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We present the early UV and optical light curve of Type IIP supernova (SN) 2010aq at z=0.0862, and compare it to analytical models for thermal emission following SN shock breakout in a red supergiant star. SN 2010aq was discovered in joint monitoring between the Galaxy Evolution Explorer (GALEX) Time Domain Survey (TDS) in the NUV and the Pan-STARRS1 Medium Deep Survey (PS1 MDS) in the g, r, i, and z bands. The GALEX and Pan-STARRS1 observations detect the SN less than 1 day after shock breakout, measure a diluted blackbody temperature of 31,000 +/- 6,000 K 1 day later, and follow the rise in the UV/optical light curve over the next 2 days caused by the expansion and cooling of the SN ejecta. The high signal-to-noise ratio of the simultaneous UV and optical photometry allows us to fit for a progenitor star radius of 700 +/- 200 R_sun, the size of a red supergiant star. An excess in UV emission two weeks after shock breakout compared to SNe well fitted by model atmosphere-code synthetic spectra with solar metallicity, is best explained by suppressed line blanketing due to a lower metallicity progenitor star in SN 2010aq. Continued monitoring of PS1 MDS fields by the GALEX TDS will increase the sample of early UV detections of Type II SNe by an order of magnitude, and probe the diversity of SN progenitor star properties., 4 pages, 3 color figures. Accepted for publication in ApJ Letters, corrections from proofs added

The net optical light curves and spectra of the supernova (SN) 2003dh are obtained from the published spectra of GRB 030329, covering about 6 days before SN maximum to about 60 days after. The bulk of the U-band flux is subtracted from the observed spectra using early-time afterglow templates, because strong line blanketing greatly depresses the UV and U-band SN flux in a metal-rich, fast-moving SN atmosphere. The blue-end spectra of the gamma-ray burst (GRB)connected hypernova SN 1998bw is used to determine the amount of subtraction. The subtraction of a host galaxy template affects the late-time results. The derived SN 2003dh light curves are narrower than those of SN 1998bw, rising as fast before maximum, reaching a possibly fainter maximum, and then declining ~ 1.2-1.4 times faster. We then build UVOIR bolometric SN light curve. Allowing for uncertainties, it can be reproduced with a spherical ejecta model of Mej ~ 7+/-3 Msun, KE ~ (3.5+/-1.5)E52 ergs, with KE/Mej ~ 5 following previous spectrum modelling, and M(Ni56) ~ (0.4 +0.15/-0.1) Msun. This suggests a progenitor main-sequence mass of about 25-40 Msun, lower than SN 1998bw but significantly higher than normal Type Ic SNe and the GRB-unrelated hypernova SN 2002ap., 18 pages, 7 figures, published by ApJ

We present quantitative studies of 8 late O and early B-type supergiants in the Magellanic Clouds using far-ultraviolet FUSE, ultraviolet IUE/HST and optical VLT-UVES spectroscopy. Temperatures, mass-loss rates and CNO abundances are obtained using the non-LTE, spherical, line-blanketed model atmosphere code of Hillier & Miller (1998). We support recent results for lower temperatures of OB-type supergiants as a result of stellar winds and blanketing, which amounts to ~2000 K at B0 Ia. In general, H$\alpha$ derived mass-loss rates are consistent with UV and far-UV spectroscopy, although from consideration of the SIV $\lambda\lambda$1063-1073 doublet, clumped winds are preferred over homogenous models. AV 235 (B0 Iaw) is a notable exception, which has an unusually strong H$\alpha$ profile that is inconsistent with the other Balmer lines and UV wind diagnostics. We also derive CNO abundances for our sample, revealing substantial nitrogen enrichment, with carbon and oxygen depletion. Our results are supported by comparison with the Galactic supergiant HD 2905 (BC0.7 Ia) for which near-solar CNO abundances are obtained. This bolsters previous suggestions that ``normal'' OB-type supergiants exhibit atmospheric compositions indicative of partical CNO processing.

The optically thick character of Wolf-Rayet winds implies that stellar continuum photons are multiply scattered, as a result of both free electron opacity and overlapping wind-broadened spectral lines. This allows the wind to accumulate a substantial excess in momentum flux relative to the driving radiation field, as is observationally required. Nevertheless, sustaining such a high degree of multiple scattering requires not only a large optical depth spatially but also substantial spectral blanketing. The latter is difficult to maintain when redistribution during scattering allows radiative flux to shift preferentially into spectral regions with fewer lines, since then the channels carrying much of the flux are also the least well blanketed. This paper parameterizes the potential severity of this effect in simple terms, using a generalization of the Rosseland mean treated in the Sobolev approximation. We show that our approach provides an informative starting point for characterizing and conceptualizing nongray effects in optically thick supersonic flows.

