunction with optical data from other observatories, evidence emerges for a composite light curve. The first interval, from 3 to 8 hr after the event, was characterized by a constant or slowly declining brightness. At a later moment, the brightness started increasing rapidly, and reached a maximum approximately 40 hr after the GRB. From that moment, the GRB brightness decayed approximately as a power law of index -1.21. The last observation, after 96 days, m_R = 24.28 ± 0.10, is brighter than the extrapolated power law, and hints that a constant component, m_R = 25.50 ± 0.40, is present. The optical transient is unresolved (FWHM 083) at the faintest magnitude level.

The brightness of the optical transient, its duration, and the general shape of the light curve set this source apart from the single other optical transient known, that of the 1997 February 28 event.

By direct hydrodynamic simulation, using the piecewise parabolic method code PROMETHEUS, we study the properties of a convective oxygen-burning shell in a SN 1987A progenitor star (20 M_☉) prior to collapse. The convection is too heterogeneous and dynamic to be well approximated by one-dimensional diffusion-like algorithms that have previously been used for this epoch. Qualitatively new phenomena are seen.

The simulations are two-dimensional, with good resolution in radius and angle, and used a large (90°) slice centered at the equator. The microphysics and the initial model were carefully treated. Many of the qualitative features of previous multidimensional simulations of convection are seen, including large kinetic and acoustic energy fluxes, which are not accounted for by mixing length theory. Small but significant amounts of ¹²C are mixed nonuniformly into the oxygen-burning convection zone, resulting in hot spots of nuclear energy production that are more than an order of magnitude more energetic than the oxygen flame itself. Density perturbations (up to 8%) occur at the "edges" of the convective zone and are the result of gravity waves generated by interaction of penetrating flows into the stable region. Perturbations of temperature and Y_e (or neutron excess η) at the base of the convective zone are of sufficient magnitude to create angular inhomogeneities in explosive nucleosynthesis products and need to be included in quantitative estimates of yields. Combined with the plumelike velocity structure arising from convection, the perturbations will contribute to the mixing of ⁵⁶Ni throughout supernovae envelopes. Runs of different resolution and angular extent were performed to test the robustness of these simulations.

We use a Monte Carlo binary synthesis code to model the formation and evolution of neutron star systems including high-mass X-ray binaries, low-mass X-ray binaries, double neutron star systems, and radio pulsars. Our focus is on the signature imprinted on such systems due to natal kicks to neutron stars over and above that imparted by orbital motions. The code incorporates the effect of the galactic potential (including rotation) on the velocities of these systems. A comparison between our models and the observations leads us to infer mean natal kicks ≳400-500 km s^-1. Moreover, to be consistent with all the data, we require a bimodal kick distribution with one peak in the distribution near 0 km s^-1 and the other above 600 km s^-1.

The color-magnitude diagram of the lower main sequence, as measured from a volume-limited sample of nearby stars, shows an abrupt downward jump between M_V ~ 12 and 13. This jump indicates that the observed mass-radius relationship steepens between 0.3 and 0.2 M_☉, but theoretical models show no such effect. It is difficult to isolate the source of this disagreement: the observational mass-radius relationship relies upon transformations that may not be sufficiently accurate, while the theoretical relationship relies upon stellar models that may not be sufficiently complete, particularly in their treatment of the complex physics governing the interior equation of state.

If the features in the observationally derived mass-radius relationship are real, their existence provides a natural explanation for the well-known gap in the orbital period distribution of cataclysmic variables. This explanation relies only upon the observed mass-radius relationship of low-mass stars and does not require ad hoc changes in magnetic braking or in the structure of cataclysmic variable secondaries. If correct, it will allow broader application of cataclysmic variable observations to problems of basic stellar physics.

In this paper we examine time-dependent and three-dimensional perturbations of spherical accretion flow onto a neutron star close to its Eddington limit. Our treatment assumes a Schwarzschild geometry for the spacetime outside the neutron star and is fully general relativistic. At all the accretion rates studied the response of the accretion flow to perturbations includes weakly damped oscillatory modes. At sufficiently high luminosities—but still well below the Eddington limit—the flows become unstable to aspherical perturbations. These unstable radiation hydrodynamic modes resemble the onset of convection and allow accretion to occur preferentially through more rapidly descending columns of gas, while the radiation produced escapes through neighboring columns in which the gas descends more slowly.

