arXiv subjects

Daniel Huber

Publications and source records attributed to Daniel Huber.

337 records · Page 19Linked to original sources

arXiv2009

Detection of solar-like oscillations from Kepler photometry of the open cluster NGC 6819

Asteroseismology of stars in clusters has been a long-sought goal because the assumption of a common age, distance and initial chemical composition allows strong tests of the theory of stellar evolution. We report results from the first 34 days of science data from the Kepler Mission for the open cluster NGC 6819 -- one of four clusters in the field of view. We obtain the first clear detections of solar-like oscillations in the cluster red giants and are able to measure the large frequency separation and the frequency of maximum oscillation power. We find that the asteroseismic parameters allow us to test cluster-membership of the stars, and even with the limited seismic data in hand, we can already identify four possible non-members despite their having a better than 80% membership probability from radial velocity measurements. We are also able to determine the oscillation amplitudes for stars that span about two orders of magnitude in luminosity and find good agreement with the prediction that oscillation amplitudes scale as the luminosity to the power of 0.7. These early results demonstrate the unique potential of asteroseismology of the stellar clusters observed by Kepler.

astro-ph.SR↗

arXiv2009

Automated extraction of oscillation parameters for Kepler observations of solar-type stars

The recent launch of the Kepler space telescope brings the opportunity to study oscillations systematically in large numbers of solar-like stars. In the framework of the asteroFLAG project, we have developed an automated pipeline to estimate global oscillation parameters, such as the frequency of maximum power (nu_max) and the large frequency spacing (Delta_nu), for a large number of time series. We present an effective method based on the autocorrelation function to find excess power and use a scaling relation to estimate granulation timescales as initial conditions for background modelling. We derive reliable uncertainties for nu_max and Delta_nu through extensive simulations. We have tested the pipeline on about 2000 simulated Kepler stars with magnitudes of V~7-12 and were able to correctly determine nu_max and Delta_nu for about half of the sample. For about 20%, the returned large frequency spacing is accurate enough to determine stellar radii to a 1% precision. We conclude that the methods presented here are a promising approach to process the large amount of data expected from Kepler.

astro-ph.SR↗

arXiv2008

MOST detects variability on tau Bootis possibly induced by its planetary companion

(abridged) There is considerable interest in the possible interaction between parent stars and giant planetary companions in 51 Peg-type systems. We demonstrate from MOST satellite photometry and Ca II K line emission that there has been a persistent, variable region on the surface of tau Boo A which tracked its giant planetary companion for some 440 planetary revolutions and lies ~68deg (phi=0.8) in advance of the sub-planetary point. The light curves are folded on a range of periods centered on the planetary orbital period and phase dependent variability is quantified by Fourier methods and by the mean absolute deviation (MAD) of the folded data for both the photometry and the Ca II K line reversals. The region varies in brightness on the time scale of a rotation by ~1 mmag. In 2004 it resembled a dark spot of variable depth, while in 2005 it varied between bright and dark. Over the 123 planetary orbits spanned by the photometry the variable region detected in 2004 and in 2005 are synchronised to the planetary orbital period within 0.0015 d. The Ca II K line in 2001, 2002 and 2003 also shows enhanced K-line variability centered on phi=0.8, extending coverage to some 440 planetary revolutions. The apparently constant rotation period of the variable region and its rapid variation make an explanation in terms of conventional star spots unlikely. The lack of complementary variability at phi=0.3 and the detection of the variable region so far in advance of the sub-planetary point excludes tidal excitation, but the combined photometric and Ca II K line reversal results make a good case for an active region induced magnetically on the surface of tau Boo A by its planetary companion.

astro-ph↗

arXiv2007

MOST photometry of the RRd Lyrae variable AQ Leo: Two radial modes, 32 combination frequencies, and beyond

