Table of contents

We report the discovery of a remarkable strong-lensing cluster from the ongoing Red-Sequence Cluster Survey (RCS). RCS 0224-0002, at a spectroscopic redshift of 0.773, lenses two to four separate background sources. These arcs are of relatively high surface brightness and hence amenable to detailed follow-up study. We provide preliminary photometry and spectroscopy of RCS 0224-0002 and discuss in detail the one lensed source for which a redshift has so far been determined. This galaxy, at a redshift of 4.8786, appears to be composed of a star-forming core enveloped in a Lyα emitting cloud. Moreover, it shows no measurable velocity structure to a limit of ∼50 km s^-1 over a region likely to be ∼1 kpc in size. The available data are also used to develop an initial lensing model, which shows RCS 0224-0002 to have a mass corresponding to a central velocity dispersion of ∼1000 km s^-1, consistent with the measured optical richness of the cluster. This preliminary model is also used to estimate redshifts for all lensed components visible in our ground-based imaging. RCS 0224-0002 is the highest-redshift cluster to show lensing with such a large number of bright arcs (comparable to the best lensing clusters at much lower redshifts), and the highest-redshift strong-lensing cluster for which both the cluster redshift and at least one source redshift have been established spectroscopically, and as such it provides a powerful opportunity to directly measure the geometry of the universe and galaxy properties at high redshift.

We report the discovery of a gravitationally lensed quasar resulting from our survey for lenses in the southern sky. Radio images of J1632-0033 with the Very Large Array and the Australia Telescope Compact Array exhibit two compact, flat-spectrum components with separation 147 and flux density ratio 13.2. Images with the Hubble Space Telescope reveal the optical counterparts to the radio components and also the lens galaxy. An optical spectrum of the bright component obtained with the first Magellan telescope reveals quasar emission lines at redshift 3.42. Deeper radio images with the Multi-Element Radio Linked Interferometry Network and the Very Long Baseline Array reveal a faint third radio component located near the center of the lens galaxy, which is either a third image of the background quasar or faint emission from the lens galaxy.

We present a comparison of three cluster-finding algorithms from imaging data using Monte Carlo simulations of clusters embedded in a 25 deg² region of Sloan Digital Sky Survey (SDSS) imaging data: the matched filter (MF; Postman et al., published in 1996), the adaptive matched filter (AMF; Kepner et al., published in 1999), and a color-magnitude filtered Voronoi tessellation technique (VTT). Among the two matched filters, we find that the MF is more efficient in detecting faint clusters, whereas the AMF evaluates the redshifts and richnesses more accurately, therefore suggesting a hybrid method (HMF) that combines the two. The HMF outperforms the VTT when using a background that is uniform, but it is more sensitive to the presence of a nonuniform galaxy background than is the VTT; this is due to the assumption of a uniform background in the HMF model. We thus find that for the detection thresholds we determine to be appropriate for the SDSS data, the performance of both algorithms are similar; we present the selection function for each method evaluated with these thresholds as a function of redshift and richness. For simulated clusters generated with a Schechter luminosity function (M = -21.5 and α = -1.1), both algorithms are complete for Abell richness ≳1 clusters up to z ∼ 0.4 for a sample magnitude limited to r = 21. While the cluster parameter evaluation shows a mild correlation with the local background density, the detection efficiency is not significantly affected by the background fluctuations, unlike previous shallower surveys.

We study the spatial distribution of Abell and X-ray–selected clusters of galaxies from the ROSAT Bright Source Catalog and determine correlation functions for both cluster samples. We find that on small scales the correlation functions depend on the cluster environment: clusters in rich superclusters have a larger correlation length and amplitude than clusters of the whole sample. On large scales the correlation functions depend on the distribution of superclusters, but for both X-ray and Abell clusters they are oscillating with a period of ∼115 h^-1 Mpc, indicating the presence of a local peak in the power spectrum at an effective wavenumber k = 0.055 h Mpc^-1.

We present a comparison of how well the large-scale structure of the universe is traced by clusters from the Abell catalog and from the Automated Plate Measuring Facility (APM). We investigate selection functions for both cluster catalogs, using samples of all clusters (including clusters with estimated redshifts) and samples of clusters with measured redshifts. We present a catalog of superclusters of galaxies, based on APM clusters up to a redshift z_lim = 0.13. We find that the distribution of rich superclusters, defined by all Abell and APM clusters, is similar in the volume covered by both cluster samples. We calculate the correlation function for Abell and APM cluster samples. We show that the supercluster-void network can be traced with both cluster samples; the network has a period of ∼120 h^-1 Mpc. However, the APM cluster sample with measured redshifts covers a small volume, which contains only a few very rich superclusters. These superclusters surround one void and have exceptionally large mutual separations. Because of this property, the secondary maximum of the correlation function of APM clusters with measured velocities is located at larger scales than the corresponding feature in the correlation function of Abell clusters. We conclude that the APM sample is not representative of the large-scale structure as a whole because of the small volume covered. The Abell cluster catalog is presently the best sample to investigate the large-scale distribution of high-density regions in the universe.

