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Astrophysics and Astrononmy
Validating Spherical Born Kernels for Meridional Flows
Natural Sciences (Astrophysics and Astrononmy)
Date of upload:
07.09.2015
Co-author:
Markus Roth, Jason Jackiewicz
Abstract:
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.
Solarnet Transnational Access and Service Programme 2013 - 2017
Natural Sciences (Astrophysics and Astrononmy)
SOLARNET - High Resolution Solar Physics Network
Date of upload:
27.08.2015
Co-author:
Alberto Escobar, Jesús Burgos, Manuel Collados
Abstract:
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.
Non-LTE inversion of spectropolarimetric and spectroscopic observations of a small active-region filament observed at the VTT
Natural Sciences (Astrophysics and Astrononmy)
Date of upload:
09.10.2015
Co-author:
P. Schwartz, H. Balthasar, C. Kuckein, J. Koza, P. Gömöry, J. Rybák, A. Kučera, P. Heinzel
Abstract:
An active-region filament in the active region NOAA 12159 (at solar disk coordinates [x,y] = [225,-440] arcsec) was observed with the Vacuum Tower Telescope at Tenerife on 11 September 2014. Full-Stokes spectropolarimetric observations of the HeI IR triplet at a wavelength around 10830 Å and of the SiI 10827 Å line were acquired using the VTT Echelle spectrograph together with the Tenerife Infrared Polarimeter (TIP I). Additional simultaneous spectroscopic observations in the CaII 8542 Å line were also obtained with the Echelle spectrograph. With the TESOS Fabry-Pérot interferometer, the filament was observed in Ha in intensity mode with a field-of-view of 25 x 25 arcsec (136 wavelength points within the range 6561 – 6564 Å). The filament shows a structure typical for an active filament composed of thin and long fluxtubes with plasma flowing along the magnetic field lines inside these fluxtubes. Thus, for diagnostics of the filament plasma, observed Ha profiles are simulated using a simple isothermal and isobaric two-dimensional non-LTE model with two finite dimensions -- vertical and across the fluxtube. The velocity of the plasma flows is also taken into account in the model because it can cause a Doppler brightening in the observed profiles. Assuming the flows are parallel to the magnetic field lines in the fluxtube, the LOS component is then projected onto a velocity vector according to the direction of the vector magnetic field that we were trying to obtain from inversions using the HAZEL inversion code (Asensio Ramos, 2008).
Asteroseismic modelling of the Binary HD 176465
Natural Sciences (Astrophysics and Astrononmy)
Date of upload:
04.10.2016
Co-author:
M. J. P. F. G. Monteiro, T. L. Campante, D. R. Reese, T. R. White, A. Garc\'{i}a Hern\'{a}ndez, C. Jiang
Abstract:
The detection and analysis of oscillations in binary star systems is critical in understanding stellar structure and evolution. This is because such systems
have the same initial chemical composition and age. Solar-like oscillations have been detected in both components of the asteroseismic binary HD 176465 by Kepler
(White et al., 2016). This study presents an independent modelling of the two stars in this binary system. Stellar models generated using MESA
(Modules for Experiment in Stellar Astrophysics) were fitted to both the observed individual frequencies and some spectroscopic parameters.
The individual theoretical oscillation frequencies for the corresponding stellar models were obtained using GYRE as the pulsation code.
A Bayesian approach was applied to find the Probability Distribution Functions of the stellar parameters using AIMS (Asteroseismic Inference on Massive Scale)
as the optimization code. The age of the individual stars was found to agree with that obtained by White et al., (2016) of about 3.0 $\pm$ 0.5 Gyr old.
