Planetary Camera Research Articles

Inner Polar Rings in Regular Lenticular Galaxies

We have investigated a sample of S0 galaxies, mostly with circumnuclear dust lanes orthogonal to their major axes, chosen from Hubble Space Telescope Wide Field Planetary Camera 2 images. Two-dimensional spectroscopy undertaken with the Multipupil Fiber Spectrograph of the 6 m telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences has revealed that indeed the ionized gas in the centers of these eight lenticular galaxies rotate in the planes nearly orthogonal to the rotation (and symmetry) planes of their central stellar components. Although almost all the galaxies are located in dense environments, an external origin of this rotation plane tilt is not obvious because all the galaxies but one are known to have extended H I disks, and in two cases where the angular resolution of H I observations allows, we find orthogonality of the external H I and inner ionized gas disks. We discuss a possible relation of the inner gas polar rings to a triaxiality of galactic potential. The stellar populations in the nuclei of all but two galaxies are very old, which excludes recent star formation bursts and proves that the polar orbits of the circumnuclear gas are rather stable. In the nuclei of NGC 2655 and NGC 4111, we have found signatures of star formation bursts some 1.5–2 Gyr ago. This finding can be related to very central gas in NGC 2655, which is coplanar to the circumnuclear stellar disk and to radial gas inflow in NGC 4111; just these gas reservoirs and not the polar rings may be responsible for fueling nuclear star formation.

Hubble Space TelescopeAstrometric Observations and Orbital Mean Motion Corrections for the Inner Satellites of Neptune

Six small inner satellites of Neptune were imaged in 1989 with Voyager 2. In 1997, we recovered the four outermost with the Hubble Space Telescope (HST) Wide Field Planetary Camera 2 for astrometric, dynamical, and photometric studies. The ring arcs were not detected in our images. Thirteen exposures were taken in each of three HST orbits: two orbits on July 3 and one on July 6. Exposures were taken in the BVI filters. Measurable images of Neptune and Triton were also obtained on the same PC1 frames with those of the faint satellites. We present here the astrometric observations of these four satellites relative to Neptune, as well as corrected orbital mean motions for them. Field distortions in the PC1 chip were corrected with both the Trauger et al. and the Anderson & King distortion models. Calibration of the scale and orientation was accomplished by comparing the measured positions of Neptune and Triton with an accurate JPL J2000 ephemeris. Separate calibrations were made for both distortion models. Small differences were detected in the calibrations, dependent on wavelength, saturation, and filter, and a small difference was found between the calibrations resulting from both distortion correction models. The resulting separation and position angle observations for the inner satellites were compared with the orbits of Owen et al. and corrections derived to their mean daily motions. A small but significant discrepancy was found for Proteus between the correction derived from the observations of separation and that from the position angles. This was shown not to be due to calibrational errors but, apparently, to the need for improvement of other orbital elements—at least for Proteus. Despite this anomaly, the mean motion accuracies were improved by almost 2 orders of magnitude as a result of the longer baseline since the Voyager observations. More HST observations of these satellites are recommended in order to improve their orbits further and for the investigation of satellite-ring interactions.

AHubble Space TelescopeStudy of Star Formation in the Inner Resonance Ring of NGC 3081

We present Hubble Space Telescope Wide Field Planetary Camera 2 images of the inner regions of NGC 3081, an absolute magnitude MB = -20.0 early-type barred spiral having four well-defined resonance rings: a nuclear ring, an inner ring, an outer R1 ring, and an outer R pseudoring. Here we focus on a photometric study of the inner ring, a feature likely associated with an inner 4 : 1 resonance near the ends of the bar. The ring is notable for its high contrast and sharp definition, which is due to a significant degree of active star formation. The ring is also notable for its significant intrinsic elongation and parallel alignment with the bar. These characteristics influence the way star-forming sites are distributed around the ring. The ring is lined by numerous blue sources, many of which appear to be slightly diffuse compared with the stellar point-spread function. These blue sources are strongly concentrated within ±60° of the bar axis and follow the Hα distribution well. The blue sources are much larger than typical Galactic open or globular and may represent young massive like the populous clusters of the LMC and objects seen previously mainly in intermediate- to late-type spiral galaxies. We also present an analysis of the integrated light of the inner ring, to deduce information on its star formation history. A profile analysis is used to separate the ring from the background old disk starlight. High-resolution Fourier analysis is used to search for wavelength-dependent phase shifts along the ring to determine if star-forming sites stay in the ring as they age. The results give an intriguing picture of a galaxy in an advanced evolutionary state where periodic orbits are clearly manifested in the morphology.

