HST FOC Research Articles

HST Observations of Sakurai's Object (V4334 Sgr) and Related Stars

We describe our ongoing program of HST observations of Sakurai's Object(V4334 Sgr). Direct WFPC2 imaging from August 1996 through August 2000 revealsno transient features (such as light echoes), and documents the decline of thestar to below 24th visual magnitude in 2000. The surrounding planetary nebulahas shown no changes from 1996 through 2000. There are no obvious peculiarfeatures (such as blobs or knots) in the immediate vicinity of the star. Wealso have in place a target-of-opportunity program to obtain UV spectra withHST in the event that the star begins to retrace its evolution back to highsurface temperature. We also present older HST FOC imaging of V605 Aql. The central object is aresolved nebula that emits in [O III] (but not in hydrogen), whose 0″.6diameter is consistent with a dust cloud ejected during the 1919 outburst. Thecentral star itself is not seen due to its being embedded in the nebula.Several other central stars (including H 3-75, IC 2120, and Abell 14) havelate-type nuclei and no evidence for hot companions. They may be furthercandidates for ‘born-again’ red-giant nuclei.

A New Way to Resolve Cepheid Binaries

AbstractWe have measured the centroid shift of low resolution HST FOC spectra as the dominant star changes from the Cepheid to the hot companion. With this approach we have resolved the AW Per system and marginally resolved the U Aql system.

Predicted HST FOC and broad band colours for young and intermediate simple stellar populations

This paper presents theoretical HST and broad band colours from population synthesis models based on an homogeneous set of stellar evolutionary tracks as computed under canonical (no overshooting) assumptions, covering the range of cluster ages from t=8 Myr to t=5 Gyr for three different metallicities (Z=0.02, 0.006, and 0.001). Statistical fluctuations in the cluster population have been investigated, assessing the predicted fluctuations of the various colours as a function of the cluster integrated absolute magnitude. We show that the red leak in HST UV filters deeply affects the predicted fluxes and colours. However, we find that for F152M-F307M < 0.5 and for F170M-F278M < 0.5 (which means ages lower than 1 Gyr) the HST UV colours can still be used to infer reliable indications on the age of distant clusters. Moreover, one finds that the age calibration of these colours is scarcely affected by the amount of original helium or by the assumed IMF. On this basis, we present a calibration of the HST UV two-colours (F152M-F307M vs F170M-F278M) in terms of cluster ages for the three above quoted metallicities. We suggest the combined use of HST UV colours and IR colours (V-K in particular) to disentangle the metallicity-age effect in integrated colours of young stellar populations (t< 1 Gyr).

HST FOC and Broad Band Colors for Young and Intermediate Simple Stellar Populations

We present theoretical HST and broad band colors from young and intermediate stellar populations with age ranging from t = 8 Myr to t = 5 Gyr and three different metallicities Z=0.02, 0.006 and 0.001. We suggest the combined use of HST UV colors and IR colors (V-K in particular) to disentangle the metallicity-age effect in integrated colors of young stellar populations (t ≤ 1 Gyr).

Modelling of metastable carbon atoms in comets: Implications for ROSETTA

The observations of CI 1931 Å emission in the ultraviolet spectra of comets show that a large fraction of carbon atoms in the cometary coma are produced in the metastable 1D excited state. A coupled chemistry-transport model is used to study the chemistry of C( 1D) atom and the mechanisms of production of CI 1931 Å in the near nucleus environment. Dissociation of CO by solar UV photon and photoelectron impact are the dominant mechanisms of the C( 1D) production in the cometary coma. The CI 1931 Å emission is found to be governed dominantly by the density distribution of CO and C( 1D). The model predict height-integrated CI 1931 Å emission intensity of 20 R on comet 46P/Wirtanen, the target of the ROSETTA mission, and intensity averaged over HST FOC field-of-view of 6.7 R. However, the observed intensity could be higher since on comet Halley the model predicted intensity (Bhardwaj, 1999) was about a factor of 5 smaller than that observed by the IUE.