Matter of solar system composition has been added to the surfaces of two initially cool carbon-oxygen (CO) white dwarfs of masses 0.5 M_☉ and 0.8 M_☉ at rates in the range 10^-8 to 10^-6M_☉ yr^-1. Four different regimes are encountered. (1) At the highest accretion rates, models become red giants after the accretion of only a very small amount of mass. As the accretion rate is decreased, models are encountered that (2) burn hydrogen at the same rate at which it is accreted, (3) experience a series of nondynamical hydrogen shell flashes followed eventually by a powerful helium shell flash, and, finally, (4) experience nova-like hydrogen shell flashes.

Although all of the regimes have been explored, special attention has been given to models that experience recurrent mild hydrogen-burning pulses or burn hydrogen at a stationary rate. For lower accretion rates, the helium flash is so powerful that the convective layer forced by helium burning penetrates deeply into the hydrogen-rich envelope; this penetration may lead to the ejection of external layers even if the helium flash would not of itself have become dynamical. For higher accretion rates, even when convection does not penetrate into hydrogen-rich layers, the helium layer expands, and much, if not most, of the accreted matter is lost during the event because of the interaction of the expanded envelope with the companion star.

Analysis of the results suggests that it is unlikely that, in the real world, a hydrogen-accreting CO white dwarf with a typical initial mass will attain the Chandrasekhar mass. Dynamical helium-burning flashes are probable.

X-ray observations of the accreting X-ray pulsar 4U 1907+09 obtained during 1996 February with the Proportional Counter Array onboard the RossiX-RayTimingExplorer (RXTE) have made possible the first measurement of the intrinsic pulse period (P_pulse) since 1984: P_pulse=440.341^{+ 0.012}_−0.017 s. 4U 1907+09 is in a binary system with a blue supergiant. The orbital parameters have been solved, which enables us to correct a measurement of P_pulse obtained in 1990 with Ginga for orbital delay effects. Thus, three spin-down rates can be extracted from four pulse periods obtained in 1983, 1984, 1990, and 1996. These are equal to within 8% to a value of _pulse=+0.225 s yr^-1. This suggests that the pulsar has perhaps been in a monotonous spin-down mode since its discovery in 1983. Furthermore, the RXTE observations showed transient ~18 s oscillations during a flare that lasted about 1 hr. These oscillations could be interpreted as Keplerian motion of an accretion disk near the magnetospheric radius. This, and the notion that the corotation radius is much larger than any conceivable value for the magnetospheric radius (because of the long spin period), renders it unlikely that this pulsar spins near equilibrium, as is suspected for other slowing accreting X-ray pulsars. We suggest as an alternative that the frequent occurrence of a retrograde transient accretion disk may be consistently slowing the pulsar down. Further observations of flares could provide more evidence of this.

We develop a simple criterion, based on the general properties of weakly excited forbidden components in neutral helium, to predict which forbidden components should be observable in the He I spectrum of extreme helium stars. On the basis of this criterion, we find that nearly a dozen such components should be visible. We then use the He I broadening data of Beauchamp et al. to calculate detailed synthetic spectra for effective temperatures and gravities representative of these objects. All the forbidden components predicted by our criterion are present in the synthetic spectra. Observation of these features in extreme helium stars will provide a valuable test of current theories of the Stark broadening of neutral helium lines.

Extensive modifications to the non-LTE radiative transfer code of Hillier have been made in order to improve the spectroscopic analysis of stars with stellar winds. The main improvement to the code is the inclusion of blanketing due to thousands of overlapping lines. To implement this effect, we have used the idea of super levels first pioneered by Anderson. In our approach, levels with similar excitation energies and levels are grouped together. Within this group, we assume that the departure coefficients are identical. Only the population (or equivalently, the departure coefficient) of the super level need be solved in order to fully specify the populations of the levels within a super level. Our approach is a natural extension of the single-level LTE assumption, and thus LTE is recovered exactly at depth.

In addition to the line blanketing modifications, the code has been improved significantly in other regards. In particular, the new code incorporates the effect of level dissolution, the influence of resonances in the photoionization cross sections, and the effect of Auger ionization. Electron scattering with a thermal redistribution can be considered, although it is normally treated coherently in the comoving frame (which still leads to redistribution in the observer's frame).