Wide field CCD imaging was carried out in the R_C band for 10 nearby clusters of galaxies and four control fields to derive the total and type-specific luminosity functions. Observation and data reduction procedures are described. We extract galaxies down to R_C ∼ 20 mag and classify them into two broad types, r^1/4-like and exponential-like, on the basis of the bulge-to-total luminosity ratio B/T estimated from Petrosian quantities. We describe our classification scheme in detail. We apply a single classification scheme to both giant and dwarf galaxies. The consistency of our classification is verified for giant galaxies using both simulated images and real data in the literature. We set the boundary of our two types at B/T = 0.35. This boundary gives 70% completeness to both the r^1/4-like sample (for E/S0 galaxies) and the exponential-like sample (for Sa–Irr galaxies). Our classification for dwarf galaxies is investigated using higher resolution images of some 20 dwarf galaxies in the Virgo Cluster. Galaxy catalogs are constructed, which include position, magnitude, and B/T. The projected sky distribution of each type of galaxies is shown for the clusters and control fields.

We investigate the total luminosity function (LF) and the type-specific luminosity functions (TSLFs)of r^1/4-like galaxies and exponential-like galaxies in 10 nearby clusters. The composite total LF of the 10clusters is described well by the Schechter function with M = -21.3 and α = -1.31 in the range -23.5 < M_R + 5 log h < -16. This value of α is consistent with the previous results for cluster galaxies. An extensive comparison of the values of α is made between clusters and fields. We find that the total LF of cluster galaxies has a steeper faint end slope than that of field galaxies. In contrast to the composite total LF, the individual total LFs of some clusters are not fitted well by the Schechter function. They show a hump around M_R ∼ -20 to -19 associated with a dip around M_R ∼ -19 to -18. The composite TSLFs of both r^1/4-like galaxies and exponential-like galaxies are fitted well by the Schechter function. The best-fit parameters are (M = -21.2, α = -1.08) for the former and (M = -21.1, α = -1.49) for the latter. When seen on a cluster to cluster basis, however, the TSLFs of the two types of galaxies show different behaviors. The TSLF of r^1/4-like galaxies shows a considerable diversity among the clusters, while the TSLF of exponential-like galaxies is similar in all the clusters. We demonstrate that the dip at M_R ∼ -19 to -18 seen in the total LFs of some clusters is almost entirely due to r^1/4-like galaxies, rather than exponential-like galaxies. We find that the dip of the TSLF of r^1/4-like galaxies is correlated with the velocity dispersion of the cluster in the sense that clusters with larger velocity dispersion show more distinct dips.

We have measured 1365 redshifts to a limiting magnitude of R ∼ 15.5 in 15 AWM/MKW clusters and have collected another 203 from the literature in MKW 4s, MKW 2, and MKW 2s. In AWM 7 we have extended the redshift sample to R ∼ 18 in the cluster center. We have identified 704 cluster members in 17 clusters; 201 are newly identified. We summarize the kinematics and distributions of the cluster galaxies and provide an initial discussion of substructure, mass and luminosity segregation, spectral segregation, velocity-dispersion profiles, and the relation of the central galaxy to global cluster properties. We compute optical mass estimates, which we compare with X-ray mass determinations from the literature. The clusters are in a variety of dynamical states, reflected in the three classes of behavior of the velocity-dispersion profile in the core: rising, falling, or flat/ambiguous. The velocity dispersion of the emission-line galaxy population significantly exceeds that of the absorption-line galaxies in almost all of the clusters, and the presence of emission-line galaxies at small projected radii suggests continuing infall of galaxies onto the clusters. The presence of a cD galaxy does not constrain the global cluster properties; these clusters are similar to other poor clusters that contain no cD. We use the similarity of the velocity-dispersion profiles at small radii and the cD-like galaxies' internal velocity dispersions to argue that cD formation is a local phenomenon. Our sample establishes an empirical observational baseline of poor clusters for comparison with simulations of similar systems.

In this paper optical positions relative to UCAC1 for 22 counterparts of extragalactic radio sources in the southern hemisphere are presented. The internal accuracy of the positions is of the order of 0060. With the help of astrometric calibration regions, choices of the centering method and the width of the subraster used for object centering are given based on the detailed analysis of the results of centers of stellar images in the CCD on the 1 m telescope at Yunnan Astronomical Observatory, and the choice of the suitable model for the reduction of CCD is also recommended. A comparison of our results with those of other authors is made.

We present high-resolution VLA images at 8.4 GHz of three Fanaroff-Riley class II (FR II) radio galaxies (RGs; 3C 42, 3C 244.1, and 3C 330) and two FR II quasars (QSRs; 3C 9 and 3C 208) as part of a study to obtain high-resolution images of the hot spots. No jet is detected in the three RGs, but the hot spots, well imaged at 022 resolution, show many details not previously observed. For the two QSRs, we have made good images of the known jets at this resolution. The overall morphology and location of the hot spots (i.e., whether they are at the outer edges of the radio lobe or embedded within it) are discussed.

We investigate the physical properties of a sample of 26 galaxies in the Bootes void and classify these galaxies based on the emission lines in their spectra. Fourteen galaxies are classified as H II galaxies with properties similar to field H II galaxies. Two of these galaxies, 1432+5302 and 1507+4554, are extreme starburst galaxies. Approximately half of the galaxies with measurable Hα+[N II] emission have elevated rates of star formation. Analysis of the galaxy continua suggests that approximately one-third of the void galaxies have large populations of blue stars. Stellar absorption features from metals in more than half the galaxies in the sample indicate the presence of late-type stars. Emission lines are detected from the H I galaxy 1517+3949 for the first time. Two systems, 1510+4727 and 1517+3956, are identified as closely interacting galaxy pairs, bringing the total number of known galaxy pairs in the void to four. The galaxy 1458+4944 is a LINER, making at least five AGNs in the void. Classifications of eight galaxies in the sample remain unknown. The galaxies in the Bootes void are similar to field emission-line galaxies with respect to stellar populations, emission-line properties, fraction of AGN and fraction of galaxy pairs. This result is inconsistent with some models of galaxy formation in low-density environments.