We have undertaken quantitative analysis of four LMC and SMC O4--9.7 extreme supergiants using far-ultraviolet FUSE, ultraviolet IUE/HSTand optical VLT UVES spectroscopy. Extended, non-LTE model atmospheres that allow for the consistent treatment of line blanketing are used to analyse wind & photospheric spectral features simultaneously. Using Halpha to constrain dM/dt, HeI-II photospheric lines reveal T_eff's which are systematically (5-7.5kK) & substantially (15-20%) lower than previously derived from unblanketed, plane-parallel, non-LTE photospheric studies. We have confidence in these revisions, since derived T_eff's generally yield consistent fits across the entire 912-7000Ang observed spectral range. In particular, we are able to resolve previous UV-optical T_eff discrepancies. `Of' classification criteria are directly linked to (strong) nitrogen enrichment (via NIII 4097) & (weak) carbon depletion (via CIII 4647-51), providing evidence for mixing of unprocessed and CNO processed material at their stellar surfaces. Oxygen abundances are more difficult to constrain, except via OII lines in the O9.7 supergiant for which it is also found to be somewhat depleted. The effect of wind clumping is also investigated, for which PV 1118-28 potentially provides a useful diagnostic in O-star winds. Revised stellar properties affect existing calibrations of (i) Lyman continuum photons - a factor of two lower for the O4 supergiant; and (ii) kinetic energy released into the ISM by O supergiants. Our results also have importance for the calibration of the wind momentum-luminosity relationship for OB stars.

We present non-LTE calculations for the ionization equilibrium of C I tailored to match the HgMn star χ Lupi. Comparisons are made with the UV C I lines observed using the Hubble Space Telescope's Goddard High Resolution Spectrograph, as well as with Voyager far-ultraviolet (FUV) measurements of the flux between 1040 and 1200 A, where C I is the dominant continuum opacity source. The non-LTE models do provide better fits to most C I line profiles than do the LTE models, but the expected increase in the FUV continuum flux due to the non-LTE underpopulation of the lower levels of carbon is largely masked by uncertainties in the Voyager data and in the FUV line blanketing. Realistic modeling of the C I ionization equilibrium required including the FUV opacity from the predicted lines of iron-group elements, and to this end the non-LTE model atmosphere program TLUSTY was modified to include opacity distribution functions of Kurucz as a fixed-opacity source. This modified code will be useful for studying non-LTE effects in other ions in χ Lupi and similar stars.

We have modeled a large sample of infrared starburst galaxies using both the PEGASE v2.0 and STARBURST99 codes to generate the spectral energy distribution of the young star clusters. PEGASE utilizes the Padova group tracks while STARBURST99 uses the Geneva group tracks, allowing comparison between the two. We used our MAPPINGS III code to compute photoionization models which include a self-consistent treatment of dust physics and chemical depletion. We use the standard optical diagnostic diagrams as indicators of the hardness of the EUV radiation field in these galaxies. These diagnostic diagrams are most sensitive to the spectral index of the ionizing radiation field in the 1-4 Rydberg region. We find that warm infrared starburst galaxies contain a relatively hard EUV field in this region. The PEGASE ionizing stellar continuum is harder in the 1-4 Rydberg range than that of STARBURST99. As the spectrum in this regime is dominated by emission from Wolf-Rayet (W-R) stars, this difference is most likely due to the differences in stellar atmosphere models used for the W-R stars. We believe that the stellar atmospheres in STARBURST99 are more applicable to the starburst galaxies in our sample, however they do not produce the hard EUV field in the 1-4 Rydberg region required by our observations. The inclusion of continuum metal blanketing in the models may be one solution. Supernova remnant (SNR) shock modeling shows that the contribution by mechanical energy from SNRs to the photoionization models is << 20%. The models presented here are used to derive a new theoretical classification scheme for starbursts and AGN galaxies based on the optical diagnostic diagrams., 36 pages, 16 figures, to be published in ApJ, July 20, 2001

Using a significantly revised non-LTE radiative transfer code that allows for the effects of line blanketing by He, C, O, Si, and Fe, we have performed a detailed analysis of the Galactic Wolf-Rayet (W-R) star HD 165763 (WR 111, WC5). Standard W-R models consistently overestimate the strength of the electron scattering wings, especially on strong lines, so we have considered models where the wind is both homogeneous and clumped. The deduced stellar parameters for HD 165763 are as follows: The stellar parameters deduced for HD 165763 are significantly different from earlier analyses. The deduced luminosity is a factor of 2 larger, and a smaller core radius is found. The smaller radius is in better agreement with expectations from stellar evolution calculations. Both of these changes can be attributed to the effects of line blanketing. The deduced C/He abundance is similar to earlier calculations, and the O/He abundance had not previously been determined. The observed iron spectrum, principally due to Fe V and Fe VI, is well reproduced using a solar iron mass fraction, although a variation of at least a factor of 2 about this value cannot be precluded. In particular, the models naturally produce the Fe V emission feature centered on 1470 A and the complex Fe emission/absorption spectrum at shorter wavelengths. Also, Fe strongly modifies the line strengths and profile shapes shortward of 1800 A and must be taken into account if we are to successfully model this region. The reddening toward HD 165763 does not follow the mean Galactic extinction law. We determine the reddening law toward HD 165763 by comparing our model continuum levels to observations. In order to simultaneously match the UV, optical, and particularly the infrared fluxes, we used the parameterized reddening law of Cardelli, Clayton, and Mathis with EB-V = 0.3 and R = 4.5, where R = AV/EB-V. Based on both observational and theoretical suggestions we have considered models in which the wind is still accelerating at large radii. In particular, we discuss models in which the velocity law can be characterized by β = 1 for r < 10R* but that undergo a substantial velocity increase (~600 km s-1) beyond 10R*. These models appear to give slightly better fits to the line profiles, but the improvements are small, and it is difficult to gauge whether observational data require such a velocity law. We cannot yet determine whether the winds of W-R stars are driven by radiation pressure, because we neglect many higher levels of iron in our model ions, and we do not include important elements such as cobalt and nickel. However, the wind problem in W-R stars is less severe than previously assumed if clumping occurs. For our clumped model, the single scattering limit is only exceeded by a factor of 10 compared to 3 times this value for a homogeneous wind. Clumping appears to be the key to explaining the apparent high mass-loss rates determined for W-R stars and is extremely important in understanding how or even whether W-R winds are driven by radiation pressure. A reduction in W-R mass-loss rates has important implications for stellar evolution calculations.

