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Astrophysics and Astrononmy
JCMT and the East Asia Observatory
Natural Sciences (Astrophysics and Astrononmy)
Date of upload:
07.02.2016
Co-author:
Jessica Dempsey
Abstract:
An overview of the James Clerk Maxwell Telescope on Mauna Kea, Hawaii and the East Asia Observatory
Photospheric and Chromospheric Observations of Dynamic Features in an Arch Filament System
Natural Sciences (Astrophysics and Astrononmy)
Sergio Javier González Manrique
Date of upload:
19.01.2017
Co-author:
S.J. González Manrique, C. Kuckein, A. Pastor Yabar, M. Collados, C. Denker, M. Verma, A. Diercke, P. Gömöry, H. Balthasar, M. Cubas Armas, A. Lagg, S.K. Solanki, and K.G. Strassmeier
Abstract:
The new generation of solar instruments provides better spectral, spatial, and temporal resolution, which is essential to investigate the physical processes that take place on the Sun. High-resolution observations often show double- or even multiple-component spectral profiles. This is particularly true for observations of the near-infrared He I 10830 Å triplet. These spectral lines provide information on the velocity and magnetic fine structure of the upper chromosphere. We present observations of an emerging flux region (EFR), including two small pores visible in the photosphere and an arch filament system (AFS) in the chromosphere. The data were taken on 2015 April 17 with the very fast spectroscopic mode (~1 min for a full scan of 180 steps) of the GREGOR Infrared Spectrograph (GRIS), one of the post-focus instruments of the 1.5-meter GREGOR solar telescope located at the Observatorio del Teide, Tenerife, Spain. Simultaneous spectroscopic observations were taken with the GREGOR Fabry-Pérot Interferometer (GFPI) of the photospheric Fe I 6302 Å line. On the island of La Palma, the Swedish Solar Telescope (SST) observed the same EFR using the CRisp Imaging Spectro-Polarimeter (CRISP), which recorded spectropolarimetric data in the photospheric Fe I 6173 Å and the chromospheric Ca II 8542 Å lines. The observed AFS connects the two opposite magnetic polarities of newly emerging flux. Supersonic downflows up to 100 km s$^{-1}$ were measured in the He I triplet, which occur near both footpoints of dark filaments, whereas loop tops rise with about 1.5–20 km s$^{-1}$. The aim of this work is to track locations of high velocities within the footpoints of the arch filaments down to the photosphere. A special question arises, if the plasma contained in chromospheric structures, which exhibits supersonic LOS downflows near the footpoints of the AFS even reaches the photosphere.
Exceptions to the rule: the X-flares of AR 2192 lacking coronal mass ejections
Natural Sciences (Physics)
Date of upload:
09.10.2015
Abstract:
The unusually large NOAA active region (AR) 2192, observed in October and November 2014, was outstanding in its productivity of major flares (GOES class M5 and larger). However, none of the X-flares was associated to a coronal mass ejection. The AR showed a predominantly north-south oriented magnetic system of arcade fields, which served as a strong, also lateral, confinement for the flares at the core of the active region. The large initial separation of the flare ribbons, together with an almost absent growth in ribbon separation suggests a confined reconnection site high up in the corona. Based on a detailed analysis of the confined X1.6 flare of Oct 22, we show how exceptional the flaring of this AR was. We find evidence for repeated energy release, presumably due to magnetic reconnection in a narrow flaring volume, closely associated to the location of hard X-ray sources. We demonstrate that a considerable portion of the magnetic energy released during the X-flare was consumed by the non-thermal flare energy.
On the Utility of Diagrams of Small Frequency vs Large Frequency Separation
Natural Sciences (Astrophysics and Astrononmy)
Date of upload:
08.09.2015
Co-author:
Saskia Hekker, Elisabeth Guggenberger, Nathalie Themessl
Abstract:
Over 35 years ago, it was proposed that there exists diagnostic potential in combining the small
and large frequency separations for solar-like oscillators.
Since then, we have been spoilt with a plethora long-timeseries photometric light curves from
which individual stochastic oscillation modes can be extracted. These light curves allow the
separations to be determined en masse and in power spectra with relatively low signal-to-noise
ratios.
