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Near the surface of any neutron star there is a thin heat blanketing envelope that produces substantial thermal insulation of warm neutron star interiors and that relates the internal temperature of the star to its effective surface temperature. Physical processes in the blanketing envelopes are reasonably clear but the chemical composition is not. The latter circumstance complicates inferring physical parameters of matter in the stellar interiors from observations of the thermal surface radiation of the stars and urges one to elaborate the models of blanketing envelopes. We outline physical properties of these envelopes, particularly, the equation of state, thermal conduction, ion diffusion and others. Various models of heat blankets are reviewed, such as composed of separate layers of different elements, or containing diffusive binary ion mixtures in or out of diffusion equilibrium. The effects of strong magnetic fields in the envelopes are outlined as well as the effects of high temperatures which induce strong neutrino emission in the envelopes themselves. Finally, we discuss how the properties of the heat blankets affect thermal evolution of neutron stars and the ability to infer important information on internal structure of neutron stars from observations., Comment: 82 pages, 26 figures, accepted for publication in Physics Reports

In the era of modern digital sky surveys, uncertainties in the flux of stellar standards are commonly the dominant systematic error in photometric calibration and can often affect the results of higher-level experiments. The Hubble Space Telescope (HST) spectrophotometry, which is based on computed model atmospheres for three hot (Teff > 30,000 K) pure-hydrogen (DA) white dwarfs, is currently considered the most reliable and internally consistent flux calibration. However many next generation facilities (e.g. Harmoni on E-ELT, Euclid and JWST) will focus on IR observations, a regime in which white dwarf calibration has not yet been robustly tested. Cool DA white dwarfs have energy distributions that peak close to the optical or near-IR, do not have shortcomings from UV metal line blanketing, and have a reasonably large sky density ($\simeq$ 4 deg$^{-2}$ at G < 20), making them, potentially, excellent calibrators. Here we present a pilot study based on STIS+WFC3 observations of two bright DA white dwarfs to test whether targets cooler than current hot primary standards (Teff < 20,000 K) are consistent with the HST flux scale. We also test the robustness of white dwarf models in the IR regime from an X-shooter analysis of Paschen lines and by cross-matching our previously derived Gaia white dwarf catalogue with observations obtained with 2MASS, UKIDSS, VHS, and WISE., 12 pages, 8 figures, 2 tables. Accepted for publication in MNRAS. The best fitting models to the two white dwarfs analyzed are available from \

The detonation of a helium shell on a white dwarf has been proposed as a possible explosion triggering mechanism for Type Ia supernovae. Here, we report ZTF 18aaqeasu (SN 2018byg/ATLAS 18pqq), a peculiar Type I supernova, consistent with being a helium-shell double-detonation. With a rise time of $\approx 18$ days from explosion, the transient reached a peak absolute magnitude of $M_R \approx -18.2$ mag, exhibiting a light curve akin to sub-luminous SN 1991bg-like Type Ia supernovae, albeit with an unusually steep increase in brightness within a week from explosion. Spectra taken near peak light exhibit prominent Si absorption features together with an unusually red color ($g-r \approx 2$ mag) arising from nearly complete line blanketing of flux blue-wards of 5000 \AA. This behavior is unlike any previously observed thermonuclear transient. Nebular phase spectra taken at and after $\approx 30$ days from peak light reveal evidence of a thermonuclear detonation event dominated by Fe-group nucleosynthesis. We show that the peculiar properties of ZTF 18aaqeasu are consistent with the detonation of a massive ($\approx 0.15$ M$_\odot$) helium shell on a sub-Chandrasekhar mass ($\approx 0.75$ M$_\odot$) white dwarf after including mixing of $\approx 0.2$ M$_\odot$ of material in the outer ejecta. These observations provide evidence of a likely rare class of thermonuclear supernovae arising from detonations of massive helium shells., Comment: 10 pages, 6 figures. Submitted to ApJL

We present a numerical parameter survey of sub-Chandrasekhar mass white dwarf (WD) explosions. Carbon-oxygen WDs accreting a helium shell have the potential to explode in the sub-Chandrasekhar mass regime. Previous studies have shown how the ignition of a helium shell can either directly ignite the WD at the core-shell interface or propagate a shock wave into the the core causing a central ignition. We examine the explosions of WDs from 0.6 - 1.2 M$_\odot$ with helium shells of 0.01, 0.05 and 0.08 M$_\odot$. Distinct observational signatures of sub-Chandrasekhar mass WD explosions are predicted for two categories of shell size. Thicker-shell models show an early time flux excess, which is caused by the presence of radioactive material in the ashes of the helium shell, and red colors due to these ashes creating significant line blanketing in the UV through the blue portion of the spectrum. Thin shell models reproduce several typical Type Ia supernova signatures. We identify a relationship between Si II velocity and luminosity which, for the first time, identifies a sub-class of observed supernovae that are consistent with these models. This sub-class is further delineated by the absence of carbon in their atmospheres. We suggest that the proposed difference in the ratio of selective to total extinction between the high velocity and normal velocity Type Ia supernovae is not due to differences in the properties of the dust around these events, but is rather an artifact of applying a single extinction correction to two intrinsically different populations of supernovae., 13 pages, 12 figures, Accepted to ApJ 30 Decemeber 2018

We study the effects of heat blanketing envelopes of neutron stars on their cooling. To this aim, we perform cooling simulations using newly constructed models of the envelopes composed of binary ion mixtures (H--He, He--C, C--Fe) varying the mass of lighter ions (H, He or C) in the envelope. The results are compared with those calculated using the standard models of the envelopes which contain the layers of lighter (accreted) elements (H, He and C) on top of the Fe layer, varying the mass of accreted elements. The main effect is that the chemical composition of the envelopes influences their thermal conductivity and, hence, thermal insulation of the star. For illustration, we apply these results to estimate the internal temperature of the Vela pulsar and to study the cooling of neutron stars of ages of 0.1 - 1 Myr at the photon cooling stage. The uncertainties of the cooling models associated with our poor knowledge of chemical composition of the heat insulating envelopes strongly complicate theoretical reconstruction of the internal structure of cooling neutron stars from observations of their thermal surface emission., 8 pages, 4 figures, MNRAS, accepted