Several example calculations are described to demonstrate the importance of line blanketing on spectroscopic analysis. We find that the inclusion of blanketing modifies the strengths of some optical CNO lines in Wolf-Rayet (W-R) stars by factors of 2-5. In particular, the strengths of the WC classification lines C III λ5696 and C IV λ5805 are both increased because of iron blanketing. This should help alleviate problems found with nonblanketed models, which were incapable of matching the strengths of these lines. We also find that, in the UV (1100-1800 Å), the influence of Fe is readily seen in both emission and absorption. The emission is sensitive to the iron abundance and should allow, for the first time, Fe abundances to be deduced in W-R stars.

The improvements made to our code should greatly facilitate the spectroscopic analysis of stars with stellar winds. We will be able to determine the importance and influence of line blanketing, as well as of several other effects that have been included in the new code. It will also allow us to better determine W-R star parameters, such as luminosity, elemental abundances, wind velocity, and mass-loss rate. With future application to related objects, such as novae and supernovae, our new code should also improve our understanding of these objects with extended outflowing atmospheres.

We have used the Röentgensatellit (ROSAT), the ExtremeUltravioletExplorer (EUVE), and the HubbleSpaceTelescope (HST) to measure X-ray and ultraviolet emissions of moderate-mass ( ~2-3 M_☉) giants in the Hertzsprung gap (spectral types early F to mid-G) and the post-helium flash "clump" ( ~G8-K0). Our motivation was to document the evolution of hot coronae (T > 10⁶ K) along the post-main-sequence trajectories traveled by such stars in order to gain insight concerning the "X-ray deficiency" of the F-G0 giants and the strong braking of stellar rotation at the red edge of the Hertzsprung gap.

With few exceptions, Hertzsprung gap and clump giants observed by ROSAT PSPC show hot (T ~ 10⁷ K) coronal energy distributions, regardless of any X-ray deficiency. EUVE spectra of gap star 31 Com (G0 III) indicate a broad coronal emission measure hump at ~10^7.2 K, while the active clump giant β Ceti (K0 III) displays a sharp peak at ~10^6.8 K, as seen previously in the mixed clump/gap binary Capella (α Aur: G8 III + G0 III). The gap giants υ Peg (F8 III) and 24 UMa (G4 III) have EUV emissions of intermediate temperature ( ~10^7.0 K).

The stars 31 Com, ψ³ Psc (G0 III), and β Cet exhibit redshifted transition zone (TZ: ~10⁵ K) lines in HST GHRS spectra, as reported earlier in Procyon (α CMi: F5 IV-V) and Capella G0. Such redshifts on the Sun are thought to signify flows in magnetic loops. β Cas (F2 III)—a rare soft coronal source among the gap stars—displays blueshifts of C IV and O IV], although emissions at cooler and hotter temperatures are near the photospheric velocity. The remarkably broad line profiles of the fastest rotating gap giants suggest that the 10⁵ K "subcoronal" emission zones extend to h ~R above the photosphere, about 50 scale heights.

In contrast to the TZ line redshifts, the upper chromospheric emissions (e.g., Mg II and Si III) of 31 Com and ψ³ Psc have blueshifted cores. Blue-asymmetric peaks in the solar Mg II lines are thought to indicate dynamical heating in the chromosphere. Observations of the H I Lyα feature of 31 Com taken 9 months apart reveal striking profile changes, reminiscent of those noted previously in the Lyα blue peak of the Capella G0 star.

We used the far-ultraviolet diagnostics, in combination with ROSAT X-ray photometry and EUVE high-excitation line strengths, to constrain physical models of the stellar outer atmospheres. Quasi-static magnetic loops can simulate the empirical coronal emission measures of the giant stars, but the inferred pressures for sensible loop lengths conflict with direct measurements of subcoronal densities. Furthermore, the high rate of emission at ~10⁵ K cannot be explained by thermal conduction down the legs of hot quasi-static loops.

On the other hand, the possible existence of elongated (l ~ R) emission structures on the gap giants leads to a speculative scenario to explain the X-ray deficiency. It is based on the increased importance of the dynamical filling phase ("explosive evaporation") of the loop life cycle; conductive cooling, yielding TZ emissions at the footpoints, when the heating is interrupted; and the possibility for transitions between "hot" and "cool" energy balance solutions owing to dynamical suspension and centrifugal trapping of the cooling gas. The long loops might represent a vestigial global "magnetosphere" inherited from the main-sequence phase, which ultimately is disrupted near ~G0 by the deepening convective envelope and growth of a more solar-like dynamo. Coronal emissions might be boosted temporarily as the X-ray deficiency is removed but soon would be quenched by wind braking previously inhibited by the magnetospheric "dead zone."

