Late-type Stellar Atmospheres Research Articles

EXOPLINES: Molecular Absorption Cross-section Database for Brown Dwarf and Giant Exoplanet Atmospheres

Abstract Stellar, substellar, and planetary atmosphere models are all highly sensitive to the input opacities. Generational differences between various state-of-the-art stellar/planetary models arise primarily because of incomplete and outdated atomic/molecular line lists. Here we present a database of precomputed absorption cross sections for all isotopologues of key atmospheric molecules relevant to late-type stellar, brown dwarf, and planetary atmospheres: MgH, AlH, CaH, TiH, CrH, FeH, SiO, TiO, VO, and H2O. The pressure and temperature ranges of the computed opacities are 10−6–3000 bar and 75–4000 K, and their spectral ranges are 0.25–330 μm for many cases where possible. For cases with no pressure-broadening data, we use collision theory to bridge the gap. We also probe the effect of absorption cross sections calculated from different line lists in the context of ultrahot Jupiter and M-dwarf atmospheres. Using 1D self-consistent radiative–convective thermochemical equilibrium models, we report significant variations in the theoretical spectra and thermal profiles of substellar atmospheres. With a 2000 K representative ultrahot Jupiter, we report variations of up to 320 and 80 ppm in transmission and thermal emission spectra, respectively. For a 3000 K M-dwarf, we find differences of up to 125% in the spectra. We find that the most significant differences arise as a result of the choice of TiO line lists, primarily below 1 μm. In summary, (1) we present a database of precomputed molecular absorption cross sections, and (2) we quantify biases that arise when characterizing substellar/exoplanet atmospheres as a result of differences in the line lists, therefore highlighting the importance of correct and complete opacities for eventual applications to high-precision spectroscopy and photometry.

3D non-LTE line formation of neutral carbon in the Sun

Carbon abundances in late-type stars are important in a variety of astrophysical contexts. However C I lines, one of the main abundance diagnostics, are sensitive to departures from local thermodynamic equilibrium (LTE). We present a model atom for non-LTE analyses of C I lines, that uses a new, physically-motivated recipe for the rates of neutral hydrogen impact excitation. We analyse C I lines in the solar spectrum, employing a three-dimensional (3D) hydrodynamic model solar atmosphere and 3D non-LTE radiative transfer. We find negative non-LTE abundance corrections for C I lines in the solar photosphere, in accordance with previous studies, reaching up to around 0.1 dex in the disk-integrated flux. We also present the first fully consistent 3D non-LTE solar carbon abundance determination: we infer log ɛC = 8.44 ± 0.02, in good agreement with the current standard value. Our models reproduce the observed solar centre-to-limb variations of various C I lines, without any adjustments to the rates of neutral hydrogen impact excitation, suggesting that the proposed recipe may be a solution to the long-standing problem of how to reliably model inelastic collisions with neutral hydrogen in late-type stellar atmospheres.

Ca line formation in late-type stellar atmospheres

Context. Departures from local thermodynamic equilibrium (LTE) distort the calcium abundance derived from stellar spectra in various ways, depending on the lines used and the stellar atmospheric parameters. The collection of atomic data adopted in non-LTE (NLTE) calculations must be sufficiently complete and accurate. Aims. We derive NLTE abundances from high-quality observations and reliable stellar parameters using a model atom built afresh for this work, and check the consistency of our results over a wide wavelength range with transitions of atomic and singly ionised calcium. Methods. We built and tested Ca I and Ca II model atoms with state-of-the-art radiative and collisional data, and tested their performance deriving the Ca abundance in three benchmark stars: Procyon, the Sun, and Arcturus. We have excellent-quality observations and accurate stellar parameters for these stars. Two methods to derive the LTE/NLTE abundances were used and compared. The LTE/NLTE centre-to-limb variation (CLV) of Ca lines in the Sun was also investigated. Results. The two methods used give similar results in all three stars. Several discrepancies found in LTE do not appear in our NLTE results; in particular the agreement between abundances in the visual and infra-red (IR) and the Ca I and Ca II ionisation balance is improved overall, although substantial line-to-line scatter remains. The CLV of the calcium lines around 6165 Å can be partially reproduced. We suspect differences between our modelling and CLV results are due to inhomogeneities in the atmosphere that require 3D modelling.

