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We present point-source catalogs for the ≈2 Ms exposure of the Chandra Deep Field North, currently the deepest X-ray observation of the universe in the 0.5–8.0 keV band. Five hundred and three (503) X-ray sources are detected over an ≈448 arcmin² area in up to seven X-ray bands. Twenty (20) of these X-ray sources lie in the central ≈5.3 arcmin² Hubble Deep Field North (13,600 sources deg^-2). The on-axis sensitivity limits are ≈2.5 × 10^-17 ergs cm^-2 s^-1 (0.5–2.0 keV) and ≈1.4 × 10^-16 ergs cm^-2 s^-1 (2–8 keV). Source positions are determined using matched-filter and centroiding techniques; the median positional uncertainty is ≈03. The X-ray colors of the detected sources indicate a broad variety of source types, although absorbed AGNs (including a small number of possible Compton-thick sources) are clearly the dominant type. We also match lower significance X-ray sources to optical counterparts and provide a list of 79 optically bright (R ≲ 23) lower significance Chandra sources. The majority of these sources appear to be starburst and normal galaxies. The average backgrounds in the 0.5–2.0 keV and 2–8 keV bands are 0.056 and 0.135 counts Ms^-1 pixel^-1, respectively. The background count distributions are very similar to Poisson distributions. We show that this ≈2 Ms exposure is approximately photon limited in all seven X-ray bands for regions close to the aim point, and we predict that exposures up to ≈25 Ms (0.5–2.0 keV) and ≈4 Ms (2–8 keV) should remain nearly photon limited. We demonstrate that this observation does not suffer from source confusion within ≈6' of the aim point, and future observations are unlikely to be source-confusion limited within ≈3' of the aim point even for source densities exceeding 100,000 deg^-2. These analyses directly show that Chandra can achieve significantly higher sensitivities in an efficient, nearly photon-limited manner and be largely free of source confusion. To allow consistent comparisons, we have also produced point-source catalogs for the ≈1 Ms Chandra Deep Field South (CDF-S). Three hundred and twenty-six (326) X-ray sources are included in the main Chandra catalog, and an additional 42 optically bright X-ray sources are included in a lower significance Chandra catalog. We find good agreement with the photometry of the previously published CDF-S catalogs; however, we provide significantly improved positional accuracy.

We have analyzed optically bright, X-ray–faint [OBXF; i.e., log (f_X/f_R)≲-2] sources identified in an 178.9 arcmin² area having high exposure (greater than 1500 ks) within the Chandra Deep Field North 2 Ms survey. We find 43 OBXF sources in this area, making up ≈15% of the X-ray sources above a 0.5–2 keV flux of ≈2.3 × 10^-17 ergs cm^-2 s^-1. We present spectroscopic identifications for 42 of the OBXF sources and optical spectra for 25, including five previously unpublished redshifts. Deep optical imaging data (either Hubble Space Telescope [HST] or ground-based) are presented for all the OBXF sources; we measure the optical morphologies of the 20 galaxies having HST imaging data. The OBXF population consists mainly of normal and starburst galaxies detected out to cosmologically significant distances (i.e., to a median redshift of z = 0.297 and a full redshift range z = 0.06–0.845). This is notable since these distances equate to look-back times of up to ≈8 Gyr; we are thus provided with a window on the X-ray emission from galaxies at redshifts much closer to the cosmic star formation peak than was possible prior to the Chandra X-Ray Observatory. The X-ray luminosity distribution of OBXF sources extends to higher luminosity than does that of "normal" galaxies, indicating that a significant fraction are likely dominated by low-luminosity active galactic nuclei or vigorous star formation. The lowest redshift galaxies (z ≈ 0.06–0.2) have very low X-ray–to–optical flux ratios [i.e., log (f_X/f_R) ≲ -3], which are consistent with those of normal galaxies in the local universe. By combining the detected X-ray counts, we find the average OBXF X-ray spectrum to be consistent with a Γ ≈ 2.0 power law. The 0.5–2 keV log N–log S for the OBXF galaxies is much steeper (α ≈ -1.7) than for the general X-ray source population. Indeed, the number of OBXF sources has doubled between the 1 and 2 Ms surveys, rising sharply in numbers at faint fluxes. The extragalactic OBXF sources are found to contribute ≈1%–2% of the soft extragalactic X-ray background. We report on the discovery of five candidate off-nuclear ultraluminous X-ray sources (L_X ≳ 10³⁹ ergs s^-1) with z ≈ 0.1–0.2 within the OBXF population. These sources are "ultraluminous" in that they are typically more X-ray luminous than, e.g., Eddington-limited accretion onto stellar-mass black holes; these sources are found to dominate the X-ray emission of their host galaxies.

We performed B- and R-band surface photometry for E/S0 galaxies in the nearby rich cluster Abell 2199 to investigate their B-R color gradients [d(B-R)/d log r]. Our aims are to study statistical properties of the color gradients and, by comparing them with those in less dense environments, to examine environmental dependence of color gradients in elliptical galaxies. We studied the distribution of the B-R color gradients in the cluster elliptical galaxies and found that the mean value of the color gradients is -0.09 ± 0.04 mag dex^-1, which can be converted to a metallicity gradient (d log Z/d log r) of -0.3 ± 0.1. The gradient seems to be comparable to that expected by a recent monolithic collapse model. We further studied the relations between the B-R color gradients and global properties of the galaxies. Our data suggest that for the galaxies brighter than an R-band magnitude of ∼15 mag, which is roughly equivalent to L* at the distance of the cluster, brighter galaxies tend to have steeper color gradients. In addition, for the galaxies with effective radii larger than ∼3'', which nearly corresponds to L* considering the correlation between galaxy luminosity and effective radius for elliptical galaxies, the galaxies with larger effective radii seem to have steeper color gradients. These trends could appear if elliptical galaxies formed through the monolithic collapse scenario. On the contrary, it is found based on the published data that such trends are not clearly seen for elliptical galaxies in less dense environments, suggesting that elliptical galaxies in sparse environments formed by galaxy mergers, although the distribution of the color gradients is quite similar to that in the rich cluster. In other words, our data and those in the literature suggest that there is an environmental dependence in the relationship between color gradient and global properties of elliptical galaxies, while the distribution of the values of color gradients is nearly independent of galaxy environment. These results do not fully support the view that elliptical galaxies in rich clusters formed through monolithic collapse, while those in sparse environments formed through galaxy mergers, because the latter elliptical galaxies are expected to have color gradients shallower on average than the former. This apparent conflict may be reconciled by taking into account star formation and subsequent chemical enrichment induced by a galaxy merger, which may allow a merger remnant to acquire a metallicity gradient and thus compensate the dilution of the existing metallicity gradients in the progenitors by mixture of stars during the galaxy merger.

