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Deep, multiband observations of high Galactic latitude fields are an essential tool for studying topics ranging from Galactic structure to extragalactic background radiation. The Hubble Deep Field (HDF-N) observations obtained in 1995 December established a standard for such narrow, deep surveys. The field has been extensively analyzed by a variety of groups and has been widely studied with imaging and spectroscopy over wavelengths ranging from 10^-3 to 2 × 10⁵ μm. We describe here a second deep field campaign (HDF-S), this time in the southern hemisphere, undertaken by the Hubble Space Telescope (HST) in 1998 October in a program very similar to the northern Hubble Deep Field. Imaging and spectroscopy of three adjacent fields in the southern continuous viewing zone were obtained simultaneously for 150 orbits, and a mosaic of flanking fields was imaged for 27 additional orbits. Two important features of the HDF-S distinguish it from the HDF-N: the campaign included parallel observations by the three main HST instruments—WFPC2, STIS, and NICMOS—and the HDF-S location was selected to place a bright z = 2.24 quasar in the STIS field of view. The HDF-S observations consist of WFPC2 images in filters close to U, B, V, and I, a deep STIS image of the field surrounding the quasar, spectroscopy of the quasar with STIS from 1150 to 3560 Å, and deep imaging of an adjacent field with NICMOS camera 3 at 1.1, 1.6, and 2.2 μm. All of the HDF-S data were fully reduced and made publicly available within 2 months of the observations, and we describe here the selection of the fields and the observing strategy that was employed. Detailed descriptions of the data and the reduction techniques for each field, together with the corresponding source catalogs, appear in separate papers.

The Hubble Deep Field South (HDF-S) observations targeted a high Galactic latitude field near QSO J2233-606. We present Wide Field Planetary Camera 2 observations of the field in four wide bandpasses centered at roughly 300, 450, 606, and 814 nm. Observations, data reduction procedures, and noise properties of the final images are discussed in detail. A catalog of sources is presented, and the number counts and color distributions of the galaxies are compared with a new catalog of the original Hubble Deep Field (HDF-N) that has been constructed in an identical manner. The two fields are qualitatively similar, with the galaxy number counts for the two fields agreeing to within 20%. The HDF-S has more candidate Lyman break galaxies at z > 2 than the HDF-N. The star formation rate per unit volume computed from the HDF-S, based on the UV luminosity of high-redshift candidates, is a factor of 1.9 higher than from the HDF-N at z ∼ 2.7, and a factor of 1.3 higher at z ∼ 4.

We present results from a study to detect variable galaxies in the Hubble Deep Field North (HDF-N). The goal of this project is to investigate the number density of active galactic nuclei (AGNs) at z ≃ 1 through the detection of variable galaxy nuclei. The advantage of the Hubble Space Telescope is its ability to do accurate photometry within smaller apertures, thus allowing us to probe much lower AGN/host galaxy luminosity ratios than can be done from the ground. The primary data sets analyzed for galactic variability follow from the original HDF-N observations in 1995 December and a second epoch obtained 2 years later. The second-epoch data consist of 36 exposures in F814W with a total integration time of 63,000 s (compared with 58 exposures and a total of 123,600 s in the original HDF-N). We have detected nuclear variability at or above the 3 σ level in eight of 633 HDF galaxies at I₈₁₄ ≲ 27. Only two detections would be expected by chance in a normal distribution. At least one of these eight has been spectroscopically confirmed as a Seyfert 1 galaxy. Based on the AGN structure function for variability, the estimated luminosity of the varying component in each galaxy lies in the range -19.5≲M_B ≲ -15.0. We construct an upper limit to the luminosity function for the variable nuclei and compare this with the local Seyfert luminosity function (LF) and the LFs for quasi-stellar objects at z ≃ 1. Assuming we have detected all Seyfert-like nuclei in the HDF-N, we find no evidence for an increase in the number density of AGNs at M_B ≃ -19 (H₀ = 75 km s^-1 Mpc^-1, q₀ = 0.5). From this study, we estimate that ∼1%–3% of field galaxies with I ≲ 27 may contain a nuclear AGN.

In order to establish an objective framework for studying galaxy morphology, we have developed a quantitative two-parameter description of galactic structure that maps closely onto Hubble's original tuning fork. Any galaxy can be placed in this "Hubble space," where the x-coordinate measures position along the early-to-late sequence while the y-coordinate measures in a quantitative way the degree to which the galaxy is barred. The parameters defining Hubble space are sufficiently robust to allow the formation of Hubble's tuning fork to be mapped out to high redshifts. In the present paper, we describe a preliminary investigation of the distribution of local galaxies in Hubble space based on the CCD imaging atlas published by Frei et al. in 1996. We find that barred, weakly barred, and unbarred galaxies are remarkably well separated on this diagnostic diagram. The spiral sequence is clearly bimodal and indeed approximates a tuning fork: strongly barred and unbarred spirals do not simply constitute the extrema of a smooth unimodal distribution of bar strength but, rather, populate two parallel sequences. Strongly barred galaxies lie on a remarkably tight sequence, strongly suggesting the presence of an underlying unifying physical process. Rather surprisingly, weakly barred systems do not seem to correspond to objects bridging the parameter space between unbarred and strongly barred galaxies but, instead, form an extension of the regular spiral sequence. This relation lends support to models in which the bulges of late-type spirals originate from secular processes driven by bars.

We present an analysis of the star formation rate in galaxies with redshifts in the range 0.7 < z < 1.8 using Near Infrared Camera and Multi-Object Spectrograph (NICMOS) grism spectral observations. We detect 163 galaxies in an area of ∼4.4 arcmin², 37 of which show possible Hα emission. We extend the observed Hα luminosity function (LF) in this redshift range to luminosities a factor of 2 fainter than earlier work and are consistent in the region of overlap. Using the Hα LF, we estimate a star formation rate (SFR) density in this redshift range of 0.166 M_⊙ yr^-1 Mpc^-3 (H₀ = 75 km s^-1 Mpc^-1), consistent with other estimates based on emission lines and supporting the order-of-magnitude increase in SFR density between z = 0 and z = 1. Our measurement of SFR density is a factor of ≈2–3 greater than that estimated from UV data, comparable to the factor observed locally, implying little evolution in the relative extinctions between UV and Hα out to z ≈ 1.3.

