Hubble Space Telescope Planetary Camera Research Articles

The First Detections of the Extragalactic Background Light at 3000, 5500, and 8000 Å. III. Cosmological Implications

We have used the Hubble Space Telescope Wide Field and Planetary Camera 2 in combination with ground-based spectroscopy to measure the integrated flux of galaxies at optical wavelengths—the extragalactic background light (EBL). We have also computed the integrated light from individual galaxy counts in the images used to measure the EBL and in the Hubble Deep Field. We find that flux in galaxies as measured by standard galaxy photometry methods has generally been underestimated by about 50%, resulting from missed flux in the outer, lower surface brightness parts of galaxies and from associated errors in the estimated sky level. Comparing the corrected, integrated flux from individual galaxies with our total EBL measurement, we find that there is yet further light that contributes to the background that is not represented by galaxy counts and that the total flux in individually detected sources is a factor of 2-3 less than the EBL from 8000 to 3000 A. We show that a significant fraction of the EBL may come from normal galaxies at z 1; while significant flux comes from galaxies beyond the current detection limits, this evolution cannot be tightly constrained by our data. Based on our measurements of the optical EBL, combined with previously published measurements of the UV and IR EBL, we estimate that the total EBL from 0.1 to 1000 μm is 100 ± 20 nW m-2 sr-1. If the total EBL were produced entirely by stellar nucleosynthesis, then we estimate that the total baryonic mass processed through stars is Ω* = 0.0062(±0.0022) h-2 in units of the critical density. For currently favored values of the baryon density ΩB this corresponds to 0.33 ± 0.12ΩB. This estimate is smaller by roughly 7% if we allow for a contribution of 7 h0.7 nW m-2 sr-1 to the total EBL from accretion onto central black holes. This estimate of Ω* suggests that the universe has been enriched to a total metal mass of 0.21 (±0.13) Z☉ ΩB, which is consistent with other observational estimates of the cumulative metal mass fraction of stars, stellar remnants, and the intracluster medium of galaxy clusters in the local universe.

On the Observational Properties of He‐burning Stars: Some Clues on the Tilt of the Horizontal Branch in Metal‐rich Clusters

We investigate the predicted color-magnitude distribution of metal-rich horizontal-branch (HB) stars, discussing selected theoretical models computed under various assumptions about the star metallicity and the efficiency of superadiabatic convection. We find that canonical zero-age horizontal branches with metallicity larger or of the order of Z = 0.002 should all be affected by a tilt by an amount that increases when the metallicity is increased and/or the mixing length is decreased, reaching a tilt of ΔV ~ 0.2 mag in the case of solar metallicity when a mixing-length value α = 1.6 is assumed (ΔV is the magnitude difference between the top of the blue HB and the fainter magnitude reached by the red HB). Uncertainties in the luminosity of the red HB due to uncertainty in the mixing-length value are discussed. We finally discuss the much larger tilt observed in the clusters NGC 6441 and NGC 6388, reporting additional evidence against suggested noncanonical evolutionary scenarios. Numerical experiments show that differential reddening could produce such sloped HBs. Furthermore, Hubble Space Telescope planetary-camera imaging of NGC 6441 gives clear indications of the occurrence of differential reddening across the cluster. However, the same imaging shows that the observed slope of the red HB is not an artifact of differential reddening. We finally show that sloping red HBs in metal-rich clusters are a common occurrence not necessarily correlated with the appearance of extended blue HBs.

ITAL]HUBBLE SPACE TELESCOPE[/ITAL][ITAL]Hubble Space Telescope[/ITAL] Photometry of Hodge 301: An “Old” Star Cluster in 30 Doradus

We present Hubble Space Telescope Planetary Camera UVI data for Hodge 301, the little-studied cluster 3' northwest of the central ionizing cluster R136 in 30 Doradus. The average reddening of Hodge 301 is found to be EB-V = 0.28 ± 0.05 mag from published infrared and ultraviolet photometry. Using two different sets of evolutionary models, we derive an age of about 20–25 Myr for Hodge 301, which makes it roughly 10 times as old as R136. Hodge 301 is the most prominent representative of the oldest population in the 30 Dor starburst region, a region that has undergone multiple star formation events. This range of ages is an important consideration for the modeling of starburst regions. Hodge 301 shows a widened upper main sequence largely caused by Be stars. We present a list of Be star candidates. The slope of the initial mass function for intermediate-mass, main-sequence stars ranging from 10 to 1.3 M⊙ is found to be Γ = -1.4 ± 0.1, in good agreement with a Salpeter law. There is no indication for a truncation or change of slope of the initial mass function (IMF) within this mass range. In accordance with the age of Hodge 301, no obvious pre–main-sequence stars are seen down to about 1 M⊙. We estimate that up to 41 ± 7 stars with masses more than 12 M⊙ may have turned into supernovae since the formation of the cluster. Multiple supernova explosions are the most likely origin of the extremely violent gas motions and the diffuse X-ray emission observed in the cluster surroundings.