We present an optical and near-infrared catalog for the X-ray sources in the ≈2 Ms Chandra observation of the Hubble Deep Field North region. We have high-quality multicolor imaging data for all 503 X-ray point sources in the X-ray–selected catalog and reliable spectroscopic redshifts for 284. We spectroscopically identify six high-redshift (z > 1) type II quasars (L_2–8keV > 10⁴⁴ ergs s^-1) in our sample. Our spectroscopic completeness for the R ≤ 24 sources is 87%. The spectroscopic redshift distribution shows two broad redshift spikes that have clearly grown over those originally seen in the ≈1 Ms exposure. The spectroscopically identified extragalactic sources already comprise 75% of the measured 2–8 keV light. Redshift slices versus 2–8 keV flux show that an impressive 54% of the measured 2–8 keV light arises from sources at z < 1 and 68% from sources at z < 2. Thus, major accretion onto supermassive black holes has occurred since the universe was half its present age. We use seven broadband colors and a Bayesian photometric redshift estimation code to obtain photometric redshifts for the X-ray sources. We find that the photometric redshifts are within 25% of the spectroscopic redshifts for 94% of the non–broad-line sources with both photometric and spectroscopic measurements. The photometrically identified sources show a smooth continuation of the spectroscopically identified sources to redder R-HK' color with increasing redshift, consistent with the galaxy tracks of evolved bulge-dominated galaxies. Fourteen have colors R-HK' > 5.7 that would classify them as extremely red objects (EROs). The photometric redshifts of these EROs are all between z ∼ 1.5 and z ∼ 2.5. We use our wide wavelength coverage to determine rest-frame colors for the X-ray sources with spectroscopic or photometric redshifts. We find that many of the X-ray sources have the rest-frame colors of evolved red galaxies and that there is very little evolution in these colors with redshift. We also determine absolute magnitudes and find that many of the non–broad-line sources are more luminous than M, even at high redshifts. We therefore infer that deep X-ray observations may provide an effective way of locating M* galaxies with colors similar to present-day early-type galaxies to high redshifts.

We report the discovery of a new gravitationally lensed quasar from the Sloan Digital Sky Survey, SDSS J092455.87+021924.9 (SDSS J0924+0219). This object was selected from among known SDSS quasars by an algorithm that was designed to select another known SDSS lensed quasar (SDSS J1226-0006A,B). Five separate components, three of which are unresolved, are identified in photometric follow-up observations obtained with the Magellan Consortium's 6.5 m Walter Baade Telescope at Las Campanas Observatory. Two of the unresolved components (designated A and B) are confirmed to be quasars with z = 1.524; the velocity difference is less than 100 km s^-1 according to spectra taken with the W. M. Keck Observatory's Keck II Telescope at Mauna Kea, Hawaii. A third stellar component, designated C, has the colors of a quasar with redshift similar to components A and B. The maximum separation of the point sources is 178. The other two sources, designated G and D, are resolved. Component G appears to be the best candidate for the lensing galaxy. Although component D is near the expected position of the fourth lensed component in a four-image lens system, its properties are not consistent with being the image of a quasar at z ∼ 1.5. Nevertheless, the identical redshifts of components A and B and the presence of component C strongly suggest that this object is a gravitational lens. Our observations support the idea that a foreground object reddens the fourth lensed component and that another unmodeled effect (such as micro- or millilensing) demagnifies it, but we cannot rule out the possibility that SDSS J0924+0219 is an example of the relatively rare class of "three-component" lens systems.

We have obtained new optical imaging and spectroscopic observations of 78 galaxies in the fields of the rich clusters Abell 1413 (z = 0.14), Abell 2218 (z = 0.18), and Abell 2670 (z = 0.08). We have detected line emission from 25 cluster galaxies plus an additional six galaxies in the foreground and background, a much lower success rate than what was found (65%) for a sample of 52 lower richness Abell clusters in the range 0.02 ≲ z ≲ 0.08. We have combined these data with our previous observations of Abell 2029 and Abell 2295 (both at z = 0.08), which yields a sample of 156 galaxies. We evaluate several parameters as a function of cluster environment: Tully-Fisher residuals, Hα equivalent width, and rotation curve asymmetry, shape, and extent. Although Hα is more easily detectable in galaxies that are located further from the cluster cores, we fail to detect a correlation between Hα extent and galaxy location in those where it is detected, again in contrast with what is found in the clusters of lesser richness. We fail to detect any statistically significant trends for the other parameters in this study. The zero point in the z ∼ 0.1 Tully-Fisher relation is marginally fainter (by 1.5 σ) than that found in nearby clusters, but the scatter is essentially unchanged.

Microlensing observations of QSO 0957+561A and QSO 0957+561B have consistently shown evidence for structure in the quasar that has not been evident in available microlensing models, where the luminous source has been consistently modeled as a single large, round structure. We show that the microlensing features can easily be reproduced by a luminous quasar model motivated by observations: a luminous inner accretion disk edge and outer ring-shaped structures where the emission lines form. Such a model can explain all of the features known from 24 years of QSO 0957+561 microlensing observations.