We obtained HubbleSpaceTelescope Goddard High-Resolution Spectrograph medium-resolution (G160M grating), phase-resolved spectroscopic observations of the prototype dwarf nova U Geminorum during dwarf nova quiescence, 13 days and 61 days following the end of a narrow outburst. The spectral wavelength ranges were centered on three different line regions: N V (1238 Å, 1242 Å), Si III (1300 Å), and He II (1640 Å). All of the quiescent spectra at both epochs are dominated by absorption lines and show no emission features. The Si III and He II absorption-line velocities versus orbital phase trace the orbital motion of the white dwarf, but the N V absorption velocities appear to deviate from the white dwarf motion. We confirm our previously reported low white dwarf rotational velocity, V sin i = 100 km s^-1. We obtain a white dwarf orbital velocity semiamplitude K₁ = 107 km s^-1. Using the γ-velocity of Wade, we obtain an Einstein redshift of 80.4 km s^-1 and hence a carbon core white dwarf mass of ~1.1 M_☉. We report the first subsolar chemical abundances of C and Si for U Gem with C/H = 0.05 times solar, almost certainly a result of C depletion due to thermonuclear processing. This C depletion is discussed within the framework of a weak thermonuclear runaway, contamination of the secondary during the common envelope phase, and mixing of C-depleted white dwarf gas with C-depleted matter deposited during a dwarf nova event. Remarkably, the T_eff of the white dwarf 13 days after outburst is only 32,000 K, anomalously cooler than previous early postoutburst measurements. Extensive cooling during an extraordinarily long (210 days) quiescence followed by accretion onto an out-of-equilibrium cooled degenerate could explain the lower T_eff.

The light curve of supernova (SN) 1993J is calculated using two approaches to radiation transport as exemplified by the two computer codes, STELLA and EDDINGTON. Particular attention is paid to shock breakout and the photometry in the U, B, and V bands during the first 120 days. The hydrodynamical model, the explosion of a 13 M_☉ star that has lost most of its hydrogenic envelope to a companion, is the same in each calculation. The comparison elucidates differences between the approaches and also serves to validate the results of both. STELLA includes implicit hydrodynamics and is able to model supernova evolution at early times, before the expansion is homologous. STELLA also employs multigroup photonics and is able to follow the radiation as it decouples from the matter. EDDINGTON uses a different algorithm for integrating the transport equation, assumes homologous expansion, and uses a finer frequency resolution. Good agreement is achieved between the two codes only when compatible physical assumptions are made about the opacity. In particular, the line opacity near the principal (second) peak of the light curve must be treated primarily as absorptive, even though the electron density is too small for collisional deexcitation to be a dominant photon destruction mechanism. Justification is given for this assumption and involves the degradation of photon energy by "line splitting," i.e., fluorescence. The fact that absorption versus scattering matters to the light curve is indicative of the fact that departures from equilibrium radiative diffusion are important. A new result for SN 1993J is a prediction of the continuum spectrum near the shock breakout (calculated by STELLA), which is superior to the results of other standard single energy group hydrocodes such as VISPHOT or TITAN. Based on the results of our independent codes, we discuss the uncertainties involved in the current time-dependent models of supernova light curves.

We describe a recalibration of the diffraction efficiency of the transmission gratings on EXOSAT. This recalibration is required in order to quantitatively interpret the soft X-ray spectra obtained with these instruments on Sco X-1. Because this source is extremely bright, the spectra are sensitive enough to reveal residual systematic effects in the grating efficiency. Specifically, we observe short-wavelength radiation dispersed into high (m > 5) spectral orders, which is usually too weak to be detectable.

We develop a simple model for the diffraction efficiency and briefly discuss some properties of the efficiency, applicable to X-ray transmission gratings in general. We explore the effect of a new degree of freedom, the cross-sectional shape of the grating bars. We then use a set of preflight calibration spectra to constrain the free parameters of the model. The new efficiency curves are in satisfactory agreement with the calibration data and quantitatively account for the anomalies seen in the spectrum of Sco X-1.

We also briefly discuss an empirical zero-order profile shape, appropriate for the analysis of spectra of very bright sources.

