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Astrophysics and Astrononmy
Modelling the effect of Meridional Flows in Time-Distance Helioseismology: Born vs. Ray approximation
Natural Sciences (Astrophysics and Astrononmy)
Date of upload:
12.07.2016
Co-author:
Markus Roth, Jason Jackiewicz, Shukur Kholikov
Abstract:
Accurate meridional flow measurements are important for understanding the solar dynamo. Recent in-
versions for meridional flows have not yet reached a consensus on the nature of the meridional flow in
depths greater than about 0.9 solar radii. In time-distance helioseismology, current modelling of the solar
interior for meridional flow inversions is performed using ray kernels, which assume that waves propagate
along infinitely thin ray paths. The Born approximation may constitute a more accurate approach as it
models the first order perturbation to the wave field in the whole solar interior. We present the current
status of an undergoing validation of a recently developed model for computing spherical Born approxi-
mation sensitivity functions suitable for inferring meridional flows. In addition, we compare Born and ray
approximations using flow models.
Analysis of a subdwarf B pulsator observed during Campaign 2 of K2
Natural Sciences (Astrophysics and Astrononmy)
Date of upload:
14.07.2016
Co-author:
A.S. Baran, M.D. Reed, R.H.Østensen, J.H. Telting, C.S. Jeffery, P. Németh
Abstract:
We present an analysis of the pulsating subdwarf B (sdB) star EPIC 203948264, observed during Campaign 2 of the extended Kepler mission. A time series analysis of the short cadence data set has revealed a rich g-mode pulsation spectrum with 20 independent pulsation periods between 0.5 and 2.8 hours. Most of the pulsations fit the asymptotic period sequences for ℓ = 1 or 2, with average period spacings of 261.34+/-0.78 and 151.18+/-0.34 s, respectively. The pulsation amplitudes are below 0.77 ppt and vary over time. Radial velocity measurements give no indication for binarity in this star. We did not find any clear rotationally induced pulsation multiplets, which indicates that the rotation period of the star is longer than about 46 days. By characterizing the various pulsation modes, we can constrain structural models of sdB stars. This is a promising approach to enhancing our understanding of horizontal branch stars.
Starspot signature on the light curve: learning about the spot distribution
Natural Sciences (Astrophysics and Astrononmy)
Date of upload:
06.09.2016
Co-author:
Margarida S. Cunha, Pedro P. Avelino, Rafael A. García, Savita Mathur
Abstract:
Starspots are cooler/darker than the stellar surface. Therefore, the total flux of a star changes when spots are visible on its surface. The presence of spots together with the stellar rotation leads to a periodic modulation on the light curve. By studying that modulation one can then learn about the stellar rotation and also magnetic activity. Recently, Reinhold & Arlt (2015) proposed a method based on the analysis of the Lomb Scargle Periodogram (LSP) of the light curve to identify the sign of the differential rotation, i.e. whether the equator rotates faster than the poles or the opposite. In this work, we study in detail the spots’ impact on the light curve and on the resulting LSP. We find that, under some conditions, the LSP can actually provide an estimate of the true spot latitudes and/or the stellar inclination angle. Moreover, we find that the impact of the spot on the ratio between the heights of the second and first harmonics of the main peaks in the LSP can be described by a single parameter, the visibility time of the spot. Finally, we also identify possible sources of false positives/negatives for the sign of the differential rotation.
MULTI-WAVELENGTH OBSERVATIONS OF VORTEX-LIKE FLOWS IN THE PHOTOSPHERE FROM GROUND-BASED AND SPACE-BORNE TELESCOPES
Natural Sciences (Astrophysics and Astrononmy)
Date of upload:
12.10.2015
Co-author:
S. Vargas Domínguez, L.A. Balmaceda, I. Cabello, V. Domingo
Abstract:
In this work we follow a series of papers on high-resolution solar observations (Balmaceda et al. 2009, Balmaceda et al. 2010, Vargas Dominguez et al. 2011, Palacios et al. 2012, Vargas Domínguez et al 2015, Cabello et al., in preparation), utilizing several long multi-wavelength data series. These were acquired from
both ground-based (SST) and space-borne (Hinode), thus obtaining high-cadence and high resolution data, including SOT-SP data, in a joint campaign of the Hinode Operation Program 14, in Sept 2007. Diffraction-limited SST data, taken in G-band and G-cont, were restored by MFBD, whilst Hinode obtained multispectral data from SOT-FG in CN, Mg II, Ca II and also SP in Fe I lines. In these series we have thoroughly studied vortex flows and their statistical occurrences, horizontal velocities by means of local correlation tracking (LCT), divergence and vorticity; but we also have studied bright point statistics and
magnetic field intensification, clearly highlighting the importance of the smallest-scale magnetic element observations.
Toward a better understanding of red giants rotation
Natural Sciences (Astrophysics and Astrononmy)
Date of upload:
14.07.2016
Co-author:
Beoît Mosser, Eric Michel
Abstract:
Red giant stars have proved to be asteroseismic targets of choice: conditions in their interior are met to couple pressure waves propagating in the envelope and gravity waves propagating in the core, so that we have a direct view on their core through mixed modes, which is not the case for main-sequence stars. In particular, asteroseismology of red giants gives us the opportunity to study their internal rotation, especially their core rotation. Rotation is known to deeply impact the evolution of stars, but including rotation in stellar evolution models is still challenging. Models predict central rotation rates at least ten times too large compared to asteroseismic measurements. This implies that angular momentum transport is at work in stellar interiors, whose physical mechanisms are not yet fully understood.
It is thus of prime importance to know how internal rotation evolves in time. This is particularly true for red giants, in order to better characterise the physical processes operating in the deepest region of these stars. Such a study requires core rotation measurements for a maximum number of red giants. In this context, I have developed an automatic method to determine the mean core rotation of red giant stars presenting different evolutionary stages with Kepler data. In the future, obtaining mean core rotation rates for thousands of red giants will improve the characterisation of the physical mechanisms causing angular momentum transport in these stars, and therefore our understanding of stellar evolution. I will present preliminary results that I obtained with a new and promising method to determine automatically core rotation rates of red giants. Such an automated measurement of the core
rotation of red giants will moreover be required to analyse the hundreds of thousands of oscillation spectra that PLATO should provide in a few years. Hence this automated method is paving the way for the future PLATO data.
Anomalous high-amplitude peaks in the Fourier spectra of Kepler red giants
Natural Sciences (Astrophysics and Astrononmy)
Date of upload:
14.07.2016
Co-author:
Daniel Huber, Tim Bedding
Abstract:
We present an analysis of 93 oscillating red giants from NASA's Kepler mission which exhibit anomalous single-frequency peaks in their amplitude spectra. These peaks may be indicative of binary star systems, with frequencies such that the companion star's orbit would be within the red giant's convective envelope. Alternatively, the observed phenomenon may be due to a close binary orbiting a red giant in a triple system, or binary systems contaminating the pixels around the target star. We eliminate 46 stars in the sample as blends. However, we find that in 47 cases the anomalous peaks are indistinguishable from the target star to within a resolution of 4", the size of a Kepler pixel. This suggests that we are either observing contamination by a line-of-sight background or foreground binary system, or a physical association. We examine a Galaxia model of the Kepler field of view to estimate background star counts and find that it is highly unlikely that all targets can be explained by chance alignments. From this, we conclude that these stars may comprise a population of physically associated systems.
RR Lyrae in M4
Natural Sciences (Astrophysics and Astrononmy)
Date of upload:
14.07.2016
Co-author:
Pawel Moskalik, Jason Drury
Abstract:
Observations by Kepler/K2 have revolutionized the study of RR Lyrae stars by allowing the detection of new phenomena, such as low amplitude additional modes and period doubling, which had not previously been seen from the ground. During campaign 2, K2 observed the globular cluster M4, providing the first opportunity to study a sizeable group of RR Lyrae stars that belong to a single population; the other RR Lyrae stars that have been observed from space are field stars in the galactic halo and thus belong to an assortment of populations. In this poster we present the results of our study of the RR Lyrae variables in M4 from K2 photometry. We have identified additional, low amplitude pulsation modes in both observed RRc stars. In 3 RRab stars we have found the Blazhko effect with periods of 16.6d, 22.4d and 44.5d.
European Helio- and Asteroseismology Network - Achievements after four years of HELAS
Natural Sciences (Astrophysics and Astrononmy)
Date of upload:
20.08.2015
Co-author:
The HELAS Board
Abstract:
HELAS had the major objective to bring together the widely dispersed European research groups active in helio- and asteroseismology. The transfer of knowledge and data analysis techniques through HELAS lead to a structuring of this field of research, as it was needed to prepare the European research community for important missions as the space missions Solar Dynamics Observatory (SDO), Kepler, CoRoT, and Picard. Here we summarize the achievements of HELAS.