We present the second gravitationally lensed quasar discovered during the course of a Hubble Space Telescope Space Telescope Imaging Spectrograph snapshot survey for small-separation gravitational lenses. CTQ 327 is a double quasar with an image separation of 122 and a g-band flux ratio of roughly 5 to 1. Spectra reveal both components to be z = 1.37 quasars, and the lensing galaxy is clearly visible after point-spread function subtraction of the two quasar components. The light profile of the lensing galaxy is well modeled by an r^1/4 law, indicative of an early-type elliptical galaxy. An estimate of the lens galaxy redshift is z ∼ 0.4–0.6, based on the Faber-Jackson relationship and photometric considerations, although values outside this range are still consistent with the present data. Resolved spectra of the two quasars show similar, but not identical, continuum and emission-line features: component A exhibits weaker emission lines with respect to the continuum than does component B, and there is evidence of intrinsic differences in the emission-line profiles between the two components. Optical monitoring of the quasar pair also shows a change in the g-band flux ratio of 0.14 mag over a 3 month period. These spectral and photometric differences may be due to microlensing fluctuations from stars in the lensing galaxy, intrinsic quasar variability coupled with the system's differential time delay, or some combination of the two. The observed variability makes CTQ 327 an attractive target for future flux monitoring, aimed at time-delay or microlensing studies.

We have constructed a sample of quasar candidates by comparing the FIRST radio survey with the 16 deg² Deeprange I-band survey carried out by Postman and coworkers. Spectroscopic follow-up of this magnitude-limited sample [I < 20.5, F_ν(20 cm) > 1 mJy] has revealed 35 quasars, all but two of which are reported here for the first time. This sample contains some unusual broad absorption line (BAL) quasars, including the first radio-loud FR II BAL previously reported by Gregg and coworkers. Comparison of this sample with the FIRST Bright Quasar survey samples selected in a somewhat bluer band and with brighter magnitude limits reveals that the I-band–selected sample is redder by 0.25–0.5 mag in B-R and that the color difference is not explained by the higher mean redshift of this sample but must be intrinsic. Our small sample contains five quasars with unusually red colors, including three that appear very heavily reddened. Our data are fitted well with normal blue quasar spectra attenuated by more than 2.5 mag of extinction in the I band. These red quasars are only seen at low redshifts (z < 1.3). Even with a magnitude limit I < 20.5, our survey is deep enough to detect only the most luminous of these red quasars at z ≲ 1; similar objects at higher redshifts would fall below our I-band limit. Indeed, the five most luminous objects (using dereddened magnitudes) with z < 1.3 are all red. Our data strongly support the hypothesis that radio quasars are dominated by a previously undetected population of red, heavily obscured objects. Unless highly reddened quasars are preferentially also highly luminous, there must be an even larger, as yet undiscovered, population of red quasars at lower luminosity. We are likely to be finding only the most luminous tip of the red quasar iceberg. A comparison of the positions of the objects in our sample with the catalog of Deeprange cluster candidates reveals that five of our six z < 1 quasars are associated with cluster candidates of similar estimated redshifts. This association is very unlikely to be the result of chance. It has some surprising implications, including the possibility that up to half of the Deeprange clusters at z ∼ 1 have associated quasars.

We have undertaken the most detailed investigation yet of the shape and variability of the radio spectrum near the spectral peak of a gigahertz-peaked spectrum radio galaxy, using the Australia Telescope Compact Array. The radio spectrum of PKS 1718-649 was measured nearly simultaneously at 30–40 different frequencies over the range of 1–9 GHz at four epochs over a 14 month period, with the aim of constraining different physical models for the absorption process that causes the spectral peak at gigahertz frequencies. We find that the two most plausible absorption processes, synchrotron self-absorption and free-free absorption, can each explain most aspects of the data. However, each process also has difficulty explaining some aspects of the data, in particular the variability of the source at frequencies below the peak. In either case, models based on homogeneous absorbers are clearly inadequate and inhomogeneities must be introduced into either the synchrotron self-absorbed source or the free-free absorbing screen. Based on our analysis we cannot strongly prefer either synchrotron self-absorption or free-free absorption as the sole process responsible for the gigahertz-peaked spectrum of PKS 1718-649. However, the consistency of the measured source size, from VLBI observations, with the observed turnover frequency favors synchrotron self-absorption as a significant effect in PKS 1718-649.

We present five X-ray quasars behind the Small Magellanic Cloud, increasing the number of known quasars behind the SMC by ∼40%. They were identified via follow-up spectroscopy of serendipitous sources from the Chandra X-Ray Observatory matched with objects from the OGLE database. All quasars lie behind dense parts of the SMC and could be very useful for proper-motion studies. We analyze X-ray spectral and timing properties of the quasars. We discuss applications of those and other recently discovered quasars behind the SMC to the studies of absorption properties of the Cloud, its proper motion, and for establishing the geometrical distance to the SMC.

We have observed 54 nearby spiral galaxies with the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope to obtain optical long-slit spectra of nuclear gas disks and STIS optical (∼R band) images of the central 5'' × 5'' of the galaxies. These spectra are being used to determine the velocity field of nuclear disks and hence to detect the presence of central massive black holes. Here we present the spectra for the successful observations. Dust obscuration can be significant at optical wavelengths, and so we also combine the STIS images with archival Near-Infrared Camera and Multi-Object Spectrometer H-band images to produce color maps to investigate the morphology of gas and dust in the central regions. We find a great variety in the different morphologies, from smooth distributions to well-defined nuclear spirals and dust lanes.

