Spectroscopy and Time Variability of Absorption Lines in the Direction of the Vela Supernova Remnant

We present high resolution (R~75,000), high signal-to-noise (S/N~100) Ca II $\lambda$3933.663 and Na I $\lambda\lambda$5889.951, 5895.924 spectra of 68 stars in the direction of the Vela supernova remnant. The spectra comprise the most complete high resolution, high S/N, optical survey of early type stars in this region of the sky. A subset of the sight lines has been observed at multiple epochs, 1993/1994 and 1996. Of the thirteen stars observed twice, seven have spectra revealing changes in the equivalent width and/or velocity structure of lines, most of which arise from remnant gas. Such time variability has been reported previously for the sight lines towards HD 72089 and HD 72997 by Danks & Sembach (1995) and for HD 72127 by Hobbs et al. (1991). We have confirmed the ongoing time variability of these spectra and present new evidence of variability in the spectra of HD 73658, HD 74455, HD 75309 and HD 75821. We have tabulated Na I and Ca II absorption line information for the sight lines in our sample to serve as a benchmark for further investigations of the dynamics and evolution of the Vela SNR.Comment: 8 pages of text, 4 tables, 16 pages of figures Accepted and to be published in ApJ

Psalm Lxxvi and Othlr Psalms for the Feast of Tabernacles

n/

The Structure of the Homunculus: I. Shape and Latitude Dependence from H2 and [Fe II] Velocity Maps of Eta Carinae

High resolution long-slit spectra obtained with the Phoenix spectrograph on Gemini South provide our most accurate probe of the three dimensional structure of the Homunculus around eta Car. The new near-infrared spectra dramatically confirm the double-shell structure inferred previously from thermal dust emission, resolving the nebula into a very thin outer shell seen in H2 21218, and a warmer, thicker inner layer seen in [Fe II] 16435. The thin H2 skin hints that the most important mass loss during the 19th century eruption had a very short duration of less than 5 yr. H2 emission traces the majority of the mass in the nebula, and has an average density of order 10^6.5 cm-3. This emission, in turn, yields our first definitive picture of the exact shape of the nebula, plus a distance of 2350pm50 pc and an inclination angle of 41deg (the polar axis is tilted 49deg from the plane of the sky). The distribution of the H2 emission provides the first measure of the latitude dependence of the speed, mass loss, and kinetic energy associated with eta Car's 19th century explosion. Almost 75 percent of the total mass and more than 90 percent of the kinetic energy in the ejecta were released at high latitudes. This rules out a model for the bipolar shape wherein an otherwise spherical explosion was pinched at the waist by a circumstellar torus. Also, the ejecta could not have been deflected toward polar trajectories by a companion star, since the kinetic energy of the polar ejecta is greater than the binding energy of the putative binary system. Instead, most of the mass appears to have been directed poleward by the explosion itself. [abridged]Comment: 25 pages, figs 2 and 3 in color. Accepted by Ap

Modeling RR Tel through the Evolution of the Spectra

We investigate the evolution of RR Tel after the outburst by fitting the emission spectra in two epochs. The first one (1978) is characterized by large fluctuations in the light curve and the second one (1993) by the slow fading trend. In the frame of a colliding wind model two shocks are present: the reverse shock propagates in the direction of the white dwarf and the other one expands towards or beyond the giant. The results of our modeling show that in 1993 the expanding shock has overcome the system and is propagating in the nearby ISM. The large fluctuations observed in the 1978 light curve result from line intensity rather than from continuum variation. These variations are explained by fragmentation of matter at the time of head-on collision of the winds from the two stars. A high velocity (500 km/s) wind component is revealed from the fit of the SED of the continuum in the X-ray range in 1978, but is quite unobservable in the line profiles. The geometrical thickness of the emitting clumps is the critical parameter which can explain the short time scale variabilities of the spectrum and the trend of slow line intensity decrease.Comment: 26 pages, LaTeX (including 5 Tables) + 6 PostScript figures. To appear in "The Astrophysical Journal

The Distances to Open Clusters from Main-Sequence Fitting. IV. Galactic Cepheids, the LMC, and the Local Distance Scale

We derive the basic properties of seven Galactic open clusters containing Cepheids and construct their period-luminosity (P-L) relations. For our cluster main-sequence fitting we extend previous Hyades-based empirical color-temperature corrections to hotter stars using the Pleiades as a template. We use BVI_{C}JHK_{s} data to test the reddening law, and include metallicity effects to perform a more comprehensive study for our clusters than prior efforts. The ratio of total to selective extinction R_V that we derive is consistent with expectations. Assuming the LMC P-L slopes, we find = -3.93 +/- 0.07 (statistical) +/- 0.14 (systematic) for 10-day period Cepheids, which is generally fainter than those in previous studies. Our results are consistent with recent HST and Hipparcos parallax studies when using the Wesenheit magnitudes W(VI). Uncertainties in reddening and metallicity are the major remaining sources of error in the V-band P-L relation, but a higher precision could be obtained with deeper optical and near-infrared cluster photometry. We derive distances to NGC4258, the LMC, and M33 of (m - M)_0 = 29.28 +/- 0.10, 18.34 +/- 0.06, and 24.55 +/- 0.28, respectively, with an additional systematic error of 0.16 mag in the P-L relations. The distance to NGC4258 is in good agreement with the geometric distance derived from water masers [\Delta (m - M)_0 = 0.01 +/- 0.24]; our value for M33 is less consistent with the distance from an eclipsing binary [\Delta (m - M)_0 = 0.37 +/- 0.34]; our LMC distance is moderately shorter than the adopted distance in the HST Key Project, which formally implies an increase in the Hubble constant of 7% +/- 8%.Comment: 28 pages, 21 figures; accepted for publication in the Ap