The spatial distribution of UV-absorbing regions on Triton

Using mosaic maps of Triton's surface made with images taken through the Voyager 2 UV and Clear filters, we have sought to: (1) distinguish between surface units containing UV-absorbing (UVA) materials and fresh volatile frosts; (2) place any such identified regions into a geological context; (3) suggest possible candidate materials for the UVAs; and (4) to interpret these regions in terms of seasonal volatile transport scenarios. In pursuit of these goals, we have identified both a region possibly containing involatile UVA materials and an adjoining region possibly covered by uncontaminated, fresh frosts. The UVA region correlates well with a distinct geological unit on Triton containing maculae and wind streaks. The location of these two regions suggests that they might be linked through the dynamical transport of volatiles between them. Hydrocarbon deposits may explain the UVA region. However, they cannot be ruled out in regions darker than average in the Clear filter. Also, we cannot rule out other UVA materials as a visible component of the UVA region, such as salts or sulfur-bearing compounds, but note that such material has not yet been detected and may be largely obscured by hydrocarbon deposits. Recent images of Triton taken by us with the post-COSTAR HST FOC in bandpasses centered near 4100 and 2780 Å will be analyzed to provide further information on the possible UVA and adjoining fresh frost regions. With resolution elements of ⋍400–700 km (depending on wavelength) at Triton's distance, the FOC images will provide the first mapping of these regions in the mid-UV (2780 Å). This wavelength corresponds to the extreme of the broad-band absorption feature seen in Triton's UV spectrum (Stern et al., 1995), suggesting that the contrast between the UVA region and frost regions may be enhanced in the HST images.

High resolution optical and UV observations of the centers of NGC 1316 and NGC 3998 with the Hubble Space Telescope

view Abstract Citations (32) References (38) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS High-Resolution Optical and UV Observations of the Centers of NGC 1316 and NGC 3998 with the Hubble Space Telescope Fabbiano, G. ; Fassnacht, C. ; Trinchieri, G. Abstract B and UV high-resolution images obtained with the HST FOC reveal centrally increasing surface brightness and UV-bright point sources at the nuclei of the early-type galaxies NGC 1316 and NGC 3998. Model fitting of the surface brightness distribution does not exclude a central flattening, which could however be the result of extinction and/or of improper evaluation of the point-source component. The present data, while not requiring the presence of central massive black holes in these two galaxies, make them possible candidates. Follow-up work is needed to study this point further. The presence of unresolved nuclei gives model- dependent support to this quest, if these sources are due to small AGN. These UV-bright sources could belong to the low-luminosity end of active nuclear emission. Publication: The Astrophysical Journal Pub Date: October 1994 DOI: 10.1086/174705 Bibcode: 1994ApJ...434...67F Keywords: Active Galactic Nuclei; Astronomical Models; Brightness Distribution; Radio Galaxies; Ultraviolet Astronomy; Ultraviolet Emission; Visible Spectrum; Astronomical Photometry; Faint Object Camera; High Resolution; Hubble Space Telescope; Interstellar Extinction; Interstellar Matter; Astronomy; GALAXIES: ACTIVE; GALAXIES: INDIVIDUAL NGC NUMBER: NGC 1316; GALAXIES: INDIVIDUAL NGC NUMBER: NGC 3998; GALAXIES: NUCLEI; ULTRAVIOLET: GALAXIES full text sources ADS | data products SIMBAD (2) NED (2) MAST (1) ESA (1)

Detection of an ultraviolet and visible counterpart of the NGC 6624 X-ray burster

view Abstract Citations (44) References (18) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Detection of an Ultraviolet and Visible Counterpart of the NGC 6624 X-Ray Burster King, I. R. ; Stanford, S. A. ; Albrecht, R. ; Barbieri, C. ; Blades, J. C. ; Boksenberg, A. ; Crane, P. ; Disney, M. J. ; Deharveng, J. M. ; Jakobsen, P. ; Kamperman, T. M. ; Macchetto, F. ; Mackay, C. D. ; Paresce, F. ; Weigelt, G. ; Baxter, D. ; Greenfield, P. ; Jedrzejewski, R. ; Nota, A. ; Sparks, W. B. ; Sosin, C. Abstract We have detected, in images taken with the HST FOC, the UV and optical counterpart of the X-ray source 4U 1820-30 in the globular cluster NGC 6624. Astrometric measurements place this object 2 sigma from the X-ray position of 4U 1820-30. The source dominates a far-UV FOC image and has the same flux at 1400 A as was seen through the large IUE aperture by Rich et al. (1993). It has a B magnitude of 18.7 but is not detected in V. It is 0.66 arcsec from the center of NGC 6624, a fact that may change the interpretation of the P-average of the 11 minute binary orbit. The flux drops between 1400 and 4300 A at a rate that is nearly as steep as that of a Rayleigh-Jeans curve. The flux is far too large to come from the neutron star directly but could accord with radiation from a heated accretion disk and/or the heated side of the companion star. Publication: The Astrophysical Journal Pub Date: August 1993 DOI: 10.1086/186973 Bibcode: 1993ApJ...413L.117K Keywords: Globular Clusters; Iue; Ultraviolet Astronomy; X Ray Sources; Astrometry; Neutron Stars; Visible Spectrum; Astrophysics full text sources ADS | data products SIMBAD (2) MAST (2) ESA (1)