From new high signal-to-noise ratio (S/N) 1-2.2 μm spectroscopy of nine extreme early-type stars—including O Iaf, O Iafpe and WN9 types—we determine stellar parameters from detailed atmospheric analysis and evaluate results from near-IR analogues of well-known spectral diagnostics in the optical. We conclude that accurate stellar parameters can be measured from near-IR spectroscopy alone, an analysis technique important to studies of luminous stars in the Galactic center and other galaxies. Derived stellar parameters—mass-loss rates, luminosities, surface abundances, temperatures—show good agreement between optical and near-IR analyses, provided that IR data are of sufficient spectral resolution (R>2000) and S/N (S/N>30). Wind velocities derived from He I 1.0830 μm are consistent with those from ultraviolet P Cygni profiles. Temperatures 200-1300 K systematically lower are determined from the near-IR diagnostics, a difference not significant in determining the stellar properties of these objects; which set of spectral lines provides the more accurate physical parameters—optical or IR—cannot at present be ascertained. The strength of He I 2.0581 μm is very sensitive to the extreme ultraviolet energy distribution where line blanketing by heavy elements plays an important role; this line should not on its own be considered a reliable temperature diagnostic. The three peculiar, extreme emission-line stars—the O Iafpe stars HD 152386, HD 152408, and HDE 313846—are more similar in both morphological and physical characteristics to WNL-type Wolf-Rayet stars than to normal O Iaf supergiants and should be classified as W-R. Their classification should be WN9ha, in which they remain a unique subgroup.

We use a spherical non-LTE fully line blanketed model atmosphere to fit the full multi-wavelength spectrum, including the extreme ultraviolet (EUV) continuum observed by the {\it Extreme Ultraviolet Explorer}, of the B2 II star \epscma. The available spectrophotometry of \epscma\ from 350 \AA\ to 25 \micron\ is best fit with model parameters \Teff = 21750\,K, \Logg = 3.5, and an angular diameter of 0.77 mas. The close agreement between the model and the measured EUV flux from \epscma\ is a result of the higher temperatures at the formation depths of the \ion{H}{1} and \ion{He}{1} Lyman continua compared to other models. The realistic model treatment of early B giants with spherical geometry and NLTE metal line blanketing results in the prediction of significantly larger EUV fluxes compared with plane-parallel models. We find that our metal line blanketed spherical models show significantly warmer temperature structures, 1-3 kK at the formation depth of the Lyman continua, and predict stronger EUV fluxes, up to a factor of 5 in the \ion{H}{1} Lyman continuum, compared with plane-parallel atmospheres that have identical model parameters. In contrast, we find spherical and plane-parallel models that do not include metal line blanketing are nearly identical. Our \Teff = 21000 K, \Logg = 3.2, spherical NLTE model predicts more than twice as many hydrogen ionizing photons and over 200 times more neutral helium ionizing photons than a standard hydrostatic plane-parallel LTE model with the same stellar parameters.

We present the first measurement of the H I Gunn-Peterson effect using the Keck 10 m telescope, observing the high-redshift QSO PKS 1937-101 (z = 3.787). The high-resolution echelle (HIRES) spectra, with FWHM ~15 km s-1 and a signal-to-noise ratio (S/N) ~50 per spectral resolution element, allows us to resolve many weak lines down to NH I = 1012 cm-2, thus reducing the line-blanketing problem compared with previous data. Based on intensity-distribution analysis, we find that a maximum likelihood best fit yields a Gunn-Peterson type of opacity τGP = 0.113 ± 0.020 in addition to a power-law Lyα absorption-line population with β of 1.7 down to NH I = 1012 cm-2. There remains systematic uncertainty in this result because of problems extrapolating the spectral continuum from the red side of the Lyα emission line. This is consistent with the previous study of the same QSO in low S/N data using weighted intensity function analysis (Fang & Crotts 1995). It indicates that this previous method succeeds in measuring the Lyα forest continuum level at low S/N, which is essential in extending the technique to possible fainter QSOs with minimum emission-line contamination for reliable continuum extrapolation. We further discuss problems of severe line blanketing, even in Keck spectra for QSOs at z ≥ 4.5, and show the effectiveness of the weighted intensity function method in measuring continuum levels in extremely crowded Lyα absorption spectra for redshifts as high as z > 5.