Spectrophotometric data have been obtained with the 2dF spectrograph at Cerro Tololo Inter-American Observatory and with Hydra at the WIYN telescope for nearly 2000 A, F, and G stars toward the south Galactic pole. Using 1305 radial velocities, 2311 uvbyHβ photometric measurements, and 1621 Yale–San Juan SPM (Southern Proper Motion) absolute proper motions, peculiar velocities were derived to determine the galactic gravitational force K(z) perpendicular to the Galactic plane, first described by Oort (published in 1932). Our results in local volume density, as derived from early-type stars (A0–F5) and giants, support Bahcall's ρ = 0.1 M_⊙ pc^-3, implying that dark matter exists in the galactic disk. Our result for the total surface density derived using late-type stars (G1 and later) is equal to 34 M_⊙ pc^-2, about 30% smaller than 46 ± 9 M_⊙ pc^-2 computed by Kuijken & Gilmore, but greater than 23 M_⊙ pc^-2 listed for the extended halo mass for z < 1.1 kpc (Cox 1999). The derived behavior of K(z) versus distance from the Galactic plane shows a well-known peak at about 400 pc. A second peak at about 1100 pc exhibited by dwarf main-sequence stars, but not for the giants, suggests that this second peak is likely due to a thick-disk population of metal-poor objects.

We have used the Hubble Space Telescope (HST) to measure the component of rotation of 47 Tuc in the plane of the sky, using background stars of the SMC as a reference. The rotation is comparable to that in the line of sight, showing that the axis of rotation of the cluster is considerably inclined. We also give an improved value of the absolute proper motion of 47 Tuc.

This investigation presents a set of transformations to Johnson B-V, Cousins V-R, and Cousins V-I, as well as bolometric corrections to V, for [Fe/H] = -3, -2, -1, -0.5, 0.0, and +0.3 and, in each case, values of log g from -0.5 to 5.0 for 3000 K ≤ T_eff ≤ 5500 K and from 2.0 to 5.0 for 6000 K ≤ T_eff ≤ 40,000 K. These transformations employ the predictions from Kurucz model atmospheres at high temperatures (T_eff ≥ 8000 K) and from MARCS model atmospheres at intermediate temperatures (from 7000 K down to a temperature in the range 4000 K ≤ T_eff ≤ 5500 K, depending on [Fe/H], where adjustments to satisfy observational constraints become necessary). Thus, theoretical color-T_eff relations are used exclusively down to a minimum temperature that is cooler than the temperatures of turnoff stars in open and globular star clusters. To better represent the color transformations obeyed by cool stars (down to 3000 K), corrections to the synthetic transformations have been determined from a careful consideration of observations for a few globular clusters (M92, M68, and 47 Tucanae), the color-magnitude diagrams (CMDs) of several open clusters (M67, the Pleiades, the Hyades, and NGC 6791), the CMDs and mass-luminosity diagram for solar neighborhood stars having good distance measurements from Hipparcos, empirical (B-V)-T_eff and (V-K)-T_eff relations, and color-color diagrams for field giants. The semiempirical color transformations that have been produced as a result of our analysis are also compared with several others that have been published in recent years: some of the deficiencies of the latter are revealed.

In a search for tidal extension features and/or streams of the probable parent satellite galaxies around the remote young globular clusters Pal 3 and Pal 4, we used wide-field VI photometry of an area a ∼ 13 × 13 around Pal 3 and an area a ∼ 13 × 09 around Pal 4, obtained with the CFH12K mosaic CCD. Applying the CMD-mask algorithm to stars in the vicinity of the clusters, we selected member star candidates that were used to examine the characteristics of the spatial distribution of stars around Pal 3 and Pal 4. The isodensity contours in the Gaussian-smoothed spatial stellar density maps around the clusters, and the Kolmogorov-Smirnov test applied to the corresponding luminosity functions, indicate tidal halos around Pal 3 at distances of up to ∼4r_t, and around Pal 4 of up to ∼6r_t. The stellar distribution around Pal 3 suggests north-south elongations along the directions of the Galactic center and anticenter, and a northeast extension in the direction of the cluster's proper motion. In the vicinity of Pal 4 an extension of a tail around the cluster in the opposite direction to the Galactic center and a possible extension of stars toward the Galactic center have also been detected in the isodensity maps and in the angular luminosity functions. We discuss the relevance of the spatial distribution of stars in possible streams around Pal 3 and Pal 4 to these spatial correlations and to the space orbits of the possible parent satellite galaxies.

We present wide-field CCD photometry of the Galactic globular cluster M92 obtained in the V and I bands with the CFH12K mosaic CCD at the Canada-France-Hawaii Telescope. A well-defined color-magnitude diagram is derived down to 5 mag fainter than the cluster main-sequence turnoff. After removing the background contribution, we obtain luminosity and mass functions, surface density profiles, and the surface number density maps of the stars belonging to the cluster. The surface density profile of all stars shows that the cluster's halo extends at least out to ∼30' from the cluster center, in agreement with previous study, but the profile of faint stars at the very outer region of the cluster shows a different gradient compared with that of bright stars. For a mass function of the form Φ(M) ∝ M^-(1+x), we find that the inner region (5' < r < 9') of the cluster has x ≃ 1.2 ± 0.2, whereas the outer region (9' < r < 15') has x ≃ 1.8 ± 0.3, clearly indicating a mass segregation of the cluster. An estimate of the photometric mass of the cluster implies that the remnant populations (white dwarfs and neutron stars) contribute at least 25% of the total cluster mass. The surface density map of M92 shows some evidence that the tidal tail of M92 may be oriented perpendicular to the direction toward the Galactic center.

Optical B- and V-band CCD data have been obtained for the young cluster Roslund 4 in conjunction with a large-scale photometric survey of open clusters. In addition, [S II] and off-line continuum CCD emission-line imaging data have also been obtained, revealing three regions in the cluster field where shock-excited gas is present. Two Herbig-Haro–like emission features are found, while a more extended emission feature, along an apparent cloud boundary, may be a result of stellar winds encountering the edge of a molecular cloud. The emission-line features are located near IRAS sources with properties consistent with star-forming regions. A previously published age for Roslund 4 (∼10 Myr) would seemingly suggest that shocked gas features should not be present in the cluster region, but an upper limit on the age (less than 4 Myr) derived from the new photometry is more consistent with their presence. Finally, the distance to Roslund 4 is found to be 1700–2000 pc, considerably closer than the previous published estimate.