Clusters of galaxies are important laboratories both for understanding galaxy evolution and for constraining cosmological quantities. Any analysis of clusters, however, is best done when one can reliably determine which galaxies are members of the cluster. While this would ideally be done spectroscopically, the difficulty in acquiring a complete sample of spectroscopic redshifts becomes rather daunting, especially at high redshift, when the background contamination becomes increasingly larger. Traditionally, an alternative approach of applying a statistical background correction has been utilized that, while useful in a global sense, does not provide information for specific galaxies. In this paper we develop a more robust technique that uses photometrically estimated redshifts to determine cluster membership. This technique can either be used as an improvement over the commonly used statistical correction method or it can be used to determine cluster candidates on an individual galaxy basis. By tuning the parameters of our algorithm, we can selectively maximize our completeness or, alternatively, minimize our contamination. Furthermore, our technique provides a statistical quantification of both our resulting completeness and contamination from foreground and background galaxies.

We present the first near-infrared spectroscopic observations of the radio-loud broad absorption line quasi-stellar object (QSO), FIRST J155633.8+351758. The spectrum is similar to that of a reddened QSO and shows strong emission lines of Hα and Hβ, as well as strong Fe II emission blends near Hβ. The redshift of the object, measured from the Hα and Hβ lines, is z_BLR = 1.5008 ± 0.0007, slightly larger than the redshift of z_metal = 1.48, estimated from the broad metal absorption features. Thus, the broad metal absorption features are blue shifted with respect to the systemic velocity. The width of the Hα emission line (FWHM ≈ 4100 km s ^-1) is typical of that observed in QSO broad-line regions, but the Balmer decrement (Hα/Hβ ≈ 5.8) is larger than that of most optically selected QSOs. Both the Balmer decrement and the slope of the rest-frame UV–optical continuum independently suggest a modest amount of extinction along the line of sight to the broad-line region (E_B-V ≈ 0.5 for SMC-type screen extinction at the redshift of the QSO). The implied gas column density along the line of sight is much less than that implied by the weak X-ray flux of the object, suggesting that either the broad emission and absorption line regions have a low dust-to-gas ratio or that the rest-frame optical light encounters significantly lower mean column density lines of sight than the X-ray emission. From the rest-frame UV-optical spectrum, we are able to constrain the stellar mass content of the system (<3 × 10¹¹M_⊙). Comparing this mass limit with the black hole mass estimated from the bolometric luminosity of the QSO, we find it possible that the ratio of the black hole to stellar mass is comparable to the Magorrian value, which would imply that the Magorrian relation is already in place at z = 1.5. However, multiple factors favor a much larger black hole–to–stellar mass ratio. This would imply that if the Magorrian relation characterizes the late history of QSOs and if the situation observed for F1556+3517 is typical of the early evolutionary history of QSOs, central black hole masses develop more rapidly than bulge masses.

We report the discovery of a new double-image quasar that was found during a search for gravitational lenses in the southern sky. Radio source PMN J1838-3427 is composed of two flat-spectrum components with separation 10, flux density ratio 14 : 1, and matching spectral indices in VLA and VLBA images. Ground-based BRI images show the optical counterpart (total I = 18.6) is also double, with the same separation and position angle as the radio components. An HST/WFPC2 image reveals the lens galaxy. The optical flux ratio (27 : 1) is higher than the radio value, probably because of differential extinction of the components by the lens galaxy. An optical spectrum of the bright component contains quasar emission lines at z = 2.78 and several absorption features, including prominent Lyα absorption. The lens galaxy redshift could not be measured, but it is estimated to be z = 0.36 ± 0.08. The image configuration is consistent with the simplest plausible models for the lens potential. The flat radio spectrum and observed variability of PMN J1838-3427 suggest that the time delay between flux variations of the components is measurable and could thus provide an independent measurement of H₀.

We present new optical observations of the z = 0.282 cluster Abell 697 from Keck Observatory. Images show asymmetric disturbed structure in the cD halo and a previously unknown source that is likely a faint gravitational lens arc. A spectrum of the arc did not yield a redshift, but its spectrum and colors suggest that it lies at z > 1.3. We construct plausible models for the inner cluster potential that are able to reproduce the arc, but in each case the models require the inner potential to be quite elliptical. The cD morphology and the best lens models suggest that this cluster may have undergone a recent merger and is in the process of forming its cD galaxy and halo. Analysis of X-ray data from ROSAT and ASCA suggests that the merging process is sufficiently advanced that the gas in the cluster has relaxed, and A697 lies along the L_X-T_X relation for normal clusters.

We present axisymmetric maximum disk mass models for a sample of 74 spiral galaxies taken from the southern sky Fabry-Perot Tully-Fisher survey by Schommer et al. The sample contains galaxies spanning a large range of morphologies and having rotation widths from 180 km s ^-1 to 680 km s ^-1. For each galaxy we have an I-band image and a two-dimensional Hα velocity field. We decompose the disk and bulge by fitting models directly to the I-band image. This method utilizes both the distinct surface brightness profiles and shapes of the projected disk and bulge in the galaxy images. The luminosity profiles and rotation curves are derived using consistent centers, position angles, and inclinations derived from the photometry and velocity maps. The distribution of mass is modeled as a sum of disk and bulge components with distinct, constant mass-to-light ratios. No dark matter halo is included in the fits. The models reproduce the overall structure of the rotation curves in the majority of galaxies, providing good fits to galaxies that exhibit pronounced structural differences in their surface brightness profiles. Of galaxies for which the rotation curve is measured to R_23.5 or beyond 75% are well fitted by a mass-traces-light model for the entire region within R_23.5. The models for about 20% of the galaxies do not fit well; the failure of most of these models is traced directly to nonaxisymmetric structures, primarily bars but also strong spiral arms. The median I-band M/L of the disk plus bulge is 2.4 ± 0.9 h₇₅ in solar units, consistent with normal stellar populations. These results require either that the mass of dark matter within the optical disk of spiral galaxies is small or that its distribution is very precisely coupled to the distribution of luminous matter.

Mid-infrared observations of the central source of NGC 1068 have been obtained with a spatial resolution in the deconvolved image of 01 (∼7 pc). The central source is extended by ∼1'' in the north-south direction but appears unresolved in the east-west direction over most of its length. About 2/3 of its flux can be ascribed to a core structure that is itself elongated north-south and does not show a distinct unresolved compact source. The source is strongly asymmetric, extending significantly farther to the north than to the south. The morphology of the mid-infrared emission appears similar to that of the radio jet and has features which correlate with the images in [O III]. Its 12.5–24.5 μm color temperature ranges from 215 to 260 K and does not decrease smoothly with distance from the core. Silicate absorption is strongest in the core and to the south and is small in the north. The core, apparently containing 2/3 of the bolometric luminosity of the inner 4'' diameter area, may be explained by a thick, dusty torus near the central active galactic nucleus (AGN) viewed at an angle of ∼65° to its plane. There are, however, detailed difficulties with existing models, especially the narrow east-west width of the thin extended mid-infrared "tongue" to the north of the core. We interpret the tongue as reprocessed visual and ultraviolet radiation that is strongly beamed and that originates in the AGN.