A Search for Faint Companions to Nearby Stars Using the Wide Field Planetary Camera 2

We have completed a direct-imaging search for faint companions (FCs) to 23 stars within 13 pc of the Sun using the Hubble Space Telescope Planetary Camera. The strategy of this search changed considerably from that reported in 1996. To maximize the image contrast between potential FCs and a target star's point-spread function, we adopted the F1042M filter (?c ~ 1.02 ?m, ?? ~ 0.04 ?m) as the primary bandpass of our search. Although our sensitivity to FCs varied with the brightness of and separation from our target stars, an ultimate 10 ? detection limit of m1042 ? 18 within 17'' of the fainter targets was achieved. As the end of the main sequence occurs at M1042 ? 12, this detection limit makes our search for FCs to nearby stars the most sensitive yet published. Despite this great sensitivity, no previously undetected FCs were found. Our survey would have detected all stellar companions within 17'' of our target stars, except for any lowest mass companions lying within 05?1'' of the brightest (V < 1.5) targets. Applying recent models of brown dwarf spectra and evolution, we find that our search was sensitive to young (0.5 Gyr), low-mass (less than 10 MJ) brown dwarf companions to the fainter targets within 5 pc. A brown dwarf with mass 40 MJ and age 5 Gyr would have been detected at separations greater than 5'' from Gl 559A (? Centauri A). Our search was not sensitive to 1 Gyr?old brown dwarfs with masses 5 MJ, nor was it sensitive to 5 Gyr?old brown dwarfs with masses 10 MJ. On the positive side, we show the first direct image of the triple system GJ 1245ABC in which all three components are well resolved. Multiband photometry of GJ 1245C suggests for that object a spectral type later than M7. The measured positions of GJ 1245AC are discordant with those expected from published photocentric-orbit elements. The observed positions of four other multiple systems (Gl 65AB, Gl 244AB, Gl 280AB, and Gl 866ABC) also differ from those expected from published orbital elements. The discrepancies noted for Gl 280AB are sufficient to resolve the long-standing inconsistency between Procyon's observed luminosity and that derived from the theoretical mass-luminosity relation. F1042M images of the astrometric binary Gl 105A do not reveal the presence of a fourth component, as has been proposed to reconcile the differences between the observed location of the M7 V companion Gl 105C and the predicted separations of the perturbing body from two independent astrometric studies.

Hubble Space TelescopeObservations of NGC 121: First Detection of Blue Stragglers in an Extragalactic Globular Cluster

We have resolved stars in the core of the old SMC globular cluster NGC 121 with images from the refurbished Hubble Space Telescope and Wide Field Planetary Camera 2. Our photometry reaches 1.5 mag below the main-sequence turnoff, with negligible field contamination. A blue straggler star (BSS) sequence of 42 candidates is clearly seen—the first such detection in an extragalactic globular cluster. The BSSs are more centrally concentrated than the subgiant, red giant, and horizontal branch stars. Various blending and completeness tests demonstrate that at least 23 of our candidates are genuine BSSs. The BSSs extend about 1.8 mag brighter than the main-sequence turnoff, in accord with similar sequences in Galactic globular clusters. One BSS candidate is 2.4 mag brighter than the turnoff, as luminous as the brightest BSS seen in Galactic globulars. The cluster red horizontal branch is clearly detected and well populated, with a modest blueward extension. The relaxation times of the cluster stars and the BSSs are such that most BSSs are in energy equipartition with the other stars. We find ΔV= 3.3 ± 0.1 mag, corresponding to a cluster age that is 2 Gyr younger than that of most Galactic globular clusters and of one of the oldest LMC globulars.