The K dwarfs in the Pleiades fall nearly mag below a main-sequence isochrone when plotted in a color-magnitude diagram utilizing V magnitude as the luminosity index and B-V as the color index. This peculiarity has been known for 40 years but has gone unexplained and mostly ignored. When compared to Praesepe members, the Pleiades K dwarfs again are subluminous (or blue) in a color-magnitude diagram using B-V as the color index. However, using V-I as the color index, stars in the two clusters are coincident to M_V ∼ 10; using V-K as the color index, Pleiades late K and M stars fall above the main-sequence locus defined by Praesepe members. We believe that the anomalous spectral energy distributions for the Pleiades K dwarfs, as compared to older clusters, are a consequence of rapid stellar rotation and may be primarily due to spottedness. If so, the required areal filling factor for the cool component has to be very large (≥50%). Weak-lined T Tauri stars have similar color anomalies, and we suspect that this is a common feature of all very young K dwarfs (spectral type >K3). The peculiar spectral energy distribution needs to be considered in deriving accurate pre–main-sequence isochrone-fitting ages for clusters like the Pleiades, since the age derived will depend on the temperature index used.

High-resolution near-infrared Hubble Space Telescope (HST) NICMOS (F160W, F222M) images and polarization (2 μm) observations were made of four bipolar proto–planetary nebulae (PPNs): IRAS 17150-3224, 17441-2411, 17245-3951, and 16594-4656. The first three of these are viewed nearly edge-on, and for the first time the central stars in them are seen. Color maps reveal a reddened torus between the bipolr lobes in the edge-on cases, with bluer lobes. The polarization values are high, with maximum values ranging from 40% to 80%. The polarization patterns are basically centrosymmetric, with some deviations in the low-polarization equatorial regions. For IRAS 17150-3224, circumstellar arcs are seen at 1.6 μm, along with a newly discovered loop in the equatorial region. Bright caps are seen at the end of the lobes, indicating that they are not open-ended. A distinct point-symmetric pattern is seen in the strengths of the polarization vectors, especially in IRAS 17150-3224. HST NICMOS observations provide a valuable complement to the WFPC2 visible images in deriving the basic structure of bipolar PPNs.

We present results from a near-infrared 2.12 μm survey covering a large portion of the ρ Ophiuchi cloud (three regions of ∼20' × 20' each, on average) in an effort to improve the detection of embedded molecular hydrogen emission objects in the cloud. We complement these observations with deep optical [S II] images of six areas of 10' × 10' each. We recovered all previously known optical jets/Herbig-Haro (HH) objects as well as H₂ outflows in the region and report the detection of four new HH objects and 13 near-infrared knots. We provide coordinates and describe the morphology of these knots, several of which are identified as belonging to the same flow on the basis of morphological and proximity arguments. We discuss likely exciting sources. A unique association of the driving star is not always possible, as several young stars lie in close proximity to these knots.

We present new spectroscopic data of 14 candidates for the weak emission-line type of central stars of planetary nebulae ([WELS]). With a medium resolution of about 2 Å, we confirmed the main spectral characteristics of these stars but found that two objects of our sample are in fact early-type [WC] ([WCE]) stars. The λ4650 and C IV λλ 5801, 5812 features are resolved, and they were used in a tentative classification. Based on the C IV lines the [WELS] stars of our sample can be divided qualitatively into two main groups: clear emission and very weak or no emission. We compared mean equivalent widths of carbon and oxygen lines of the [WELS] stars with those of the [WCE] and [WCL] types. The results are not conclusive but point in favor of an increase in temperature from [WCL] to [WCE], followed perhaps by a decrease from [WCE] to [WELS].

We report the discovery of new Herbig-Haro objects in the Pelican Nebula (IC 4050). HH 555 is a bipolar jet emerging from the tip of a major elephant trunk protruding into the Pelican Nebula from the adjacent molecular cloud. Both beams of HH 555 bend toward the west, indicating deflection by a side wind. A chain of three nearly equally spaced bow shocks, HH 563, HH 564, and HH 565, trace a bent flow bursting out of the southern rim of the Pelican molecular cloud, possibly driven by the moderate-luminosity Class I protostar IRAS 20489+4406. Object HH 570 is a highly collimated jet emerging from a compact cloud located about 15' southeast of the Pelican molecular cloud. A parallel outflow, possibly driven by IRAS 20496+4354, powers the bright bow shock HH 569. These observations demonstrate that vigorous star formation is still occurring within the clouds that surround the evolved North America/Pelican Nebula complex.

We present an analysis of recently published photometry of OW Geminorum during its 2002 eclipse season. The photometric data are analyzed simultaneously with previously published radial velocities. The results show that OW Gem consists of an F2 Ib–II primary with M₁ = 5.8 ± 0.2 M_⊙, R₁ = 30.1 ± 0.3 R_⊙ and a G8 IIb secondary with M₂ = 3.9 ± 0.1 M_⊙, R₂ = 31.7 ± 0.3 R_⊙. We discuss the evolutionary implications of these results.

We present 88 Hα profiles for 24 pulsating variable stars with periods between 1 and 3 days in order to explore the behavior of this line in type II as compared with classical Cepheids. Surprisingly, large velocity differences were found between Hα and the metal lines in some type II Cepheids. Strong emission was observed in three stars, VZ Aql, NW Lyr, and V439 Oph, while line filling by incipient emission is present in seven others. All of the stars with emission and most with incipient emission belong to Diethelm's AHB2 class, and the emission is associated with the secondary bump on the rising branch of the light curve. Two stars, BF Ser and MQ Aql, show doubling of the core near maximum light, and asymmetry of the line is noted in some spectra.

