Author name code: kiselman ADS astronomy entries on 2022-09-14 author:"Kiselman, Dan" ------------------------------------------------------------------------ Title: Physical properties of a fan-shaped jet backlit by an X9.3 flare Authors: Pietrow, A. G. M.; Druett, M. K.; de la Cruz Rodriguez, J.; Calvo, F.; Kiselman, D. Bibcode: 2022A&A...659A..58P Altcode: 2021arXiv211010541P Context. Fan-shaped jets sometimes form above light bridges and are believed to be driven by the reconnection of the vertical umbral field with the more horizontal field above the light bridges. Because these jets are not fully opaque in the wings of most chromospheric lines, it is not possible to study their spectra without highly complex considerations of radiative transfer in spectral lines from the atmosphere behind the fan.
Aims: We take advantage of a unique set of observations of the Hα line along with the Ca II 8542 Å and Ca II K lines obtained with the CRISP and CHROMIS instrument of the Swedish 1-m Solar Telescope to study the physical properties of a fan-shaped jet that was backlit by an X9.3 flare. For what we believe to be the first time, we report an observationally derived estimate of the mass and density of material in a fan-shaped jet.
Methods: The Hα flare ribbon emission profiles from behind the fan are highly broadened and flattened, allowing us to investigate the fan with a single slab via Beckers' cloud model, as if it were backlit by a flat spectral profile of continuum emission. Using this model we derived the opacity and velocity of the material in the jet. Using inversions of Ca II 8542 Å emission via the STockholm inversion Code, we were also able to estimate the temperature and to cross-check the velocity of the material in the jet. Finally, we used the masses and the plane-of-sky and line-of-sight velocities as functions of time to investigate the downward supply of energy and momentum to the photosphere in the collapse of this jet, and evaluated it as a potential driver for a sunquake beneath.
Results: We find that the physical properties of the fan material are reasonably chromospheric in nature, with a temperature of 7050 ± 250 K and a mean density of 2 ± 0.3 × 10−11 g cm−3.
Conclusions: The total mass observed in Hα was found to be 3.9 ± 0.7 × 1013 g and the kinetic energy delivered to the base of the fan in its collapse was nearly two orders of magnitude below typical sunquake energies. We therefore rule out this jet as the sunquake driver, but cannot completely rule out larger fan jets as potential drivers. Title: Inference of the chromospheric magnetic field configuration of solar plage using the Ca II 8542 Å line Authors: Pietrow, A. G. M.; Kiselman, D.; de la Cruz Rodríguez, J.; Díaz Baso, C. J.; Pastor Yabar, A.; Yadav, R. Bibcode: 2020A&A...644A..43P Altcode: 2020arXiv200614486P Context. It has so far proven impossible to reproduce all aspects of the solar plage chromosphere in quasi-realistic numerical models. The magnetic field configuration in the lower atmosphere is one of the few free parameters in such simulations. The literature only offers proxy-based estimates of the field strength, as it is difficult to obtain observational constraints in this region. Sufficiently sensitive spectro-polarimetric measurements require a high signal-to-noise ratio, spectral resolution, and cadence, which are at the limit of current capabilities.
Aims: We use critically sampled spectro-polarimetric observations of the Ca II 8542 Å line obtained with the CRISP instrument of the Swedish 1-m Solar Telescope to study the strength and inclination of the chromospheric magnetic field of a plage region. This will provide direct physics-based estimates of these values, which could aid modelers to put constraints on plage models.
Methods: We increased the signal-to-noise ratio of the data by applying several methods including deep learning and PCA. We estimated the noise level to be 1 × 10-3 Ic. We then used STiC, a non-local thermodynamic equilibrium inversion code to infer the atmospheric structure and magnetic field pixel by pixel.
Results: We are able to infer the magnetic field strength and inclination for a plage region and for fibrils in the surrounding canopy. In the plage we report an absolute field strength of |B| = 440 ± 90 G, with an inclination of 10° ±16° with respect to the local vertical. This value for |B| is roughly double of what was reported previously, while the inclination matches previous studies done in the photosphere. In the fibrillar region we found |B| = 300 ± 50 G, with an inclination of 50° ±13°. Title: Solar off-limb emission of the O I 7772 Å line Authors: Pazira, H.; Kiselman, D.; Leenaarts, J. Bibcode: 2017A&A...604A..49P Altcode: 2017arXiv170506459P
Aims: The aim of this paper is to understand the formation of the O I line at 7772 Å in the solar chromosphere.
Methods: We used SST/CRISP observations to observe O I 7772 Å in several places around the solar limb. We compared the observations with synthetic spectra calculated with the RH code in the one-dimension spherical geometry mode. New accurate hydrogen collisional rates were included for the RH calculations.
Results: The observations reveal a dark gap in the lower chromosphere, which is caused by variations in the line opacity as shown by our models. The lower level of the 7772 Å transition is populated by a downward cascade from the continuum. We study the effect of Lyman-β pumping and hydrogen collisions between the triplet and quintet system in O I. Both have a small but non-negligible influence on the line intensity. Title: Non-LTE line formation of Fe in late-type stars - IV. Modelling of the solar centre-to-limb variation in 3D Authors: Lind, K.; Amarsi, A. M.; Asplund, M.; Barklem, P. S.; Bautista, M.; Bergemann, M.; Collet, R.; Kiselman, D.; Leenaarts, J.; Pereira, T. M. D. Bibcode: 2017MNRAS.468.4311L Altcode: 2017arXiv170304027L Our ability to model the shapes and strengths of iron lines in the solar spectrum is a critical test of the accuracy of the solar iron abundance, which sets the absolute zero-point of all stellar metallicities. We use an extensive 463-level Fe atom with new photoionization cross-sections for Fe I and quantum mechanical calculations of collisional excitation and charge transfer with neutral hydrogen; the latter effectively remove a free parameter that has hampered all previous line formation studies of Fe in non-local thermodynamic equilibrium (NLTE). For the first time, we use realistic 3D NLTE calculations of Fe for a quantitative comparison to solar observations. We confront our theoretical line profiles with observations taken at different viewing angles across the solar disc with the Swedish 1-m Solar Telescope. We find that 3D modelling well reproduces the observed centre-to-limb behaviour of spectral lines overall, but highlight aspects that may require further work, especially cross-sections for inelastic collisions with electrons. Our inferred solar iron abundance is log (ɛFe) = 7.48 ± 0.04 dex. Title: SOLARNET WP50: The SOLARNET data pipeline activity Authors: Kiselman, Dan Bibcode: 2017psio.confE..95K Altcode: No abstract at ADS Title: JP3D compression of solar data-cubes: photospheric imaging and spectropolarimetry Authors: Del Moro, Dario; Giovannelli, Luca; Berrilli, Francesco; Pietropaolo, Ermanno; Ermolli, Ilaria; Kiselman, Dan Bibcode: 2017psio.confE.121D Altcode: No abstract at ADS Title: Stable umbral chromospheric structures Authors: Henriques, V. M. J.; Scullion, E.; Mathioudakis, M.; Kiselman, D.; Gallagher, P. T.; Keenan, F. P. Bibcode: 2015A&A...574A.131H Altcode: 2014arXiv1412.6100H
Aims: We seek to understand the morphology of the chromosphere in sunspot umbra. We investigate if the horizontal structures observed in the spectral core of the Ca II H line are ephemeral visuals caused by the shock dynamics of more stable structures, and examine their relationship with observables in the H-alpha line.
Methods: Filtergrams in the core of the Ca II H and H-alpha lines as observed with the Swedish 1-m Solar Telescope are employed. We utilise a technique that creates composite images and tracks the flash propagation horizontally.
Results: We find 0.̋15 wide horizontal structures, in all of the three target sunspots, for every flash where the seeing is moderate to good. Discrete dark structures are identified that are stable for at least two umbral flashes, as well as systems of structures that live for up to 24 min. We find cases of extremely extended structures with similar stability, with one such structure showing an extent of 5''. Some of these structures have a correspondence in H-alpha, but we were unable to find a one-to-one correspondence for every occurrence. If the dark streaks are formed at the same heights as umbral flashes, there are systems of structures with strong departures from the vertical for all three analysed sunspots.
Conclusions: Long-lived Ca II H filamentary horizontal structures are a common and likely ever-present feature in the umbra of sunspots. If the magnetic field in the chromosphere of the umbra is indeed aligned with the structures, then the present theoretical understanding of the typical umbra needs to be revisited.