KIC 6220497: a new Algol-type eclipsing binary with multiperiodic Pulsations
Natural Sciences (Astrophysics and Astrononmy)
Date of upload:
14.07.2016
Co-author:
Jae Woo Lee, Kyeongsoo Hong, Seung-Lee Kim, and Jae-Rim Koo
Abstract:
We present both binarity and pulsation of KIC 6220497 from the {\it Kepler} observations. The light curve synthesis shows that the eclipsing system is a semi-detached Algol with parameters of $q$ = 0.243$\pm$0.001, $i$ = 77.3$\pm$0.3 deg, and $\Delta T$ = 3,372$\pm$58 K, in which the detached primary component fills its Roche lobe by $\sim$87\%. A multiple frequency analysis of the eclipse-subtracted light residuals reveals 33 frequencies in the range of 0.75$-$20.22 d$^{-1}$ with amplitudes between 0.27 and 4.56 mmag. Among these, four are pulsation frequencies in fundamental ($f_1$, $f_5$) and $p$ ($f_2$, $f_7$) modes, and six are orbital frequency ($f_8$, $f_{31}$) and its harmonics ($f_6$, $f_{11}$, $f_{20}$, $f_{24}$), which can be attributed to tidally excited modes. For the pulsation frequencies, the pulsation constants of 0.16$-$0.33 d and the period ratios of $P_{\rm pul}/P_{\rm orb}$ = 0.042$-$0.089 indicate that the primary component is a $\delta$ Sct pulsating star and, thus, KIC 6220497 is an oscillating eclipsing Algol (oEA) star. The dominant pulsation period of 0.1174051$\pm$0.0000004 d is significantly longer than that expected from empirical relations that link the pulsation period with the orbital period. The surface gravity of $\log g_1$ = 3.78$\pm$0.03 is clearly smaller than those of the other oEA stars with similar orbital periods. The pulsation period and the surface gravity of the pulsating primary demonstrate that KIC 6220497 would be the more evolved EB, compared with normal oEA stars.
Stellar Parameters in an Instant with Machine Learning
Natural Sciences (Astrophysics and Astrononmy)
Date of upload:
15.07.2016
Co-author:
George C. Angelou, Saskia Hekker, Sarbani Basu, Warrick Ball, Elisabeth Guggenberger
Abstract:
We use machine learning to build a constrained multiple regression model for rapidly estimating the fundamental stellar parameters of main-sequence solar-like stars (Bellinger & Angelou 2016). We train a random forest of decision trees with scikit-learn on a matrix of stellar models generated with MESA that we varied quasi-randomly in mass, initial helium and metallicity abundances, mixing length, the strength of convective overshooting, and the efficiency of gravitational settling. We additionally compute frequencies of each stellar model using ADIPLS and summarize them to obtain averaged large and small frequency separations and frequency ratios.
We supply the global asteroseismic properties and other observable quantities of our stellar models to the random forest algorithm and produce a statistical model relating observable quantities to fundamental stellar parameters. We validate this technique on a hare-and-hound exercise and the Sun, and then apply it to 16 Cyg A & B and finally 34 planet-hosting candidates.
Asteroseismic signature of a starspot
Natural Sciences (Astrophysics and Astrononmy)
Date of upload:
13.07.2016
Abstract:
Stellar acoustic oscillations are affected by magnetic activity, however it is unclear how a single starspot would affect the power spectrum of oscillations. Since the starspot rotates with the star, it causes a perturbation that is unsteady in the observer’s frame. Each (n, l) multiplet appears as (2l + 1) 2 blended peaks in the power spectrum, whose amplitudes depend on the star’s inclination and on the latitude of the starspot. We simulate example power spectra using both perturbation theory and numerical simulations.
Improving 1D Stellar Models with 3D Atmospheres
Natural Sciences (Astrophysics and Astrononmy)
Date of upload:
13.10.2016
Co-author:
Víctor Silva Aguirre, Achim Weiss, Jørgen Christensen-Dalsgaard and Regner Trampedach
Abstract:
How to improve traditional 1D models of stellar structure and evolution using results from sophisticated 3D simulations of stellar atmospheres. More specifically, how T-tau relations from Trampedach et al. (2014, MNRAS, 442, 805) and variable mixing-length parameter from Trampedach et al. (2014, MNRAS, 445, 4366) have been implemented in the Garching Stellar Evolution Code (GARSTEC, see Weiss & Schlattl, 2008, Astrophys. Space Sci., 316, 99).