Using three elements of the Navy Prototype Optical Interferometer and observing in 20 spectral channels covering 520-850 nm, we have implemented a phase bootstrapping technique in which short baselines with high visibilities are used to keep the longer baselines with low visibilities in phase. Using this method, we have been able to extend the spatial frequency coverage beyond the first zero of the stellar visibility function for two K giants α Arietis, and α Cassiopeiae. The data are inconsistent with a uniform-disk model and confirm the presence of limb-darkened radial profiles. Adopting a particular limb-darkening law enables us to determine the diameter with small formal errors (one part in 1000). In addition, we have measured closure phases for both stars. The closure phases show a jump of 180° at the first zero in the visibility amplitude, which was expected.

The abundances of cosmic-ray helium isotopes between 0.2 and 3.7 GeV nucleon^-1 were measured by the Isotope Matter Antimatter Experiment (IMAX) during a flight from Lynn Lake, Manitoba, Canada on 1992 July 16-17. The IMAX balloon-borne magnetic spectrometer realized a direct measurement of the charge, the velocity, and the rigidity of cosmic rays using plastic scintillators, a high-resolution time-of-flight system, and two silica-aerogel Cerenkov counters in conjunction with a drift chamber/multiwire proportional chamber tracking system. About 75,000 helium isotopes are identified by their mass using the velocity versus magnetic rigidity technique. The measured ³He/⁴He ratios are corrected to the top of the atmosphere, and a comparison with previous data is given. The observed isotopic composition is found to be generally consistent with the predictions of a standard leaky box model of cosmic-ray transport in the Galaxy.

We show that there are no dynamic screening corrections to Salpeter's enhancement factor in the weak-screening limit.

The Homestake Solar Neutrino Detector, based on the inverse beta-decay reaction ν_e +³⁷Cl →³⁷Ar + e^-, has been measuring the flux of solar neutrinos since 1970. The experiment has operated in a stable manner throughout this time period. All aspects of this detector are reviewed, with particular emphasis on the determination of the extraction and counting efficiencies, the key experimental parameters that are necessary to convert the measured ³⁷Ar count rate to the solar neutrino production rate. A thorough consideration is also given to the systematics of the detector, including the measurement of the extraction and counting efficiencies and the nonsolar production of ³⁷Ar. The combined result of 108 extractions is a solar neutrino-induced ³⁷Ar production rate of 2.56 ± 0.l6 (statistical) ± 0.16 (systematic) SNU.

We study a simplified model of solar acoustic oscillations and show how asymmetries in spectral lines depend both on the acoustic source depth, as previously recognized, and on the acoustic source type. We provide a unified description of modal line asymmetries and high-frequency pseudomode locations, suggesting an inversion on power spectra minima to determine source properties and a correction to Lorentzian line shapes based upon the relative locations of spectral peaks and valleys. We also consider nonadiabatic effects due to Newtonian cooling and demonstrate that these do not lead to notable differences between velocity and intensity power spectral line shapes. We argue more generally that it is unlikely that any nonadiabatic effect can be responsible for the observed differences. Finally, we discuss the importance of both multiplicative and additive background power to the spectra and show how additive noise can reduce the apparent line asymmetry of a mode. We note that information on solar convective motions can be potentially extracted from three components of the acoustic power spectra: the additive background yielding information on the spectrum of nonoscillatory motions at the height of observation, the multiplicative background reflecting the source spectrum, and the power minima providing the source depth and physical nature. For stochastically excited linear waves only the first of these contributes significantly to spectral differences between observed variables.

The High Energy X-Ray Timing Experiment (HEXTE) is one of three scientific instruments aboard the RossiX-RayTimingExplorer (RXTE), which was launched on 1995 December 30. RXTE performs timing and spectral studies of bright X-ray sources to determine the physical parameters of these systems. The HEXTE consists of two independent clusters of detectors, each cluster containing four NaI(Tl)/CsI(Na) phoswich scintillation counters sharing a common 1° FWHM field of view. The field of view of each cluster is switched on and off source to provide near real-time background measurements. The net open area of the eight detectors is 1600 cm², and each detector covers the energy range 15-250 keV with an average energy resolution of 15.4% at 60 keV. The in-flight performance of the HEXTE is described, the light curve and spectrum of the Crab Nebula/pulsar is given, and the 15-240 keV spectrum of the weak source, active galaxy MCG +8-11-11 is presented to demonstrate the weak source spectral capabilities of HEXTE.