Imaging polarimetry at 1.65 and 2.2 μm is presented for the classic starburst galaxy M82 and the advanced merger system Cen A. Polarimetry at near-IR wavelengths allows the magnetic field geometry in galaxies to be probed much deeper into dusty regions than optical polarimetry. In M82, the magnetic field throughout the nucleus has a polar geometry, presumably due to the massive vertical flow that is a result of the intense star formation there. Fully two-thirds of the line-of-sight dust through to the center of M82 contains a vertical magnetic field. In Cen A, the prominent dust lane shows a normal planar field geometry. There is no indication of significant disturbance in the field geometry in the dust lane and the polarization strength is near normal for the amount of extinction. Either the magnetic field geometry was well maintained during the merger, or it reestablished itself very easily.

We present new radial velocities for 87 globular clusters around the elliptical galaxy NGC 4472 and combine these with our previously published data to create a data set of velocities for 144 globular clusters around NGC 4472. We utilize this data set to analyze the kinematics of the NGC 4472 globular cluster system. The new data confirm our previous discovery that the metal-poor clusters have significantly higher velocity dispersion than the metal-rich clusters in NGC 4472. We also find very little or no rotation in the more spatially concentrated metal-rich population, with a resulting upper limit for this population of (v/σ)_proj < 0.34 at a 99% confidence level. The very small angular momentum in the metal-rich population requires efficient angular momentum transport during the formation of this population, which is spatially concentrated and chemically enriched. Such angular momentum transfer can be provided by galaxy mergers, but it has not been achieved in other extant models of elliptical galaxy formation that include dark matter halos. We also calculate the velocity dispersion as a function of radius and show that it is consistent with roughly isotropic orbits for the clusters and the mass distribution of NGC 4472 inferred from X-ray observations of the hot gas around the galaxy.

Using Hubble Space Telescope images we have carried out a study of cluster populations in the nearby S0 galaxy NGC 1023. In two WFPC2 pointings we have identified 221 cluster candidates. The small distance (∼9 Mpc) combined with deep F555W and F814W images allows us to reach about 2 mag below the expected turnover of the globular cluster luminosity function. NGC 1023 appears to contain at least three identifiable cluster populations: the brighter clusters show a clearly bimodal color distribution with peaks at (V-I)₀ = 0.92 and at (V-I)₀ = 1.15, and in addition there are a number of fainter, more extended objects with predominantly red colors. Among the brighter clusters, we find that the blue clusters have somewhat larger sizes than the red ones with mean effective radii of R_e ∼ 2 and R_e ∼ 1.7 pc, respectively. These clusters have luminosity functions (LFs) and sizes consistent with what is observed for globular clusters in other galaxies. Fitting Gaussians to the LFs of the blue and red compact clusters, we find turnover magnitudes of M_TO(blue) = -7.58 and M_TO(red) = -7.37 in V and dispersions of σ_V(blue) = 1.12 and σ_V(red) = 0.97. The fainter, more extended clusters have effective radii up to R_e ∼ 10–15 pc, and their LF appears to rise at least down to M_V ∼ -6, few of them being brighter than M_V = -7. We suggest that these fainter objects may have a formation history distinct from that of the brighter GCs.

We observed 17 nearby low-luminosity FR I radio galaxies using the NRAO Very Long Baseline Array (VLBA) at 1.67 GHz, as part of a multiwavelength study of a complete sample of 21 sources selected by radio flux density from the Uppsala General Catalogue of Galaxies. We detected radio emission from all 17 galaxies. At a FWHM resolution of ≈10 × 4 mas, five galaxies show only an unresolved radio core, 10 galaxies show core-jet structures, and two galaxies show twin-jet structures. Comparing these VLBA images with images previously obtained with the NRAO VLA, we find that all detected VLBA jets are well aligned on parsec scales with the VLA jets on kiloparsec scales and that the jet-to-counterjet surface brightness ratios, or the sidedness, decreases systematically with increasing distance along the jet. We attribute the sidedness to the Doppler boosting effect and its decline to the deceleration of the jets. We show that a distribution of Lorentz factor centered near Γ = 5 can reproduce our VLBA detection statistics for core, core-jet, and twin-jet sources. We also note that the luminosity per unit length, L_j, of the VLBA jets drops quickly with distance, r, along the jet, approximately as L_j ∝ r^-2.0. We discuss three different mechanisms to explain this jet fading: (1) the decrease of Doppler boosting due to jet deceleration, (2) synchrotron losses, and (3) expansion losses in constant velocity but adiabatically spreading jets. Mechanisms (1) and (2) are inconsistent with the observations, while mechanism (3) is consistent with the observations provided the magnetic field lines in the jets are aligned perpendicular to the jet axis. This implies that the deceleration of the jets required by the unified scheme does not occur on the tens of parsec scales but must occur on larger scales.

Arcsecond-resolution X-ray imaging of the nucleus of the nearby starburst galaxy NGC 253 with Chandra reveals a well-collimated, strongly limb-brightened, kiloparsec-scale conical outflow from the central starburst region. The outflow is very similar in morphology to the known Hα outflow cone, on scales down to ≲20 pc. This provides, for the first time, robust evidence that both X-ray and Hα emission come from low volume filling factor regions of interaction between the fast energetic wind of SN ejecta and the denser ambient interstellar medium and not from the wind fluid itself. We provide estimates of the (observationally and theoretically important) filling factor of the X-ray–emitting gas, of between ∼4% and 40%, consistent with an upper limit of ∼40%, based directly on the observed limb-brightened morphology of the outflow. Only ≲20% of the observed X-ray emission can come from the volume-filling, metal-enriched, wind fluid itself. Spatially resolved spectroscopy of the soft diffuse thermal X-ray emission reveals that the predominant source of spectral variation along the outflow cones is due to strong variation in the absorption on scales of ∼60 pc, there being little change in the characteristic temperature of the emission. We show that these observations are easily explained by, and fully consistent with, the standard model of a superwind driven by a starburst of the observed power of NGC 253. If these results are typical of all starburst-driven winds, then we do not directly see all the energy and gas (in particular the hot metal-enriched gas) transported out of galaxies by superwinds, even in X-ray emission.

We present the results from a comparative study of the atomic hydrogen (H I) and optical properties of a sample of 16 dwarf galaxies, chosen to investigate the effects of star formation on the properties of low-mass systems. The violent star formation bursts believed to occur in these low-mass systems suggest a possible connection between the actively star-forming blue compact dwarfs (BCDs), and the quiescent low surface brightness dwarfs (LSBDs). It has been suggested that LSBDs, upon undergoing a burst of star formation, will evolve into BCDs and then back into LSBDs when the star formation slows or stops as the H I column density falls below the critical threshold necessary to support it. We have examined the location and kinematics of H I in eight BCDs and eight LSBDs of similar H I masses and a range of color indices to investigate this "evolutionary" sequence. The starburst episodes in these low-mass galaxies should lead to (1) a dispersal/depletion of the H I seen in the eight LSB dwarfs and (2) more centrally concentrated and agitated H I in the eight BCDs. The results of this project indicate that the quiescent LSBD galaxies have more diffuse H I distributions and often show a ringlike structure, while the active galaxies have more highly centrally concentrated H I reservoirs. The bluer, more recently active systems of both types also have higher internal H I velocity dispersions, indicating that energy has been pumped into the interstellar medium of these galaxies. These observations are consistent with an evolutionary scheme wherein the H I reservoirs in these galaxies take on different characteristics depending upon their star formation histories.

We present new Hα and [O III] λ5007 narrowband images of the starbursting dwarf galaxy NGC 4214, obtained with the Wide Field and Planetary Camera (WFPC2) on board the Hubble Space Telescope (HST), together with VLA observations of the same galaxy. The HST images resolve features down to physical scales of 2–5 pc, revealing several young (<10 Myr) star-forming complexes of various ionized gas morphologies (compact knots, complete or fragmentary shells) and sizes (∼10–200 pc). Our results are consistent with a uniform set of evolutionary trends: The youngest, smaller, filled regions that presumably are those just emerging from dense star-forming clouds tend to be of high excitation and are highly obscured. Evolved, larger shell-like regions have lower excitation and are less extincted owing to the action of stellar winds and supernovae. In at least one case we find evidence for induced star formation, which has led to a two-stage starburst. Age estimates based on W(Hα) measurements do not agree with those inferred from wind-driven shell models of expanding H II regions. The most likely explanation for this effect is the existence of an ≈2 Myr delay in the formation of superbubbles caused by the pressure exerted by the high-density medium in which massive stars are born. We report the detection of a supernova remnant embedded in one of the two large H II complexes of NGC 4214. The dust in NGC 4214 is not located in a foreground screen but is physically associated with the warm ionized gas.

Eight dwarf irregular galaxies, in the two nearby groups of galaxies Sculptor and Centaurus A (at 2.5 Mpc and 3.5 Mpc), have been imaged in neutral hydrogen (H I) with the Australia Telescope and the Very Large Array. These galaxies have absolute magnitudes ranging from M_B = -15.7 to -11.3. Yet they are mostly rotationally supported, with maximum velocities going from 19 to 67 km s^-1. Multicomponent mass models have been fitted to the rotation curves to investigate the properties of their dark matter halos and the scaling laws of dark matter halo parameters. Dwarf galaxies have, on average, a higher dark to luminous mass ratio, as well as higher dark halo central densities than spiral galaxies. They have a larger dispersion of their dark matter properties both in terms of their total dark matter amount and of their dark halo parameters, compared to spiral galaxies. It is therefore very difficult to predict a dwarf galaxy rotation curve shape based only on its optical properties. Dwarfs are not well fitted by cold dark matter (CDM) halos of the type proposed by Navarro, Frenk, & White, even for ΛCDM models with Ω₀ as low as 0.3. For two of our dwarfs we also have Hα rotation curves confirming the H I velocities, so the discrepancy with the CDM models cannot be attributed to beam-smearing effects.

We present Hubble Space Telescope Wide Field Planetary Camera 2 photometry of the central regions of the Phoenix dwarf galaxy. Accurate photometry allows us to (1) confirm the existence of the horizontal branch previously detected by ground-based observations and use it to determine a distance to Phoenix, (2) clearly detect the existence of multiple ages in the stellar population of Phoenix, (3) determine a mean metallicity of the old red giant branch stars in Phoenix and suggest that Phoenix has evolved chemically over its lifetime, and (4) extract a rough star formation history for the central regions, which suggests that Phoenix has been forming stars roughly continuously over its entire lifetime.

We have developed a robust, automated method, hereafter designated HIIphot, which enables accurate photometric characterization of H II regions while permitting genuine adaptivity to irregular source morphology. HIIphot utilizes object recognition techniques to make a first guess at the shapes of all sources and then allows for departure from such idealized "seeds" through an iterative growing procedure. Photometric corrections for spatially coincident diffuse emission are derived from a low-order surface fit to the background after exclusion of all detected sources. We present results for the well-studied, nearby spiral M51 in which 1229 H II regions are detected above the 5 σ level. A simple, weighted power-law fit to the measured Hα luminosity function (H II LF) above log L_Hα = 37.6 gives α = -1.75 ± 0.06, despite a conspicuous break in the H II LF observed near L_Hα = 10^38.9. Our best-fit slope is marginally steeper than measured by Rand, perhaps reflecting our increased sensitivity at low luminosities and to notably diffuse objects. H II regions located in interarm gaps are preferentially less luminous than counterparts which constitute M51's grand design spiral arms and are best fitted with a power-law slope of α = -1.96 ± 0.15. We assign arm/interarm status for H II regions based upon the varying surface brightness of diffuse emission as a function of position throughout the image. Using our measurement of the integrated flux contributed by resolved H II regions in M51, we estimate the diffuse fraction to be approximately 0.45—in agreement with the determination of Greenawalt et al. Automated processing of degraded narrowband data sets is undertaken in order to gauge (distance-related) systematic effects associated with limiting spatial resolution and sensitivity.

We present optical spectroscopy of four star clusters and six H II regions in the nearby, dwarf irregular galaxy NGC 6822. From the data, we estimate ages of 2.5 × 10⁷, 2.0 × 10⁸, 1.4 × 10⁹, and 10 ± 2 × 10⁹ yr for clusters H VI, C21, H VIII, and H VII, respectively. For clusters H VI and H VII we estimate [Fe/H] ≈ -1.46 ± 0.26 and ∼-2.0, lower than Large and Small Magellanic Cloud clusters at similar ages. Mass estimates for H VI, H VII, and H VIII demonstrate that clusters with typical globular cluster masses (>10⁴M_⊙) have formed over the lifetime of NGC 6822. For six H II regions, ionic abundances are derived for element species of N, O, S, and Ne. These show that there is a correlation of abundance with position, the highest oxygen abundance H II regions being located off the main body of the galaxy.

We present the first spectra of an RV Tauri star in the Large Magellanic Cloud. MACHO* 04:55:43.2-67:51:10 is a recently identified RV Tauri variable with a double period of 112.448 days. Three low-resolution (210 Å mm^-1, or 7 Å resolution) spectrograms obtained of this star show strong C₂ bands with a pronounced phase dependence, their strength being greatest at minimum light. The overall spectrum of MACHO*04:55:43.2-67:51:10 is similar to those observed in proto–planetary nebulae. This unusual star therefore combines the main spectral anomaly of the proto–planetary nebulae with RV Tauri–like photometric variability. The occurrence of a luminous, carbon-rich RV Tauri–like star may be explained by assuming that it is a star of 1–2 solar masses that has evolved through the M-S-C transition on the extended asymptotic giant branch (AGB) and is now moving to the blue in the pre–planetary nebula stage. This star may therefore represent a link between these two types of star and strengthen the post-AGB interpretation of RV Tauri stars.

Hubble Space Telescope (V, I) photometry has been obtained for the inner halo globular cluster NGC 6652. The photometry reaches ∼4 mag below the turnoff and includes a well-populated horizontal branch. This cluster is located close to the Galactic center, at R_GC ≃ 2.0 kpc, with a reddening of E(V-I) = 0.15 ± 0.02, and has a metallicity of [Fe/H] ≃ -0.85. Based upon ΔV, NGC 6652 is 11.7 ± 1.6 Gyr old. Using ΔV, precise differential ages for 47 Tuc (a thick disk globular cluster), M107, and NGC 1851 (both halo clusters) were obtained. NGC 6652 appears to be the same age as 47 Tuc and NGC 1851 (within ±1.2 Gyr), while there is a slight suggestion that M107 is older than NGC 6652 by 2.3 ± 1.5 Gyr. As this is a less than 2 σ result, this issue needs to be investigated further before a definitive statement regarding the relative age of M107 and NGC 6652 may be made.

CCD photometry extending below the turnoff of the globular cluster NGC 6397 on the uvbyHβ system is presented and analyzed. Restricting the sample to stars with small photometric errors and highly probable cluster membership, the reddening from approximately 1500 stars at the turnoff is E(b-y) = 0.127 ± 0.002 (standard error of the mean), equivalent to E(B-V) = 0.179 ± 0.003. The photometric abundance for the same sample, on the revised spectroscopic scale for globular clusters, is [Fe/H] = -1.82 ± 0.04 (standard error of the mean). From 220 cluster red giants, well-defined relations are established for m₁ and c₁ as a function of b-y. The reddening from the giants is systematically smaller than found for the turnoff stars, while the m₁-based metallicity ranges from [Fe/H] = -2.4 for the bluest stars to greater than -1.7 for the reddest. In conjunction with previous results for ω Cen, M22, M55, and M13, it is concluded that the globular cluster red giants do not follow the same c₁ versus b-y and m₁ versus b-y relations as the field giants, even if the cluster sample is restricted to CN-weak giants. Comparison of the cluster turnoff stars with field dwarfs of comparable [Fe/H] in the c₁ versus b-y diagram implies that, within the uncertainties, there is no difference in age between the two samples. From a handful of field dwarfs with comparable [Fe/H] and accurate parallaxes, the modulus of NGC 6397, (V - M_V)₀, is calculated to be 12.15. Comparison with isochrones of the appropriate [Fe/H] with [α/H] = +0.3 results in an age of 12.0 ± 0.8 Gyr. The ultraviolet color-magnitude diagram of the cluster is examined and compared with earlier work.

We present CCD photometry for the Galactic globular cluster NGC 4833. Our BVI color-magnitude diagrams extend from above the tip of the red giant branch to several magnitudes below the main-sequence turnoff. The principal sequences of the cluster show the effects of differential reddening. We have created a local extinction map, consistent with IRAS and COBE DIRBE dust maps of the region. We use our map to correct the colors and magnitudes of each star to the value at the cluster center. The cluster horizontal branch (HB) is predominately blueward of the instability strip with 13 confirmed RR Lyrae variables and five additional RR Lyrae candidates. Using the 11 confirmed RR Lyraes measured on our images and the differential reddening corrected photometry, we calculate V(HB) = 15.56 ± 0.063. We have used the simultaneous reddening and metallicity method of Sarajedini to find the mean reddening of the cluster E(B-V) = 0.32 ± 0.03, and the mean metallicity [Fe/H] = -1.83 ± 0.14. As for the age of NGC 4833, we find provisional evidence that NGC 4833 is 2 ± 1 Gyr older than the average of M92 and M5. A more definitive conclusion must await higher resolution imaging of the cluster core where the effects of differential reddening are minimized.

We use the 2MASS Second Incremental Release Point Source Catalog to investigate the spatial distribution of young stars in the Perseus, Orion A, Orion B, and MonR2 molecular clouds. After subtracting a semiempirical model of the field star contamination from the observed star counts, stellar surface density maps are used to identify compact clusters and any stellar population found more uniformly distributed over the molecular cloud. Each cloud contains between two and seven clusters, with at least half of the cluster population found in a single, rich cluster. In addition, a distributed stellar population is inferred in the Orion A and MonR2 molecular clouds within the uncertainties of the field star subtraction with a surface density between 0.013 and 0.083 arcmin^-2. Sensitivity calculations suggest, however, that the number of stars in the distributed population may be underestimated by a factor of 2 or more if stars have been forming with a Miller-Scalo IMF at a constant star formation rate for longer than 10 Myr. After considering the possible evolutionary status of the distributed population, the global star formation efficiency implied by the sum of the distributed and cluster populations ranges between 1% and 9% among the four clouds. The fraction of the total stellar population contained in clusters for the nominal extinction model ranges from ≈50% to 100% if the distributed population is relatively young (<10 Myr), to ≈25%–70% if it is relatively old (≈100 Myr). The relatively high fraction of stars contained in clusters regardless of the age of the distributed population, in conjunction with the young ages generally inferred for embedded clusters in nearby molecular clouds, indicates that a substantial fraction of the total stellar population in these regions has formed within the past few million years in dense clusters. This suggests that either the star formation rate in each these clouds has recently peaked if one assumes clouds have ages greater than 10 Myr or molecular clouds are younger than typically thought if one assumes that the star formation rate has been approximately constant in time.

We report the results of a sensitive near-infrared JHKL imaging survey of the Trapezium cluster in Orion. We use the JHKL colors to obtain a census of infrared excess stars in the cluster. Of (391) stars brighter than 12th magnitude in the K and L bands, 80% ± 7% are found to exhibit detectable infrared excess on the J-H, K-L color-color diagram. Examination of a subsample of 285 of these stars with published spectral types yields a slightly higher infrared excess fraction of 85%. We find that 97% of the optical proplyds in the cluster exhibit excess in the JHKL color-color diagram indicating that the most likely origin of the observed infrared excesses is from circumstellar disks. We interpret these results to indicate that the fraction of stars in the cluster with circumstellar disks is between 80%–85%, confirming earlier published suggestions of a high disk fraction for this young cluster. Moreover, we find that the probability of finding an infrared excess around a star is independent of stellar mass over essentially the entire range of the stellar mass function down to the hydrogen burning limit. Consequently, the vast majority of stars in the Trapezium cluster appear to have been born with circumstellar disks and the potential to subsequently form planetary systems, despite formation within the environment of a rich and dense stellar cluster. We identify 78 stars in our sample characterized by K-L colors suggestive of deeply embedded objects. The spatial distribution of these objects differs from that of the rest of the cluster members and is similar to that of the dense molecular cloud ridge behind the cluster. About half of these objects are detected in the short wavelength (J and H) bands, and these are found to be characterized by extreme infrared excess. This suggests that many of these sources could be protostellar in nature. If even a modest fraction (i.e., ∼50%) of these objects are protostars, then star formation could be continuing in the molecular ridge at a rate comparable to that which produced the foreground Trapezium cluster.

An analysis of the multiplicity of 14 sources driving giant Herbig-Haro flows has revealed an observed binary frequency between 79% and 86%, of which half are higher order multiples. These sources represent the hitherto youngest sample of stars examined for binarity. I postulate that the dynamical decay of triple or multiple systems leads to strong outflow activity. It is well known that a large fraction of nonhierarchical triple systems rapidly break up and eject the lightest member. At the same time a closer binary in a highly eccentric orbit is formed. Massive disk truncation results, accompanied by large-scale accretion, with a consequent burst of outflow activity, which produces the observed giant HH bow shocks. Some of the material culled from the individual circumstellar disks may settle into a circumbinary disk around the newly bound stellar pair. The small remaining and truncated circumstellar disks are fed from the circumbinary disk through gas streams, and this as well as other dynamical effects cause the binary orbit to shrink. Gas streams together with disk interactions at periastron drive cyclic accretion modulated on an orbital timescale. As the stellar components gradually spiral toward each other, the increasingly frequent mass-loss events form chains of HH objects until eventually the binary has a semimajor axis of only 9–12 AU, at which point the closely spaced shocked ejecta appear as a finely collimated jet. Thus, such HH flows can be read as a fossil record of the evolution of orbital motions of a binary, newly formed in a triple disintegration event, as it shrinks from a typical separation of 100 AU or more to 10 AU or less. When the triple system disintegrates and a single star is ejected, the newly formed binary recoils, and as a result both components (star and close binary) leave their nascent envelope. While one component becomes visible as a T Tauri star, the other will be obscured for a while by the envelope and will appear as a bright near-infrared object. For typical parameters, this geometry persists for only 5000 yr or so. If the ejected star does not escape, cyclic motion of a hierarchical triple begins. This explains the so-called IRC binaries that are infrequently found in star-forming regions. The standard model of early stellar evolution states that young stars gradually and smoothly make the transitions from Class 0 through Class I and II objects to eventually become Class III objects. In contrast, stars born in multiple systems can abruptly transit from a Class 0 or I object to a visible T Tauri star. The main accretion phase may be terminated by the stochastic process of triple decay. Depending on the moment of triple disintegration, the ejected objects can range from stellar embryos, which will emerge as very low mass stars or even brown dwarfs, to essentially fully built-up stars. In this picture, the initial mass function toward its low-mass end has an important stochastic component that can only be described by the half-life of the decay processes. Because the ejected stars can take only limited circumstellar material with them, they will soon lose their classical T Tauri characteristics and join the halo of weak-line T Tauri stars that surround star-forming clouds. Differences in ejection may explain why two apparently similar T Tauri stars of about the same age can have major differences in the size of their circumstellar disks.

As part of a systematic study of the environment of Galactic WR stars, a region along the line of sight to Cygnus (l = 77.5°, b = 0°) has been studied. The neutral hydrogen 21 cm line distribution shows the existence of a cavity expanding from the Wolf-Rayet star WR 143. This cavity, created by the stellar wind of the star, has a mean radius of 7 pc (assuming a distance of about 1 kpc), an expansion velocity of more than 8 km s^-1, and a missing mass of some 150 M_⊙ and seems to be surrounded by an H I shell, likely made up of the gas pushed by the star. Although this hole (also called bubble) is quite conspicuous in H I, there seems to be no trace of it at the other wavelengths studied (radio continuum and infrared). All radio observations were obtained at the Dominion Radio Astrophysical Observatory as part of the Canadian Galactic Plane Survey.

We present the most intensive, high-quality spectroscopic monitoring of optical Wolf-Rayet emission lines ever obtained. The Wolf-Rayet star HD 192103 (=WR 135; subtype WC8) was observed in the 5650–5840 Å regime alternately from both the William Herschel Telescope and the Canada-France-Hawaii Telescope. The final data consist of a series of 197 spectra spread over 64 hr, each with a resolving power λ/Δλ ≃ 20,000 and a signal-to-noise ratio in the continuum ≃450 per 3 pixel resolution element. We clearly and unambiguously identify stochastic, structured patterns of intrinsic variability at the 1%–2% level of the line flux in the broad C III λ5696 emission line. The λ5801/12 doublet emission is also found to be variable at the 0.2%–0.5% level of the line flux. We find a correlation between the variability patterns observed in C III and C IV, which suggests a significant overlap in the emission volumes of these transitions, although C IV is known to arise somewhat closer to the star. We attempt to reproduce the observed line profile variation patterns using a simple phenomenological model, which assumes the wind to be fully clumped. With a minimal set of assumptions, we are able to reproduce both the shape and the variability in the C III λ5696 emission profile. We show that the variability pattern provides constraints on the radial extent of WR 135's wind where C III is produced, as well as on the local wind acceleration rate. However, our simple clump model does not reproduce the lower variability in the C IV doublet unless we assume the C IV emission to occur in a much larger volume than C III, implying that significant C IV emission occurs farther out in the wind than C III. We suggest that while some C IV emission might occur farther out, possibly because of reionization from shocks, a more likely explanation is that wind clumping significantly increases with distance from the star, leading to larger variability levels in C III, formed farther out than most of C IV. Alternatively, optical depth effects and/or local ionization gradients within clumps could conspire to attenuate clumping effects in the C IV emission line while enhancing them in the C III line.

