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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 construct new models of outer heat blanketing envelopes of neutron stars composed of binary ion mixtures (H - He, He - C, C - Fe) in and out of diffusive equilibrium. To this aim, we generalize our previous work on diffusion of ions in isothermal gaseous or Coulomb liquid plasmas to handle non-isothermal systems. We calculate the relations between the effective surface temperature Ts and the temperature Tb at the bottom of heat blanketing envelopes (at a density rhob= 1e8 -- 1e10 g/cc) for diffusively equilibrated and non-equilibrated distributions of ion species at different masses DeltaM of lighter ions in the envelope. Our principal result is that the Ts - Tb relations are fairly insensitive to detailed distribution of ion fractions over the envelope (diffusively equilibrated or not) and depend almost solely on DeltaM. The obtained relations are approximated by analytic expressions which are convenient for modeling the evolution of neutron stars., Comment: 12 pages, 7 figures, MNRAS, published

We present non-LTE time-dependent radiative transfer simulations for ejecta produced by the detonation of an helium shell at the surface of a low-mass carbon/oxygen white dwarf (WD). This mechanism is one possible origin for supernovae (SNe) with faint and fast-decaying light curves, such as .Ia SNe and Ca-rich transients. Our initial ejecta conditions at 1d are given by the 0.18B explosion model COp45HEp2 of Waldman et al.. The 0.2Msun ejecta initially contains 0.11Msun of He, 0.03Msun of Ca, and 0.03Msun of Ti. We obtain a ~5d rise to a bolometric maximum of 3.59x10^41 erg/s, primarily powered by 48V decay. Multi-band light curves show distinct morphologies, with a rise to maximum magnitude (-14.3 to -16.7mag) that varies between 3 to 9d from the U to the K bands. Near-IR light curves show no secondary maximum. Because of the presence of both HeI and SiII lines at early times we obtain a hybrid Type Ia/Ib classification. During the photospheric phase line blanketing is caused primarily by TiII. At nebular times, the spectra show strong CaII lines in the optical (but no [OI] 6300--6364A emission), and TiII in the near-IR. Overall, these results match qualitatively the very disparate properties of .Ia SNe and Ca-rich transients. Although the strong TiII blanketing and red colors that we predict are rarely observed, they are seen, for example, in OGLE-2013- SN-079. Furthermore, we obtain a faster light-curve evolution than, for example, PTF10iuv, indicating an ejecta mass >0.2Msun. An alternate scenario may be the merger of two WDs, one or both composed of He., Accepted to MNRAS

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.

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.

SN 1998bw holds the record for the most energetic Type Ic explosion, one of the brightest radio supenovae and probably the first supernova associated with a gamma-ray burst. In this paper we present spectral observations of SN 1998bw observed in a cooperative monitoring campaign using the AAT, UKST and the SSO 2.3-m telescope. We investigate the evolution of the spectrum between 7 and 94 days after V_band maximum in comparison to well-studied examples of Type Ic SNe in order to quantify the unusual properties of this supernova event. Though the early spectra differ greatly from the observations of classical Ic SNe, we find that the evolution from the photospheric to the nebular phases is slow but otherwise typical. The spectra differ predominantly in the extensive line blending and blanketing which has been attributed to the high velocity of the ejecta. We find that by day 19, the absorption line minima blueshifts are 10% - 50% higher than other SNe and on day 94 emission lines are 45% broader, as expected if the progenitor had a massive envelope. However, it is difficult to explain the extent of line blanketing entirely by line broadening, and we argue that additional contribution from other species is present, indicating unusual relative abundances or physical conditions in the envelope., 8 pages, 6 figures

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.

