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Astrophysics and Astrononmy
Accurate mean density and surface gravity of Delta Scuti stars using Asteroseismology
Natural Sciences (Astrophysics and Astrononmy)
Date of upload:
19.07.2016
Co-author:
M. J. P. F. G. Monteiro, S. Martín, Z. Guo, D. R. Reese, J. C. Suárez, J. Pascual-Granado, A. Moya, R. Garrido
Abstract:
In the work we present here, we empirically demonstrate that a simple relation exists between a periodic pattern present in the frequency spectra of Delta Scuti stars and their mean density. This relation is homologous to that of solar-type stars. But the most important result is that this relation is indeed independent of the rotational velocity, becoming an ideal tool to constrain the pyhisical parameters of the star, even at high rotation rates.
Moreover, we pushed the data one step further. Using also an estimation of the luminosity of each object, we demonstrate that it is possible, once the periodic pattern is determined, to derived the surface gravity of the star. The typical uncertainty we found for this quantity is the same as with high resolution spectroscopic data. This result might settle the degeneracy problem of the surface gravity determination with the spectra of A-type stars.
AAL Multi Messenger Working Group
Natural Sciences (Astrophysics and Astrononmy)
Date of upload:
17.12.2015
Abstract:
AAL Multi Messenger Working Group
The Envelope Spectrum
Natural Sciences (Astrophysics and Astrononmy)
Date of upload:
30.09.2015
Co-author:
Ariane Schad, Wiebke Herzberg, and Markus Roth
Abstract:
Context. 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.
Aims. We want to develop a robust method to determine the large and small frequency separations for time series with low signal-tonoise ratio. For this purpose, we analyse a time series of the Sun from the GOLF instrument aboard SOHO and a time series of the star KIC 5184732 from the NASA Kepler satellite by employing a combination of Fourier and Hilbert transform.
Methods. We use the analytic signal of filtered stellar oscillation 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.
Results. With the described 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 the regularities in the periodogram of the time series.
Halo formation in 3D MHD simulations of sunspots
Natural Sciences (Astrophysics and Astrononmy)
Date of upload:
31.01.2017
Co-author:
Vigeesh G., Roth, M.
Abstract:
Halo formation in realistic 3D MHD simulations
Thaler I., Vigeesh G., Roth, M.
Since the first discovery of acoustic halos around magnetic regions by
(Brown 1992, Toner and LaBonte 1993) many observational and
theoretical studies have been undertaken to better understand this
phenomena. The currently most accepted theory for its formation is the
refraction of fast magnetic waves in the solar atmosphere, which
then leads to the observed power access in doppler velocity maps
compared to the quiet sun (Khomenko & Collados 2009, e.g. Rijs 2016).
Along with that we want to investigate if we can confirm these results
using non-linearized wave propagation through a sunspot stripe in fully
convective 3D MHD simulations using the STAGGER code.
Dynamical model forspindown of solar type stars
Natural Sciences (Astrophysics and Astrononmy)
Date of upload:
09.09.2015
Abstract:
We for the first time propose a spin-down model where the loss of angular momentum by magnetic fields is dynamically treated,
instead of being kinematically prescribed. To this end, we evolve stellar rotation and magnetic field simultaneously over the
stellar evolution time by incorporating the nonlinear feedback mechanisms on rotation and magnetic fields and examine the behaviour
of rotation rate $\Omega$ with time $t$, magnetic field strength $|B|$ and frequency of magnetic field $\omega_{cyc}$ with rotation rate $\Omega$.
Initially, rotation rate is found to decrease very rapidly with time until there is a sudden transition
from fast to slow spin down of stars. The dependence of rotation rate on time illustrates exponential spin-down for rapid rotators and power
law spin-down for slow rotators. For fast rotators, the strength $|B|$ is found to saturate for large $\Omega$ while for slow rotators, $|B|$ increases almost linearly with
$\Omega$. The analysis of the local frequency of magnetic fields reveals the existence of the two (active and inactive) branches of magnetic
fields for stars with different frequencies $\omega_{cyc}$ which have different scalings with rotation rate $\Omega$: the active and inactive branches
with power law scaling exponents 0.85 and 1.16, respectively. The transition from fast to slow rotators occurs very rapidly with the disappearance of the active branch.