Hubble Space TelescopeImaging of HD 44179, The Red Rectangle

We present Hubble Space Telescope (HST) Wide Field Planetary Camera 2 and deep ground-based images of the Red Rectangle (RR), a bipolar proto–planetary nebula associated with the post-AGB binary system HD 44179. The high-resolution HST images reveal complex new structures, many of them unique to this object. The RR nebula is dominated by a discontinuous whose bright, sharp linear edges give the nebula an overall X-shaped appearance. The edges of the bicone are connected by a remarkable series of linear features elongated perpendicular to the radius vector, giving the object a ladder-like structure. The rungs of the ladder structure show a quasi-periodic spacing, suggesting that they have arisen from discrete episodes of mass loss from the central star, separated by a few hundred years. The total timescale over which mass has been shed into the visible nebula is of order 14,000 yr. Outside the X-shaped bicone, parabolas curl inward, resembling wineglasses, which terminate on the bicone edges in large, limb-brightened vortices. The central object is bisected by a dark band, indicating that the star is not seen directly but is instead obscured by a surrounding opaque dust disk.

Stellar Populations in the Outer Reaches of M31 and M32 from WFPC2 Photometry11Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS 5‐26555.

ABSTRACTHubble Space Telescope Wide Field Planetary Camera 2 photometry in a field 7 effective radii from the nucleus of M32 and a comparison field in the outer disk of M31, already examined for the presence of RR Lyrae variable stars, are further compared. No relative distance difference between the two fields could be measured using the luminosity of red clump stars. Since M32 is projected on the half of the M31 disk inclined away from us, we argue that M32 most likely lies beyond M31. The color‐magnitude diagrams of the two fields are similar and yield abundance distributions that are similar, with the exception that M32 appears to have a narrower abundance distribution than the comparison field in M31. This conclusion is unlikely to be compromised by age effects. M32 has far fewer metal‐poor stars than the simple one‐zone model of chemical evolution predicts. The deepest combined image shows no trace of a main sequence younger than about 1 Gyr.

Deep Photometry in a Remote M31 Major-Axis Field Near G1

We present photometry from Hubble Space Telescope (HST)/Wide Field Planetary Camera 2 parallel imagery of a remote M31 field at a projected distance of about 34 kpc from the nucleus near the SW major axis. This field is near the globular cluster G1, and near one of the candidate tidal plumes identified by Ferguson et al. (2002). The F606W (V) and F814W (I) images were obtained in parallel with Space Telescope Imaging Spectrograph spectroscopy of G1 (GO-9099) and total 7.11 hours of integration time -- the deepest HST field in the outer disk of M31 to date, reaching to V ~ 28. The color-magnitude diagram of the field shows a clearly-defined red clump at V = 25.25 and a red giant branch consistent with [Fe/H] ~ -0.7. The lack of a blue horizontal branch contrasts with other M31 halo fields, the Andromeda dwarf spheroidals, and with the nearby globular cluster G1. Comparing the observed luminosity function to the Padova models, we find that at least some of the stellar population must be younger than 6 - 8 Gyr. The outermost detected neutral hydrogen gas disk of M31 lies only 2 kpc in projection from our field. The finding that some giants in the field have radial velocities close to that of the neutral hydrogen gas (Reitzel, Guhathakurta, & Rich 2003) leads us to conclude that our field samples the old, low-surface-brightness disk rather than the true Population II spheroid.