We present a high resolution synthetic spectral library, INTRIGOSS, designed for studying FGK stars. The library is based on atmosphere models computed with specified individual element abundances via ATLAS12 code. Normalized SPectra (NSP) and surface Flux SPectra (FSP), in the 4830-5400 A, wavelength range, were computed with the SPECTRUM code. INTRIGOSS uses the solar composition by Grevesse et al. 2007 and four [alpha/Fe] abundance ratios and consists of 15,232 spectra. The synthetic spectra are computed with astrophysical gf-values derived by comparing synthetic predictions with a very high SNR solar spectrum and the UVES-U580 spectra of five cool giants. The validity of the NSPs is assessed by using the UVES-U580 spectra of 2212 stars observed in the framework of the Gaia-ESO Survey and characterized by homogeneous and accurate atmospheric parameter values and by detailed chemical compositions. The greater accuracy of NSPs with respect to spectra from the AMBRE, GES_Grid, PHOENIX, C14, and B17 synthetic spectral libraries is demonstrated by evaluating the consistency of the predictions of the different libraries for the UVES-U580 sample stars. The validity of the FSPs is checked by comparing their prediction with both observed spectral energy distribution and spectral indices. The comparison of FSPs with SEDs derived from ELODIE, INDO--U.S., and MILES libraries indicates that the former reproduce the observed flux distributions within a few percent and without any systematic trend. The good agreement between observational and synthetic Lick/SDSS indices shows that the predicted blanketing of FSPs well reproduces the observed one, thus confirming the reliability of INTRIGOSS FSPs., 32 pages, 12 figures

Aims: We systematically explore a plausible subset of the parameter space involving effective temperatures and metallicities of the ionizing stellar sources, the effects of the hardening of their radiation by surrounding leaky HII regions with different escape fractions, as well as different scenarios for the clumpiness of the DIG, and compute the resulting line strength ratios for a number of diagnostic optical emission lines. Methods: For the ionizing fluxes we compute a grid of stellar spectral energy distributions (SEDs) from detailed, fully non-LTE model atmospheres that include the effects of stellar winds and line blocking and blanketing. To calculate the ionization and temperature structure in the HII regions and the diffuse ionized gas we use spherically symmetric photoionization models as well as state-of-the-art three-dimensional (3D) non-LTE radiative transfer simulations, considering hydrogen, helium, and the most abundant metals. Results: We provide quantitative predictions of how the line ratios from HII regions and the DIG vary as a function of metallicity, stellar effective temperature, and escape fraction from the HII region. The range of predicted line ratios reinforces the hypothesis that the DIG is ionized by (filtered) radiation from hot stars; however, comparison of observed and predicted line ratios indicates that the DIG is typically ionized with a softer SED than predicted by the chosen stellar population synthesis model. Even small changes in simulation parameters like the clumping factor can lead to considerable variation in the ionized volume. Both for a more homogeneous gas and a very inhomogeneous gas containing both dense clumps and channels with low gas density, the ionized region in the dilute gas above the galactic plane can cease to be radiation-bounded, allowing the ionizing radiation to leak into the intergalactic medium., Comment: 21 pages, 9 figures, accepted by A&A

We derive analytic approximations for the neutrino luminosities and the heat capacities of neutron stars with isothernal nucleon cores as functions of the mass and radius of stars. The neutrino luminosities are approximated for the three basic neutrino emission mechanisms, and the heat capacities for the five basic combinations of the partial heat capacities. The approximations are valid for for a wide class of equations of state of dense nucleonmatter. The results significantly simplify the theoretical interpretation of observations of cooling neutron stars as well as of quasistationary thermal states of neutron stars in X-ray transients. For illustration, we present an analysis of the neutrino cooling functions of nine isolated neutron stars taking into account the effects of their magnetic fields and of the presence of light elements in their heat blanketing envelopes. These results allow one to investigate the superfluid properties of neutron star cores., 15 pages, 9 figures, 5 tables

Extreme helium stars (EHe stars) are hydrogen-deficient supergiants of spectral type A and B. They are believed to result from mergers in double degenerate systems. In this paper we present a detailed quantitative non-LTE spectral analysis for BD+10$^\circ$2179, a prototype of this rare class of stars, using UVES and FEROS spectra covering the range from $\sim$3100 to 10 000 {\AA}. Atmosphere model computations were improved in two ways. First, since the UV metal line blanketing has a strong impact on the temperature-density stratification, we used the Atlas12 code. Additionally, We tested Atlas12 against the benchmark code Sterne3, and found only small differences in the temperature and density stratifications, and good agreement with the spectral energy distributions. Second, 12 chemical species were treated in non-LTE. Pronounced non-LTE effects occur in individual spectral lines but, for the majority, the effects are moderate to small. The spectroscopic parameters give $T_\mathrm{eff}$ = 17 300$\pm$300 K and $\log g$ = 2.80$\pm$0.10, and an evolutionary mass of 0.55$\pm$0.05 $M_\odot$. The star is thus slightly hotter, more compact and less massive than found in previous studies. The kinematic properties imply a thick-disk membership, which is consistent with the metallicity $[$Fe/H$]\approx-1$ and $\alpha$-enhancement. The refined light-element abundances are consistent with the white dwarf merger scenario. We further discuss the observed helium spectrum in an appendix, detecting dipole-allowed transitions from about 150 multiplets plus the most comprehensive set of known/predicted isolated forbidden components to date. Moreover, a so far unreported series of pronounced forbidden He I components is detected in the optical-UV., Comment: Accepted for publication in MNRAS, 26 pages, 19 Figures

