Dan Kiselman

Dark cores in sunspot penumbral filaments

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 100 km.

Boron in Very Metal-poor Stars*

We have observed the B I λ2497 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 non-LTE (NLTE) effects on the line formation. We have also reassessed 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 λ2497 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 log N(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 of ~1. Furthermore, the B/Be ratio found is constant at a value of ~20 for stars in the range -3 < [Fe/H] < -1. These results point to spallation reactions of ambient protons and α-particles with energetic particles enriched in CNO as the origin of boron and beryllium in halo stars.

Is the solar spectrum latitude-dependent? - An investigation with SST/TRIPPEL

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.

Non-LTE line formation of Fe in late-type stars - IV. Modelling of the solar centre-to-limb variation in 3D

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 m ...

A tilted interference filter in a converging beam

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-Pinterferometers and Lyot filters. At the Swedish 1-meter Solar Telescope, such a setup is used to scan through the blue wing of the Caii 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, di erent 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 di erent tilt angles. Knowing the broadened profile is important for the interpretation of the solar images. Compensating the images for the degrading e ects 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 e ects. We test the hypothesis that they are separable from the e ects 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 di ers significantly from the specifications. We demonstrate how to compensate for the image-degrading e ects. Because the filter tilt e ects 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.

Ca II H sunspot tomography from the photosphere to the chromosphere

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 Teles ...

Stable Umbral Chromospheric Structures

Aims. To understand the morphology of the chromosphere in sunspot umbra. We investigate if the horizontal structures observed in the spectral core of the Caii 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 Caii 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 sun spots, for every flash where the seeing was moderate to good. Discrete dark structures are identified that are sta ble for at least two umbral flashes, as well as systems of struc tures that live for up to 24 minutes. We find cases of extremely extended s tructures with similar stability, with one such structure s howing 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 then there are systems of structures with strong departures from the vertical for all three analysed sunspot s. Conclusions. Long-lived Caii H filamentary horizontal structures are a common and likely e ver-present feature in the umbra of sunspots. If the magnetic field in the chromosphere of the umb ra is indeed aligned with the structures, then the present th eoretical understanding of the typical umbra needs to be revisited.

Solar off-limb emission of the O i 7772 Å line

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 severa ...

Commission 36: Theory of Stellar Atmospheres

The members of the Commission 36 Organizing Committee attending the IAU General Assembly in Rio de Janeiro met for a business session on August 7. Both members from the previous (2006–2009) and the new (2009–2012) Organizing Committee partook in the discussions. Past president John Landstreet described the work he had done over the past three years in terms of supporting proposed conferences on the topic. He has also spent significant amount of time establishing an updated mailing list of all >350 members of the commission, which is unfortunately not provided automatically by the IAU. Such a list is critical for a rapid dissemination of information to the commission members and for a correct and smooth running of elections of IAU officials. Everyone present thanked John effusively for all of his hard work over the past three years to stimulate a high level of activity within the discipline.