Atmospheric structure and mass loss in Miras

The cool molecular region of late-type stellar atmospheres, the ‘MOLsphere’ located between the photosphere and expanding circumstellar shell, is explored by using time series of near-IR spectra of one large amplitude AGB (Mira) variable. The ∼⃒1400 K MOLsphere of the Mira R Cas is shown to undergo large amplitude velocity changes that are aperiodic with time scales many times longer than the photospheric pulsation. The mass of the MOLsphere is ∼⃒3×10−5 M⊙. Initial analysis suggests that MOLsphere is thin compared to the stellar radius.

Mg line formation in late-type stellar atmospheres

Context. Mg is the α element of choice for Galactic population and chemical evolution studies because it is easily detectable in all late-type stars. Such studies require precise elemental abundances, and thus departures from local thermodynamic equilibrium (LTE) need to be accounted for.

Mg line formation in late-type stellar atmospheres

Mg is often traced in late-type stars using lines of neutral magnesium, which is expected to be subject to departures from LTE. The astrophysical importance of Mg as well as its relative simplicity from an atomic physics point of view, makes it a prime target and test bed for detailed ab initio non-LTE modelling in stellar atmospheres. For the low-lying states of Mg i, electron collision data were calculated using the R-matrix method. Calculations for collisional broadening by neutral hydrogen were also performed where data were missing. These calculations, together with data from the literature, were used to build a model atom. First, the modelling was tested by comparisons with observed spectra of benchmark stars with well-known parameters. Second, the spectral line behaviour and uncertainties were explored by extensive experiments in which sets of collisional data were changed or removed. The modelled spectra agree well with observed spectra. The line-to-line scatter in the derived abundances shows improvements compared to LTE. The observed Mg emission features at 7 and 12 microns in the spectra of the Sun and Arcturus are reasonably well reproduced. The modelling predicts non-LTE abundance corrections in dwarfs, both solar metallicity and metal-poor, to be very small (of order 0.01 dex), even smaller than found in previous studies. In giants, corrections vary greatly between lines, but can be as large as 0.4 dex. Our results emphasise the need for accurate data of Mg collisions with both electrons and H atoms for precise non-LTE predictions of stellar spectra, but demonstrate that such data can be calculated and that ab initio non-LTE modelling without resort to free parameters is possible. Grids of departure coefficients and abundance corrections for a range of stellar parameters are planned for a forthcoming paper.

NLTE analysis of Sr lines in spectra of late-type stars with new R-matrix atomic data

We investigate statistical equilibrium of neutral and singly-ionized strontium in late-type stellar atmospheres. Particular attention is given to the completeness of the model atom, which includes new energy levels, transition probabilities, photoionization and electron-impact excitation cross-sections computed with the R-matrix method. The NLTE model is applied to the analysis of Sr I and Sr II lines in the spectra of the Sun, Procyon, Arcturus, and HD 122563, showing a significant improvement in the ionization balance compared to LTE line formation calculations, which predict abundance discrepancies of up to 0.5 dex. The solar Sr abundance is log A = 2.93 \pm 0.04 dex, in agreement with the meteorites. A grid of NLTE abundance corrections for Sr I and Sr II lines covering a large range of stellar parameters is presented.