This is the first of two papers where we present the analysis of an HST/NICMOS2 near-infrared (NIR) snapshot survey in the F160W (H) filter for a sample of 78 spiral galaxies selected from the UGC and ESOLV catalogs. For 69 of these objects we provide nuclear color information derived by combining the H data either with additional NICMOS F110W (J) images or with V WFPC2/HST data. Here we present the NIR images and the optical-NIR color maps. We focus our attention on the properties of the photometrically distinct "nuclei" which are found embedded in most of the galaxies and provide measurements of their half-light radii and magnitudes in the H (and when available in the J) band. We find that (1) in the NIR the nuclei embedded in the bright early- to intermediate-type galaxies span a much larger range in brightness than the nuclei which are typically found embedded in bulgeless late-type disks: the nuclei embedded in the early- to intermediate-type galaxies reach, on the bright end, values up to H_AB ∼ -17.7 mag; (2) nuclei are found in both nonbarred and barred hosts, in large-scale (≳1 kpc) as well as in nuclear (up to a few 100 pc) bars; (3) there is a significant increase in half-light radius with increasing luminosity of the nucleus in the early/intermediate types (a decade in radius for ≈8 mag brightening), a correlation which was found in the V band and which is also seen in the NIR data; (4) the nuclei of early/intermediate-type spirals cover a large range of optical-NIR colors, from V-H ≈ -0.5 to 3. Some nuclei are bluer and others redder than the surroundinggalaxy, indicating the presence of activity or reddening by dust in many of these systems; (5) someearly/intermediate nuclei are elongated and/or slightly offset from the isophotal center of the host galaxy. On average, however, these nuclei appear as centered, star-cluster–like structures similar to those whichare found in the late-type disks.

We present surface brightness profiles for 56 of the 78 spiral galaxies observed in the HST/NICMOS2 F160W snapshot survey introduced in Paper I of this series, as well as surface brightness profiles for 23 objects out of the 41 that were also observed in the F110W filter. We fit these surface brightness profiles with the Nuker law of Lauer et al. and use the smooth analytical descriptions of the data to compute the average nuclear stellar cusp slopes ⟨γ⟩ in the 01–05 radial range. Our main result is the startling similarity between the nuclear stellar cusp slopes ⟨γ⟩ in the near-infrared compared with those derived in the visual passband. This similarity has several implications: (1) Despite the significant local color variations that are found in the nuclear regions of spirals and that are documented in Paper I, there are typically little or no optical-NIR global color gradients, and thus no global stellar population variations, inside ∼50–100 pc from the nucleus in nearby spirals. (2) The large observed range of the strength of the nuclear stellar cusps seen in the HST optical study of spiral galaxies reflects a physical difference between galaxies and is not an artifact caused by nuclear dust and/or recent star formation. (3) The dichotomy between R^1/4 bulges, with steep nuclear stellar cusps ⟨γ⟩ ∼ 1, and exponential bulges, with shallow nuclear stellar cusps ⟨γ⟩ < 0.3, is also not an artifact of the effects of dust or recent star formation. (4) The presence of a surrounding massive disk appears to have no effect on the rise of the stellar density distribution within the innermost hundred parsecs of the R^1/4 spheroids. These results imply a breakdown within the family of exponential bulges of the nuclear versus global relationships that have been found for the R^1/4 spheroids. Such a breakdown is likely to have significant implications concerning the formation of exponential bulges and their connection with the R^1/4 spheroids.

This work presents new surface photometry and two-dimensional modeling of the light distribution of the polar ring galaxy NGC 4650A, based on near-infrared (NIR) observations and high-resolution optical imaging acquired during the Hubble Heritage program. The NIR and optical integrated colors of the S0 galaxy and the polar ring and their scale parameters are compared with those for standard galaxy morphological types. The polar structure appears to be a disk of a very young age, while the colors and light distribution of the host galaxy do not resemble those of a typical early-type system. We compare these observational results with the predictions from different formation scenarios for polar ring galaxies. The peculiarities of the central S0 galaxy, the polar disk structure, and stellar population ages suggest that the polar ring galaxy NGC 4650A may be the result of a dissipative merger event rather than of an accretion process.

We have developed photometric techniques that can be applied to images with highly variable backgrounds, as well as to slightly extended objects (object size comparable to or smaller than point-spread function [PSF] size). We have shown that ordinary stellar PSF-fitting photometry can be applied to slightly extended objects provided that one applies a systematic correction to the photometry that is a function primarily of the observed sharpness. Applying these techniques to the Cepheid target NGC 3627, we find that we are successfully able to photometer the stars and clusters, as well as discriminate the cluster population with a negligible number of false detections.