WP80: Synoptic Observations: Solar Physics Research Network Group (SPRING)
Natural Sciences (Astrophysics and Astrononmy)
Date of upload:
16.01.2017
Co-author:
Sanjay Gosain, Frank Hill, Michael Thompson
Abstract:
High-resolution telescopes (such as SST, GREGOR and
the future EST and ATST) allow observations of only a small
fraction of the solar surface. Real-time context data showing the
large-scale dynamics and magnetism at different layers of the solar
atmosphere are crucial to understand the global behavior of solar
phenomena. However, despite the amount of information coming
from space and ground-based full-Sun telescopes, real-time
information about the variation of important parameters such as
velocities, magnetic field and intensity at different solar layers is
stilllacking. To this aim, a network of telescopes with a small
aperture but a large field-of-view can provide useful data to prepare
observing campaigns with large-aperture high-resolution telescopes
and complement the data taken with them. Distributed in a worldwide
network, these small apertures can represent an invaluable
supporting tool for coordinated observations with the major
infrastructures. Within this Joint Research Activity under Solarnet,
the definition of an adequate network of small telescopes, as well as
the most suited instrumentation was addressed.
Measurements of the Absorption and Scattering Cross Sections for the Interactions of Solar Acoustic Waves with Sunspots
Natural Sciences (Astrophysics and Astrononmy)
Date of upload:
09.09.2015
Co-author:
Dean-Yi Chou(Physics Department, National Tsing Hua University, Hsinchu, Taiwan)
Abstract:
The solar acoustic waves are modified by the interaction with sunspots. The interaction can be treated as a scattering problem: an incident wave propagating toward a sunspot is scattered by the sunspot into different modes. The absorption cross section and scattering cross section are two important parameters in the scattering problem. In this study, we use the wavefunction of the scattered wave, measured with a deconvolution method, to compute the absorption cross section σab and the scattering cross section σsc for the radial order n = 0 − 5 for two sunspots, NOAAs 11084 and 11092. In the computation of the cross sections, the random noise and dissipation in the measured acoustic power are corrected. For both σab and σsc, the value of NOAA 11092 is greater than that of NOAA 11084, but their overall n dependences are similar: decreasing with n. The ratio of σabs of two sunspots is approximately equal to the ratio of sunspot radii for all ns, while the ratio of σscs of two sunspots is greater than the ratio of sunspot radii and increases with n. This suggests that σab is approximately proportional to the sunspot radius, while the change of σsc with radius is faster than the linear increase.
Radial mode widths in red giant star spectra observed by Kepler
Natural Sciences (Astrophysics and Astrononmy)
Date of upload:
06.09.2016
Co-author:
Barban, C & Mosser, B
Abstract:
The Kepler space mission has observed many solar-like pulsators, and helped to decipher their main characteristics (e.g: mass, radius, rotation). Most of the achievements recently obtained in that domain consist of the analysis of the mode frequency positions. However, unique information on non-adiabatic physics derives from the height and width of the modes. In this study, we aim to measure the mode widths in thousands of Kepler red giants and to analyze their behavior in function of stellar parameters as well as seismic parameters. The results unveil a clear dependence on mass and stellar evolution for the star mode width, thus their mode damping.
Quiet sun magnetic field evolution observed with Hinode SOT and IRIS
Natural Sciences (Astrophysics and Astrononmy)
Date of upload:
06.10.2015
Co-author:
N. Bello González, R. Rezaei
Abstract:
We have calibrated and aligned a co-spatial and co-temporal 3 hour quiet sun time series taken with the Hinode and IRIS satellites. Hinode SOT observed full Stokes profiles of the Fe I 630 line pairs with the Spectro-polarimeter (SP) scanning a 8.7 arcsec region with a cadence of about 70 seconds. The Broadband Filter Imager (BFI) observed in Ca II H with a cadence of 31 s which was complemented by magnetograms and dopplergrams constructed from Na D I 589,6 nm measurements observed by the Narrowband Filter Imager (NFI). The IRIS satellite recorded slit jaw images centered at 2832 Å (112 s cadence), 1390 Å (23 s) and with a cadence of 19 s at 2796 Å. The spectrograph recorded spectra in several passbands including the Mg II k and h lines performing a 2 step raster with a 9.5 s step cadence.
We study magnetic elements seen in SP and NFI data undergoing a magnetic field intensification process accompanied by the development of bright points in the Ca II H images which we identify as convective collapse events. In addition we study pairs of magnetic elements involved in flux cancellation. The magnetic elements pass the IRIS slit at different stages of these processes. The IRIS Magnesium lines spectra are analyzed to investigate the response of the chromosphere to the magnetic events in the photosphere below. We present the temporal evolution of the atmospheric parameters from the photosphere to the transition region during these occurrences and compare the results to convective collapse and flux cancellation models.
There are no papers within this category so far.