Three-dimensional surface convection simulations of metal-poor stars

Three-dimensional (3D) radiative hydrodynamic model atmospheres of metal-poor late-type stars are characterized by cooler upper photospheric layers than their 1D counterparts. This property of 3D models can dramatically affect elemental abundances derived from temperature-sensitive spectral lines. We investigate whether the cool surface temperatures predicted by metal-poor 3D models can be ascribed to the approximated treatment of scattering in the radiative transfer. We use the Bifrost code to test three different ways to handle scattering in 3D model atmospheres of metal-poor stars. First, we solve self-consistently the radiative transfer equation for a source function with a coherent scattering term. Second, we solve the radiative transfer equation for a Planckian source function, neglecting the contribution of continuum scattering to extinction in the optically thin layers; this has been the default mode in previous models of ours. Third, we treat scattering as pure absorption everywhere, which is the standard case in CO5BOLD models. We find that the second approach produces temperature structures with cool upper photospheric layers very similar to the correct coherent scattering solution. In contrast, treating scattering as pure absorption leads to significantly hotter and shallower temperature stratifications. The main differences in temperature structure between our published models and those generated with the CO5BOLD code can be traced to the different treatments of scattering. Neglecting the contribution of continuum scattering to extinction in optically thin layers provides a good approximation to the full radiative transfer solution for metal-poor stars. Our results demonstrate that the cool temperature stratifications predicted for metal-poor late-type stellar atmospheres by previous models of ours are not an artifact of the approximated treatment of scattering.

Electron-impact excitation of neutral oxygen

Aims.Our goal was to calculate transition rates from ground and excited states in neutral oxygen atoms due to electron collisions for non-LTE modelling of oxygen in late-type stellar atmospheres, thus enabling the reliable interpretation of oxygen lines in stellar spectra.

Hydrodynamical simulations of convection-related stellar micro-variability

Local-box hydrodynamical model atmospheres provide statistical information about the spatial dependence, as well as temporal evolution, of a star's emergent radiation field. Here, we consider late-type stellar atmospheres for which temporal changes of the radiative output are primarily related to convective (granular) surface flows. We derived relations for evaluating the granulation-induced, disk-integrated thus observable fluctuations of the stellar brightness and location of the photocenter from radiation intensities available from a local model. Apart from their application in the context of hydrodynamical stellar atmospheres, these formulae provide some broader insight into the nature of the fluctuations under consideration. Brightness fluctuations scale inversely proportional to the square root of the number of convective cells (the statistically independently radiating surface elements) present on the stellar surface and increase with more pronounced limb-darkening. Fluctuations of the stellar photocentric position do not depend on the number of cells and are largely insensitive to the degree of limb-darkening. They amount to a small fraction of the typical cell size, and can become a limiting factor for high-precision astrometry in the case of extreme giants. The temporal brightness and positional fluctuations are statistically uncorrelated but closely related in magnitude.

Effects of line-blocking on the non-LTE Fe I spectral line formation

The effects of background line opacity (line-blocking) in statistical equilibrium calculations for Fe in late-type stellar atmospheres have been investigated using an extensive and up-to-date model atom with radiative data primarily from the IRON Project. The background metal line opacities have been computed using data from the MARCS stellar model atmospheres. While accounting for this line opacity is important at solar metallicity, the differences between calculations including and excluding line-blocking at low metallicity are insignificant for the non-local thermodynamic equilibrium (non-LTE) abundance corrections for Fe I lines. The line-blocking has no impact on the non-LTE effects of Fe II lines. The dominant uncertainty in Fe non-LTE calculations for metal-poor stars is still the treatment of the inelastic H I collisions, which have here been included using scaling factors to the classical Drawin formalism, and whether or not thermalisation of the high Fe I levels to Fe II ground state should be enforced. Without such thermalisation, the Fe I non-LTE abundance corrections are substantial in metal-poor stars: about 0.3 dex with efficient (i.e. Drawin-like) H I collisions and about 0.5 dex without. Without both thermalisation and H I collisions, even Fe II lines show significant non-LTE effects in such stars.

