M.S.Xiang, X.F.Zhang,T.D.Li,S.L.Bi,X.W.Liu,Y.Huang,J.N.Fu,K.Liu,Z.J.Tian,Z.S.Ge,J.H.Zhang

Main sequence turn-off (MSTO) stars are advantaged indicators of Galactic evo- lution since their ages could be robustly estimated by atmospheric parameters. Hundreds of thousands of MSTO stars have been selected from the LAMOST Galactic survey to study the evolution of the Galaxy, and it will be benefited from accurate estimates of stellar param- eters. In this work, we select 150 MSTO candidates in the LAMOST MSTO stars sample and constrain their stellar parameters with theoretical models. Asteroseismic properties ∆ν, νmaxobtained from Kepler data are considered for accurate estimates. And furthermore, we examine the contamination rate of the LAMOST MSTO stars sample and validate the age estimation by the LAMOST spectra in Xiang et al. (2015). The results show that 79 of the candidates are MSTO stars and others are main-sequence stars or sub giants and a comparison between ages from this work and what have been obtained through interpolation method in Xiang et al. (2015) shows that there is a mean difference of 0.53 Gyr (7%) and a dispersion of 2.71 Gyr (28%). A considerable fraction of oldest stars in the LAMOST MSTO stars sample are likely contamination stars from much younger dwarf/subgiant stars. The main cause for high contamination rates are likely relatively large systematic bias of surface gravity and may not be ignored in the study of Galactic structure.

Emmanuel Hecht, Markus Roth

In current approaches to time-distance helioseismology, the line-of-sight projection effect on the traveltimes is not fully taken into account. Furthermore, filtering of full-disc data induces leakage due to the projection onto the CCD, which has so far not been accounted for. We develop a theoretical approach to consider these effects when computing sensitivity functions. As the formulas obtained do not seem to give results for spherical Born approximation sensitivity functions in a reasonable computation time, we develop tests to estimate the strength of the effects.

Markus Roth, Wolfgang Zima, Aaron C. Birch, Laurent Gizon

We extend an existing Born approximation model for calculating the linear sensitivity of helioseismic travel-times to flows from Cartesian to spherical geometry. This development is necessary to use the Born approximation for inferring large-scale flows in the deep solar interior. Two consistency tests show that results for our sensitivity kernels agree with reference values to within a few percent. Consequently, we evaluate the impact of different data analysis filters on the kernels for a meridional travel-distance of 42 degrees. When mainly low-degree modes are used (roughly l < 70), the sensitivity is concentrated in deeper regions and it visually best resembles a ray-path like structure, otherwise the sensitivity is concentrated near the surface. Among the different low-degree filters used, we find the phase-speed filtered kernel to be best localized at depth.

Sarbani Basu

We create isochrones of M67 using the Yale Rotating Stellar Evolution Code. In addition to metallicity, parameters that are traditionally held fixed, such as the mixing length parameter and initial helium abundance, also vary. The amount of convective overshoot is also changed in different sets of isochrones. Models are constructed both with and without diffusion. From the resulting isochrones that fit the cluster, the age range is between 3.6 and 4.8 Gyr and the distance is between 755 and 868 pc. We also confirm Michaud et al. (2004) claim that M67 can be fit without overshoot if diffusion is included.

