Structure Of Stellar Atmosphere Research Articles

EMISSA: Exploring millimetre indicators of solar-stellar activity III. Comparison of Ca II indices and millimetre continua in a 3D model atmosphere

Amongst several spectral lines, some of the strongest chromospheric diagnostics are offered by the Ca II H K lines. These lines can be used to gauge the temperature stratification of the atmosphere since the line core and wings are formed in different regions of the solar atmosphere. Furthermore, the Ca II lines act as tracers for the magnetic structure of the solar atmosphere, as the line cores are formed in the upper chromosphere even though they are formed in non-local thermodynamic equilibrium (NLTE). In contrast, the formation of millimetre (mm) continuum radiation occurs under local thermodynamic equilibrium (LTE) conditions. As a result, the brightness temperatures obtained from observations with the Atacama Large Millimetre/Submillimetre Array (ALMA) offer a complementary perspective on the activity and thermal structure of stellar atmospheres. The overall aim is to establish more robust solar/stellar activity indicators using ALMA observations in comparison with classical diagnostics, such as the s index and infrared triplet (IRT) index. We employed the 1.5D radiative transfer codes RH1.5D and advanced radiative transfer (ART) to compute the synthetic spectra for the Ca II lines and the millimetre (mm) continua, respectively. These calculations were performed using an enhanced network atmosphere model, which incorporates non-equilibrium hydrogen ionisation generated by the state-of-the-art 3D radiation magnetohydrodynamics (rMHD) Bifrost code. To account for the limited spatial resolution of ALMA, we simulated the effect using a Gaussian point spread function (PSF). Additionally, we analysed the correlations and slopes of scatter plots between the Ca II indices and mm continuum for the original and degraded resolutions, focusing on the entire simulation box, quiet Sun regions, and enhanced network patches separately. The activity indices generated from these lines could further be used to compare the spectra of Sun-like stars with the solar spectrum. We present a comparative study between synthetic continuum brightness temperature maps at mm wavelengths (0.3 mm to 8.5 mm) and the Ca II activity indices; namely, the s index and infrared triplet (IRT) index. The Ca II activity indices and mm brightness temperatures are weakly correlated at the high resolution, with the highest correlation observed at a wavelength of 0.3 mm, corresponding to ALMA band 10. As the resolution decreases, the correlation consistently increases. Conversely, the slopes exhibit a decreasing trend with increasing wavelength, while the degradation of resolution does not noticeably affect the calculated slopes. As the spatial resolution decreases, the standard deviations of the Ca II activity indices and brightness temperatures decrease, while the correlations between them increase. However, the slopes do not exhibit significant changes. Consequently, these relationships could be valuable for calibrating the mm continuum maps obtained through ALMA observations.

The Shapes of Stellar Spectra

Stellar atmospheres separate the hot and dense stellar interiors from the emptiness of space. Radiation escapes from the outermost layers of a star, carrying direct physical information. Underneath the atmosphere, the very high opacity keeps radiation thermalized and resembling a black body with the local temperature. In the atmosphere the opacity drops, and radiative energy leaks out, which is redistributed in wavelength according to the physical processes by which matter and radiation interact, in particular photoionization. In this article, I will evaluate the role of photoionization in shaping the stellar energy distribution of stars. To that end, I employ simple, state-of-the-art plane-parallel model atmospheres and a spectral synthesis code, dissecting the effects of photoionization from different chemical elements and species, for stars of different masses in the range of 0.3 to 2 M⊙. I examine and interpret the changes in the observed spectral energy distributions of the stars as a function of the atmospheric parameters. The photoionization of atomic hydrogen and H− are the most relevant contributors to the continuum opacity in the optical and near-infrared regions, while heavier elements become important in the ultraviolet region. In the spectra of the coolest stars (spectral types M and later), the continuum shape from photoionization is no longer recognizable due to the accumulation of lines, mainly from molecules. These facts have been known for a long time, but the calculations presented provide an updated quantitative evaluation and insight into the role of photoionization on the structure of stellar atmospheres.

Center-to-limb variation of spectral lines and continua observed with SST/CRISP and SST/CHROMIS

Context.Observations of center-to-limb variations (CLVs) of spectral lines and continua provide a good test for the accuracy of models with a solar and stellar atmospheric structure and spectral line formation. They are also widely used to constrain elemental abundances, and are becoming increasingly more important in atmospheric studies of exoplanets. However, only a few such data sets exist for chromospheric lines.Aims.We aim to create a set of standard profiles by means of mosaics made with the CRISP and CHROMIS instruments of the Swedish 1-m Solar Telescope (SST), as well as to explore the robustness of said profiles obtained using this method.Methods.For each spectral line, we used a mosaic that ranges from the center to the limb. Each of these mosaics were averaged down to 50 individual spectral profiles and spaced by 0.02 in theμscale. These profiles were corrected for p-mode oscillations, and their line parameters (equivalent width, line shift, full-width at half-maximum, and line depth) were then compared against literature values whenever possible.Results.We present a set of 50 average profiles that are spaced equidistantly along the cosine of the heliocentric angle (μ) by steps of 0.02 for five continuum points between 4001 and 7772 Å, as well as ten of the most commonly observed spectral lines at the SST (Ca II H & K, Ηβ, Mg I 5173 Å, C I 5380 Å, Fe I 6173 Å, Fe I 6301 Å, Ha, O I 7772 Å, and Ca II 8542 Å).Conclusions.The CLV of line profiles and continua are shared in the CDS as machine readable tables, providing a quantitative constraint on theoretical models that aim to model stellar atmospheres.

Direct Observation of the Radial Magnetic Field Gradient in HD 58260 from Spectropolarimetry of NLTE Lines in Emission

Abstract Because of the unquestionable presence of magnetic fields in stars, their role in the structure of stellar atmospheres has for a long time been a subject of speculation. In our contribution to this discussion we present spectropolarimetric evidence of the decrease of the radial component of the magnetic field with altitude in the atmosphere of HD 58260, a B-type magnetic star on the main sequence. We show that the Stokes V profiles of metal lines in emission of the outer atmosphere are evidence for a field three times weaker than absorption lines from inner layers. The extra flow of energetic particles due to the magnetic-gradient pumping mechanism could be at the origin of the magnetospheres surrounding this class of stars and at the basis of the high-energy phenomena observed. We also list a series of spectral lines useful for measuring the surface field of early-type stars.

Chromospheric activity in 55 Cancri – I. Results from theoretical wave studies

ABSTRACT We present theoretical models of chromospheric heating for 55 Cancri, an orange dwarf of relatively low activity. Self-consistent, non-linear, and time-dependent ab initio numerical computations are pursued encompassing the generation, propagation, and dissipation of waves. We consider longitudinal waves operating among arrays of flux tubes as well as acoustic waves pertaining to non-magnetic stellar regions. Additionally, flux enhancements for the longitudinal waves are also taken into account as supplied by transverse tube waves. The Ca ii K fluxes are computed (multi-ray treatment) assuming partial redistribution as well as time-dependent ionization (TDI). The self-consistent treatment of TDI (especially for hydrogen) greatly impacts the atmospheric temperatures and electron densities (especially behind the shocks); it also affects the emergent Ca ii fluxes. Particularly, we focus on the influence of magnetic heating on the stellar atmospheric structure and the emergent Ca ii emission, as well as the impact of non-linearities. Our study shows that a higher photospheric magnetic filling factor entails a larger Ca ii emission; however, an increased initial wave energy flux (e.g. associated with mode coupling) is of little difference. Comparisons of our theoretical results with observations will be conveyed in forthcoming Paper II.

Interferometric Fringe Visibility Null as a Function of Spatial Frequency: A Probe of Stellar Atmospheres