Oscillating Blue Stragglers in the Core of 47 Tucanae

Six SX Phe variables detected in a 39 hr time series of Hubble Space Telescope Planetary Camera exposures in the F336W (U) filter are discussed. Two of these stars show both the fundamental and first-overtone modes and are among the first double-mode SX Phe stars reported in globular clusters. One of the double-mode stars shows evidence for a third mode near the expected period for the second overtone. Two of the other SX Phe variables in 47 Tuc are shown to oscillate simultaneously in the fourth and fifth radial overtones. The remaining two variables both have multiple oscillations excited which include nonradial modes. To support interpretation of these variables we provide evolutionary computations using current opacities and physics allowing comparison of the blue straggler stars (BSSs) and several cluster-magnitude diagram features for 47 Tuc. Linear nonadiabatic oscillation analysis is performed for a significant range of envelope and full evolutionary models in order to (1) establish the theoretical characteristics of SX Phe pulsations (e.g., location of primary driving and adiabatic nature), (2) establish the theoretical red and blue edges of the instability strip, and (3) derive the theoretical relations necessary for obtaining evolutionary and pulsational masses. Masses inferred from fundamental period-period ratio relations for the double-mode oscillators are well above the turnoff mass in 47 Tucanae and are consistent with expected masses for these blue stragglers based on position in the color-magnitude diagram (CMD) and comparison with theoretical evolutionary tracks. Combining the evolution and pulsation constraints results in mass estimates for the four double-mode BSSs in 47 Tuc of M = 1.35 ± 0.1 to 1.6 ± 0.2 M☉.

TheHubble Space TelescopeKey Project on the Extragalactic Distance Scale. XI. The Cepheids in NGC 4414

We report the Discovery of Cepheid variable stars in the galaxy NGC 4414, as part of the Hubble Space Telescope (HST) Key Project on the Extragalactic Distance Scale. Observations were obtained with the HST Wide Field and Planetary Camera 2 (WFPC2) for 13 epochs at V (F555W) and 4 at I (F814W).

Externally Illuminated Young Stellar Environments in the Orion Nebula: [ITAL]Hubble[/ITAL] [ITAL]Space[/ITAL] [ITAL]Telescope[/ITAL] Planetary Camera and Ultraviolet Observations

We present new 005 resolution (22 AU) narrowband Hubble Space Telescope Planetary Camera images of externally illuminated young stellar objects embedded in the Orion Nebula. We also present 002 resolution (9 AU) UV images of seven externally illuminated protostellar environments and the first UV spectra that cover the spectral range between 1400 and 3000 Å. We discuss 43 objects for which the angular resolution has been improved over previous data by more than a factor of 2. These young stellar objects are either embedded inside the Orion Nebula and externally illuminated by the Trapezium stars or located in front of the nebula and seen in silhouette. The visibility of young stars surrounded by diffuse matter is dominated by intense line emission from ionization fronts (IFs) propagating into material photoablated from circumstellar disks by soft UV radiation. Near the Trapezium stars, the electron density at these IFs is around 105 to 106 cm-3 and the radial intensity profiles of emission lines decrease roughly as r-3, consistent with an approximately constant-velocity diverging flow with an r-2 density profile. However, some radial intensity profiles are better fitted with an exponential function. Low ionization fraction near the IF and the heating and acceleration of the photoablation flow can explain deviations from the r-3 intensity profiles. Many young stars located within 30'' of θ1 Ori C are surrounded by concentric arcs of [O III] and Hα emission located 05 to 3'' from the IF facing θ1 Ori C. These arcs may trace bow shocks formed by the interaction of the expanding photoablation flow with the fast stellar wind from θ1 Ori C. The [O III] emission may be enhanced by UV radiation from θ1 Ori C, thermal conduction, and/or turbulent mixing of weakly shocked, photoablated gas with the thermalized shocked stellar wind. About 30% of the bright externally illuminated young stellar objects contain dark regions seen in silhouette against background nebular emission in Hα and the forbidden transitions of common ions that may trace circumstellar protoplanetary disks. In some sources, the regions seen in silhouette in Hα and ionic transitions are bright in the 6300 Å [O I] line. Most externally illuminated young stellar objects have dusty tails pointing radially away from the source of ionizing photons. Tails have an average length of 500 AU, independent of the projected distance from θ1 Ori C, are limb-brightened in emission lines, and are sometimes seen in silhouette against background nebular light, indicating that they contain large column densities of gas and dust. We discuss a variety of tail formation mechanisms and conclude that initial conditions probably play a key role in their formation. The large fraction of young stellar objects with extended circumstellar structure, the mass limits on this structure, and the estimated mass-loss rates are combined to produce an estimate for the photoionization age of the Orion Nebula. The derived photoionization age of the Orion Nebula is less than 105 yr and possibly as short as 104 yr.

