John C. Brandt

Mercury in the HgMn stars chi Lupi and HR 7775

Observations of mercury lines in the HgMn stars chi Lupi and HR 7775 made with the Hubble Space Telescope Goddard High Resolution Spectrograph are presented and analyzed. In chi Lupi we find that all observed lines are consistent with the same isotopic mixture (essentially pure (204)Hg). Strong ionization anomalies are present, with UV Hg I lines being too weak and Hg III lines too strong for the abundance derived from lines of the majority ionization state, Hg rr. Observations of mercury in I;IR 7775 show less extreme isotope and ionization anomalies. We find that the ionization anomaly in the Hg I resonance lines can be plausibly explained as a non-LTE effect, but the same non-LTE calculations show that the Hg III ionization anomaly in chi Lupi cannot be explained in this way. Radiative force calculations show that the observed mercury abundance cannot be supported in the atmosphere by the radiative forces alone. We suggest that weak mixing brings mercury into the line-forming region from below the photosphere, while a wind of order 10(-14) M. yr(-1) supports a cloud of Hg III at very small optical depths. (Less)

A Goddard High Resolution Spectrograph Atlas of Echelle Observations of the HgMn Star χ Lupi

Observations of the ultra-sharp-lined, chemically peculiar star chi Lupi taken by the Goddard High Resolution Spectrograph in echelle mode are presented. Thirty-six intervals of the spectral region between 1249 and 2688 Angstrom are covered with resolving powers in the range 75,000-93,000. Line identifications are provided, and the observed spectra are compared with synthetic spectra calculated using the SYNTHE program and associated line lists with changes to the line lists. The significance of these spectra for the chi Lupi Pathfinder Project and the closely related atomic physics effort is discussed in a companion paper.

Interplanetary Gas. V. a. Hydrogen Cloud of Terrestrial Origin.

Evidence is presented which indicates that the nighttime Lyman-alpha observations can be attributed to solar radiation scattered by a cloud of hydrogen of terrestial origin located at geocentric distances greater than about 5 to 10 earth radii. It also appears that the earth has a comet-like tail of hydrogen in the antisolar direction. (auth)

The Bismuth Abundance in the HGMN Stars chi Lupi and HR 7775 and Improved Atomic Data for Selected Transitions of BI I, BI II, and BI III

High-resolution spectra of the chemically peculiar HgMn stars chi Lupiand HR 7775, obtained with the Hubble Space Telescope/Goddard HighResolution Spectrograph, are investigated for their abundance of bismuthby comparison with LTE synthetic spectrum modeling. HR 7775, previouslyknown from International Ultraviolet Explorer spectra to display stronglines of Bi II, is determined to have bismuth present at an enhancementlevel of nearly 5 orders of magnitude from the lines Bi IIlambdalambda1436, 1902 and Bi III lambda1423. The bismuthenhancement for chi Lupi is found to be near a level of 1.5 dex, andan ionization anomaly between Bi+ and Bi++ isapparent. HR 7775 abundance enhancements of the heavy elements platinum,[Pt/H]=4.7 dex, and gold, [Au/H]=3.8 dex, have also been determined. Newlaboratory measurements for wavelengths and hyperfine structure patternsof Bi I/Bi II/Bi III lines are presented, as well as the results ofcalculations for hyperfine structure constants and oscillator strengthsfor selected lines of Bi II and Bi III. Based on observations with theNASA/ESA Hubble Space Telescope, obtained at the Space Telescope ScienceInstitute, which is operated by the Association of Universities forResearch in Astronomy, Inc., under NASA contract NAS5-26555. (Less)

An Ultraviolet Spectral Atlas of 10 Lacertae Obtained with the Goddard High Resolution Spectrograph on the Hubble Space Telescope

Observations of the narrow-lined O-type star 10 Lacertae taken with the Goddard High Resolution Spectrograph in 1992 November are presented. The spectra cover the wavelength range 1181–1777 A with a resolution of 15 km s-1 and signal-to-noise ratio greater than 100:1. Absorption lines arising in the interstellar medium, the photosphere, and the stellar wind are identified and discussed.

INTERPLANETARY GAS. IV. NEUTRAL HYDROGEN IN A MODEL SOLAR CORONA

The problem of the distribution of neutral hydrogen in a model solar corona is investigated. Hydrogen is considered as created by recombinations, destroyed by collisions and photoionization, and transported by diffusion. The diffusion cross section is taken as that for charge exchange, and the effects of radiation pressure from solar Lyman- alpha are included. A model with an electron density near the orbit of earth of the order of 10/sup 2//cm/sup 3/ yields sufficient hydrogen beyond the earth to back-scatter solar Lyman- alpha and produce the observed night-sky radiation. (auth)

Echelle Spectroscopy of Interstellar Absorption toward μ Columbae with the Goddard High Resolution Spectrograph

Goddard High Resolution Spectrograph echelle-mode observations of the interstellar absorption lines of Mg II, Si IV, C IV, and N V toward μ Columbae (HD 38666) are presented. The observations have a spectral resolution of 3.5 km s-1 and signal-to-noise ratios (S/Ns) of 20–200. The μ Col sight line (l = 2373, b = -271, d = 0.40 kpc, z = -0.18 kpc) extends though the Local Bubble and the warm neutral, warm ionized, and hot ionized phases of the interstellar medium (ISM). The high-ionization column densities toward μ Col are log N(Si IV) = 12.16 ± 0.05, log N(C IV) = 12.88 ± 0.02, and logN(N V) = 11.8–12.3. Profile fits to Copernicus satellite measures of O VI absorption toward μ Col yield log N(O VI) = 13.82±0.01 and b = 38.7 km s-1. This implies N(C IV)/N(O VI) = 0.11 ± 0.01, which is typical of the values found for the hot ISM of the Galactic disk. The O VI profile is twice as broad as the C IV and N V profiles, even though these species have roughly similar average velocities. Some of the C IV, N V, and O VI absorption toward μ Col may occur at the interface of the Local Cloud and Local Bubble, although additional contributions to these ions probably also occur in more distant gas along the sight line. A substantial part of the Si IV absorption likely arises in warm photoionized gas in an H II region surrounding μ Col. The profile width differences among the high-ionization lines of C IV, N V, and O VI could be produced if the line of sight passes through a highly evolved supernova remnant. The observations for μ Col and for other stars observed at high resolution with the GHRS reveal that multiple gas types (warm and hot) contribute to the absorption by the highly ionized atoms along both nearby and distant sight lines. Disentangling the relative contributions from the different gas types requires high-resolution and high-S/N observations. The Mg II observations, combined with a solar Mg reference abundance, imply that the Mg depletion toward μ Col is -0.31 dex. As observed for other sight lines through the warm neutral medium, the gas-phase observations of Mg, when combined with results for Fe and Si, suggest that Mg and Fe are more deficient from the gas phase than one would expect if these elements are only contained in silicate dust grains.

A POSSIBLE COMPANION TO PROXIMA CENTAURI

We report Hubble Space Telescope observations that provide evidence for a companion to Proxima Centauri (Gl 551), the closest star to the Sun. Data acquired with the Faint Object Spectrograph (FOS), used as a coronagraphic camera, show excess light that can be interpreted as being from a substellar object within 05 of Proxima Cen. Two observations of Proxima Cen separated by 103 days indicate a point source (or a feature) superposed on the wing of the point-spread function in the FOS images. This feature moves relative to the aperture, and on the plane of the sky. Comparisons with other FOS images of stars acquired using the coronagraphic mode reveal no comparable features or evidence that this feature can be explained by any instrumental anomaly. If this feature denotes a companion to Proxima Cen, it has an apparent separation corresponding to ~0.5 AU at Proxima Cen and is ≈7 mag fainter than Proxima Cen in the bandpass of the FOS red detector. The small apparent separation could result from a highly eccentric orbit, which could project a close companion. Alternately, the small separation could imply a short (~1 yr) period. Further coronagraphic observations, using the Space Telescope Imaging Spectrograph, the Near-Infrared Camera and Multiobject Spectrometer, or some other instrumentation, are needed to verify this tentative result.

Hubble Space Telescope/Faint Object Spectrograph imaging of Wolf 424 AB: Is this binary a double brown dwarf system?

The low-mass binary system Wolf 424 AB (GJ 473 AB) was spatially resolved in an image obtained on 1996 April 16 with the imaging mode of the Hubble Space TelescopeFaint Object Spectrograph. The separation was measured to be 00 0 .03 at a position angle of 3537 . 7 The new measurement was 42 5 02 5 2 combined with previously published orbital positions to update the orbital elements and obtain an estimate of the total mass. Spectroscopy and photometry of these objects suggest they are nearly equal in mass. The derived orbital solution, using all of the observations since 1938 appropriately weighted, yields a total mass of m 1 1 of 0.143 M,, and component masses of »0.07 M,, just under the theoretical limit for a brown dwarf. An m2 alternate orbital solution derived solely from speckle observations since 1986 and the Faint Object Spectrograph image yields a total mass of 0.232 M,, placing both components well above the brown-dwarf limit. The mass depends on the value for the semimajor axis, which is controlled in the latter solution by an optical speckle observation made near the last period of maximum separation. Further high-resolution observations during the upcoming maximum separation, and through the next secondary maximum, are needed to constrain the orbit.

Radar observations and the extent of coronal heating.

Coronal heating extent placed at 1.6 solar radii by 38 MHz radar investigation studies, noting agreement with coronal density calculation