Extensive modifications to the non-LTE radiative transfer code of Hillier have been made in order to improve the spectroscopic analysis of stars with stellar winds. The main improvement to the code is the inclusion of blanketing due to thousands of overlapping lines. To implement this effect, we have used the idea of super levels first pioneered by Anderson. In our approach, levels with similar excitation energies and levels are grouped together. Within this group, we assume that the departure coefficients are identical. Only the population (or equivalently, the departure coefficient) of the super level need be solved in order to fully specify the populations of the levels within a super level. Our approach is a natural extension of the single-level LTE assumption, and thus LTE is recovered exactly at depth. In addition to the line blanketing modifications, the code has been improved significantly in other regards. In particular, the new code incorporates the effect of level dissolution, the influence of resonances in the photoionization cross sections, and the effect of Auger ionization. Electron scattering with a thermal redistribution can be considered, although it is normally treated coherently in the comoving frame (which still leads to redistribution in the observer's frame). Several example calculations are described to demonstrate the importance of line blanketing on spectroscopic analysis. We find that the inclusion of blanketing modifies the strengths of some optical CNO lines in Wolf-Rayet (W-R) stars by factors of 2-5. In particular, the strengths of the WC classification lines C III λ5696 and C IV λ5805 are both increased because of iron blanketing. This should help alleviate problems found with nonblanketed models, which were incapable of matching the strengths of these lines. We also find that, in the UV (1100-1800 A), the influence of Fe is readily seen in both emission and absorption. The emission is sensitive to the iron abundance and should allow, for the first time, Fe abundances to be deduced in W-R stars. The improvements made to our code should greatly facilitate the spectroscopic analysis of stars with stellar winds. We will be able to determine the importance and influence of line blanketing, as well as of several other effects that have been included in the new code. It will also allow us to better determine W-R star parameters, such as luminosity, elemental abundances, wind velocity, and mass-loss rate. With future application to related objects, such as novae and supernovae, our new code should also improve our understanding of these objects with extended outflowing atmospheres.

We have made a detailed comparison of results of spectroscopic analysis using three differents types of model atmospheres: classical non-LTE H-He models; approximate non-LTE line-blanketed models (with only a subset of Fe and Ni lines: those originating from transitions between levels with measured energies); and non-LTE fully blanketed models. The three models were applied to the sdO star BD +75°325, adopted as a test case. We demonstrate that the effects of line blanketing are very important: the best fit of the observed H and He lines is achieved for Teff = 58,000 K when using H-He models, Teff = 55,000 K for approximate non-LTE line-blanketed models, and Teff = 52,000 K for fully blanketed non-LTE models. Using the high-resolution Goddard High Resolution Spectrograph spectrum of BD +75°325 and our final fully blanketed model, we have derived reliable abundances of He, C, N, O, Si, Fe, and Ni. We find that BD +75°325 is an He-enriched star (He/H = 1 by number), whose surface exhibits CNO-cycle products, i.e., N-rich (AN = 1.5 × 10-3 by number, or 4.2 × 10-3 by mass fraction), and C and O deficient (by factor of about 100 with respect to the solar value). We also find a significant surface depletion of silicon and an enhancement of iron and nickel. We argue that these anomalous abundances reflect some mixing with processed material from the core, with subsequent modification at the surface by diffusion processes. Finally, BD +75°325 possesses a weak wind. Using a simplified description of the wind, we have derived a preliminary value of the mass loss rate: =1.5 × 10−11 M☉ yr-1.

Broad-band ultraviolet photometry from the TD-1 satellite and low dispersion spectra from the short wavelength camera of IUE have been used to investigate a long-standing proposal of Bohm-Vitense that the normal main sequence A- and early-F stars may divide into two different temperature sequences: (1) a high temperature branch (and plateau) comprised of slowly rotating convective stars, and (2) a low temperature branch populated by rapidly rotating radiative stars. We find no evidence from either dataset to support such a claim, or to confirm the existence of an "A-star gap" in the B-V color range 0.22, 40 pages, 14 figures, 1 table, AAS LaTex, to appear in The Astrophysical Journal

We present new observations of the [S III] 19 μm, [O III] 52 and 88 μm, and [N III] 57 μm lines toward 18 compact and ultracompact (UC) H II regions. These data were combined with data from the literature and high-resolution radio continuum maps to construct detailed statistical equilibrium and ionization equilibrium models of 34 compact H II regions located at galactocentric distances (DG) 0-12 kpc. Our models simultaneously fitted the observed IR fine-structure lines and high-resolution radio continuum maps. Abundance gradients are found of the form [S/H] = (-4.45 ± 0.04) - (0.063 ± 0.006) DG (kpc), [N/H] = (-3.58 ± 0.04) - (0.072 ± 0.006) DG (kpc), and [O/H] = (-2.85 ± 0.06) - (0.064 ± 0.009) DG (kpc), and we derive Te = (4560 ± 220) + (390 ± 40) DG (kpc). The Te gradient is consistent with the Te gradient determined independently via radio recombination lines (Afflerbach et al.). We observe no dependence of S/O, N/O, or Teff on DG. Gradients in N++/O++ and O++/S++ are observed in the sense of increasing ionization with increasing DG. This is entirely consistent with the decreased line blanketing with increasing DG required by the above abundance gradients. All three gradients are best fitted by a linear dependence on DG. The abundances are consistent with production of sulfur, nitrogen, and oxygen by primary nucleosynthesis. Comparison with abundances in other galaxies implies a Hubble type between Sab and Sb for our Galaxy and an unbarred or mixed galactic structure (Vila-Costas & Edmunds). Our derived Teff is 2000-10,000 K lower than Teff expected from ZAMS stars of the same Lyman continuum flux (Panagia; Vacca et al.), probably owing to uncertainties in the UV flux of stellar models for E ≥ 35.1 eV, uncertainties in the luminosity-Teff calibration, and/or ionization of H II regions by multiple stars in some sources.

