Change search
Refine search result
1 - 32 of 32
CiteExportLink to result list
Permanent link
Cite
Citation style
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf
Rows per page
  • 5
  • 10
  • 20
  • 50
  • 100
  • 250
Sort
  • Standard (Relevance)
  • Author A-Ö
  • Author Ö-A
  • Title A-Ö
  • Title Ö-A
  • Publication type A-Ö
  • Publication type Ö-A
  • Issued (Oldest first)
  • Issued (Newest first)
  • Created (Oldest first)
  • Created (Newest first)
  • Last updated (Oldest first)
  • Last updated (Newest first)
  • Disputation date (earliest first)
  • Disputation date (latest first)
  • Standard (Relevance)
  • Author A-Ö
  • Author Ö-A
  • Title A-Ö
  • Title Ö-A
  • Publication type A-Ö
  • Publication type Ö-A
  • Issued (Oldest first)
  • Issued (Newest first)
  • Created (Oldest first)
  • Created (Newest first)
  • Last updated (Oldest first)
  • Last updated (Newest first)
  • Disputation date (earliest first)
  • Disputation date (latest first)
Select
The maximal number of hits you can export is 250. When you want to export more records please use the Create feeds function.
  • 1.
    de la Cruz Rodríguez, Jaime
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Kiselman, Dan
    Stockholm University, Faculty of Science, Department of Astronomy.
    Carlsson, Mats
    University of Oslo.
    Solar velocity references from 3D HD photospheric modelsArticle in journal (Refereed)
  • 2. Fabbian, D.
    et al.
    Asplund, M.
    Barklem, P. S.
    Carlsson, M.
    Kiselman, D.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Neutral oxygen spectral line formation revisited with new collisional data: large departures from LTE at low metallicity2009In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, ISSN ISSN 0004-6361, Vol. 500, p. 1221-1238Article in journal (Refereed)
    Abstract [en]

    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.

  • 3. Fabbian, D.
    et al.
    Asplund, M.
    Carlsson, M.
    Kiselman, Dan
    Stockholm University, Faculty of Science, Department of Astronomy. Institutet för solfysik.
    The non-LTE line formation of neutral carbon in late-type stars2006In: Astronomy & Astrophysics, ISSN 0004-6361, Vol. 458, no 3, p. 899-914Article in journal (Refereed)
    Abstract [en]

    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.

  • 4.
    Henriques, V. M. J.
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Kiselman, D.
    Temperature stratification in the Sun's photosphere in high horizontal resolution using Ca II H filtergrams.2009In: Memorie della Societa Astronomica Italiana, Vol. 80, p. 639-Article in journal (Other academic)
    Abstract [en]

    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.

  • 5. Henriques, V. M. J.
    et al.
    Scullion, E.
    Mathioudakis, M.
    Kiselman, Dan
    Stockholm University, Faculty of Science, Department of Astronomy.
    Gallagher, P. T.
    Keenan, F. P.
    Stable umbra chromospheric structures2015In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 574, article id A131Article in journal (Refereed)
    Abstract [en]

    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. Filtergrarns in the core of the Call 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 rind 0215 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 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 Cart 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.

  • 6.
    Henriques, Vasco M. J.
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy. Albanova Univ Ctr, Royal Swedish Acad Sci, Inst Solar Phys.
    Kiselman, Dan
    Stockholm University, Faculty of Science, Department of Astronomy. Albanova Univ Ctr, Royal Swedish Acad Sci, Inst Solar Phys.
    Ca II H sunspot tomography from the photosphere to the chromosphere2013In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 557, p. A5-Article in journal (Refereed)
    Abstract [en]

    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.

  • 7.
    Henriques, Vasco
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Kiselman, Dan
    Stockholm University, Faculty of Science, Department of Astronomy.
    van der Noort, Michiel
    Stockholm University, Faculty of Science, Department of Astronomy.
    Temperature structure from Ca II H wing inversions2008Conference paper (Refereed)
  • 8.
    Henriques, Vasco
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Kiselman, Dan
    Stockholm University, Faculty of Science, Department of Astronomy.
    van Noort, Michiel
    Stockholm University, Faculty of Science, Department of Astronomy.
    Temperature structure from Ca II H wing inversions2008Conference paper (Refereed)
  • 9.
    Kiselman, D.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Solar 3D models versus observations—a few comments2008In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. 133, p. 4016-Article in journal (Other academic)
    Abstract [en]

    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.