Only recently has it been possible to find an effective temperature and composition for the hot DA white dwarf G191–B2B which can consistently match the optical, far-UV and EUV data simultaneously, by using new non-LTE model calculations that include the effects of line blanketing from more than 9 million (mainly Fe and Ni) transitions. However, to maintain agreement below the He ii 228-A Lyman limit requires the inclusion of additional He ii opacity in the form of photospheric, circumstellar or interstellar material. If photospheric, the homogeneous H+He structure adopted predicted a He ii 1640-A line strength well above the IUE detection limit, while an unusally high He ionization fraction was required in this particular line of sight if the material was in the local ISM. We show that, by considering the effects of a stratified H upon He photospheric structure within the heavy-element blanketed models, a lower interstellar He ii column density is obtained, leading to a more reasonable He ionization fraction for the G191–B2B line of sight. In addition, the absence of a He ii 1640-A feature in the far-UV data is consistent with the equivalent width predicted by the stratified model. However, while this alternative interpretation offers a better description of certain aspects of the G191–B2B optical-to-EUV spectra, a new problem is introduced, namely that the He ii Lyman lines are predicted to be stronger than can be comfortably accommodated by the observational data.

We present detailed atmospheric analyses of two [WC10] central stars of planetary nebulae (PNe), CPD−56° 8032 and He 2-113, the results of which are compared with parameters inferred from their nebular properties. Our quantitative study is based on non-LTE modelling of diagnostic He i–II, C II–IV and O II–III lines. We find that spectroscopic similarities are reflected in the derived stellar properties: for both stars we obtain Teff∼30 kK and log L/L⊙=3.7; log (˙M/M⊙ yr−1)=−5.4 and v∞=225 km s−1 for CPD−56° 8032, while log (˙M/M⊙ yr−1)=−6.1 and v∞=160 km s−1 for He 2-113. The derived stellar properties are fairly consistent with the recent study of Leuenhagen et al., when adjusted for the different distances adopted. We find excellent agreement between our C2+ wind temperature and the independent determination by De Marco et al., supporting our assumption of radiative equilibrium. We find that both stars are highly enriched in carbon and oxygen, but with stellar hydrogen undetectable for He 2-113 (in contrast to Leuenhagen et al.). Our final abundance determinations indicate H:He:C:O mass fractions of 0:34:52:14 for CPD−56° 8032 and 0:34:54:13 for He 2-113. We confront our model flux distributions with observed nebular properties using Zanstra and photoionization techniques, and identify a major discrepancy between the observed and predicted nebular properties for these particular PNe. The hydrogen ionizing fluxes predicted by our WR non-LTE models greatly exceed those implied by nebular observations. The lack of heavy-element line blanketing in our wind models could be responsible. However, the geometry and high nebular densities of these PNe suggest that they represent poor probes of the Lyman continuum flux of their central stars.

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.

Carbon and nitrogen abundances have been obtained for a sample of 11-F- and G-type dwarfs covering a range in Fe/H abundance ratio from -0.8 to +0.3. Model atmospheres, which included the effects of convection and line blanketing, were used to calculate synthetic spectra of the CH, CN, and NH molecular bands. Effective oscillator strengths for the bands studied were found by matching synthetic spectra calculated from a model solar atmosphere with the observed solar bands. Many of the metal-poor stars, and particularly the high-velocity stars, were found to have substantial nitrogen over-deficiencies, suggesting that N is manufactured mostly in a secondary manner. The carbon-to-iron ratios were similar to the solar ratio, although there may be slight C over-deficiencies in metal-poor stars. However, the variation in C/Fe is not as marked as that found recently by Hearnshaw (1974). A comprehensive discussion of the theoretical errors is given, and some applications to Galactic evolution are noted.

Two computer programs have been used to compute flux constant model atmospheres, which include the effects of convection as well as line blanketing, for solar type stars. One program computes a model atmosphere and then calculates the integrated radiative flux and convective flux as a function of optical depth. After applying a line-blanketing correction to the fluxes, temperature corrections are calculated and a new model atmosphere computed. A second program computes the line blocking of a model atmosphere as a function of optical depth. The line-blocking data, expressed as a function of the radiative flux computed without allowance for line absorption, is used by the model atmosphere program to allow for line absorption effects. This method of com. puting line-blanketed model atmospheres, the flux-fraction method, does not make the opacity correlation assumption that is inherent in the giant-line method. Model atmospheres have been computed for the Sun, a metal deficient star (STAA/H! = -0.6) and Groombridge 1830 and the colors of these models are discussed. The models do predict the observed variation of ultraviolet excess, delta (U-B), with metal abundance. (auth)