Observational Properties of the Orion Nebula Proplyds

?????Hubble Space Telescope Planetary Camera images have been used to quantitatively determine key parameters characterizing emission-line proplyds in the Orion Nebula. A previous conclusion that the objects are radiation-bounded ionization fronts is confirmed and found to be in excellent agreement with the observations. Only a very loose correlation of proplyd size with distance from the ionizing star ?1 Ori C was found. The brightness distribution around the bright cusp facing ?1 Ori C is found to not be in agreement with simple spherical models. The brightness distribution along radial lines passing through the center of the proplyds was subjected to careful analysis, including the finite resolution of the camera. It was found that the proplyd atmospheres are more compact than the r-3 relation expected for a freely expanding gas; rather, the best fit is to a surface brightness decreasing exponentially with a scale height of 7 ? 1014 cm. This observation argues that the proplyds are not rapidly losing material and hence may not be short-lived objects, a conclusion also reached through consideration of their frequency and location. This also requires a constraining force that is isotropic, since the scale height does not depend upon the orientation with respect to ?1 Ori C. Arguments are presented that this constraining force is provided by radiation pressure of nebular Ly? photons acting upon the dust mixed with the proplyd gas.

Simultaneous Spectroscopy and Imaging of the Jovian Aurora with the Hopkins Ultraviolet Telescope and theHubble Space Telescope

Simultaneous ultraviolet spectra and images of the north polar aurora of Jupiter were obtained on 1995 March 9 with the Hopkins Ultraviolet Telescope (HUT) and the Hubble Space Telescope (HST). This unique data set contains a well-calibrated flux measurement that includes all of the H2 Lyman and Werner band emission from 900 to 1650 A, as well as information about the spatial extent of the aurora. For these observations, we oriented the 10'' × 57'' slit of the HUT spectrograph parallel to the equator of Jupiter so that it was underfilled by the aurora and minimally contaminated by extraneous reflected solar light from low to mid latitudes. The aurora was imaged simultaneously using the HST Wide Field and Planetary Camera 2 (WFPC2). Images were taken through a Wood's filter and also through a Wood's filter/CaF2 filter combination. These images have recorded the aurora in the light of atomic and molecular hydrogen with relatively low reflected solar contamination. The molecular hydrogen aurora is found to extend over 4.5 ± 0.5 × 10-10 sr, with an average brightness of 106 ± 12 kR integrated over the band of wavelengths from 900 to 1650 A, excluding H I λ1216. We measure a 13% larger emitting area (5.1 ± 0.5 × 10-10 sr) and significantly different morphology for the combined atomic and molecular hydrogen aurora (Lyα + H2), although these differences could be a result of time variability. Color ratios derived from the data indicate a total auroral energy deposition rate of 13 ergs s-1 cm-2 by 3-30 keV electrons. We find little evidence for ions as primary auroral particles in the HUT spectra. Our analysis places upper limits on sulfur ion emissions that are an order of magnitude lower than those from the IUE observations, and, therefore, the emissions could be entirely from the Io torus in the background of the planet.

Astrometry with reconstructed hubble space telescope planetary camera (WF/PC 1) images

AbstractThe effect of image reconstruction on the astrometric precision of the Hubble Space Telescope Planetary Camera (WF/PC 1) has been investigated. Depending on the crowded nature and exposure of the stellar images, the reconstruction process may provide benefits for the image centering, especially in terms of the number of stars which can be detected. Unfortunately, there is also evidence that the positions derived from reconstructed images are biased toward the center of individual pixels, an astrometrically unacceptable condition.

