Table of contents

We report Keck LRIS observations of an arclike background galaxy near the center of Abell 2152 (z = 0.043), one of the three clusters comprising the Hercules supercluster. The background object has a redshift z = 0.1423 and is situated 25'' north of the primary component of the A2152 brightest cluster galaxy (BCG). The object is about 15'' in total length and has a reddening-corrected R-band magnitude of m_R = 18.55 ± 0.03. Its spectrum shows numerous strong emission lines, as well as absorption features. The strength of the Hα emission would imply a star formation rate SFR ≈ 3 h^-2M_⊙ yr^-1 in the absence of any lensing. However, the curved shaped of this object and its tangential orientation along the major axis of the BCG suggest lensing. We model the A2152 core mass distribution including the two BCG components and the cluster potential. We present velocity and velocity dispersion profile measurements for the two BCG components and use these to help constrain the potential. The lens modeling indicates a likely magnification factor of ∼1.9 for the lensed galaxy, making A2152 the nearest cluster in which such significant lensing of a background source has been observed. Finally, we see evidence for a concentration of early-type galaxies at z = 0.13 near the centroid of the X-ray emission previously attributed to A2152. We suggest that emission from this background concentration is the cause of the offset of the X-ray center from the A2152 BCG. The background concentration and the dispersed mass of the Hercules supercluster could add further to the lensing strength of the A2152 cluster.

We present an estimation of the expected number of arcs and arclets in a survey of nearby (z < 0.1) clusters of galaxies, which takes into account the observational constraints. We show that strong lensing effects are not common, but also that they are not as rare as usually stated. Indeed, for a given cluster, the predicted number of arcs strongly depends on the magnitude limit of the survey and the actual value of the seeing. We also describe the procedures and results of a search for gravitational arcs and arclets in a sample of 33 galaxy clusters, representative of the local cluster distribution and spanning the redshift range of 0.014 < z < 0.076. Only one new arc candidate was discovered, located ∼3 arcmin away from the center of the cluster Abell 3266 (z = 0.059), whose redshift was found to be z = 0.073. The assumption that a mass concentration around a bright cluster elliptical 166 away from the arc candidate could explain this arc as a gravitational image requires the presence of a very massive substructure in this cluster (M/L_V ∼ 163 M_⊙/L_⊙). Although such substructure cannot be completely discarded, the nature of the arc candidate most possibly corresponds to a disk galaxy belonging to A3266. Our simplified theoretical model predicts that the probability of finding at least one gravitational arc in the sample investigated is 25%, in good agreement with the actual number of arcs found, N = 1 or 0, either adopting the arc in A3266 as a gravitational image or not. We conclude that a deep survey of the central regions of low-z galaxy clusters, with very good image quality, should allow significant progress in the mapping of their central mass concentration.

Two new samples of QSOs have been constructed from recent surveys to test the hypothesis that the redshift distribution of bright QSOs is periodic in log (1 + z). The first of these comprises 57 different redshifts among all known close pairs or multiple QSOs with image separations ≤10'', and the second consists of 39 QSOs selected through their X-ray emission and their proximity to bright, comparatively nearby active galaxies. The redshift distributions of the samples are found to exhibit distinct peaks with a periodic separation of ∼0.089 in log (1 + z) identical to that claimed in earlier samples, but now extended out to higher redshift peaks z = 2.63, 3.45, and 4.47, predicted by the formula but never seen before. The periodicity is also seen in a third sample, the 78 QSOs of the 3C and 3CR catalogs. It is present in these three data sets at a high overall significance level (∼10^-5) and appears not to be explicable by spectroscopic or similar selection effects. Possible interpretations are briefly discussed.

This is the third paper in a series aimed at finding high-redshift quasars from five-color (u'g'r'i'z') imaging data taken along the celestial equator by the Sloan Digital Sky Survey (SDSS) during its commissioning phase. In this paper, we first present the observations of 14 bright, high-redshift quasars (3.66 ≤ z ≤ 4.77, i* ≲ 20) discovered in the SDSS fall equatorial stripe, and the SDSS photometry of two previously known high-redshift quasars in the same region of the sky. Combined with the quasars presented in Paper I and by Schneider et al., we define a color-selected flux-limited sample of 39 quasars at 3.6 < z < 5.0 and i* ≲ 20, covering a total effective area of 182 deg². From this sample, we estimate the average spectral power-law slope in the rest-frame UV for quasars at z ∼ 4 to be -0.79 with a standard deviation of 0.34, and the average rest-frame equivalent width of the Lyα + N V emission line to be 69 Å with a standard deviation of 18 Å. The selection completeness of this multicolor sample is determined from the model colors of high-redshift quasars, taking into account the distributions of emission-line strengths, intrinsic continuum slope, the line and continuum absorption from intervening material, and the effects of photometric errors. The average completeness of this sample is about 75%. The selection function calculated in this paper will be used to correct the incompleteness of this color-selected sample and to derive the high-redshift quasar luminosity function in a companion paper. In an appendix, we present the observations of an additional 18 faint quasars (3.57 ≤ z ≤ 4.80, 20.1 < i* < 20.8) discovered in the region on the sky that has been imaged twice. Several quasars presented in this paper exhibit interesting properties, including a radio-loud quasar at z = 4.77 and a narrow-line quasar (FWHM = 1500 km s^-1) at z = 3.57.

This is the fourth paper in a series aimed at finding high-redshift quasars from five-color (u'g'r'i'z') imaging data taken along the celestial equator by the Sloan Digital Sky Survey during its commissioning phase. In this paper, we use the color-selected sample of 39 luminous high-redshift quasars presented in Paper III to derive the evolution of the quasar luminosity function over the range 3.6 < z < 5.0 and -27.5 < M₁₄₅₀ < -25.5 (Ω = 1, H₀ = 50 km s^-1 Mpc^-1). We use the selection function derived in Paper III to correct for sample incompleteness. The luminosity function is estimated using three different methods: (1) the 1/V_a estimator; (2) a maximum likelihood solution, assuming that the density of quasars depends exponentially on redshift and as a power law in luminosity; and (3) Lynden-Bell's nonparametric C^- estimator. All three methods yield consistent results. The luminous quasar density decreases by a factor of ∼6 from z = 3.5 to z = 5.0, consistent with the decline seen from several previous optical surveys at z < 4.5. The luminosity function follows ψ(L) ∝ L^-2.5 for z ∼ 4 at the bright end, significantly flatter than the bright-end luminosity function ψ(L) ∝ L^-3.5 found in previous studies for z < 3, suggesting that the shape of the quasar luminosity function evolves with redshift as well, and that the quasar evolution from z = 2 to z = 5 cannot be described as pure luminosity evolution. Possible selection biases and the effect of dust extinction on the redshift evolution of the quasar density are also discussed.

The KPNO International Spectroscopic Survey (KISS) is a new objective-prism survey for extragalactic emission-line objects. It combines many of the features of previous slitless spectroscopic surveys with the advantages of modern CCD detectors and is the first purely digital objective-prism survey for emission-line galaxies. Here we present the first list of emission-line galaxy candidates selected from our red spectral data, which cover the spectral range 6400 to 7200 Å. In most cases, the detected emission line is Hα. The current survey list covers a 1°-wide strip located at δ = 29°30' (B1950.0) and spanning the right ascension range 12^h15^m to 17^h0^m. An area of 62.2 deg² is covered. A total of 1128 candidate emission-line objects have been selected for inclusion in the survey list (18.1 deg^-2). We tabulate accurate coordinates and photometry for each source, as well as estimates of the redshift and emission-line flux and equivalent width based on measurements of the digital objective-prism spectra. The properties of the KISS emission-line galaxies are examined using the available observational data.

