Fourier Analysis of Gapped Time Series: Improved Estimates of Solar and Stellar Oscillation Parameters

Quantitative helio- and asteroseismology require very precise measurements of the frequencies, amplitudes, and lifetimes of the global modes of stellar oscillation. It is common knowledge that the precision of these measurements depends on the total length (T), quality, and completeness of the observations. Except in a few simple cases, the effect of gaps in the data on measurement precision is poorly understood, in particular in Fourier space where the convolution of the observable with the observation window introduces correlations between different frequencies. Here we describe and implement a rather general method to retrieve maximum likelihood estimates of the oscillation parameters, taking into account the proper statistics of the observations. Our fitting method applies in complex Fourier space and exploits the phase information. We consider both solar-like stochastic oscillations and long-lived harmonic oscillations, plus random noise. Using numerical simulations, we demonstrate the existence of cases for which our improved fitting method is less biased and has a greater precision than when the frequency correlations are ignored. This is especially true of low signal-to-noise solar-like oscillations. For example, we discuss a case where the precision on the mode frequency estimate is increased by a factor of five, for a duty cycle of 15%. In the case of long-lived sinusoidal oscillations, a proper treatment of the frequency correlations does not provide any significant improvement; nevertheless we confirm that the mode frequency can be measured from gapped data at a much better precision than the 1/T Rayleigh resolution.Comment: Accepted for publication in Solar Physics Topical Issue "Helioseismology, Asteroseismology, and MHD Connections

Seismic constraints on rotation of Sun-like star and mass of exoplanet

Rotation is thought to drive cyclic magnetic activity in the Sun and Sun-like stars. Stellar dynamos, however, are poorly understood owing to the scarcity of observations of rotation and magnetic fields in stars. Here, inferences are drawn on the internal rotation of a distant Sun-like star by studying its global modes of oscillation. We report asteroseismic constraints imposed on the rotation rate and the inclination of the spin axis of the Sun-like star HD 52265, a principal target observed by the CoRoT satellite that is known to host a planetary companion. These seismic inferences are remarkably consistent with an independent spectroscopic observation (rotational line broadening) and with the observed rotation period of star spots. Furthermore, asteroseismology constrains the mass of exoplanet HD 52265b. Under the standard assumption that the stellar spin axis and the axis of the planetary orbit coincide, the minimum spectroscopic mass of the planet can be converted into a true mass of 1.85 (+0.52,-0.42) M_Jupiter, which implies that it is a planet, not a brown dwarf.Comment: Published in Proceedings of the National Academy of Sciences (5 pages, 5 figures, 3 tables). Available at http://www.pnas.org/cgi/doi/10.1073/pnas.130329111

Zeitreihenanalyse sonnenähnlicher Oszillationen Anwendung auf Beobachtungen der Sonne und HD52265

Die Untersuchung von Schwingungen in Sternen erlaubt es, den inneren Aufbau und die Dynamik der Sterne mit einer Genauigkeit zu bestimmen, die mit keiner anderen Methode erreicht werden kann. Den Schwerpunkt der vorliegenden Dissertation bildet die Messung der Parameter (Frequenzen, Amplituden, Linienbreiten und rotationsbedingte Frequenzaufspaltung) der globalen Moden sonnenähnlicher Schwingungen, die in der Sonne und dem sonnenähnlichen Stern HD52265 beobachtet wurden. Die Arbeit ist in drei Hauptteile gegliedert: (i) Die Implementierung und Validierung eines globalen Fits stellarer Schwingungsspektren anhand von Sonnenbeobachtungen, (ii) die Anwendung dieser Methode auf eine viermonatige CoRoT-Beobachtung des sonnenähnlichen Sterns HD52265, der außerdem von einem Planeten umkreist wird, sowie (iii) die Erweiterung der Auswertungsmethode auf Zeitreihen mit Unterbrechungen. Die Hauptergebnisse meiner Arbeit betreffen HD52265: Die Schwingungsfrequenzen der Radial-, Dipol- und Quadrupolmoden wurden mit der höchsten, jemals für einen sonnenähnlichen Stern erreichten Genauigkeit gemessen. Zum ersten Mal konnte der Effekt der stellaren Rotation auf die Schwingungsmoden zweifelsfrei für einen sonnenähnlichen Stern nachgewiesen werden. Außerdem erlaubte uns die seismologische Messung der Neigung der Rotationsachse des Sterns, die absolute Masse des Begleiters abzuschätzen. Demzufolge legt die asteroseismologische Untersuchung nahe, dass es sich bei dem Begleiter um einen Planeten und nicht um einen Braunen Zwerg handelt

Capturing time-varying brain dynamics

The human brain is a complex network of interacting nonstationary subsystems, whose complicated spatial–temporal dynamics is still poorly understood. Deeper insights can be gained from recent improvements of time-series-analysis techniques to assess strength and direction of interactions together with methodologies for deriving and characterizing evolving networks from empirical time series. We here review these developments, and by taking the example of evolving epileptic brain networks, we discuss the progress that has been made in capturing and understanding brain dynamics that varies on time scales ranging from seconds to years