Highly precise and nearly uninterrupted optical photometry of the RR Lyrae star AQ Leo was obtained with the MOST (Microvariability & Oscillations of STars) satellite over 34.4 days in February-March 2005. AQ Leo was the first known double-mode RR Lyrae pulsator (RRd star). Three decades after its discovery, MOST observations have revealed that AQ Leo oscillates with at least 42 frequencies, of which 32 are linear combinations (up to the sixth order) of the radial fundamental mode and its first overtone. Evidence for period changes of these modes is found in the data. The other intrinsic frequencies may represent an additional nonradial pulsation mode and its harmonics (plus linear combinations) which warrant theoretical modeling. The unprecedented number of frequencies detected with amplitudes down to millimag precision also presents an opportunity to test nonlinear theories of mode growth and saturation in RR Lyrae pulsators.

astro-ph↗

arXiv2005

Cooperative Dynamics in a Network of Stochastic Elements with Delayed Feedback

Networks of globally coupled, noise activated, bistable elements with connection time delays are considered. The dynamics of these systems is studied numerically using a Langevin description and analytically using (1) a Gaussian approximation as well as (2) a dichotomous model. The system demonstrates ordering phase transitions and multi-stability. That is, for a strong enough feedback it exhibits nontrivial stationary states and oscillatory states whose frequencies depend only on the mean of the time delay distribution function. Other observed dynamical phenomena include coherence resonance and, in the case of non-uniform coupling strengths, amplitude death and chaos. Furthermore, an increase of the stability of the trivial equilibrium with increasing non-uniformity of the time delays is observed.

cond-mat.stat-mech↗

arXiv2005

A Mean-Field Model for Extended Stochastic Systems with Distributed Time Delays

A network of noisy bistable elements with global time-delayed couplings is considered. A dichotomous mean field model has recently been developed describing the collective dynamics in such systems with uniform time delays near the bifurcation points. Here the theory is extended and applied to systems with nonuniform time delays. For strong enough couplings the systems exhibit delay-independent stationary states and delay-dependent oscillatory states. We find that the regions of oscillatory states in the parameter space are reduced with increasing width of the time delay distribution function; that is, nonuniformity of the time delays increases the stability of the trivial equilibrium. However, for symmetric distribution functions the properties of the oscillatory states depend only on the mean time delay.

cond-mat.stat-mech↗

arXiv2003

Dynamics in a Bistable-Element-Network with Delayed Coupling and Local Noise

The dynamics of an ensemble of bistable elements under the influence of noise and with global time-delayed coupling is studied numerically by using a Langevin description and analytically by using 1) a Gaussian approximation and 2) a dichotomous model. We find that for a strong enough positive feedback the system undergoes a phase transition and adopts a non-zero stationary mean field. A variety of coexisting oscillatory mean field states are found for positive and negative couplings. The magnitude of the oscillatory states is maximal for a certain noise temperature, i.e., the system demonstrates the phenomenon of coherence resonance. While away form the transition points the system dynamics is well described by the Gaussian approximation, near the bifurcations it is more adequately described by the dichotomous model.

cond-mat.stat-mech↗

arXiv2002

Long-Range Correlations in Self-Gravitating N-Body Systems

Observed self-gravitating systems reveal often fragmented non-equilibrium structures that feature characteristic long-range correlations. However, models accounting for non-linear structure growth are not always consistent with observations and a better understanding of self-gravitating $N$-body systems appears necessary. Because unstable gravitating systems are sensitive to non-gravitational perturbations we study the effect of different dissipative factors as well as different small and large scale boundary conditions on idealized $N$-body systems. We find, in the interval of negative specific heat, equilibrium properties differing from theoretical predictions made for gravo-thermal systems, substantiating the importance of microscopic physics and the lack of consistent theoretical tools to describe self-gravitating gas. Also, in the interval of negative specific heat, yet outside of equilibrium, unforced systems fragment and establish transient long-range correlations. The strength of these correlations depends on the degree of granularity, suggesting to make the resolution of mass and force coherent. Finally, persistent correlations appear in model systems subject to an energy flow.