We report the first detection of emission-line gas within the host galaxies of high-redshift quasi-stellar objects (QSOs). This was done using narrowband imaging at the redshifted wavelengths of [O III] and Hα, using the PUEO adaptive optics camera of the Canada-France-Hawaii Telescope. The QSOs are all radio-quiet or very compact radio sources. In all five observed QSOs, which have redshifts 0.9 to 2.4, we find extended line emission that lies within 05 (a few kiloparsecs) of the nucleus. The emission (redshifted) equivalent widths range from 35 to 300 Å. Where there is radio structure, the line emission is aligned with it. We also report on continuum fluxes and possible companions. Two of the QSOs are very red and have high resolved continuum flux.

BL Lac was observed in the V and R bands on 11 nights in 1997 July during a major optical outburst. It varied continuously, exhibiting nightly variations ranging from 0.1 to 0.6 mag. Over the course of the 11 nights, BL Lac was seen to vary by 1.8 R magnitudes. Color variations were also observed, with BL Lac becoming bluer as it brightened. The influence of variable seeing conditions on the observed variations was investigated. No correlation was found between the observed microvariability and the local seeing conditions.

We present near-infrared spectroscopy for a complete sample of 33 ultraluminous infrared galaxies at a resolution of R ≈ 1000. Most of the wavelength range from 1.80–2.20 μm in the rest frame is covered, including the Paα and Brγ hydrogen recombination lines, and the molecular hydrogen vibration-rotation 1–0 S(1) and S(3) lines. Other species, such as He I, [Fe II], and [Si VI] appear in the spectra as well, in addition to a number of weaker molecular hydrogen lines. Nuclear extractions for each of the individual galaxies are presented here, along with spectra of secondary nuclei, where available. The Paα emission is seen to be highly concentrated on the nuclei, typically with very little emission extending beyond a radius of 1 kpc. This survey was carried out primarily to search for signatures of active nuclei via velocity-broadened hydrogen recombination or the presence of the [Si VI] coronal line. These signatures are rare in the present sample, occurring in only two of the 33 galaxies. The extinction to the hydrogen recombination lines is investigated via the Paα/Brγ line ratio. It is found that visual extinctions to the nuclei in excess of 10 mag are relatively common among ULIRGs and that visual extinctions greater than 25 mag are necessary to conceal a QSO emitting half the total bolometric luminosity. The ionized hydrogen regions in many ULIRGs are sufficiently obscured that dust-enshrouded active galactic nuclei would remain hidden at 2 μm at the current level of sensitivity. The vibration-rotation lines of molecular hydrogen appear to be predominantly thermal in origin, with effective temperatures generally around 2200 K. The relative nuclear velocities between double nucleus ULIRGs are investigated, through which it is inferred that the maximum deprojected velocity difference is ∼200 km s^-1. This figure is lower than the velocities predicted by physical models of strong interactions/mergers of large, gas-rich galaxies.

It was once common to regard Seyfert and starburst galaxies as completely different types of object, but there is growing recognition that these classifications refer to the extremes of a continuous spectrum of galaxy types. In a previous study we investigated a sample of galaxies with ambiguous optical emission-line ratios and concluded from near-infrared spectroscopic observations that the sample consisted of composite galaxies, containing both a starburst and an active galactic nucleus (AGN). We now extend our study using radio synthesis and long-baseline interferometer observations made with the Australia Telescope, together with far-infrared IRAS observations, to discuss the relative contribution of starburst and AGN components to the overall luminosity of the composite galaxies. We find that only a small fraction of the radio emission (<10%) can be attributed to an AGN and that the majority of the far-infrared emission (>90%) is probably due to the starburst component. We also show that an AGN contribution to the optical emission of as little as 10% is sufficient to account for the ambiguous line-ratio diagnostics.

We measure the frequency of bulk gas-stellar counterrotation in a sample of 67 galaxies drawn from the Nearby Field Galaxy Survey, a broadly representative survey of the local galaxy population down to M_B ∼ -15. We detect four counterrotators among 17 E/S0's with extended gas emission (24 %). In contrast, we find no clear examples of bulk counterrotation among 38 Sa–Sbc spirals, although one Sa does show peculiar gas kinematics. This result implies that, at 95% confidence, no more than 8% of Sa–Sbc spirals are bulk counterrotators. Among types Sc and later, we identify only one possible counterrotator, a Magellanic irregular. We use these results together with the physical properties of the counterrotators to constrain possible origins for this phenomenon.

We present V and I CCD photometry of suspected low surface brightness dwarf galaxies detected in a survey covering ∼2.4 deg² around the central region of the Dorado group of galaxies. The low surface brightness galaxies were chosen based on their sizes and magnitudes at the limiting isophote of 26 Vμ. The selected galaxies have magnitudes brighter than V ≈ 20 (M_V ≈ -11 for an assumed distance to the group of 17.2 Mpc), with central surface brightnesses μ₀ > 22.5 V mag arcsec^-2, scale lengths h > 2'', and diameters ≥14'' at the limiting isophote. Using these criteria, we identified 69 dwarf galaxy candidates. Four of them are large very low surface brightness galaxies that were detected on a smoothed image, after masking high surface brightness objects. Monte Carlo simulations performed to estimate completeness, photometric uncertainties and to evaluate our ability to detect extended low surface brightness galaxies show that the completeness fraction is, on average, >80% for dwarf galaxies with -17 < M_V < -10.5 and 22.5 < μ₀ < 25.5 V mag arcsec^-2, for the range of sizes considered by us (D ≥ 14''). The V-I colors of the dwarf candidates vary from -0.3 to 2.3 with a peak on V-I = 0.98, suggesting a range of different stellar populations in these galaxies. The projected surface density of the dwarf galaxies shows a concentration toward the group center similar in extent to that found around five X-ray groups and the elliptical galaxy NGC 1132 studied by Mulchaey & Zabludoff, suggesting that the dwarf galaxies in Dorado are probably physically associated with the overall potential well of the group.

We present a spectrophotometric study based on VLT/FORS I observations of one of the most metal-deficient blue compact dwarf (BCD) galaxies known, Tol 1214-277 (Z ∼ Z_⊙/25). The data show that roughly half of the total luminosity of the BCD originates from a bright and compact starburst region located at the northeastern tip of a faint dwarf galaxy with cometary appearance. The starburst has ignited less than 4 Myr ago and its emission is powered by several thousand O7V stars and ∼170 late-type nitrogen Wolf-Rayet stars located within a compact region with ≲500 pc in diameter. For the first time in a BCD, a relatively strong [Fe V] λ4227 emission line is seen which together with intense He II λ4686 emission indicates the presence of a very hard radiation field in Tol 1214-277. We argue that this extraordinarily hard radiation originates from both Wolf-Rayet stars and radiative shocks in the starburst region. The structural properties of the low surface brightness (LSB) component underlying the starburst have been investigated by means of surface photometry down to 28 B mag arcsec^-2. We find that, for a surface brightness level fainter than ∼24.5 B mag arcsec^-2, an exponential fitting law provides an adequate approximation to its radial intensity distribution. The broadband colors in the outskirts of the LSB component of Tol 1214-277 are nearly constant and are consistent with an age below one Gyr. This conclusion is supported by the comparison of the observed spectral energy distribution (SED) of the LSB host with theoretical SEDs.