.The Vaughan-Preston gap is consistently explained in our model by the shortest spin-down timescale in this transition from fast to slow rotators.
All these results successfully reproduce the key observations and capture the V-P gap in a self-contained model.
VLBI with Mopra: 2016-2018
Natural Sciences (Astrophysics and Astrononmy)
Date of upload:
17.12.2015
Abstract:
VLBI with Mopra: 2016-2018
L4+L5 Mission as an Ideal Project for International Collaboration
Natural Sciences (Astrophysics and Astrononmy)
Date of upload:
09.07.2017
Co-author:
Mario Bisi, Anatoli Petrukovich, Ying Liu
Abstract:
Having satellites positioned in-orbit at both Lagrangian L5 and L4 points offers several major advantages. For example, the L5 vantage point provides an early view of the solar surface, which Earth will be facing 4-5 days later. In turn, the L4 viewing point enables a better view of the source regions of eruptions responsible for SEPs affecting the near-Earth environment. Taken together, observations from L4 and L5 cover about 83% of solar surface, which will significantly improve both short- and long-term forecasts. However, in the most likely scenario that funding will support only a single L5 mission, not both, one alternative that the space weather community may want to explore is to encourage other spacefaring nations such as Russia, China, and India, to launch their own spacecraft to L4 in close coordination with the L5 mission. Launching two separate spacecraft to L4 and L5 will allow us to reap the benefits of having two new vantage points for space weather in addition to the L1 vantage point, to more-fully share the costs of such combined missions, and avoid the restrictions related to the transfer of technology (predominantly affected the L5 and L1 concepts to date).
Measurement of the amplitude of the solar cross-covariance function
Natural Sciences (Astrophysics and Astrononmy)
Date of upload:
14.09.2015
Co-author:
Kaori Nagashima, Laurent Gizon, Aaron C. Birch, Damien Fournier
Abstract:
In current time-distance helioseismology analyses the (phase) travel time is commonly used. However, other than the travel time, there are parameters of the cross-covariance functions of the solar oscillation field that are affected by solar interior structure and dynamics. Including these parameters in helioseismology analyses might improve the analyses. Therefore, here we focus on one such parameter, the amplitude. We formulate a two-parameter fit of the cross-covariance function in the regime where the amplitude and travel-time differences of the cross-covariance function are small, and measure amplitudes as well as travel times in several areas on the Sun. We find that the amplitude of the center-to-annulus cross-covariance function in the quiet Sun shows the supergranulation pattern, although the amplitude is noisier than the travel times; the out-in amplitude difference shows positive correlation with the out-in travel-time difference. We also detect significant amplitude reduction due to sunspots, which is consistent with Liang et al. (2013).
Stellar magnetic activity and their effects on planets
Natural Sciences (Astrophysics and Astrononmy)
Date of upload:
16.07.2016
Abstract:
In this talk, I review the latest results on surveys of stellar magnetism and present evidence and suggestions that planets could affect the magnetic activity of their host stars.
Non-adiabatic pulsations in ESTER models
Natural Sciences (Astrophysics and Astrononmy)
Date of upload:
21.07.2016
Co-author:
M.-A. Dupret, M. Rieutord
Abstract:
One of the greatest challenges in interpreting the pulsations of rapidly rotating stars is mode identification, i.e. correctly matching theoretical modes to observed pulsation frequencies. Indeed, the latest observations as well as current theoretical results show the complexity of pulsation spectra in such stars, and the lack of easily recognisable patterns. In the present contribution, I will describe the latest results on non-adiabatic effects in such pulsations, and show how these come into play when identifying modes. These calculations fully take into account the effects of rapid rotation, including centrifugal distortion, and are based on models from the ESTER project, currently the only rapidly rotating models in which thermal equilibrium, a prerequisite for calculating non-adiabatic effects, is achieved. Non-adiabatic effects determine which modes are excited and play a key role in the near-surface pulsation-induced temperature variations which intervene in multi-colour amplitude ratios and phase differences, as well as line profile variations.