We describe the Navy Prototype Optical Interferometer (NPOI), a joint project of the Naval Research Laboratory (NRL) and the US Naval Observatory (USNO) in cooperation with Lowell Observatory. The NPOI has recently begun operations at the Lowell Observatory site near Flagstaff, Arizona, obtaining its first images, of a binary star, in 1996 May and June and its first limb-darkening observations during 1996 November to 1997 February. This paper gives an overview of the NPOI, including the characteristics of optical interferometry that affect its design.

The NPOI includes subarrays for imaging and for astrometry. The imaging subarray consists of six moveable 50 cm siderostats feeding 12 cm apertures, with baseline lengths from 2.0 to 437 m. The astrometric subarray consists of four fixed 50 cm siderostats feeding 12 cm apertures (35 cm apertures to be installed in 1998), with baseline lengths from 19 m to 38 m. The shared back end covers 450-850 nm in 32 channels. The NPOI features vacuum feed and delay systems, active group-delay fringe tracking, and a high degree of automation. The astrometric subarray also includes an extensive site laser metrology system to measure the motions of the siderostats with respect to one another and to the bedrock.

For imaging stellar surfaces, arrays with equal spacing between elements are superior to arrays that have been laid out to optimize (u, v) coverage and that therefore have unequal spacing. The imaging subarray of the NPOI provides a number of equally spaced configurations with linear scales at ratios of ≈ 1.64. Unequally spaced configurations are available for a variety of other imaging programs. Coherence across either type of imaging configuration is maintained by "phase bootstrapping": the phases on the longest baselines, on which fringes may be too weak to track, are stabilized by tracking fringes on the shortest baselines.

In principle, the four elements of the astrometric subarray provide enough independent baselines to solve for stellar positions and the array geometry simultaneously while observing each of 11 stars only once.

The anticipated magnitude limit is 7 mag or better with 12 cm apertures and average seeing; with 35 cm apertures, we expect the limit to be one or more magnitudes fainter. The anticipated wide-angle astrometric precision of the NPOI is ≈ 2 mas. The best angular resolution of the imaging subarray will be ≈ 200 μas. Our experience with the Mark III interferometer suggests that we will be able to measure stellar diameters as small as 200 μas with 1% precision and binary star separations as small as ρ ≈ 65 μas (for Δm ≈ 0 mag) or ρ ≈ 200 μas (for Δm ≈ 3-4 mag). With its large bandwidth and phase bootstrapping, the imaging subarray should be able to make images ≳10 resolution elements across the disks of nearby late-type stars.

We consider fireball models where the ejecta have a range of bulk Lorentz factors, so that the inner (lower Γ) parts may carry most of the mass, or even most of the energy. The outer shock and contact discontinuity decelerate as the fireball sweeps up external matter. This deceleration allows slower ejecta to catch up, replenishing and reenergizing the reverse shock and boosting the momentum in the blast wave. In consequence, the energy available to power the afterglow may substantially exceed that of the burst itself. Such models allow a wide range of possibilities for the afterglow evolution, even in the case of spherically symmetric expansion.

We identify the extended Einstein IPC X-ray source 1E 0657-56 with a previously unknown cluster of galaxies at a redshift of z=0.296. Optical CCD images show the presence of a gravitational arc in this cluster, and galaxy spectra yield a cluster velocity dispersion of 1213^{+ 352}₋₁₉₁ km s⁻¹. X-ray data obtained with the ROSAT HRI and ASCA indicate that 1E 0657-56 is a highly luminous cluster in which a merger of subclusters may be occurring. The temperature of the hot gas in 1E 0657-56 is kT=17.4±2.5 keV, which makes it an unusually hot cluster, with important cosmological implications.

We report the discovery of the brightest X-ray source hosted by a faint (M_B=-16) dwarf galaxy in the immediate vicinity of the ultraluminous IRAS merging galaxy Markarian 273. The dwarf galaxy, 13 away from Mrk 273, is at the tip of a faint northeast plume of Mrk 273. Its spectrum exhibits strong [O III], Hα, and [N II] emission lines, which establish the redshift of the dwarf galaxy, z=0.0376, the same as that of Mrk 273. The emission-line ratios are typical of Seyfert galaxies. The X-ray emission is consistent with a pointlike source coincident with the center of the dwarf galaxy. The intrinsic X-ray luminosity, 6.3×10⁴¹ ergs s⁻¹, in the 0.1-2.4 keV energy range, is about 7 times larger than the B-band luminosity. The X-ray spectrum of the source can be fitted with a power law. All the evidence is consistent with the source being a Seyfert galaxy. Out of ~10 faint objects in the same field, only one is detected by ROSAT, and its ratio of soft X-ray to optical luminosity is as high as those for BL Lacertae objects and few active galactic nuclei (AGNs). If there is a population of such dwarf AGNs hidden as companions of major merger galaxies (such as Mrk 273), they may contribute to the luminosity function of AGNs and the cosmic X-ray background at the faint end.

