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We have analyzed the far-UV spectrum of two Galactic O4 stars, the O4If+ supergiant HD190429A and the O4V((f)) dwarf HD96715, using archival FUSE and IUE data. We have conducted a quantitative analysis based on the two NLTE model atmosphere and wind codes, TLUSTY and CMFGEN. We have derived the stellar and wind parameters and the surface composition of the two stars. The surface of HD190429A has a composition typical of an evolved O supergiant (N-rich, C and O-poor), while HD96715 exhibits surface N enhancement similar to the enrichment found in SMC O dwarfs and attributed to rotationally-induced mixing. We find that homogeneous wind models could not match the observed profile of O V1371 and require very low phosphorus abundance to fit the P V1118-1128 resonance lines. However, we are able to match the O V and P V lines using clumped wind models. We find that N IV1718 is also sensitive to wind clumping. For both stars, we have calculated clumped wind models that match well all these lines from different species and that remain consistent with Halpha data. These fits therefore provide a coherent and thus much stronger evidence of wind clumping in O stars than earlier claims. We find that the wind of these two stars is highly clumped, as expressed by very small volume filling factors, namely f=0.04 for HD190429A and f=0.02 for HD96715. In agreement with our analysis of SMC stars, clumping starts deep in the wind, just above the sonic point. The most crucial consequence of our analysis is that the mass loss rates of O stars need to be revised downward significantly, by a factor of 3 and more. Accounting for wind clumping is essential when determining the wind properties of O stars. Our study therefore calls for a fundamental revision in our understanding of mass loss and of O-type star winds. (abridged), Comment: To appear in Astronomy & Astrophysics; 16 pages; accepted version after minor revision

High-resolution UCLES/AAT spectra are presented for nine B-type supergiants in the SMC, chosen on the basis that they may show varying amounts of nuclear-synthetically processed material mixed to their surface. These spectra have been analysed using a new grid of approximately 12 000 non-LTE line blanketed {\sc tlusty} model atmospheres to estimate atmospheric parameters and chemical composition. The abundance estimates for O, Mg and Si are in excellent agreement with those deduced from other studies, whilst the low estimate for C may reflect the use of the \ion{C}{ii} doublet at 4267\AA. The N estimates are approximately an order of magnitude greater than those found in unevolved B-type stars or \ion{H}{ii} regions but are consistent with the other estimates in AB-type supergiants. These results have been combined with results from a unified model atmosphere analysis of UVES/VLT spectra of B-type supergiants (Trundle et al. \cite{Tru04}) to discuss the evolutionary status of these objects. For two stars that are in common with those discussed by Trundle et al., we have undertaken a careful comparison in order to try to understand the relative importance of the different uncertainties present in such analyses, including observational errors and the use of static or unified models. We find that even for these relatively luminous supergiants, {\sc tlusty} models yield atmospheric parameters and chemical compositions similar to those deduced from the unified code {\sc fastwind}., Comment: Submitted to astronomy and astrophysics

We reinvestigate the UV spectrum of NGC1535 by means of recently developed fully line-blanketed non-LTE models. These new models account for the wind in spherical geometry while handling the atomic data in a very similar way to the TLUSTY code. This approach ensures at the same time realistic predictions of the photospheric absorption lines and of the emission lines formed in the wind. Our analysis confirms the results of previous studies. We derive T_* = 70kK, log M_dot = -7.8M_sun/year and v_infty = 2000km/s., Comment: 4 pages, to appear in: ASP Conference Series, Asymmetric Planetary Nebulae III

With the goal of producing a reliable set of model atoms and singly-ionized ions for use in building NLTE model atmospheres, we have combined measured energy levels, critically-compiled line transition probabilities, and resonance-averaged calculations of photoionization cross-sections. A majority of the elements from Li to Ca are considered, covering most of the important species in late-type atmospheres. These include elements which contribute free electrons and/or continuous opacity in the ultraviolet (e.g., Mg, and Si), as well as trace elements whose abundance determinations rely on ultraviolet lines (e.g., B from B I lines). The new data complement and, for the species in common, supersede a previous collection of model atoms originally designed for use in studies of early-type stars., Comment: 6 pages, 4 figures, to appear in ApJS