We introduce an observational tool based on visibility nulls in optical spectro-interferometry fringe data to probe the structure of stellar atmospheres. In a preliminary demonstration, we use both Navy Precision Optical Interferometer (NPOI) data and stellar atmosphere models to show that this tool can be used, for example, to investigate limb darkening. Using bootstrapping with either multiple linked baselines or multiple wavelengths in optical and infrared spectro-interferometric observations of stars makes it possible to measure the spatial frequency [Formula: see text] at which the real part of the fringe visibility [Formula: see text] vanishes. That spatial frequency is determined by [Formula: see text], where [Formula: see text] is the projected baseline length, and [Formula: see text] is the wavelength at which the null is observed. Since [Formula: see text] changes with the Earth’s rotation, [Formula: see text] also changes. If [Formula: see text] is constant with wavelength, [Formula: see text] varies in direct proportion to [Formula: see text]. Any departure from that proportionality indicates that the brightness distribution across the stellar disk varies with wavelength via variations in limb darkening, in the angular size of the disk, or both. In this paper, we introduce the use of variations of [Formula: see text] with [Formula: see text] as a means of probing the structure of stellar atmospheres. Using the equivalent uniform disk diameter [Formula: see text], given by [Formula: see text], as a convenient and intuitive parameterization of [Formula: see text], we demonstrate this concept by using model atmospheres to calculate the brightness distribution for [Formula: see text] Ophiuchi and to predict [Formula: see text], and then comparing the predictions to coherently averaged data from observations taken with the NPOI.

Line-dependent veiling in very active classical T Tauri stars

Context. The T Tauri stars with active accretion disks show veiled photospheric spectra. This is supposedly due to non-photospheric continuum radiated by hot spots beneath the accretion shocks at stellar surface and/or chromospheric emission lines radiated by the post-shocked gas. The amount of veiling is often considered as a measure of the mass-accretion rate. Aim. We analysed high-resolution photospheric spectra of accreting T Tauri stars LkHα 321, V1331 Cyg, and AS 353A with the aim of clarifying the nature of the line-dependent veiling. Each of these objects shows a strong emission line spectrum and powerful wind features indicating high rates of accretion and mass loss. Methods. Equivalent widths of hundreds of weak photospheric lines were measured in the observed spectra of high quality and compared with those in synthetic spectra of appropriate models of stellar atmospheres. Results. The photospheric spectra of the three T Tauri stars are highly veiled. We found that the veiling is strongly line-dependent: larger in stronger photospheric lines and weak or absent in the weakest ones. No dependence of veiling on excitation potential within 0 to 5 eV was found. Different physical processes responsible for these unusual veiling effects are discussed in the framework of the magnetospheric accretion model. Conclusions. The observed veiling has two origins: (1) an abnormal structure of stellar atmosphere heated up by the accreting matter, and (2) a non-photospheric continuum radiated by a hot spot with temperature lower than 10 000 K. The true level of the veiling continuum can be derived by measuring the weakest photospheric lines with equivalent widths down to ≈10 mÅ. A limited spectral resolution and/or low signal-to-noise ratio results in overestimation of the veiling continuum. In the three very active stars, the veiling continuum is a minor contributor to the observed veiling, while the major contribution comes from the line-dependent veiling.

Observations of Strong Magnetic Fields in Nondegenerate Stars

We review magnetic-field measurements of nondegenerate stars across the Hertzprung–Russell diagram for main sequence, premain sequence, and postmain sequence stars. For stars with complex magnetic-field morphologies, which includes all G–M main sequence stars, the analysis of spectra obtained in polarized vs unpolarized light provides very different magnetic measurements because of the presence or absence of cancellation by oppositely directed magnetic fields within the instrument’s spatial resolution. This cancellation can be severe, as indicated by the spatially averaged magnetic field of the Sun viewed as a star. These averaged fields are smaller by a factor of 1000 or more compared to spatially resolved magnetic-field strengths. We explain magnetic-field terms that characterize the fields obtained with different measurement techniques. Magnetic fields typically control the structure of stellar atmospheres in and above the photosphere, the heating rates of stellar chromospheres and coronae, mass and angular momentum loss through stellar winds, chemical peculiarity, and the emission of high energy photons, which is critically important for the evolution of protoplanetary disks and the habitability of exoplanets. Since these effects are governed by the star’s magnetic energy, which is proportional to the magnetic-field strength squared and its fractional surface coverage, it is important to measure or reliably infer the true magnetic-field strength and filling factor across a stellar disk. We summarize magnetic-field measurements obtained with the different observing techniques for different types of stars and estimate the highest magnetic-field strengths. We also comment on the different field morphologies observed for stars across the H–R diagram, typically inferred from Zeeman-Doppler imaging and rotational modulation observations,