Paulo Garcia

Wave front sensing for solar telescopes is commonly implemented with Shack-Hartmann sensors. The Shack-Hartmann lenslet sub-aperture solar image shifts/slopes are usually estimated with correlation algorithms. The sub-pixel precision image shifts are computed by applying a peak-finding algorithm to the correlation peak. Usually, the measured image displacements consist of systematic errors due to pixel locking effects, because correlation matching is limited only to an integer pixel grid. The amplitude of the systematic error depends on the combination of the correlation algorithm chosen to compute the correlation peak with the type of peak-finding algorithm chosen. To study the systematic errors in detail, solar sub-aperture synthetic images are constructed by using a Swedis Solar Telescope solar granulation image. The performance of different cross-correlation peak finding algorithms is investigated. The algorithms are: parabola; quadratic polynomial; threshold center of gravity; Gaussian and Pyramid. It is found that pyramid fit is the most robust to pixel locking effects. The RMS error analysis study reveals that threshold centre of gravity behaves better in low SNR although systematic errors in the measurement are large. It is found that no peak finding model is good enough in attenuating both systematic errors and RMS error. A new method is proposed to overcome the above limitations. It works in two steps. In the first, the cross-correlation is executed at the original image spatial resolution grid (1 pixel). In the second, the cross-correlation is performed with a sub-pixel level grid and by confining the field of view to 4 x 4 pixels centered at the first step delivered initial position. The generation of these sub-pixel grid based search windows from the spatially discrete target image is achieved with bi-linear interpolation. This method is called as cross-correlation executed at continuous grid (CCC). This technique was previously reported in electronic speckle photography. This technique is applied to wave front sensing. The combination of coarse level grid search executed in large field followed by quasi-continuous grid search executed in a small field enables one to achieve high accuracy wave front estimation by reducing the systematic errors with a low computational cost. The results show that the proposed method outperforms all the approaches in the first study. It improves the wave front estimation accuracy to a factor of 5 in terms of both systematic error and RMS error (75% systematic error reduction, for 0.2 pixel sub-sampling grid), at the expense of twice the computational cost. The CCC method is strongly recommended for wave front sensing in solar telescopes, particularly in open loop adaptive optics, for measuring large the dynamic shifts.

T. Zic, J. Calogovic, M. Dumbovic

We present a new space weather online forecast-tool for predicting the arrival of Interplanetary Coronal Mass Ejections (ICMEs). The forecast-tool is based on the “Drag-Based Model” (DBM), developed in the frame of the European Commission FP7 Project SOTERIA (SOlar-TERrestrial Investigations and Archives) and advanced within FP7 Project COMESEP (Coronal Mass Ejections and Solar Energetic Particles) and the Croatian Scientific Foundation Project SOLSTEL (Solar and Stellar Variability). The DBM is based on a hypothesis that the driving Lorentz force that launches CME ceases in the upper corona, and that beyond certain distance the dynamics becomes governed solely by the interaction of the ICME and the ambient solar wind. This assumption is founded on the fact that in the interplanetary space fast ICMEs decelerate, whereas slow ones accelerate, showing a tendency to adjust their velocity with the ambient solar wind. In particular, we consider the option where the drag acceleration has the quadratic dependence on the ICME relative speed, which is expected in the collisionless environment, where the drag is caused primarily by emission of MHD waves. This is the simplest version of DBM, where the equation of motion can be solved analytically, providing explicit solutions for the Sun-Earth ICME transit time and the impact speed. DBM offers easy handling and straightforward application in the real-time space-weather forecasting. DBM results are compared with remotely-measured interplanetary kinematics of several ICMEs, whereas forecasting abilities are validated on the statistical basis by employing in situ measurements. Finally, the advantages and drawbacks of DBM are summarized.

Markus Roth, Frank Hill, Michael Thompson

SPRING is an evolving concept for next generation solar synoptic observations network. It is envisaged that the new network will cater to the needs of (i) Helioseismology community, by providing improved resolution Doppler observations at multiple heights in solar atmosphere, (ii) Space weather research community, by providing full disk vector magnetograms at a cadence of few minutes and in multiple heights in the solar atmosphere, and (iii) Large solar telescopes, such as DKIST and EAST, by providing high resolution fulldisk context imaging in multiple wavelengths. We will present the conceptual designs currently being explored for SPRING.