We present a comprehensive multiwavelength study of the nearby interacting dwarf galaxy NGC 3077 (member of the M81 triplet). High-resolution VLA H I observations show that most of the atomic gas (∼90%) around NGC 3077 is situated in a prominent tidal arm with a complex velocity structure. Little H I (∼5 × 10⁷M_⊙) is associated with NGC 3077 itself. High-resolution Owens Valley Radio Observatory (OVRO) observations of the molecular component (CO) reveal the presence of 16 molecular complexes near the center of NGC 3077 (total mass: ∼1.6 × 10⁶M_⊙). A virial mass analysis of the individual complexes yields a lower CO-to-H₂ conversion factor in NGC 3077 than the Galactic value—a surprising result for a dwarf galaxy. The lower conversion factor can be explained by extreme excitation conditions and the metallicity of the molecular gas. The total (atomic and molecular) gas content in the center of NGC 3077 is displaced from the stellar component of NGC 3077—this implies that not only the gas at large galactocentric radii is affected by the interaction within the triplet but also the center. We speculate that the starburst activity of NGC 3077 was triggered by this redistribution of gas in the center: Hα, as well as Paα, images show the presence of violent central star formation, as well as dramatic ionized supershells reaching galactocentric distances of ∼1 kpc. Some of these supershells are surrounded by neutral hydrogen. In a few cases, the rims of the ionized supershells are associated with dust absorption. The most prominent star-forming region in NGC 3077 as probed by Paα observations is hidden behind a dust cloud that is traced by the molecular complexes. Correcting for extinction we derive a star-forming rate of 0.05 M_⊙ yr^-1; i.e., given the reservoir in atomic and molecular gas in NGC 3077, star formation may proceed at a similar rate for a few times 10⁸ yr. The efficiency of forming stars out of molecular gas in NGC 3077 is similar to that in M82.

We present the latest results of our ongoing homogeneous cool C star survey of Local Group dwarf galaxies. We apply our two-color photometric technique to the study of two small galaxies: the Sagittarius dwarf (SagDIG) and Leo I. We identify 16 C stars in SagDIG and 13 C stars in Leo I. Even though both galaxies have a known C star population, we identify seven previously unknown C stars in each galaxy. The photometric properties of all the known C stars in each galaxy are presented. It is shown that our definition of a C star, based on our photometric criteria, produces a subset of carbon stars with homogeneous properties useful for population comparison.

We have determined the distance to M33 using the tip of the red giant branch (TRGB) and the red clump (RC), from the VI photometry of stars in 10 regions of M33 based on HST/WFPC2 images. The regions used in this study are located at R = 26–178 from the center of M33. The distance modulus to M33 obtained in this study, for an adopted foreground reddening of E(B-V) = 0.04, is (m - M)_0,TRGB = 24.81 ± 0.04(random)(systematic) from the TRGB, and (m - M)_0,RC = 24.80 ± 0.04(random) ± 0.05(systematic) from the RC, showing an excellent agreement between the two [corresponding to a distance of 916 ± 17(random) kpc and 912 ± 17(random) kpc, respectively]. These results are ≈0.3 mag larger than the Cepheid distances based on the same HST/WFPC2 data and ground-based data. This difference is considered partially due to the uncertainty in the estimates of the total reddening for Cepheids in M33.

The ambient interstellar environment of wind- and supernova-driven superbubbles strongly affects their evolution, but its properties are rarely well determined. We have therefore obtained H I aperture synthesis imaging of the environment around three similar optically selected superbubble nebulae in the Large Magellanic Cloud. The resulting H I maps show that the ambient gas distributions around these superbubbles differ to an extreme: DEM L25 shows no neutral shell component but is nestled within an H I hole; DEM L50 shows a massive neutral shell component but is otherwise within an H I void; and DEM L301 shows no correspondence at all between the optical nebula and H I distribution. There is also poor correspondence between the H I and optical kinematics. These results strongly caution against inferring properties of the ambient neutral environment of individual superbubbles without direct observations. Finally, all three objects show some evidence of shock activity.

Using the ESO 2.2 m telescope with the 8K × 8K mosaic CCD, we have surveyed 2.8 deg² (∼16 × 17) of the Small Magellanic Cloud (SMC) to search for faint planetary nebulae (PNs); 34 PNs were previously known in this central region. We identified 25 new PNs, all faint and spectroscopically confirmed. Most of these are spatially extended with typical diameters of ∼1 pc, but a few are as large as ∼3 pc. Based on the total number of PNs previously known (∼80), we can now estimate that there should be ∼139 PNs in the SMC to the limits of a survey such as this one, which is complete to 6 mag down the planetary nebulae luminosity function (PNLF). For a complete survey (8 mag down the PNLF), there should be about 216 PNs. Because no new bright PNs were found in this survey, the bright end of the PNLF remains unchanged from that reported by Jacoby, Walker, & Ciardullo. Consequently, the distance modulus to the SMC, derived using the PNLF technique, is still ∼19.1. However, a strong new feature is now evident in the PN brightness distribution that may be attributed to central stars evolving from a relatively young population. This feature may serve as an indicator of the ages of the PN progenitors. The survey spectra that were used to confirm the candidates as PNs also provide a clue to the nature of the very faint PNs. Statistically, the fainter PNs of our survey exhibit a high incidence (∼28%) of strong [N II] emission [where I([N II])/I(Hα) > 1] relative to the bright Sanduleak et al. sample (∼6%) reported by Meatheringham & Dopita, and comparable to the intermediate-brightness Jacoby sample (∼26%) reported by Boroson & Liebert. This incidence of strong [N II] is higher than in the Kingsburgh & Barlow sample (∼17%) of Galactic PNs, despite the ∼3 times higher abundance of nitrogen in the Galaxy. We propose that the very faint SMC PNs are selectively biased toward the chemically enriched Type I objects derived from younger, more massive progenitors and are partially obscured by their own dust. This brightness-dependent population change is also seen in the Large Magellanic Cloud.

