Relativistic effects in the solar EOS

We study the sensitivity of the sound speed to relativistic corrections of the equation of state (EOS) in the standard solar model by means of a helioseismic forward analysis. We use the latest GOLF/SOHO data for $\ell = 0,1,2,3$ modes to confirm that the inclusion of the relativistic corrections to the adiabatic exponent $\Gamma_1$ computed from both OPAL and MHD EOS leads to a more reliable theoretical modelling of the innermost layers of the Sun.Comment: 3 pages, 3 figures, aa.cls, to appear on Astronomy and Astrophysic

The star formation history of ultra-faint dwarf galaxies

Se propone realizar un estudio homogéneo de la historia de formación estelar (edad y metalicidad de las estrellas de una galaxia desde el instante de su formación, HFE) y estructura espacial de una selección de galaxias enanas ultra-débiles del Grupo Local: Bootes I, Canes Venatici II y Leo IV a partir de datos fotométricos de banda ancha (V,I) del Telescopio Subaru de 8.2 m (Bootes I) y del telescopio espacial HST (Canes Venatici II y Leo IV en los filtros F606W y F814W). Parte de los datos están ya disponibles (datos de archivo) ampliandose las observaciones en filtros ultravioleta para completar el análisis. Las herramientas principal serán los diagramas color-magnitud (DCM) en combinación con los códigos de obtención de la historia de formación estelar desarrollados por nuestro grupo. Mediante el análisis de la HFE se pretende abordar el efecto de varios procesos físicos en la formación y evolución de las galaxias enanas, tales como la reionización, retroalimentación por supernovas, barrido por presión cinética o interacciones de marea. Comprender el alcance que dichos procesos tienen en la formación de las galaxias enanas es fundamental para conocer cómo se forman galaxias mayores, como la Vía Láctea (V.L)

Satellite observations of reconnection between emerging and pre-existing small-scale magnetic fields

We report multi-wavelength ultraviolet observations taken with the IRIS satellite, concerning the emergence phase in the upper chromosphere and transition region of an emerging flux region (EFR) embedded in the unipolar plage of active region NOAA 12529. The photospheric configuration of the EFR is analyzed in detail benefitting from measurements taken with the spectropolarimeter aboard the Hinode satellite, when the EFR was fully developed. In addition, these data are complemented by full-disk, simultaneous observations of the SDO satellite, relevant to the photosphere and the corona. In the photosphere, magnetic flux emergence signatures are recognized in the fuzzy granulation, with dark alignments between the emerging polarities, cospatial with highly inclined fields. In the upper atmospheric layers, we identify recurrent brightenings that resemble UV bursts, with counterparts in all coronal passbands. These occur at the edges of the EFR and in the region of the arch filament system (AFS) cospatial to the EFR. Jet activity is also found at chromospheric and coronal levels, near the AFS and the observed brightness enhancement sites. The analysis of the IRIS line profiles reveals the heating of dense plasma in the low solar atmosphere and the driving of bi-directional high-velocity flows with speeds up to 100 km/s at the same locations. Furthermore, we detect a correlation between the Doppler velocity and line width of the Si IV 1394 and 1402 \AA{} line profiles in the UV burst pixels and their skewness. Comparing these findings with previous observations and numerical models, we suggest evidence of several long-lasting, small-scale magnetic reconnection episodes between the emerging bipole and the ambient field. This process leads to the cancellation of a pre-existing photospheric flux concentration of the plage with the opposite polarity flux patch of the EFR. [...]Comment: 4 pages, 2 figures, to be published in "Nuovo Cimento C" as proceeding of the Third Meeting of the Italian Solar and Heliospheric Communit

Height dependence of the penumbral fine-scale structure in the inner solar atmosphere

We studied the physical parameters of the penumbra in a large and fully-developed sunspot, one of the largest over the last two solar cycles, by using full-Stokes measurements taken at the photospheric Fe I 617.3 nm and chromospheric Ca II 854.2 nm lines with the Interferometric Bidimensional Spectrometer. Inverting measurements with the NICOLE code, we obtained the three-dimensional structure of the magnetic field in the penumbra from the bottom of the photosphere up to the middle chromosphere. We analyzed the azimuthal and vertical gradient of the magnetic field strength and inclination. Our results provide new insights on the properties of the penumbral magnetic fields in the chromosphere at atmospheric heights unexplored in previous studies. We found signatures of the small-scale spine and intra-spine structure of both the magnetic field strength and inclination at all investigated atmospheric heights. In particular, we report typical peak-to-peak variations of the field strength and inclination of $\approx 300$ G and $\approx 20^{\circ}$, respectively, in the photosphere, and of $\approx 200$ G and $\approx 10^{\circ}$ in the chromosphere. Besides, we estimated the vertical gradient of the magnetic field strength in the studied penumbra: we find a value of $\approx 0.3$ G km$^{-1}$ between the photosphere and the middle chromosphere. Interestingly, the photospheric magnetic field gradient changes sign from negative in the inner to positive in the outer penumbra.Comment: 14 page, 9 figures, accepted for Ap

Recent insights on the penumbra formation process

Using high-resolution spectropolarimetric data acquired by \textit{IBIS}, as well as \textit{SDO}/HMI observations, we studied the penumbra formation in AR NOAA 11490 and in a sample of twelve ARs appeared on the solar disk on 2011 and 2012, which were characterized by $\beta$-type magnetic field configuration. The results show that the onset of the classical Evershed flow occurs in a very short time scale, 1-3 hours. Studying the formation of the first penumbral sector around the following proto-spot, we found that a stable penumbra forms in the area facing the opposite polarity, which appears to be co-spatial with an AFS, i.e. in a flux emergence region, in contrast with the results of \cite{Schlichenmaier2010} concerning the leading polarity of AR NOAA 11490. Conversely, analyzing the sample of twelve ARs, we noticed that there is not a preferred location for the formation of the first penumbral sector. We also observed before the penumbra formation an inverse Evershed flow, which changes its sign when the penumbra appears. This confirms the observational evidence that the appearance of the penumbral filaments is correlated with the transition from the inverse Evershed to the classical Evershed flow. Furthermore, the analysis suggests that the time needed to form the penumbra may be related to the location where the penumbra first appears. New high-resolution observations, like those that will be provided by the European Solar Telescope, are expected to increase our understanding of the penumbra formation process.Comment: 3 pages, 2 figures, to be published in "Nuovo Cimento C" as proceeding of the Third Meeting of the Italian Solar and Heliospheric Communit