  • 10.
    Kiselman, Dan
    Stockholm University, Faculty of Science, Department of Astronomy.
    Extremt rymdväder2018In: Fysikaktuellt, ISSN 0283-9148, no 1, p. 24-26Article in journal (Other (popular science, discussion, etc.))
  • 11.
    Kiselman, Dan
    Stockholm University, Faculty of Science, Department of Astronomy. KVA:s institut för solfysik.
    Solar 3D models vs. observations - a few comments2008In: Physica Scripta, Vol. T133, p. 014016-Article in journal (Other academic)
    Abstract [en]

    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.

  • 12.
    Kiselman, Dan
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Lundqvist, Sophia
    Utsikt från jorden 19: Det bara händer2007In: Populär astronomi, no 2, p. 39-Article in journal (Other (popular science, discussion, etc.))
  • 13.
    Kiselman, Dan
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Lundqvist, Sophia
    Utsikt från jorden 20: Longitud2007In: Populär astronomi, no 3, p. 46-Article in journal (Other (popular science, discussion, etc.))
  • 14.
    Kiselman, Dan
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Lundqvist, Sophia
    Utsikt från jorden 21: Lik en diamant i skyn2007In: Populär astronomi, no 4, p. 50-Article in journal (Other (popular science, discussion, etc.))
  • 15.
    Kiselman, Dan
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy. KVA:s institut för solfysik.
    Lundqvist, Sophia
    Utsikt från jorden 22: När natt blir dag2008In: Populär astronomi, Vol. 9, no 1, p. 1-Article in journal (Other (popular science, discussion, etc.))
  • 16.
    Kiselman, Dan
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy. KVA:s institut för solfysik.
    Lundqvist, Sophia
    Utsikt från jorden 23: Allting har en ände2008In: Populär astronomi, Vol. 9, no 2, p. 46-Article in journal (Other (popular science, discussion, etc.))
  • 17.
    Kiselman, Dan
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Löfdahl, Mats
    Stockholm University, Faculty of Science, Department of Astronomy.
    Scharmer, Göran
    Stockholm University, Faculty of Science, Department of Astronomy.
    EST: the European Solar Telescope2007Conference paper (Other (popular science, discussion, etc.))
  • 18.
    Kiselman, Dan
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Löfdahl, Mats
    Stockholm University, Faculty of Science, Department of Astronomy.
    Scharmer, Göran
    Stockholm University, Faculty of Science, Department of Astronomy.
    The Swedish 1-m Solar Telescope2007Conference paper (Other (popular science, discussion, etc.))
    Abstract [en]

    Poster, Astronomdagar i Kiruna

  • 19.
    Kiselman, Dan
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Pereira, Tiago M. D.
    Gustafsson, Bengt
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Asplund, Martin
    Meléndez, Jorge
    Langhans, Kai
    Stockholm University, Faculty of Science, Department of Astronomy.
    Is the solar spectrum latitude-dependent? An investigation with SST/TRIPPEL2011In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 535, p. A14-Article in journal (Refereed)
    Abstract [en]

    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.

  • 20.
    Langhans, Kai
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Scharmer, Göran
    Stockholm University, Faculty of Science, Department of Astronomy.
    Kiselman, Dan
    Stockholm University, Faculty of Science, Department of Astronomy.
    Löfdahl, Mats
    Stockholm University, Faculty of Science, Department of Astronomy.
    Observations of dark-cored filaments in sunspot penumbrae2007In: Astronomy & Astrophysics, Vol. 464, p. 763-774Article in journal (Refereed)
  • 21. Lind, K.
    et al.
    Amarsi, A. M.
    Asplund, M.
    Barklem, P. S.
    Bautista, M.
    Bergemann, M.
    Collet, R.
    Kiselman, Dan
    Stockholm University, Faculty of Science, Department of Astronomy.
    Leenaarts, Jorrit
    Stockholm University, Faculty of Science, Department of Astronomy.
    Pereira, T. M. D.
    Non-LTE line formation of Fe in late-type stars - IV. Modelling of the solar centre-to-limb variation in 3D2017In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 468, no 4, p. 4311-4322Article in journal (Refereed)
    Abstract [en]

    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(epsilon(Fe)) = 7.48 +/- 0.04 dex.

  • 22.
    Löfdahl, Mats G.
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Henriques, Vasco M. J.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Kiselman, Dan
    Stockholm University, Faculty of Science, Department of Astronomy.
    A tilted interference filter in a converging beam2011In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 533Article in journal (Refereed)
    Abstract [en]

    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-Perot 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.