We report on the discovery and analysis of 14 short-period variable stars in the field of the southern globular cluster NGC 3201, located within roughly 2 mag on either side of the main-sequence turnoff. Eleven of these variable stars are eclipsing binaries, one is an RR Lyrae, and two are thus far unclassified systems. Among the eclipsing binary stars, nine are of the W Ursae Majoris (W UMa) type, one an Algol (EA) system, and one a detached system. Using spectroscopic follow-up observations, as well as analysis of the variables' locations in the color-magnitude diagram of the cluster, we find that only one variable star (a W UMa type blue straggler) is actually a member of NGC 3201. We present the phased photometry light curves for all the variable star systems, as well as their locations in the field of view and in the color-magnitude diagram.

We present UBVI CCD photometry of the intermediate-age open cluster NGC 2516. From this new photometry we derive the cluster parameters [V₀ - M_V = 7.77 ± 0.11 mag, E(B-V) = 0.112 ± 0.024 mag], age [log (age) = 8.2 ± 0.1], and a photometric abundance ([Fe/H] = -0.10 ± 0.04) from a comparison with theoretical isochrones of the Padua group. We find that the X-ray bright stars [log L_X(ergs s^-1)⪆29.0] are systematically bluer in B-V and U-B. It was necessary to modify the photometric membership criterion used by Sung & Bessell due to the UV excess of X-ray bright stars and due to the abrupt increase in the slope of the (M_V, B-V) ZAMS relation at B-V⪆1.4. A minimum binary frequency 40% (±5%) was estimated from the distribution of distance moduli of unevolved photometric members. The photometric abundance obtained in this study is similar to that of the Pleiades and therefore the lower metallicity is not the cause of strong X-ray emission of F-type stars in NGC 2516. We believe that this results from them being close binaries, as suggested by their different distribution of distance moduli compared to other cluster members and surmise that the tidal forces between the two stars must increase their X-ray activity.

The B0 V star BD +46°3474 lies near the front surface of a dense molecular cloud and illuminates the emission/reflection nebula IC 5146. The HAeBe variable BD +46°3471 is embedded in the same cloud, about 10' (3.5 pc) away. CCD photometry in BVRI (to V = 22) and in JHK (to about K = 16.5) has been obtained for the young clusters surrounding each of these two bright stars. Some 100 emission-Hα stars brighter than R = 20.5 have been found in the area, most of them in IC 5146. (Among these are two that have spectra resembling a high-excitation Herbig-Haro [HH] object plus a stellar continuum.) A distance of 1.2 kpc follows from the photometry of several late-type IC 5146 cluster members; the average extinction from 38 stars classified spectroscopically is A_V = 3.0 ± 0.2 mag. Although optical photometry is available for 700 stars in the IC 5146 field, only about half (including all the Hα emitters) lie above the main sequence, while a substantial fraction of these are estimated to be foreground. A number of such interlopers have been identified on the basis of proper motion or abnormally low A_V. The age distribution of the Hα emitters has been estimated by reference to several sets of theoretical isochrones. There is substantial disagreement, but the median age does appear to be near 1 Myr. The spectrum of +46°3474 is unexceptional except for an unusually low v sin i (10 km s^-1), but +46°3471 has a complex emission plus absorption spectrum. Our interpretation of the structure of IC 5146 on the basis of optical and radio radial velocities follows a proposal by Roger & Irwin in 1982, namely, that +46°3474 formed near the near surface of the present cloud and evacuated a blister cavity out of which gas and dust are now flowing through a funnel-shaped volume in the approximate direction of the Sun. It is suggested that the IC 5146 cluster stars formed in a dense foreground section of the molecular cloud that was dissipated following the appearance of +46°3474.

New spectra of the Type Ic supernovae 1997B and 1997X at nebular phase are shown. Although one of them, SN 1997X, was studied with some detail around its maximum and its properties compared with other well-known SNe Ib/c, both of these events were later followed within its nebular phase. Our aim is to explore the behavior of these supernovae when they reached the nebular phase. Comparisons with data of other SNe Ic at similar nebular phases, mainly by using spectra from the Canarias Database, are also performed. The results derived from these comparisons reinforce the quite heterogeneous behavior found in SNe Ic nebular spectra. In particular, the widths of the [O I] and [Ca II] emissions found in SN 1997B are wider than the average for Type Ic events, while the [O I] width of SN 1997X is narrower than the average and is nearly comparable to typical [O I] widths found in SNe Ib.

Using the Australia Telescope Compact Array, we have carried out a survey of the H I emission in the direction of the barrel-shaped supernova remnant (SNR) G320.4-1.2 (MSH 15-52) and its associated young pulsar B1509-58. The angular resolution of the data is 40 × 27, and the rms noise is of order 30 mJy beam^-1 (∼0.5 K). The H I observations indicate that the north-northwest radio limb has encountered a dense H I filament (density ∼12 cm^-3) at the same LSR velocity as that of the SNR (V_LSR ∼ -68 km s^-1). This H I concentration would be responsible for the flattened shape of the northwestern lobe of G320.4-1.2 and for the formation of the radio-optical–X-ray nebula RCW 89. The emission associated with the bright knots in the interior of RCW 89 can be explained as arising from the interaction between the collimated relativistic outflow from the pulsar and the denser part of this H I filament (density ∼15 cm^-3). The south-southeastern half of the SNR, on the other hand, seems to have rapidly expanded across a lower density environment (density ∼0.4 cm^-3). The H I data also reveal an unusual H I feature aligned with a collimated outflow generated by the pulsar, suggestive of association with the SNR. The anomalous kinematical velocity of this feature (V_LSR ∼ 15 km s^-1), however, is difficult to explain.