Numerical simulations of magneto-acoustic wave propagation from the upper convection zone into the chromosphere.
Natural Sciences (Astrophysics and Astrononmy)
Date of upload:
08.07.2015
Co-author:
O. Steiner, M. Roth
Abstract:
The contribution of acoustic waves to the chromospheric heating is still an open question. To discuss this Issue, it is crucial to understand the propagation of waves from the convection zone, where the waves are excited, into the higher layers of the solar atmosphere. Traveling upwards through the atmosphere the waves interact with the magnetic field that is present in the photosphere and the chromosphere. This specific interaction takes place in the mode conversion zone, where the sound speed equals the Alfvén speed. Using numerical simulations of wave propagation in a realistic solar model atmosphere, we show how dramatically this interaction influences the propagation of magneto-acoustic waves in the solar atmosphere. Our results demonstrate that due to mode conversion the waves are partially refracted back towards the convection zone and are partially transmitted into the chromosphere. Furthermore, we investigate our simulations for observational quantities to infer properties of the photospheric and chromospheric magnetic field.
The additional-mode garden of RRab stars
Natural Sciences (Astrophysics and Astrononmy)
Date of upload:
15.07.2016
Co-author:
Emese Plachy, Péter Klagyivik, Áron L. Juhász, Róbert Szabó, Zachary Dalessandro, Benjamin Kratz, Justin Ortega, Shashi Kanbur
Abstract:
Space-based photometric missions revealed a surprising abundance of millimagnitude-level additional modes in RR Lyrae stars. The modes that appear in the modulated fundamental-mode (RRab) stars can be ordered into four major categories. Period doubling, caused by the resonant ninth overtone, and the mode at ~0.6 P0 that may potentially correspond to the second overtone, are relatively abundant in these stars. In more rare cases (including RR Lyr itself), the additional modes can be identified with the first overtone, and finally some fall outside the above mentioned categories, so they are most likely non-radial modes. Here we present a census of the additional modes based on the K2 Two Wheel Engineering Test run and Campaigns 0-3 and 6 of the K2 mission, the largest sample obtained with space-based photometry so far.
Looking for Magnetic Field in Kepler Red Giants
Natural Sciences (Astrophysics and Astrononmy)
Date of upload:
14.07.2016
Co-author:
M. Aurière, S. Deheuvels, R. A. García, F. Lignières, S. Mathur, P. Petit
Abstract:
Spectropolarimetric measurements allow us to detect and measure magnetic field through Zeeman effect in stellar photospheres. We observed with the spectropolarimeters Narval (at Telescope Bernard Lyot) and ESPADONS (at CFHT) a few Kepler Red Giants in 2012 and 2013, especially four stars showing depressed dipolar modes. Interestingly, recent seismic studies by Fuller et al. (2015) suggest these stars had a magnetic history. After presenting shortly the technique, I will detail the detections we did and the constraints we put in surface magnetic fields, then I will discuss the origin of these fields by comparing these observations to a broader sample of red giants observed by Aurière et al. (2015).
ON POSSIBLE EXPLANATIONS OF PULSATIONS IN MAIA STARS
Natural Sciences (Astrophysics and Astrononmy)
Jadwiga Daszynska-Daszkiewicz
Date of upload:
21.07.2016
Co-author:
Przemyslaw Walczak, Alexey Pamyatnykh
Abstract:
The long-time photometric surveys in a few young open clusters allowed to identify the light variability in stars located between the well defined Scuti variables and Slowly Pulsating B-type stars (Mowlavi et al, 2013, 2015). The period of these changes is in the range of about 0.1 - 0.7 [d] corresponding to the frequencies of about 1.4 - 10 [d-1]. Several objects of this type were suggested also from the analysis of the Kepler data (Balona et al. 2015a). Assuming the pulsational origin of this variability, we test a few hypotheses. Because all these open clusters are very young, we consider only the ZAMS models with the metallicity Z=0.015.
There are no papers within this category so far.