Hα‐derived Star Formation Rates for thez= 0.84 Galaxy Cluster Cl J0023+0423B

We present Hα-derived star formation rates (SFRs) for the galaxy cluster Cl J0023+0423B at z = 0.845. Our 3 σ flux limits correspond to an SFR of 0.24 h M☉ yr-1, and our minimum reliable Hα + [N ] equivalent width is greater than 10 A, demonstrating that near-infrared narrowband imaging can sample the star-forming galaxy population in distant clusters. Comparison with spectroscopy shows that the number of false detections is low (9% ± 6%) and that our Hα equivalent widths are correlated with spectroscopically determined [O II] equivalent widths. A magnitude-limited spectroscopic survey conducted over the same area missed 70% of the star-forming galaxies and 65% of the integrated star formation. Using Hubble Space Telescope Wide Field Planetary Camera 2 Archive images, we fit Sersic profiles to all galaxies with significant narrowband equivalent widths and find that equivalent width decreases as the steepness of galaxy profile increases. We find no significant population of early-type galaxies with ongoing star formation. The integrated SFR per cluster mass of Cl J0023+0423B is a factor of 10 higher than that of the three z ~ 0.2 clusters in the literature with available Hα observations. A larger sample of z ~ 0.8 clusters spanning a range of cluster masses is needed to determine whether this variation is due to a difference in cluster mass or redshift.

Study of the vertical structure of Saturn's atmosphere using HST/WFPC2 images

We have studied the vertical structure of hazes at six different latitudes (−60°, −50°, −30°, −10°, +30°, and +50°) on Saturn's atmosphere. For that purpose we have compared the results of our forward radiative transfer model to limb-to-limb reflectivity scans at four different wavelengths (230, 275, 673.2, and 893 nm). The images were obtained with the Hubble Space Telescope Wide Field Planetary Camera 2 in September 1997, during fall on Saturn's northern hemisphere. The spatial distribution of particles appears to be very variable with latitude both in the stratosphere and troposphere. For the latitude range +50° to −50°, an atmospheric structure consisting of a stratospheric haze and a tropospheric haze interspersed by clear gas regions has been found adequate to explain the center to limb reflectivities at the different wavelengths. This atmospheric structure has been previously used by Ortiz et al. (1996, Icarus 119, 53–66) and Stam et al. (2001, Icarus 152, 407–422). In this work the top of the tropospheric haze is found to be higher at the southern latitudes than at northern latitudes. This hemispherical asymmetry seems to be related to seasonal effects. Different latitudes experience different amount of solar insolation that can affect the atmospheric structure as the season varies with time. The haze optical thickness is largest (about 30 at 673.2 nm) at latitudes ±50 and −10 degrees, and smallest (about 18) at ±30 degrees. The stratospheric haze is found to be optically thin at all studied latitudes from −50 to +50 degrees being maximum at −10° ( τ=0.033). At −60° latitude, where the UV images show a strong darkening compared to other regions on the planet, the cloud structure is remarkably different when compared to the other latitudes. Here, aerosol and gas are found to be uniformly mixed down to the 400 mbar level.

Discovery of Another Peculiar Radial Distribution of Blue Stragglers in Globular Clusters: The Case of 47 Tucanae

We have used the high-resolution Wide Field Planetary Camera (WFPC2) on the Hubble Space Telescope (HST) and wide-field ground-based observations to construct a catalog of blue straggler stars (BSSs) in the globular cluster 47 Tuc spanning the entire radial extent of the cluster. The BSS distribution is highly peaked in the cluster center, rapidly decreases at intermediate radii, and finally rises again at larger radii. The observed distribution closely resembles that discovered in M3 by Ferraro and coworkers. To date, complete BSS surveys covering the full radial extent (from HST for the center and wide-field CCD, ground-based observations for the exterior) have only been performed for these two clusters. Both show a bimodal radial distribution despite their different dynamical properties. BSS surveys covering the full spatial extent of more globular clusters are clearly required to determine how common bimodality is and what its consequences are for theories of BSS formation and cluster dynamics.