We present new observations of the ionized gas, molecular gas, and cool dust in the Helix Nebula (NGC 7293). The ionized gas is observed in the form of an Hα image, which is constructed using images from the Southern Hα Sky Survey Atlas. The molecular emission was mapped using the H₂v = 1 → 0 S(1) line at 2.122 μm. The far-infrared (FIR) observations were obtained using ISOPHOT on the Infrared Space Observatory. The Hα observations are more sensitive than previous measurements and show the huge extent of the Helix, confirming it as a density-bounded nebula and showing previously unseen point-symmetric structures. The H₂ observations show that the molecular gas follows the distribution of molecular material shown in previous work. The molecular emission is confined to that part of the nebula seen in the classic optical image. Furthermore, comparison of the H₂ emission strength with time-dependent models for photodissociation regions (PDRs) shows that the emission arises from thermal excitation of the hydrogen molecules in PDRs and not from shocks. The FIR observations, at 90 and 160 μm, represent mostly contributions from thermal dust emission from cool dust grains but include a small contribution from ionized atomic lines. Comparison of the FIR emission with the Hα observation shows that the dust and ionized gas are coincident and extend to ∼1100'' radius. This equates to a spatial radial extent of more than 1 pc (assuming a distance to the Helix of ∼200 pc). Assuming that the outer layers of the circumstellar shell have spherical symmetry, radiative transfer modeling of the emission in Hα gives a shell mass of ∼1.5 M_⊙. However, the modeling does not cover the outermost part of the shell (beyond ∼600'' radius), and therefore this is a lower limit for the shell mass. Moreover, the models suggest the need for very large dust grains, with ∼80% of the dust mass in grains larger than 3.5 μm. Comparison of these new observations with previous observations shows the large-scale stratification of the Helix in terms of ionized gas and dust, as well as the coexistence of molecular species inside the ionized zones, where molecules survive in dense condensations and cometary knots.

We present new, deep Hα and [S II] images of the HH 34 jet and bow shock obtained with the Wide Field Planetary Camera 2 on board the Hubble Space Telescope (HST), which reveal the structure of this fine HH flow with unprecedented detail. Many of the knots in the jet appear to form small working surfaces with bright [S II] cores and thin Hα filaments where the mini–bow shocks extend into the surrounding medium. In combination with earlier, short-exposure HST images we have determined very precise proper-motion vectors for the various shock structures in the outflow. The jet becomes visible within about an arcsecond of the source, where a new knot has emerged between our two epoch images; it has a space velocity of at least 300 km s^-1, as derived from the proper motions and correcting for the 30° angle of the flow to the line of sight. The jet rapidly slows down to a mean space velocity of about 220 km s^-1, with a standard deviation of 20 km s^-1 among the jet knots. Such low internal velocities lead to weak shocks, consistent with the high [S II]/Hα ratio along the jet body and in accordance with the internal working surface model for jets. The jet motion appears to be ballistic, with no evidence for a turbulent boundary layer. The jet is well resolved and steadily expands with a half-opening angle of 04. The large HH 34 working surface shows a multitude of knots, all of which are enveloped by a series of very thin, limb-brightened Hα-emitting filaments immediately behind the shock front where the flow faces into the preshock medium. One of these filaments developed four regularly spaced tiny knots between the two epochs, possibly due to a Rayleigh-Taylor instability along the filament or caused by the presence of small, dense clumps in the ambient medium. Proper motions of the HH 34 working surface show an obvious expansion due to material being squirted sideways. In addition to the large-scale S-shaped symmetry of the giant HH 34 flow, the jet shows a marked and surprisingly abrupt change in flow direction during a 65 yr interval that ended 10 yr ago, suggesting that the jet-disk system may have been influenced by powerful tidal effects by a companion star during a recent periastron passage. A second, smaller bowlike flow, called HH 534, possibly emanates from the HH 34 source region, and if so this supports the contention that the source is a binary. This data set is a testament to the unique abilities of the HST to follow morphological, photometric, and excitation changes on cooling timescales in the shocks of flows from young stars.

Interstellar gas comes with empirical physical relations governing mean parameters such as the gas density n, diameter D, magnetic field B, and gas line width σ, with the forms ⟨n⟩ ∼ D^c, ⟨B⟩ ∼ ⟨D⟩^p, ⟨B⟩ ∼ ⟨n⟩^k, ⟨σ⟩ ∼ ⟨D⟩^q, and p = ck. Using these results, I derive simple formulae for the energy components in turbulent young stellar objects of Classes 0 and I as a function of clump size D (D < 0.5 pc) from empirical fits to observations. Here I find the relative importance of the energy components in molecular clumps, each one of the form ⟨E⟩ ∼ D^x, with x ranging from 0 to 3. Magnetic and gravitational energies are more important for small D (<0.1 pc), while for large D (>0.1 pc) turbulent and gravitational energies become dominant (x = 2). In addition, the expected relationship between turbulent width and magnetic field, ⟨σ⟩ ∼ ⟨B⟩^q/p, is not steep, and the observational data to date bear this out. Damping scenarios, magnetic field shapes, and ambipolar diffusion relevant to observations are discussed.

We present VRI photometry of 320,917 stars with 11 ≲ R ≲ 18 throughout the λ Ori star-forming region. Using the more spatially limited spectroscopic surveys of Dolan & Mathieu to define the color-magnitude domain of young low-mass members of the association and removing statistically the field stars in this domain, we use our photometry to identify a representative pre–main-sequence (PMS) population throughout the interior of the molecular ring. The spatial distribution of this population shows a concentration of PMS stars around λ Ori and in front of the B35 dark cloud. However, few PMS stars are found outside these pockets of high stellar density, suggesting that star formation was concentrated in an elongated cloud extending from B35 through λ Ori to the B30 cloud. We find a lower limit for the global stellar mass of about 500 M_⊙. We find that the global ratio of low- to high-mass stars is similar to that predicted by the field initial mass function, but this ratio varies strongly as a function of position in the star-forming region. Locally, the star formation process does not produce a universal initial mass function. Using our derived stellar ages across the region, we construct a history of the star-forming complex. This history incorporates a recent supernova to explain the distribution of stars and gas today. We infer that most of the present molecular ring was formed by ejecta from the center driven by the supernova blast about 1 Myr ago. However, we suggest that the B30 and B35 clouds were primordial, and massive enough to be mostly little disturbed by the shock. The stars that we see today trace the former extent of the cloud complex. Given the kinematics of the stellar population, we predict that the association will disperse into the field within a few tens of megayears.

As an investigation of the origin of "α-poor" halo stars, we analyze kinematic and abundance data for 73 intermediate-metallicity stars (-1 > [Fe/H] ≥ -2) selected from Paper I of this series. We find evidence for a connection between the kinematics and the enhancement of certain element-to-iron ([X/Fe]) ratios in these stars. Statistically significant correlations were found between [X/Fe] and galactic rest-frame velocities (v_RF) for Na, Mg, Al, Si, Ca, and Ni, with marginally significant correlations existing for Ti and Y as well. We also find that the [X/Fe] ratios for these elements all correlate with a similar level of significance with [Na/Fe]. Finally, we compare the abundances of these halo stars against those of stars in nearby dwarf spheroidal (dSph) galaxies. We find significant differences between the abundance ratios in the dSph stars and halo stars of similar metallicity. From this result, it is unlikely that the halo stars in the solar neighborhood, including even the "α-poor" stars, were once members of disrupted dSph galaxies similar to those studied to date.

YY Draconis (likely the same variable called DO Draconis) is one of a small number of intermediate polars (IPs) that show outburst behavior. We report results from Target of Opportunity observations with the Rossi X-Ray Timing Explorer, together with ground-based optical photometry during outbursts in 1999 September and 2000 November. Similar behavior was evident in both outbursts. At outburst, the X-ray flux increased by more than a factor of 12, and the spectrum became hotter and more absorbed compared to quiescence. While the spin pulse at 529 s is clearly present in the X-ray data at 2–4 days past outburst peak and during quiescence, it was not detected in the X-ray data closest to outburst (1.5 days). This is contrary to the large increase in spin pulse amplitude that has been seen during outbursts of the IPs GK Per, XY Ari, and EX Hya. The differences in YY Dra are likely due to its unique geometry, with two relatively equal poles located near the white dwarf equator. The equal enhancement of both poles near outburst could account for the low pulse amplitude, while unequal feeding of the poles as the magnetosphere recedes during decline could explain the changes in amplitude and pulse shape. The changing height of the shocks may also have an effect on the visibility of both poles.

We present simultaneous EUV and infrared (J, K) observations of the polar HU Aquarii obtained during 1998 August when the star was in a high mass accretion state. EUV and IR light curves and EUV spectra are presented and compared with previous observations. The accretion region on the white dwarf has increased in temperature (124,000 to 240,000 K) and radius (0.04R_WD to 0.06R_WD) compared with previous EUV observations made during low mass accretion states. The EUV and IR photometric observations are shown to have a similar appearance as a function of orbital phase. The EUV photometry shows rapid changes and provides evidence for mass accretion via blobs. The high-state IR light curves present an asymmetric double-humped shape with J = 14.8 and K = 14.1. We applied an ellipsoidal model fit to the observations, and the result indicates that the cause of the modulated shape is due to both ellipsoidal variations from the Roche lobe filling secondary star and a complex flux combination probably dominated at all orbital phases by cyclotron emission. The source of maximum cyclotron emission appears to be in the accretion column high above the white dwarf surface.

The commissioning year of the Sloan Digital Sky Survey (SDSS) has demonstrated that many cataclysmic variables (CVs) have been missed in previous surveys with brighter limits. We report the identification of 22 CVs, of which 19 are new discoveries and three are known systems (SW UMa, BH Lyn, and OU Vir). A compendium of positions, colors, and characteristics of these systems obtained from the SDSS photometry and spectroscopy is presented, along with data obtained during follow-up studies with the Apache Point Observatory and Manastash Ridge Observatory telescopes. We have determined orbital periods for three of the new systems, two show dwarf nova outbursts, and the third is a likely magnetic system with eclipses of its region of line emission. Based on these results, we expect the completed survey to locate at least 400 new CVs. Most of these will be faint systems with low accretion rates that will provide new constraints on binary evolution models.

CCD photometric observation of the short-period eclipsing binary V432 Persei was carried out in the B and V bands at the Yunnan Observatory of China. The light curves of the system are obviously asymmetrical, with the primary maximum brighter than the secondary maximum, which is known as the O'Connell effect. The presented light curves are analyzed by means of the latest version of the Wilson-Devinney program. A grid of solutions for several fixed values of the mass ratio was calculated. The best fitting possible is for a mass ratio of 0.269 and a low degree of contact. The results show that V432 Per is seen to be a W-subtype W Ursae Majoris contact binary in poor thermal contact. The difference between the mean temperatures of the components is about 850 K. The asymmetry of the light curves is explained by a cool spot on the secondary component. The nature of the overluminosity of the secondary of the system suggests that there should be very great energy transfer from the primary to the secondary.