Hubble Space Telescope observations in U, B, V, and I passbands of the interacting spiral galaxies IC 2163 and NGC 2207 are used to measure extinctions in the cloud and intercloud regions and ages and luminosities of the star-forming regions. The extinction in the part of NGC 2207 seen in projection against IC 2163 was determined by using the method of White & Keel. The extinctions there and elsewhere were also determined from radiative transfer models of the magnitude differences between clouds and their surroundings. The intercloud extinction in V band ranges from 0.5 to 1 mag on the line of sight, and the cloud extinction ranges from 1 to 2 mag. The measured star-forming regions in these galaxies have a power-law relation between size and luminosity and a power-law luminosity distribution function. These power laws are consistent with a fractal dimension for the star formation that is the same as that for interstellar gas, D ∼ 2.2, extending over scales ranging from 20 to 1000 pc. Fifteen compact massive star clusters that are analogous to super–star clusters found in starburst regions are in the spiral arms of NGC 2207. Nothing is peculiar about these regions except for a high H I velocity dispersion (∼50 km s^-1). Two more super–star clusters are in the tidally compressed oval of IC 2163. These clusters have masses ranging from ∼10⁴ to 2 × 10⁵M_⊙ and ages of a few times 10⁶ yr.

We report on a detailed kinematic study of the galactic-scale outflow in the Seyfert galaxy NGC 2992. The TAURUS-2 imaging Fabry-Perot interferometer was used on the 3.9 m Anglo-Australian Telescope to derive the two-dimensional velocity field of the Hα-emitting gas over the central arcminute of NGC 2992. The complete two-dimensional coverage of the data combined with simple kinematic models of rotating axisymmetric disks allow us to differentiate the outflowing material from the line-emitting material associated with the galactic disk. The kinematics of the disk component out to R = 3.0 kpc are well modeled by pure circular rotation in a plane inclined at i = 68° ± 3° from the plane of the sky and with kinematic major axis along P.A. = 32° ± 3°. The outflow component is distributed into two wide cones with opening angle ≈125°–135° and extending ∼2.8 kpc (18'') on both sides of the nucleus at nearly right angles (ϕ ≈ 116° ± 5°) to the disk kinematic major axis. The outflow on the southeastern side of the nucleus is made of two distinct kinematic components interpreted as the front and back walls of a cone. The azimuthal velocity gradient in the back-wall component reflects residual rotational motion, which indicates either that the outflowing material was lifted from the disk or that the underlying galactic disk is contributing slightly to this component. A single outflow component is detected in the northwestern cone. A biconical outflow model with velocities ranging from 50 to 200 km s^-1 and oriented nearly perpendicular to the galactic disk can explain the data. The broad-line profiles and asymmetries in the velocity fields suggest that some of the entrained line-emitting material may lie inside the biconical structure rather than only on the surface of the bicone. The mass involved in this outflow is of order ∼1 × 10⁷nM_⊙, and the bulk and "turbulent" kinematic energies are ∼6 × 10⁵³n ergs and ∼3 × 10⁵⁴n ergs, respectively. The most likely energy source is a hot, bipolar, thermal wind powered on a subkiloparsec scale by the active galactic nucleus and diverted along the galaxy's minor axis by the pressure gradient of the ISM in the host galaxy. The data are not consistent with a starburst-driven wind or a collimated outflow powered by radio jets.

We have undertaken a study of the globular cluster (GC) systems of a large sample of elliptical and spiral galaxies in order to test specific predictions of galaxy formation models. Here we present results for the first elliptical in the study, the giant Virgo cluster galaxy NGC 4472 (M49). The galaxy was observed in three filters (BVR) using the wide-field Mosaic Imager on the Kitt Peak 4 m telescope. The Mosaic images roughly double the radial coverage of previous CCD observations of NGC 4472. We have combined the Mosaic data with published spectroscopic data and archival HST observations in order to study NGC 4472's GC system in detail, and to fully characterize the amount of contamination in our sample of GC candidates. We find that the radial profile of the GC system is fit fairly well by a de Vaucouleurs law of the form log σ_GC = (3.38 ± 0.08) - (1.56 ± 0.05) r^1/4 out to 17' (∼80 kpc), but that the observed profile falls slightly below the de Vaucouleurs law between 17' and 23', the limit of the data. The bimodal color distribution observed in previous studies is apparent in our data. We find a small metallicity gradient in the inner 8' of the total GC system due to the increasing ratio of blue to red clusters, consistent with results from past studies. The gradient vanishes, however, when the entire radial extent of the data is taken into account. We estimate a total of ∼5900 GCs in NGC 4472 out to 23', yielding a specific frequency of 3.6 ± 0.6 within this radius. This specific frequency value is smaller than that found by previous studies of NGC 4472. We examine the implications of these results with regard to predictions made by four different galaxy formation models, and we find that all four models have at least one inconsistency with the data.

This paper presents new near-infrared images and surface photometry and Hα Fabry-Perot radial velocities for NGC 1433 and NGC 6300, two large and previously well-studied nearby ringed barred spiral galaxies. Stellar absorption-line radial velocities and new optical surface photometry are also presented for NGC 1433. The data are intended to set the stage for dynamical modeling of the two galaxies, with the principal goals being to derive mass distributions, bar mass-to-light ratios, bar pattern speeds, and resonance locations, parameters which have been derived for very few ringed disk galaxies. The presence of strong rings and pseudorings in the two galaxies allows the possibility to link specific orbital resonances with the observed rings. The new data allow us to derive a fairly complete composite rotation curve of NGC 1433, leading us to make some interesting preliminary judgments about the structure of the galaxy. The new data also verify the previous finding that the radial velocity of the Seyfert nucleus in NGC 6300 differs from the actual systemic velocity of the galaxy by nearly 100 km s^-1. We demonstrate in this paper that the offset is not an artifact of significant extinction in the inner regions of the galaxy.

The intrinsic shape and orientation of the elliptical galaxy NGC 3379 are estimated by dynamical modeling. The maximal-ignorance shape estimate, an average over the parameter space, is axisymmetric and oblate in the inner parts, with an outward triaxiality gradient. The 1 σ limits on total mass triaxiality T are T < 0.13 at 0.33 kpc and T = 0.08 ± 0.07 at 3.5 kpc from the center. The luminous short-to-long axis ratio c_L = 0.79 inside 0.82 kpc, flattening to c_L = 0.66 at 1.9 kpc. The results are similar if the galaxy is assumed to rotate about its short axis. Estimates for c_L are robust, but those for T are dependent on whether the internal rotation field is disklike or spheroid-like. Short-axis inclinations i between 30° and 50° are preferred for nearly axisymmetric models, but triaxial models in high inclination are also allowed, which can affect central black hole mass estimates. The available constraints on orientation rule out the possibility that the nuclear dust ring at R ≈ 15 is in a stable equilibrium in one of the galaxy's principal planes. The ring is thus a decoupled nuclear component not linked to the main body of the galaxy. It may be connected with ionized gas that extends to larger radii, since the projected gas rotation axis is near the minor axis of the ring. The gas and dust may both be part of a strongly warped disk; however, if caused by differential precession, the warp will wind up on itself in a few times 10⁷ yr. The decoupling with the stellar component suggests that the gas has an external origin, but no obvious source is present.