Inelastic H+Li and H-+Li+collisions and non-LTE Li I line formation in stellar atmospheres

Rate coefficients for inelastic collisions between Li and H atoms covering all transitions between the asymptotic states Li(2s,2p,3s,3p,3d,4s,4p,4d,4f)+H(1s) and Li + +H − are presented for the temperature range 2000-8000 K based on recent cross-section calculations. The data are of sufficient completeness for non-LTE modelling of the Li I 670.8 nm and 610.4 nm features in late-type stellar atmospheres. Non-LTE radiative transfer calculations in both 1D and 3D model atmospheres have been carried out for test cases of particular interest. Our detailed calculations show that the classical modified Drawin-formula for collisional excitation and de-excitation (Li ∗ + H Li ∗� + H) over-estimates the cross-sections by typically several orders of magnitude and consequently that these reactions are negligible for the line formation process. However, the charge transfer reactions collisional ion-pair production and mutual neutralization (Li ∗ + H Li + + H − ) are of importance in thermalizing Li. In particular, 3D non-LTE calculations of the Li I 670.8 nm line in metal-poor halo stars suggest that 1D non-LTE results over-estimate the Li abundance by up to about 0.1 dex, aggrevating the discrepancy between the observed Li abundances and the primordial Li abundance as inferred by the WMAP analysis of the cosmic microwave background.

On the interaction between dust and gas in late-type stellar atmospheres and winds

An assumption inherent to most models of dust-driven winds from cool, evolved stars is that the radiative and collisional drag forces acting on an individual dust grain are in balance throughout the flow. We have checked the validity of this supposition of 'complete momentum coupling' by comparing the grain motion obtained from such a model with that derived from solution of the full grain equation of motion. For physical conditions typical of the circumstellar envelopes of oxygen-rich red giants, we find that silicate grains with initial radii smaller than about 5 x 10 exp -6 cm decouple from the ambient gas near the base of the outflow. The implications of these results for models of dust-driven mass loss from late-type giants and supergiants are discussed.

The Si II intercombination multiplet in late-type stars

New atomic data are used to calculate the relative intensities of transitions within the Si II|$3{s}^{2}3{p}^{2}P-3s3{p}^{2}\,^{4}{P}$| intercombination multiplet for plasma parameters appropriate to late-type stellar atmospheres. These ratios are found to be significantly different from those of Dufton & Kingston, principally due to changes in the radiative rates. A comparison with line ratios for the Sun (from Skylab S082B spectra) and late-type stars (from IUE data) indicates that although the new theoretical line ratios are in better agreement with observation, significant discrepancies still exist. Possible explanations for these discrepancies are briefly discussed.

Interspecies emission-line ratios as electron density diagnostics for late-type stellar atmospheres

view Abstract Citations (3) References (36) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Interspecies Emission-Line Ratios as Electron Density Diagnostics for Late-Type Stellar Atmospheres Keenan, F. P. ; Dufton, P. L. ; Kingston, A. E. Abstract Electron-impact excitation rates for transitions in S IV, Al III, and N IV, determined with the R-matrix code, are used to calculate relative level populations for electron temperatures log Te = 4.4 - 5.4 and densities Ne between 10 to the 8th and 10 to the 12th/cu cm. These data are combined with similar results for C III and O III to derive emission-line ratios which should be useful as electron-density diagnostics when applied to the UV spectra of late-type stellar atmospheres. Publication: The Astrophysical Journal Pub Date: April 1990 DOI: 10.1086/168653 Bibcode: 1990ApJ...353..636K Keywords: Electron Density (Concentration); Late Stars; Stellar Atmospheres; Stellar Spectra; Ultraviolet Spectra; Chromosphere; Electron Energy; Emission Spectra; Iue; Line Spectra; Astrophysics; ATOMIC PROCESSES; STARS: ATMOSPHERES; STARS: CHROMOSPHERES; STARS: LATE-TYPE; ULTRAVIOLET: SPECTRA full text sources ADS |

Observational Evidence by Fourier Transform Spectroscopy of Convective Motion in Late-Type Stellar Atmospheres

We have obtained FTS spectra of 11 stars of spectral types from MO to M4 III in the infrared K band. The shifts at line bottom of the lines of CO and Fe I show a dependence on excitation energy and line depth similar to that caused by convective motions in the photosphere of the Sun but with a larger amplitude. Time variability is small but important differences appear between individual stars of similar types. From these data it should be possible to obtain quantitative estimates of the convective velocities and horizontal temperature inhomogeneities.