We report on observations of the candidate supernova remnant (SNR) G106.3+2.7 with the Dominion Radio Astrophysical Observatory's Synthesis Telescope in the continuum at both 408 and 1420 MHz and in the 21 cm line of neutral hydrogen. The general morphology of the object and its spectral index (α ≈ 0.57 ± 0.04, where S_ν ∝ ν^-α) confirm it as an SNR. The object consists of two distinct components, the head and the tail. The tail component is of lower surface brightness and has a marginally steeper spectral index than the head component. A deficiency of neutral hydrogen at an LSR velocity of about -105 km s^-1 is most likely due to the effect of the SNR, suggesting that the SNR is expanding at a velocity of about 15 km s ^-1, it is at a kinematic distance of 12 kpc, and its largest angular extent is of the order of 200 pc. These parameters are shown to be consistent with a dynamical model in which the SNR is in a very late stage of its isothermal evolution, where the pressure inside the SNR is approaching the pressure of the ambient interstellar medium. We also describe the H II region Sh 141, which is about 20' north of G106.3+2.7.

We present photometric observations in B and V, as well as spectroscopic observations of the detached, eccentric 6.6 day double-lined eclipsing binary GG Ori, a member of the Orion OB1 association. Absolute dimensions of the components, which are virtually identical, are determined to high accuracy (better than 1% in the masses and better than 2% in the radii) for the purpose of testing various aspects of theoretical modeling. We obtain M_A = 2.342 ± 0.016 M_⊙ and R_A = 1.852 ± 0.025 R_⊙ for the primary, and M_B = 2.338 ± 0.017 M_⊙ and R_B = 1.830 ± 0.025 R_⊙ for the secondary. The effective temperature of both stars is 9950 ± 200 K, corresponding to a spectral type of B9.5. GG Ori is very close to the zero-age main sequence, and comparison with current stellar evolution models gives ages of 65–82 Myr or 7.7 Myr, depending on whether the system is considered to be burning hydrogen on the main sequence or still in the final stages of pre–main-sequence contraction. Good agreement is found in both scenarios for a composition close to solar. We have detected apsidal motion in the binary at a rate of = 000061 ± 000025 cycle^-1, corresponding to an apsidal period of U = 10,700 ± 4500 yr. A substantial fraction of this (∼70%) is due to the contribution from general relativity, and our measurement is entirely consistent with theory. The eccentric orbit of GG Ori is well explained by tidal evolution models, but both theory and our measurements of the rotational velocity of the components are as yet inconclusive as to whether the stars are synchronized with the orbital motion.

The Tycho-2 Catalogue provides astrometric and photometric information for 2.5 million stars. Within the Tycho-2 Catalogue, while there is no separate listing of double-star parameters, there is a wealth of double-star data. For all measured double-star systems, each component has its own entry in Tycho-2 (this corresponds to the Hipparcos "double-entry systems"), and the classical double-star parameters of separation, position angle, and magnitude difference can be calculated for these systems. Most double stars in the Tycho-2 Catalogue are identified by a code indicating a double, failed double, or photocentric solution. Stars flagged in this manner include 6251 known double stars and 1234 new double-star systems (designated in the Washington Double Star Catalog [WDS] with a "TDS" discovery code). Also, 4726 systems solved via Tycho-2 "single-star treatment" not only matched one component of a WDS pair but also had a nearby star that closely approximated the position and magnitude of the other WDS component. Finally, 1133 WDS systems were seen as single in Tycho-2 whose historical double-star parameters would indicate that Tycho-2 should have measured them. The observational statistics of these various classes of objects are presented together with catalog statistics for those with single-star solutions. A quantitative assessment of Tycho-2 double-star measures is provided via a comparison of Tycho-2 O-C residuals to known orbits. Finally, a subset of systems measured by Tycho-2 have been verified via speckle interferometry using the 82 inch (2 m) telescope of McDonald Observatory. These results are presented here.

We have obtained a series of spectroscopic measurements of the high proper-motion star BPM 6502 showing Balmer emission superposed upon the absorption lines. Radial velocity measurements determined by tracing optical absorption and emission lines show that the star is a binary system comprised of a DA white dwarf and an M dwarf with an orbital period of 0.33678 days. The space motion of the system falls within the general white dwarf population, and the system is not expected to initiate mass transfer within a Hubble time.

Infrared radial velocities have been used to determine orbital elements for the cool giants of five well-known symbiotic systems, Z And, AG Dra, V443 Her, AX Per, and FG Ser, all of which have orbital periods near the two-year mean period for S-type symbiotics. The new orbits are in general agreement with previous orbits derived from optical velocities. From the combined optical and infrared velocities, improved orbital elements for the five systems have been determined. Each of the orbital periods has been determined solely from the radial-velocity data. The orbits are circular and have quite small mass functions of 0.001–0.03 M_⊙. The infrared velocities of AG Dra do not show the large orbital velocity residuals found for its optical radial velocities.

We have obtained spectroscopy and photometry of HD 95559 and photometry of Gliese 410 = DS Leonis. HD 95559 consists of a pair of essentially identical K1 V stars, whose orbital period we refine to 1.52599775 ± 0.00000104 days. The system is photometrically variable with a mean period of 1.5264 ± 0.0003 days. Despite minimum masses greater than 0.8 M_⊙ for each component, a search for eclipses proved negative. The lithium abundances of the components of HD 95559 indicate that the system is younger than the Hyades cluster, and its components may even have just arrived on the zero-age main sequence. Gl 410 = DS Leo is also a photometric variable, but we conclude that the photometric period originally ascribed to this star is an alias of the period for HD 95559. We find periods of 13.99 and 15.71 days for the first and second seasons of observation, respectively. Both HD 95559 and Gl 410 are BY Draconis variables, with variability resulting from the rotational modulation of starspots. We also find HR 4269, the check star for our photometry of HD 95559 and Gl 410, to be a variable K4 III with a photometric period of 26.4 days in the first season of observation and periods of 13.96 and 83 days in the second. We suggest that its variability mechanism is radial pulsation, the same as that for M giant semiregular variables.

Matrix light-curve inversion (MLI) is a technique for deducing the surface brightness distributions of rapidly rotating spotted stars or the surface albedo distributions of planets (in particular Pluto) from their rotational light curves. When applied to the stellar problem, it has the significant advantage over "spot models" that it makes no a priori assumptions about the number of spots on the stellar surface or their shapes. We demonstrate the viability of MLI for determining the locations and sizes of dark spots on stellar surfaces and explore its potential and its limitations by presenting the results of inversions of synthetic light curves corresponding to model stars with known surface features. We show that when light curves acquired through different photometric filters are simultaneously inverted, significant improvements can be achieved compared to when only a single filter is used. In particular, it becomes possible to detect the presence of high-latitude activity, presenting the possibility of corroborating Doppler images that imply high-latitude spots.