Movies associated to Figs. 3 and 4 are available in electronic form at http://www.aanda.org Title: Ca II H sunspot tomography from the photosphere to the chromosphere Authors: Henriques, V. M. J.; Kiselman, D. Bibcode: 2013A&A...557A...5H Altcode:
Aims: We aim at gaining insight into the thermal properties of different small-scale structures related to sunspots.
Methods: We use filtergrams in the Ca ii H filter at the Swedish 1-m Solar Telescope to study the relationship between fine structure at different heights in a sunspot.
Results: The methods for destretching and aligning the different image data work well. The magnetic spine structure in the outer parts of the sunspot penumbra is found to be associated with higher intensities in the Ca ii H wing passbands but with less steep vertical temperature gradients. Dark lanes in a light bridge behave very similarly to dark cores in penumbral filaments. Fibril structures are seen in the line-core images over the umbra and penumbra.
Conclusions: The observations add support to the idea that penumbral filaments, light bridges, and umbral dots are caused by similar processes of overturning convection. Observations in the Ca ii H & K wings are a promising observable, complementing others, for testing simulation results for sunspots at high spatial resolution. Title: Is the solar spectrum latitude-dependent?. An investigation with SST/TRIPPEL Authors: Kiselman, D.; Pereira, T. M. D.; Gustafsson, B.; Asplund, M.; Meléndez, J.; Langhans, K. Bibcode: 2011A&A...535A..14K Altcode: 2011arXiv1108.4527K Context. In studies of the solar spectrum compared to spectra of solar twin stars, it has been found that the chemical composition of the Sun seems to depart systematically from those of the twins. One possible explanation could be that the effect is caused by the special aspect angle of the Sun when observed from Earth compared with the aspect angles of the twins. This means that a latitude dependence of the solar spectrum, even with the heliocentric angle constant, could lead to the observed effects.
Aims: We explore a possible variation in the strength of certain spectral lines that are used in the comparisons between the composition of the Sun and the twins at loci on the solar disk with different latitudes but at constant heliocentric angle.
Methods: We use the TRIPPEL spectrograph at the Swedish 1-m Solar Telescope on La Palma to record spectra in five spectral regions to compare different locations on the solar disk at a heliocentric angle of 45°. Equivalent widths and other parameters are measured for fifteen different lines representing nine atomic species. Spectra acquired at different times are used in averaging the line parameters for each line and observing position.
Results: The relative variations in equivalent widths at the equator and at solar latitude ~45° are found to be less than 1.5% for all spectral lines studied. Translated into elemental abundances as they would be measured from a terrestrial and a hypothetical pole-on observer, the difference is estimated to be within 0.005 dex in all cases.
Conclusions: It is very unlikely that latitude effects could cause the reported abundance difference between the Sun and the solar twins. The accuracy obtainable in measurements of small differences in spectral line strengths between different solar disk positions is very high, and can be exploited in studies of, e.g. weak magnetic fields or effects of solar activity on atmospheric structure. Title: A tilted interference filter in a converging beam Authors: Löfdahl, M. G.; Henriques, V. M. J.; Kiselman, D. Bibcode: 2011A&A...533A..82L Altcode: 2011arXiv1108.1234L Context. Narrow-band interference filters can be tuned toward shorter wavelengths by tilting them from the perpendicular to the optical axis. This can be used as a cheap alternative to real tunable filters, such as Fabry-Pérot interferometers and Lyot filters. At the Swedish 1-meter Solar Telescope, such a setup is used to scan through the blue wing of the Ca ii H line. Because the filter is mounted in a converging beam, the incident angle varies over the pupil, which causes a variation of the transmission over the pupil, different for each wavelength within the passband. This causes broadening of the filter transmission profile and degradation of the image quality.
Aims: We want to characterize the properties of our filter, at normal incidence as well as at different tilt angles. Knowing the broadened profile is important for the interpretation of the solar images. Compensating the images for the degrading effects will improve the resolution and remove one source of image contrast degradation. In particular, we need to solve the latter problem for images that are also compensated for blurring caused by atmospheric turbulence.
Methods: We simulate the process of image formation through a tilted interference filter in order to understand the effects. We test the hypothesis that they are separable from the effects of wavefront aberrations for the purpose of image deconvolution. We measure the filter transmission profile and the degrading PSF from calibration data.
Results: We find that the filter transmission profile differs significantly from the specifications. We demonstrate how to compensate for the image-degrading effects. Because the filter tilt effects indeed appear to be separable from wavefront aberrations in a useful way, this can be done in a final deconvolution, after standard image restoration with Multi-Frame Blind Deconvolution/Phase Diversity based methods. We illustrate the technique with real data. Title: Detection of Convective Downflows in a Sunspot Penumbra Authors: Scharmer, G. B.; Henriques, V. M. J.; Kiselman, D.; de la Cruz Rodríguez, J. Bibcode: 2011Sci...333..316S Altcode: The fine structure and dynamics of sunspots and the strong outflow in their outer filamentary part—the penumbra—have puzzled astronomers for more than a century. Recent theoretical models and three-dimensional numerical simulations explain the penumbral filaments and their radiative energy output as the result of overturning convection. Here, we describe the detection of ubiquitous, relatively dark downward flows of up to 1 kilometer per second (km/s) in the interior penumbra, using imaging spectropolarimetric data from the Swedish 1-meter Solar Telescope. The dark downflows are omnipresent in the interior penumbra, distinguishing them from flows in arched flux tubes, and are associated with strong (3 to 3.5 km/s) radial outflows. They are thus part of a penumbral convective flow pattern, with the Evershed flow representing the horizontal component of that convection. Title: Solar velocity references from 3D HD photospheric models Authors: de la Cruz Rodríguez, J.; Kiselman, D.; Carlsson, M. Bibcode: 2011A&A...528A.113D Altcode: 2011arXiv1101.2671D Context. The measurement of Doppler velocities in spectroscopic solar observations requires a reference for the local frame of rest. The rotational and radial velocities of the Earth and the rotation of the Sun introduce velocity offsets in the observations. Normally, good references for velocities are missing (e.g. telluric lines), especially in filter-based spectropolarimetric observations.
Aims: We determine an absolute reference for line-of-sight velocities measured from solar observations for any heliocentric angle, calibrating the convective line shift of spatially-averaged profiles on quiet sun from a 3D hydrodynamical simulation. This method works whenever there is quiet sun in the field-of-view, and it has the advantage of being relatively insensitive to uncertainties in the atomic data.
Methods: We carry out radiative transfer computations in LTE for selected C i and Fe i lines, whereas the Ca ii infrared lines are synthesized in non-LTE. Radiative transfer calculations are done with a modified version of Multi, using the snapshots of a non-magnetic 3D hydrodynamical simulation of the photosphere.
Results: The resulting synthetic profiles show the expected C-shaped bisector at disk center. The degree of asymmetry and the line shifts, however, show a clear dependence on the heliocentric angle and the properties of the lines. The profiles at μ = 1 are compared with observed profiles to prove their reliability, and they are tested against errors induced by the LTE calculations, inaccuracies in the atomic data and the 3D simulation.
Conclusions: Theoretical quiet-sun profiles of lines commonly used by solar observers are provided to the community. Those can be used as absolute references for line-of-sight velocities. The limb effect is produced by the projection of the 3D atmosphere along the line of sight. Non-LTE effects on Fe i lines are found to have a small impact on the convective shifts of the lines, reinforcing the usability of the LTE approximation in this case. We estimate the precision of the disk-center line shifts to be approximately 50 m s-1, but the off-center profiles remain to be tested against observations.

The spectral profiles are only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/528/A113 Title: VizieR Online Data Catalog: Synthetic lines in the Sun (de la Cruz Rodríguez+, 2011) Authors: de la Cruz Rodríguez, J.; Kiselman, D.; Carlsson, M. Bibcode: 2011yCat..35289113D Altcode: We have computed synthetic spectra from a realistic 3D numerical simulation of the solar photosphere. We provide the spatially averaged spectra for selected lines that are commonly used on solar applications. These data can be used to calibrate Doppler velocity measurements in the solar photosphere. The calculations are carried out along the solar disk from heliocentric angle mu=1.0 to mu=0.3.

