STIS Spectra Research Articles

HST/STIS observations of the high-velocity interstellar cloud HVC 291.2-41.2+80: a warm, mainly ionized high-velocity cloud

We present intermediate-resolution HST/STIS spectra of a high-velocity interstellar cloud (vLSR = +80 km s−1) towards DI 1388, a young star in the Magellanic Bridge located between the Small and Large Magellanic Clouds. The STIS data have a signal-to-noise ratio (S/N) of 20–45 and a spectral resolution of about 6.5 km s−1 (FWHM). The high-velocity cloud absorption is observed in the lines of C ii, O i, Si ii, Si iii, Si iv and S iii. Limits can be placed on the amount of S ii and Fe ii absorption that is present. An analysis of the relative abundances derived from the observed species, particularly C ii and O i, suggests that this high-velocity gas is warm (Tk∼103–104 K) and predominantly ionized. This hypothesis is supported by the presence of absorption produced by highly ionized species, such as Si iv. This sightline also intercepts two other high-velocity clouds that produce weak absorption features at vLSR = +113 and +130 km s−1 in the STIS spectra.

Damped [CLC]Lyα[/CLC] Absorption from a Nearby Low Surface Brightness Galaxy

Ground-based & HST images of the nearby galaxy SBS 1543+593 (z=0.009) show it to be a Low Surface Brightness (LSB) galaxy with a central surface brightness of mu_B(0)=23.2 mag/arcsec-2 and scale length 0.9 h-1 kpc, values typical for the local LSB galaxy population. The galaxy lies directly in front of the QSO HS 1543+5921 (z=0.807); an HST STIS spectrum of the quasar reveals a damped Lyman-alpha (DLA) line at the redshift of the interloper with an HI column density of log N(HI) = 20.35, as well as several low-ionization metal lines with strengths similar to those found in the Milky Way interstellar medium. Our data show that LSB galaxies are certainly able to produce the DLA lines seen at higher redshift, and fuels the speculation that LSB galaxies are a major contributor to that population of absorbers.

Light‐Element Abundance Patterns in the Orion Association. I.Hubble Space TelescopeObservations of Boron in G Dwarfs

The boron abundances for two young solar-type members of the Orion association, BD -6°1250 and HD 294297, are derived from Hubble Space Telescope STIS spectra of the B I transition at 2496.771 A. The best-fit boron abundances for the target stars are 0.13 and 0.44 dex lower than the solar meteoritic value of log (B) = 2.78. An anticorrelation of boron and oxygen is found for Orion when these results are added to previous abundances obtained for four B-type stars and the G-type star BD -5°1317. An analysis of the uncertainties in the abundance calculations indicates that the observed anticorrelation is probably real. The B versus O relation observed in the Orion association does not follow the positive correlation of boron versus oxygen that is observed for the field stars with roughly solar metallicity. The observed anticorrelation can be accounted for by a simple model in which two poorly mixed components of gas (supernova ejecta and boron-enriched ambient medium) contribute to the new stars that form within the lifetime of the association. This model predicts an anticorrelation for Be as well, at least as strong as for boron.

ITAL]HUBBLE SPACE TELESCOPE[/ITAL][ITAL]Hubble Space Telescope[/ITAL] STIS Observations of the Cygnus Loop: Spatial Structure of a Nonradiative Shock

We present a spatially resolved ultraviolet spectrum of a nonradiative shock front in the Cygnus Loop, obtained with the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope. The spectrum covers the wavelength range 1118–1716 A with an effective spectral resolution of ~12 A. The 01 spatial resolution of these data provides a huge improvement over earlier ultraviolet spectra, allowing us to study the spatial distribution of high-ionization line emissions directly behind the shock front. We are able to isolate individual shock features in our spectrum by comparing the STIS spectrum with a Wide Field Planetary Camera 2 Hα image of the region. Isolating the brightest shock tangency, we identify lines of N V λ1240, C IV λ1549, He II λ1640, O V λ1371, O IV], Si IV λ1400, and N IV] λ1486, as well as the hydrogen two-photon continuum. The N V line peaks ~03 behind the C IV and He II emission and is spatially broader than the other emissions. Also, the observed line ratios of C IV and He II to N V are higher in our bright-shock spectrum than in previous observations of the same filament obtained through much larger apertures (and little or no spatial resolution), indicating that there must be a more widely distributed component of the N V emission. We calculate shock models and show that the observed separation between the C IV and N V emission zones and observed line intensities constrain the combinations of shock velocity and preshock density that are allowed.