Adaptive optics–corrected images obtained with the Coronagraphic Imager with Adaptive Optics instrument at the 8.2 m Subaru Telescope show the presence of two subarcsecond companions to the nearby (d = 19.3 pc) young star GJ 900, which was previously classified as a single member of the IC 2391 supercluster. The two companions are redder than the primary and share the same proper motion. The projected separations of B and C from the primary are 10 and 14.5 AU, respectively. The estimated masses for the two new companions depend strongly on the age of the system. For the range of ages found in the literature for IC 2391 supercluster members (from 35 to 200 Myr), the expected masses range from 0.2 to 0.4 M_⊙ for the B component and from 0.09 to 0.22 M_⊙ for the C component. The determination of the dynamic mass of the faintest component of GJ 900 will yield the age of the system using theoretical evolutionary tracks. The apparent separations of the GJ 900 system components meet the observational criterion for an unstable Trapezium-type system, but this could be a projection effect. Further observations are needed to establish the nature of this interesting low-mass multiple system.

In a continuation of our systematic search for high proper motion stars in the Digitized Sky Survey, we have completed the analysis of northern sky fields at Galactic latitudes above 25°. With the help of our SUPERBLINK software, a powerful automated blink comparator developed by us, we have identified 1146 stars in the magnitude range 8 < r < 20 with proper motions 0500 yr^-1 < μ < 2000 yr^-1. These include 1080 stars previously listed in Luyten's proper-motion catalogs (the Luyten Half-Second catalog and the New Luyten Two-Tenths catalog), nine stars not previously listed in the Luyten catalogs but reported elsewhere in the literature (including one previously reported by our team), and 57 new objects reported here for the first time. This paper includes a list of positions, proper motions, magnitudes, and finder charts for all the new high proper motion stars. Combined with our previous study of low Galactic latitude fields (see Paper I), our survey now covers over 98% of the northern sky. We conclude that the Luyten catalogs were ≃90% complete in the northern sky for stars with 05 < μ < 20 down to magnitude r = 19. We discuss the incompleteness of the old Luyten proper-motion survey and estimate completeness limits for our new survey.

For a complete 12 μm flux-limited sample of 207 IRAS sources (F₁₂ ≥ 150 Jy, |b| ≥ 5°), the majority of which are AGB stars (∼87%), we have extracted light curves in seven infrared bands between 1.25 and 60 μm using the database of the Diffuse Infrared Background Experiment (DIRBE) instrument on the Cosmic Background Explorer (COBE) satellite. Using previous infrared surveys, we filtered these light curves to remove data points affected by nearby companions and obtained time-averaged flux densities and infrared colors, as well as estimates of their variability at each wavelength. In the time-averaged DIRBE color-color plots, we find clear segregation of semiregulars, Mira variables, carbon stars, OH/IR stars, and red giants without circumstellar dust (i.e., V-[12] < 5) and with little or no visual variation (ΔV < 0.1 mag). The DIRBE 1.25–25 μm colors become progressively redder and the variability in the DIRBE database increases along the oxygen-rich sequence nondusty slightly varying red giants→SRb/Lb→SRa→Mira→OH/IR and the carbon-rich SRb/Lb→Mira sequence. This supports previous assertions that these are evolutionary sequences involving the continued production and ejection of dust. The carbon stars are redder than their oxygen-rich counterparts for the same variability type, except in the F₁₂/F₂₅ ratio, where they are bluer. Of the 28 sources in the sample not previous noted to be variable, 18 are clearly variable in the DIRBE data, with amplitudes of variation of ∼0.9 mag at 4.9 μm and ∼0.6 mag at 12 μm, consistent with them being very dusty Mira-like variables. We also present individual DIRBE light curves of a few selected stars. The DIRBE light curves of the semiregular variable L₂ Pup are particularly remarkable. The maxima at 1.25, 2.2, and 3.5 μm occur 10–20 days before those at 4.9 and 12 μm, and, at 4.9 and 12 μm, another maximum is seen between the two near-infrared maxima.

We present time series UV and optical (1150–8000 Å) spectrophotometry of the nova-like (NL) cataclysmic variable V348 Pup through and after eclipse. Outside of eclipse the spectrum is characterized by a blue continuum, strong line emission, and a broad dip from 2000 to 3000 Å. The continuum eclipse depth is roughly the same over the entire wavelength range covered, and there are no signatures of the white dwarf eclipse in the light curves. Model steady-state accretion disk spectra provide a poor fit to the shape of the UV-optical spectrum outside of eclipse. All of these properties are consistent with a disk that is self-shielding, hiding the inner disk and the white dwarf. In eclipse the emission lines and the broad dip are occulted less than the continuum, indicating vertical extension in these components. The dip is consistent with the presence of a vertical "Fe II–absorbing curtain" in the system. Archival UV-optical spectra of the lower inclination NL UX UMa show a bluer continuum and weaker line emission than in V348 Pup, and no broad dip. There is strong flickering in V348 Pup. The flickering spectrum shows that the flickering occurs only in the continuum. We did not detect any periodicities associated with the rapid variability. The properties of V348 Pup are consistent with its previous identification as a member of the SW Sextantis subclass of NL variables.

We report final results from our 2.5 yr infrared parallax program carried out with the European Southern Observatory 3.5 m New Technology Telescope and the SOFI infrared camera. Our program targeted precision astrometric observations of 10 T-type brown dwarfs in the J band. Full astrometric solutions (including trigonometric parallaxes) for nine T dwarfs are provided along with proper-motion solutions for a further object. We find that HgCdTe-based infrared cameras are capable of delivering precision differential astrometry. For T dwarfs, infrared observations are to be greatly preferred over the optical, both because they are so much brighter in the infrared, and because their prominent methane absorptions lead to similar effective wavelengths through the J filter for both target and reference stars, which in turn results in a dramatic reduction in differential color refraction effects. We describe a technique for robust bias estimation and linearity correction with the SOFI camera, along with an upper limit to the astrometric distortion of the SOFI optical train. Color-magnitude and spectral type–magnitude diagrams for both L and T dwarfs are presented that show complex and significant structure, with major import for luminosity function and mass function work on T dwarfs. Based on the width of the early L dwarf and late T dwarf color magnitude diagrams, we conclude the brightening of early T dwarfs in the J passband (the "early T hump") is not an age effect, but due to the complexity of brown dwarf cooling curves. Finally, empirical estimates of the "turn on" magnitudes for methane absorption in field T dwarfs and in young stars clusters are provided. These make the interpretation of the T6 dwarf σ Ori J053810.1-023626 as a σ Ori member problematic.