Hubble Space Telescope Planetary Camera Observations of the Stellar Populations Near the Nucleus of M33

view Abstract Citations (38) References (51) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Hubble Space Telescope Planetary Camera Observations of the Stellar Populations Near the Nucleus of M33 Mighell, Kenneth J. ; Rich, R. Michael Abstract We have studied the stellar populations near the nucleus of the nearby spiral galaxy M33 using archival observations of the Hubble Space elescope Planetary Camera in the F555W and F785LP passbands. The I vs (V-I) color-magnitude diagram reveals a young stellar population with a blue main sequence and red supergiants, and stars in a blue loop that are Cepheid candidates. An old red giant branch is clearly visible, with a break at I = 20.7 mag which is, within the observational errors, identical to the giant branch termination point of the Mould & Kristian (1986, ApJ, 305,591) halo field. However, red giants are seen up to I= 19.5 mag, and many bright red giants are undetected in the V band. We find evidence for a wide abundance spread of at least 1.5 dex from metal- poor (~M92) to metal-rich (>= 47 Tucanae) in the Population II stars, in sharp contrast to the M33 halo field of Mould & Kristian (1986) which is metal-poor (~M92) with a narrow abundance range. We show that the various stellar populations have different spatial distributions. The Population II stars are more centrally concentrated than the Population I stars. The red giants brighter than I = 20.7 (the termination point for a Galactic globular cluster giant branch) are found to be more centrally concentrated than the (presumably) old red giants. The most-centrally concentrated bright Population II giants are probably associated with the many bright red giants that are undetected in the V band. These infrared- bright giants were probably created in an intermediate-age burst of star formation that took place well after the formation of the oldest halo stars but long before the recent burst of star formation seen in the nearby OB associations. This stellar population probably corresponds to the bulge identified in the infrared H band by Minniti et al. (1993, ApJ, 410, L79). Publication: The Astronomical Journal Pub Date: October 1995 DOI: 10.1086/117638 Bibcode: 1995AJ....110.1649M Keywords: GALAXIES: INDIVIDUAL: M33; GALAXIES: STELLAR CONTENT; GALAXIES: NUCLEI full text sources ADS | data products SIMBAD (5) NED (1) MAST (1) ESA (1) CDS (1)

The Intermediate Stellar Mass Population in R136 Determined from Hubble Space Telescope Planetary Camera 2 Images

The Intermediate Stellar Mass Population in R136 Determined from Hubble Space Telescope Planetary Camera 2 Images

The discovery of a jetlike feature from the massive star Herschel 36

view Abstract Citations (18) References (25) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Discovery of a Jetlike Feature from the Massive Star Herschel 36 Stecklum, B. ; Henning, T. ; Eckart, A. ; Howell, R. R. ; Hoare, M. G. Abstract The O7 V star Hershcel 36 has been studied in the near-infrared by ground-based high-resolution techniques. These observations and image restoration of Hubble Space Telescope (HST) planetary camera hydrogen alpha and (S II) images led to the detection of a jetlike feature. The interpretation of that feature as a jet is discussed in relation to a circumstellar disk for which evidence exists. Publication: The Astrophysical Journal Pub Date: June 1995 DOI: 10.1086/187912 Bibcode: 1995ApJ...445L.153S Keywords: Gas Jets; H Alpha Line; Infrared Astronomy; Interstellar Matter; Massive Stars; Plasma Jets; Stellar Envelopes; Stellar Mass Ejection; Sulfur; Hubble Space Telescope; Light Curve; Near Infrared Radiation; Stellar Luminosity; Astronomy; ISM: JETS AND OUTFLOWS; STARS: CIRCUMSTELLAR MATTER; STARS: IMAGING full text sources ADS | data products SIMBAD (4) MAST (1) ESA (1)

Star counts from the HST Snapshot Survey. 2: Wide binaries

We report on a study of the first sample of wide binaries whose members are primarily in the old component of the Galactic disk. The binaries were discovered in 446 fields imaged with the Hubble Space Telescope (HST) Planetary Camera to an average magnitude limit of V = 2.13, as part of the Snapshot Survey for gravitational lenses. Most of the fields are at moderate to high Galactic latitude. Their total area is 0.15 deg squared. Because of the relatively faint magnitude limit, the observations are sensitive primarily to binaries in the old disk with heights above the Galactic plane of 500-1500 pc, absolute magnitudes of M(sub v) = 6-11, and physical separations of 130-4000 AU. By contrast, most previous surveys have been sensitive to binaries composed of earlier-type stars lying closer to the plane, that is, to a younger population. We find that 4% of the disk stars in the Snapshot sample have fainter companions within 1.5 mag, and separations in the range 0.13 sec is less than theta which is less than 4.0 sec. The distribution of angular separations can be fitted to a power law F(theta) is proportional to theta(exp -l), where l = 1.2 +/- 0.4, in agreement with most previous studies. The binaries in the Snapshot sample have bluer colors than would be expected for random pairs of field stars.