The SOHO–Stellar Connection

The ‘solar– stellar connection’ bridges the daytime and nighttime communities; an essential link between the singular, but detailed, views of our Sun, and the broad, but coarse, glimpses of the distant stars. One area in particular – magnetic activity – has profited greatly from the two way traffic in ideas. In that spirit, I present an evolutionary context for coronal activity, focusing on the very different circumstances of low-mass main-sequence stars like the Sun, compared with more massive stars. The former are active mainly very early in their lives, whereas the latter become coronal only near the end of theirs, during the brief incursion into the cool half of the Hertzsprung–Russell diagram as yellow, then red, giants. I describe tools at the disposal of the stellar astronomer; especially spectroscopy in the ultraviolet and X-ray bands where coronae leave their most obvious imprints. I compare HST STIS spectra of solar-type dwarfs – ζ Dor (F7 V), an active coronal source, and α Cen A (G2 V), near twin of the Sun – to the SOHO SUMER UV solar atlas. I also compare the STIS line profiles of the active coronal dwarf to the corresponding features in the mixed activity ‘hybrid chromosphere’ bright giant α TrA (K2 II) and the archetype ‘non-coronal’ red giant Arcturus (α Boo; K2 III). The latter shows dramatic evidence for a ‘cool absorber’ in its outer atmosphere that is extinguishing the ‘hot lines’ (like Si iv λ1393 and N v λ1238) below about 1500 A; the corona of the red giant seems to lie beneath its extended chromosphere, rather than outside as in the Sun. I present an early taste of the moderate resolution spectra we can expect from the recently launched Chandra X-ray Observatory (CXO), and contemporaneous STIS high resolution UV measurements of the CXO calibration star Capella (α Aur; G8 III + G1 III). Last, I describe preliminary results from a May 1999 observing campaign involving SOHO SUMER, TRACE, and the Kitt Peak Infrared Imaging Spectrometer (IRIS). The purpose was to explore the dynamics of the quiet solar atmosphere through the key ‘magnetic transition zone’ that separates the kinetically dominated deep photosphere from the magnetically dominated coronal regime. Linking spatially and temporally resolved solar phenomena to properties of the average line shapes (widths, asymmetries, intensity ratios, and Doppler shifts) is a crucial step in carrying physical insights from the solar setting to the realm of the distant stars.

Kinematics and Morphology of the Resolved Narrow Line Region in NGC 4151

A complement of HST narrow band images, STIS slitless and long-slit spectra have been employed to probe the morphology, kinematics, dynamics, and ionization structure of the near-nuclear region in NGC 4151. The images illustrate the inner biconical cloud distribution which exhibits recessional velocities relative to the nucleus to the NE and approaching velocities to the SW of the nucleus. The STIS spectra of the NLR map the velocity field from 1.2 kpc to within 13 pc (Ho = 75 km s-1 Mpc-1) of the nucleus. We find evidence for at least two kinematic cloud populations. One is characterized by Low Velocities and Low Velocity Dispersions (LVLVD clouds: |v| < 400km s-1,and Δ v <130 km s-1). These clouds extend through the NLR and may be gravitationally associated with the host galaxy. Another component is characterized by High Velocities and High Velocity Dispersions (HVHVD clouds: 400 < |v| ≲1700 km s-1, Δ v ≥ 130 km s-1). This set of clouds is located within 1.1”(~70 pc) of the nucleus and has radial velocities which are too high to be gravitational in origin, show no strong correlation with the knots in the radio jet, but are consistent with wind driven outflow.

Pharmaceutical Centers

Lindal et al. (Lindal, G.F., Lyons, J.R., Sweetnam, D.N., Eshleman, V.R., Hinson, D.P. [1987]. J. Geophys. Res. 92 (11), 14987–15001) presented a range of temperature and methane profiles for Uranus that were consistent with 1986 Voyager radio occultation measurements of refractivity versus altitude. A localized refractivity slope variation near 1.2 bars was interpreted to be the result of a condensed methane cloud layer. However, models fit to near-IR spectra found particle concentrations much deeper in the atmosphere, in the 1.5–3 bar range (Sromovsky, L.A., Irwin, P.G.J., Fry, P.M. [2006]. Icarus 182, 577–593; Sromovsky, L.A., Fry, P.M. [2010]. Icarus 210, 211–229; Irwin, P.G.J., Teanby, N.A., Davis, G.R. [2010]. Icarus 208, 913–926), and a recent analysis of STIS spectra argued for a model in which aerosol particles formed diffusely distributed hazes, with no compact condensation layer (Karkoschka, E., Tomasko, M. [2009]. Icarus 202, 287–309). To try to reconcile these results, we reanalyzed the occultation observations with the He volume mixing ratio reduced from 0.15 to 0.116, which is near the edge of the 0.033 uncertainty range given by Conrath et al. (Conrath, B., Hanel, R., Gautier, D., Marten, A., Lindal, G. [1987]. J. Geophys. Res. 92 (11), 15003–15010). This allowed us to obtain saturated mixing ratios within the putative cloud layer and to reach above-cloud and deep methane mixing ratios compatible with STIS spectral constraints. Using a 5-layer vertical aerosol model with two compact cloud layers in the 1–3 bar region, we find that the best fit pressure for the upper compact layer is virtually identical to the pressure range inferred from the occultation analysis for a methane mixing ratio near 4% at 5°S. This strongly argues that Uranus does indeed have a compact methane cloud layer. In addition, our cloud model can fit the latitudinal variations in spectra between 30°S and 20°N, using the same profiles of temperature and methane mixing ratio. But closer to the pole, the model fails to provide accurate fits without introducing an increasingly strong upper tropospheric depletion of methane at increased latitudes, in rough agreement with the trend identified by Karkoschka and Tomasko (Karkoschka, E., Tomasko, M. [2009]. Icarus 202, 287–309).