astro-ph↗

arXiv2001

Lumpy Structures in Self-Gravitating Disks

Following Toomre & Kalnajs (1991), local models of slightly dissipative self-gravitating disks show how inhomogeneous structures can be maintained over several galaxy rotations. Their basic physical ingredients are self-gravity, dissipation and differential rotation. In order to explore the structures resulting from these processes on the kpc scale, local simulation of self-gravitating disks are performed in this paper in 2D as well as in 3D. The third dimension becomes a priori important as soon as matter clumping causes a tight coupling of the 3D equations of motion. The physically simple and general framework of the model permits to make conclusions beyond the here considered scales. A time dependent affine coordinate system is used, allowing to calculate the gravitational forces via a particle-mesh FFT-method, increasing the performance with respect to previous direct force calculations. Persistent patterns, formed by transient structures, whose intensity and morphological characteristic depend on the dissipation rate are obtained and described. Some of our simulations reveal first signs of mass-size and velocity dispersion-size power-law relations, but a clear scale invariant behavior will require more powerful computer techniques.

astro-ph↗

arXiv2001

Fragmentation in Kinematically Cold Disks

Gravity is scale free. Thus gravity may form similar structures in self-gravitating systems on different scales. Indeed, observations of the interstellar medium, spiral disks and cosmic structures, reveal similar characteristics. The structures in these systems are very lumpy and inhomogeneous. Moreover some of these structures do not seem to be of random nature, but obey certain power laws. Models of slightly dissipative self-gravitating disks show how such inhomogeneous structures can be maintained on the kpc-scale. The basic physical processes in these models are self-gravity, dissipation and differential rotation. In order to explore the structures resulting from these processes, local simulations of self-gravitating disks are performed in 2D and 3D. We observe persistent patterns, formed by transient structures, whose intensity and morphological characteristic depend on the dissipation rate.

astro-ph↗

arXiv2001

N-Body Simulations of Open, Self-Gravitating Systems

Astrophysical systems differ often in two points from classical thermodynamical systems: 1.) They are open and 2.) gravity is a dominant factor. Both modifies the homogeneous equilibrium structure, known from classical thermodynamics. In order to study the consequence for structure formation in astrophysical systems, we carry out N-body simulations of self-gravitating systems, subjected to an energy-flow. The simulations show that physically realistic, time-dependent boundary conditions can maintain a molecular cloud in a statistically steady state, out of thermodynamic equilibrium. Moreover we perform some simple "gravo-thermal" N-body experiments and compare them with theoretical results. We find negative specific heat in an energy range predicted by Follana and Laliena (1999).

astro-ph↗

arXiv2000

Self-Gravitating N-Body Systems out of Equilibrium

Real physical systems are often maintained off equilibrium by energy or matter flows. If these systems are far from equilibrium then the thermodynamical branch become unstable and fluctuations can lead them to other more stable states. These new states are often endowed with higher degrees of organization. In order to explore whether an energy-flow in combination with self-gravity can lead to complex, inhomogeneous structures, like observed in the interstellar medium (ISM), we perform N-body simulations of self-gravitating systems subjected to an energy-flow. Moreover we perform some simple gravo-thermal N-body experiments and compare them with theoretical results. We find negative specific heat in an energy range as predicted by Follana & Laliena (1999).

astro-ph↗

arXiv1999

Scaling Laws in Self-Gravitating Disks

The interstellar medium (ISM) reveals strongly inhomogeneous structures at every scale. These structures do not seem completely random since they obey certain power laws. Larson's law (\citeyear{Larson81}) $σ\propto R^δ$ and the plausible assumption of virial equilibrium justify to consider fractals as a possible description of the ISM. In the following we investigate how self-gravitation, differential rotation and dissipation affect the matter distribution in galaxies. To this end we have performed 3D-simulations for self-gravitating local boxes embedded in a larger disk, extending the 2D-method of Toomre & Kalnajs (\citeyear{Toomre91}) and Wisdom & Tremaine (\citeyear{Wisdom88}). Our simulations lead to the conclusion that gravitation, shearing and dissipation can be dominantly responsible for maintaining an inhomogeneous and eventually a fractal distribution of the matter.

astro-ph↗