The structure and environment of young stellar clusters in spiral galaxies

A search for stellar clusters has been carried out in 18 nearby spiral galaxies, using archive images from the Wide Field Planetary Camera 2 on board the Hubble Space Telescope. All of the galaxies have previously been imaged from the ground in UBVI. A catalogue of structural parameters, photometry and comments based on visual inspection of the clusters is compiled and used to investigate correlations between cluster structure, environment, age and mass. Least-squares fits to the data suggest correlations between both the full-width at half-maximum (FWHM) and half-light radius (Reff )o f the clusters and their masses (M) at about the 3σ level. Although both relations show a large scatter, the fits have substantially shallower slopes than for a constant-density relation (size ∝ M 1/3 ). However, many of the youngest clusters have extended halos which make the Reff determinations uncertain. There is no evidence for galaxy-to-galaxy variations in the mean cluster sizes. In particular, the mean sizes do not appear to depend on the host galaxy star formation rate surface density. Many of the youngest objects (age < 10 7 years) are located in strongly crowded regions, and about 1/3-1/2 of them are double or multiple sources. The HST images are also used to check the nature of cluster candidates identified in a previous ground-based survey. The contamination rate in the ground-based sample is generally less than about 20%, but some cluster identifications remain ambiguous because of crowding even with HST imaging, especially for the youngest objects.

Relative Frequencies of Blue Stragglers in Galactic Globular Clusters: Constraints for the Formation Mechanisms

We discuss the main properties of the Galactic globular cluster (GC)blue straggler stars (BSSs), as inferred from our new catalog containingnearly 3000 BSSs. The catalog has been extracted from thephotometrically homogeneous V versus (B-V) color-magnitude diagrams(CMDs) of 56 GCs, based on Wide Field Planetary Camera 2 images of theircentral cores. In our analysis, we used consistent relative distancesbased on the same photometry and calibration. The number of BSSs hasbeen normalized to obtain relative frequencies (FBSS) andspecific densities (NS) using different stellar populationsextracted from the CMD. The cluster FBSS is significantlysmaller than the relative frequency of field BSSs. We find a significantanticorrelation between the BSS relative frequency in a cluster and itstotal absolute luminosity (mass). There is no statistically significanttrend between the BSS frequency and the expected collision rate. Thevalue of FBSS does not depend on other cluster parameters,apart from a mild dependence on the central density. Post-core-collapseclusters act like normal clusters as far as the BSS frequency isconcerned. We also show that the BSS luminosity function for the mostluminous clusters is significantly different, with a brighter peak andextending to brighter luminosities than in the less luminous clusters.These results imply that the efficiency of BSS production mechanisms andtheir relative importance vary with the cluster mass.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS 5-26555. (Less)

The Dynamical Distance to M15: Estimates of the Cluster’s Age and Mass and of the Absolute Magnitude of Its RR Lyrae Stars

Newly determined high-precision relative proper motions determined from the Hubble Space Telescope Wide Field Planetary Camera 2 are used along with radial velocity measurements to determine the dynamical distance to the globular cluster M15. A comparison of the proper motion and radial velocity dispersions from a sample of 237 stars, located at an average radial distance of about 10'' from the cluster center, yields a cluster distance of 9.98 ± 0.47 kpc. This distance agrees to within the stated errors to other distance estimates but places this object about 5% closer than the currently adopted value of 10.4 kpc. Using this new distance, we estimate that RR Lyrae stars having [Fe/H] = -2.15 have a value of Mv(RR) = 0.51 ± 0.11. We also estimate that M15 has an age of about 13.2 Gyr, which places it among the oldest of the Galactic globular clusters. From a comparison of the observed velocity dispersion with results from recent N-body calculations, we derive a total cluster mass for M15 of MC = 4.5 × 105 M☉.

Did Carinae's Weigelt Blobs Originate circa 1941?

We report astrometric measurements of η Carinae's Weigelt blobs C and D (also known as speckle objects C and D) derived from observations with the Hubble Space Telescope Wide Field Planetary Camera 2. Using three epochs of data with a temporal baseline of 5.9 yr, we measure blob D's proper motion relative to the central star to be 4.4 ± 1.4 mas yr-1, implying a date of origin of 1934.1. Similarly, using two epochs with a 1.7 yr baseline, we measure blob C's relative proper motion to be 3.8 ± 5.6 mas yr-1, indicating a probability of ejection after 1910 of ≈60%; we view this as confirming the more accurate blob D results (assuming coevality), with a combined post-1910 ejection indicated at a ≈90% confidence level. The blob D date roughly coincides with the sudden brightening of the central star observed in 1941 and provides a possible explanation for the onset of certain narrow spectral features, associated with the Weigelt blobs, that have been observed only since the mid-1940s. We propose that the ejection of Weigelt blobs C and D was related to this 1941 event.

RR Lyrae Stars in M32: Signatures of an Ancient Population

We report the discovery of variable stars in the dwarf elliptical galaxy M32 using a series of images obtained with the Wide Field Planetary Camera 2 on the Hubble Space Telescope. The colors, luminosities, and approximate periods of the majority of these stars indicate that they are RR Lyrae stars. Comparisons with the number of such stars identified in a nearby control field reveals that the majority of these variable stars are M32 members and not contaminants from the bulge, halo, or thick disk of the Andromeda Galaxy, M31. This discovery of RR Lyrae stars in M32 provides strong evidence of the presence of an ancient stellar population in this enigmatic galaxy. Although it is not possible with our data to provide a precise age estimate of the population from which the M32 RR Lyrae stars arise, we argue that these stars almost certainly represent a component that is older than 10 Gyr. Due to the sensitivity of the horizontal branch—and therefore the population of the instability strip—to age and metallicity among other parameters, we roughly estimate that this old and metal-poor component represents at least 2.3% of the total stellar population of M32.

The Stellar Content of the Polar Rings in the Galaxies NGC 2685 and NGC 4650A

We present the results of stellar photometry of polar ring galaxies NGC 2685 and NGC 4650A, using the archival data obtained with the Hubble Space Telescope's Wide Field Planetary Camera 2. Polar rings of these galaxies were resolved into ~800 and ~430 stellar objects in the B, V, and IC bands, a considerable part of which are blue supergiants located in the young stellar complexes. The stellar features in the CM diagrams are best represented by isochrones with metallicity Z = 0.008. The process of star formation in the polar rings of both galaxies was continuous, and the age of the youngest detected stars is about 9 Myr for NGC 2685 and 6.5 Myr for NGC 4650A.

Stellar Associations in the Tail of NGC 4038

We have used the Hubble Space Telescope and Wide Field Planetary Camera 2 to image the putative tidal dwarf galaxy located at the tip of the Southern tidal tail of NGC 4038/9, the Antennae. We resolve individual stars, and identify two stellar populations. Hundreds of massive stars are present, concentrated into tight OB associations on scales of 200 pc, with ages ranging from 2-100 Myr. An older stellar population is distributed roughly following the outer contours of the neutral hydrogen in the tidal tail; we associate these stars with material ejected from the outer disks of the two spirals. The older stellar population has a red giant branch tip at I = 26.5 ± 0.2 from which we derive a distance modulus (m - M)0 = 30.7 ± 0.25. The implied distance of 13.8 ± 1.7 Mpc is nearly a factor of two closer than commonly quoted distances for NGC 4038/9. In contrast to the previously studied core of the merger, we find no super star clusters. One might conclude that SSCs require the higher pressures found in the central regions in order to form, while spontaneous star formation in the tail produces the kind of O-B star associations seen in dwarf irregular galaxies.

The Recent Cluster Formation Histories of NGC 5253 and NGC 3077: Environmental Impact on Star Formation

We present multicolor photometry of bright star cluster candidates in the nearby starburst galaxies NGC 3077 and NGC 5253, observed with the Hubble Space Telescope Wide Field Planetary Camera 2 in both broadband (F300W, F547M, and F814W) and narrowband (F487N and F656N) filters. By comparing the photometry with theoretical population synthesis models, we estimate the age and mass of each star cluster, which provides constraints on the recent star formation histories of the host galaxies. We compare the star cluster populations in these dwarf starburst galaxies with those of the nuclear starburst in the barred spiral M83 and discuss the implications for our understanding of the nature and evolution of starburst events.

Cosmic Gravitational Shear from the Hubble Space Telescope Medium Deep Survey

We present a measurement of cosmic shear on scales ranging from 10'' to 2' in 347 Wide Field Planetary Camera 2 (WFPC2) images of random fields. Our result is based on shapes measured via image fitting and on a simple statistical technique; careful calibration of each step allows us to quantify our systematic uncertainties and to measure the cosmic shear down to very small angular scales. The WFPC2 images provide a robust measurement of the cosmic shear signal decreasing from 5.2% at 10'' to 2.2% at 130''.

The Stellar Content of the Southern Tail of NGC 4038/4039 and a Revised Distance

We have used the Hubble Space Telescope and Wide Field Planetary Camera 2 to image the putative tidal dwarf galaxy located at the tip of the Southern tidal tail of NGC 4038/4039, the Antennae. We resolve individual stars and identify two stellar populations. Hundreds of massive stars are present, concentrated into tight OB associations on scales of 200 pc, with ages ranging from 2 to 100 Myr. An older stellar population is distributed roughly following the outer contours of the neutral hydrogen in the tidal tail; we associate these stars with material ejected from the outer disks of the two spirals. The older stellar population has a red giant branch tip at I = 26.5 ± 0.2 from which we derive a distance modulus (m - M)0 = 30.7 ± 0.25. The implied distance of 13.8 ± 1.7 Mpc is significantly smaller than commonly quoted distances for NGC 4038/4039. In contrast to the previously studied core of the merger, we find no super–star clusters (SSCs). One might conclude that SSCs require the higher pressures found in the central regions in order to form, while spontaneous star formation in the tail produces the kind of OB star associations seen in dwarf irregular galaxies. The youngest population in the putative tidal dwarf has a total stellar mass of ≈2 × 105 M⊙, while the old population has a stellar mass of ≈7 × 107 M⊙. If our smaller distance modulus is correct, it has far-reaching consequences for this prototypical merger. Specifically, the luminous to dynamical mass limits for the tidal dwarf candidates are significantly less than 1, the central SSCs have sizes typical of Galactic globular clusters, rather than being 1.5 times as large, and the unusually luminous X-ray population becomes both less luminous and less populous.

Post–Asymptotic Giant Branch Evolution in the Large Magellanic Cloud: A Study of the Central Stars of Planetary Nebulae

We present medium- and broadband Hubble Space Telescope (HST) photometry of a sample of 35 central stars (CSs) of planetary nebulae (PNs) in the Large Magellanic Cloud (LMC). The observations were made with the Wide Field Planetary Camera 2 (WFPC2) and Space Telescope Imaging Spectrograph instruments on board the HST. By observing LMC objects, our sample is free of the distance uncertainty that is the dominant source of error in the determination of CS luminosities in Galactic PNs. By observing with the HST we resolve the nebula, and therefore we often detect the CSs unambiguously. We obtain core masses of 16 of the objects by comparing their positions on the H-R diagram with theoretical evolutionary tracks, once we determine the stellar effective temperature through Zanstra analysis. This sample of CS masses is the largest and most reliable set obtained in an extragalactic environment. We find an average mass of 0.65 M?, although a few of the objects have very high mass. This average value is consistent with the average mass of the white dwarf population in the Galaxy. As the immediate precursors of white dwarfs, the study of the mass distribution of PN CSs should help to constrain the initial-to-final mass relation within environments of differing metallicity. Finally, by exploring the physical connections between the star and the nebula, we establish the importance of the study of PNs in the LMC to constrain the energy input from the wind during the post-asymptotic giant branch phase.