The small-frequency separation in low-mass main-sequence stars and in subgiants is sensitive
to the molecular weight gradient in the inner regions whilst the large-frequency separation is
indicative of the mean density of the star.
We investigate how accurately these parameters must be determined in order to place
constraints on stellar parameters such as mass and age. We discuss some difficulties
associated with calculating the necessary large grids of models (with different input physics)
as well as the degeneracy that exists in later evolutionary phases.
Stellar prominences: the cases of HK Aqr and PZ Tel indications of prominence oscillations and prominence eruptions
Natural Sciences (Astrophysics and Astrononmy)
Date of upload:
09.06.2016
Co-author:
P. Odert, T. Zaqarashvili, R. Greimel, A. Hanslmeier, H. Lammer
Abstract:
Prominences are manifestations of solar/stellar coronal magnetic fields. Coronal magnetic field supports cool dense prominence plasma against gravity, which may be kept for several rotations (quiescent prominences) or may be ejected because of disturbances in the stellar plasma causing the plasma to accelerate very fast (eruptive prominences).
On the Sun, prominences are known to exhibit oscillations, so-called small- and/or large-amplitude oscillations, which show amplitudes of a few km/s for small amplitude oscillations and >20 km/s for large amplitude oscillations. The periods of large amplitude oscillations are in the range of 6-150~min. So far no prominence oscillations have been detected on stars. With existing observatories the stellar analogue of large-amplitude oscillations can be searched on stars which are known to host prominences. Furthermore, stars hosting prominences are good targets for searching for stellar mass ejections, because erupting filaments and mass ejections (CMEs) are closely correlated on the Sun, moreover the CME core often represents the filament itself.
We present the analysis of six nights of optical spectroscopic monitoring of the young and fast rotating stars HK Aqr and PZ Tel, which are known to host prominences. We detect on both stars prominences. In two prominences on HK Aqr we detect indications of prominence oscillations reminiscent to solar large amplitude oscillations, at least with period. We detect no oscillations in the prominences of PZ Tel. Furthermore we find no eruptive prominences on both stars. We explain the non-existence of eruptive prominences using geometrical considerations.
Asteroseismic modelling of the Binary HD 176465
Natural Sciences (Astrophysics and Astrononmy)
Date of upload:
16.08.2016
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.
An estimate of chromospheric heating by acoustic waves - reloaded
Natural Sciences (Physics)
Date of upload:
07.06.2016
Co-author:
P. Heinzel, M. Švanda, J. Jurčák, D. del Moro, and F. Berrilli
Abstract:
Several mechanisms may heat the solar chromosphere: acoustic waves, magnetoacoustic waves (slow, fast, and Alfven waves), and small-scale magnetic reconnections. Based on observations in the Ca II 854.2 nm line, the contribution of acoustic waves to the heating of quiet and plage regions in the chromosphere is discussed. The point is to compare the energy released by radiative losses with the energy deposited by acoustic waves. Radiative losses are computed using a grid of semi-empirical chromospheric models. The deposited acoustic flux is calculated using power spectra of Doppler oscillations measured in the Ca II line core. The comparison shows that the spatial correlation of maps of radiative losses and acoustic flux is 72 %. The deposited acoustic flux covers only 15 % of radiative losses in quiet chromosphere but 23 % in network and 54 % in plage areas. This estimate is a lower limit of the real acoustic energy flux.
Investigating the origin of flares in A-type stars
Natural Sciences (Astrophysics and Astrononmy)
Date of upload:
08.09.2016
Co-author:
Victoria Antoci (SAC, Aarhus University) and Heidi Korhonen (Dark Cosmology Centre, University of Copenhagen)
Abstract:
Recent studies show evidence of flare-like features in 33 Kepler A-type stars which were interpreted to be intrinsic, contradicting theory. Flares in late-type stars are generated through the reconnection of magnetic field lines in stellar atmospheres. For magnetic fields to be sufficiently strong to emerge at the surface and form flares a dynamo is required, which is operated by a convective envelope. A-type stars only have shallow convective envelopes of the order of 1.3% of the total stellar radius and therefore are not expected to support flaring. On the other hand X-ray flares have been observed in strongly magnetic A- and B- type stars and through colliding winds in massive binary systems. Strong stellar winds vanish for late B-type stars and normal A-type stars only have weak global magnetic fields. Therefore, neither reconnection or strong stellar winds should support flaring in A-type stars. We analyse the 33 A-type stars previously found to be flaring, setting specific criteria in order to identify these flares. Our results strongly disagree with the numbers found in literature. However, the positive detections follow the expected correlation between flare duration and intensity. We present results on our investigation of the origin of these flares, considering effects from contamination and combined light as well as study binarity from spectroscopy and possible circumstellar disks.