Stellar Rotation in Young Clusters. II. Evolution of Stellar Rotation and Surface Helium Abundance

We derive the effective temperatures and gravities of 461 OB stars in 19 young clusters by fitting the H-gamma profile in their spectra. We use synthetic model profiles for rotating stars to develop a method to estimate the polar gravity for these stars, which we argue is a useful indicator of their evolutionary status. We combine these results with projected rotational velocity measurements obtained in a previous paper on these same open clusters. We find that the more massive B-stars experience a spin down as predicted by the theories for the evolution of rotating stars. Furthermore, we find that the members of binary stars also experience a marked spin down with advanced evolutionary state due to tidal interactions. We also derive non-LTE-corrected helium abundances for most of the sample by fitting the He I 4026, 4387, 4471 lines. A large number of helium peculiar stars are found among cooler stars with Teff < 23000 K. The analysis of the high mass stars (8.5 solar masses < M < 16 solar masses) shows that the helium enrichment process progresses through the main sequence (MS) phase and is greater among the faster rotators. This discovery supports the theoretical claim that rotationally induced internal mixing is the main cause of surface chemical anomalies that appear during the MS phase. The lower mass stars appear to have slower rotation rates among the low gravity objects, and they have a large proportion of helium peculiar stars. We suggest that both properties are due to their youth. The low gravity stars are probably pre-main sequence objects that will spin up as they contract. These young objects very likely host a remnant magnetic field from their natal cloud, and these strong fields sculpt out surface regions with unusual chemical abundances.Comment: 50 pages 18 figures, accepted by Ap

An analysis of ultraviolet spectra of Extreme Helium Stars and new clues to their origins

Abundances of about 18 elements including the heavy elements Y and Zr are determined from Hubble Space Telescope Space Telescope Imaging Spectrograph ultraviolet spectra of seven extreme helium stars (EHes): LSE 78, BD+10 2179, V1920 Cyg, HD 124448, PV Tel, LS IV -1 2, and FQ Aqr. New optical spectra of the three stars -- BD+10 2179, V1920 Cyg, and HD 124448 were analysed. The abundance analyses is done using LTE line formation and LTE model atmospheres especially constructed for these EHe stars. The stellar parameters derived from an EHe's UV spectrum are in satisfactory agreement with those derived from its optical spectrum. Adopted abundances for the seven EHes are from a combination of the UV and optical analyses. Published results for an additional ten EHes provide abundances obtained in a nearly uniform manner for a total of 17 EHes, the largest sample on record. The initial metallicity of an EHe is indicated by the abundance of elements from Al to Ni; Fe is adopted to be the representative of initial metallicity. Iron abundances range from approximately solar to about one-hundredth of solar. Clues to EHe evolution are contained within the H, He, C, N, O, Y, and Zr abundances. Two novel results are (i) the O abundance for some stars is close to the predicted initial abundance yet the N abundance indicates almost complete conversion of initial C, N, and O to N by the CNO-cycles; (ii) three of the seven stars with UV spectra show a strong enhancement of Y and Zr attributable to an s-process. The observed compositions are discussed in light of expectations from accretion of a He white dwarf by a CO white dwarf.Comment: 126 pages, 15 figures, 20 Tables, accepted for publication in the Ap

An outflow from the nebula around the LBV candidate S 119

We present an analysis of the kinematic and morphological structure of the nebula around the LMC LBV candidate S 119. On HST images, we find a predominantly spherical nebula which, however, seems to be much better confined in its eastern hemisphere than in the western one. The filamentary western part of the nebula is indicative of matter flowing out of the nebula's main body. This outflow is even more evidenced by our long-slit echelle spectra. They show that, while most of the nebula has an expansion velocity of 25.5 km/s, the outflowing material reaches velocities of almost 140 km/s, relative to the systemic one. A ROSAT HRI image shows no trace of S 119 and thus no indications of hot or shocked material.Comment: 8 pages, 8 figures, Accepted in A&A, paper with images in full resolution at http://www.mpifr-bonn.mpg.de/staff/kweis/publications.htm

Triphenylsulfonium topophotochemistry

The products from the 193 nm irradiation of triphenylsulfonium nonaflate (TPS) embedded in a poly(methyl methacrylate) (PMMA) film have been characterized. The analysis of the photoproduct formation was performed using chromatographic techniques including HPLC, GPC and GC-MS as well as UV-vis and NMR spectroscopic methods. Two previously unreported TPS photoproducts, triphenylene and dibenzothiophene, were detected; additionally, GPC and DOSY-NMR spectroscopic analyses after irradiation suggested that TPS fragments had been incorporated into the polymer film. The irradiation of acetonitrile solutions containing 10% w/v PMMA and 1% w/v TPS in a 1 cm-path-length cuvette showed only a trace amount of triphenylene or dibenzothiophene, indicating that topochemical factors were important for the formation of these molecules. The accumulated evidence indicates that both products were formed by in-cage, secondary photochemical reactions: 2-(phenylthio)biphenyl to triphenylene, and diphenylsulfide to dibenzothiophene