Using limb darkening to measure fundamental parameters of stars

Context. Limb darkening is an important tool for understanding stellar atmospheres, but most observations measuring limb darkening assume various parameterizations that yield no significant information about the structure of stellar atmospheres. Aims. We use a specific limb-darkening relation to study how the best-fit coefficients relate to fundamental stellar parameters from spherically symmetric model stellar atmospheres. Methods. Using a grid of spherically symmetric Atlas model atmospheres, we compute limb-darkening coefficients, and develop a novel method to predict fundamental stellar parameters. Results. We find our proposed method predicts the mass of stellar atmosphere models given only the radius and limb-darkening coefficients, suggesting that microlensing, interferometric, transit and eclipse observations can constrain stellar masses. Conclusions. This novel method demonstrates that limb-darkening parameterizations contain important information about the structure of stellar atmospheres, with the potential to be a valuable tool for measuring stellar masses.

Accretion-powered chromospheres in classical T Tauri stars

(Abridged) Optical spectra of classical T Tauri stars (cTTS) are rich in emission lines of low-excitation species that are composed of narrow and broad components, related to two regions with different kinematics, densities, and temperatures. The photospheric spectrum is often veiled by an excess continuous emission. This veiling is usually attributed to radiation from a heated region beneath the accretion shock. The aim of this research is to clarify the nature of the veiling, and whether the narrow chromospheric lines of Fe I and other metals represent a standard chromosphere of a late-type star, or are induced by mass accretion. From high-resolution spectroscopy of DR Tauri we found that the amount of veiling in this star varies from practically nothing to factors more than 10 times the stellar continuum intensity, and that the veiling is caused by both a non-photospheric continuum and chromospheric line emission filling in the photospheric absorption lines. This effect can be shown to exist in several other T Tauri stars. We conclude that enhanced chromospheric emission in cTTS is linked not only to solar-like magnetic activity, but is powered to a greater extent by the accreting gas. We suggest that the area of enhanced chromospheric emission is induced by mass accretion, which modifies the local structure of stellar atmosphere in an area that is more extended than the hot accretion spot. The narrow emission lines from this extended area are responsible for the extra component in the veiling through line-filling of photospheric absorption lines.

ATLAS Versus NextGen Model Atmospheres: A Combined Analysis of Synthetic Spectral Energy Distributions

We carried out a critical appraisal of the two theoretical models, Kurucz' ATLAS9 and PHOENIX/NextGen, for stellar atmosphere synthesis. Our tests relied on the theoretical fit of spectral energy distributions (SEDs) for a sample of 334 target stars along the whole spectral-type sequence, from the classical optical catalogs of Gunn & Stryker and Jacoby et al. The best-fitting physical parameters (Teff, log g) of stars allowed an independent calibration of the temperature and bolometric scale versus empirical classification parameters (i.e., spectral type and MK luminosity class); in addition, the comparison of the synthetic templates from the ATLAS and NextGen grids allowed us to probe the capability of the models to match spectrophotometric properties of real stars and assess the impact of the different input physics. We can sketch the following main conclusions of our analysis: (1) Fitting accuracy of both theoretical libraries drastically degrades at low Teff at which both ATLAS and NextGen models still fail to properly account for the contribution of molecular features in the observed SED of K–M stars. (2) Compared with empirical calibrations, both ATLAS and NextGen fits tend, on average, to predict slightly warmer (by 4%–8%) Teff for both giant and dwarf stars of fixed spectral type, but ATLAS provides, in general, a sensibly better fit (a factor of 2 lower σ of flux residuals) than NextGen. (3) There is a striking tendency of NextGen to label target stars with an effective temperature and surface gravity higher than that of ATLAS. The effect is especially evident for MK I–III objects for which about one in four stars is clearly misclassified by NextGen in log g. This is a consequence of some "degeneracy" in the solution space, partly induced by the different input physics and geometry constraints in the computation of the integrated emerging flux (ATLAS model atmospheres assume standard plane-parallel layers, while NextGen adopts, for low-gravity stars, a spherical-shell geometry). A different T(τ) vertical structure of stellar atmosphere seems also required for NextGen synthetic SEDs in order to better account for limb-darkening effects in cool stars, as supported by the recent observations of the EROS BLG2000-5 microlensing event.