(11 data files). Title: Photospheric Temperatures from Ca IIH Authors: Henriques, V. M. J.; Kiselman, D.; van Noort, M. Bibcode: 2010ASSP...19..511H Altcode: 2010mcia.conf..511H The temperature stratification in the upper photosphere can be extracted from Ca II H&K spectrograms following Shine and Linsky (1974) by assuming LTE, the Eddington-Barbier approximation, hydrostatic equilibrium, and that Ca II is mostly in the ground state. Rouppe van der Voort (2002) confirmed that these assumptions were solid for a wide range in the Ca IIK wings and further developed the method including forward computation using MULTI (Carlsson 1986). Title: Oxygen lines in solar granulation. II. Centre-to-limb variation, NLTE line formation, blends, and the solar oxygen abundance Authors: Pereira, T. M. D.; Asplund, M.; Kiselman, D. Bibcode: 2009A&A...508.1403P Altcode: 2009arXiv0909.2310P Context: There is a lively debate about the solar oxygen abundance and the role of 3D models in its recent downward revision. These models have been tested using high-resolution solar atlases of flux and disk-centre intensity. Further testing can be done using centre-to-limb variations.
Aims: Using high-resolution and high S/N observations of neutral oxygen lines across the solar surface, we seek to test that the 3D and 1D models reproduce their observed centre-to-limb variation. In particular we seek to assess whether the latest generation of 3D hydrodynamical solar model atmospheres and NLTE line formation calculations are appropriate to derive the solar oxygen abundance.
Methods: We use our recent observations of O i 777 nm, O i 615.81 nm, [O i] 630.03 nm, and nine lines of other elements for five viewing angles 0.2≤μ≤ 1 of the quiet solar disk. We compared them with the predicted line profiles from the 3D and 1D models computed with the most up-to-date line formation codes and line data and allowing for departures of LTE. The centre-to-limb variation of the O i 777 nm lines is also used to obtain an empirical correction for the poorly known efficiency of the inelastic collisions with H i.
Results: The 3D model generally reproduces the centre-to-limb observations of the lines very well, particularly the oxygen lines. From the O i 777 nm lines we find that the classical Drawin recipe slightly overestimates H i collisions (S_H≈ 0.85 agrees best with the observations). The limb observations of the O i 615.82 nm line allow us to identify a previously unknown contribution of molecules for this line, prevalent at the solar limb. A detailed treatment of the [O i] 630.03 nm line that includes the recent nickel abundance shows that the 3D modelling closely agrees with the observations. The derived oxygen abundances with the 3D model are 8.68 (777 nm lines), 8.66 (630.03 nm line), and 8.62 (615.82 nm line).
Conclusions: These additional tests have reinforced the trustworthiness of the 3D model and line formation for abundance analyses.

SST spectra are available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/508/1403 Title: Oxygen lines in solar granulation. I. Testing 3D models against new observations with high spatial and spectral resolution Authors: Pereira, T. M. D.; Kiselman, D.; Asplund, M. Bibcode: 2009A&A...507..417P Altcode: 2009arXiv0909.2307P Aims: We seek to provide additional tests of the line formation of theoretical 3D solar photosphere models. In particular, we set out to test the spatially-resolved line formation at several viewing angles, from the solar disk-centre to the limb and focusing on atomic oxygen lines. The purpose of these tests is to provide additional information on whether the 3D model is suitable to derive the solar oxygen abundance. We also aim to empirically constrain the NLTE recipes for neutral hydrogen collisions, using the spatially-resolved observations of the O i 777 nm lines.
Methods: Using the Swedish 1-m Solar Telescope we obtained high-spatial-resolution observations of five atomic oxygen lines (as well as several lines for other species, mainly Fe i) for five positions on the solar disk. These observations have a high spatial (sub-arcsecond) and spectral resolution, and a continuum intensity contrast up to 9% at 615 nm. The theoretical line profiles were computed using the 3D model, with a full 3D NLTE treatment for oxygen and LTE for the other lines.
Results: At disk-centre we find an excellent agreement between predicted and observed line shifts, strengths, FWHM and asymmetries. At other viewing angles the agreement is also good, but the smaller continuum intensity contrast makes a quantitative comparison harder. We use the disk-centre observations we constrain SH, the scaling factor for the efficiency of collisions with neutral hydrogen. We find that SH=1 provides the best match to the observations, although this method is not as robust as the centre-to-limb line variations to constrain SH.
Conclusions: Overall there is a very good agreement between predicted and observed line properties over the solar granulation. This further reinforces the view that the 3D model is realistic and a reliable tool to derive the solar oxygen abundance.

2D spectrograms are available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/507/417 Title: VizieR Online Data Catalog: Oxygen lines in solar granulation. II. (Pereira+, 2009) Authors: Pereira, T. M. D.; Asplund, M.; Kiselman, D. Bibcode: 2009yCat..35081403P Altcode: We obtained solar observations of several lines at several positions of the solar disk. Data were obtained in May 2007 with the TRIPPEL spectrograph at the Swedish 1-m Solar Telescope (SST). The spectra were obtained for three distinct wavelength windows at ~615, 630 and 777nm. Each window covers approx. 1nm. Here we provide the spatially and temporally averaged spectra. Each spectrum was the average of 50 frames (mu=1.0) or 25 frames (other positions). The frames were selected by the highest continuum contrast, and thus are not regularly spaced in time between the start and end Julian dates.

The observations are given for five positions in the solar disk, ordered by mu, the cosine of the heliocentric angle. The objective was to study the centre-to-limb variation of the lines. Active sun regions were avoided.

Important note: the wavelength scales have NOT been corrected for solar rotation or gravitational redshift.

(4 data files). Title: Testing 3D solar models against observations Authors: Pereira, Tiago M. D.; Asplund, Martin; Kiselman, Dan Bibcode: 2009arXiv0909.4121P Altcode: We present results from a series of observational tests to 3D and 1D solar models. In particular, emphasis is given to the line formation of atomic oxygen lines, used to derive the much debated solar oxygen photospheric abundance. Using high-quality observations obtained with the Swedish Solar Telescope (SST) we study the centre-to-limb variation of the O I lines, testing the models and line formation (LTE and non-LTE). For the O I 777 nm triplet, the centre-to-limb variation sets strong constraints in the non-LTE line formation, and is used to derive an empirical correction factor (S_H) to the classical Drawin recipe for neutral hydrogen collisions. Taking advantage of the spatially-resolved character of the SST data, an additional framework for testing the 3D model and line formation is also studied. From the tests we confirm that the employed 3D model is realistic and its predictions agree very well with the observations. Title: VizieR Online Data Catalog: Oxygen lines in solar granulation. I. (Pereira+, 2009) Authors: Pereira, T. M. D.; Kiselman, D.; Asplund, M. Bibcode: 2009yCat..35070417P Altcode: We obtained solar observations of several lines at several positions of the solar disk. Data were obtained in May 2007 with the TRIPPEL spectrograph at the Swedish 1-m Solar Telescope (SST). The spectra were obtained for three distinct wavelength windows at approx 615, 630 and 777nm. Each window covers ~1nm. The spectra are spatially-resolved in the solar surface, hence are given as 2D spectrograms. One axis covers the wavelength and the other spatial position.

The observations are given for five positions in the solar disk, ordered by mu, the cosine of the heliocentric angle. The objective was to study the centre-to-limb variation of the lines. Active sun regions were avoided. In total 150 spectrograms are given for each wavelength window: 50 for the solar disk-centre (mu=1) and 25 for the other four positions (mu=0.8, 0.6, 0.4, 0.2). The images were selected by continuum contrast, hence their observed times are not the same for the three windows.

The spectrograms are given as FITS files. Each file has two Header/Data Units (HDU). The first HDU contains the reduced spectrogram, a 2D array. The first dimension of the array contains (FITS NAXIS1) corresponds to wavelength, and the second (FITS NAXIS2) to spatial coordinate. The second HDU contains the continuum levels obtained for each spatial point (1D array, length equal to the number of spatial points in the spectrogram). To obtain the normalized reduced spectrogram one has to divide each spectrum in the spectrogram by the corresponding continuum level. The spectrograms have been corrected for stray light and to minimize noise a Fourier filter has been applied (details in the paper).

Important note: the wavelength scales have NOT been corrected for solar rotation or gravitational redshift.

(3 data files). Title: Neutral oxygen spectral line formation revisited with new collisional data: large departures from LTE at low metallicity Authors: Fabbian, D.; Asplund, M.; Barklem, P. S.; Carlsson, M.; Kiselman, D. Bibcode: 2009A&A...500.1221F Altcode: 2009arXiv0902.4472F Aims: A detailed study is presented, including estimates of the impact on elemental abundance analysis, of the non-local thermodynamic equilibrium (non-LTE) formation of the high-excitation neutral oxygen 777 nm triplet in model atmospheres representative of stars with spectral types F to K.
Methods: We have applied the statistical equilibrium code MULTI to a number of plane-parallel MARCS atmospheric models covering late-type stars (4500 ≤ T_eff ≤ 6500 K, 2 ≤ log g ≤ 5 [cgs], and -3.5 ≤ [Fe/H] ≤ 0). The atomic model employed includes, in particular, recent quantum-mechanical electron collision data.