Cataclysmic and close binaries in star clusters. 2: Probing the core of NGC 6752 with Hubble Space Telescope

view Abstract Citations (33) References (28) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Cataclysmic and Close Binaries in Star Clusters. II. Probing the Core of NGC 6752 with Hubble Space Telescope Shara, Michael M. ; Drissen, Laurent ; Bergeron, Louis E. ; Paresce, F. Abstract The cores of globular clusters have been predicted to be rich in close, interacting binaries. We have used the Hubble Space Telescope (HST) to test this prediction by monitoring 730 stars in the core of NGC 5752 for rapid variability. Thirty-one Faint Object Camera (FOC) images at 2200 A, with time resolution of 11 minutes, and spanning 7 hr have been searched for variables. Artificial star tests demonstrate that we should have found virtually all variables brighter than m2200 = 20 (i.e., M220 less than or = 6.75) with variability amplitude Delta m greater than or = 0.25 mag and most variables brighter than m2200 = 22 (i.e., M2200 less than or = 8.75) with Delta m greater than or = 0.35 mag. No variables were found; eight were expected based on tidal capture models in the one-fourth of the core that we observed. Our results strongly constrain the number of cataclysmic and contact binaries (with the above variability and luminosity limits) in the core of NGC 6752 to be less than or = 10 in total with 95% probability. A color-magnitude diagram reveals the presence of four UV-excess stars below the turnoff, all located within 6 arcsec of the cluster center. None show detectable variability (sigma less than or = 0.1 mag) during the 7 hr FOC observing sequence. Two of them are detected in archival HST planetary camera (PC) R and H-alpha images, but show no H-alpha excess. The lack of variability, the absence of H-alpha emission, as well as the fact that none of them appear in outburst in any of four epochs, strongly suggest that they are not cataclysmic variables. This is the strongest observational test to date of simple tidal capture theory. NGC 6752 appears to have significantly fewer tidal capture binaries than the simple tidal capture theory predicts. Five blue stragglers are discovered, as well as 14 horizontal-branch stars which completely dominate the cluster light at 2200 A. The main sequence luminosity function of NGC 6752 is remarkably flat at the cluster center. This is in contrast to the rising value found by Da Costa at 10.1 arcmin less than or = r less than 13.2 arcmin and Richer et al. at 6 arcmin less than or = r less than 10 arcmin. Mass segregation probably accounts for the difference. The preponderance of low-mass stars in the outer parts of NGC 6752 may be due to enrichment (i.e., depletion of the core and transport outward of these low-mass stars) rather than being primordial. If this is true, then the suggestion that dissolved globular cluster low-mass Population II stars may be an important component of the mass budget of the Galactic halo is further weakened. Publication: The Astrophysical Journal Pub Date: March 1995 DOI: 10.1086/175387 Bibcode: 1995ApJ...441..617S Keywords: Cataclysmic Variables; Eclipsing Binary Stars; Globular Clusters; Image Analysis; Spaceborne Astronomy; Color-Magnitude Diagram; Faint Object Camera; Hubble Space Telescope; Light Curve; Main Sequence Stars; Stellar Luminosity; Stellar Spectrophotometry; Astronomy full text sources ADS | data products SIMBAD (6) MAST (1) ESA (1) Related Materials (3) Part 1: 1994ApJ...429..767S Part 3: 1995ApJ...448..203S Part 4: 1996ApJ...471..804S

Discovery of candidate cataclysmic variables in the post-core-collapse globular cluster NGC 6397