Year- to decade-long cyclic orbital period changes have been observed in several classes of close binary systems, including Algol, W Ursae Majoris, and RS Canum Venaticorum systems and the cataclysmic variables. The origin of these changes is unknown, but mass loss, apsidal motion, magnetic activity, and the presence of a third body have all been proposed. In this paper, we use new CCD observations and the century-long historical record of the times of primary eclipse for WW Cygni to explore the cause of these period changes. WW Cyg is an Algol binary whose orbital period undergoes a 56 yr cyclic variation with an amplitude of ≈0.02 days. We consider and reject the hypotheses of mass transfer, mass loss, apsidal motion, and the gravitational influence of an unseen companion as the cause for these changes. A model proposed by Applegate, which invokes changes in the gravitational quadrupole moment of the convective and rotating secondary star, is the most likely explanation of this star's orbital period changes. This finding is based on an examination of WW Cyg's residual O-C curve and an analysis of the period changes seen in 66 other Algols. Variations in the gravitational quadrupole moment are also considered to be the most likely explanation for the cyclic period changes observed in several different types of binary systems.

Low-dispersion optical spectra have been obtained with the Hobby-Eberly Telescope of 22 very red objects found in early imaging data from the Sloan Digital Sky Survey (SDSS). The objects are assigned spectral types on the Two Micron All Sky Survey (2MASS) system and are found to range from late M to late L. The red and near-infrared colors from SDSS and 2MASS correlate closely with each other, and most of the colors are closely related to spectral type in this range; the exception is the i*-z* color, which appears to be independent of spectral type between about M7 and L4. The spectra suggest that this independence is due to the disappearance of the TiO and VO absorption in the i band for later spectral types, the presence of strong Na I and K I absorption in the i band, and the gradual disappearance of the 8400 Å absorption of TiO and FeH in the z band.

We present narrowband M photometry of nine low-mass dwarfs with spectral types ranging from M2.5 to L0.5. Combining the (L'-M') colors derived from our observations with data from the literature, we find colors consistent with a Rayleigh-Jeans flux distribution for spectral types earlier than M5, but enhanced F_3.8/F_4.7 flux ratios (negative [L'-M'] colors) at later spectral types. This probably reflects increased absorption at M' due to the CO fundamental band. We compare our results against recent model predictions and briefly discuss the implications.

We present an absolute parallax and relative proper motion for the fundamental distance scale calibrator, RR Lyrae. We obtain these with astrometric data from FGS 3, a white-light interferometer on the Hubble Space Telescope. We find π_abs = 3.82 ± 0.2 mas. Spectral classifications and VRIJHKT₂M and DDO 51 photometry of the astrometric reference frame surrounding RR Lyr indicate that field extinction is low along this line of sight. We estimate ⟨A_V⟩ = 0.07 ± 0.03 for these reference stars. The extinction suffered by RR Lyr becomes one of the dominant contributors to the uncertainty in its absolute magnitude. Adopting theaverage field absorption ⟨A_V⟩ = 0.07 ± 0.03, we obtain M = 0.61. This provides a distance modulus for the Large Magellanic Cloud (LMC) of m - M = 18.38–18.53, with the average extinction-corrected magnitude of RR Lyrae variables in the LMC, ⟨V(RR)⟩, remaining a significant uncertainty. We compare this result with more than 80 other determinations of the distance modulus of the LMC.

The Sloan Digital Sky Survey (SDSS) is an imaging and spectroscopic survey that will eventually cover approximately one-quarter of the celestial sphere and collect spectra of ≈10⁶ galaxies, 100,000 quasars, 30,000 stars, and 30,000 serendipity targets. In 2001 June, the SDSS released to the general astronomical community its early data release, roughly 462 deg² of imaging data including almost 14 million detected objects and 54,008 follow-up spectra. The imaging data were collected in drift-scan mode in five bandpasses (u, g, r, i, and z); our 95% completeness limits for stars are 22.0, 22.2, 22.2, 21.3, and 20.5, respectively. The photometric calibration is reproducible to 5%, 3%, 3%, 3%, and 5%, respectively. The spectra are flux- and wavelength-calibrated, with 4096 pixels from 3800 to 9200 Å at R ≈ 1800. We present the means by which these data are distributed to the astronomical community, descriptions of the hardware used to obtain the data, the software used for processing the data, the measured quantities for each observed object, and an overview of the properties of this data set.

We present Hubble Space Telescope Faint Object Camera observations of the asteroid 1 Ceres at near-, mid-, and far-UV wavelengths (λ = 3636, 2795, and 1621 Å, respectively) obtained on 1995 June 25. The disk of Ceres is well resolved for the first time, at a scale of ∼50 km. We report the detection of a large, ∼250 km diameter surface feature for which we propose the name "Piazzi"; however, it is presently uncertain whether this feature is due to a crater, albedo variegation, or other effect. From limb fits to the images, we obtainsemimajor and semiminor axes of R₁ = 484.8 ± 5.1 km and R₂ = 466.4 ± 5.9 km, respectively, for the illumination-corrected projected ellipsoid. Although albedo features are seen, they do not allow for a definitive determination of the rotational motion and pole position of Ceres, particularly because of the sparse sampling (two epochs) of the 9 hr rotation period. From full-disk integrated albedo measurements, we find that Ceres has a red spectral slope from the mid- to near-UV, and a significant blue slope shortward of the mid-UV. Despite the presence of Piazzi, we detect no significant global differences in the integrated albedo as a function of rotational phase for the two epochs of data we obtained. From Minnaert surface fits to the near- and mid-UV images, we find an unusually large Minnaert parameter of k ≈ 0.9, suggesting a more Lambertian than lunar-like surface.