Results: We confirm that the O i triplet lines form under non-LTE conditions in late-type stars, suffering negative abundance corrections with respect to LTE. At solar metallicity, the non-LTE effect, mainly attributed in previous studies to photon losses in the triplet itself, is also driven by an additional significant contribution from line opacity. At low metallicity, the very pronounced departures from LTE are due to overpopulation of the lower level (3s ^5S^o) of the transition. Large line opacity stems from triplet-quintet intersystem electron collisions, a form of coupling previously not considered or seriously underestimated. The non-LTE effects generally become severe for models (both giants and dwarfs) with higher T_eff. Interestingly, in metal-poor turn-off stars, the negative non-LTE abundance corrections tend to rapidly become more severe towards lower metallicity. When neglecting H collisions, they amount to as much as |Δlog ɛ_O| ~ 0.9 dex and ~1.2 dex, respectively at [Fe/H] = -3 and [Fe/H] = -3.5. Even when such collisions are included, the LTE abundance remains a serious overestimate, correspondingly by |Δlog ɛ_O| ~ 0.5 dex and ~0.9 dex at such low metallicities. Although the poorly known inelastic hydrogen collisions thus remain an important uncertainty, the large metallicity-dependent non-LTE effects seem to point to a resulting “low” (compared to LTE) [O/Fe] in metal-poor halo stars.
Conclusions: Our results may be important in solving the long-standing [O/Fe] debate. When applying the derived non-LTE corrections, the LTE oxygen abundance inferred from the 777 nm permitted triplet will be decreased substantially at low metallicity. If the classical Drawin formula is employed for O+H collisions, the derived [O/Fe] trend becomes almost flat below [Fe/H] ~ -1, in better agreement with recent literature estimates generally obtained from other oxygen abundance indicators. A value of [O/Fe] ⪉ +0.5 may therefore be appropriate, as suggested by standard theoretical models of type II supernovae nucleosynthetic yields. If neglecting impacts with H atoms instead, [O/Fe] decreases towards lower [Fe/H], which would open new questions. Our tests using ATLAS model atmospheres show that, though non-LTE corrections for metal-poor dwarfs are smaller (by ~0.2 dex when adopting efficient H collisions) than in the MARCS case, our main conclusions are preserved, and that the LTE approach tends to seriously overestimate the O abundance at low metallicity. However, in order to finally reach consistency between oxygen abundances from the different available spectral features, it is of high priority to reduce the large uncertainty regarding H collisions, to undertake a full investigation of the interplay of non-LTE and 3D effects, and to clarify the issue of the temperature scale at low metallicity. Title: Temperature stratification in the Sun's photosphere in high horizontal resolution using Ca II H filtergrams. Authors: Henriques, V. M. J.; Kiselman, D. Bibcode: 2009MmSAI..80..639H Altcode: A method to extract the temperature stratification in the Sun's photosphere using filtergrams is presented along with some high resolution results. The data was acquired with the Swedish 1-m Solar Telescope (SST) using a tunable filter in the Ca II H blue wing. Each full scan is completed in the order of seconds thus allowing for the full resolution of the SST and reasonable depth sampling to be obtained simultaneously in a shorter time than that of the evolution time scale of the photosphere. We test the quality of the method by applying it to a set of synthetic images (obtained through radiative transfer on 3D HD and MHD simulation snapshots followed by degradation) and comparing the output with the known 3D simulated atmosphere. Fine structure around bright points becomes evident in both the temperature gradient maps computed from a set of test observations and synthetic images obtained from MHD simulations. Title: Testing 3D solar models against observations . Center-to-limb variations of oxygen lines, spatially-resolved line formation and probing for departures from LTE Authors: Pereira, T. M. D.; Asplund, M.; Kiselman, D. Bibcode: 2009MmSAI..80..650P Altcode: We present results from a series of observational tests to 3D and 1D solar models. In particular, emphasis is given to the line formation of atomic oxygen lines, used to derive the much debated solar oxygen photospheric abundance. Using high-quality observations obtained with the Swedish Solar Telescope (SST) we study the center-to-limb variation of the O I lines, testing the models and line formation (LTE and non-LTE). For the O I 777 nm triplet, the center-to-limb variation sets strong constraints in the non-LTE line formation, and is used to derive an empirical correction factor (SH) to the classical Drawin recipe for neutral hydrogen collisions. Taking advantage of the spatially-resolved character of the SST data, an additional framework for testing the 3D model and line formation is also studied. From the tests we confirm that the employed 3D model is realistic and its predictions agree very well with the observations. Title: CRISP Spectropolarimetric Imaging of Penumbral Fine Structure Authors: Scharmer, G. B.; Narayan, G.; Hillberg, T.; de la Cruz Rodriguez, J.; Löfdahl, M. G.; Kiselman, D.; Sütterlin, P.; van Noort, M.; Lagg, A. Bibcode: 2008ApJ...689L..69S Altcode: 2008arXiv0806.1638S We discuss penumbral fine structure in a small part of a pore, observed with the CRISP imaging spectropolarimeter at the Swedish 1-m Solar Telescope (SST), close to its diffraction limit of 0.16''. Milne-Eddington inversions applied to these Stokes data reveal large variations of field strength and inclination angle over dark-cored penumbral intrusions and a dark-cored light bridge. The mid-outer part of this penumbra structure shows ~0.3'' wide spines, separated by ~1.6'' (1200 km) and associated with 30° inclination variations. Between these spines, there are no small-scale magnetic structures that easily can be identified with individual flux tubes. A structure with nearly 10° more vertical and weaker magnetic field is seen midway between two spines. This structure is cospatial with the brightest penumbral filament, possibly indicating the location of a convective upflow from below. Title: Solar 3D models versus observations—a few comments Authors: Kiselman, D. Bibcode: 2008PhST..133a4016K Altcode: Some properties of 3D solar photospheric models and the observations used to test them are discussed: granulation contrast and spectral line variation in the granulation pattern and over the solar disc. The recent use of the techniques of solar physics to tackle abundance problems is encouraged. Title: Spectropolarimetry of Sunspots at 0.16 ARCSEC resolution Authors: Scharmer, G.; Henriques, V.; Hillberg, T.; Kiselman, D.; Löfdahl, M.; Narayan, G.; Sütterlin, P.; van Noort, M.; de la Cruz Rodríguez, J. Bibcode: 2008ESPM...12..2.5S Altcode: We present first observations of sunspots with the imaging spectropolarimeter CRISP, recently installed at the Swedish 1-m Solar Telescope (SST) on La Palma. This spectropolarimeter is based on a high-fidelity dual Fabry-Perot filter system.

Two liquid crystals and a polarizing beam splitter are used to reduce seeing induced I,Q,U,V crosstalk by simultaneously recording images with two 1kx1k back-illuminated Sarnoff CCD's. A third CCD simultaneously records broadband images through the pre-filter of the FPI filter system, allowing image reconstruction and co-alignment of images of different polarization states and at different wavelengths in Zeeman sensitive spectral lines.