We have determined new relations between $UBV$ colors and mass-to-light ratios ($M/L$) for dwarf irregular (dIrr) galaxies, as well as for transformed $g^\prime - r^\prime$. These $M/L$ to color relations (MLCRs) are based on stellar mass density profiles determined for 34 LITTLE THINGS dwarfs from spectral energy distribution fitting to multi-wavelength surface photometry in passbands from the FUV to the NIR. These relations can be used to determine stellar masses in dIrr galaxies for situations where other determinations of stellar mass are not possible. Our MLCRs are shallower than comparable MLCRs in the literature determined for spiral galaxies. We divided our dwarf data into four metallicity bins and found indications of a steepening of the MLCR with increased oxygen abundance, perhaps due to more line blanketing occurring at higher metallicity., Comment: 5 pages, 4 figures, 2 tables

We address the physical nature of subdwarf A-type (sdA) stars and their possible link to extremely low mass (ELM) white dwarfs (WDs). The two classes of objects are confused in low-resolution spectroscopy. However, colors and proper motions indicate that sdA stars are cooler and more luminous, and thus larger in radius, than published ELM WDs. We demonstrate that surface gravities derived from pure hydrogen models suffer a systematic ~1 dex error for sdA stars, likely explained by metal line blanketing below 9000 K. A detailed study of five eclipsing binaries with radial velocity orbital solutions and infrared excess establishes that these sdA stars are metal-poor ~1.2 Msun main sequence stars with ~0.8 Msun companions. While WDs must exist at sdA temperatures, only ~1% of a magnitude-limited sdA sample should be ELM WDs. We conclude that the majority of sdA stars are metal-poor A-F type stars in the halo, and that recently discovered pulsating ELM WD-like stars with no obvious radial velocity variations may be SX Phe variables, not pulsating WDs., Comment: 17 pages, ApJ accepted

A small fraction of hot OBA stars host global magnetic fields with field strengths of the order of 0.1-10 kG. This leads to the creation of persistent surface structures (spots) in stars with sufficiently weak winds as a result of the radiative diffusion. These spots become evident in spectroscopic and photometric variability. This type of variability is not expected in stars with strong winds, where the wind inhibits the radiative diffusion. Therefore, a weak photometric variability of the magnetic Of?p star HD 191612 is attributed to the light absorption in the circumstellar clouds. We study the nature of the photometric variability of HD 191612. We assume that the variability results from variable wind blanketing induced by surface variations of the magnetic field tilt and modulated by stellar rotation. We used our global kinetic equilibrium (NLTE) wind models with radiative force determined from the radiative transfer equation in the comoving frame (CMF) to predict the stellar emergent flux. Our models describe the stellar atmosphere in a unified manner and account for the influence of the wind on the atmosphere. The models are calculated for different wind mass-loss rates to mimic the effect of magnetic field tilt on the emergent fluxes. We integrate the emergent fluxes over the visible stellar surface for individual rotational phases, and calculate the rotationally modulated light curve of HD 191612. The wind blanketing that varies across surface is able to explain a part of the observed light variability in this star. The mechanism is able to operate even at relatively low mass-loss rates. The remaining variability is most likely caused by the flux absorption in circumstellar clouds. The variable wind blanketing is an additional source of the light variability in massive stars. The presence of the rotational light variability may serve as a proxy for the magnetic field., 5 pages, accepted for publication in A&A

We analyze a high-resolution spectrum of the A3m star HD27411. We compare abundances derived from ATLAS9 model atmospheres with those using the more computationally-intensive ATLAS12 code. We found very little differences in the abundances, suggesting that ATLAS9 can be used for moderate chemical peculiarity. Our abundances agree well with the predictions of diffusion theory, though for some elements it was necessary to calculate line profiles in non-thermodynamic equilibrium to obtain agreement. We investigate the effective temperatures and luminosities of Am/Fm stars using synthetic Stromgren indices derived from calculated spectra with the atmospheric abundances of HD27411. We find that the effective temperatures of Am/Fm stars derived from Stromgren photometry are reliable, but the luminosities are probably too low. Caution is required when deriving the reddening of these stars owing to line blanketing effects. A comparison of the relative proportions of pulsating and non-pulsating Am stars with delta Scuti stars shows quite clearly that there is no significant decrease of helium in the driving zone, contrary to current models of diffusion.

We explore the thermal state of the neutron star in the Cassiopeia A supernova remnant using the recent result of Ho & Heinke (Nature, 462, 71 (2009)) that the thermal radiation of this star is well-described by a carbon atmosphere model and the emission comes from the entire stellar surface. Starting from neutron star cooling theory, we formulate a robust method to extract neutrino cooling rates of thermally relaxed stars at the neutrino cooling stage from observations of thermal surface radiation. We show how to compare these rates with the rates of standard candles -- stars with non-superfluid nucleon cores cooling slowly via the modified Urca process. We find that the internal temperature of standard candles is a well-defined function of the stellar compactness parameter $x=r_g/R$, irrespective of the equation of state of neutron star matter ($R$ and $r_g$ are circumferential and gravitational radii, respectively). We demonstrate that the data on the Cassiopeia A neutron star can be explained in terms of three parameters: $f_\ell$, the neutrino cooling efficiency with respect to the standard candle; the compactness $x$; and the amount of light elements in the heat blanketing envelope. For an ordinary (iron) heat blanketing envelope or a low-mass ($\lesssim 10^{-13}\,M_\odot$) carbon envelope, we find the efficiency $f_\ell \sim 1$ (standard cooling) for $x \lesssim 0.5$ and $f_\ell \sim 0.02$ (slower cooling) for a maximum compactness $x\approx 0.7$. A heat blanket containing the maximum mass ($\sim 10^{-8}\,M_\odot$) of light elements increases $f_\ell$ by a factor of 50. We also examine the (unlikely) possibility that the star is still thermally non-relaxed.

New flux data are presented for nine nonvariable stars and 14 evolved variable stars with spectral types M and C. The data are from measurements of 21 passbands in the wavelength range from 7440 A to 10834 A, and they are comparable to measurements made by Wing some 40 years ago. Because the extinction algorithm applied to the new data is based partly on up-to-date calculations of telluric water-vapor effects, those calculations are tested for accuracy. In addition, methods used to calibrate standard stars both outside and inside the Paschen confluence are explained. After reddening corrections are applied to the flux data for the variable stars, those data are used to calculate color temperatures. In turn, those temperatures are used to derive blanketing corrections to color temperatures measured in the Wing filter system. Indices of absorption strength are calculated by comparing the flux data to blackbody colors derived from the color temperatures. It is found that the standard errors of those temperatures range from 3% to less than 1%. For the variable stars, the standard errors for the flux data range from 6.8 mmag to 11.6 mmag. For the nonvariable stars, the corresponding standard error is about 6.0 mmag.

Aims. The fast improvement of spectroscopic observations makes mandatory a strong effort on the theoretical side to better reproduce the spectral energy distribution (SED) of stars at high spectral resolution. In this regard, relying on the Kurucz Atlas/Synthe original codes we computed the Bluered library, consisting of 832 synthetic SED of stars, that cover a large parameter space at very high spectral resolution (R = 500 000) along the 3500−7000 A wavelength range. Methods. Bluered synthetic spectra have been used to assess in finer detail the intrinsic reliability and the performance limits of the Atlas theoretical framework. The continuum-normalized spectra of the Sun, Arcturus, and Vega, plus a selected list of 45 bright stars with high-quality SEDs from the Prugniel & Soubiran Elodie catalog, form our sample designed to probe the global properties of synthetic spectra across the entire range of H-R parameters. Results. Atlas models display a better fitting performance with increasing stellar temperature. High-resolution spectra of Vega, the Sun, and Arcturus have been reproduced at R = 100 000, respectively, within a 0.7%, 4.5%, and 8.8% relative scatter in residual flux. In all the three cases, the residual flux distribution shows a significant asymmetry (skewness parameter γ = −2.21, −0.98, −0.67, respectively), which neatly confirms an overall “excess” of theoretical line blanketing. For the Sun, this apparent discrepancy is alleviated, but not recovered, by a systematic decrease (−40%) of the line oscillator strengths, log(gf ), especially referring to iron transitions. Definitely, a straight “astrophysical” determination of log(gf ) for each individual atomic transition has to be devised to overcome the problem. By neglecting overblanketing effects in theoretical models when fitting high-resolution continuum-normalized spectra of real stars, we lead to a systematically warmer effective temperature (between +80 and +300 K for the solar fit) and a slightly poorer metal content.

We present synthetic spectra around maximum for the type II supernova SN 2003Z, which was first detected on January 29.7 2003. Comparison with observed spectra aim at the determination of physical parameters for SN 2003Z. Synthetic spectra are calculated with our stellar atmosphere code PHOENIX. It solves the special relativistic equation of radiative transfer, including large NLTE-calculations and line blanketing by design, in 1-dimensional spherical symmetry. The observed spectra were obtained at the 3.5 meter telescope at Calar Alto. The TWIN instrument was used so that a spectral range from about 3600 to 7500 Angstroem was covered. The spectra were taken on Feb. 4, 5, 9, and 11, 2003. The physical parameters of the models give the luminosities, a range of possible velocity profiles for the SN, an estimate of the colour excess, and the observed metalicity., 8 figures

The central compact object in the supernova remnant HESS J1731-347 appears to be the hottest observed isolated cooling neutron star. The cooling theory of neutron stars enables one to explain observations of this star by assuming the presence of strong proton superfluidity in the stellar core and the existence of the surface heat blanketing envelope which almost fully consists of carbon. The cooling model of this star is elaborated to take proper account of the neutrino emission due to neutron-neutron collisions which is not suppressed by proton superfluidity. Using the results of spectral fits of observed thermal spectra for the distance of 3.2 kpc and the cooling theory for the neutron star of age 27 kyr, new constraints on the stellar mass and radius are obtained which are more stringent than those derived from the spectral fits alone., 9 pages, 5 figures, MNRAS, accepted

We present new grids of Potsdam Wolf-Rayet (PoWR ) model atmospheres for Wolf-Rayet stars of the nitrogen sequence (WN stars). The models have been calculated with the latest version of the PoWR stellar atmosphere code for spherical stellar winds. The WN model atmospheres include the non-LTE solutions of the statistical equations for complex model atoms, as well as the radiative transfer equation in the co-moving frame. Iron-line blanketing is treated with the help of the superlevel approach, while wind inhomogeneities are taken into account via optically thin clumps. Three of our model grids are appropriate for Galactic metallicity. The hydrogen mass fraction of these grids is 50%, 20%, and 0%, thus also covering the hydrogen-rich late-type WR stars that have been discovered in recent years. Three grids are adequate for LMC WN stars and have hydrogen fractions of 40%, 20%, and 0%. Recently, additional grids with SMC metallicity and with 60%, 40%, 20%, and 0% hydrogen have been added. We provide contour plots of the equivalent widths of spectral lines that are usually used for classification and diagnostics.

Observations of thermal radiation from neutron stars can potentially provide information about the states of supranuclear matter in the interiors of these stars with the aid of the theory of neutron-star thermal evolution. We review the basics of this theory for isolated neutron stars with strong magnetic fields, including most relevant thermodynamic and kinetic properties in the stellar core, crust, and blanketing envelopes., Comment: 57 pages, 3 tables, 12 figures, invited topical review in The Strongest Magnetic Fields in the Universe, eds. V. S. Beskin, A. Balogh, M. Falanga, M. Lyutikov, S. Mereghetti, T. Piran, R. A. Treumann (Space Sciences Series of ISSI, Springer). In v.2, a typo in Eq.(3) is fixed. In v.3, Eqs.(32)-(33) are corrected, and references updated

We present a recently developed code for detailed NLTE calculations of accretion disc spectra of cataclysmic variables and compact X-ray binaries. Assuming a radial structure of a standard alpha-disc, the disc is divided into concentric rings. For each disc ring the solution of the radiation transfer equation and the structure equations, comprising the hydrostatic and radiative equilibrium, the population of the atomic levels as well as charge and particle conservation, is done self-consistently. Metal-line blanketing and irradiation by the central object are taken into account. As a first application, we show the influence of different disc parameters on the disc spectrum for the helium cataclysmic variable AM CVn., 7 pages, 11 figures to be published in A&A

We study mass-loss and wind momentum rates of 29 Galactic O-type stars with luminosity classes I, III and V by means of a pure Hα profile analysis and investigate to what extent the results compare to those originating from a state-of- the-art, complete spectral analysis. Our investigation relies on the approximate method developed by Puls et al. (1996) which we have modified to account for the effects of line-blanketing. Effective temperatures and gravities needed to obtain quanti- tative results from such a simplified approach have been derived by means of calibrations based on most recent spectroscopic NLTE analyses and models of Galactic stars by Repolust et al. (2003) and Martins et al. (2002). Comparing (i) the derived wind- densities to those determined by Repolust et al. (2003) for eleven stars in common and (ii) the Wind-momentum Luminosity Relationship (WLR) for our sample stars to those derived by other investigations, we conclude that our approximate approach is actually able to provide consistent results. Additionally, we studied the consequences of "fine tuning" some of the direct and indirect parameters entering the WLR, especially by accounting for different possible values of stellar reddening and distances. Combining our data set with the corresponding data provided by Herrero et al. (2002) and Repolust et al. (2003) we finally study the WLR for the largest sample of Galactic O-type stars gathered so far, including an elaborate error treatment. The established disagreement between the theoretical predictions and the "observed" WLRs being a function of luminosity class is suggested to be a result of wind clumping. Different strategies to check this hypothesis are discussed, particularly by comparing the Hα mass-loss rates with the ones derived from radio observations.

The influence of the blanketing effect on the intensities of Fe I lines in solar and stellar atmospheres is investigated. More than 18,000 spectral lines of 14 elements were taken into account. It was demonstrated that ignoring the line-blanketing effect produces an error in the estimation of iron abundance of 0.04 dex in the case of the Sun and solar-type stars. (The iron abundance is given in the logarithmic scale and dex means the decimal logarithm estimation error.)

Techniques and results of computations of model atmospheres are discussed for M and C giants and for giants with abundance anomalies. The SAM12 code, a modification of the ATLAS12 code developed by Kurucz, is used. Blanketing effects due to atomic and molecular line absorption are taken into account using the opacity-sampling approach. The computed model atmosphere for the Sun (G2V) is used as a test of the SAM12 code. The model red-giant atmospheres are compared with models reported in other studies. Comparisons between the computed and observed spectral energy distributions are given for the C giant WX Cyg (C-J6) and Sakurai's object (V4334 Sgr).

We present a new grid of ionizing fluxes for O and Wolf-Rayet stars for use with evolutionary synthesis codes and single star H II region analyses. A total of 230 expanding, non-LTE, line-blanketed model atmospheres have been calculated for five metallicities (0.05, 0.2, 0.4, 1 and 2 solar) using the WM-basic code of Pauldrach et al. (2001) and the CMFGEN code of Hillier & Miller (1998). The stellar wind parameters are scaled with metallicity for both O and W-R stars. We incorporate the new models into Starburst99 (Leitherer et al. 1999) and compare the ionizing outputs with Schaerer & Vacca (1998) and Leitherer et al. (1999). The changes in the output ionizing fluxes are dramatic, particularly below 228 A. We also find lower fluxes in the He I continuum for Z > 0.4 solar and ages < 7 Myr because of the increased line blanketing. We test the accuracy of the new models by constructing photoionization models. We show that for the dwarf O star grid, He I 5876/H beta decreases between Z = 1 and twice solar in a similar manner to observations (e.g. Bresolin et al. 1999) due to the increased effect of line blanketing. We therefore suggest that a lowering of the upper mass limit at high abundances is not required to explain the observations. For the case of an instantaneous burst, we plot the softness parameter "eta prime" against the abundance indicator R_23. The new models are coincident with the data of Bresolin et al. (1999), particularly during the W-R phase, unlike previous models which over-predict the hardness of the ionizing radiation.

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 rediscuss the temperature of O dwarfs based on new non-LTE line blanketed atmosphere models including stellar winds computed with the CMFGEN code of Hillier & Miller (1998). Compared to the latest calibration of Vacca et al. (1996), the inclusion of line blanketing leads to lower effective temperatures, typically by 4000 to 1500 K for O3 to O9.5 dwarf stars. The dependence of the Teff-scale on stellar and model parameters - such as mass loss, microturbulence, and metallicity - is explored, and model predictions are compared to optical observations of O stars. Even for an SMC metallicity we find a non-negligible effect of line blanketing on the Teff-scale. The temperature reduction implies downward revisions of luminosities by 0.1 dex and Lyman continuum fluxes Q0 by approximately 40% for dwarfs of a given spectral type., 6 pages, 4 figures. To be published in A&A

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

To assess the uncertainties in primordial helium abundance determination by nebular codes, we calculate a grid of OB stellar atmospheres at low metallicities, including both non-local thermodynamic equilibrium (NLTE) and metal line blanketing effects. The more sophisticated stellar atmosphere models we use can differ from LTE models by as much as 40% in the ratio of He to H-ionizing photons.

We calculate a grid of spherically symmetric OB stellar atmospheres at low metallicities, including both non-local thermodynamic equilibrium (NLTE) and metal line blanketing effects. This is done to assess the uncertainties in helium abundance determination by nebular codes due to input stellar atmosphere models. The more sophisticated stellar atmosphere models we use can differ from LTE models by as much as 40 percent in the ratio of He to H-ionizing photons.

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.

Absorption due to HeII Ly-alpha has been detected at low resolution in the spectra of four quasars between redshifts z = 2.74-3.29. We assess these observations, giving particular attention to the radiative transfer of the ionizing background radiation, cloud diffuse emission and ionization physics, and statistical fluctuations. We use high-resolution observations of HI absorption towards quasars to derive improved models for the opacity of the intergalactic medium (IGM) and use these models to calculate the HI and HeII photoionization history in both optically-thin and self-shielded clouds, and the average line-blanketing contribution to HeII absorption. The derived ionization rate at z = 2-4, Gamma(HI) = (1-3)x10^-12 s^-1, is consistent with the ionizing background intensity inferred from the proximity effect, but it remains larger than that inferred by N-body hydrodynamical simulations of the Lya absorber distribution. The HeII observations are con- sistent with line blanketing from clouds with N(HI) > 10^12 cm^-2, although a contribution from a more diffuse IGM would help to explain the observed opacity. We compute the expected HeII optical depth and examine the sizable fluc- tuations that arise from variations in the cloud numbers and ionizing radiation field. We assess how HeII absorption constrains the intensity and spectrum of the ionizing radiation and the fractional contributions of the dominant sources (quasars and starburst galaxies). Finally, we demonstrate how high-resolution UV observations can distinguish between absorption from the diffuse IGM and the Lya forest clouds and determine the source of the ionizing background.

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

The model atmosphere programs FASTWIND and CMFGEN are both elegantly designed to perform non-LTE analyses of the spectra of hot massive stars, and include sphericity and mass-loss. The two codes differ primarily in their approach towards line blanketing, with CMFGEN treating all of the lines in the co-moving frame and FASTWIND taking an approximate approach which speeds up execution times considerably. Although both have been extensively used to model the spectra of O-type stars, no studies have used the codes to independently model the same spectra of the same stars and compare the derived physical properties. We perform this task on ten O-type stars in the Magellanic Clouds. For the late-type O supergiants, both CMFGEN and FASTWIND have trouble fitting some of the He I lines, and we discuss causes and cures. We find that there is no difference in the average effective temperatures found by the two codes for the stars in our sample, although the dispersion is large, due primarily to the various difficulties each code has with He I. The surface gravities determined using FASTWIND are systematically lower by 0.12 dex compared to CMFGEN, a result we attribute to the better treatment of electron scattering by CMFGEN. This has implications for the interpretation of the origin of the so-called mass discrepancy, as the masses derived by FASTWIND are on average lower than inferred from stellar evolutionary models, while those found by CMFGEN are in better agreement., Accepted by the Astrophysical Journal

The coalescence of compact objects are a promising astrophysical sources of gravitational wave (GW) signals. The ejection of r-process material from such mergers may lead to a radioactively-powered electromagnetic counterpart which, if discovered, would enhance the science return of a GW detection. As very little is known about the optical properties of heavy r-process elements, previous light curve models have adopted opacities similar to those of iron group elements. Here we report that the presence of heavier elements, particularly the lanthanides, increase the ejecta opacity by several orders of magnitude. We include these higher opacities in time dependent, multi-wavelength radiative transport calculations to predict the broadband light curves of one-dimensional models over a range of parameters (ejecta masses from 0.001 to 0.1 solar masses and velocities from 0.1 to 0.3c). We find that the higher opacities lead to much longer duration light curves which can last a week or more. The emission is shifted toward the infrared bands due to strong optical line blanketing, and the colors at later times are representative of a blackbody near the recombination temperature of the lanthanides (T ~ 2500K). We further consider the case in which a second mass outflow, composed of 56Ni, is ejected from a disk wind, and show that the net result is a distinctive two component spectral energy distribution, with a bright optical peak due to 56Ni and an infrared peak due to r-process ejecta. We briefly consider the prospects for detection and identification of these transients., Comment: submitted to ApJ

We present the first non-LTE time-dependent radiative-transfer simulations of supernovae (SNe) II-Plateau (II-P) covering both the photospheric and nebular phases, from similar to 10 to greater than or similar to 1000 d after the explosion, and based on 1.2 B piston-driven ejecta produced from a 15 M-circle dot and a 25 M-circle dot non-rotating solar-metallicity star. The radial expansion of the gradually cooling photosphere gives rise to a near-constant luminosity up to greater than or similar to 100 d after the explosion. The photosphere remains in the outer 0.5 M-circle dot of the ejecta for up to similar to 50 d after the explosion. As the photosphere reaches the edge of the helium core, the SN luminosity drops by an amount mitigated by the progenitor radius and the 56Ni mass. Synthetic light curves exhibit a bell-shaped morphology, evolving faster for more compact progenitors, and with an earlier peak and narrower width in bluer filters. UV and U-band fluxes are very sensitive to line blanketing, the metallicity and the adopted model atoms. During the recombination epoch synthetic spectra are dominated by H i and metal lines, and are largely insensitive to the differing H/He/C/N/O composition of our two progenitor stars. In contrast, synthetic nebular-phase spectra reveal a broader/stronger O i doublet line in the higher-mass progenitor model, reflecting the larger masses of oxygen and nickel that are ejected. Our simulations overestimate the typical luminosity and the visual rise time of standard SNe II-P, most likely a consequence of our progenitor stars being too big and/or too hydrogen rich. Comparison of our simulations with photospheric-phase observations of SN1999em of the same colour is satisfactory. Our neglect of non-thermal excitation/ionization leads to a fast disappearance of continuum radiation and Balmer-line emission at the end of the plateau phase. With the exception of H i lines, our nebular spectra show a striking similarity to contemporaneous observations of SN1999em.