Markus Roth, Jason Jackiewicz

We present the current status of an undergoing validation of a recently developed model for computing spherical Born approximation sensitivity functions for flows. In a first step, power spectra and reference cross-correlations from the model and a simulation of Hartlep et al. (2013) are matched. Some difficulties in obtaining such a match are discussed. In a second step, travel times from the forward model and from the simulation, which includes a standard meridional flow profile, are to be compared. The analysis procedure including the use of phase-speed filters is identical to the one employed in Jackiewicz et al. (2015). Furthermore, we present a novel approach for a fast computation of integrated sensitivity functions which can be used for interpreting rotationally symmetric flows such as differential rotation and meridional flow.

Ariane Schad, Wiebke Herzberg, and Markus Roth

Solar-like oscillations exhibit a regular pattern of frequencies. This pattern is dominated by the small and large frequency separations between modes. The accurate determination of these parameters is of great interest, because they give information about e.g. the evolutionary state and the mass of a star. Here, we present a robust method to determine the large and small frequency separations for time series with low signal-to-noise ratio. For this purpose, we analyse a time series of the star KIC 5184732 from the NASA Kepler satellite by employing a combination of Fourier and Hilbert transform. We use the analytic signal of the filtered time series to compute the signal envelope. Spectral analysis of the signal envelope then reveals frequency differences of dominant modes in the periodogram of the stellar time series. With this method, the large frequency separation $\Delta\nu$ can be extracted from the envelope spectrum even for data of poor signal-to-noise ratio. A modification of the method allows for an overview of regularities in a periodogram.

Alberto Escobar, Jesús Burgos, Manuel Collados

As a part of the SOLARNET project, the Transnational Access and Service Programme supports the access of the European solar physics community to some of the best European telescopes. To enhance the efficiency of data usage, external observers will receive also support for postfactum reduction of data, while standard pipelines are not fully developed, with the aim of providing them science-ready data. A successful Programme, which will bring together researchers of different nationalities, forms the basis for a long-term perspective of solar physics in Europe and for the operation of the European Solar Telescope, when it become a reality.

S. Rappaport, B. Kalomeni, and T. Borkovits

There were some 3000 binaries discovered in the Kepler main mission, and there is a growing collection of binaries that have been found to date in the 2-wheel extension of the Kepler mission, called ‘K2’. Among this impressive collection of mostly eclipsing binaries, some 220 triple stars have been detected, mostly through eclipse timing variations, but some via 3rd-body eclipses. In addition to the large sample of triple-star systems, a number of higher-order multiple star systems have also been discovered using Kepler data plus follow-up ground-based observations, like the quadruple systems KIC 4247791 (Lehmann et al. 2012) and KIC 7177553 (Lehmann et al. 2016), or the quintuple system KIC 4150611 (Shibahashi & Kurtz 2012, Prsa et al. 2016). We report on the first quintuple star system found in the K2 fields, and one of the few that contain two eclipsing and spectroscopic binaries.

Georgia Tsiropoula, Kostas Tziotziou

We report preliminary results on the evolution of a small-scale bipolar magnetic feature, from its emergence at the photosphere to its brightening at the corona. We use imaging and spectral observations from the space-born Hinode (SOT/BFI, SOT/SP, EIS & XRT), TRACE (1550 Å, 1600 Å, 1700 Å) and SoHO (MDI hi-res) as well as the ground-based Dutch Open Telescope (G-band, CaII H and five positions along the Hα profile). The small-scale feature emerges, adjacent to the chromospheric network and shows all morphological characteristics of a small-scale magnetic bubble. The magnetic flux density increases, reaching a maximum value while fine-scale CaII H brightenings coalesce forming clusters of positive and negative polarity footpoints of a bipolar feature. The corresponding emerging magnetic flux tubes make their way to the chromosphere, pushing aside the ambient magnetic field, producing Doppler-shifted absorption features. At the upper chromosphere and transition region, imaged by EIS, the emission gradually increases. The connectivity of the quiet-Sun network gradually changes and part of the existing network connects to the newly emerged bipole. A few minutes after the bipole has reached its maximum magnetic flux density, the bipole brightens in soft X-rays forming a coronal bright point. The brightening is observed in all EIS transition region and coronal windows and is accompanied by Doppler-shifted Hα features.