Many disk galaxies are lopsided; their brightest inner parts are displaced from the center of the outer isophotes or the outer contours of the H I disk. This asymmetry is particularly common in small, low-luminosity galaxies. We argue here that long-lived lopsidedness is a consequence of the disk lying off-center in the potential of the galaxy's extended dark halo and spinning in a sense retrograde to its orbit about the halo center. The stellar velocity field predicted by our gravitational N-body simulations is clearly asymmetric.

The origin and sustenance of large-scale galactic magnetic fields has been a long-standing and controversial astrophysical problem. Here an alternative to the "standard" α-Ω mean field dynamo and primordial theories is pursued. The steady supply of supernovae-induced turbulence exponentiates the total field energy, providing a significant seed mean field that can be stretched linearly by shear. The observed microgauss fields would be produced primarily within one vertical diffusion time since it is only during this time that linear stretching can compete with diffusion. This approach does not invoke exponential mean field dynamo growth from the helicity α-effect but does employ turbulent diffusion, which limits the number of large-scale reversals. The approach could be of interest if the helicity effect is suppressed independently of the turbulent diffusion. This is an important but presently unresolved issue.

We have studied the optical counterparts of X1755-338 and X1658-298 in their X-ray "off" states. The first observations of X1755-338 in quiescence show that the counterpart, V4134 Sgr, has faded by more than 3.5 mag in V. If the mass donor in the system is an M0 V star, as implied by the period, our upper limits on the brightness of the counterpart suggest that it is more distant than 4 kpc.

We observed V2134 Oph, the optical counterpart of X1658-298, on several occasions in 1997 April/May and found the source only ~1 mag fainter than when it is X-ray bright. Contemporaneous X-ray data confirm that the source remains in the quiescent state during our optical observations. Our optical light curve, folded on the 7.1 hr orbital period, does not show any modulation across the binary cycle. It is possible that the absence of detectable X-ray emission, despite the indication for an accretion disk and activity in the system, is related to a structure in the disk that permanently obscures the central X-ray source. The optical properties of V2134 Oph are unique among the known X-ray transients.

We identify the X-ray-reflected component in the Ginga spectra of Nova Muscae 1991, a black hole transient system used as the prototype for the recent model of Esin, McClintock, & Narayan based on advection-dominated disk solutions. We see that the reflected spectrum is generally significantly relativistically smeared and use this, together with the amplitude of reflection, to track the innermost extent of the accretion disk. The optically thick disk switches from being highly ionized to nearly neutral during the transition from high to low state, and the inner radius of the disk moves outward during the low-state decline.

Qualitatively, this overall trend is compatible with the Esin et al. model, but quantitatively, the retreat of the inner disk during the high- to low-state transition is much slower than predicted. The hard (low-state) spectra are not produced solely by an optically thin accretion flow: optically thick material within ~20-100R_g is generally present.

We have used the Hubble Space Telescope's (HST) Goddard High-Resolution Spectrograph to detect a photospheric metallic absorption line, Si III λ1206, in the spectrum of the magnetic white dwarf component of the Hyades precataclysmic binary V471 Tauri. The Si III feature is modulated on the soft X-ray/EUV/optical 9.25 minute rotational period of the white dwarf and is strongest at the time of soft X-ray/EUV minimum and optical maximum. A model in which the soft X-ray/EUV magnetic pole is dark because of metallic and helium absorption, and bright in the optical due to flux redistribution, is strongly supported. We derive a Si abundance of 0.10^{+ 0.03}_−0.07 times solar in the accretion cap. Assuming equilibrium between mass accretion onto—and diffusion of Si out of—the photosphere, we find the white dwarf to be accreting from its dK companion's wind at only 3.8×10⁻¹⁸M_☉ yr^-1, some 5 orders of magnitude lower than the Bondi-Hoyle fluid rate. This strongly suggests operation of a magnetic-centrifugal propeller mechanism that rejects most of the material that attempts to accrete. We tentatively detect Zeeman splitting of the Si III line, implying a polar field strength of ~350 kG. V471 Tau is destined, in the distant future, to become a DQ Her-type cataclysmic binary.