The star G191-B2B is one of a number of hot DA white dwarfs whose atmospheres have been found to contain significant quantities of heavy elements, including C, N, O, Si, Fe, and Ni. Several earlier studies have measured their abundances using IUE echelle data in conjunction with synthetic spectra derived from theoretical model atmosphere calculations of varying degrees of sophistication. However, predictions of the EUV spectrum based on these observations failed completely to match either its shape or absolute flux level. We present here the results of new non-LTE model calculations including the effects of line blanketing from more than 9 million (mainly Fe and Ni) transitions. For the first time, we are able to find an effective temperature and composition that can consistently match the optical, FUV and EUV data. However, to maintain this agreement below the He ii Λ228 Lyman limit, it is necessary to incorporate additional He ii opacity in the form of photospheric, circumstellar or interstellar material.

We present the XMM-Newton discovery of X-ray emission from the planetary nebula (PN) A78, the second born-again PN detected in X-rays apart from A30. These two PNe share similar spectral and morphological characteristics: They harbor diffuse soft X-ray emission associated with the interaction between the H-poor ejecta and the current fast stellar wind, and a point-like source at the position of the central star (CSPN). We present the spectral analysis of the CSPN, using for the first time a NLTE code for expanding atmospheres which takes line blanketing into account for the UV and optical spectra. The wind abundances are used for the X-ray spectral analysis of the CSPN and the diffuse emission. The X-ray emission from the CSPN in A78 can be modeled by a single C VI emission line, while the X-ray emission from its diffuse component is better described by an optically thin plasma emission model with temperature $kT$=0.088 keV ($T\approx$1.0$\times$10${^6}$ K). We estimate X-ray luminosities in the 0.2--2.0 keV energy band of $L_{\mathrm{X,CSPN}}$=(1.2$\pm$0.3)$\times$10$^{31}$ erg~s$^{-1}$ and $L_{\mathrm{X,DIFF}}$=(9.2$\pm$2.3)$\times$10$^{30}$ erg~s$^{-1}$ for the CSPN and diffuse components, respectively., 12 pages, 10 figures, 2 tables; Accepted to Astrophysical Journal

We present the Pan-STARRS1 discovery of PS1-10afx, a unique hydrogen-deficient superluminous supernova (SLSN) at z=1.388. The light curve peaked at z_P1=21.7 mag, making PS1-10afx comparable to the most luminous known SNe, with M_u = -22.3 mag. Our extensive optical and NIR observations indicate that the bolometric light curve of PS1-10afx rose on the unusually fast timescale of ~12 d to the extraordinary peak luminosity of 4.1e44 erg/s (M_bol = -22.8 mag) and subsequently faded rapidly. Equally important, the SED is unusually red for a SLSN, with a color temperature of 6800 K near maximum light, in contrast to previous H-poor SLSNe, which are bright in the UV. The spectra more closely resemble those of a normal SN Ic than any known SLSN, with a photospheric velocity of 11,000 km/s and evidence for line blanketing in the rest-frame UV. Despite the fast rise, these parameters imply a very large emitting radius (>5e15 cm). We demonstrate that no existing theoretical model can satisfactorily explain this combination of properties: (i) a nickel-powered light curve cannot match the combination of high peak luminosity with the fast timescale; (ii) models powered by the spindown energy of a rapidly-rotating magnetar predict significantly hotter and faster ejecta; and (iii) models invoking shock breakout through a dense circumstellar medium cannot explain the observed spectra or color evolution. The host galaxy is well detected in pre-explosion imaging with a luminosity near L*, a star formation rate of 15 M_sun/yr, and is fairly massive (2e10 M_sun), with a stellar population age of 1e8 yr, also in contrast to the dwarf hosts of known H-poor SLSNe. PS1-10afx is distinct from known examples of SLSNe in its spectra, colors, light-curve shape, and host galaxy properties, suggesting that it resulted from a different channel than other hydrogen-poor SLSNe., 17 pages, 12 figures, accepted to ApJ, minor revisions, including expanded discussion of lensing hypothesis

We first present our new grids of model atmospheres and spectra for hot subdwarf O (sdO) stars: standard non-LTE (NLTE) H+He models with no metals, NLTE line-blanketed models with C+N+O, and NLTE line-blanketed models with C+N+O+Fe. Using hydrogen and helium lines in the optical range, we make detailed comparisons between theoretical spectra of different grids in order to characterize the line-blanketing effects of metals. We find these effects to be dependent on both the effective temperature and the surface gravity. Moreover, we find that the helium abundance also influences in an important way the effects of line blanketing on the resulting spectra. We further find that the addition of Fe (solar abundance) leads only to incremental effects on the atmospheric structure as compared with the case where the metallicity is defined by C+N+O (solar abundances). We use our grids to perform fits on a 9 A resolution, high signal-to-noise ratio (~300 blueward of 5000 A) optical spectrum of SDSS J160043.6+074802.9, the only known pulsating sdO star. Our best and most reliable result is based on the fit achieved with NLTE synthetic spectra that include C, N, O, and Fe in solar abundances, leading to the following parameters : T eff = 68,500 ± 1770 K, log g = 6.09 ± 0.07, and log N(He)/N(H) = –0.64 ± 0.05 (formal fitting errors only). This combination of parameters, particularly the comparatively high helium abundance, implies that line-blanketing effects due to metals are not very large in the atmosphere of this sdO star.

The recent Hubble Space Telescope (HST) observations of the z(q) = 3.286 quasar Q0302-003 (Jakobsen et at. 1994) and the z(q) = 3.185 quasar Q1935-67 by Tytler (1995) show absorption edges at the redshifted wavelength of He II 304 A. A key goal is to distinguish between contributions from discrete Ly-alpha forest clouds and a smoothly distributed intergalactic medium (IGM). We model the contributions from each of these sources of He II absorption, including the distribution of line Doppler widths and column densities, the 'He II proximity effect' from the quasar, and a self-consistent derivation of the He II opacity of the universe as a function of the spectrum of ionizing sources, with the assumption that both the clouds and the IGM are photoionized. The He II edge can be fully accounted for by He II line blanketing for reasonable distributions of line widths and column densities in the Ly-alpha forest, provided that the ionizing sources have spectral index alpha(s) greater than 1.5, and any He II proximity effect is neglected. Even with some contribution from a diffuse IGM, it is difficult to account for the edge observed by Jakobsen et al. (1994) with a 'hard' source spectrum (alpha(s) less than 1.3). The proximity effect modifies the relative contributions of the clouds and IGM to tau(He II) near the quasar (z approx. less than z(q)) and markedly increases the amount of He II absorption required. This implies, for example, that to account for the He II edge with line blanketing alone, the minimum spectral index alpha(s) must be increased from 1.5 to 1.9. We demonstrate the need for higher resolution observations that characterize the change in transmission as z approaches z(q) and resolve line-free gaps in the continuum. We set limits on the density of the diffuse IGM and suggest that the IGM and Ly-alpha clouds are likely to be a significant repository for dark baryons.

We present our initial results of model atmosphere calculations for cool M dwarfs using an opacity sampling method and a new list of H2O lines. We obtain significantly improved fits to the infrared spectrum of the M dwarf VB10 when compared to earlier models. H2O is by far the dominant opacity source in cool stars. To illustrate this, we show the Rosseland mean of the total extinction under various assumptions. Our calculations demonstrate the importance of a good treatment of the water opacities in cool stars and the improvements possible by using up-to-date data for the water line absorption.

It has been suggested by several authors that the sodium and aluminum abundance variations correlating with CN-band strength, frequently observed in CN-bimodal globular cluster giants, could be spurious manifestations of different temperature structures in the 'CN-strong' and 'CN-weak' stars, caused by different molecular line blanketing related to the C, N, and O trio. For stellar parameters generally appropriate to giants in the intermediate metallicity CN-bimodal cluster M4, we demonstrate through new model atmosphere calculations, employing opacity sampling and spherical geometry, that the observed abundance anomalies cannot be the result of atmospheric temperature structure. Our results using spherical geometry are compared to identical calculations performed with plane-parallel geometry: the effects of atmospheric extension on derived abundances for all lines considered amount to less than 0.1 dex.

A detailed analysis of the optical spectrum of the DAO white dwarf Feige 55 is presented. We show that the atmospheric parameters obtained from fits to the Hβ line and to higher Balmer lines, respectively, are discrepant. This problem is akin to that reported in previous analyses of hot DAO central stars of planetary nebulae. Model atmosphere calculations are used to demonstrate that the discrepancy is likely to be due to the presence of small quantities of heavy elements in the atmosphere of Feige 55. We then show that line blanketing by these elements indeed produces enough cooling of the upper layers to change significantly the predicted line profiles

We have developed a model to estimate the amplitude and wavelength dependence of broad-band linear polarization (BLP) from magnetic regions on cool stars. The model includes corrections both for line blends and for the partial cancellation of polarization in the vector sum over the stellar disk. Our results agree with recent calculations of BLP in the red, but show larger amplitudes and a different wavelength dependence in the blue. We find that the detailed wavelength dependence of the polarization is complex and varies with effective temperature and gravity due to changes in line blanketing. The BLP amplitudes depend strongly on field strength, blanketing, and magnetic region filling factor and geometry. We make rough estimates of the maximum BLP for stars of various spectral types and demonstrate a method for deriving a lower limit to the filling factor from the maximum observed BLP. This lower limit is related to the fractional area covered by the spatially asymmetric component of magnetic regions.

UV spectropolarimetric observations of the early-type supergiant stars P Cygni and Kappa Cassiopeiae obtained with the Wisconsin UV Photo-Polarimeter Experiment (WUPPE) are presented. Contrary to model predictions, the intrinsic polarization of P Cyg remains constant in the Balmer continuum except for a broad dip between 2600 and 3000 A. This decrease in polarization is discussed in terms of possible Fe line blanketing effects. The intrinsic position angle is essentially constant in the UV except for a rotation across the feature between 2600 and 3000 A. The intrinsic polarization of Kappa Cas is small; most of the observed polarization is interstellar in nature. The Serkowski curve extrapolated into the UV is found to fall consistently below the observed polarization. A slight rotation in position angle from optical to UV wavelengths is noted. Very good agreement is found between the mean continuum polarization obtained by WUPPE and the broadband UV polarization measurement obtained by Gehrels (1968).

Color-magnitude diagrams for the metal-rich bulge clusters NGC 6528, NGC 6553, and Terzan 1 are calibrated in M I0 versus (V-I) 0 , and compared to that of 47 Tuc. In such diagrams the blanketing effects for increasing metallicities are directly estimated. Such studies are important for the interpretation of CMDs of metal-rich populations in Local Group external galaxies. We compare our results to literature color-magnitude diagrams of M31 and M32 bulges.

A 256 × 256 Hg:Cd:Te array has been used to obtaine J, H, and K images of a field in Baade's Window. The data extend down to K∼16, which is near the base of the red giant branch at the distance of the Galactic bulge. The distribution of data points on the (K, J-K) and (J-H, H-K) diagrams is similar to that defined by previous aperture measurements. Furthermore, the redward edge of the data envelope on the (K, J-K) diagram roughly follows the giant branch of the cluster M67 to K∼15, indicating that the blanketing agent thought to dominate the broad-band photometric properties of super metal-rich stars must be present over a broad range of photospheric temperatures and surface gravities.

Solar irradiance spectra obtained at or near the maximum and minimum phases of the solar magnetic activity cycle are compared. The characteristics of these spectra are related to the irradiance measurements available from the Active Cavity Radiometer Irradiance Monitor (ACRIM) of the SMM. An interpretation based on sunspots and faculae being added to the atmosphere at solar maximum can account for less than 4 percent of the ACRIM decrease to be arising in the 500-560 nm range. The UV, with its much greater line-blanketing, is more important, in agreement with the finding of Lean (1989).

CCD spectrophotometry is presented of 140 white dwarfs at high SNR and is analyzed in detail. Energy distributions at 14,000 A are given at bandpasses from 3571 to 8300 A, and equivalent widths of lines of H, He I, metals, and atomic and molecular carbon are given as functions of color for DB, DQ, DZ, and DA stars. New forbidden H I transitions at 6068 A and 6632 A are found in at least the two hottest DB stars, new metallic features are found in cooler DZ stars, and the presence of Ca I in vMa 2 is confirmed. The spectrum of the hot DQAB star G227 - 5 and the pressure-shifted carbon bands seen in 0038-226 are discussed in detail. Comparison of the optical energy distribution of the latter with published IR fluxes shows that the 1-2 micron region is strongly depressed, with extensive blanketing. Equivalent widths, central depths, and width parameters are presented for H-alpha in 73 DA stars in the sample, and their dependences on color are studied. 64 refs.

This paper presents model atmospheres for the first week of SN 1987A, based on the luminosity and density/velocity structure from hydrodynamic models of Woosley (1988). The models account for line blanketing, expansion, sphericity, and departures from LTE in hydrogen and helium and differ from previously published efforts because they represent ab initio calculations, i.e., they contain essentially no free parameters. The formation of the UV spectrum is dominated by the effects of line blanketing. In the absorption troughs, the Balmer line profiles were fit well by these models, but the observed emissions are significantly stronger than predicted, perhaps due to clumping. The generally good agreement between the present synthetic spectra and observations provides independent support for the overall accuracy of the hydrodynamic models of Woosley. The question of the accuracy of the Baade-Wesselink method is addressed in a detailed discussion of its approximations. While the application of the standard method produces a distance within an uncertainty of 20 percent in the case of SN 1987A, systematic errors up to a factor of 2 are possible, particularly if the precursor was a red supergiant.

We have measured the circular polarization of sunspots with a sensitivity of less than 3 x 10/sup -6/, over the range from 3700 A to 4.5 ..mu..m. Through all of our passbands of widths > or =0.1 ..mu..m, V has a single sign for a given spot, namely: if H is toward the observer, V is negative, i.e., the E vector rotates clockwise in a stationary plane over the spot. The rough magnitude, sign, and to some extent the spectral shape of V(lambda) for lambda> or =1.5 ..mu..m agree with a continuum magneto-opacity model. But the UV-visible polarization is anomalously large, e.g., > or approx. =0.05% in the B band, seemingly correlated with line blanketing. A new mechanism for this is proposed, magneto-emission by atoms oriented by anisotropic radiation.

High resolution, high signal-to-noise spectra have provided Li abundances for 31 M giants and supergiants. The spectrum around the Li I 6707 A doublet is depressed by unresolved TiO lines. A spectrum synthesis technique was developed to account for the TiO line blanketing and to extract the Li abundance.

The method of artificial absorption edges is applied to a grid of model atmospheres to include the effects of hydrogen and metal line blanketing on the atmospheric structure. A best-fit model of Sirius is obtained which is free of the discrepancies inherent in unblanketed or partially blanketed models. All major spectral features calculated from the model agree with the corresponding observed features to within the observational uncertainty. Some additional properties of the models are discussed.

A statistical solution for the problem of non-LTE radiative transfer in the millions of atomic transitions responsible for line blanketing is proposed which takes into account the influence of scattering on the atomic populations. A multifrequency/multigray algorithm is used to analyze the thermal equilibrium. Although CO is shown to cool the upper atmosphere to a boundary temperature of 2640 K, the molecule has little influence in the region of the solar temperature minimum.

The ultraviolet spectral energy distribution of the O5f star Zeta Pup and the WN5 star HD 50896 are derived from OAO-2 observations with the calibration of Bless, Code, and Fairchild (1976). An estimate of the interstellar reddening (0.12 magnitude) of the Wolf-Rayet star is determined from the size of the characteristic interstellar extinction bump at 4.6 inverse microns. After correction for extinction, both stars show a flat energy distribution in the ultraviolet. The distribution of HD 50896 from 1100 A to 2 microns is in good agreement with results of extended model atmospheres, but some uncertainty remains because of the interstellar-extinction correction. The absolute energy distribution of Zeta Pup is fitted by a 42,000-K plane-parallel model if the model's flux is adjusted for the effects of electron scattering in the stellar wind and for UV line blanketing that was determined empirically from high-resolution Copernicus satellite observations. To achieve this fit, it is necessary to push both the spectroscopically determined temperature and the ultraviolet calibration to the limits of their probable errors.

An opacity sampling (OS) technique for treating the radiative opacity of large numbers of atomic and molecular lines in cool stellar atmospheres is presented. Tests were conducted and results show that the structure of atmospheric models is accurately fixed by the use of 1000 frequency points, and 500 frequency points is often adequate. The effects of atomic and molecular lines are separately studied. A test model computed by using the OS method agrees very well with a model having identical atmospheric parameters computed by the giant line (opacity distribution function) method.

Metallic line-blanketing coefficients have been measured over the wavelength range from 3800 to 4700 A in a sample of 20 Am and normal A stars. An expression for the logarithmic iron abundance has been calibrated in terms of effective temperature and a mean line-blocking coefficient. This makes it possible to determine the atmospheric iron abundance in sharp-lined Am and A stars with an accuracy of better than plus or minus 0.1 dex, using only line-blocking measurements and a photometric effective temperature.

Newly calculated model atmospheres, including both the effects of Lyman and Balmer line blanketing and the effects of convection, are used to determine the surface gravities of white-dwarf stars by the use of the separation of stars with different surface gravities in the Stroemgren and multichannel two-color diagrams. The mean surface gravity of 40 stars in the appropriate temperature range with multichannel colors is log g=8.04 +- 0.25, and the mean value for 35 stars with Stroemgren photomety is log g=7.86 +- 0.25. Comparison of all available investigations of white-dwarf masses shows that they are internally consistent with the possible exception of masses derived from gravitational redshifts, where a slight, marginally significant discrepancy may exist. The weighted mean of all mass determination is 0.75 M/sub sun/, allowing for selection effects. The 1 sigma range of white-dwarf masses extends from 0.5 M/sub sun/ to 1.0 M/sub sun/; we do not find evidence for a smaller mass range.

The diffusion of calcium and strontium in Hg-Mn and magnetic Ap stars is shown to lead to apparent overabundances of strontium and about normal abundances of calcium. This is in agreement with the observations of those elements. Detailed non-TLE calculations of the radiative forces were needed because the radiative forces must be known in the outermost regions of the atmosphere. They were carried out for atmospheres with T/sub eff/ = 10,000; 12,500; and 15,000 K; log g = 4. The effects of line saturation and UV blanketing are discussed. Radiative acceleratons are calculated deep enough in the envelope to determine the maximum possible enrichment.

Hydrogen and helium line profiles with high signal-to-noise ratios were obtained for four stars of spectral type 09.5 (Alpha Cam, Xi Ori A, Delta Ori A,AE Aur) that form a sequence in luminosity: Ia, Ib, II, V. The basic stellar parameters of these stars are determined by fitting the observed line profiles of weak photospheric absorption lines with profiles from models which include the effect of radiation scattered back onto the photosphere from their stellar winds, an effect referred to as wind blanketing. For these stars, the inclusion of wind blanketing is significant only for the most luminous star, Alpha Cam, for which the effective temperature was shifted about -2000 K relative to an unblanketed model.

The effects of an enhanced helium abundance on the visible spectra of luminous F and G stars are small and liable to escape detection. These results have important implications for recent work on the possible helium enrichment in the envelopes of Cepheid variables. In addition we confirm the results of similar studies of early-type stars on the apparent enhancement of the wings of the hydrogen Balmer lines with increasing helium abundance. We also find that the presence of microturbulence tends to narrow strong pressure-broadened lines by providing an alternative atmospheric support mechanism to gas pressure. The effect of enhanced helium abundance on the continuum flux and the Stroemgren colors is also discussed. By and large we find that the effects produced by the large helium enrichment are small and unlikely to be separated from the effects of microturbulence and UV line blanketing.

New hydrodynamic techniques coupled with improved opacities have produced light curves that can now be compared directly with the observations. A new dynamic zoning method due to J. Castor has resolved the front of the hydrogen ionization region, allowing effects due to zoning to be separated from the real effects occurring in the limited-amplitude nonlinear pulsation models. New low-temperature opacities have corrected for blanketing in the atmosphere to produce color-T/sub eff/ relationships which are in general agreement with the observations (Kraft-Oke) and a color-color loop for a 10 day Cepheid that is in good agreement with the observation of the Cepheid S Nor (Fernie).

Model atmospheres for the K2 III star Arcturus (alpha Boo), calculated with various representations of the atomic and molecular opacity, are compared with the observed flux distribution and a published semiempirical temperature structure. A model computed with the opacity-sampling method and with an effective temperature of 4250 K and a surface gravity of 50 cm/sec per sec matches well, while models with other representations of the opacity produce distinctly worse fits. The effective temperature is well determined at 4250 + or - 100 K.

O and B type stars neutral He lines, studying UV line blanketing effects on predicted departures from LTE

B1 V star model atmosphere calculated and iterated for flux constancy, considering blanketing by strongest lines in UV