We present spectroscopic observations of the classical nova CP Crucis (Nova Crux 1996) obtained with the Infrared Space Observatory Short Wavelength Spectrometer (ISO SWS) and the Anglo-Australian Telescope using both the Infrared Imaging Spectrograph and the Royal Greenwich Observatory Spectrograph. From the expansion parallax, we find that CP Crucis lies at a distance of 2.6 ± 0.5 kpc and reached a maximum M_V of -8.7 at 0.96 days after discovery. We find abundance enhancements versus solar by mass of 75, 17, and 27 for N, O, and Ne, respectively. Additionally, we constrain the Mg abundance in the ejecta to be approximately solar. Abundance analysis suggests CP Crucis is an old Population I binary system. Combining the strong N and Ne abundances with the relatively low Mg abundance and a Ne/O ratio of 0.5, we propose that CP Crucis is an example of the "missing link" between CO and ONeMg novae.

We have monitored the photometric variability of nine field L and T brown dwarfs for 10 nights during the course of 1 month. Observations were obtained in the K_s band with the Palomar 60 inch (1.5 m) telescope Near-Infrared Camera. Results of statistical analyses indicate that at least three of the nine targets show significant evidence for variability, and three more are possibly variable. Fractional deviations from the median flux vary from 5% to 25%. Two of the variable targets, 2MASS 0030-14 (L7) and SDSS 0151+12 (T1), have marginally significant peaks in their periodograms. The phased light curves show evidence for periodic behavior on timescales of 1.5 and 3.0 hr, respectively. No significant correlations between variability amplitude and spectral type or J-K_s color are found. While it is clear that variability exists in objects near the L/T dwarf boundary, we find no evidence that variability near the L/T boundary is more likely than it is for early L dwarfs.

We have analyzed over 6 years of MACHO optical photometry and a smaller amount of OGLE-II data for the X-ray/Be star source RX J0058.2-7231, in the Small Magellanic Cloud. The brightness is clearly modulated with a 60 day period. During this period, the light curve shows a rapid brightening by ∼0.05 mag and a much slower decline. The amplitude of the brightening increases with wavelength, so that the system is redder in its bright phase. At R and I, there is evidence of a shallow eclipse with a duration of ∼0.1P that occurs within the bright portion of the cycle. The 60 day clock most likely represents the orbit of the X-ray source around the Be star. We suggest that the brightening may be related to changes in the Be star's disk at periastron, assuming the neutron star is in an eccentric orbit. There is some evidence that the period may be decreasing at a rate of ∼0.01 days per cycle. A single spectrum, taken during the fainter part of the light curve, shows a typical Be star, with emission lines at Hα and Hβ and absorptions at higher Balmer and He I lines. A comparison is made with the LMC X-ray/Be star system A0538-66, which has a highly eccentric orbit.

An initial assessment is made of white dwarf and hot subdwarf stars observed in the Sloan Digital Sky Survey. In a small area of sky (190 square degrees), observed much like the full survey will be, 269 white dwarfs (WDs) and 56 hot subdwarfs are identified spectroscopically where only 44 white dwarfs and five hot subdwarfs were known previously. Most are ordinary DA (hydrogen atmosphere) and DB (helium) types. In addition, in the full survey to date, a number of WDs have been found with uncommon spectral types. Among these are blue DQ stars displaying lines of atomic carbon; red DQ stars showing molecular bands of C₂ with a wide variety of strengths; DZ stars where Ca and occasionally Mg, Na, and/or Fe lines are detected; and magnetic WDs with a wide range of magnetic field strengths in DA, DB, DQ, and (probably) DZ spectral types. Photometry alone allows identification of stars hotter than 12,000 K, and the density of these stars for 15 < g < 20 is found to be ∼2.2 deg^-2 at Galactic latitudes of 29°–62°. Spectra are obtained for roughly half of these hot stars. The spectra show that for 15 < g < 17, 40% of hot stars are WDs, and the fraction of WDs rises to ∼90% at g = 20. The remainder are hot sdB and sdO stars.

We present a method for obtaining atmospheric temperature, pressure, and number density profiles for small bodies through inversion of light curves recorded during stellar occultations. This method avoids the assumption that the atmospheric scale height is small compared with the radius of the body, and it includes the variation of gravitational acceleration with radius. First we derive the integral equations for temperature, scale height, pressure, number density, refractivity, and radius in terms of the light-curve flux. These are then cast into summation form suitable for numerical evaluation. Equations for the errors in these quantities caused by Gaussian noise in the occultation light curve are also derived. The method allows for an arbitrary atmospheric boundary condition above the inversion region, and one particular boundary condition is implemented through least-squares fitting. When the inversion equations are applied to noiseless test data for a simulated isothermal atmosphere, numerical errors in the calculated temperature profile are less than 5 parts in 10⁴. Nonisothermal test cases are also presented. We explore the effects of (1) the boundary condition, (2) data averaging (in the time, observer-plane, and body-plane domains), (3) systematic errors in the zero stellar flux level, and (4) light-curve noise on the accuracy of the inversion results. A criterion is presented for deciding whether inversion would be an appropriate analysis for a given stellar occultation light curve, and limitations to the radial resolution of the inversion results are discussed. The inversion method is then employed on the light curves for the 1988 June 9 occultation by Pluto observed with the Kuiper Airborne Observatory and the 1997 November 4 occultation by Triton observed with the Hubble Space Telescope. Under the (possibly incorrect) assumption that no extinction effects are present in the occultation light curve, the Pluto inversion yields a 110 K isothermal profile down to approximately 1215 km radius, at which point a strong thermal gradient, 3.9 ± 0.6 K km^-1, abruptly appears, reaching 93 K at the end of the inversion. The Triton inversion yields a differently shaped profile, which has an upper level thermal gradient, ∼0.4 K km^-1, followed by a ∼51 K isothermal profile at lower altitudes. The Triton inversion shows wavelike temperature variations in the lower atmosphere, with amplitudes of ∼1 K and wavelengths of ∼20 km, that could be caused by horizontal or vertical atmospheric waves.