  • 23.
    M. J. Henriques, Vasco
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Kiselman, Dan
    Stockholm University, Faculty of Science, Department of Astronomy.
    Ca II H sunspot tomography from the photosphere to the chromosphereManuscript (preprint) (Other academic)
  • 24.
    Pazira, Hiva
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Kiselman, Dan
    Stockholm University, Faculty of Science, Department of Astronomy.
    Leenaarts, Jorrit
    Stockholm University, Faculty of Science, Department of Astronomy.
    Solar off-limb emission of the O I 7772 angstrom line2017In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 604, article id A49Article in journal (Refereed)
    Abstract [en]

    Aims. The aim of this paper is to understand the formation of the O I line at 7772 angstrom in the solar chromosphere. Methods. We used SST/CRISP observations to observe O I 7772 angstrom 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 angstrom transition is populated by a downward cascade from the continuum. We study the effect of Lyman-beta 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.

  • 25. Pereira, T. M. D.
    et al.
    Asplund, M.
    Kiselman, D.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Oxygen lines in solar granulation.: II. Centre-to-limb variation, NLTE line formation, blends, and the solar oxygen abundance2009In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 508, p. 1403-1416Article in journal (Refereed)
    Abstract [en]

    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

  • 26. Pereira, T. M. D.
    et al.
    Asplund, M.
    Kiselman, D.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Testing 3D solar models against observations .: Center-to-limb variations of oxygen lines, spatially-resolved line formation and probing for departures from LTE2009In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, ISSN ISSN 0004-6361, Vol. 80, p. 650-Article in journal (Refereed)
    Abstract [en]

    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.

  • 27. Pereira, T. M. D.
    et al.
    Kiselman, D.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Asplund, M.
    Oxygen lines in solar granulation.: I. Testing 3D models against new observations with high spatial and spectral resolution2009In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 507, p. 417-432Article in journal (Refereed)
    Abstract [en]

    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

  • 28.
    Scharmer, Göran B.
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Henriques, Vasco M. J.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Kiselman, Dan
    Stockholm University, Faculty of Science, Department of Astronomy.
    de la Cruz Rodríguez, J.
    Detection of Convective Downflows in a Sunspot Penumbra2011In: Science, ISSN 0036-8075, E-ISSN 1095-9203, Vol. 333, no 6040, p. 316-319Article in journal (Refereed)
    Abstract [en]

    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.

  • 29.
    Scharmer, Göran B.
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Narayan, Gautam
    Stockholm University, Faculty of Science, Department of Astronomy.
    Hillberg, Tomas
    Stockholm University, Faculty of Science, Department of Astronomy.
    de la Cruz Rodriguez, Jaime
    Stockholm University, Faculty of Science, Department of Astronomy.
    Löfdahl, Mats G.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Kiselman, Dan
    Stockholm University, Faculty of Science, Department of Astronomy.
    Sütterlin, Peter
    Stockholm University, Faculty of Science, Department of Astronomy.
    van Noort, Michiel
    Stockholm University, Faculty of Science, Department of Astronomy.
    Lagg, Andreas
    CRISP spectropolarimetric imaging of penumbral fine structure2008In: Astrophysical Journal Letters, ISSN 2041-8205, Vol. 689, no 1, p. L69-Article in journal (Refereed)
    Abstract [en]

    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.

  • 30.
    Scharmer, Göran
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Henriques, Vasco
    Stockholm University, Faculty of Science, Department of Astronomy.
    Hillberg, Tomas
    Stockholm University, Faculty of Science, Department of Astronomy.
    Kiselman, Dan
    Stockholm University, Faculty of Science, Department of Astronomy.
    Löfdahl, Mats
    Stockholm University, Faculty of Science, Department of Astronomy.
    Narayan, Gautam
    Stockholm University, Faculty of Science, Department of Astronomy.
    Sütterlin, Peter
    van Noort, Michiel
    Stockholm University, Faculty of Science, Department of Astronomy.
    de la Cruz Rodriguez, Jaime
    Stockholm University, Faculty of Science, Department of Astronomy.
    Spectropolarimetry of Sunspots at 0.16 ARCSEC resolution2008In: 12th European Solar Physics Meeting, 2008Conference paper (Other academic)
  • 31.
    Scharmer, Göran
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Langhans, Kai
    Stockholm University, Faculty of Science, Department of Astronomy.
    Kiselman, Dan
    Stockholm University, Faculty of Science, Department of Astronomy.
    Löfdahl, Mats
    Stockholm University, Faculty of Science, Department of Astronomy.
    Recent High Resolution Observations and Interpretations of Sunspot Fine Structure2007In: New Solar Physics with Solar-B Mission, 2007, p. 71-Conference paper (Other academic)
  • 32. Spite, M.
    et al.
    Kiselman, Dan
    Stockholm University.
    Commission 36: Theory of Stellar Atmospheres2007In: Transactions of the International Astronomical Union, Series A, 2007, p. 215-219Conference paper (Other academic)
1 - 32 of 32
CiteExportLink to result list
Permanent link
Cite
Citation style
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf