Rotational Mode Coupling Research Articles

Asteroseismology of the fast-rotating high-amplitude δ Scuti star V2367 Cygni

ABSTRACT We present the comprehensive analysis of the high-amplitude $\delta$ Sct star V2367 Cygni. First, we perform the frequency analysis for the whole available set of the Kepler and TESS photometry. Most of the frequency peaks are harmonics or combinations of the three known independent frequencies with the highest amplitudes, i.e.$\nu _1=5.661\,06$ d$^{-1}$, $\nu _2=7.14898$ d$^{-1}$, and $\nu _3=7.77557$ d$^{-1}$. The total number of independent frequencies is 26 and 25 from the Kepler and TESS light curve, respectively. Then, using the ${\it UBVRI}$ time-series photometry, we unambiguously identify the dominant frequency $\nu _1$ as the radial mode, whereas in the case of frequencies $\nu _2$ and $\nu _3$ the most probable mode degrees are $\ell =0$ or $\ell =2$. However, only the frequency $\nu _2$ can be associated with a radial mode, and only if higher order effects of rotation are taken into account. Including the rotational mode coupling, we constructed complex seismic models of V2367 Cyg, which fit $\nu _1$ and $\nu _2$ as radial modes, and reproduce the amplitude of bolometric flux variations (the parameter f) for the dominant mode. The empirical values of f are derived from the ${\it UBVRI}$ amplitudes and phases. We rely on the Bayesian analysis based on Monte Carlo simulations to derive constraints on evolutionary stage, mass, rotation, overshooting from the convective core, and efficiency of convective transport in the envelope. Our seismic analysis clearly indicates that V2367 Cyg is in a post-main sequence phase of evolution. This is the first extensive seismic modelling that takes into account the effect of rotational coupling between pulsation modes.

Refining the asteroseismic model for the youngδScuti star HD 144277 using HARPS spectroscopy

HD 144277 was previously discovered by MOST space photometry to be a young and hot delta Scuti star showing regular groups of pulsation frequencies. The first asteroseismic models required lower than solar metallicity to fit the observed frequency range based on a purely photometric analysis. High-resolution, high S/N spectroscopic data obtained with the HARPS spectrograph were used to determine the fundamental parameters and chemical abundances of HD 144277. These values were put into context alongside the results from asteroseismic models. The effective temperature, Teff, of HD 144277 was determined as 8640(+300)(-100) K, log g is 4.14 +/- 0.15 and the projected rotational velocity, vsini, is 62.0 +/- 2.0 km/s. As the vsini value is significantly larger than previously assumed, we refined the first asteroseimic model accordingly. The overall metallicity Z was determined to be 0.011 where the light elements He, C, O, Na, and S show solar chemical composition, but the heavier elements are significantly underabundant. In addition, the radius of HD 144277 was determined to be 1.55 +/- 0.65 Rsun from spectral energy distribution fitting, based on photometric data taken from the literature. From the spectroscopic observations, we could confirm our previous assumption from asteroseismic models that HD 144277 has less than solar metallicity. The fundamental parameters derived from asteroseismology, Teff, log g, L/Lsun and R\Rsun, agree within one sigma to the values found from spectroscopic analysis. As the vsini value is significantly higher than assumed in the first analysis, near-degeneracies and rotational mode coupling were taken into account in the new models. These suggest that HD 144277 has an equatorial rotational velocity of about 80 km/s and is seen equator-on. The observed frequencies are identified as prograde modes.

Approaching asteroseismology of δ Scuti stars: problems and prospects

Inhalt (u.a.): M. BREGER: Delta Scuti stars: Observational aspects; M.S. Cunha: Theory of rapidly oscillating Ap stars; M. SACHKOV et al.: Vertical structure of pulsations in roAp stars; G. HOUDEK: Theoretical asteroseismology of solar-like oscillations; B.L. POPIELSKI: Core modes as a seismic probe of mixing beyond the convective core; M. GODART: Temperature gradients in the core overshooting region; W.A. DZIEMBOWSKI – J. DASZYŃSKA-DASZKIEWICZ – A.A. PAMYATNYKH: Interpretation of the Be star HD 163868 oscillation spectrum based on the MOST observations; D. KILKENNY: Pulsating Hot Subdwarfs – An Observational Review; M.H. MONTGOMERY: Mapping Convection using Pulsating White Dwarf Stars; M. ROTH: The Network Activities in HELAS; I. KUDZEJ et al.: A New Slovak Observatory 500 km from Vienna; W.W. WEISS: Microsatellites; G. KOPACKI – S. FRANDSEN: RR Lyrae stars in M4

Ab initio molecular dynamics of liquid hydrogen chloride

We carried out an ab initio molecular dynamics simulation of liquid hydrogen chloride (l-HCl) at a temperature of 313 K. Comparison with inelastic neutron scattering data shows that the simulation achieves an overall good description of the structural correlations, improving significantly upon a description based on classical interaction potentials. Despite some minor differences between theory and experiment in the H-H partial structure factor, the simulation gives a description of the hydrogen bonding in impressive agreement with experiment, for both the amount and the bond-length distribution of the bonds. In the simulation, 40% of the molecules are nonbonded, while the hydrogen-bonded chains are short, principally consisting of dimers (25%) and trimers (15%). Neighboring molecules in the simulation are found to form L-shaped arrangements, like in the isolated (HCl)(2) dimer and in crystalline phases of HCl. The time correlation of the molecular-axis orientation is found to be characterized by a very short decay time (0.13 ps), consistent with the short length of the hydrogen-bonded chains. Other dynamical properties investigated in this work include the diffusion coefficient and the vibrational density of states. We evaluated the molecular dipole of the HCl molecule in the liquid using a definition based on the coupling of rotational modes to an external electric field. The average dipole moment (1.53 D) derived in this way is found to be considerably larger than for the isolated molecule (1.11 D). Our results show that the dipole moment in [script-l]-HCl undergoes large fluctuations, both in orientation and in modulus. Upon the onset of an external field, such dipole fluctuations concur to reduce the fluctuations of the dielectric response.

Translational and rotational mode coupling in disordered ferroelectric KTa 1− xNb xO 3 studied by Raman spectroscopy

The coupling of translational modes to the reorientational motion is an essential property of systems with internal orientational degrees of freedom. Due to their high complexity most of those systems (molecular crystals, glasses, etc.) present a major puzzle for scientists. In this paper we analyze the Raman scattering of a relatively simple ferroelectric system, KTa 1− x Nb x O 3, which may serve as a model for more complicated cases. We show the presence of a strong coupling between translational and reorientational motion in the crystal. Our data suggest that this coupling is the main reason for the depolarized component of the second-order Raman spectra and that it is also responsible for the frequency decrease (softening) of the transverse acoustic mode down to the third of three transitions, below which reorientational motion is no longer allowed.

Photometric Nonadiabatic Observables in Rotating β Cephei Models

We study how moderate rotation affects photometric observables inβ Cephei stars. The most important effect is the rotationalmode coupling of modes with harmonic degree, l, differing by2 and the same azimuthal order, m, if the frequencies are close.This is not an uncommon situation among unstable modes in βCep stars. Positions of the coupled modes in the amplitude ratiovs. phase difference diagrams are aspect, i, – andm-dependent. Inference from the diagrams becomes morecomplicated.

Photometric amplitudes and phases of nonradial oscillation in rotating stars

Effects of rotational mode coupling on photometric parameters of stellar oscillations are studied. At moderate rotation rates, a strong coupling between modes of spherical harmonic degree, $\ell$, differing by 2 and of the same azimuthal order, $m$, takes place if the frequencies are close. This is a common situation amongst main sequence pulsators. Numerical results for a sequence of $\beta$ Cephei star models are reported for the two- and three-mode couplings. One consequence of mode coupling is that modes of higher degree should be considered in photometric mode identification. Modes with nominal degree $\ell>2$ acquire substantial $\ell\le2$ components and therefore are more likely to reach detectable amplitudes. Coupled mode positions in the amplitude ratio - phase difference diagrams, based on multicolour photometry, become both aspect- and $m$-dependent. Examples of the mode path in the diagram with varying aspect are given. The diagrams remain a useful tool for mode identification in rotating stars but the tool must be used with care.

An Effect of Deuteration on Ion Motions and Hydrogen Bondings in Guanidinium Tetrafluoroborate

Abstract The fluorine spin-lattice relaxation time as well as NMR second moment for perdeuterated guanidinium tetrafluoroborate were studied over a wide range of temperature. An analytical solution of a set of coupled differential equations describing the time variation of nuclear magnetisations for four unlike spin systems was applied to analyse all cross-relaxation effects in the compound. Activation parameters EF a = 19.3 kJ/mole and τF U= 9 • 10"14 s for the isotropic anion reorientation were derived. A coupling of rotational modes of cation and anion was found. Significant lowering of the melting point explained by a weakening of the hydrogen bonds involves diminishing of the ion activation energies due to the large positive isotope effect

Coupling of rotational modes in a phase transition of a charge-transfer crystal

ESR spectra of triplet excitons in the charge-transfer crystal anthracene- d 10− s-tetracyanobenzene (TCNB) have been used to investigate changes in the molecular orientation. The data yield an order parameter describing the orientational phase transition at 196 K. This transition is described using Landau theory and is shown to be of first order. The change of a second order parameter has also been detected. A coupling relation between the two order parameters and the influence of this coupling on the nature of the phase transition are discussed.