The first data, recorded in April 2008, demonstrate the capability of this system to record high cadence, high S/N polarimetric data with a spatial resolution at or close to the diffraction limit of the SST at 630 nm, 0.16 arcsec. We discuss the analysis of first spectropolarimetric data for sunspots, based on Milne-Eddington inversion techniques. Title: Commission 36: Theory of Stellar Atmospheres Authors: Spite, Monique; Landstreet, John D.; Asplund, Martin; Ayres, Thomas R.; Balachandran, Suchitra C.; Dravins, Dainis; Hauschildt, Peter H.; Kiselman, Dan; Nagendra, K. N.; Sneden, Christopher; Tautvaišiené, Grazina; Werner, Klaus Bibcode: 2007IAUTB..26..160S Altcode: The business meeting of Commission 36 was held during the General Assembly in Prague on 16 August. It was attended by about 15 members. The issues presented included a review of the work made by members of Commission 36, and the election of the new Organising Committee. We note that a comprehensive report on the activities of the commission during the last triennium has been published in Reports on Astronomy, Transactions IAU Volume XXVIA. The scientific activity of the members of the commission has been very intense, and has led to the publication of a large number of papers. Title: Recent High Resolution Observations and Interpretations of Sunspot Fine Structure Authors: Scharmer, G. B.; Langhans, K.; Kiselman, D.; Löfdahl, M. G. Bibcode: 2007ASPC..369...71S Altcode: We review analyses made of highly resolved filtergrams, magnetograms and Dopplergrams of sunspots, recorded with the Swedish 1-meter Solar Telescope (SSTSST) on La Palma. Dark cores in penumbral filaments are shown to be directly linked to peripheral umbral dots and to dark lanes in light bridges, suggesting similar or related underlying physics. The visibility of dark cores is found to depend strongly on the azimuth angle already for spots located at small heliocentric distances. It is shown that dark cores are clearly visible close to the center of the Ca II H line, formed approximately 150--200~km above the photosphere. We conclude that the τ = 1 layer of dark-cored filaments outlines a strongly warped surface, consistent with the finding that the magnetic field strength is strongly reduced in dark cores. We show that several properties of dark-cored filaments derived from SSTSST data are consistent with results of inversions of low-resolution Stokes spectra, but also find important discrepancies with the interpretation that penumbral filaments can be identified with flux tubes. Our data are consistent with the model proposed by Spruit & Scharmer (2006), explaining dark cores as signatures of field-free convection occurring just below the visible surface of the penumbra. We discuss recent simulations of light bridges and umbral dots, providing additional support to that model. Title: Commission 36: Theory of Stellar Atmospheres Authors: Spite, Monique; Landstreet, John; Asplund, M.; Ayres, T.; Balachandran, S.; Dravins, D.; Hauschildt, P.; Kiselman, D.; Nagendra, K. N.; Sneden, C.; Tautvaišiené, G.; Werner, K. Bibcode: 2007IAUTA..26..215S Altcode: Commission 36 covers all the physics of stellar atmospheres. The scientific activity in this large field has been very intense during the last triennium and led to the publication of a large number of papers which makes an exhaustive report practically not feasible. As a consequence we decided to keep the format of the preceding report: first a list of areas of current research, then web links for obtaining further information. Title: Observations of dark-cored filaments in sunspot penumbrae Authors: Langhans, K.; Scharmer, G. B.; Kiselman, D.; Löfdahl, M. G. Bibcode: 2007A&A...464..763L Altcode: Context: The recent discovery of dark-cored penumbral filaments suggests that we are resolving the building blocks of sunspot penumbrae. Their properties are largely unknown but provide important clues to understanding penumbral fine structure.
Aims: Our observations provide new constraints for the different scenarios put forward to explain the structure of sunspot penumbrae.
Methods: We present an analysis of dark-cored penumbral filaments, based on intensity filtergrams (G-band, continuum and Ca II H line wing), magnetograms and Dopplergrams, obtained at heliocentric distances between 15° and 55°.
Results: In general, the visibility of dark cores degrades with increasing heliocentric distance. Based on Ca II H wing images we conclude that this is due to a geometrical 3D-effect and not due to a simple formation height effect. Only in the center-side penumbra are dark-cored filaments visible at all observed heliocentric distances. We observe that dark-cored filaments frequently split in the umbra, forming a Y-shape that disappears after a few minutes, leaving a shortened filamentary structure and a bright dot in the umbra. The dark-cored filaments have life times ≥ 90 min. The dark cores are related to a much weaker and a more horizontal magnetic field than their lateral brightenings. Where the dark-cored filaments appear in the umbra, the magnetic field is inclined by 40° with respect to the solar surface normal for both the dark core and the bright edges. With increasing distance from the umbra, the magnetic field inclination in the dark cores increases rapidly within a few thousand km. Both the magnetic field strength and inclination in the lateral brightenings show very small variations with spot-center radial distance. The velocity field possesses a strong horizontal component within the dark cores. The absolute line-of-sight (LOS) velocity is larger within the dark cores than in their lateral brightenings. The Evershed flow apparently is present primarily in the dark cores. Title: The non-LTE line formation of neutral carbon in late-type stars Authors: Fabbian, D.; Asplund, M.; Carlsson, M.; Kiselman, D. Bibcode: 2006A&A...458..899F Altcode: 2006astro.ph..8284F Aims.We investigate the non-Local Thermodynamic Equilibrium (non-LTE) line formation of neutral carbon in late-type stars in order to remove some of the potential systematic errors in stellar abundance analyses employing C i features.
Methods: .The statistical equilibrium code MULTI was used on a grid of plane-parallel 1D MARCS atmospheric models.
Results: .Within the parameter space explored, the high-excitation C i lines studied are stronger in non-LTE due to the combined effect of line source function drop and increased line opacity due to overpopulation of the lower level for the transitions considered; the relative importance of the two effects depends on the particular combination of T{eff}, log g, [Fe/H] and [C/Fe] and on the analysed C i line. As a consequence, the non-LTE abundance corrections are negative and can be substantially so, for example ∼ -0.4 dex in halo turn-off stars at [Fe/H]∼ -3. The magnitude of the non-LTE corrections is rather insensitive to whether inelastic H collisions are included or not.
Conclusions: .Our results have implications on studies of nucleosynthetic processes and on Galactic chemical evolution models. When applying our calculated corrections to recent observational data, the upturn in [C/O] at low metallicity might still be present (thus apparently still necessitating contributions from massive Pop. III stars for the carbon production), but at a lower level and possibly with a rather shallow trend of ∼ -0.2 dex/dex below [O/H]∼ -1. Title: Inclination of magnetic fields and flows in sunspot penumbrae Authors: Langhans, K.; Scharmer, G. B.; Kiselman, D.; Löfdahl, M. G.; Berger, T. E. Bibcode: 2005A&A...436.1087L Altcode: An observational study of the inclination of magnetic fields and flows in sunspot penumbrae at a spatial resolution of 0.2 arcsec is presented. The analysis is based on longitudinal magnetograms and Dopplergrams obtained with the Swedish 1-m Solar Telescope on La Palma using the Lockheed Solar Optical Universal Polarimeter birefringent filter. Data from two sunspots observed at several heliocentric angles between 12 ° and 39 ° were analyzed. We find that the magnetic field at the level of the formation of the Fe i-line wing (630.25 nm) is in the form of coherent structures that extend radially over nearly the entire penumbra giving the impression of vertical sheet-like structures. The inclination of the field varies up to 45 ° over azimuthal distances close to the resolution limit of the magnetograms. Dark penumbral cores, and their extensions into the outer penumbra, are prominent features associated with the more horizontal component of the magnetic field. The inclination of this dark penumbral component - designated B - increases outwards from approximately 40 ° in the inner penumbra such that the field lines are nearly horizontal or even return to the solar surface already in the middle penumbra. The bright component of filaments - designated A - is associated with the more vertical component of the magnetic field and has an inclination with respect to the normal of about 35 ° in the inner penumbra, increasing to about 60 ° towards the outer boundary. The magnetogram signal is lower in the dark component B regions than in the bright component A regions of the penumbral filaments. The measured rapid azimuthal variation of the magnetogram signal is interpreted as being caused by combined fluctuations of inclination and magnetic field strength. The Dopplergrams show that the velocity field associated with penumbral component B is roughly aligned with the magnetic field while component A flows are more horizontal than the magnetic field. The observations give general support to fluted and uncombed models of the penumbra. The long-lived nature of the dark-cored filaments makes it difficult to interpret these as evidence for convective exchange of flux tubes. Our observations are in