Electron density diagnostics for late-type stellar atmospheres

view Abstract Citations (4) References (38) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Electron Density Diagnostics for Late-Type Stellar Atmospheres Keenan, F. P. ; Dufton, P. L. ; Aggarwal, K. M. ; Kingston, A. E. Abstract R-matrix calculations of electron excitation rates in C III, O III, and Si IV have been used to obtain several theoretical emission-line ratios for a range of electron temperatures and densities applicable to late-type stellar atmospheres. Data are derived for the ratios I(C III 1908 A)/I(O III 1666 A), I(O III 1666 A)/I(Si IV 1402 A), and I(C III 1908 A)/I(Si IV 1402 A). It is suggested that differences found between the results and those of Doschek et al. (1978) are due to the present adoption of improved atomic data. Publication: The Astrophysical Journal Pub Date: January 1988 DOI: 10.1086/165963 Bibcode: 1988ApJ...324.1068K Keywords: Electron Density (Concentration); Late Stars; Plasma Diagnostics; Stellar Atmospheres; Stellar Spectra; Ultraviolet Spectra; Carbon; Electron Energy; Emission Spectra; Oxygen; Silicon; Astrophysics; ATOMIC PROCESSES; STARS: ATMOSPHERES; STARS: LATE-TYPE; ULTRAVIOLET: SPECTRA full text sources ADS |

Selective excitation of Fe II in the laboratory and late-type stellar atmospheres

Les spectres a faible resolution de geantes froides et supergeantes observes par l'IUE contiennent des structures d'emission pour lesquelles il est difficile d'obtenir des identifications sans ambiguites. Les spectres a haute resolution de RR Tel et β Gru montrent des raies pouvant etre attribuees a Fe II. Confirmation en laboratoire

The Role of Magnetic Fields in Stellar Chromospheres and Transition Regions

In this review based largely on observations with the IUE and Einstein satellites, I will summarize the different roles that magnetic fields play in controlling the structure and energy balance in the chromospheres and transition regions of late-type stars. Solar observations clearly show that magnetic flux tubes are the dominant structural element in the solar atmosphere, but the rotational modulation of plages (structures that are bright in ultraviolet emission lines) that overlie dark starspots provide strong evidence that magnetic flux tubes are the dominant structural elements in late-type stellar atmospheres as well. The wide range of radiative loss rates (and thus heating rates) observed in chromospheric and transition region emission lines also provides evidence for the importance of magnetic fields, but it is not yet clear whether the most active stars can be understood in terms of a large fractional coverage by solar-like magnetic flux tubes or whether brighter flux tubes are needed. I propose that the existence of a boundary between solar-like stars and those with little or no hot plasma, as well as the different types of G-K giants and supergiants, can be understood in terms of the fractional surface coverage by closed magnetic structures. Transition region downflows, the chromospheric heating mechanism, and the relative heating rates at different layers can be simply explained by the control of the energy balance by magnetic fields. Finally, I will intercompare models computed for active and quiet regions on the Sun with similar models computed for active and quiet stars, that is stars with intrinsically bright or weak emission lines.

Polynomial partition function approximations of 344 atomic and molecular species

view Abstract Citations (184) References (8) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Polynomial partition function approximations of 344 atomic and molecular species. Irwin, A. W. Abstract Approximations of 344 atomic and molecular partition functions are given for a temperature range from 1000 to 16000 K. These approximations are useful for the calculation of the equation of state and the line and continuum opacity of intermediate and late-type stellar atmospheres. Publication: The Astrophysical Journal Supplement Series Pub Date: April 1981 DOI: 10.1086/190730 Bibcode: 1981ApJS...45..621I Keywords: Approximation; Atmospheric Optics; Molecular Gases; Partitions (Mathematics); Stellar Atmospheres; Functions (Mathematics); Opacity; Polynomials; Tables (Data); Astrophysics full text sources ADS |