Uncertainties in the solar photospheric oxygen abundance

The purpose of this work is to better understand the confidence limits of the photospheric solar oxygen abundance derived from three-dimensional models using the forbidden [OI] line at 6300 \AA , including correlations with other parameters involved. We worked with a three-dimensional empirical model and two solar intensity atlases. We employed Bayesian inference as a tool to determine the most probable value for the solar oxygen abundance given the model chosen. We considered a number of error sources, such as uncertainties in the continuum derivation, in the wavelength calibration and in the abundance/strength of Ni. Our results shows correlations between the effects of several parameters employed in the derivation. The Bayesian analysis provides robust confidence limits taking into account all of these factors in a rigorous manner. We obtain that, given the empirical three-dimensional model and the atlas observations employed here, the most probable value for the solar oxygen abundance is $\log(\epsilon_O) = 8.86\pm0.04$. However, we note that this uncertainty does not consider possible sources of systematic errors due to the model choice.Comment: Accepted for publication in Astronomy and Astrophysic

Milne-Eddington inversions of the He I 10830 {\AA} Stokes profiles: Influence of the Paschen-Back effect

The Paschen-Back effect influences the Zeeman sublevels of the He I multiplet at 10830 {\AA}, leading to changes in strength and in position of the Zeeman components of these lines. We illustrate the relevance of this effect using synthetic Stokes profiles of the He I 10830 {\AA} multiplet lines and investigate its influence on the inversion of polarimetric data. We invert data obtained with the Tenerife Infrared Polarimeter (TIP) at the German Vacuum Tower Telescope (VTT). We compare the results of inversions based on synthetic profiles calculated with and without the Paschen-Back effect being included. We find that when taking into account the incomplete Paschen-Back effect, on average 16% higher field strength values are obtained. We also show that this effect is not the main cause for the area asymmetry exhibited by many He I 10830 Stokes V-profiles. This points to the importance of velocity and magnetic field gradients over the formation height range of these lines.Comment: Accepted for publication in A&A on Jun 12th 200

Clasificación de PAEV aditivos de una etapa con cantidades discretas relativas

Este estudio presenta una organización exhaustiva y aporta una nueva clasificación de las situaciones y problemas, basada en las cantidades, medidas y números enteros, del dominio de aplicación del campo conceptual aditivo de las magnitudes discretas

Bayesian Inversion of Stokes Profiles

[abridged] Inversion techniques are the most powerful methods to obtain information about the thermodynamical and magnetic properties of solar and stellar atmospheres. In the last years, we have witnessed the development of highly sophisticated inversion codes that are now widely applied to spectro-polarimetric observations. The majority of these inversion codes are based on the optimization of a complicated non-linear merit function. However, no reliable and statistically well-defined confidence intervals can be obtained for the parameters inferred from the inversions. A correct estimation of the confidence intervals for all the parameters that describe the model is mandatory. Additionally, it is fundamental to apply efficient techniques to assess the ability of models to reproduce the observations and to what extent the models have to be refined or can be simplified. Bayesian techniques are applied to analyze the performance of the model to fit a given observed Stokes vector. The posterior distribution, is efficiently sampled using a Markov Chain Monte Carlo method. For simplicity, we focus on the Milne-Eddington approximate solution of the radiative transfer equation and we only take into account the generation of polarization through the Zeeman effect. However, the method is extremely general and other more complex forward models can be applied. We illustrate the ability of the method with the aid of academic and realistic examples. We show that the information provided by the posterior distribution turns out to be fundamental to understand and determine the amount of information available in the Stokes profiles in these particular cases.Comment: 15 pages, 12 figures, accepted for publication in A&

SPINOR: Visible and Infrared Spectro-Polarimetry at the National Solar Observatory

SPINOR is a new spectro-polarimeter that will serve as a facility instrument for the Dunn Solar Telescope at the National Solar Observatory. This instrument is capable of achromatic polarimetry over a very broad range of wavelengths, from 430 up to 1600 nm, allowing for the simultaneous observation of several visible and infrared spectral regions with full Stokes polarimetry. Another key feature of the design is its flexibility to observe virtually any combination of spectral lines, limited only by practical considerations (e.g., the number of detectors available, space on the optical bench, etc).Comment: To appear in Solar Physics. Note: Figures are low resolution versions due to file size limitation

Spectropolarimetric observations of the Ca II 8498 A and 8542 A lines in the quiet Sun

The Ca II infrared triplet is one of the few magnetically sensitive chromospheric lines available for ground-based observations. We present spectropolarimetric observations of the 8498 A and 8542 A lines in a quiet Sun region near a decaying active region and compare the results with a simulation of the lines in a high plasma-beta regime. Cluster analysis of Stokes V profile pairs shows that the two lines, despite arguably being formed fairly close, often do not have similar shapes. In the network, the local magnetic topology is more important in determining the shapes of the Stokes V profiles than the phase of the wave, contrary to what our simulations show. We also find that Stokes V asymmetries are very common in the network, and the histograms of the observed amplitude and area asymmetries differ significantly from the simulation. Both the network and internetwork show oscillatory behavior in the Ca II lines. It is stronger in the network, where shocking waves, similar to those in the high-beta simulation, are seen and large self-reversals in the intensity profiles are common.Comment: 23 pages, 17 figures, accepted to ApJ some figures are low-res, for high-res email [email protected]

A Synthetic Stellar Polarization Atlas from 400 to 1000 nm

% context heading (optional) {With the development of new polarimeters for large telescopes, the spectro-polarimetric study of astrophysical bodies is becoming feasible and, indeed, more frequent. In particular, this is permitting the observational study of stellar magnetic fields} % aims heading (mandatory) {With the aim to optimize and interpret this kind of observations, we have produced a spectral atlas of circular polarization in a grid of stellar atmospheric models with effective temperatures between 3500 and 10000 K, surface gravities $\log(g)=3.5-5$, metallicities between 10$^{-2}$ and 1, and magnetic field strengths of 100, 1000 and 5000~G} % methods heading (mandatory) {We have computed the emergent Stokes $I$ and $V$ flux spectra in LTE of more than 10$^5$ spectral lines} % results heading (mandatory) {The atlas and several numerical tools are available in electronic format and may be downloaded from http://download.hao.ucar.edu/pub/PSA/. In this paper we review and discuss some of its most relevant features, such as which spectral regions and individual lines harbor the strongest signals, what are interesting lines to observe, how to disentangle field strength from filling factor, etc.}Comment: To appear in A&

Resolving the Azimuthal Ambiguity in Vector Magnetogram Data with the Divergence-Free Condition: Application to Discrete Data

We investigate how the divergence-free property of magnetic fields can be exploited to resolve the azimuthal ambiguity present in solar vector magnetogram data, by using line-of-sight and horizontal heliographic derivative information as approximated from discrete measurements. Using synthetic data we test several methods that each make different assumptions about how the divergence-free property can be used to resolve the ambiguity. We find that the most robust algorithm involves the minimisation of the absolute value of the divergence summed over the entire field of view. Away from disk centre this method requires the sign and magnitude of the line-of-sight derivatives of all three components of the magnetic field vector.Comment: Solar Physics, in press, 20 pages, 11 figure