We discuss a new one-dimensional non-LTE time-dependent radiative-transfer technique for the simulation of supernova (SN) spectra and light curves. Starting from a hydrodynamical input characterizing the homologously-expanding ejecta at a chosen post-explosion time, we model the evolution of the entire ejecta, including gas and radiation. Non-LTE, which holds in all regions at and above the photosphere, is accounted for. The effects of line blanketing on the radiation field are explicitly included, using complex model atoms and solving for all ion level populations appearing in the statistical-equilibrium equations. Here, we present results for SN1987A, evolving the model "lm18a7Ad" of Woosley from 0.27 to 20.8d. The fastest evolution occurs prior to day 1, with a spectral energy distribution peaking in the range 300-2000A, subject to line blanketing from highly ionized metal and CNO species. After day 1, our synthetic multi-band light curve and spectra reproduce the observations to within 10-20% in flux in the optical, with a greater mismatch for the faint UV flux. We do not encounter any of the former discrepancies associated with the HeI and HI lines in the optical, which can be fitted well with a standard Blue-supergiant-star surface composition and no contribution from radioactive decay. The effects of time dependence on the ionization structure, discussed in Dessart & Hillier, are recovered, and thus nicely integrated in this new scheme. Despite the 1D nature of our approach, its high physical consistency and accuracy will allow reliable inferences to be made on explosion properties and pre-SN star evolution.

Extreme helium stars are hydrogen‐deficient supergiants of spectral type A and B. They are believed to result from mergers in double degenerate systems, if one white dwarf is of C/O‐type and the other of He‐type. We calculate a new grid of model atmospheres accounting for metal line blanketing with ATLAS12 and compare it to the benchmark code STERNE. Synthetic spectra are calculated accounting for NLTE effects for many ions and applied to high‐quality optical spectra of the prototype extreme helium star BD+10°2179.

We present our new grids of NLTE model atmospheres for hot subdwarf O (sdO) stars computed with the public codes TLUSTY and SYNSPEC and including the effects of metal line blanketing. Until recently, large grids of NLTE models could not be practically computed over reasonable timescales, and have remained unavailable for quantitative spectroscopic analyses of this category of hot stars. We solved the problem by adapting TLUSTY and SYNSPEC to run in parallel mode on a small dedicated cluster of 80 fast processors. We present a comparison of three different NLTE grids including different elements. By fitting synthetic spectra from a given grid with another one, we obtain “maps” showing the line blanketing effects of metals, i.e., the differences between the real atmospheric parameters (Teff and log g) and the ones inferred by the fitting procedure. Finally, we present the results of the spectral analysis of SDSS J1600+0748, this unique pulsating sdO star, using our different grids of NLTE model atmospheres.

In this paper we have carried out an analysis of the predictions of the radiatively driven stellar winds theories on 63 stars belonging to clusters or associations. The spectral types in our sample range from O3 to B0 and all classes of luminosity are considered. The study has been carried out starting from the relationship between the stellar-wind velocity (v edge) obtained from the resonance doublet ofCIV for stars observed with the IUE, and the escape velocity. The stellar masses have been obtained from the evolutionary tracks of Maeder and Meynet. Results from recent NLTE analyses with blanketing of lines and winds have been used for the effective temperature.

The information content of (photospheric) stellar spectra and the accuracy of abundance determinations are discussed. Besides the physical properties of the line spectra, the spectral resolution, the signal-to-noise ratio, but also the incompleteness of the knowledge of the contributing blends limit the abundance information. For a perfect model atmosphere the main factors determining the accuracy of the abundances are S/N, the line saturation, and the location of the continuum (or, in a synthetic spectrum, the background of numerous lines). In addition systematic errors introduced by missing atomic data and an imperfect model atmosphere (non-LTE, line blanketing, hydrodynamics) are important. The typical accuracy of abundance determinations is discussed for main sequence stars for which the most reliable data are available, and for a few selected other stellar types.

A grid of NLTE model atmospheres for early-type stars is presented. Model atoms of eleven different elements with various abundances and, for a subset of the models, effects of wind blanketing, are taken into account. These model atmospheres supply realistic emergent fluxes which can be used in ionization calculations for circum- and interstellar gas. The dependence of the ionizing radiation of these stars on spectral type, luminosity class, evolutionary state and stellar environment is discussed.

AA Tau was observed for about 5h per XMM orbit (2 days) over 8 successive orbits, which covers two optical eclipse periods (8.2 days). The XMM optical/UV monitor simultaneously provided UV photometry with a ~15 min sampling rate. Some V-band photometry was also obtained from the ground during this period in order to determine the dates of the eclipses. Two X-ray and UV measurements were secured close to the center of the eclipse. The UV flux is the highest just before the eclipse starts and the lowest towards the end of it. We model the UV flux variations with a weekly modulation (inner disk eclipse), plus a daily modulation, which suggests a non-steady accretion. No eclipses are detected in X-rays. For one measurement, the X-ray count rate was nearly 50 times stronger than the minimum observed level, and the plasma temperature reached 60 MK, i.e., a factor of 2-3 higher than in the other observations. This X-ray event, observed close to the center of the optical eclipse, is interpreted as an X-ray flare. We identify the variable column density with the low-density accretion funnel flows blanketing the magnetosphere. The lack of X-ray eclipses indicates that X-ray emitting regions are located at high latitudes. Furthermore, the occurrence of a strong X-ray flare near the center of the optical eclipse suggests that the magnetically active areas are closely associated with the base of the high-density accretion funnel flow. We speculate that the impact of this free falling accretion flow onto the strong magnetic field of the stellar corona may boost the X-ray emission (abridged)., 17 pages and 9 Figures. Accepted by A&A