We suggest an alternative explanation for the apparently low metal abundances observed in some very active late-type stellar coronae: that the He abundance in these coronae is enhanced, causing lower line-to-continuum ratios. The dependence of the total free-free Gaunt factor on the square of the nuclear charge means that He can begin to dominate the EUV and X-ray thermal continua of a hot plasma with only a doubling of the abundance of He^{2 +} nuclei relative to H⁺ nuclei. Using model continua, we calculate the relation between He abundance and apparent metal deficiency. We suggest that He might be selectively enhanced by preferential mechanisms for retention of He nuclei in the corona or by diffusion processes, as recent sophisticated calculations indicated might be the case under certain coronal conditions. While it might seem unlikely that He nuclei could be enhanced in the hot gas phase of elliptical galaxies, until the possible presence of small-scale structure and spatial inhomogeneities in their interstellar media are better understood, diffusion and other element fractionation processes should perhaps not be ruled out entirely in the context of trying to understand the puzzlingly low metal abundances found from X-ray studies of elliptical galaxies.

The effect of a strong magnetic field on photospheric convection in a cool star like the Sun can be established by relating high-resolution solar observations to results from nonlinear models that rely on computation. The patterns of motion in numerical experiments on three-dimensional, compressible magnetoconvection depend not only on the strength of the imposed vertical magnetic field but also on the aspect ratio λ of the computational box. In a wide box (λ=8) with a moderately strong field, the flow organizes itself so that magnetic flux is separated from the motion. There are regions with strong fields and small-scale oscillatory convection next to almost field-free regions with clusters of broad and vigorous convective plumes. In the solar photosphere, this corresponds to the difference between the patterns of granulation in plage regions (with fields greater than 100 G) and in the adjacent quiet Sun.

Using a 1988-1997 data set of original photospheric vector magnetograms from the Solar Magnetic Field Telescope (SMFT) of the Huairou Solar Observing Station of Beijing Astronomical Observatory, we computed the local current helicity B_z(∇×B)_z for 422 active regions. We found that any given active region contained mixed signs of current helicity, but in most cases current helicity with a particular sign was dominant over a whole region area. In our data set, 84% of the active regions in the northern hemisphere have negative helicity, and 81% in the southern hemisphere have positive helicity. It is estimated that the noise and error in our calculation are at the 2 σ level.

In addition, we have studied the evolution of the large-scale surface current helicity during the cycle 22, which is a running mean of absolute current helicity of the active regions observed over a Carrington rotation period. By comparing with monthly mean sunspot number, we found that the average current helicity has a good correlation with solar activity.

Consideration is given to the possible mechanisms responsible for the production of the atomic sodium tail of comet Hale-Bopp. It is shown that both photosputtering and ion sputtering of nonvolatile dust grains might not be the main driving force. Instead, the generation of impact vapor by collisional interaction between the cometary dust coma of micron-sized particles and the very small grains (VSGs) of 10-100 Å size could possibly account for the observations if a large amount of VSGs existed in the coma region.

The rotational transitions of ²⁸Si¹⁸O in the ground vibrational state were observed by laboratory microwave spectroscopy for astronomical use. The SiO molecules were generated in a free-space absorption cell by dc-glow discharge in a mixture of tetra-methyl silane, Si(CH₃)₄, and ¹⁸O₂. Ten rotational transitions were precisely measured in the 40-444 GHz region. The rotational and centrifugal distortion constants were determined by a least-squares analysis of measured line frequencies: B₀=20176.4394(43) and D₀=0.025809(32) MHz, with the 3 σ deviations in parentheses.

The pure rotational transitions of the protonated hydrogen cyanide ion, HCNH⁺, and its isotopic species, HCND⁺ and DCND⁺, were measured in the 107-482 GHz region with a source-modulated microwave spectrometer. The ions were generated in a cell with a magnetically confined DC-glow discharge of HCN and/or DCN. The rotational constant B₀ and the centrifugal distortion constant D₀ for each ion were precisely determined by a least-squares fitting to the observed spectral lines. The observed rotational transition frequencies by laboratory spectroscopy and the predicted ones are accurate to about 30-40 kHz and are useful as rest frequencies for astronomical searches of HCNH⁺ and HCND⁺.