Simulation of a Double/Triple-Fabry-Pérot-Spectropolarimeter for the new solar telescope DKIST
Natural Sciences (Astrophysics and Astrononmy)
Matthias Johannes Schubert
Date of upload:
22.09.2016
Abstract:
Context:
Our Sun is an unique high-energy plasma physics laboratory, which can be
studied with exceptional spatial and temporal resolution. Using dedicated telescopes, it
is possible to verify and enhance our knowledge about aspects of modern physics unaccessible
to any experiments on earth. State of the art magneto-hydrodynamic simulations
of the solar photosphere have a spatial resolution of 6km at a wavelength $\lambda$ = 500nm
and model the dynamics of solar pores, sunspots and coronal mass ejections. The theoretical
models have to be evaluated in different atmospheric heights with highly resolving
spectro-polarimetric observations. Therefore, the international solar community started
to build a 4 m-class telescope. Furthermore an imaging spectro-polarimeter for visible
light is developed. This is an unique tool to access highly dynamical, small scale processes
on the solar photosphere and chromosphere for ground breaking scientific research.
Method:
Since a similar, yet smaller instrument is operated at the Vacuum-Tower-
Telescope on the Canarian Island Tenerife, it was used as a test bed to characterize
realistic instrumental induced errors. This defined the relevant parameters of the filter
instrument for scientific research which were modelled: the etalons surface micro roughness,
a varying reflectivity, plate figure errors, the photon noise, the relative aperture
and the distance of the individual etalons to a defined focal plane of the telescope. Micro
roughness, reflectivity and plate figure errors will shift and broaden the observed line
profiles. Thus an instrumental error is induced in calculated Doppler velocity and full
width half maximum maps of the solar surface. Additionally, the magnetic measurement
sensitivity is limited by noise . Therefore, simulated observations of the quiet Sun were
performed for two instrument configurations to study their measurement capabilities.
One instrument is simulated with three etalons and a spectral bandwidth of $\delta\ambda$ = 3.8pm
and the other with two etalons and $\delta\ambda$ = 6.1pm (spectral bandwidth is given for wavelength
$\lambda$=630 nm). To study the effect of a defocused mounting of the etalons on the
optical axis, the simulations were carried out for the instruments theoretical in a focal
plane and at specified distances.
Waves inside Stars: Observational Evidence
Natural Sciences (Astrophysics and Astrononmy)
Date of upload:
04.09.2015
Abstract:
Waves inside Stars: Theory, Simulations, Observational Signatures, and Lab
Experiments
Splinter session, Monday 31 August, 9h25 - 12h30, during
SOLARNET III / HELAS VII / SpaceInn Conference "The Sun, the stars, and
solar-stellar relations"
http://www.iac.es/congreso/solarnet-3meeting/
Organisers:
Tamara Rogers (Newcastle University, UK)
Conny Aerts (Leuven University, B)
Abstract:
Waves are as ubiquitous in stars as they are on Earth. Just as on Earth, waves
can transport angular momentum and mix species within stellar interiors,
steering their rotational and chemical evolution. Waves also set up standing
modes which can be observed through helio- and asteroseismology.
Helioseismology has revolutionized our picture of the Sun, constraining the
internal rotation profile and convective undershooting in the solar interior.
Asteroseismology is not far behind, recently constraining core-envelope
differential rotation and core convective overshooting in more massive stars.
Indeed, the observations of waves through helio- and asteroseismology places the
tightest constraints on the dynamical evolution those same waves induce.
This 3-hour splinter session aims to bring together researchers doing theory,
simulations, and observations of waves in stars (gravity, pressure and mixed)
with the hope that the synergy between the three (often disparate) fields could
lead to tests and comparisons which would further our understanding of stellar
interiors. Moreover, we include also studies of wave generation by convection
in laboratory experiments to search for connections between those and stellar
physics. We begin this session with four short talks on each of the sub-topics
and will then continue with a guided discussion on how these fields can work
together to advance our understanding.
Programme: Monday 31 August, 09:25 - 12:30
09:25 Welcome
09:30 - 10:00 Theory: Stephane Mathis (Saclay, France)
10:00 - 10:30 Simulations: Tami Rogers (Newcastle, UK)
10:30 - 11:00 Coffee Break
11:00 - 11:15 Observational Signatures: Conny Aerts (Leuven, B)
11:15 - 11:30 Lab Experiments: Santiago Andres Triana (Leuven, B)
11:30 - 12:15 Guided discussion, participants are encouraged to bring 1 slide
12:15 - 12:30 Summary of Synergies & Future Steps
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