Search

Several studies have shown that a number of stars pulsating in p modes lie between the $\beta$ Cep and $\delta$ Sct instability strips in the Hertzsprung-Russell (HR) Diagram. At present, there is no certain understanding of how p~modes can be excited in this $T_{\rm eff}$ range. The goal of this work is to disprove the conjecture that all stars pulsating in p modes and lying in this $T_{\rm eff}$ range are the result of incorrect measurements of $T_{\rm eff}$, contamination, or the presence of unseen cooler companions lying in the $\delta$ Sct instability strip (given the high binary fraction of stars in this region of the HR Diagram). Using TESS data, we show that the A0Vnne star HD 42477 has a single p mode coupled to several r modes and/or g modes. We rule out a contaminating background star with a pixel-by-pixel examination, and we essentially rule out the possibility of a companion $\delta$ Sct star in a binary. We model the pulsations in HD 42477 and suggest that the g modes are excited by overstable convective core modes. We also conjecture that the single p mode is driven by coupling with the g modes, or that the oblateness of this rapidly-rotating star permits driving by He II ionization in the equatorial region., Comment: 11 pages, 6 figures, 2 tables, accepted to MNRAS

HD49330 is a Be star that underwent an outburst during its five-month observation with the CoRoT satellite. An analysis of its light curve revealed several independent p and g pulsation modes, in addition to showing that the amplitude of the modes is directly correlated with the outburst. We modelled the results obtained with CoRoT. We modelled the flattening of the structure of the star due to rapid rotation in two ways: Chandrasekhar-Milne's expansion and 2D structure computed with ROTORC. We then modelled kappa-driven pulsations. We also adapted the formalism of the excitation and amplitude of stochastically excited gravito-inertial modes to rapidly rotating stars, and we modelled those pulsations as well. We find that while pulsation p modes are excited by the kappa mechanism, the observed g modes are a result of stochastic excitation. In contrast, g and r waves are stochastically excited in the convective core and transport angular momentum to the surface, increasing its rotation rate. This destabilises the external layers of the star, which then emits transient stochastically excited g waves. These transient waves produce most of the low-frequency signal detected in the CoRoT data and ignite the outburst. During this unstable phase, p modes disappear at the surface because their cavity is broken. Following the outburst and ejection of the surface layer, relaxation occurs, making the transient g waves disappear and p modes reappear. This work includes the first coherent model of stochastically excited gravito-inertial pulsation modes in a rapidly rotating Be star. It provides an explanation for the correlation between the variation in the amplitude of frequencies detected in the CoRoT data and the occurrence of an outburst. This scenario could apply to other pulsating Be stars, providing an explanation to the long-standing questions surrounding Be outbursts and disks., Comment: Accepted in A&A, 15 pages, abstract shown here has been abridged to fit Arxiv's character limit

The Be phenomenon, that is the ejection of matter from Be stars into a circumstellar disk, has been a long lasting mystery. In the last few years, the CoRoT (Convection, Rotation and planetary Transits) satellite brought clear evidence that Be outbursts are directly correlated with pulsations. We found that it may be the transport of angular momentum by waves or pulsation modes that brings the already rapid stellar rotation to its critical value at the surface, and allows the star to eject material. The recent discovery of stochastically excited gravito-inertial modes by CoRoT in a hot Be star strengthens this scenario. We present the CoRoT observations and modeling of several Be stars and describe the new picture of the Be phenomenon which arose from these results., Comment: 6 pages, proceedings of a conference held in Hakone, in press

We calculate axisymmetric toroidal modes of magnetized neutron stars with a solid crust. We assume the interior of the star is threaded by a poloidal magnetic field that is continuous at the surface with the outside dipole field whose strength $B_p$ at the magnetic pole is $B_p\sim 10^{16}$G. Since separation of variables is not possible for oscillations of magnetized stars, we employ finite series expansions of the perturbations using spherical harmonic functions to represent the angular dependence of the oscillation modes. For $B_p\sim 10^{16}$G, we find distinct mode sequences, in each of which the oscillation frequency of the toroidal mode slowly increases as the number of radial nodes of the eigenfunction increases. The frequency spectrum of the toroidal modes for $B_p\sim 10^{16}$G is largely different from that of the crustal toroidal modes of the non-magnetized model, although the frequency ranges are overlapped each other. This suggests that an interpretation of the observed QPOs based on the magnetic toroidal modes may be possible if the field strength of the star is as strong as $B_p\sim 10^{16}$G., Comment: submitted to mn

The Microvariability and Oscillations of stars (MOST) satellite has detected low-amplitude light variations ($\Delta m\sim$1 mmag) in the Be star $\beta$ CMi (B8Ve). The observations lasted 41 days and the variations have typical periods $\sim 0.3$ days. We demonstrate that the dominant frequencies are consistent with prograde high-order g-modes of $m=-1$ excited by the Fe-bump of opacity in an intermediate-mass ($\approx 3.5 M_\odot$) star with a nearly critical rotation period of 0.38 days. This is the first detection of nonradial g-mode pulsations in a Be star later than B6 leading to the possibility that pulsations are excited in all classical Be stars., Comment: 17 pages, 6 figures; Astrophysical Journal part 1 in press

Recently, multiple time scale characteristics of heart dynamics have received much attention for distinguishing healthy and pathologic cardiac systems. Despite structural peculiarities of the fetal cardiovascular system, the fetal heart rate(FHR) displays multiple time scale characteristics similar to the adult heart rate due to the autorhythmicity of its different oscillatory tissues and its interaction with other neural controllers. In this paper, we investigate the event and time scale characteristics of the normal and two pathologic fetal heart rate groups with the help of the new measure, called the Unit Time Block Entropy(UTBE), which approximates the entropy at each event and time scale based on symbolic dynamics. This method enables us to match the measurement time and the number of words between fetal heart rate data sets simultaneously. We find that in the small event scale and the large time scale, the normal fetus and the two pathologic fetus are completely distinguished. We also find that in the large event scale and the small time scale, the presumed distress fetus and the acidotic distress fetus are significantly distinguished., Comment: 15pages, 18figures, submitted to Physica A