Thirty-eight long-period (P > 10 days) apparently contact binary stars discovered by the OGLE-II project in the SMC show EB-type light curves and an "inverted" period-color relation with longer orbital periods for redder systems. The strong light variations between eclipses can be explained within a semidetached model in which ellipsoidal variations of a large, evolved, Roche lobe–filling component dominates over eclipse effects in the systemic light changes. The model requires further spectroscopic and color-curve support before it can be fully accepted. It is noted that the dominant role of the Roche lobe–filling component in the total systemic luminosity can explain the new period-luminosity-color (PLC) relation, which has been established for the long-period EB (LP–EB) systems. We call it the PLC-β relation, to distinguish it from the Cepheid relation. Two versions of the PLC-β relation—based on the (B-V)₀ or (V-I)₀ color indices—have been calibrated for 33 systems with (V-I)₀ > 0.25 spanning the orbital period range of 11 to 181 days (it was found that blue systems with (V-I)₀ ≤ 0.25 do not follow the same calibration). The relations can provide maximum-light, absolute-magnitude estimates accurate to M_V ≃ 0.35 mag within the approximate range -3 < M_V < +1. In terms of their number in the SMC, the LP–EB binaries are about 50 times less common than the Cepheids. Nevertheless, their large luminosities coupled with continuous light variations make these binaries very easy to spot in nearby galaxies, so that the PLC-β relation can offer an auxiliary and entirely independent method of distance determination to nearby stellar systems rich in massive stars.

Observations of the interstellar and circumstellar absorption components obtained with the Hubble Space Telescope Space Telescope Imaging Spectrograph (STIS) along the line of sight toward the Wolf-Rayet–luminous blue variable (LBV) system HD 5980 in the Small Magellanic Cloud are analyzed. Velocity components from C I, C I*, C II, C II*, C IV, N I, N V, O I, Mg II, Al II, Si II, Si II*, Si III, Si IV, S II, S III, Fe II, Ni II, Be I, Cl I, and CO are identified, and column densities estimated. The principal velocity systems in our data are (1) interstellar medium (ISM) components in the Galactic disk and halo (V_hel = 1.1 ± 3, 9 ± 2 km s^-1); (2) ISM components in the SMC (V_hel = +87 ± 6, +110 ± 6, +132 ± 6, +158 ± 8, +203 ± 15 km s^-1); (3) SMC supernova remnant SNR 0057-7226 components (V_hel = +312 ± 3, +343 ± 3, +33, +64 km s^-1); (4) circumstellar (CS) velocity systems (V_hel = -1020, -840, -630, -530, -300 km s^-1); and (5) a possible system at -53 ± 5 km s^-1 (seen only in some of the Si II lines and marginally in Fe II) of uncertain origin. The supernova remnant SNR 0057-7226 has a systemic velocity of +188 km s^-1, suggesting that its progenitor was a member of the NGC 346 cluster. Our data allow estimates to be made of T_e ∼ 40,000 K, n_e ∼ 100 cm^-3, N(H) ∼ (4–12) × 10¹⁸ cm^-2 and a total mass between 400 and 1000 M_⊙ for the supernova remnant (SNR) shell. We detect C I absorption lines primarily in the +132 and +158 km s^-1 SMC velocity systems. As a result of the LBV-type eruptions in HD 5980, a fast-wind/slow-wind circumstellar interaction region has appeared, constituting the earliest formation stages of a windblown H II bubble surrounding this system. Variations over a timescale of 1 year in this circumstellar structure are detected.

We critically examine the use of the Washington photometric system (with the DDO51 filter) for identifying distant halo giants. While this is the most powerful photometric technique for isolating G and K giant stars, spectroscopic follow-up of giant candidates is vital. There are two situations in which interlopers outnumber genuine giants in the diagnostic M-51/M-T₂ plot and are indistinguishable photometrically from the giants. (1) In deep surveys covering tens of square degrees, very metal-poor halo dwarfs are a significant contaminant. An example is our survey of the outer halo, where these metal-poor dwarfs dominate the number of photometric giant candidates at magnitudes fainter than V = 18 and cannot be isolated photometrically. (2) In deep surveys of smaller areas with low photometric precision, most objects in the giant region of the color-color plot are dwarfs whose photometric errors have moved them there. Color errors in M-51 and M-T₂ need to be smaller than 0.03 mag to avoid this problem. An example of a survey whose photometric errors place the giant identifications under question is the survey for extratidal giants around the Carina dwarf spheroidal galaxy of Majewski et al. Accurate photometry and spectroscopic follow-up of giant candidates are essential when using the Washington system to identify the rare outer halo giants.

We present a model for the local system of gas and stars that we associate essentially with Gould's Belt, the Sirius supercluster, and the Local arm. We consider that these subsystems were formed in different epochs within a supercloud that was initially moving almost ballistically in the Galactic field until an encounter with a major spiral arm started a braking process. The stars of older generations, i.e., the Sirius supercluster, tended to conserve the kinematics of the prebraking phase of the supercloud, while the remaining gas and the early star complexes reflect the recent kinematics, resulting from the braking process. We have calculated back in time the epicyclic orbits of the Sirius supercluster, as well as of the postulated supercloud, starting from adequate initial conditions and taking into account the action of a friction force on the gas. From the condition that the Sirius supercluster and the postulated supercloud shared the same orbits before the separation of gas and stars because of the braking forces, we have determined the free parameters of the model. The main evidence supporting our hypothesis is that the supercloud's track derived from the model coincides with a large "tunnel" in the distribution of the local interstellar matter, toward l ≈ 240°.

We conclude that a supercloud of ≈2 × 10⁷M_☉ and a radius of ≈400 pc was the common precursor of the Sirius supercluster, Gould's Belt, and the Local arm. The Sirius supercluster was born ≈500 Myr ago in the supercloud, rotating in the Galactic sense and coexisting with the supercloud ever since. We show that the theoretical velocity distribution of the Sirius supercluster derived from the model is consistent with the observed vertex deviation and velocity dispersions of the Sirius supercluster. The supercloud entered into a major spiral arm 100 Myr ago. The interaction with the Galactic shock and the subsequent gas streaming generated a process of braking and compression of the gas in the supercloud. This led to the drifting out of the stars of the Sirius supercluster from the gas and to the formation of Gould's Belt and the Local arm.

The results of the model suggest that the interaction with the Galactic shock compressed the gas of the central regions of the supercloud into a flattened disk, precursor of Gould's Belt, whereas the peripheral gas generated an expanding superring, precursor of the Local arm. The observed inclination of Gould's Belt and the Galactic longitude of its nodal line are consistent with the results of the model. To study the initial configuration and the evolution of the kinematics of the local superring in the Galactic force field with interstellar friction forces, we assumed that initially the superring (1) had a cylindrical form with its axis perpendicular to the Galactic plane, (2) was centered at the barycenter that the supercloud had 100 Myr ago, and (3) had a velocity field resulting from the velocity of its own barycenter and the velocity of rotation about it (both conserved from the supercloud), plus a velocity of radial expansion from the center of the ring. The model of the superring fits the configuration of the Local arm and the kinematics of its interstellar matter, i.e., Lindblad's feature C/H, reasonably well. Computing the Sun's orbit for the last 100 Myr, with the help of the supercloud model, we find that the Sun has been attached to the local system. We quote that this interesting fact could give a firm foundation for a Galactic theory of terrestrial catastrophism. Finally, we describe the probable origins of the main stellar streams, i.e., the superclusters in the solar neighborhood, from the point of view of the supercloud model.