We present the results of a detailed UV and optical spectral analysis of the properties of 6 dwarf O-type stars in the SMC H II region NGC 346. Stellar parameters, chemical abundances, and wind parameters have been determined using NLTE line blanketed models calculated with the photospheric code, Tlusty, and with the wind code, CMFGEN. The results, in particular iron abundances, obtained with the two NLTE codes compare very favorably, demonstrating that basic photospheric parameters of O dwarfs can be reliably determined using NLTE static model atmospheres. The two NLTE codes require a microturbulent velocity to match the observed spectra. Our results hint at a decrease of the microturbulent velocity from early O stars to late O stars. Similarly to several recent studies of galactic, LMC and SMC stars, we derive effective temperatures lower than predicted from the widely-used relation between spectral type and Teff, resulting in lower stellar luminosities and lower ionizing fluxes. From evolutionary tracks in the HR diagram, we find an age of 3 10^6 years for NGC 346. A majority of the stars in our sample reveal CNO-cycle processed material at their surface during the MS stage, indicating thus fast stellar rotation and/or very efficient mixing processes. We obtain an overall metallicity, Z = 0.2 Zsun, in good agreement with other recent analyses of SMC stars. The derived mass loss rate of the three most luminous stars agrees with recent theoretical predictions. However, the three other stars of our sample reveal very weak wind signatures. We obtain mass loss rates that are significantly lower than 10^{-8} Msun/yr, which is below the predictions of radiative line-driven wind theory by an order of magnitude or more. (abridged version), Comment: 61 pages, 17 figures; to appear in ApJ, 595 (Oct 1, 2003); minor revisions and additions

The recent spectral analysis of LBDS 53W091 by Spinrad and his collaborators has suggested that this red galaxy at z=1.55 is at least 3.5 Gyr old. This imposes an important constraint on cosmology, suggesting that this galaxy formed at z > 6.5, assuming recent estimates of cosmological parameters. We have performed chi^2 tests to the continuum of this galaxy using its UV spectrum and photometric data (RJHK). We have used the updated Yi models that are based on the Yale tracks. We find it extremely difficult to reproduce such large age estimates, under the assumption of the most probable input parameters. Using the same configuration as in Spinrad et al. (solar abundance models), our analysis suggests an age of approximately 1.4 -- 1.8 Gyr. The discrepancy between Spinrad et al.'s age estimate (based on the 1997 Jimenez models) and ours originates from the large difference in the model integrated spectrum: the Jimenez models are much bluer than the Yi models and the Bruzual \& Charlot (BC) models. Preliminary tests favor the Yi and BC models. The updated age estimate of LBDS 53W091 would suggest that this galaxy formed approximately at z=2-3., Comment: LaTeX, 18 eps files Accepted for publication in ApJ (Feb 10, 2000, vol 530), uses emulateapj.sty

We present high-quality HST/GHRS spectra in the Hydrogen L alpha spectral region of Vega and Sirius-A. Thanks to the signal-to-noise ratio achieved in these observations and to the similarity of the two spectra, we found clear evidence of emission features in the low flux region, 1190-1222 A. These emission lines can be attributed unambiguously to Fe II and Cr II transitions. In this spectral range, silicon lines are observed in absorption. We built a series of non-LTE model atmospheres with different, prescribed temperature stratification in the upper atmosphere and treating Fe II with various degrees of sophistication in non-LTE. Emission lines are produced by the combined effect of the Schuster mechanism and radiative interlocking, and can be explained without the presence of a chromosphere. Silicon absorption lines and the L alpha profile set constraints on the presence of a chromosphere, excluding a strong temperature rise in layers deeper than tau_R ~ 10^{-4}., Comment: 14 pages including 11 figures; accepted for publication in Astronomy & Astrophysics

An echelle spectrogram (R = 30,000) of the 2300-3100 A region in the ultraviolet spectrum of the F8V star 9 Comae is presented. The observation is used to calibrate features in the mid-ultraviolet spectra of similar stars according to age and metal content. In particular, the spectral break at 2640 A is interpreted using the spectral synthesis code SYNSPEC. We use this feature to estimate the time since the last major star formation episode in the z=1.55 early-type galaxy LBDS 53W091, whose rest frame mid-ultraviolet spectrum, observed with the Keck Telescope, is dominated by the flux from similar stars that are at or near the main-sequence turnoff in that system (Spinrad et al. 1997). Our result, 1 Gyr if the flux-dominating stellar population has a metallicity twice solar, or 2 Gyr for a more plausible solar metallicity, is significantly lower than the previous estimate and thereby relaxes constraints on cosmological parameters that were implied by the earlier work., Comment: 8 pages, Latex, 6 figures. To appear in ApJ Letters