view Abstract Citations (85) References (61) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Discovery of Candidate Cataclysmic Variables in the Post--Core-Collapse Globular Cluster NGC 6397 Cool, Adrienne M. ; Grindlay, Johnathan E. ; Cohn, Haldan N. ; Lugger, Phyllis M. ; Slavin, Shawn D. Abstract A photometric search for objects with H alpha emission in the globular cluster NGC 6397 has been carried out with the Hubble Space Telescope (HST) Planetary Camera. Images were obtained through the F675W (similar to Johnson R) and F656N (H alpha) filters, and photometry carried out on approximately 900 stars detected in the central approximately 38 x 35 arcseconds of the cluster. Limiting magnitudes of R approximately 21 and 19.5 were reached in the F675W and F656N images, respectively. Three H alpha-bright stars are found which have apparent magnitudes in the range R = 17.8-19.5. The corresponding absolute magnitudes (MR = 5.6-7.3) and inferred H alpha emission-line strengths (EW(H alpha) greater than or approximately equal to 15-26 A) make these objects good candidates to be cataclysmic variables (CVs) in this nearby, post-collapse cluster. All three H alpha-bright objects are found to be UV-bright in a photometric comparison of the F675W images with archival HST Faint Object Camera (FOC) images of the cluster center through the F220W and F346M filters. Such UV excesses further support the identification of these objects as candidates to be CVs in the cluster. The H alpha-bright stars are within the error circles of three X-ray sources previously found with the ROSAT High Resolution Imager (HRI) and are likley to be the dominant source of the X-ray emission. The implied X-Ray to optical flux ratios (fx/fv approximately 0.8-4.2) are consistent with CVs known elsewhere in the Galaxy. Two additional stars with possible H alpha emission are also discussed, along the stars identified on the basis of UV emisssion. Preliminary completeness estimates suggest that these observations should be sensitive to approximately one-third to two-thirds of most varieties of CVs in the cluster, and that therefore greater than or approximately 5-10 CVs are likely to be present in the observed section of NGC 6397. Scaling these numbers to the cluster as a whole would increase these numbers by up to a factor of 2. A total CV population on the order of 5-20 is in reasonable with previous estimates made on the basis of X-ray observations, and with theoretical predictions scaled to this cluster. Publication: The Astrophysical Journal Pub Date: February 1995 DOI: 10.1086/175209 Bibcode: 1995ApJ...439..695C Keywords: Cataclysmic Variables; Globular Clusters; Novae; X Ray Stars; Emission Spectra; Faint Object Camera; H Alpha Line; Hubble Space Telescope; Stellar Magnitude; X Ray Spectra; Astrophysics full text sources ADS | data products SIMBAD (12) MAST (1) ESA (1)

Astrometry with the HST Planetary Camera

The astrometric capability of the Hubble Space Telescope Planetary Camera (WF/PC1) is investigated, motivated by a study of the internal velocity distribution of globular clusters. The astrometric accuracy of the HST PC will be determined ultimately by 1) the accuracy to which the aberrated images can be ‘centered’, and 2) the accuracy to which the distortions across the PC field can be modeled. A series of overlapping exposures of two clusters, NGC 6752 and M15, are utilized to examine these issues.We have made use of maximum-likelihood image reconstruction to address the first issue, with good success. Reconstruction improves both the detectability and precision of the image centers. A preliminary exploration of the second issue, that of modeling the distortion across the PC field, is also presented, using positions derived from the multiple overlapping exposures.

Additional constraints on circumstellar disks in the Trapezium Cluster

We discuss the new constraints on the population of compact ionized sources in the Trapezium Cluster thought to arise from the ionization by the central OB stars of circumstellar disks around low-mass pre-main sequence stars. We present new Hubble Space Telescope (HST) Planetary Camera observations of two of these candidate disk sources, resolving extended nebulosity around them. One source shows a small-scale (greater than or approximately = 100 AU) bow-shock structure, previously seen on larger scales by O'Dell et al. We show that the circumstellar disk model is the most likely one for the majority of sources, although is remains plausible that some of the larger objects could be equilibrium globules. We combine the most complete censuses of compact radio sources and stars in the core region to derive the fraction of the stellar population that may be associated with a circumstellar disk. Our estimate of 25%-75% is comparable to that found for pre-main sequence (PMS) stars in the Taurus-Auriga dark clouds, indicating that the dense cluster environment of the Trapezium has not drastically reduced the frequency of disks seen around PMS stars.

A Hubble Space Telescope Study of Posteclipse Brightening and Albedo Changes on Io

Images of Io taken by the Hubble Space Telescope Planetary Camera using filters at 7120 and 3577 Å in April and May 1992 shortly after Io emerged from Jupiter's shadow have been analyzed for the effect of posteclipse brightening first reported by Binder and Cruikshank (1964, Icarus 3,299-305). The classic effect displays a 10% increase in brightening lasting for 10 to 15 min. This was not seen in any of the eclipses that were studied.A comparison of the 3577-Å images with UV maps produced from images taken with Voyager 1 (McEwen 1988,Icarus 73, 385-426) showed little change in 13 years. The striking, detailed similarity of the two demonstrate that the Hubble Space Telescope Planetary Camera has the capability to monitor temporal changes on Io's surface.