We present B, V, and I photometry of the stellar field SA 98 in the plane of the Galaxy. This field is located on the opposite side of the Galactic center, in the direction of the Monoceros region. The magnitude and color counts predicted by Méndez & van Altena's Galactic structure and reddening model, based on the reddening maps derived from the work of FitzGerald and Neckel & Klare or on Schlegel, Finkbeiner, & Davis's total reddening values, do not agree with the observations. Detailed inspection by means of color-color diagrams of selected Landolt standard stars within SA 98 shows a strong clumpiness and inhomogeneity of the reddening distribution along the line of sight. These color-color diagrams allow an optimization of the model-predicted color counts. A disk scale length of about 2.0 kpc based on the color counts of giant stars seems to be favored. Our findings also provide evidence to suggest that a supplemental reddening-heliocentric distance relation other than a simple exponential function must be used to apply the Schlegel et al. values at low Galactic latitude to construct reddening maps.

We have obtained new CCD BV photometry of the little-studied southern Galactic globular cluster NGC 5986, including light curves of five of its RR Lyrae variables. The cluster's red giant branch bump is detected for the first time, at V = 16.47 ± 0.03. We derive a reddening and true distance modulus of E(B-V) = 0.29 ± 0.02 and (m - M)₀ = 15.15 ± 0.10, respectively. The cluster's color-magnitude diagram reveals a mostly blue horizontal branch, like that of M13 or M2, and quite unlike M3; yet all of these clusters have nearly identical metallicities ([Fe/H]_CG97 = -1.35). We show that the RR Lyrae variables in NGC 5986 are about 0.2 mag brighter on average than those in M3, an important exception to the often-employed, universal M_V(RR)-[Fe/H] relation. Finally, we note that NGC 5986 contains two luminous stars with spectral types A–F, which are likely to be post–asymptotic giant branch (PAGB) objects. The V-band luminosity function of such yellow PAGB stars is a promising standard candle. We suggest that the luminosity function is sharply peaked at M_V(PAGB) = -3.28 ± 0.07.

We present an analysis of high-dispersion spectra (R ∼ 40,000) of three red clump stars in the old open cluster NGC 6819. The spectra were obtained with SARG, the high-dispersion spectrograph of the Telescopio Nazionale Galileo. The spectra were analyzed using both equivalent widths measured with an automatic procedure and comparisons with synthetic spectra. NGC 6819 is found to be slightly metal-rich ([Fe/H] = +0.09 ± 0.03, internal error); there are no previous high-resolution studies to compare. Most element-to-element abundance ratios are close to solar; we find a slight excess of Si and a significant Na overabundance. Our spectra can also be used to derive the interstellar reddening toward the cluster by comparing the observed colors with those expected from line excitation: we derive E(B-V) = 0.14 ± 0.04, in agreement with the most recent estimate for this cluster.

We present results from a 36 ks observation of the core of the Pleiades open cluster using ACIS-I on the Chandra X-Ray Observatory. We have detected 57 sources, most of which do not have previously known optical counterparts. Follow-up photometry indicates that many of the detections are likely to be active galactic nuclei, in accordance with extragalactic source counts, but some of the sources may be previously undiscovered low-mass members of the Pleiades. We discuss our data set and our findings about X-ray emission from early-type stars, as well as very late-type stars. In particular, the large X-ray fluxes, lack of variability, and hardness ratios of the four Pleiades B6 IV–F4 V stars suggest a tentative conclusion that Pleiades stars in this spectral type range are intrinsic X-ray sources rather than previously unknown binaries in which the X-ray emission is from a late-type companion. Also, the sensitivity of Chandra allowed us to detect nonflare X-ray emission from late M stars.

A field of about 38' × 38' around the supernova remnant (SNR) G349.7+0.2 has been surveyed in the CO J = 1–0 transition with the 12 m telescope of the NRAO, using the on-the-fly technique. The resolution of the observations is 54''. We have found that this remnant is interacting with a small CO cloud, which, in turn, is part of a much larger molecular complex, which we call the "large CO shell." The large CO shell has a diameter of about 100 pc, an H₂ mass of 9.3 × 10⁵M_⊙, and a density of 35 cm^-3. We investigate the origin of this structure and suggest as a suitable hypothesis that an old supernova explosion occurred about 4 × 10⁶ yr ago. Analyzing the interaction between G349.7+0.2 and the large CO shell, it is possible to determine that the shock front currently driven into the molecular gas is a nondissociative shock (C-type), in agreement with the presence of OH 1720 MHz masers. The positional and kinematic coincidence of one of the CO clouds that constitute the large CO shell, an IRAS pointlike source, and an ultracompact H II region indicates the presence of a recently formed star. We suggest that the formation of this star was triggered during the expansion of the large CO shell and suggest the possibility that the same expansion also created the progenitor star of G349.7+0.2. The large CO shell would then be one of the few observational examples of supernova-induced star formation.

Hubble archival images of NGC 6543 reveal a series of at least nine regularly spaced concentric circular rings that surround the famous nebular core, known as the Cat's Eye Nebula. The rings are almost certainly spherical bubbles of periodic isotropic nuclear mass pulsations that preceded the formation of the core. The bubbles are detected solely in the lines of Hα, [O III], and [N II]. The core and the bubbles around it appear to have very similar temperature, ionization, and chemical properties. Assuming a distance of 1 kpc and an outflow velocity of 10 km s^-1, a good fit to the Hα surface brightness distribution suggests that the bubbles were ejected with constant mass (∼0.01 M_⊙) and thickness (∼1000 AU) approximately every 1500 years. The combined mass of the visible bubbles, ∼0.1 M_⊙, is comparable to that of the present mass of the core (∼0.05 M_⊙). Since the bubbles are evenly spaced and there is no sign of bubble-bubble collisions, the bubble ejection mechanism regulates the outflow speed to better than 10%. The line widths of the bubbles, ∼30 km s^-1, argue that the bubbles are in the process of thickening and merging on timescales ≲10³ yr. Their ejection period is consistent with a suggestion that quasi-periodic shells are launched every few hundred years in dust-forming asymptotic giant branch (AGB) winds but not consonant with the predictions of extant models of core thermal pulses (∼10⁵ yr) and surface pulsations (∼10 yr). It appears that regular isotropic AGB mass pulses can precede the formation of brighter, denser, and more complex planetary nebula cores that are formed when an abrupt change of mode of mass loss occurs. Disruptive binary companion mergers or the sudden emergence of a magnetic field might account for the mode change.

Infrared spectroscopy from 0.8 to 2.5 μm is presented for the planetary nebula IC 5117. The emission lines of IC 5117 span a wide range of ionization that includes He II, [S III], [S II], [N I], and H₂. The reddening measured from the hydrogen lines is E(B-V) = 0.79, most of which is probably interstellar in origin. The He/H abundance ratio is 0.113 ± 0.015, with approximately 10% of the helium being doubly ionized. Using our measurements of [S II] and [S III] lines and published observations of [S IV], we find a sulfur abundance, relative to hydrogen, of N(S)/N(H) = 7.8 × 10^-6, or approximately half the solar value. Fluxes and flux limits for several lines of molecular hydrogen are presented. Measurements of 1–0 transitions, together with the limits on 2–1 transitions, indicate T_vib ∼ T_rot = 1900 K, suggesting a purely collisional excitation mechanism. The ortho-to-para ratio is ∼3, a value that is also indicative of collisional excitation. The presence of [C I] λ9850 is consistent with previous studies of IC 5117 that indicated carbon is more abundant than oxygen. IC 5117 follows the trend of planetary nebulae that display bipolar outflows and H₂ emission to be carbon-rich. We confirm the results of Zhang & Kwok, who reported infrared continuum emission substantially in excess of that produced by the ionized gas. This emission is most likely due to hot dust (T ∼ 1300 K) and accounts for roughly half of the continuum between 1.5 and 2 μm.

We have carried out 8.3 GHz hydrogen and helium radio recombination line and continuum observations of three Galactic H II regions with the NRAO 140 Foot Telescope and the Very Large Array. These observations measure the spatial variations in the singly ionized helium-to-hydrogen abundance ratio, ⁴He⁺/H⁺ (y⁺), and probe for any localized enhancements in y⁺. The three observed regions (W43, NGC 6334A, and K3-50) are morphologically complex, and high-resolution, wide field observations are crucial to measure the helium abundances on scales that would be associated with individual massive stars or clusters of stars. The nearly constant y⁺ value across the W43 source (_W43 = 7.7% ± 1%) indicates that any ionization correction over the spatial scales investigated is small. Such ionization corrections are important for calculating total abundances of the light element ³He, which is measured in the Galaxy via a singly ionized hyperfine transition. Localized enhancements in the y⁺ value have been detected in both NGC 6334A (y⁺ = 20% ± 4%) and K3-50A (y⁺ = 24% ± 5%). The ⁴He abundance results are discussed along with other measurements of the physical properties of these H II regions, such as the kinematics and excitation.

Absorption in the J = 2₁₁–2₁₂ transition of formaldehyde at 2 cm toward the ultracompact H II regions C1 and C2 of W58 has been observed with the Very Large Array with an angular resolution of ∼02 and a velocity resolution of ∼1 km s^-1. The high-resolution continuum image of C1 (also known as ON) shows a partial shell that opens to the northeast. Strong H₂CO absorption is observed against W58C1. The highest optical depth (τ > 2) occurs in the southwest portion of C1 near the edge of the shell, close to the continuum peak. The absorption is weaker toward the nearby, more diffuse compact H II region C2, τ ≤ 0.3. The H₂CO velocity (-21.2 km s^-1) toward C1 is constant and agrees with the velocity of CO emission, main-line OH masers, and the H76α recombination line, but differs from the velocity of the 1720 MHz OH maser emission (∼-13 km s^-1). Observations of the absorption in the J = 1₁₀–1₁₁ transition of formaldehyde at 6 cm toward W58C1 and C2 carried out earlier with the Westerbork Aperture Synthesis Telescope at lower resolution (∼4'' × 7'') show comparable optical depths and velocities to those observed at 2 cm. Based on the mean optical depth profiles at 6 and 2 cm, the volume density of molecular hydrogen n(H₂) and the formaldehyde column density N(H₂CO) were determined. The n(H₂) is ∼6 × 10⁴ cm^-3 toward C1. N(H₂CO) for C1 is ∼8 × 10¹⁴ cm^-2, while that toward C2 is ∼8 × 10¹³ cm^-2.

We have imaged the large-scale radio continuum structure in the Orion region with the Very Large Array at 330 MHz. Arcminute-resolution morphology of the extended emission in the H II regions M42 and M43 (NGC 1976 and NGC 1982) and in the NGC 1973-75-77 nebulosity to the north are presented. A low surface brightness thermal radio halo is detected in the H II region M42: comparison with an optical photograph indicates that the radio emission distinguishes optical emission structures from reflection nebulosities. In NGC 1977 we have discovered a compact, steep-spectrum radio source coincident with a bright optical rim.

Bally & Reipurth have recently discovered a number of Herbig-Haro (HH) jets in the periphery of the Orion Nebula (M42). In many cases both these jets and their counterjets curve away from the central regions of M42. These curved outflows appear to be evidence of an interaction of the jets with an outward flow in the H II region. In this paper, we present three-dimensional gasdynamic models of HH jet–side wind interactions, showing that structures similar to those observed in the newly discovered jets are straightforwardly obtained. In particular, we concentrate on modeling the HH 505 jet and obtain one of the best agreements ever reported with gasdynamic models for emission-line maps of HH objects.

A 2° field centered at the optical position of the Wolf-Rayet (WR) star HD 193793 (≡ WR 140) was surveyed in the H I 21 cm line and in the 1420 MHz radio continuum, using the Dominion Radio Astrophysical Observatory interferometer. The aim of this observations was to look for evidence of an interaction between the WR star and its local interstellar medium (ISM). Our H I data displays a large oval minimum at a velocity of V_sys ≃ -12 km s^-1, close to the optical position of HD 193793. We believe this H I void was created by WR 140 and its massive companion (HD 193793 is a member of a well-known binary system). The major axis of the H I void is ∼11.5 pc and its axial ratio, the ratio of major to minor axis, is ∼1.4. This structure is observable over a velocity range of around 11 km s^-1. The H I minimum appears surrounded by a clumpy shell of H I in emission that has a total mass of neutral hydrogen of M_HI ∼ 1300 solar masses. The kinematical distance corresponding to a local standard of rest velocity of -12 km s^-1 is approximately 5.4 kpc, which disagrees with the distance of HD 193793, namely, around 1 kpc. Therefore, it is concluded that the H I bubble was blown in an ISM having nonnegligible peculiar motions. Hipparcos proper-motion measurements indicate that HD 193793 has a tangential velocity of 28 ± 3 km s^-1along the major axis of one of the H I minima detected in the interior of the main H I cavity. To reconcile this motion with the physical size of the H I structure blown by the winds of the binary system, it is concluded that such tangential velocity was acquired by the system a short while ago, namely, ∼1.3 × 10⁵ yr. Maps from the IRAS database show a large-scale IR feature, with good spatial correlation with the H I shell surrounding the main H I void. We believe the emission from the IR shell arises from dust grains heated by the energetic stellar continuum photons emitted by the stellar objects.

Most OH/IR stars discovered by color selection from the IRAS Point Source Catalog have 5''–15'' quality positions, whereas the density of 2MASS sources around them is typically ∼0.005 arcsec^-2. About 40% of positional associations within 5'' are thus false. This confusion can be reduced by a factor of 8 by discarding potential associations with inappropriate J-K_s colors. It is further reduced for OH/IR stars within 45 of the Galactic plane by the availability of ∼ 2'' quality positions from the Midcourse Space Experiment. We use a sample of IRAS-derived OH/IR stars with radio positions from the VLA and extant 2MASS data to assess the likelihood of finding valid near-IR identifications for stars with thick circumstellar shells, and the residual probability of making a false 2MASS identification. Two noteworthy results are that (1) 19440+2251 cannot be associated with a 2MASS source satisfying J-K_s> 2; and (2) the candidate hypergiant star 19566+3423, alias MSX5C G070.7763+02.6837, which has an LRS 36 type, has a positional association with 2MASSI 1958322+343133 and a K_s = 14.2 ± 0.1.

The results of new spectroscopic analyses of 20 recently reported extrasolar planet parent stars are presented. The companion of one of these stars, HD 10697, has recently been shown to have a mass in the brown dwarf regime; we find [Fe/H] = +0.16 for it. For the remaining sample, we derive [Fe/H] estimates ranging from -0.41 to +0.37, with an average value of +0.18 ± 0.19. If we add the 13 stars included in the previous papers of this series and six other stars with companions below the 11 M_J limit from the recent studies of Santos et al., we derive ⟨[Fe/H]⟩ = +0.17 ± 0.20. Among the youngest stars with planets with F or G0 spectral types, [Fe/H] is systematically larger than young field stars of the same Galactocentric distance by 0.15 to 0.20 dex. This confirms the recent finding of Laughlin that the most massive stars with planets are systematically more metal-rich than field stars of the same mass. We interpret these trends as supporting a scenario in which these stars accreted high-Z material after their convective envelopes shrunk to near their present masses. Correcting these young star metallicities by 0.15 dex still does not fully account for the difference in mean metallicity between the field stars and the full parent stars sample. The stars with planets appear to have smaller [Na/Fe], [Mg/Fe], and [Al/Fe] values than field dwarfs of the same [Fe/H]. They do not appear to have significantly different values of [O/Fe], [Si/Fe], [Ca/Fe], or [Ti/Fe], though. The claim made in Paper V that stars with planets have low [C/Fe] is found to be spurious, due to unrecognized systematic differences among published studies. When corrected for these differences, they instead display slightly enhanced [C/Fe] (but not significantly so). If these abundance anomalies are due to the accretion of high-Z matter, it must have a composition different from that of the Earth.

The line profile of the resonance line of K I at 7698 Å has been analyzed in the halo turnoff star HD 84937, using a high-resolution (λ/Δλ = 110,000), high signal-to-noise ratio (S/N = 550) spectrum. Three different groups have reported detecting ⁶Li in this star, based on a red asymmetry in the Li I λ6707 line profile (⁶Li displays an isotopic shift of about 0.15 Å to the red, relative to ⁷Li, in this line). It is possible, however, that convection could introduce this red asymmetry by mass motions of ⁷Li in the star's atmosphere. At the metallicity of HD 84937, the K I resonance line at 7698 Å is expected to have a similar line strength to the Li I λ6707 feature, and both these lines are resonance transitions. In addition, both potassium and lithium have similar first ionization potentials. The result of these similarities is that the Li I and K I lines are formed at nearly identical regions in the atmosphere of HD 84937. This study presents a line profile analysis of the K I line, which has negligible isotopic splitting and is effectively a single-component line, in HD 84937. Any possible convective motions of sufficient magnitude to produce a spurious detection of ⁶Li should also produce detectable asymmetries in the K I line. No such asymmetries are found here, strengthening the case that the previously reported detections of ⁶Li in HD 84937 are real.

A simple approach to detecting metal-poor stars is to measure a magnesium index, which depends on the Mg H band plus the three nearby Mg b lines and is derived through intermediate-band interference filters. An empirically established line of demarcation in the Mg index versus B-V diagram separates metal-poor stars from solar-abundance stars. A further separation between metal-poor dwarfs and giants depends on B-V; primarily dwarfs for B-V < 0.55, giants for B-V > 0.7, with both dwarfs and giants falling in the transition region. For the metal-poor giants the distance from the demarcation line correlates well with [Fe/H], permitting estimates of stellar abundances. Stars in two regions on the sky in the vicinity of the north Galactic pole have been observed with such a set of filters. Eighteen stars (6% of the population of 299) in the sample covering the V range 8.7 to 15.6 and 48 stars (31% of the population of 163) in a deeper probe to V = 19.9 found through this process are suspected metal-poor stars according to their Mg indices. Twenty-three are specifically deemed giants, with ⟨[Fe/H]⟩ ≤ -1.5.

We have measured the diameters of the Cepheid variables δ Cephei (18 nights) and η Aquilae (11 nights) with the Navy Prototype Optical Interferometer. The primary results of these observations are the mean angular diameters ⟨θ_LD⟩ of these Cepheids: 1.520 ± 0.014 milliseconds of arc (mas) for δ Cep and 1.69 ± 0.04 mas for η Aql. We also report limb-darkened diameters for the check stars in this program: for β Lac, θ_LD = 1.909 ± 0.011 mas and for 12 Aql, θ_LD = 2.418 ± 0.010 mas. When combined with radius estimates from period-radius relations in the literature, the Cepheid angular diameters suggest distances slightly smaller than, but still consistent with, the Hipparcos distances. Pulsations are weakly detected at a level of ∼1.5 to 2 σ for both Cepheids.

We have obtained time-series infrared photometry for the highly magnetic cataclysmic variable AR UMa. Our J- and K-band observations occurred during a low state, and they show a distinctive double-humped structure. Using detailed models for the expected ellipsoidal variations in the infrared caused by the nonspherical secondary star, we find that the most likely value for the system inclination is 70°. We also model low-state V-band photometry and find that its observed double-humped structure is caused not by ellipsoidal variations, to which it has been ascribed, but by beamed cyclotron radiation. We use this result to estimate the magnetic field strength of the active southern accretion region (B ≲ 190 MG) and its magnetic longitude (ψ_S ∼ 330°).

We present analysis of Hubble Space Telescope (HST) planetary camera images of 20 L dwarfs identified in the course of the Two Micron All Sky Survey. Four of the targets, 2MASSW J0746425+200032, 2MASSs J0850359+105716, 2MASSW J0920122+351742, and 2MASSW J1146345+223053, have faint red companions at separations between 007 and 029 (1.6–7.6 AU). Ground-based infrared imaging confirms the last as a common proper motion companion. The surface density of background sources with comparable colors is extremely low, and we identify all four as physical binaries. In three cases, the bolometric magnitudes of the components differ by less than 0.3 mag. Since the cooling rate for brown dwarfs is a strong function of mass, similarity in luminosities implies comparable masses. The faint component in the 2M0850 system, however, is over 1.3 mag fainter than the primary in the I band and ∼0.8 mag fainter in M_bol. Indeed, 2M0850B is ∼0.8 mag fainter in I than the lowest luminosity L dwarf currently known, while the absolute magnitude we deduce at J is almost identical with M_J for Gl 229B. We discuss the implications of these results for the temperature scale in the L/T transition region. 2M0850 is known to exhibit 6708 Å Li I absorption, indicating that the primary has a mass less than 0.06 M_⊙. Theoretical models predict that the magnitude difference implies a mass ratio of ≈0.75. The apparent binary fraction of the current sample, 20%, is comparable to the results of previous surveys of late-type M dwarfs in the field and in the Hyades. However, the mean separation of the L dwarf binaries in the current sample is smaller than the M dwarf value by a factor of 2, and only one system would be detected at the distance of the Hyades. We discuss the likely binary frequency among L dwarfs in light of these new data.

We present the mass distribution, gravitational redshifts, radial velocities, and space motions of white dwarf stars in common proper motion binary systems. The mass distribution we derive for the 41 DA white dwarfs in this study has a mean of 0.68 ± 0.04 M_⊙. This distribution has a slightly higher mean and larger dispersion than most previous white dwarf studies. We hypothesize that this is due to a higher fraction of cool (average T_eff ∼ 10,000 K), hence old, white dwarfs in our sample. Our results indicate that samples made up of predominantly cool, old white dwarf stars tend to have a bimodal distribution with a second mass peak at ∼1.0 M_⊙, which skews the mean toward a higher mass. Both the mean and individual white dwarf masses we report here are in better agreement with those determined from model atmosphere spectroscopic fits to line profiles than with most previous gravitational redshift studies of cool white dwarfs. Our results indicate that measurement biases and weak geocoronal emission lines in the observed spectra may have affected previous gravitational redshift measurements. These have been minimized in our study. We present measurements for some previously unobserved white dwarfs, as well as independent new measurements for some that have been reported in the literature. A list of complete space motions for 50 wide binary white dwarfs is presented, derived from radial velocity measurements of their nondegenerate companions. We find that the UVW space motions and dispersions of the common proper motion binaries that contain white dwarf components are consistent with those of old, metal-poor disk stars.

A set of four test regions covering a total celestial area of 520 deg² at low galactic latitude is investigated on a set of 130 century-old astrographic plates. About 8% of the 2 × 10⁵ stars with apparent B magnitude 10–14 and with total proper-motion component larger than 002 yr^-1 show a significant flickering greater than 0.4 mag in B over a timescale shorter than 20 minutes. About 25% of these stellar flare candidates concern stars with B-V ∼ 0.2 (spectral type A7). A procedure to select pre–main-sequence stellar candidates from the distance-limited sample of 16 × 10³ disk dwarfs with detected flickering in B magnitude ≥0.4 is presented in view of a mass loss of 10^-8M_☉ yr^-1, derived from the analysis of the angular momentum distribution of stellar orbits. Follow-up UBV photoelectric photometry at the present epoch of 200 nearby stellar flares along the Vulpecula Rift shows an effect of reddening in B-V and of blueing in U-B for about 10% of the targets. The detection of optically thin or thick disks around the selected targets is suggested.

The success rate in detecting optical flares in nearby stars of ∼1.5 M_☉ with as much as 5 × 10^-6M_☉ dumped onto the central star during the past 100 years, from deep astrographic plates coupled with 1 m–class telescope is estimated to 10% with a kinematical bias that can be dealt with.

We present new near-infrared (1.1 μm) observations of the circumstellar environment of the planet-bearing star 55 Cancri. With these Hubble Space Telescope (HST) images we are unable to confirm the observation of bright scattered radiation at longer NIR wavelengths previously reported by Trilling and coworkers. NICMOS coronagraphic images with detection sensitivities to ∼100 μJy arcsec^-2 at 1.1 μm in the region 28–60 AU from the star fail to reveal any significant excess flux in point-spread function (PSF) subtracted images taken in two HST orbits. These new observations place flux densities in the 19–28 AU zone at a factor of 10 or more below the reported ground-based observations. Applying a suite of a dozen well-matched coronagraphic reference PSFs, including one obtained in the same orbits as the observations of 55 Cnc, yielded consistently null results in detecting a disk. We also searched for and failed to find a suggested flux-excess anisotropy in the ratio of ∼1.7 : 1 in the circumstellar background along and orthogonal to the plane of the putative disk. We suggest that, if such a disk does exist, then the total 1.1 μm spectral flux density in an annular zone 28–42 AU from the star must be no more than ∼0.4 mJy, at least 10 times smaller than suggested by Trilling and Brown, upon which their very large estimate for the total dust mass (0.4 M_⊕) was based. Based on the far-infrared and submillimeter flux of this system and observations of scattered light and thermal emission from other debris disks, we also expect the intensity of the scattered light to be at least an order of magnitude below our upper limits.

We describe gravitational stirring models of planetary debris disks using a new multiannulus planetesimal evolution code. The current code includes gravitational stirring and dynamical friction; future studies will include coagulation, fragmentation, Poynting-Robertson drag, and other physical processes. We use the results of our calculations to investigate the physical conditions required for small bodies in a planetesimal disk to reach the shattering velocity and begin a collisional cascade. Our results demonstrate that disks composed primarily of bodies with a single size will not undergo a collisional cascade that produces small dust grains at 30–150 AU on timescales of 1 Gyr or smaller. Disks with a size distribution of bodies reach conditions necessary for a collisional cascade in 10 Myr to 1 Gyr if the disk is at least as massive as a minimum mass solar nebula and if the disk contains objects with radii of 500 km or larger. The estimated ∼500 Myr survival time for these disks is close to the median age of ∼400 Myr derived for nearby stars with dusty disks.

Pluto and its satellite Charon regularly occulted or transited each other's disks from 1985 through 1990. The light curves resulting from these events (collectively called "mutual events") have been used to determine albedo maps of Pluto's sub-Charon hemisphere. We now use a data set of four light curves that were obtained in both B and V Johnson filters to construct a two-color map of Pluto's surface. We are able to resolve the central part of Pluto's sub-Charon hemisphere. We find that the dark albedo feature that forms a band below Pluto's equator is comprised of several distinct color units. We detect ratios of V-filter/B-filter normal reflectances ranging from ∼1.15 to ∼1.39 on Pluto's sub-Charon hemisphere.

We have conducted a large-area search for the brightest members of the trans-Neptunian and Centaur/scattered-disk asteroid populations by reprocessing archival scans from the Spacewatch 0.9 m telescope at Kitt Peak. Our survey encompasses 331 scans taken from 1995 September to 1999 September and has a raw sky coverage of 1483.8 deg². We discovered five trans-Neptunians and five Centaur/scattered-disk objects using an automated motion detection code. In addition, we serendipitously found four trans-Neptunians and two Centaur/scattered-disk objects that had been previously discovered. This survey is unique in that it involves a method that has a reasonable chance to reacquire its lost objects. In this paper we develop techniques to aid our understanding of our software efficiency and survey procedures. We use this understanding to "convolve" our raw sky coverage with our measured detection efficiency and a model of our scan coverage to estimate what fraction of survey areas can be considered "new." Our large sky coverage extends the cumulative luminosity function of the trans-Neptunians into a region previously constrained only by upper limits, and it allows a power-law fit to be attempted to the Centaur cumulative luminosity function. In objects per square degree brighter than R = 21.5, we find cumulative surface densities of Centaurs to be 0.017 ± 0.011, of trans-Neptunians to be 0.040 ± 0.018, and scattered-disk objects to be 0.007 ± 0.004. We extrapolate these values to estimate the number of each class in the ecliptic brighter than R = 21.5: 100 Centaurs, 400 trans-Neptunians, and 70 scattered-disk objects. Orbit analysis by the Minor Planet Center suggests that three of our five trans-Neptunians are resonators: 1998 VG₄₄ is in the 3 : 2, 1995 SM₅₅ appears to be in the 5 : 3, and 1998 SN₁₆₅ appears to be in the 7 : 5 resonance.

The differential equations of motion of the elliptical restricted problem of three bodies with variable masses are derived with the help of Meshcherskii's transformation. Here we see that these equations of motion differ from the equations of motion of the elliptical restricted problem of three bodies with constant mass only by a factor Δ = (β² - γα).