We report 1.1–2.4 μm multiband photometry of the small Uranian satellites Puck and Portia and of the small Neptunian satellite Proteus obtained with the Hubble Space Telescope and its near-infrared camera NICMOS. The surface of Puck (and likely Portia) displays an absorption feature matching the 2.0 μm band of water ice. In the case of Proteus, a shallower absorption is detected in the long-wavelength region of the spectrum. Although water ice cannot be entirely ruled out, this feature could also be produced by C–H– or C≡N–bearing material. Future spectroscopic observations at high angular resolution are needed to further investigate the nature of the compounds covering the surface of the small moons of Uranus and Neptune.

CCD photometric observations of the two near-Earth asteroids (NEAs) 2001 YB₅ and 2001 TX₁₆ were carried out in 2002 January with the 0.6/0.9 m Schmidt telescope of the National Astronomical Observatories of China (NAOC). Analysis of the light curves of these two objects reveals rotation periods of 3.20 ± 0.03 hr with amplitude 0.21 ± 0.02 mag for 2001 YB₅ and 4.8005 ± 0.0003 hr with amplitude 0.51 ± 0.01 mag for 2001 TX₁₆. Spectroscopic observations of the two NEAs were made with the NAOC 2.16 m telescope, ranging from 5000 to 9000 Å. The reflectance spectrum of 2001 YB₅ is a little bluish, with a possible weak absorption band from 8000 to 9000 Å, which is consistent with the spectra of B-type asteroids. That of 2001 TX₁₆ is spectrally flat, with a shallow absorption band centered near 7000 Å, consistent with the spectra of Ch-type asteroids.

Element by element, we have combined the optical components in the three cameras of the Two Micron All Sky Survey (2MASS), and incorporated detector quantum efficiency curves and site-specific atmospheric transmissions, to create three relative spectral response curves (RSRs). We provide the absolute 2MASS attributes associated with "zero magnitude" in the JHK_s bands so that these RSRs may be used for synthetic photometry. The RSRs tie 2MASS to the "Cohen-Walker-Witteborn" framework of absolute photometry and stellar spectra for the purpose of using 2MASS data to support the development of absolute calibrators for the Infrared Array Camera and pairwise cross-calibrators between all three SIRTF instruments. We examine the robustness of these RSRs to changes in water vapor within a night. We compare the observed 2MASS magnitudes of 33 stars (converted from the precision optical calibrators of Landolt and Carter-Meadows into absolute infrared calibrators from 1.2 to 35 μm) with our predictions, thereby deriving 2MASS "zero-point offsets" from the ensemble. These offsets are the final ingredients essential to merge 2MASS JHK_s data with our other absolutely calibrated bands and stellar spectra, and to support the creation of faint calibration stars for SIRTF.

Extending Nacozy's idea of manifold correction by using the concept of the integral invariant relation, we propose a new approach to numerically integrate quasi-Keplerian orbits. The method integrates the time evolution of the Kepler energy and the usual equation of motion simultaneously. Then it directly adjusts the integrated position and velocity by a spatial scale transformation in order to satisfy the Kepler energy relation rigorously at every integration step. The scale factor is determined by solving an associated cubic equation precisely with the help of Newton's method. In treating multiple bodies, the Kepler energies are integrated for each body and the scale factors are adjusted separately. The implementation of the new method is simple, the added cost of computation is low, and its applicability is wide. Numerical experiments show that the scaling reduces the integration error drastically. In the case of pure Keplerian orbits, the truncation error grows linearly with respect to time, and the round-off error grows more slowly than that. When perturbations exist, a component that grows with the second or a higher power of time appears in the truncation error, but its magnitude is reduced significantly as compared with the case without scaling. The rate of decrease varies roughly as the 5/4 to 5/2 power of the strength of the perturbing acceleration, where the power index depends on the type of perturbation. The method seems to suppress the accumulation of round-off errors in the perturbed cases, although the details remain to be investigated. The new approach provides a fast and high-precision device with which to simulate the orbital motions of major and minor planets, natural and artificial satellites, comets, and space vehicles at a negligible increase in computational cost.

Simple modifications to the analysis used in the Kuhn, Lin, & Loranz flat-field CCD calibration method yield significant improvements in both speed and accuracy. In this method, multiple exposures are taken of a time-independent signal at different spatial positions. The flat field is then expressed in the form of a Jacobi relaxation solution to Poisson's equation. By applying the technique of simultaneous overrelaxation, we have improved the convergence rate to require approximately the square root of the number of iterations () needed by the Jacobi method. For large arrays, where r is correspondingly large, this improvement is significant. Furthermore, we have improved the accuracy by extending the method to account for fractional pixel shifts.

Symplectic integrators are a popular tool applied to a variety of numerical problems. Many symplectic methods are known, but those higher than second order must include some substeps that travel backward compared with the main integration. To compensate for this, some substeps must have large coefficients. This produces large error terms and reduces the efficiency of high-order symplectic algorithms. The constraint equations for the substeps of high-order algorithms often have complex solutions in addition to the known real ones. The complex solutions typically generate symplectic integrators with small substeps, so these algorithms have substantially smaller error terms than conventional algorithms. This makes them more efficient in principle. Here third-, fourth-, and sixth-order algorithms are developed and tested for problems in which the Hamiltonian is split into kinetic and potential energy terms, and also for perturbed Kepler problems. In addition, symplectic correctors are developed for the third- and fourth-order methods. It is shown that complex integrators with leading error terms that have strictly imaginary coefficients effectively behave as if they are 1 order higher than expected.