Recommendations for assessing motor performance in individuals with dementia:suggestions of an expert panel - a qualitative approach

Background: Recommendations for assessing motor performance in individuals with dementia (IWD) are rare, and most existing assessment tools previously applied in IWD were initially developed for healthy older adults. However, IWD and their healthy counterparts differ in motor and cognitive capabilities, which needs to be considered when designing studies for this population. This article aims to give recommendations for motor assessments for IWD and to promote standardisation based on a structured discussion of identified assessment tools used in previous trials.Methods: Appropriateness and standardisation of previously applied motor assessments for IWD were intensively discussed using a qualitative approach during an expert panel. Furthermore, the use of external cues and walking aids, as well as psychometric properties were considered. Starting with a comprehensive overview of current research practice, the discussion was gradually specified and resulted in the elaboration of specific recommendations.Results: The superior discussion emphasised the need for tailoring motor assessments to specific characteristics of IWD and attaching importance to standardised assessment procedures. Specific recommendations include the use of sequential approaches, which incorporate a gradual increase of complexity from simple to more difficult tasks, a selection of motor assessments showing sufficient relative reliability and appropriateness for IWD, as well as allowing external cues and walking aids when restricted to repeated instructions and commonly used devices, respectively.Conclusions: These are the first recommendations for assessing motor performance in IWD based on a comprehensive qualitative approach. Due to limited evidence, it was not possible to address all existing questions. It is therefore important to evaluate these recommendations in studies with IWD. Besides tailoring and evaluating available assessments, future research should focus on developing specific tools for IWD. Moreover, further progress in standardisation is necessary to enhance comparability between different trials. This article provides initial approaches for overcoming existing limitations in trials with IWD by giving recommendations and identifying future research questions, and therefore contributes to enhancing evidence regarding efficacy and effectiveness of physical activity interventions.</p

Recommendations for assessing motor performance in individuals with dementia: suggestions of an expert panel - a qualitative approach

Background: Recommendations for assessing motor performance in individuals with dementia (IWD) are rare, and most existing assessment tools previously applied in IWD were initially developed for healthy older adults. However, IWD and their healthy counterparts differ in motor and cognitive capabilities, which needs to be considered when designing studies for this population. This article aims to give recommendations for motor assessments for IWD and to promote standardisation based on a structured discussion of identified assessment tools used in previous trials.Methods: Appropriateness and standardisation of previously applied motor assessments for IWD were intensively discussed using a qualitative approach during an expert panel. Furthermore, the use of external cues and walking aids, as well as psychometric properties were considered. Starting with a comprehensive overview of current research practice, the discussion was gradually specified and resulted in the elaboration of specific recommendations.Results: The superior discussion emphasised the need for tailoring motor assessments to specific characteristics of IWD and attaching importance to standardised assessment procedures. Specific recommendations include the use of sequential approaches, which incorporate a gradual increase of complexity from simple to more difficult tasks, a selection of motor assessments showing sufficient relative reliability and appropriateness for IWD, as well as allowing external cues and walking aids when restricted to repeated instructions and commonly used devices, respectively.Conclusions: These are the first recommendations for assessing motor performance in IWD based on a comprehensive qualitative approach. Due to limited evidence, it was not possible to address all existing questions. It is therefore important to evaluate these recommendations in studies with IWD. Besides tailoring and evaluating available assessments, future research should focus on developing specific tools for IWD. Moreover, further progress in standardisation is necessary to enhance comparability between different trials. This article provides initial approaches for overcoming existing limitations in trials with IWD by giving recommendations and identifying future research questions, and therefore contributes to enhancing evidence regarding efficacy and effectiveness of physical activity interventions

EXOTIME: searching for planets around pulsating subdwarf B stars

peer reviewedIn 2007, a companion with planetary mass was found around the pulsating subdwarf B star V391 Pegasi with the timing method, indicating that a previously undis- covered population of substellar companions to apparently single subdwarf B stars might exist. Following this serendip- itous discovery, the EXOTIME (http://www.na.astro.it/ ~silvotti/exotime/) monitoring program has been set up to follow the pulsations of a number of selected rapidly pul- sating subdwarf B stars on time scales of several years with two immediate observational goals: (1) determine P ̇ of the pulsational periods P (2) search for signatures of substellar companions in O– C residuals due to periodic light travel time variations, which would be tracking the central star’s companion- induced wobble around the centre of mass These sets of data should therefore, at the same time, on the one hand be useful to provide extra constraints for classical asteroseismological exercises from the P ̇ (comparison with “local” evolutionary models), and on the other hand allow one to investigate the preceding evolution of a target in terms of possible “binary” evolution by extending the otherwise unsuccessful search for companions to potentially very low masses. While timing pulsations may be an observationally ex- pensive method to search for companions, it samples a dif- ferent range of orbital parameters, inaccessible through or- bital photometric effects or the radial velocity method: the latter favours massive close-in companions, whereas the timing method becomes increasingly more sensitive toward wider separations. In this paper we report on the status of the on-going ob- servations and coherence analysis for two of the currently five targets, revealing very well-behaved pulsational charac- teristics in HS 0444+0458, while showing HS 0702+6043 to be more complex than previously thought