Ray dynamics of gravito-inertial modes in rotating stars
Natural Sciences (Astrophysics and Astrononmy)
Date of upload:
03.09.2015
Co-author:
François Lignières (IRAP), Jérôme Ballot (IRAP)
Abstract:
Seismology of intermediate-mass and massive stars is limited by our lack of understanding of
the effect of fast rotation on gravity modes.
In particular, in this regime perturbative methods are unable to identify observed modes.
We therefore develop an asymptotic theory for adiabatic gravito-inertial modes in uniformly
rotating stars.
We first derived a generalized dispersion equation taking the Coriolis force and the centrifugal
deformation into account.
The corresponding ray dynamics allowed us to explore the structure of the phase space thanks
to a ray-tracing code.
We observed three coexisting types of structures: (i) nearly integrable structures similar to non-
rotating structures, (ii) island chains around stable periodic orbits, (iii) large chaotic zones.
These three different types of structures are expected to give three different families of modes.
Tight asteroseismic constraints on core overshooting and diffusive mixing in massive stars
Natural Sciences (Astrophysics and Astrononmy)
Date of upload:
07.09.2015
Co-author:
Conny Aerts, Peter Papics, Santiago Triana
Abstract:
Thanks to the unprecedented high quality space photometry provided by CoRoT and Kepler
missions, our view on stellar oscillations in B-type stars and the physics of the upper
HRD is progressively improving. O- and B-type stars harbor a fully mixed convective cores,
and a radiative envelope. However, the interface between these two layers -- the so-called
overshooting layer -- is not understood from first principles. Thus, the width and the mixing
efficiency of the overshooting layer is always treated by simplistic schemes like step-function
or exponentially diffusive mixing prescriptions. We modelled two rich main sequence pulsators
observed by Kepler and CoRoT which also turn out to be very slowly rotating pulsating B stars.
They are KIC 10526294 and HD 50230, respectively.
Based on forward seismic modelling, we derive the width of the overshooting layer on top of their
receding cores. Additionally, we show that extra diffusive mixing of 100 to 10 000 cm$^2$ sec$^{-1}$
in the radiative envelope of these two stars is essential to better fit their observed g-mode
frequencies. We also compare the classical step-function versus exponential diffusive overshoot.
The diffusive overshooting prescription outperforms the other to fit the observed frequencies by
a factor 2 to 3 (in $\chi^2$ sense). The derived values for the overshooting and diffusive mixing
coefficients are weakly dependent on the choice of opacities and chemical mixtures, and are
considered robust constraints.
“Old” data – new science or why do we need long-term synoptic programs
Natural Sciences (Astrophysics and Astrononmy)
Date of upload:
04.05.2017
Abstract:
It is not uncommon to see the references to recent (cycle 24) solar activity as “exceptionally low” or as an “extended long-term decline”. But how would we know that the current level of activity is unusual if we did not have historical data taken over many cycles? How would we know, for example, what are the strongest field strengths in sunspots and how they change with time or that the amplitude of next cycle could be defined by the strength of polar field in previous cycle if we did not have long-term records of solar activity? Truly, synoptic observations feed future research to solve issues that may not be identified at the time when data are acquired. In my talk, I will discuss the current state of long-term synoptic programs and present results of my recent projects on reconstructing the solar activity using historical data.
Fitting Sun-as-a-star GOLF & VIRGO acoustic spectra
Natural Sciences (Astrophysics and Astrononmy)
Date of upload:
02.10.2015
Abstract:
The development of a new fitting tool to handle with high-resolution acoustic spectra of the Sun is of central interest to allow for measuring the asteroseismic properties of solar oscillations in a robust and reliable way. A Bayesian approach implemented in the recently released code DIAMONDS and an extension of this code applied to the case of Sun-as-a-star datasets from both GOLF and VIRGO instruments are presented.
There are no posters within this category so far.
There are no papers within this category so far.