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

CONTEXT: Very massive stars pass through the Wolf-Rayet (WR) stage before they finally explode. Details of their evolution have not yet been safely established, and their physics are not well understood. Their spectral analysis requires adequate model atmospheres, which have been developed step by step during the past decades and account in their recent version for line blanketing by the millions of lines from iron and iron-group elements. However, only very few WN stars have been re-analyzed by means of line-blanketed models yet. AIMS: The quantitative spectral analysis of a large sample of Galactic WN stars with the most advanced generation of model atmospheres should provide an empirical basis for various studies about the origin, evolution, and physics of the Wolf-Rayet stars and their powerful winds. METHODS: We analyze a large sample of Galactic WN stars by means of the Potsdam Wolf-Rayet (PoWR) model atmospheres, which account for iron line blanketing and clumping. The results are compared with a synthetic population, generated from the Geneva tracks for massive star evolution. RESULTS: We obtain a homogeneous set of stellar and atmospheric parameters for the Galactic WN stars, partly revising earlier results. CONCLUSIONS: Comparing the results of our spectral analyses of the Galactic WN stars with the predictions of the Geneva evolutionary calculations, we conclude that there is rough qualitative agreement. However, the quantitative discrepancies are still severe, and there is no preference for the tracks that account for the effects of rotation. It seems that the evolution of massive stars is still not satisfactorily understood., 19 pages, 11 figures, A&A, in press, additional Online-material on http://www.astro.physik.uni-potsdam.de/abstracts/galwn.html

The progress over the last years in modelling the atmospheres and winds of PN central stars is reviewed. We discuss the effect of the inclusion of the blanketing by millions of metal lines in NLTE on the diagnostics of photospheric and stellar wind lines, which can be used to determine stellar parameters such as effective temperature, gravity, radius, mass loss rate and distance. We also refer to recent work on the winds of massive O-type stars, which indicates that their winds are possibly inhomogeneous and clumped. We investigate implications from this work on the spectral diagnostics of PN central stars and introduce a method to determine wind clumping factors from the relative strengths of Halpha and HeII 4686. Based on new results we discuss the wind properties of CSPN., 8 pages, 12 figures; Proceedings, IAU Symposium No. 234, 2006, "Planetary Nebulae in our Galaxy and Beyond", M.J. Barlow and R.H. Mendez, eds

The parameters of ten supersoft X-ray sources (RX J0439.8-6809, RX J0513.9-6951, RX J0527.8-6954, CAL 87, CAL 83, 1E 0035.4-7230, RX J0048.4-7332, 1E 0056.8-7154, RX J0019.8 +2156, RX J0925.7-4758) observed by ROSAT obtained using blanketing LTE model atmospheres are analyzed. The consistency of the resulting parameters with a model with stable/recurrent burning on the surface of the white dwarf is studied. The luminosity and sizes of seven of the sources are in good agreement with this model. The masses of the white dwarfs in these sources are estimated. A formula that can be used to estimate the masses of white dwarfs in classical supersoft sources based on their effective temperatures is presented., 8 pages, 2 tables, 4 figures

The technique of model atmosphere calculation for magnetic Ap and Bp stars with polarized radiative transfer and magnetic line blanketing is presented. A grid of model atmospheres of A and B stars are computed. These calculations are based on direct treatment of the opacities due to the bound-bound transitions that ensures an accurate and detailed description of the line absorption and anomalous Zeeman splitting. The set of model atmospheres was calculated for the field strengths between 1 and 40 kG. The high-resolution energy distribution, photometric colors and the hydrogen Balmer line profiles are computed for magnetic stars with different metallicities and are compared to those of non-magnetic reference models and to the previous paper of this series. The results of modelling confirmed the main outcomes of the previous study: energy redistribution from UV to the visual region and flux depression at 5200A. However, we found that effects of enhanced line blanketing when transfer for polarized radiation takes place are smaller in comparison to those obtained in our first paper where polarized radiative transfer was neglected. Also we found that the peculiar photometric parameter delta_a is not able to clearly distinguish stellar atmospheres with abundances other than solar, and is less sensitive than delta(V_1-G) or Z to a magnetic field for low effective temperature (Teff=8000K). Moreover we found that the back determination of the fundamental stellar atmosphere parameters using synthetic Stromgren photometry does not result in significant errors., 12 pages, 6 figures. The final version, Sect. 3.4 revised, typos and mistakes corrected

We present results of calculations of scattering dominated atmospheres in order to interpret the spectra of type II supernovae. We assume spherical geometry and a density profile which is either a power law or given by the expansion of a stellar structure. For hydrogen up to 8 energy levels are allowed to deviate from LTE. Effects of sphericity are most important for the continuum forming regions and are responsible for much higher colour temperatures in the optical wavelength range than the corresponding effective temperature would imply. Whereas non-LTE effects are small for the continuum flux in the optical range, they become most important for the hydrogen lines. The strong influence of line blanketing on the observed spectra is demonstrated. This effect can lead to significant errors in the interpretation of the spectra and in the Baade Wesselink method for using supernovae as distance indicators. The application of our model on the SN1987a imply a distance of 46∓ 4 kpc for the LMC.

Very early observations with the Swift satellite of gamma-ray burst (GRB) afterglows reveal that the optical component is not detected in a large number of cases. This is in contrast to the bright optical flashes previously discovered in some GRBs (e.g. GRB 990123 and GRB 021211). Comparisons of the X-ray afterglow flux to the optical afterglow flux and prompt gamma-ray fluence is used to quantify the seemingly deficient optical, and in some cases X-ray, light at these early epochs. This comparison reveals that some of these bursts appear to have higher than normal gamma-ray efficiencies. We discuss possible mechanisms and their feasibility for explaining the apparent lack of early optical emission. The mechanisms considered include: foreground extinction, circumburst absorption, Ly-alpha blanketing and absorption due to high redshift, low density environments, rapid temporal decay, and intrinsic weakness of the reverse shock. Of these, foreground extinction, circumburst absorption, and high redshift provide the best explanations for most of the non-detections in our sample. There is tentative evidence of suppression of the strong reverse shock emission. This could be because of a Poynting-flux-dominated flow or a pure non-relativistic hydrodynamical reverse shock., 22 pages, 5 figures. Accepted for publication in ApJ