broad agreement with the two component model of Bellot Rubi et al. (2003), but do not rule out the embedded flux tube model of Solanki & Montavon (1993). Title: Line formation in solar granulation. IV. [O I], O I and OH lines and the photospheric O abundance Authors: Asplund, M.; Grevesse, N.; Sauval, A. J.; Allende Prieto, C.; Kiselman, D. Bibcode: 2005A&A...435..339A Altcode: A&A, 417, 751-768 (2004), DOI:10.1051/0004-6361:20034328 Title: CI non LTE spectral line formation in late-type stars Authors: Fabbian, D.; Asplund, M.; Carlsson, M.; Kiselman, D. Bibcode: 2005IAUS..228..255F Altcode: 2005astro.ph..8063F We present non-Local Thermodynamic Equilibrium (non-LTE) calculations for neutral carbon spectral line formation, carried out for a grid of model atmospheres covering the range of late-type stars. The results of our detailed calculations suggest that the carbon non-LTE corrections in these stars are higher than usually adopted, remaining substantial even at low metallicity. For the most metal-poor stars in the sample of Akerman et al (2004), the effects are of the order of ΔlogɛC ≃ -0.35ldots-0.45 (when neglecting H collisions). Applying our results to those observations, the apparent [C/O] upturn seen in their LTE analysis is no longer present, thus revealing no need to invoke contributions from Pop. III stars to the carbon nucleosynthesis. Title: Line formation in solar granulation. IV. [O I], O I and OH lines and the photospheric O abundance Authors: Asplund, M.; Grevesse, N.; Sauval, A. J.; Allende Prieto, C.; Kiselman, D. Bibcode: 2004A&A...417..751A Altcode: 2003astro.ph.12290A The solar photospheric oxygen abundance has been determined from [O I], O I, OH vibration-rotation and OH pure rotation lines by means of a realistic time-dependent, 3D, hydrodynamical model of the solar atmosphere. In the case of the O I lines, 3D non-LTE calculations have been performed, revealing significant departures from LTE as a result of photon losses in the lines. We derive a solar oxygen abundance of log ɛO = 8.66 ± 0.05. All oxygen diagnostics yield highly consistent abundances, in sharp contrast with the results of classical 1D model atmospheres. This low value is in good agreement with measurements of the local interstellar medium and nearby B stars. This low abundance is also supported by the excellent correspondence between lines of very different line formation sensitivities, and between the observed and predicted line shapes and center-to-limb variations. Together with the corresponding down-ward revisions of the solar carbon, nitrogen and neon abundances, the resulting significant decrease in solar metal mass fraction to Z = 0.0126 can, however, potentially spoil the impressive agreement between predicted and observed sound speed in the solar interior determined from helioseismology. Title: Penumbral structure at 0.1 arcsec resolution. I. General appearance and power spectra Authors: Rouppe van der Voort, L. H. M.; Löfdahl, M. G.; Kiselman, D.; Scharmer, G. B. Bibcode: 2004A&A...414..717R Altcode: We analyse sunspot filtergrams of unprecedented quality obtained by \citet{scharmer02dark} with the Swedish 1-m Solar Telescope on La Palma. The observations comprise images in three different wavelength bands: 488, 436, and 430 nm (G-band). We find that there are still unresolved penumbral filaments which must have widths smaller than 80 km. The fine structuring along the filaments is limited. Penumbral grains have internal structure and look like they are split or crossed by narrow dark structures. We calculate intensity power spectra of the penumbra from images that are corrected for seeing using the Phase Diversity technique. The effects of high order aberrations that are not corrected for are estimated to be too low to be consistent with a flat power spectrum. The penumbral power spectra do not show any signs of bumps or peaks that could correspond to a preferred scale at 0\farcs35 for the width of penumbral filaments. We argue that the power spectrum is not a very reliable source of information concerning preferred scales. Title: First Results from the Swedish 1-m Solar Telescope Authors: Scharmer, G. B.; Kiselman, D.; Löfdahl, M. G.; Rouppe van der Voort, L. H. M. Bibcode: 2003ASPC..307....3S Altcode: No abstract at ADS Title: The Granulation Fingerprints of Spectral Lines Authors: Kiselman, D.; Asplund, M. Bibcode: 2003IAUS..210P.E62K Altcode: No abstract at ADS Title: Dark cores in sunspot penumbral filaments Authors: Scharmer, Göran B.; Gudiksen, Boris V.; Kiselman, Dan; Löfdahl, Mats G.; Rouppe van der Voort, Luc H. M. Bibcode: 2002Natur.420..151S Altcode: Sunspot umbrae-the dark central regions of the spots-are surrounded by brighter filamentary penumbrae, the existence of which remains largely inexplicable. The penumbral filaments contain magnetic fields with varying inclinations and are associated with flowing gas, but discriminating between theoretical models has been difficult because the structure of the filaments has not hitherto been resolved. Here we report observations of penumbral filaments that reveal dark cores inside them. We cannot determine the nature of these dark cores, but their very existence provides a crucial test for any model of penumbrae. Our images also reveal other very small structures, in line with the view that many of the fundamental physical processes in the solar photosphere occur on scales smaller than 100km. Title: NLTE effects on oxygen lines Authors: Kiselman, Dan Bibcode: 2002HiA....12..429K Altcode: The NLTE effects affecting oxygen-abundance determination of solar-type stars are discussed. LTE is safe for the forbidden lines. The permitted triplet at 777 nm is expected to show NLTE effects so that assuming LTE overestimates the abundance, but the magnitude of the effects is dependent on the poorly known cross sections of collisional excitation by collisions with neutral hydrogen atoms. Little is known about the NLTE effects on molecular line formation. Title: NLTE effects on oxygen lines Authors: Kiselman, Dan Bibcode: 2001NewAR..45..559K Altcode: 2000astro.ph.10300K The NLTE effects affecting oxygen-abundance determinations of solar-type stars are discussed. LTE is perfectly safe for the forbidden lines. The permitted triplet at 777 nm is expected to show NLTE effects on the order of a few tenths of a dex (always in the sense that LTE overestimates the abundance), but the magnitude of the effects is dependent on the still very uncertain cross sections of collisional excitation by collisions with neutral hydrogen atoms. Little is known about the NLTE effects on molecular line formation. Title: Spatially Resolved Solar Lines as Diagnostics of NLTE Effects (CD-ROM Directory: contribs/kiselman) Authors: Kiselman, D.; Asplund, M. Bibcode: 2001ASPC..223..684K Altcode: 2001csss...11..684K No abstract at ADS Title: Departures from LTE in be Line Formation Authors: García Perez, A. E.; Asplund, M.; Kiselman, D. Bibcode: 2001coev.conf..131G Altcode: Stellar abundances of Be can be affected by NLTE effects. We have calculated the NLTE corrections to the LTE abundances obtained from the Bell doublet at 313.0 and 313.1 nm for a range of stellar parameters. These lines are not formed in LTE due to over-ionization and over-excitation but the NLTE corrections are in general small (~0.1 dex). Title: The formation of G-band bright points I: Standard LTE modelling Authors: Kiselman, D.; Rutten, R. J.; Plez, B. Bibcode: 2001IAUS..203..287K Altcode: Assuming LTE, we investigate the formation of the G band in models of quiet solar photosphere and a semiempirical flux-tube model (Briand & Solanki 1995). Preliminary results agree with observations of of G-band bright-point contrast, though this a sensitive function of the amount of scattered light in the observations. Thus LTE line modelling in models constructed under the LTE assumptions seems to fit observations. This does not, however, necessarily imply that LTE is valid here. We also present LTE synthetic spectra of the same models for the full wavelength range from UV to IR. This serves to point out other promising pass bands for the observations of flux-tube structures. Title: Proxy Magnetometry of the Photosphere: Why are G-Band Bright Points so Bright? Authors: Rutten, R. J.; Kiselman, D.; Rouppe van der Voort, L.; Plez, B. Bibcode: 2001ASPC..236..445R Altcode: 2001aspt.conf..445R No abstract at ADS Title: The formation of G-band bright points. I: Standard LTE modelling Authors: Kiselman, D.; Rutten, R. J.; Plez, B. Bibcode: 2000astro.ph.10390K Altcode: Assuming LTE, we synthesise solar G band spectra from the semiempirical flux-tube model of Briand Solanki (1995). The results agree with observed G-band bright-point contrasts within the uncertainty set by the amount of scattered light. We find that it is the weakening of spectral lines within the flux tube that makes the bright-point contrast in the G band exceed the continuum contrast. We also synthesise flux-tube spectra assuming LTE for the full wavelength range from UV to IR, and identify other promising passbands for flux-tube observations. Title: NLTE effects on oxygen lines Authors: Kiselman, Dan Bibcode: 2000IAUJD...8E...5K Altcode: I will review the problems involved in NLTE modelling of lines used for oxygen-abundance determinations of cool stars. Where are the most important uncertainties and what could be done about them? Could a complete understanding of the NLTE effects on OH, OI, and [OI] lines solve the reported abundance discrepancies? Title: Oxygen Line Formation in 3D Hydrodynamical Model Atmospheres Authors: Asplund, M.; Carlsson, M.; Garcia Perez, A. E.; Kiselman, D. Bibcode: 2000IAUJD...8E...8A Altcode: 2000astro.ph.11043A The new generation of realistic 3-dimensional, time-dependent, hydrodynamical model atmospheres have been applied to the line formation of {O}{I}, [{O}{I}] and OH lines. Additionally detailed 3D non-LTE calculations have been performed for {O}{I} in order to study the influence of temperature inhomogeneities on the line formation process. Implications in terms of the evolution of oxygen abundance with metallicity will be discussed, partly based on new VLT/UVES observations of metal-poor stars. Title: Scattered Light from Envelopes around N-type Stars Authors: Gustafsson, Bengt; Eriksson, Kjell; Kiselman, Dan; Olander, Nils; Olofsson, Hans; Schwarz, Hugo E. Bibcode: 2000IAUS..177..409G Altcode: No abstract at ADS Title: Effects of NLTE and Granulation on LiBeB Abundance Determinations Authors: Kiselman, Dan Bibcode: 1999ASPC..171...85K Altcode: 1999astro.ph..2375K; 1999lcrr.conf...85K NLTE effects - the errors caused by assuming local thermodynamic equilibrium (LTE) - on LiBeB abundance determinations for cool stars are discussed. These NLTE effects are significant in many cases for B I and Li I lines. The Be II 313 nm lines are not formed under LTE circumstances, but the NLTE effects on equivalent widths seem to be rather small. Reasons for doing LTE abundance analysis are discussed and confronted with the reasons for doing NLTE abundance analysis. The impact of three-dimensional photospheric inhomogeneity on line formation is discussed - there are not many definite results on this yet, but there may be soon. Title: Boron in Very Metal Poor Stars Authors: Garcia Lopez, Ramon J.; Lambert, David L.; Edvardsson, Bengt; Gustafsson, Bengt; Kiselman, Dan; Rebolo, Rafael Bibcode: 1998ApJ...500..241G Altcode: 1998astro.ph..1167G; 1998astro.ph..1167L We have observed the B I 2497 A line to derive the boron abundances of two very metal-poor stars selected to help in tracing the origin and evolution of this element in the early Galaxy: BD +23 3130 and HD 84937. The observations were conducted using the Goddard High Resolution Spectrograph on board the Hubble Space Telescope. A very detailed abundance analysis via spectral synthesis has been carried out for these two stars, as well as for two other metal-poor objects with published spectra, using both Kurucz and OSMARCS model photospheres, and taking into account consistently the NLTE effects on the line formation. We have also re-assessed all published boron abundances of old disk and halo unevolved stars. Our analysis shows that the combination of high effective temperature (Teff > 6000 K, for which boron is mainly ionized) and low metallicity ([Fe/H]<-1) makes it difficult to obtain accurate estimates of boron abundances from the B I 2497 A line. This is the case of HD 84937 and three other published objects (including two stars with [Fe/H] ~ -3), for which only upper limits can be established. BD +23 3130, with [Fe/H] ~ -2.9 and logN(B)_NLTE=0.05+/-0.30, appears then as the most metal-poor star for which a firm measurement of the boron abundance presently exists. The evolution of the boron abundance with metallicity that emerges from the seven remaining stars with Teff < 6000 K and [Fe/H]<-1, for which beryllium abundances were derived using the same stellar parameters, shows a linear increase with a slope ~ 1. Furthermore, the B/Be ratio found is constant at a value ~ 20 for stars in the range -3<[Fe/H]<-1. These results point to spallation reactions of ambient protons and alpha particles with energetic particles enriched in CNO as the origin of boron and beryllium in halo stars. Title: The 671nm Li I line in solar granulation Authors: Kiselman, Dan Bibcode: 1998A&A...333..732K Altcode: 1998astro.ph..2049K The astrophysically very interesting Li I 671nm line has been observed with high spatial resolution in solar granulation with the intention to diagnose departures from local thermodynamic equilibrium (LTE) in the line formation. The spectral feature is very weak, so this is also a test on the limits of such observations. The observations of the Li I line and other weak lines nearby are compared with the results of synthetic line calculations in three-dimensional granulation simulations. The dependence of line-centre velocities on photospheric continuum brightness is well described by the simulations. The observed equivalent width of the Li I line show an approximately flat dependence on continuum brightness, contrary to the theoretical LTE results. Detailed modelling of the line radiative transfer, with an approximate inclusion of three-dimensional effects, gives a better agreement with observations. The match is not perfect and various interesting reasons for this are considered. However, the possibility of systematic errors caused by the sensitivity of the Li I equivalent width to continuum placement calls for cautiousness in the conclusions. Title: Formation of Li I Lines in Photospheric Granulation Authors: Kiselman, Dan Bibcode: 1997ApJ...489L.107K Altcode: 1997astro.ph..8198K The possibility of significant systematic errors due to the use of one-dimensional homogeneous atmospheres in lithium-abundance determinations of cool stars motivates a study of non-local thermodynamic equilibrium (NLTE) effects on Li I line formation in a three-dimensional solar granulation simulation snapshot. The NLTE effect on the equivalent width of the 671 nm resonance line is small in one-dimensional models or in integrated light from the granulation model. The line-strength variations over the granulation pattern are, however, markedly different in NLTE compared with LTE--observations of this may provide diagnostics to NLTE effects. The effects of horizontal photon exchange found in the granulation model are moderate and due entirely to bound-bound processes; ultraviolet overionization is unimportant. Title: (Erratum) Line-blanketed model atmospheres for R Coronae Borealis stars and hydrogen-deficient carbon stars. Authors: Asplund, M.; Gustafsson, B.; Kiselman, D.; Eriksson, K. Bibcode: 1997A&A...323..286A Altcode: Erratum to Astron. Astrophys. 318, 521-534 (1997) Title: KI emission from envelopes around N-type stars. Spectroscopic observations and interpretations. Authors: Gustafsson, B.; Eriksson, K.; Kiselman, D.; Olander, N.; Olofsson, H. Bibcode: 1997A&A...318..535G Altcode: Circumstellar envelopes around three bright N-type stars, R Scl, X TrA, and V Aql have been detected in emission in resonance lines from KI. This radiation, which is most probably scattered photospheric radiation, was first found spectroscopically, but has later been imaged with coronographic and polarimetric techniques. In the present paper, which is the first in a series, the spectroscopic KI observations are discussed. From the observations of the KI 769.9nm emission we find systemic and expansion velocities in fair agreement with those obtained from the CO millimetre lines. We find a decline of the emission with distance from the star, in rough agreement with the assumption of a constant expansion velocity, mass-loss rate and KI abundance. Our mass loss rate estimates from the KI line observations agree rather well with those obtained from CO (ranging from 1/4 to 1/1 of the CO mass loss), which suggests that a considerable fraction of the potassium stays neutral through the envelope. This puts strong upper limits on the photoionizing chromospheric UV emission from the stars. Some indirect indications that the envelopes have inhomogeneous structures, clumps, are discussed. Title: Line-blanketed model atmospheres for R Coronae Borealis stars and hydrogen-deficient carbon stars. Authors: Asplund, M.; Gustafsson, B.; Kiselman, D.; Eriksson, K. Bibcode: 1997A&A...318..521A Altcode: We have constructed line-blanketed model atmospheres for the hydrogen-deficient and carbon-rich R Coronae Borealis (RCrB) stars, as well as for the similar hydrogen-deficient carbon (HdC) stars and the cool extreme helium (EHe) stars. Improved continuum opacities have been used together with realistic line absorption data for atomic and molecular transitions. The observed dereddened fluxes of R CrB are compared with the calculated model fluxes and found to agree best with a model effective temperature of 6900K, while the infrared flux method gives between 6600 and 6900K, depending on the nature of the flux excess in the J and H bands compared to the model fluxes. The excess may correspond to a recently formed dust cloud close to the star, with a typical temperature around 2000K and a dust mass of ~10^-11^Msun_. The agreement for the ultraviolet flux distribution is also very satisfactory as seen from IUE spectra of RCrB. Theoretical broad band photometry is presented and effective temperatures of RCrB and HdC stars estimated. The constructed models show a significantly steeper temperature gradient compared to previously existing models as a result of the line opacity. Due to the cool surface and high abundance of carbon, molecular bands of e.g. C_2_ and CO are visible in the spectra even at as high effective temperatures as 7000K. Furthermore, the high temperatures encountered at depth explain the observed Hei and CII lines for T_eff_ down to ~7000K. In the inner layers (τ_Ross_ > 3) the models show density inversions related to the ionization zone of helium. For certain low gravity models the luminosity exceeds the local Eddington limit and hence gas pressure inversions occur as well, which could be related to the decline events of RCrB stars. Title: The NLTE formation of neutral-boron lines in cool stars. Authors: Kiselman, D.; Carlsson, M. Bibcode: 1996A&A...311..680K Altcode: 1996astro.ph..1144K We study the formation of BI lines in a grid of cool stellar model atmospheres without the assumption of local thermodynamic equilibrium (LTE). The non-LTE modelling includes the effect of other lines blending with the BI resonance lines. Except for the cases where the BI lines are very strong, the departures from LTE relevant for the resonance lines can be described as an overionisation effect and an optical-pumping effect. This causes the lines to be weaker than in LTE so that an abundance analysis assuming LTE will underestimate stellar boron abundances. We present non-LTE abundance corrections useful to improve on abundances derived from the Bi250nm and 209nm lines under the LTE assumption. Application of the results on literature data indicates that the B/Fe ratio in metal-poor stars is constant. Title: 3D non-LTE line formation in the solar photosphere and the solar oxygen abundance. Authors: Kiselman, D.; Nordlund, A. Bibcode: 1995A&A...302..578K Altcode: 1995astro.ph..5037K We study the formation of OI and OH spectral lines in three-dimensional hydrodynamic models of the solar photosphere. The line source function of the OI 777nm triplet is allowed to depart from local thermodynamic equilibrium (lte), within the two-level-atom approximation. Comparison with results from 1D models show that the 3D models alleviate, but do not remove, the discrepancy between the oxygen abundances reported from non-lte work on the 777nm triplet and from the [OI] 630nm and OH lines. Results for the latter two could imply that the solar oxygen abundance is below 8.8. If this is confirmed, the discrepancy between theory and observation for the 777nm triplet lines might fall within the range of errors in equivalent-width measurements and f-values. The line source function of the 777nm triplet in the 1.5D approximation is shown to differ insignificantly from the full 3D non-lte result. Title: A non-LTE study of Li I lines in AGB stars. Authors: Kiselman, D.; Plez, B. Bibcode: 1995MmSAI..66..429K Altcode: 1994astro.ph.11074K We study the formation of Li lines in a case study of an S-type AGB star in the SMC. AGB stars may be important providers of $^7$Li to the interstellar medium. Abundance analysis of these stars is difficult due to the heavy molecular blanketing, uncertainties in the model atmospheres and possible departures from LTE in the Li atomic level populations. We address here the latter problem and somewhat also that of background molecular line formation. We do not find non-LTE effects big enough to alone rule out the possibility that the reported super-Li-rich stars exist. There are, however, several other complications connected to the formation of the Li lines in these stars. Title: Non-LTE Effects on Be and B Abundance Determinations in Cool Stars Authors: Kiselman, D.; Carlsson, M. Bibcode: 1995lea..conf..372K Altcode: 1994astro.ph..9064K We discuss the nature of non-LTE effects affecting abundance analysis of cool stars. The departures from LTE of importance for the B I lines in solar-type stars are described and some new results are presented. Boron abundances derived under the LTE assumption have significant systematic errors, especially for metal-poor stars. For beryllium, current results suggest that departures from LTE will not affect abundance analysis significantly. Title: The impact of granulation on the line formation Authors: Kiselman, D. Bibcode: 1995IAUS..176P.110K Altcode: No abstract at ADS Title: Boron in the extreme Population II star HD 140283 and the production of light elements in the Early Galaxy Authors: Edvardsson, B.; Gustafsson, B.; Johansson, S. G.; Kiselman, D.; Lambert, D. L.; Nissen, P. E.; Gilmore, G. Bibcode: 1994A&A...290..176E Altcode: 1994astro.ph..3002E Using observations of the 2496.7A Bi line with the HST GHRS at a nominal resolving power of 90,000, we have found the abundance of boron of HD140283 to be logɛ_B_(=12+log(N_B_/N_H_))=0.34+/-0.20. This result is found when a significant non-LTE effect in the formation of the Bi line is taken into account. The resulting N_B_/N_Be_ ratio is about 17 (in the range 9-34), which is in very good agreement with what is expected from spallation by cosmic rays. We conclude that this origin of Be and B in the Early Galaxy is the most probable of recently suggested formation mechanisms. Title: A NLTE study of neutral boron in solar-type stars Authors: Kiselman, D. Bibcode: 1994A&A...286..169K Altcode: 1994astro.ph..1018K The formation of the resonance lines of neutral boron in solar-type stellar atmospheres is investigated taking into account effects of departures from local thermodynamic equilibrium (NLTE effects). The latter are due to a combination of overionisation and optical pumping in resonance lines, both caused by the hot, non-local, ultraviolet radiation fields in the line-forming regions. They lead to an underestimation of the boron abundance when analysis methods assuming local thermodynamic equilibrium (LTE) are used. The abundance correction, for the 249.7 nm resonance line, amounts to +0.56 dex for the metal-poor star HD 140283 and +0.4 dex for Procyon. No significant NLTE effects are predicted for the Sun. Applying the abundance correction on the results for HD 140283 of Duncan et al. (1992) leads to a B/Be ratio well above the minimum value required by spallation production of beryllium. The reliability of the results in view of atomic and atmospheric uncertainties is discussed. With the possible exception of photospheric inhomogeneity, it seems unlikely that these could remove the effect for HD 140283. Title: High-spatial-resolution solar observations of spectral lines used for abundance analysis Authors: Kiselman, D. Bibcode: 1994A&AS..104...23K Altcode: Solar observations of spectral lines that are of importance for abundance analysis have been obtained with high spatial resolution in order to study the effect of granulation on these lines. Data are presented in several plots and tables demonstrating both the detailed behaviour of the lines over the granulation pattern and the statistical properties of the variation of the different line properties. The variation of line equivalent width with continuum intensity is generally linear and looks similar for different kinds of lines. Thus no effects have been found that could signal significant problems for abundance analysis using homogeneous model atmospheres. Title: The 777 NM oxygen triplet in the Sun and solar-type stars, and its use for abundance analysis Authors: Kiselman, D. Bibcode: 1993A&A...275..269K Altcode: The use of the neutral oxygen triplet lines at 777 nm and other atomic lines for abundance analysis of the Sun and solar-type stars is investigated and discussed taking into account the effects on these lines by departures from local thermodynamic equilibrium (NLTE effects). It is found that the discrepant behaviour of the 777 nm triplet in the Sun is not possible to explain within the context of one-dimensional homogeneous model atmospheres. It seems very likely that the inhomogeneous properties of the solar photosphere (granulation) must be taken inteo account to reproduce these lines with the accuracy needed for precision abundance analysis. Experiments with line calculations in a solar granulation simulation snapshot are made, leading to one possible mechanism for the suppression of the NLTE effect that is predicted for the 777 nm triplet lines by one-dimensional model atmospheres. Title: Reliability of classical abundance analysis: boron and oxygen Authors: Kiselman, D. Bibcode: 1993fces.conf...35K Altcode: No abstract at ADS Title: Implications of departures from LTE and homogeneity in the sun and solar-type stars Authors: Kiselman, Dan Bibcode: 1993PhDT.......224K Altcode: No abstract at ADS Title: Implications of departures from LTE and homogeneity in the sun and solar-type stars Authors: Kiselman, Dan Bibcode: 1993idlh.book.....K Altcode: No abstract at ADS Title: Non-LTE effects on oxygen abundance determinations for solar-type stars Authors: Kiselman, Dan Bibcode: 1991A&A...245L...9K Altcode: Non-LTE effects on oxygen abundance determinations for solar-type stars have been investigated using a model atom of 15 levels plus continuum. The computed equivalent widths of the 777 nm triplet lines in the sun are too large compared to observations, the corresponding LTE values seem to give a better agreement. There are, however, several reasons not to believe that the lines are formed in LTE, and the discrepancy could be due to photospheric inhomogeneities. The implications of the non-LTE effects for oxygen lines on the shape of the galactic (O/Fe)-(Fe/H) relation are discussed. Title: A uvby-like photometric system for the WF/PC of the HST. Authors: Kiselman, D.; Oja, T.; Gustafsson, B. Bibcode: 1990A&A...238..265K Altcode: The possibility of combining four filters of the Hubble Space Telescope Wide Field/Planetary Camera into a uvby-like photometric system is discussed. On the basis of earth-based observations with appropriate filters, such a system consisting of WF/PC filters F 368M, F 413M, F 492M, and F 547M was calibrated for the determination of fundamental parameters of F dwarf stars. A good agreement was found between this empirical calibration and a theoretical one by Bell (1988). Good estimates of effective temperatures and metallicities can be obtained with this system, while gravity determinations require a UV filter at shorter wavelengths. Title: Astrophysical processes and structures in the universe. Abstracts. Authors: Kiselman, D.; Lagerkvist, C. -I. Bibcode: 1990UppOR..52.....K Altcode: No abstract at ADS Title: Non-LTE effects on oxygen abundance determinations for solar-type stars. Authors: Kiselman, D. Bibcode: 1990apsu.conf...38K Altcode: No abstract at ADS Title: Nordic-Baltic Astronomy Meeting : proceedings of a meeting held at the Astronomical Observatory of the Uppsala University, June 17-21, 1990, celebrating the 250th anniversary of the Celsius Observatory Authors: Lagerkvist, C. -I.; Kiselman, D.; Lindgren, M. Bibcode: 1990nba..meet.....L Altcode: No abstract at ADS