Local simulations of the outer edge of the Encke gap are presented that use particles of approximately the proper size and optical depth but ignore self-gravity. These simulations clearly show the formation and damping of wakes caused by the moonlet Pan. In this paper we focus primarily on shear reversal and the values of the pressure tensor in this system. We observe angular momentum luminosity reversal in the simulations, lending support to the prediction that this process could be responsible for the sharp edges seen at the Encke gap. We also find evidence for vertical splashing of particles out of the ring plane at the wake peaks. This vertical splashing violates assumptions made in many analytic treatments of this region but does not appear to invalidate the main conclusions drawn from those models. In addition, we find that the maximum amplitude of the wakes is limited by localized displacements of the particles' semimajor axes near the wake peaks.

We have investigated excitations of orbital eccentricities and inclinations of Kuiper belt objects (KBOs) caused by the sweeping secular resonances during the primitive solar nebula depletion. Since nebula gravitational potential rotates the longitudes of perihelia and ascending nodes, the nebula depletion leads to migration of secular resonances. In the outer (classical) Kuiper belt (the region beyond 42 AU), inclinations and eccentricities are respectively distributed up to 0.6 (radian) and 0.2, and their root mean squares are about 0.2 (radian) and 0.1. These large values are not explained by present planetary perturbations alone. We have investigated the sweeping secular resonances in the Kuiper belt with both direct orbital integration and the analytical method and have found that the sweeping secular resonances can account for the eccentricity and inclination in the outer belt. Inclinations of objects in the outer belt are excited to the observational level if the residual nebula with about 0.1% of the density in the minimum mass nebula model is depleted in a timescale of 10⁷–10⁸ yr. For inclination excitation, Jovian perturbations and nebula potential are the most important, and Neptunian perturbations do not play an important role during the residual nebula depletion, although Neptune with more than one-fifth of its present mass is needed for enough eccentricity excitation. If further observation of the KBOs at semimajor axis ≳50 AU confirms our model, it would give important clues about Neptune's formation and the depletion of the solar nebula.

We present a procedure for determination of positions and orbital elements, and associated uncertainties, of outer solar system planets. The orbit-fitting procedure is greatly streamlined compared with traditional methods because acceleration can be treated as a perturbation to the inertial motion of the body. These techniques are immediately applicable to Kuiper belt objects, for which recovery observations are costly. Our methods produce positional estimates and uncertainty ellipses even in the face of the substantial degeneracies of short-arc orbit fits; the sole a priori assumption is that the orbit should be bound or nearly so. We use these orbit-fitting techniques to derive a strategy for determining Kuiper belt orbits with a minimal number of observations.

We confirm that the positional error of a perturbed two-body problem expressed in the Kustaanheimo-Stiefel (K-S) variable is proportional to the fictitious time s, which is the independent variable in the K-S transformation. This property does not depend on the type of perturbation, on the integrator used, or on the initial conditions, including the nominal eccentricity. The error growth of the physical time evolution and the Kepler energy is proportional to s if the perturbed harmonic oscillator part of the equation of motion is integrated by a time-symmetric integration formula, such as the leapfrog or the symmetric multistep method, and is proportional to s² when using traditional integrators, such as the Runge-Kutta, Adams, Störmer, and extrapolation methods. Also, we discovered that the K-S regularization avoids the step size resonance/instability of the symmetric multistep method that appears in the unregularized cases. Therefore, the K-S regularized equation of motion is useful to investigate the long-term behavior of perturbed two-body problems, namely, those used for studying the dynamics of comets, minor planets, the Moon, and other natural and artificial satellites.

We present results from the application of a global photometric calibration (GPC) procedure to calibration data from the first 2 years of the Two Micron All Sky Survey (2MASS). The GPC algorithm uses photometry of both primary standards and moderately bright "tracer" stars in 35 2MASS calibration fields. During the first 2 years of the survey, each standard was observed on approximately 50 nights, with about 900 individual measurements. Based on the photometry of primary standard stars and secondary tracer stars and under the assumption that the nightly zero-point drift is linear, the GPC ties together all calibration fields and all survey nights simultaneously, producing a globally optimized solution. Calibration solutions for the Northern and Southern Hemisphere observatories are found separately and are tested for global consistency based on common fields near the celestial equator. Several results from the GPC are presented, including establishing candidate secondary standards, monitoring of near-infrared atmospheric extinction coefficients, and verification of the global validity of the standards. The solution gives long-term averages of the atmospheric extinction coefficients (in magnitudes per air mass), A_J = 0.096, A_H = 0.026, A = 0.066 (north) and A_J = 0.092, A_H = 0.031, A = 0.065 (south), with formal error of 0.001. The residuals show small seasonal variations, most likely due to changing atmospheric content of water vapor. Extension of the GPC to ∼100 field stars in each of the 35 calibration fields yields a catalog of more than 2000 photometric standards ranging from the 10th to 14th magnitude, with photometry that is globally consistent to ∼1%.

The growing level of radio frequency interference (RFI) is a recognized problem for research in radio astronomy. This paper describes an intuitive but powerful RFI cancellation technique that is suitable for radio spectroscopy where time-averages are recorded. An RFI "reference signal," is constructed from the cross power spectrum of the signals from the two polarizations of a reference horn pointed at the source of the RFI signal. The RFI signal paths obey simple phase and amplitude closure relations, which allows computation of the RFI contamination in the astronomical data and the corrections to be applied to the astronomical spectra. Since the method is immune to the effects of multipath scattering in both the astronomy and reference signal channels, "clean copies" of the RFI signal are not required. The method could be generalized (1) to interferometer arrays, (2) to correct for scattered solar radiation that causes spectral "standing waves" in single-dish spectroscopy, and (3) to pulsar survey and timing applications where a digital correlator plays an important role in broadband pulse dedispersion. Future large radio telescopes, such as the proposed LOFAR and SKA arrays, will require a high degree of RFI suppression and could implement the technique proposed here with the benefit of faster electronics, greater digital precision and higher data rates.

We describe our methods to validate the absolute calibration of the infrared (IR) radiometry of the Midcourse Space Experiment (MSX) Point Source Catalog, version 1.2 (PSC1.2). These are based upon stars drawn from our current all-sky network of absolute calibrators and upon other stars that also support the calibration of ESA's Infrared Space Observatory and of several other satellites. Based on the mean ratios of (observed/predicted) irradiances for this ensemble of stars, all four of MSX's mid-IR bands are consistent with this network; i.e., there are no mean ratios that deviate more than 3 σ from unity. However, the two narrowest bands (near 4.3 μm) are significantly discrepant by a few percent. This paper probes the radiometry of stars fainter than those validated by an independent study of the primary and secondary MSX standards by Cohen et al. To provide the user of the PSC1.2 with a basis for comparison, we derive the IR properties of normal stars from measurements by MSX's IR focal planes.