Extreme Ultraviolet ExplorerSpectra of the 1993 March Flares on AD Leonis: The Differential Emission Measure and Implications for Coronal Structure

The flare star AD Leonis was observed by the Extreme Ultraviolet Explorer (EUVE) from 1993 March 1 to 3 UT. Two flares were detected by the EUVE Deep Survey detector and spectrometer and also seen in optical photometry on 1993 March 2 UT. The DS Lexan/boron-band and optical results have been discussed in the previous paper by Hawley et al. In this paper, we describe the spectra observed by EUVE during quiescence, the peaks of the flares, and the decay phase following the first flare and analyze the spectra to investigate the stellar atmospheric structure during these time periods. The spectra show that the observed EUV emission from AD Leo is dominated by iron lines from a hot coronal plasma. Two methods were used to estimate the differential emission measure distribution (DEM) of the stellar corona. In the Pottasch method, we fitted Gaussian line profiles to the strongest lines in the spectra and estimated the DEM at the formation temperature of those lines. Upper limits to the DEM were obtained in the case of no detection. We also used a regularized inversion technique, together with a weighting scheme based on information contained in the plasma-emission model and on the signal-to-noise ratio of the data, to find the DEM. The weighting was designed to prevent the noisy pixels in our low-signal-to-noise ratio data from dominating the solution. The results produced by the two methods are consistent in the temperature regimes where strong lines are present. The inversion method provides additional information where no strong single lines dominate the spectra. The ability to use lines from the entire wavelength region covered by the spectra allowed us to investigate the hydrogen column NH and iron abundance [Fe/H]. We found that [Fe/H] in the corona of AD Leo was essentially unconstrained by our data, but NH was well determined, yielding NH ~ (3 ± 1) × 1018 cm-2. We assumed both a solar-coronal value of [Fe/H] and a value one tenth of this and computed the DEM distribution of the stellar corona for both cases. The DEM of the quiescent corona is dominated by a broad plateau of emission ranging from 106.8 to 107.2 K, with the DEM of plasma near 106.2 K about an order of magnitude less. We interpret the plateau of the DEM in terms of a broad distribution of loops with differing peak temperatures. We discuss and compare these results with those of Giampapa et al., who analyzed ROSAT soft X-ray data from AD Leo taken during a different time period. The DEM of the flare plasma is strongly peaked at temperatures greater than 107 K, indicative of hot flare loops, while that of the decay phase consists of a smaller peak at temperatures less than 107 K, as might be expected from the cooling and condensation of previously heated flare loops. These results are consistent with a flare model that includes strong evaporation and condensation as in our previous paper. The EUVE spectral analysis leads to lower peak flare temperatures than those used in our previous paper, but the basic conclusion reached—that the dominant flaring emission originates from long loops with L ~ R* and with peak flare densities ranging from 109 to 1011 cm-3—remains unchanged. This conclusion is not qualitatively affected by the value of [Fe/H] used in our DEM analysis.

Observing stellar surface structure with the ESO-VLT interferometer

The ESO Very Large Telescope (VLT) has the capability to coherently combine the light from four 8m telescopes and from smaller Auxiliary telescopes with baselines 4m and 200m (VLT Interferometer or VLTI). The resulting resolution approaches 0.5 milli-arcsec at visible wavelengths and 1 milli-arcsec in the near infra-red. We estimate that about 2000 nearby bright stars can be resolved with these baselines. The surface of about 400 stars, mainly K and M giants, can be mapped with a resolution of a dozen pixels or better across the stellar surface. This resolution permits detailed studies of the structure of stellar atmospheres, their hydrodynamics and magneto-hydrodynamics. VLTI can easily resolve starspots and very likely also convection cells on an active K-type giant at 10pc distance. A marginal detection of the largest starspots on a very active solar-type star at the same distance may be possible, but it appears very unlikely that the surface structure on an inactive solar-type star can be imaged.We present computer simulations of stellar surfaces to study in detail the response of the interferometer to an extended, complex source. First results which simulate the end-to-end process in a simplified manner indicate that useful maps can be produced provided that a sufficient number of baselines are combined in Earth-rotation synthesis mode. We also give arguments that high spectral resolution will prove essential to constrain reconstructed maps.

Dominating Molecules in the Photospheres of Cool Stars

Molecular data bases (including very different numbers of molecular lines, and based on computations spanning a wide range in quality) have been used in the construction of models of the stellar atmospheric structure, and effects of various molecules on the stellar structure and evolution have been described in the literature. A systematic study showing which molecules are of importance in which types of stars is, nevertheless, still lacking, and the line lists used in various model computations necessarily reflect what was at hand at the time of computation rather than a systematic effort to solve the molecular opacity problem. A pioneering investigation was presented in two extensive papers by Tsuji (1964, 1973) where the foundation of the reaction scheme used in most models today was laid down. Later Tsuji (1986) has given a review of the existing work on molecules known in stars, and Johnson (1986) and Gustafsson (1989) have touched upon the same subject in reviews covering a broader range of stellar aspects. Jørgensen (1992) has recently summarized the existing molecular data (and discussed the methods used) available for model atmosphere construction.

Velocity structure of stellar atmospheres - R Scuti

view Abstract Citations (21) References (28) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Velocity Structure of Stellar Atmospheres: R Scuti Mozurkewich, David ; Gehrz, R. D. ; Hinkle, K. H. ; Lambert, D. L. Abstract Ten 2 micron spectra of R Scuti, the brightest RV Tauri variable, are analyzed. The period, determined from the atomic absorption lines, is 142 days, not 70 days as proposed to explain the visible spectrum. The infrared atomic absorption lines trace out a continuous, repeatable velocity curve which is interpreted as the radial pulsation of the star. The mean velocity is 37 km/s and the net displacement during a cycle is 5 x 10 to the 12th cm. The CO lines form in the cooler upper atmosphere. The dominant feature is material falling toward the star at about 20 km/s. The region is supported by a shock wave which passes through it once each pulsation; material heated by the shock is responsible for the formation of the CO emission lines. An extended envelope where the atomic emission lines form surrounds the star. Publication: The Astrophysical Journal Pub Date: March 1987 DOI: 10.1086/165053 Bibcode: 1987ApJ...314..242M Keywords: Stellar Atmospheres; Stellar Oscillations; Stellar Spectra; Variable Stars; Velocity Distribution; Absorption Spectra; Carbon Monoxide; Emission Spectra; Infrared Spectra; Shock Waves; Astrophysics; SHOCK WAVES; STARS: EMISSION-LINE; STARS: INDIVIDUAL CONSTELLATION NAME: R SCUTI; STARS: PULSATION; STARS: RV TAURI; STARS: VARIABLES full text sources ADS | data products SIMBAD (1)

The Effects of Molecules on the Structure of Stellar Atmospheres

Spectra from cool stars are dominated by lines from different molecules. The effects of these molecular lines on the structure of the atmospheres are discussed. The structure is demonstrated as being often severely dependent on the element abundances due to the effects of the molecules, and in some cases the data adopted for the molecules are very crucial. This implies that careful abundance analysis of carbon stars, for example, is complicated.

Carbon dioxide and climate: an astrophysical perspective

In this survey the earth is viewed from the astrophysical perspective, i.e. using global mean values of environmental parameters. The role of carbon dioxide is described in the processes of energy transfer from the earth's surface to space, which determine “global climate” as measured by the mean surface temperature. Analogies and differences between the problems of the terrestrial atmosphere and those of the solar and stellar atmospheres are examines, both in the computation of model atmosphere and in remote sensing of atmospheric temperature and composition. Subsequently, the temporal astrophysical perspective, with a review of the evolution of CO 2 abundance and climate on astrophysical or geological time scales, on earth as on Venus (the runaway greenhouse) and on Mars is introduced. Variation of CO 2 may have been critical to the maintenance of an environment in which life could originate and evolve, and may itself have been affected by life. On human time scales, the recent and continuing increase in atmospheric CO 2 raises new problems, which are briefly surveyed. It is argued, that the differential greenhouse effect of increased CO 2 in the earth's atmosphere is essentially identical to the “blanketing effect” of spectral lines on the temperature structure of stellar atmospheres. The methods used by astrophysicists in such studies are reviewed and compared with those used to evaluate the differential greenhouse effect of CO 2 in radiative-convective models of the earth's atmosphere. The latter methods remain relatively crude, but recent results by different authors are in reasonably good agreement; however, the astrophysical perspective, i.e. the use of one-dimensional global mean models, remains a gross simplification of the real complexity of the earth's climate system, which is also true in stellar atmospheres.

A method for empirical determination of stellar atmospheric structure

A technique for obtaining information on the temperature structure of a stellar atmosphere from spectral line data where only flux observations are available is discussed. The direct inversion of the flux integral to obtain the line source function can be circumvented by making the physically plausible assumptions of (1) source function equality in a multiplet and (2) the dominance of line absorption over continuum absorption at line center. Consistency of the technique is demonstrated by treating a synthetic spectrum as input data and attempting to recover the temperature structure of the input atmosphere. Using high quality solar spectrum scans obtained from K.P.N.O. we demonstrate the accuracy of source function equality for several Fe i multiplets and use one of these multiplets to obtain an empirical outer atmosphere for the Sun. Our empirical atmosphere agrees well with current solar models.

Radiative Opacity in Stellar Atmospheres. III. Effect of the Hydrogen-To Ratio on the Atmospheric Structure

view Abstract Citations (13) References (11) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Radiative Opacity in Stellar Atmospheres. III. Effect of the Hydrogen-To Ratio on the Atmospheric Structure Matsushima, Satoshi ; Terashita, Yoichi Abstract A grid of line-blanketed model atmospheres with different hydrogen-to-helium ratios is constructed for wide ranges of Teff and log g in order to investigate the effect of differing hydrogen content on the structure of stellar atmospheres. The changes in the emergent flux distribution with wavelength, UBV color indices, and the fly and Ho profiles are also considered. It is found that a small reduction in the H-to-He ratio produces a large increase in the electron pressure as well as in the gas pressure, and that the change in Pe produces considerable effects on the observable features. On the other hand, the effect of hydrogen content on the temperature stratification is small until the H-to-He ratio is reduced to about 0.1. Theoretical interpretations are presented to account for the variations in T, Pe, and P with varying hydrogen content in terms of differential variations in the relative importance of H, H, and He as sources of atmospheric opacity coupled with Lyman- and Balmer-line blanketing. Since an increase in Fe also results from an increased surface gravity, the variations of the observable quantities with hydrogen content or with surface gravity are generally indistinguishable. A general for- mula is derived to relate the errors due to uncertainties in log g and those in hydrogen content. In a hydrogen-rich atmosphere, the accuracies in the determinations of the surface gravity and hydrogen content from the color indices are estimated to be `-~.-`O 1 in log g and about 15 per cent in the hydrogen content for a model with Teff 150000 K and logg 7. Since a small change in hydrogen content has lit- tle effect on Teff, the determination of Teff from observed color indices appears to be a promising method for white dwarfs. For stars with lower g, however, the use of hydrogen line profiles may be superior to the use of color indices in determining the atmospheric parameters, although in this case it is not possible to determine Teff independently from g and the H-to-He ratio Publication: The Astrophysical Journal Pub Date: April 1969 DOI: 10.1086/149957 Bibcode: 1969ApJ...156..183M full text sources ADS | Related Materials (2) Part 1: 1968ApJ...154..689T Part 2: 1968ApJ...154..715M

Structure of Stellar Atmospheres. II.

view Abstract Citations (6) References (4) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Structure of Stellar Atmospheres. II. Krook, Max Abstract The structure equations for non-gray atmospheres can generally be solved only by means of iteration procedures. A moment method is presented for solving the equation of transfer within the framework of such an iteration scheme. Certain average coefficients are evaluated from solutions of monochromaticmoment equations in which the argument T of the Planek function is identified with a known "trial" temperature. Those coefficients are then inserted in a set of formal integrated-moment equations in which T is now identified with the required "improved" value of the temperature distribution. Publication: The Astrophysical Journal Pub Date: May 1959 DOI: 10.1086/146670 Bibcode: 1959ApJ...129..724K full text sources ADS |